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Sample records for amphibolites

  1. Radiometric dating of metamorphites of amphibolite facies from the Rudnany deposit, Spissko-Gemerske Rudohorie Mts

    International Nuclear Information System (INIS)

    Eruptive rocks from granodiorites to ultrabasic and rocks of amphibolite facies from the siderite deposit of Rudnany are discussed. They reach the Carboniferous, are characterized by intensive hydrothermal and metasomatic alterations. The genesis and mutual relations have so far been little cleared. The metamorphism into amphibolite facies is uncommon in the epimetamorphosed Spissko-gemerske rudohorie Mts. Its age ranging has been questionable. By the argon method the ages of 324, 320 and 281 mil. y were proved at amphiboles from amphibolites. These results disprove the views on the Alpine age of metamorphism and are an evidence of its Variscan age. (author)

  2. Growth of early continental crust controlled by melting of amphibolite in subduction zones.

    Science.gov (United States)

    Foley, Stephen; Tiepolo, Massimo; Vannucci, Riccardo

    2002-06-20

    It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process. PMID:12075348

  3. New Occurrence of Garnet Amphibolite and its Tectonic Implications in the Western Dabie Block, Eastern China

    Science.gov (United States)

    Tsai, C.; Zhou, H.; Iizuka, Y.

    2001-12-01

    We report a new occurrence of deformed garnet amphibolite as discontinuous pods enclosed within impure marble in the western rim of the Dabie tectonic block. The rock association (115\\deg 4' E, 31\\deg 2' N) is located close to the Macheng fault, the western limit of the Dabie block, and situated along the previously postulated boundary of the amphibolite unit (AU) and the Northern Orthogneiss unit (NOU). The field occurrence resembles those of well-documented coesite-bearing eclogites enclosed in marbles and schists in the southeastern Dabie. The studied marble contains mainly calcite with accessory phases of phlogopitic mica, quartz, plagioclase (andesine), and tremolitic amphibole. The mineral assemblage indicates metamorphic equilibrium under lower-to-middle amphibolite-facies conditions. In one garnet amphibolite, porphyroblastic almandine-rich (50-57 mol%) garnet commonly surrounded by a corona of symplectitic sodic plagioclase and hornblende indicates retrograde metamorphic reactions between the garnet porphyroblast and matrix precursor sodic clinopyroxene. In another sample, fine-grained intergrowths of diopsidic clinopyroxene (Na2O = 0.6-0.8 wt%) and sodic plagioclase (oligoclase) represent breakdown products from precursor omphacitic clinopyroxene. Domains of magnesio-hornblende and plagioclase (oligoclase-andesine) in textural equilibrium appear to have postdated the porphyroblastic garnet and inferred omphacitic pyroxene. In this regard, the garnet amphibolite must have been eclogite(s) overprinted by variant degrees of granulite- and amphibolite-facies metamorphism. A clockwise P-T path thus can be reconstructed. Such a finding confirms that the lack of ultrahigh-pressure (UHP) metamorphic records in the NOU and AU, in contrast to the wide distribution of eclogites and associated rocks in the southeastern part of the Dabie block, is best explained by post-UHP mid-to-high temperature metamorphic overprint involving hydration reactions during slab

  4. Scheelite distribution a long of amphibolitic belt from greenstone belt Barbacena, Minas Gerais, Brazil

    International Nuclear Information System (INIS)

    In the middle southern portion of the Minas Gerais state a 60 Km long and 12 Km wide tungsten belt was discovered, and related to the amphibolitic rocks of the Barbacena Greenstone. Tungsten, present as scheelite, is associated with amphibolites, amphibole schists and amphibole gneisses, with chemical characteristics indicating an igneous origin. Chemical analyses on pan concentrates by I.C.P. showed high values on lead, tin, yttrium, lanthanum, cerium and zirconium, and average values for zinc and copper. The scheelite mineralization is probably strata bound and has a possible submarine exhalative origin. (author)

  5. Emerald mineralization and metasomatism of amphibolite, khaltaro granitic pegmatite - Hydrothermal vein system, Haramosh Mountains, Northern Pakistan

    Science.gov (United States)

    Laurs, B.M.; Dilles, J.H.; Snee, L.W.

    1996-01-01

    Emerald mineralization is found within 0.1- to 1-m-thick hydrothermal veins and granitic pegmatites cutting amphibolite within the Nanga Parbat - Haramosh massif, in northern Pakistan. The amphibolite forms a sill-like body within garnet-mica schist, and both are part of a regional layered gneiss unit of Proterozoic (?) age. The 40Ar/39Ar data for muscovite from a pegmatite yield a plateau age of 9.13 ?? 0.04 Ma. Muscovite from mica schist and hornblende from amphibolite yield disturbed spectra with interpreted ages of 9 to 10 Ma and more than 225 Ma, respectively, which indicate that peak Tertiary metamorphism reached 325 to 550??C prior to 10 Ma. Pegmatites were emplaced after peak metamorphism during this interval and are older than pegmatites farther south in the massif. At Khaltaro, simply zoned albite-rich miarolitic pegmatites and hydrothermal veins containing various proportions of quartz, albite, tourmaline, muscovite, and beryl are associated with a 1- to 3-m-thick heterogeneous leucogranite sill, that is locally albitized. The pegmatites likely crystallized at 650 to 600??C at pressures of less than 2 kbar. Crystals of emerald form within thin (0.20, 0.54-0.89 wt%), to pale blue beryl (emerald results from introduction of HF-rich magmatic-hydrothermal fluids into the amphibolite, which caused hydrogen ion metasomatism and released Cr and Fe into the pegmatite-vein system.

  6. Petrophysical properties and durability of the Touro amphibolite s (A Coruna, Spain) used in civil engineering

    International Nuclear Information System (INIS)

    Nowadays, Touro amphibolites (La Coruna, Spain) are exploited as crushed-rock aggregates in civil engineering. These materials are mainly used as ballast for road construction. Therefore, it is interesting to study these materials in function of several parameters obtained directly in quarry faces. For this purpose, the same techniques and tests previously applied in other fields such as natural stone have been used. The aim of this work is to study the quality and durability of Touro amphibolites when subjected to extreme temperature, water and salt (NaCl) actions. The NaCl is included in this study because it is commonly used as fluxing material in pavements and roads. The quality of these amphibolites, a classification based on the concrete and road tests and the techniques applied, is good. The best properties of these materials are their low porosity and high ability to compact. They also have a high durability under the action of extreme temperatures, salts and water. These results also indicate that these amphibolites can better support high temperature rather than low temperature degradation. (Author)

  7. Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions

    Science.gov (United States)

    Rushmer, Tracy

    1991-03-01

    A large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase. As a consequence, H2O-undersaturated or fluid-absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events. In an attempt to determine the systematics of melting under fluid-absent conditions, a series of piston-cylinder experiments was conducted on two natural amphibolites; one, a meta-alkali basalt (ABA) with a total water content of ˜ wt% contained in hornblende, and the other, a meta-island-arc tholeiite (IAT) which has ˜1 1.3 wt% water contained in hornblende, cummingtonite and biotite. The experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures, and sta temperature interval over which amphibole was stable, were controlled by the amphibolites' different bulk compositions and their resulting metamorphic assemblages. The volume % of melt produced by melting of the two amphibolites were compared with estimated amounts, based on Burnham's (1979) water-melt solubility model and the fluid-absent melting model presented by Clemens and Vielzeuf (1987). The observed melt volumes were greater than estimated. As the water content of melt largely detemines the volume % of melt produced, independent measurements of the water-content of the glass formed during partial melting in the ABA were made by thermogravimetric analyses. The water content of the ABA glass is ˜2 wt%, which is less than the assumed “melt-water” content (water content of the melt) used in previous modeling of fluidabsent anatexis in mafic lithologies. As a consequence, more melt can be expected during fluid-absent partial melting of mafic lower crust, as is observed in the experiments. A modification of the Clemens and Vielzeuf (1987) fluid-absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems

  8. Dehydration melting of solid amphibolite at 2.0 GPa: Effects of time and temperature

    Institute of Scientific and Technical Information of China (English)

    ZHOU; Wenge; XIE; Hongsen; LIU; Yonggang; ZHENG; Xiaogang

    2005-01-01

    Two sets of dehydration-melting with a natural solid amphibolite, collected from North Himalayan structure zone, Tibet, have been carried out in multi-anvil apparatus at 2.0 GPa and 800―1000℃, for 12―200 h. One is keeping the pressure at 2.0 GPa and the annealing time of 12 h, changing the temperature (800―1000℃). The other is keeping the pressure at 2.0 GPa and temperature at 850℃, varying the annealing time (12―200 h). The products are inspected with microscope and electron probe. The results indicate that at 2.0 GPa, annealing time of 12 h, garnets, melts and clinopyroxenes occur in amphibolite gradually with increasing temperature and the chemical compositions of melt vary from tonalite to granodiorite, and then to tonalite. However, at 2.0 GPa and 850℃, with the annealing time increasing, the garnets, melts and clinopyroxenes also occur in amphibolite gradually and the chemical compositions of melt vary from tonalite to granodiorite. In both cases, melts interconnect with each other when the contents of melt are over the 5 vol.%. the viscosities of the melt produced in amphibolite at temperature higher than 850℃ are on a level with 104 Pa·s. The interconnected melt with such a viscosity may segregate from the source rock and form the magma over reasonable geological time. Therefore, it is believed that at the lower part of the overthickened crust, the tonlitic and granodioritic magma may be generated through the dehydration melting of amphibolite.

  9. Geotectonic significance of Neoproterozoic amphibolites from the Central Eastern Desert of Egypt: A possible dismembered sub-ophiolitic metamorphic sole

    Science.gov (United States)

    Farahat, E. S.

    2011-07-01

    Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc-boninitic and MORB/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic association and intruded by late- to post-orogenic granitoids and gabbros. The WUG rocks are metamorphosed under epidote amphibolite to common amphibolite facies. The amphiboles are calcic and represented by actinolitic hornblende to magnesio-hornblende in the epidote amphibolites and magnesio- to ferro-hornblende in the amphibolites. Plagioclase composition varies from pure albite (An3-8) in the epidote amphibolites to andesine and labradorite (An36-65) in the amphibolites. The estimated P-T conditions are in favor of their metamorphism under epidote amphibolite (c. 550-600 °C and 2-3 ± 1.5 kbar) and amphibolite (c. 618-720 °C and 3-6 ± 1.5 kbar) facies. The peak metamorphic conditions point to a burial depth of c.15-20 km. Geochemically, the WUG amphibolites show basaltic to andesitic compositions of tholeiitic affinity. They display LILE-enriched MORB-normalized patterns with negative Nb anomalies characteristic of the subduction-related rocks. However, their chondrite-normalized rare-earth element (REE) patterns vary from LREE-depleted (LaN/YbN = 0.29 to 0.49) to LREE-enriched (LaN/YbN = 2.97 to 3.74). Few samples show major and trace element contents typical of boninitic rocks, including U-shaped REE pattern. On the standard tectonic discrimination diagrams the WUG amphibolites plot mostly in the island-arc fields with some samples of MORB and boninitic affinities. Greenschist facies metamorphosed NCEDO obviously share these geochemical characteristics, implying formation in the same tectonic environment, i.e. forearc basin

  10. Mid amphibolite facies metamorphism of harzburgites in the Neoproterozoic Cerro Mantiqueiras Ophiolite, southernmost Brazil

    Directory of Open Access Journals (Sweden)

    HARTMANN LÉO A.

    2003-01-01

    Full Text Available Valuable information is retrieved from the integrated investigation of the field relationships, microstructure and mineral compositions of harzburgites from the Neoproterozoic Cerro Mantiqueiras Ophiolite. This important tectonic marker of the geological evolution of southernmost Brazilian Shield was thoroughly serpentinized during progressive metamorphism, because the oldest mineral assemblage is: olivine + orthopyroxene + tremolite + chlorite + chromite. This M1 was stabilized in mid amphibolite facies - 550-600ºC as calculated from mineral equilibria. No microstructural (e.g. ductile deformation of olivine or chromite or compositional (e.g. mantle spinel remnant of mantle history was identified. A metamorphic event M2 occurred in the low amphibolite facies along 100 m-wide shear zones, followed by intense serpentinization (M3 and narrow 1-3 m-wide shear zones (M4 containing asbestos.

  11. Amphibolites and other metamorphic mafic rocks of the blastomylonitic graben in Western Galicia, NW Spain: field relations and petrography

    OpenAIRE

    Arps, C.E.S.

    1981-01-01

    Within the strongly migmatized axial zone of the Hesperian massif in western Galicia a graben-like structure has been distinguished, characterized essentially by the presence of non-migmatic rocks that comprise orthogneisses with blastomylonitic textures, leucocratic gneisses, plagioclase-blastbearing paragneisses, pelitic schists, and numerous amphibolitic layers and lenses. In the southern and central part of the graben and at the borders in the north the majority of the amphibolites are me...

  12. The Minerageny of Two Groups of Zircons from Plagioclase- Amphibolite of Mayuan Group in Northern Fujian

    CERN Document Server

    Bao, Xuezhao

    1996-01-01

    Zircon can crystallize in a wide range of physical and chemical conditions. At the same time, it has very high stability and durability. Therefore zircon can grow and survive in a variety of geological processes. In addition, the diffusivity of chemical compositions in its crystal is very low. Consequently, we can trace back the evolution history of the planetary materials containing zircon by zircon U-Th-Pb geochronology and geochemistry studies. However, this depends on our ability to decipher its genesis, namely magmatic or metamorphic origins. In this paper, magmatic and metamorphic zircons were found from plagioclase-amphibolite samples. Their geneses have been determined by zircon morphology, chemical composition zonations and geological field setting combined with their zircon U-Th-Pb ages. We have found obvious differences in micro-scale Raman spectra between these magmatic and metamorphic zircons. The magmatic zircons exhibit a high sloping background in their Raman spectra, but the metamorphic zirco...

  13. Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina schist, southern California

    Energy Technology Data Exchange (ETDEWEB)

    Sorensen, S.S. (Smithsonian Institution, Washington, DC (USA)); Grossman, J.N. (Geological Survey, Reston, VA (USA))

    1989-12-01

    The abundance, P-T stability, solubility, and element-partitioning behaviour of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600{degree}C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace element among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM.

  14. Bald Friar Metabasalt and Kennett Square Amphibolite: Two Iapetan Ocean Floor Basalts

    Science.gov (United States)

    Smith, R.C., II

    2006-01-01

    The Bald Friar Metabasalt (BFM) and Kennett Square Amphibolite (KSA) are basaltic units found in the Piedmont of southeastern Pennsylvania. The BFM is also recognized in northern Maryland. Both are believed to represent fragments of the floor of the Iapetus Ocean, but are not known occur in direct association with one another. The BFM typically occurs as small fragments having typical stratigraphic thicknesses of 2.5 m, and composed of greenish, fine-grained chlorite-epidote-actinolite-albite metabasalt in ophiolite me??lange. One bed of pillow basalt has been found at the type locality, Bald Friar, Cecil County, Maryland. Even though outcrops of BFM are highly discontinuous, they have a remarkable chemical uniformity over a strike length of 143 km and appear to be equivalent to the Caldwell Group 1b metabasalt of the Thetford, Quebec, area. The BFM is typically associated with ultramafic fragments and may be affiliated with the Baltimore Mafic Complex (BMC), from which a baddeleyite date of 442 +/- 7 Ma (Silurian) has been obtained. The BFM is probably a back arc basin basalt (BABB). Pod and schlieren chromite compositions suggest an island arc environment for the BMC itself. The poorly defined, informal "Conowingo Creek metabasalt" of Lancaster County, Pennsylvania, occurs on the north margin of the BMC and appears to be a fore arc boninite. The BFM and associated ultramafic fragments serve as a field-mappable marker for the structural equivalent of the Baie Verte-Brompton line in southeastern Pennsylvania and northern Maryland. Steatization of the associated ultramafic fragments has produced zones of extremely low competence that facilitated and localized thrusts of presumed Silurian age and later Alleghanian folding. The KSA typically occurs as much larger bodies having lengths of 3 km and composed of dark, medium-grained hornblende-plagioclase-clinopyroxene gneiss. No ultramafic rocks or me??lange have been recognized with the KSA. In Pennsylvania, the KSA

  15. Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

    OpenAIRE

    LÉO A. HARTMANN; João O. S Santos; Jayme A.D. Leite; Carla C. Porcher; McNaughton, Neal J.

    2003-01-01

    The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at ...

  16. Protolith age of Santa Maria Chico granulites dated on zircons from an associated amphibolite-facies granodiorite in southernmost Brazil

    International Nuclear Information System (INIS)

    U-Pb dating of zircon was undertaken with the Beijing SHRIMP II (sensitive high resolution ion microprobe) on an amphibolite facies granodiorite and an almandine-albite granulite from the Santa Maria Chico Granulitic Complex, southern Brazilian Shield. This work was also done to unravel protolith ages which are often hidden in the array of partly reset data. The obtained metamorphic ages of the granodiorite gneiss and the granulite are 2035 ± 9 Ma and 2006 ± 3 Ma, respectively. These data are within the range of metamorphic ages determined in previous studies (2022 ± 18 Ma and 2031 ± 40 Ma). However, protolith ages for the granodiorite (2366 ± 8 Ma) and the granulite (2489 ± 6 Ma) were obtained which are outside the previously recognized range (> 2510-2555 Ma). The magmatic protolith age of the granodiorite refers to a previously little known magmatic event in the shield. Further investigations may demonstrate that amphibolite facies zircon crystals are useful as a window into geological events in associated granulites, because zircon ages are blurred in the studied granulites. (author)

  17. Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizario ultramafic amphibolite, Encantadas Complex, southernmost Brazil

    International Nuclear Information System (INIS)

    The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizario ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702+- 21 Ma during a greenschist facies event M2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257 ± 12 Ma) and Camboriu Orogeny (∼1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702 ± 21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton. (author)

  18. Quantitative analysis of material transfer during the ascent of garnet-amphibolite mass in the Sambagawa metamorphic belt, Japan

    Science.gov (United States)

    Uno, M.; Toriumi, M.

    2010-12-01

    mineral zoning of garnet and amphibole at each z. Garnet amphibolites in the amphibolite mass of the Sambagawa metamorphic belt were analyzed. The precise pressure-temperature-time history of the amphibolite was obtained by application of the Gibbs’ method to both amphibole and garnet zonings. The obtained P-T path was heating decompression path with steep dP/dT, with the rim condition of 600oC, 11kbar and the peak condition of at least 550oC, 15kbar. To estimate mass divergence dm(z,t) that has occurred in the amphibolite along the estimated metamorphic P-T paths, the new method was applied to the mineral banding of the garnet amphibolite. It is inferred from the results, that Na, Mg, Fe, Ca and Al components have transferred among and/or within the layers. It was quantitatively confirmed that the mass transfer has occurred as if it intensifies the heterogeneity of rock composition and mineral modes, rather than to homogenize them.

  19. Geochronological and Geochemical evidence of amphibolite from the Hualong Group, northwest China: Implication for the early Paleozoic accretionary tectonics of the Central Qilian belt

    Science.gov (United States)

    Wang, Tao; Wang, Zongqi; Yan, Zhen; Ma, Zhenhui; He, Shengfei; Fu, Changlei; Wang, Dongsheng

    2016-04-01

    The Hualong Group, located in the Central Qilian belt, northwest China, consists mainly of schist, amphibolite, quartzite, and marble, ranging from greenschist to amphibolite facies metamorphism. On the basis of the medium-grade metamorphism, the group has been considered to comprise Proterozoic basement rocks. In this study, geochemical, Sr-Nd isotopic, and zircon U-Pb geochronological analyses were performed on lentoid amphibolites from the Hualong Group, to characterize their age, petrogensis, and tectonic setting. Uranium-lead zircon dating of amphibolite revealed a formation age of 456 ± 2 Ma and a metamorphic age of 440 ± 1 Ma. Major, trace, and rare earth element data indicate that the amphibolites are predominantly basaltic-andesitic to andesitic rocks, with island arc affinities. The trace element patterns show enrichment in large-ion lithophile elements and depletion in high field strength elements relative to the N-MORB which confirm their island arc signatures. Obviously enriched light REEs ((La/Yb)N = 2.5-16.9) to chondrite normalized REE patterns further support this interpretation. The εNd(t) values for the amphibolites range from 4.6 to + 2.1, indicating subducted sediments as a larger endmember in the source. Geochemical data for these rocks suggest an island arc setting, and the rocks were derived from the depleted mantle that was enriched by melts of subducted sediments in an active continental margin setting at ca. 456 Ma. Together with regional evidence it suggests that the Hualong Group is an accretionary complex that was incorporated into the Central Qilian belt during the 440-400 Ma orogenic event.

  20. High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone related to the Tapo ultramafic body, Eastern Cordillera of Central Peru

    OpenAIRE

    Willner, Arne P.; Castroviejo Bolibar, Ricardo; Rodrigues, Jose F.; Acosta, Jorge; Rivera Feijóo, Miguel

    2010-01-01

    A discontinuos belt of elongated ultramafic rock bodies (mostly serpentinites) occurs in the Eastern Cordillera of the central Peruvian Andes. One of the main occurrences is the Tapo Massif, a lense-shaped serpentinite body, ~2 km x 5 km, comprising small podiform chromitite deposits (Castroviejo et al., 2009) and bands or lenses of garnet-amphibolite, both strongly sheared and thrust upon the upper Paleozoic sediments of the Ambo Group (Fig. 1). Metabasite geochemistry suggests a mid-oce...

  1. Kwangsian and Indosinian reworking of the eastern South China Block: Constraints on zircon U-Pb geochronology and metamorphism of amphibolites and granulites

    Science.gov (United States)

    Wang, Yuejun; Wu, Chunming; Zhang, Aimei; Fan, Weiming; Zhang, Yanhua; Zhang, Yuzhi; Peng, Touping; Yin, Changqin

    2012-10-01

    To constrain temporal-spatial patterns of the Kwangsian and Indosinian orogenic events and the Phanerozoic tectonic evolution of the eastern South China Block, this paper documents the zircon U-Pb geochronology of the high-grade metamorphic rocks for key outcrops in the eastern SCB and the P-T path for the Indosinian metamorphic amphibolites. Our data show that the representative amphibolites from the originally-defined Badou, Mayuan, Chencai, Zhoutan and Yunkai Groups and associated granulites yielded zircon U-Pb metamorphic ages of 428-468 Ma. The weighted mean ages of 221-246 Ma are also given by the metamorphic zircons from amphibolites and granulites in the Yunkai, Shiwandashan, northern Wuyi and southern Hunan regions. The two age groups are followed by the 40Ar/39Ar plateau ages of 406-438 Ma and 195-217 Ma for synkinematic minerals from major shear zones in the eastern South China Block, respectively. The age-spans of 406-468 Ma and 195-246 Ma roughly coincide with the formation time of the Kwangsian (396-462 Ma) and Indosinian (202-248 Ma) granites of the eastern South China Block, respectively. The strongly compressive deformation, metamorphism and granitic magmatism are roughly contemporaneous for the Kwangsian and Indosinian tectonothermal events in the eastern South China Block. They occurred over a similar spatial extent, only reaching westward to the Jiangnan-Xuefeng Domains across the Jiangshan-Shaoxing Fault. The amphibolites from Zhouyuan (SW Zhejiang Province) of the Cathaysia Block, which have the Indosinian zircon U-Pb metamorphic ages of 243-246 Ma, underwent three episodes of metamorphism (M1-M3) and yielded a near-isothermal decompression clockwise P-T path. In combination with other geological data, it is proposed that the ages of ca. 430 Ma and ca.230 Ma might be interpreted as the transformational time from the prograde- to retro-metamorphism for the Kwangsian and Indosinian events, respectively. The Kwangsian and Indosinian orogenic

  2. Strain localization and fluid infiltration during subduction initiation: the record from sheared mafic amphibolites at the base of the New Caledonian ophiolite

    Science.gov (United States)

    Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Vitale-Brovarone, Alberto; Monié, Patrick; Chauvet, Alain; Whitechurch, Hubert

    2015-04-01

    Most of our knowledge on subduction inception and obduction processes comes from metamorphic soles structurally associated with peridotite tectonites at the base of many ophiolites, and from early-obduction, rarely deformed, magmatic dikes emplaced at different level of the mantle sequence. These dikes record a partial refertilization of obducted ophiolites through subduction-derived fluids. However, these dikes are rarely deformed and/or metamorphosed. Here, we study the base of the New Caledonian ophiolite, using a combination of structural field studies and petrological-geochemical-geochronological analysis, with the aim of linking deformation and metasomatism through fluid infiltration and recrystallization. We report the existence of strongly sheared mafic amphibolites within the base of the New Caledonian obducted ophiolite, ~ 50-100 m above the basal thrust contact and peridotites), highly boudinaged and amphibolitized at high temperatures (750-800 °C), providing evidence that strain localized at the base of the ophiolite. Mafic protoliths of these amphibolites consisted of plagioclase and orthopyroxene (± olivine and calcic amphibole in places). We show that deformation is intimately associated to at least three major stages of fluid infiltration on mafic intrusions. The first stage of deformation and metasomatism coincides with amphibolitization and controlled the later channelization of fluids. The formation of calcic amphiboles records the percolation of Ca and Al-rich aqueous fluids. Amphibole-plagioclase geothermobarometry indicates high temperature and low pressure conditions (i.e. 750-800 °C; 3-5 kbar). Thermochronological data from hornblende (40Ar/39Ar) suggest that this deformation episode occurred at ~ 55 Ma, coinciding with E-dipping subduction initiation and incipient obduction. The main metasomatic stage is evidenced by a phlogopite-rich matrix wrapping peridotite and amphibolite boudins. The formation of phlogopite records the percolation

  3. Late Triassic post-collisional slab break-off along the Ailaoshan suture: insights from OIB-like amphibolites and associated felsic rocks

    Science.gov (United States)

    Liu, Huichuan; Wang, Yuejun; Guo, Xiaofei; Fan, Weiming; Song, Jingjing

    2016-08-01

    What dynamic processes the South China-Indochina collision had gone through is still a pending problem. Our recent investigations identified a Late Triassic gabbroic intrusion in Mengdong village and several granitic plutons near Wana village, Yunnan province (SW China). Both have underwent strong metamorphism and been altered to amphibolite and granitic gneiss, respectively. We carried out SIMS and LA-ICPMS zircon U-Pb dating for the amphibolites and granitic gneisses, respectively. Dating results yield weighted mean 206Pb/238U ages of 221.5 ± 5.3 Ma for the amphibolites and 224.0 ± 1.8 and 235.4 ± 0.6 Ma for the granitic gneisses. The amphibolite samples have low Mg# (41.7-42.8), high TiO2 (~3.7 wt%) and Na2O (Na2O/K2O = 1.89-2.68) contents and depleted Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.707647 and ɛ Nd(t) = +1.17]. They exhibit OIB-like REE patterns and spidergrams, with strongly enriched LREE contents, insignificant Eu anomalies (Eu* = 0.99-1.03) and moderate negative Sr anomalies (Sr* = 0.46-0.56). The protolith gabbro of the Mengdong amphibolites is derived from low-degree partial melting of a homogeneous OIB-type mantle source in the garnet stability field and experienced significant fractionation of olivine, clinopyroxene and plagioclase during magma evolution. The Wana samples are strongly peraluminous (A/CNK > 1.1) and K-enriched (K2O/Na2O = 2.93-3.63). They show enriched Sr-Nd isotopic compositions with (87Sr/86Sr)i = 0.718589-0.719754 and ɛ Nd(t) = -11.34 to -10.92. The Wana plutons are typical S-type granite and product of the dehydration melting of meta-sedimentary rocks. We summarized Late Triassic age data in the literature along the South China-Indochina suture zone and recognized a significant Late Triassic magmatic flare-up. In combination with previous studies on the Late Triassic HP-UHP metamorphic rocks, we proposed that during the Late Triassic the South China-Indochina suture zone had transformed into post-collisional setting

  4. Metamorphic rocks of amphibolite facies from the Atokura Klippe in the Minano town, Kanto Mountains and a K-Ar age determination

    International Nuclear Information System (INIS)

    Regional metamorphic rocks mainly composed of hornblende schists and quartzo-feldspathic rocks of amphibolite facies are exposed in southern margin of the Sanbagawa metamorphic belt of the Kanto Mountains. The metamorphic rocks are in contact with quartz diorite and Atokura formation by faults, and intruded by granitic rocks. A K-Ar age of hornblende separated from a hornblende schist is 105+-5 Ma, older than those of nearby Sanbagawa and Ryoke metamorphic rocks. The metamorphic studied here are one of geologic units belonging to the Atokura Nappe. (author)

  5. Geochemical Characteristics and LA-ICP-MS Zircon U-Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

    Institute of Scientific and Technical Information of China (English)

    LIU Liang; CHEN Danling; ZHANG Anda; ZHANG Chengli; YUAN Honglin; LUO Jinhai

    2004-01-01

    Geochemical studies on the amphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The amphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N =0.33-0.55, (La/Sm)N= 0.45-0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N-MORB. The second group of rocks have (La/Yb)N=0.63-0.95, (La/Sm)N = 0.69-0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82,0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N-MORB and E-MORB but closer to the former. The two groups of rocks both exhibit flat pattems from Th to Yb in the highly incompatible elements spider diagram, but the first group of rocks have lower element abundances than the modem N-MORB, indicating a derivation of their mantle source from more depleted mantle source than the present N-MORB. The abundances of Th, Ta, Nb, La and Ce in the second group of rocks are slightly higher than those of the present N-MORB, and other elements, such as Hf, Zr,Sm, Ti, Y and Yb, are close to those of the N-MORB, indicating that the original magma was derived from depleted mantle but mixed with the enriched mantle. These characteristics, combined with the regional geology and previous studies,provide further evidence that the mafic-ultramafic rocks have the features of a typical ophiolite.Zircon grains from the amphibolite are generally rounded, and in most of them a distinguishable core-mantle texture is preserved as shown in the cathodoluminescence (CL) images. The core or core-mantle parts of the zircon grains are also rounded, same as those

  6. Strongly foliated garnetiferous amphibolite clasts in ophiolitic melanges, Yarlung Zangbo Suture Zone, Tibet; Early Cretaceous disruption of a back-arc basin?

    Science.gov (United States)

    Guilmette, C.; Hebert, R.; Wang, C.; Indares, A. D.; Ullrich, T. D.; Dostal, J.; Bedard, E.

    2007-12-01

    Metre to decameter-size clasts of amphibolite are found embedded in ophiolitic melanges underlying the Yarlung Zangbo Suture Zone Ophiolites, South Tibet, China. These ophiolites and melanges occur at the limit between Indian and Tibetan-derived rocks and represent remnants of an Early Cretaceous intraoceanic supra-subduction zone domain, the Neo-Tethys. In the Saga-Dazuka segment (500 km along-strike), we discovered new occurrences of strongly foliated amphibolites found as clasts in the ophiolitic melange. In garnet-free samples, hornblende is green-blue magnesio-hornblende and cpx is low-Al diopside. In garnet- bearing samples, garnet is almandine with a strong pyrope component (up to 30 mol%) whereas coexisting hornblende is brown Ti-rich tschermakite and clinopyroxene is Al-diopside. Plagioclase composition was ubiquitously shifted to albite during a late metasomatic event. Geochemistry of these rocks indicates that their igneous protoliths crystallized from a slightly differentiated tholeiitic basaltic liquid that did not undergo major fractionation. Trace element patterns reveal geochemical characteristics identical to those of the overlying ophiolitic crust. These are 1) trace element abundances similar to that of N-MORBs or BABBs, 2) a slight depletion of LREE and 3) a moderate to strong Ta-Nb negative anomaly and a slight Ti anomaly. Such characteristics suggest genesis over a spreading center close to a subduction zone, possibly a back-arc basin. Step-heating Ar/Ar plateau ages were obtained from hornblende separates. All ages fall in the range of 123-128 Ma, overlapping the crystallization ages from the overlying ophiolite (126-131 Ma). Pseudosections were built with the THERMOCALC software in the system NCFMASH. Results indicate that the observed assemblage Hb+Pl+Gt+Cpx is stable over a wide range of P-T conditions, between 10-18 kbars and at more than 800°C. Measured mineral modes and solid solution compositions were successfully modeled, indicating

  7. Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

    Directory of Open Access Journals (Sweden)

    Léo A. Hartmann

    2003-09-01

    Full Text Available The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702±21 Ma during a greenschist facies eventM2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257±12 Ma and Camboriú Orogeny (~ 1989 Ma of the Trans-Amazonian Cycle, followed by an orogenic event (702±21 Ma of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.O entendimento dos processos evolutivos do Complexo Encantadas no sul do Brasil foi aperfeiçoado através do estudo integrado do metamorfismo de um anfibolito ultramáfico e da geocronologia U-Pb SHRIMP de zircão. Os núcleos herdados de alguns cristais de zircão tem idades em torno de 2257 ±12 Ma e constituem a única evidência preservada do protólito ígneo, que pode ter sido um basalto magnesiano ou um piroxenito. O metamorfismo M de fácies anfibolito formou abundante hornblenda na amostra investigada, possivelmente há 1989 ±21 Ma. Esta rocha ultramáfica foi re-metamorfizada talvez há cerca de 702 ±21 Ma durante um evento M de fácies xistos verdes do metamorfismo regional. Durante o evento M, a hornblenda foi recristalizada e formou a assembléia actinolita + oligoclásio + microclínio + epidoto + titanita + monazita. Estes eventos foram a manifesta

  8. Trace and rare-earth element geochemistry: A tool for petrogenetic and geotectonic modeling of ensi-matic ortho-amphibolites from Pan-African belt of Obudu Plateau, Southeastern Nigeria

    Institute of Scientific and Technical Information of China (English)

    Ukaegbu V.U.; Beka F.T.

    2009-01-01

    A model for the petrogenetic affinity and original geotectonic setting of ortho-amphibolites from the Obudu Plateau was tested using the distribution patterns of trace and rare-earth elements from the geochemical analyses of twelve representative amphibolite samples. Discrimination plots, normalized patterns of the incompatible trace elements against average MORB, low ratios of Ba/Nb (9-23) and Ba/Ta (130-327) and other geochemical characteristics suggest that the protoliths were dominantly of tholeiitic MORB composition. The values of the ratios of La/Ta (8.13-10.8), Rb/Sr (0.044).07), Th/U (mainly 4.43-5.43) and Hf/Ta (2.35-2.88) further indicate that the ortho-amphibolites demonstrate E-type MORB characteristics. These features are related to substantial ocean floor divergent tectonic boundary setting rather than marginal basin tectonic setting. This evolutionary pattern appears to be controlled by limited progressive partial melting and fractional crystallization of a single mantle source region, irrespective, however, of variations due to local source heterogeneities. The tholeiitic magmas were most probably generated by hotspot activities on the constructive plate margins and subsequently modified by subduction-related low-K tholeiitic chemistry due to narrowing of a proto oceanic basin between the West African craton and eastern Sahara plate. A likely model, therefore, is that the amphibolites of the Obudu Plateau represent ophiolitic suites with characteristics of enriched mid-ocean floor tholeiites, which were deformed and metamorphosed during a reversed plate motion involving subduction and collision within the Obudu Plateau in the Pan-African orogenic episode.

  9. Anomalous Seismic Velocity Drop in Iron and Biotite Rich Amphibolite to Granulite Facies Transitional Rocks from Deccan Volcanic Covered 1993 Killari Earthquake Region, Maharashtra (India): a Case Study

    Science.gov (United States)

    Pandey, O. P.; Tripathi, Priyanka; Vedanti, Nimisha; Srinivasa Sarma, D.

    2016-07-01

    65 Ma Deccan Volcanic Province of western India forms one of the largest flood basaltic eruptions on the surface of the earth. The nature of the concealed crust below this earthquake prone region, which is marked by several low velocity zones at different depths has hardly been understood. These low velocity zones have been invariably interpreted as fluid-filled zones, genetically connected to earthquake nucleation. While carrying out detailed geological and petrophysical studies on the Late Archean basement cores, obtained from a 617 m deep KLR-1 borehole, drilled in the epicentral zone of 1993 Killari earthquake region of the southern Deccan Volcanic Province, we came across several instances where we observed remarkable drop in measured P-wave velocity in a number of high density cores. We provide detailed petrographic and geological data on 11 such anomalous samples which belong to mid-crustal amphibolite to granulite facies transitional rocks. They are associated with a mean P-wave velocity of 6.02 km/s (range 5.82-6.22 km/s) conforming to granitic upper crust, but in contrast have a high mean density of 2.91 g/cm3 (range 2.75-3.08 g/cm3), which characterise mid to lower crust. This velocity drop, which is as much as 15 % in some cores, is primarily attributed to FeOT enrichment (up to about 23 wt%) during the course of mantle-fluid driven retrogressive metasomatic reactions, caused by exhumation of deep-seated mafic rocks. Presence of Iron content (mainly magnetite), widely seen as opaques in thin sections of the rocks, seems to have resulted into sharp increase in density, as well as mean atomic weight. Our study indicates that the measured V p is inversely related to FeOT content as well as mean atomic weight of the rock.

  10. 40Ar/39Ar constraints on the timing and history of amphibolite facies gold mineralisation in the Southern Cross area, Western Australia

    International Nuclear Information System (INIS)

    The Southern Cross Greenstone Belt in Western Australia contains structurally controlled, hydrothermal gold deposits which are thought to have formed at or near the peak of amphibolite facies regional metamorphism during the Late Archaean. Although the geological features of deposits in the area are well documented. conflicting genetic models and ore-fluid sources have been used to explain the observed geological data. This paper presents new 40Ar/39Ar data which suggest that the thermal history of the Southern Cross area after the peak of regional metamorphism was more complex than has previously been suggested. After the main gold mineralisation event prior to ca 2620 Ma, the 40Ar/39Ar ages from amphiboles and biotites sampled from the alteration selvages of gold-bearing veins indicate that temperatures remained elevated in the region of 500 deg C for between 20 and 70 million years. These amphiboles and biotites from individual deposits yield ages that are in good agreement with one another to a high precision. implying increased cooling rates after the long period of elevated temperatures. Along the Southern Cross Greenstone Belt. however. amphibole-biotite pairs from the alteration selvages of gold-bearing quartz veins. while remaining in good agreement with one another, vary between deposits from ca 2560 Ma to ca 2440 Ma. Amphiboles from metabasalts that are associated with regional metamorphism and not hydrothermal alteration contain numerous exsolution lamellae that reduce the effective closure temperature of the amphiboles and yield geologically meaningless ages. These age relationships show that the thermal history of the area did not follow a simple cooling path and the area may have been tectonically active for a long period after the main gold mineralisation event before ca 2620 Ma. Such data may provide important constraints on subsequent genetic modelling of gold mineralisation and metamorphism. Copyright (1998) Blackwell Science Asia

  11. Constraints on Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust from a survey of orogenic eclogites and amphibolites

    Science.gov (United States)

    Zirakparvar, N. Alex

    2016-04-01

    To further understand Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust, this paper evaluates all available Lu-Hf garnet isochron ages and initial ɛHf values in conjunction with present-day bulk-rock Lu-Hf isotope and trace element (K, Nb, Ta, Zr, and Ti in addition to Lu-Hf) data from the world's orogenic eclogites and amphibolites (OEAs). Approximately half of OEAs exhibit Lu-Hf and Nb-Ta systematics mimicking those of unsubducted oceanic crust whereas the rest exhibit variability in one or both systems. For the Lu-Hf system, mixing calculations demonstrate that subduction-related phase transformations, in conjunction with open system behavior, can shift subducted oceanic crust toward higher Lu/Hf, or toward lower Lu/Hf that can also be associated with unradiogenic ɛHf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEAs have Nb-Ta systematics that are decoupled from Lu-Hf and the behavior of K, Zr, and Ti. Nonetheless, the global data set demonstrates that the association between unradiogenic ɛHf and elevated Nb/Ta observed in some kimberlitic eclogite xenoliths can be inherited from processes that occurred during subduction of their oceanic crustal protoliths. This allows for a geologically based estimate of the Nb concentration in a reservoir composed of deeply subducted oceanic crust. However, mass balance calculations confirm that such a reservoir, when considered as a whole, likely has a Nb concentration similar to unsubducted oceanic crust and is therefore not the solution to the problem of the Earth's "missing" Nb.

  12. Anomalous Seismic Velocity Drop in Iron and Biotite Rich Amphibolite to Granulite Facies Transitional Rocks from Deccan Volcanic Covered 1993 Killari Earthquake Region, Maharashtra (India): a Case Study

    Science.gov (United States)

    Pandey, O. P.; Tripathi, Priyanka; Vedanti, Nimisha; Srinivasa Sarma, D.

    2016-03-01

    65 Ma Deccan Volcanic Province of western India forms one of the largest flood basaltic eruptions on the surface of the earth. The nature of the concealed crust below this earthquake prone region, which is marked by several low velocity zones at different depths has hardly been understood. These low velocity zones have been invariably interpreted as fluid-filled zones, genetically connected to earthquake nucleation. While carrying out detailed geological and petrophysical studies on the Late Archean basement cores, obtained from a 617 m deep KLR-1 borehole, drilled in the epicentral zone of 1993 Killari earthquake region of the southern Deccan Volcanic Province, we came across several instances where we observed remarkable drop in measured P-wave velocity in a number of high density cores. We provide detailed petrographic and geological data on 11 such anomalous samples which belong to mid-crustal amphibolite to granulite facies transitional rocks. They are associated with a mean P-wave velocity of 6.02 km/s (range 5.82-6.22 km/s) conforming to granitic upper crust, but in contrast have a high mean density of 2.91 g/cm3 (range 2.75-3.08 g/cm3), which characterise mid to lower crust. This velocity drop, which is as much as 15 % in some cores, is primarily attributed to FeOT enrichment (up to about 23 wt%) during the course of mantle-fluid driven retrogressive metasomatic reactions, caused by exhumation of deep-seated mafic rocks. Presence of Iron content (mainly magnetite), widely seen as opaques in thin sections of the rocks, seems to have resulted into sharp increase in density, as well as mean atomic weight. Our study indicates that the measured V p is inversely related to FeOT content as well as mean atomic weight of the rock.

  13. Geochemistry of amphibolites and related graphitic gneisses from the Suchý and Malá Magura Mountains (central Western Carpathians – evidence for relics of the Variscan ophiolite complex

    Directory of Open Access Journals (Sweden)

    Ivan Peter

    2015-10-01

    Full Text Available Three small bodies of amphibolites and associated graphitic gneisses from the Suchý and Malá Magura Mountains (Tatric Megaunit, central Western Carpathians have been studied by petrographic and geochemical methods. Isolated, fault-bounded bodies first hundreds of meters in size are located in the complex of the Early Paleozoic paragneisses and migmatites intruded by the Lower Carboniferous granitoid rocks. Amphibolites (locally actinolite schists were formed from effusive basalts, dolerites or isotropic gabbros hydrothermally altered and veined before the regional metamorphic transformation. Distribution of the trace elements relatively immobile during the metamorphic alteration (HFSE, REE, Cr, V, Sc is similar to E-MORB type in the Malá Magura Mountain or to N-MORB/E-MORB types in the Suchý Mountain. Graphitic gneisses to metacherts are rich in silica (up to 88 wt. % and Ctot, poor in other major element contents and display negative Ce-anomaly, enrichment in HREE, V, Cr and U. They were probably originally deposited as non-carbonate and silica-rich deep-sea sediments in anoxic conditions. The oceanic provenance of amphibolites and related graphitic gneisses clearly indicates their oceanic crust affinity and identity with the uppermost part of the ophiolite sequence. Ophiolite bodies from the Suchý and Malá Magura Mountains are supposed to be relic fault blocks identical with the Upper Devonian Pernek Group which represents a Variscan ophiolite nappe preserved to large extent in the Malé Karpaty Mountains located in the Tatric Megaunit further to the southwest. All these ophiolite relics are vestiges of the original ophiolite suture created by oceanic closure in the Lower Carboniferous.

  14. 辽吉地区含硼岩系中斜长角闪岩地球化学%GEOCHEMISTY OF PLAGIOCLASE-AMPHIBOLITE OF THE BORON-BEARING ROCK SERIES IN LIAONING AND JILIN PROVINECES AND ITS ORIGIN

    Institute of Scientific and Technical Information of China (English)

    肖晔; 刘长学

    2009-01-01

    辽吉地区古元古界含硼岩系经过了中高级区域变质和混合岩化作用,主要由黑云母变粒岩、浅粒岩、电气石变粒岩,斜长角闪岩及富镁质含矿岩石组成,局部形成层状混合花岗岩.斜长角闪岩在含硼岩系内各个岩组中都有分布,岩石化学分析显示斜长角闪岩具有富钠,富镁、高铁、高钛特征,MgO含量成多众数值分布,辽东地区斜长角闪岩MgO含量高于集安地区,并且各类斜长角闪岩中硼含量都较高.辽东地区B、Cr、Ni明显高于集安地区,显示为海相地幔岩浆岩特征更为明显,集安地区则显示为变质热液改造特征.辽东地区斜长角闪岩的稀土地化显示为热液改造的幔源岩浆特征,集安地区的斜长角闪岩的稀土地化显示为壳源沉积岩特征.含硼岩系斜长角闪岩与浅粒岩稀土配分模式相似,表明部分斜长角闪岩与浅粒岩具有岩浆成因特征,而电气变粒岩、黑云变粒岩与混合岩稀土富集模式显示海相沉积泥岩变质成因,混合岩化及变质热液对原岩具有明显改造作用.斜长角闪岩石的地球化学特征表明原岩环境为富钠、富硼的海相环境,原岩主要是海相富镁拉斑玄武岩和海相泥质岩.%The Paleoproterozoic boron-bearing rock series in Liaoning-Jilin provinces, which underwent intermediate and advanced level regional metamorphism and migmatitization, consists mainly of biotite granulitite, leucoleptite, tourmaline granulitite, plagioclase-amphibolite, and Mg-rich ore-bearing rocks. Layered migmatitic granite can be obersered locally. Plagioclase-amphibolite is widely distributed among all the boron-bearing rock series. Geochemistry of plagioclase-amphibolite in this study are studied in detail so as to better understand forming environment of protolith. Lithochemical analysis shows that plagioclase-amphibolite is characteristized by enrichment of Na, Mg and Ti, with MgO abundance especial high. MgO content of

  15. Influence of deformation mechanisms and metamorphic reactions during strain localization in the continental crust under lower amphibolite facies conditions: an example from the Gotthard massif

    Science.gov (United States)

    Oliot, E.; Goncalves, P.; Schulmann, K.; Marquer, D.

    2009-04-01

    Ductile shear zones are the result of the process of strain localization in the continental crust. Depending on the metamorphic conditions during deformation, strain localization is coeval with dramatic changes in microstructures, mineralogy and mass transfers, due to the interactions with externally-derived fluid. Therefore, to accurately model the mechanisms of strain localization, it is critical to identify deformation mechanisms related to the recrystallization of the quartzo-feldspathic assemblages, and to better constrain the role of metamorphic reactions during deformation. The aim of this contribution is to characterize the mineralogical, geochemical, textural and microstructural evolution of a high strain zone from the Fibbia granite, which is located in the Gotthard Massif (External Crystalline Massif, Central Alps). This variscan massif has been affected by Alpine Tertiary metamorphism and deformation under lower amphibolite facies conditions. The strain gradient is approximately a meter width. The rock texture evolves from a weakly deformed granite, toward an orthogneiss, a mylonite and a ~10 cm-wide ultramylonite. The mineralogical assemblage changes from a metastable magmatic assemblage consisting of Qtz + Kspar + Pla + Bio ± Pheng ± Grt ± Ep to a fine banded texture consisting of a quartzo-feldspathic matrix, with metamorphic phyllosilicates (biotite and phengite) and garnet in the ultramylonite. Cathodoluminescence (CL) imaging has been used to quantify the modal proportions of phases in the quartzo-feldspathic matrix in this strain gradient. More specifically, in the orthogneiss and the mylonite, CL imaging reveals a subtle layering consisting of alternating bands of quartz-rich ribbons, K-feldspars and coupled quartz- and plagioclase-rich ribbons. The texture in the ultramylonite is more homogeneous with isolated single quartz and K-feldspar grains. CL imaging has also revealed chemical zoning, as "core and mantle" texture in plagioclases. With

  16. SIMS U-Pb, Sm-Nd isotope and geochemical study of an arkosite-amphibolite suite, Peräpohja Schist Belt: evidence for ca. 1.98 Ga A-type felsic magmatism in northern Finland

    Directory of Open Access Journals (Sweden)

    Eero Hanski

    2005-01-01

    Full Text Available In the northern and north-eastern part of the Peräpohja Schist Belt, northern Finland, an extensive supracrustal rock unit has been identified which is composed of alternating amphibolitic and arkositic components. The amphibolites form layers whose thickness varies from one millimeter to some tens of meters, being most often a few tens of centimeters. They represent mafic tuff beds deposited concurrently with more abundant arkositic rocks. Most of the arkosites have a modal and major and trace element compositionsimilar to that of A2-type granites. For example, they exhibit high LREE/HREE, negative Eu anomalies, and flat HREE and are moderately enriched in Nb, Zr, and Y. The genesis of the arkosites is enigmatic as they show features supporting either a volcaniclastic or an epiclastic origin. In the latter case, they were derived via erosion of a source dominated by A2-type granitic rocks. Previous conventional ID-TIMS and new SIMS U-Pb dating of zircons from two arkosite samples and one mica schist sample, all three picked from the northern part of the schist belt, indicate that these rocks contain a single population of zircons with an age of ca. 1975 Ma suggesting that they are among the youngest supracrustal rocks in the schist belt. In contrast, one mica schist sample from the western part of the belt revealed only the presence of Archean zircons. The samples do not differ markedly in terms of their Nd isotopecomposition as they all have a moderately negative εNd(1900 Ma. Regardless of the genesis of the arkosites, their isotopic and geochemical data suggest a previously unknown occurrence of extensive A-type felsic magmatism at ca. 1.98 Ga, contemporaneously withsome continental flood basalts. However, concrete evidence for this felsic A-type magmatism in the form of ca. 1.98 Ga felsic plutonic rocks is virtually absent in the presently exposed Fennoscandian Shield.

  17. Dynamic evolution in a Cretaceous high-P/T subduction channel evidenced by the juxtaposition of amphibolite blocks with different P-T paths: an example from the Kamuikotan belt, northern Japan

    Science.gov (United States)

    Okamoto, A.; Takeshita, T.

    2013-12-01

    A subduction channel developed at the boundary between a subducting oceanic plate and an overlying plate could be geologically defined as the place, where accretionary sediments were dragged down to great depth with an oceanic plate to suffer a high-P/T type metamorphism, and transformed to high-P/T metamorphic rocks (e.g. blueschist and eclogite). In the study area, while typical high-P/T metamorphic rocks (blueschist), which originated from Cretaceous accretionary sediments, amphibolites and metacherts also occur as tectonic blocks in mélange surrounded by either serpentinite or pelitic matrix, which originally suffered intermediate-P/T type metamorphism, but later the same high-P/T type metamorphism as the sediments did. In this research, we have analyzed mineral assemblages in these amphibolites and metacherts, and conducted micro-chemical analyses of compositional zoning in amphibole and garnet from these rocks with an EPMA. As a result, compositional zoning in some constituent amphibole can be divided into 3 types. Type I is a dominant type, where actinolite is overgrown by glaucophane, indicating pressure increase. Type II, which has been found in only one sample, is defined as the compositional zoning in amphibole consisting of magnesiohornblende, actinolite and glaucophane from core to rim. The compositional zoning shows a change of the temperature gradient from low-P/T (or intermediate-P/T) type to high-P/T type, which could reflect a cooling of the subduction channel with time from the onset of subduction to a steady state. Type III is characterized by the compositional zoning in amphibole from tschermakite to glaucophane-magnesioriebeckite. This also shows a cooling of the subduction channel with time. In this sample, garnet also shows a compositional zoning from a Mn-rich and Ca-poor inner core to a Mn-poor and Ca-rich outer core, which is surrounded by a Mn-rich rim, showing a compositional discontinuity across the core-rim boundary. The

  18. The metamorphic evolution from ultrahigh-temperature to amphibolite facies metamorphism in the Odaesan area after the collision between the North and South China Cratons in the Korean Peninsula

    Science.gov (United States)

    Lee, Byung Choon; Oh, Chang Whan; Kim, Tae Sung; Yi, Keewook

    2016-07-01

    The Odaesan Gneiss Complex (Odesan Gneiss Complex) is the eastern end of the Hongseong-Odaesan collision belt in the Korean Peninsula, which is an extension of the Dabie-Sulu collision belt between the North and South China cratons. The Odaesan Gneiss Complex mainly consists of banded and migmatitic gneisses with porphyritic granitoids and amphibolites. The garnet-bearing banded gneisses can be subdivided into garnet-biotite and garnet-orthopyroxene banded gneisses. At the beginning of the post-collision stage, the banded gneisses underwent regional ultrahigh-temperature metamorphism (902-950 °C/8.8-9.4 kbar) at ca. 247-245 Ma due to the heat supplied from underplated basic magma, which was generated by the partial melting of the lithospheric mantle caused by the heat supplied from the asthenospheric mantle. As a result of the continuous extensional force, the study area (lower crust) uplifted onto the middle crust depths, and then the study area underwent prograde granulite facies metamorphism from 660 °C and 8.7 kbar to 750-760 °C and 6.3-6.5 kbar at ca. 227 Ma, causing migmatization, which erased the ultrahigh-temperature metamorphism in most of the study area. The ultrahigh-temperature metamorphism was preserved only in the garnet-orthopyroxene banded gneisses due to their very low water contents. During migmatization, the garnet-biotite banded gneisses were retrograded into upper granulite facies due to the relatively abundant water compared with the garnet-orthopyroxene gneisses. Finally, the study area was uplifted to a shallow depth and locally underwent amphibolite facies retrograde metamorphism (575-680 °C and 3.1-4.5 kbar). In addition, Paleoproterozoic metamorphic (ca. 1930-1886 Ma) and post-collisional magmatic events (ca. 1847 Ma) are identified based on SHRIMP age dating. These ages agree well with the regional Paleoproterozoic metamorphic and post-collisional magmatic activities reported from other areas of the Gyeonggi Massif.

  19. 胶东谭格庄地区奥长花岗质片麻岩和斜长角闪岩的野外地质和锆石SHRIMP定年%Geology and zircon dating of trondhjemitic gneiss and amphibolite in the Tangezhuang area, eastern Shandong

    Institute of Scientific and Technical Information of China (English)

    颉颃强; 万渝生; 王世进; 刘敦一; 谢士稳; 刘守偈; 董春艳; 马铭株

    2013-01-01

    ~2. 5Ga and ~ 1.9Ga metamorphic events are widely recorded all over the North China Craton, being important for understanding the formation and evolution of the craton. Eastern Shandong is located in the eastern North China Craton and contains metamorphic rocks with ca. 2. 5Ga and ca. 1. 9Ga metamorphic zircon ages. For further constraining the Early Precambrian tectono-thermal events in the area, we carried out detailed field observation and SHRIMP U-Pb zircon dating in a road cutting section near Tangezhuang, Laixi County, eastern Shandong. The outcrop consists of interlayered and folded trondhjemitic gneisses and amphibolites. The foliation of the rocks is completely parallel to each other and shows thin interlayer in strong deformation domain, suggesting that thin interlayers of different components cannot be considered as a fact that the protoliths are sedimentary in origin. SHRIMP U-Pb zircon dating yielded an age of 2496 ± 10Ma (MSWD = 0. 45) for magmatic zircons from a trondhjemitic gneiss sample ( S1105), with metamorphic rims having ages of ca. 2. 5Ga and ca. 1. 9Ga. Two amphibolite samples ( S1238 and S0812) contain metamorphic zircons with ages of 1842 ± 3Ma ( MSWD = 1.6) and 1833 ± 13Ma (MSWD = 1. 0) , respectively, but no magmatic zircons have been identified. Combining with previous researches, we suggest that the Late Neoarchean raetamorphism widespread over the whole Jiaodong terrane, leading to anatexis and recycling of older crust material, and the Late Paleoproterozoic metamorphism shows a zonal feature as a result of orogenesis.%对胶东谭格庄地区一地质剖面进行了详细野外观察和锆石SHRIMP年代学研究.剖面主要是由奥长花岗质片麻岩和斜长角闪岩组成,两者互层产出并一起发生褶皱变形,在强变形域两者完全平行化.所以,在强变形地区,不同成分岩性的薄层状互层产出不能作为变质沉积岩的识别标志.奥长花岗质片麻岩(S1105)形成于2496±10Ma,遭

  20. Humic acids quality of Cambisols developed on gneiss and amphibolite

    Czech Academy of Sciences Publication Activity Database

    Pospíšilová, L'.; Žigová, Anna; Šťastný, Martin; Liptaj, T.

    2012-01-01

    Roč. 9, č. 4 (2012), s. 503-510. ISSN 1214-9705 R&D Projects: GA ČR GA526/08/0434 Institutional support: RVO:67985831 Keywords : Cambisols * parent material * arable soil, * grassland soil * X-ray diffraction * humic acids * SRATR FTIR and 13C NMR spectroscopy Subject RIV: DF - Soil Science Impact factor: 0.530, year: 2011 http://www.irsm.cas.cz/materialy/acta_content/2012_04/8.Pospisilova.pdf

  1. Partial melting of metavolcanics in amphibolite facies regional metamorphism

    Indian Academy of Sciences (India)

    Alan Bruce Thompson

    2001-12-01

    Metavolcanic rocks containing low-Ca amphiboles (gedrite, cummingtonite) and biotite can undergo substantial dehydration-melting. This is likely to be most prominent in Barrovian Facies Series (kyanite-sillimanite) and occurs at the same time as widespread metapelite dehydration- melting. In lower pressure facies series, metavolcanics will be represented by granulites rich in orthopyroxene when dehydration occurs at much lower temperatures than melting. In higher pressure facies series it is not well known whether metavolcanic rocks dehydrate or melt at temperatures lower or similar to that of metapelites.

  2. Radiometric discrimination of pre-Variscan amphibolites in the Ediacaran Serie Negra (Ossa-Morena Zone, SW Iberia)

    Science.gov (United States)

    Sánchez-Lorda, M. E.; Ábalos, B.; García de Madinabeitia, S.; Eguíluz, L.; Gil Ibarguchi, J. I.; Paquette, J.-L.

    2016-06-01

    New results on the geochronology of metabasites hosted by the Neoproterozoic (late Ediacaran) Serie Negra of the Ossa-Morena Zone (Iberian Massif) are presented. These rocks record a protracted subduction at least since the latest Cryogenian, followed in a continuum by early Cambrian rifting. The Serie Negra country rocks are continental-crust forearc segments of the thinned continental margin of Gondwana. The metabasite suite exhibits E- and N-MORB, as well as volcanic arc signatures, and discloses the existence of a diverse Late Ediacaran-Early Cambrian magmatism across the Ossa-Morena Zone. Protoliths of the E-MORB metabasites are clearly Ediacaran. They exhibit geochemical characteristics typical of younger island arcs, notably of their forearc zones affected by extension soon after the initiation of subduction. Radiometric dating of N-MORB metabasites shows that these rocks are significatively younger than their country rocks, whose age has been stratigraphically and paleontologically constrained as Ediacaran. They record early Cambrian rifting developed in a continuum after a protracted subduction. Metabasite petrological zonation permits to reconstruct a single Ediacaran subduction zone located to the S of the current OMZ and dipping towards the N (in present day geographical coordinates).

  3. Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

    Science.gov (United States)

    Leslie, S. R.; Mahan, K. H.; Regan, S.; Williams, M. L.; Dumond, G.

    2015-02-01

    The deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8-1.0 GPa, 725-850 °C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4-0.6 GPa, 550-650 °C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolution-precipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

  4. Early Cretaceous amphibolite dehydratation melting preserved within the Tertiary Sabzevar ophiolitic suture (NE Iran): significance for the closure of the Alpine Tethyan oceans in central Iran

    Science.gov (United States)

    Rossetti, Federico; Nasrabady, Mohsen; Vignaroli, Gianluca; Theye, Thomas; Gerdes, Axel; Razavi, Mohammad Hossein; Moin Vaziri, Hosein

    2010-05-01

    The tectono-metamorphic signature of the oceanic-derived units marking orogenic suture zones provides key elements to decipher modes and regimes of oceanic subduction and continental accretion, and to constrain tectonic reconstructions at paleo-convergent margins. The remnants of the Tethyan oceanic realm form the most remarkable of these suture zones, running from the Mediterranean through East Europe, Middle East to Asia. These ophiolitic rocks record a polyphase and prolonged history of oceanic construction (the Paleozoic-Early Mesozoic Paleo-Tethys and the Mesozoic-Tertiary Neo-Tethys oceanic realms) and consumption during a sequence of Late Paleozoic to Cenozoic subduction/obduction/collision stages localized along the Eurasian active plate margin (e.g. Stampfli and Borel, 2002). The Iranian ophiolites are an integrant part of this evolving scenario, with the Neotethyan remnants distributed to mark diachronous closures of various oceanic branches during the Alpine-Himalayan convergence history. Despite these peculiar characteristics, few modern studies have addressed the characterization of the tectono-metamorphic evolution of the Neotethyan Iranian ophiolites. Furthermore, most of these studies focused on the Zagros orogen (e.g. Agard et al., 2006), and the ophiolitic mélanges surrounding the Central East Iranian Microcontinent are still lacking of a full petrological and geochronological characterisation. The ophiolitic mélange exposed in the Sabzevar Range of NE Iran is a remnant of one of the Neo-Tethyan oceanic branches of Central Iran, closed during the Paleocene-Eocene Arabia-Eurasia convergence. In this study, we document occurrence of km-scale, variably retrogressed mafic high-pressure granulitic (Am + Grt + Cpx + Pl + Qtz) slices embedded within this suture zone. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at 1.1 ± 0.1 GPa and 810 ± 80 °C. In situ U(-Th)-Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. This provides evidence for an unknown episode of high-grade subduction zone metamorphism in the region and argues for juxtaposition of an older ophiolitic suture along the Paleocene-Eocene Sabzevar orogen. When combined with the existing reconstructions, these new data (i) impose reconsideration of the current paleotectonic models of the Eurasia convergent margin during the Early Cretaceous, and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region. Agard P., P. Monié, W. Gerber, J. Omrani, M. Molinaro, L. Labrousse, B. Vrielynck, B. Meyer, L. Jolivet and P. Yamato (2006): J. Geophys. Res., 111, doi: 10.1029/2005JB004103. Stampfli, G. M. and G. D. Borel (2002): Earth Planet. Sci. Lett., 196, 17-33.

  5. Constraints on mineralisation and hydrothermal alteration in the Nalunaq gold deposit, South Greenland

    DEFF Research Database (Denmark)

    Bell, Robin-Marie Fairbairn

    -and - medium grained amphibolite of the Nanortalik Nappe. Detailed petrographic and geochronological studies have revealed a multi-stage hydrothermal alteration system, with alteration pre-and post-dating gold mineralisation. The hydrothermal alteration records a transition from upper-amphibolite facies to......- and-medium grained wall rock amphibolite. Late-stage faults have haloes of low gold-grades, which may indicate that gold remobilisation has occurred. Stable oxygen, hydrogen and carbon isotope analysis suggests that the fluids which formed the early amphibolite alteration have a crustal source, the...

  6. Plate tectonics 2.5 billion years ago - Evidence at Kolar, south India

    Science.gov (United States)

    Krogstad, E. J.; Hanson, G. N.; Balakrishnan, S.; Rajamani, V.; Mukhopadhyay, D. K.

    1989-01-01

    The Archean Kolar Schist Belt, south India, is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accreted. Amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics suggesting that their volcanic protoliths were derived from different mantle sources. The amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago (Ma) along a zone marked by the Kolar Schist Belt. This history of accretion of discrete crustal terranes resembles those of Phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 Ma.

  7. Petrology of an eclogite- and pyrigarnite-bearing polymetamorphic rock complex at Cabo Ortegal, NW Spain

    NARCIS (Netherlands)

    Vogel, D.E.

    1967-01-01

    At Cabo Ortegal, paragneisses are found in association with amphibolites, metagabbros, amphibolized eclogites, amphibolized (plagio) pyrigarnites, and serpentinized ultrabasic rocks. On the basis of petrographical and chemical evidence, their geological history was reconstructed as follows: Precambr

  8. Preliminary Report for the location of drilling in the Raigon aquifer area

    International Nuclear Information System (INIS)

    This work is about the geological and hydrogeological characteristics of Santa Lucia basin belongs to the Raison Aquifer. The tectonic basin of Santa Lucia is a structural complex constituted by the crystalline basement, gneisses, amphibolite s, mica schists and metamorfites

  9. The origin and age of the metamorphic sole from the Rogozna Mts., Western Vardar Belt: New evidence for the one-ocean model for the Balkan ophiolites

    Science.gov (United States)

    Borojević Šoštarić, S.; Palinkaš, A. L.; Neubauer, F.; Cvetković, V.; Bernroider, M.; Genser, J.

    2014-04-01

    This study brings new geochronological and petrochemical data from the metamorphic sole beneath the Rogozna Mts., Western Vardar ophiolite belt. The Rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc-chlorite schists. The Rogozna amphibolites are medium- to fine-grained rocks with nematoblastic texture and pronounced foliation. They consist of green amphibole (~ 70 vol.%) with variable silica contents (6.4 to 7.8 Si apfu), as well as Mg# (molMg/[Mg + Fetot]; 0.53 to 0.77) and variably albitized plagioclase (~ 30 vol.%; Ab24-Ab98). Amphibolites are overprinted by a retrograde assemblage containing actinolite, epidote, clinoclore, sericite, chlorite, and magnetite. The amphibolites formed due to metamorphism of two basaltic suites: subalkaline/tholeiitic and alkaline. Subalkaline/tholeiitic amphibolites possess low Zr, Nb, Y, Th, Hf, TiO2, and P2O5 values and a LREE-depleted pattern, typical for the N-MORB (normal mid ocean ridge basalt) to BAB (back-arc basalt) origin. Alkaline amphibolites show elevated concentrations of Zr, Nb, Y, Th, Hf, TiO2, and P2O5 with a LREE-enriched pattern typically displayed by OIB (ocean island basalt). Amphibolites were crystallized during intra-oceanic thrusting at temperatures between 685 °C and 765 °C and at a depth of 12-17 km. 40Ar/39Ar cooling ages of amphibole, ranging from 165 to 170 Ma, slightly postdate the sole formation. Talc-chlorite schists are related to retrograde greenschist-facies metamorphism. They consist of Fe-rich talc and Cr-rich chlorite (peninite-diabantite) pseudomorphs after amphibole and MORB-type Cr-Al spinel, surrounded by Al- and Mg-poor ferrit chromite. The occurrence of ferrit chromite is related to earlier, amphibolite facies metamorphism. Chlorite pseudomorphs after amphibole were formed at 300-410 °C.

  10. METALLOGENY OF SOUTH TISIA - MOSLAVAČKA MT., PSUNJ, PAPUK AND KRNDIJA

    OpenAIRE

    Ivan Jurković

    2013-01-01

    Core of the Moslavačka Mt. is built of S-type granite and granodiorite, wrapped up in migmatites. Outer zone is represented with metamorphites of amphibolite facies. The Moslavačka Mt. is a product of regional metamorphism on Lower Paleozoic pelito-psammitic protholite. Numerous quarries exploited granite, amphibolite, gabbro, orthogneiss. Stronger concentrations of sillimanite, andalusite, cordierite, garnet, and tourmaline were observed in different types of rocks. The pegmatite phase gener...

  11. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: Pre-Nagssugtoqidian crustal evolution in West Greenland: geology, geochemistry and deformation of supracrustal and granitic rocks north-east of Kangaatsiaq

    Directory of Open Access Journals (Sweden)

    Watt, Gordon R.

    2006-12-01

    Full Text Available The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kangaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are comprised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite paragneisses. Their association and geochemical compositions are consistent with a metamorphosed volcano-sedimentary basin (containing both tholeiitic and calc-alkali lavas and is similar to other Archaean greenstone belts. The Kangaatsiaq granite forms a 15 × 3 km flat, subconcordant body of deformed,pink, porphyritic granite occupying the core of the supracrustal synform, and is demonstrably intrusive into the amphibolites. The granite displays a pronounced linear fabric (L or L > S. Thepost-granite deformation developed under lower amphibolite facies conditions (400 ± 50°C, and is characterised by a regular, NE–SW-trending subhorizontal lineation and an associated irregular foliation, whose poles define a great circle; together they are indicative of highly constrictional strain. The existence of a pre-granite event is attested by early isoclinal folds and a foliation within the amphibolites that is not present in the granite, and by the fact that the granite cuts earlier structures in the supracrustal rocks. This early event, preserved only in quartz-free lithologies, resulted in high-temperature fabrics being developed under upper amphibolite to granulite facies conditions.

  12. The P-T-t history of blocks in serpentinite-matrix melange, west-central Baja California

    Energy Technology Data Exchange (ETDEWEB)

    Baldwin, S.L.; Harrison, T.M. [State Univ. of New York, Albany, NY (United States)

    1992-01-01

    Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<<1 km) in serpentinite-matrix melanges. 67 refs., 16 figs., 4 tabs.

  13. Linear geologic structure and magic rock discrimination as determined from infrared data

    Science.gov (United States)

    Offield, T. W.; Rowan, L. C.; Watson, R. D.

    1970-01-01

    Color infrared photographs of the Beartooth Mountains, Montana show the distribution of mafic dikes and amphibolite bodies. Lineaments that cross grassy plateaus can be identified as dikes by the marked constrast between the dark rocks and the red vegetation. Some amphibolite bodies in granitic terrain can also be detected by infrared photography and their contacts can be accurately drawn due to enchanced contrast of the two types of rock in the near infrared. Reflectance measurements made in the field for amphibolite and granite show that the granite is 25% to 50% more reflective in the near infrared than in the visible region. Further enhancement is due to less atmospheric scattering than in the visible region. Thermal infrared images of the Mill Creek, Oklahoma test site provided information on geologic faults and fracture systems not obtainable from photographs. Subtle stripes that cross outcrop and intervening soil areas and which probably record water distribution are also shown on infrared photographs.

  14. Influence of phosphorus on Cu sorption kinetics: Stirred flow chamber experiments

    International Nuclear Information System (INIS)

    A stirred flow reactor was used to study the influence of phosphorus on the adsorption and desorption kinetics of copper in two acid soils on granite and amphibolite. The presence of P was found to significantly increase Cu adsorption in both soils, albeit at different types of sites (mainly in slow adsorption sites in the soil on granite, and both in fast and slow adsorption sites in that on amphibolite). The increased Cu sorption at fast sites in the amphibolite soil was due to its high content in Fe oxyhydroxides, which bound P and released OH- as a result, thereby raising the pH and leading to a higher sorption capacity during fast reactions. On the other hand, the increased Cu sorption at slow adsorption sites was due to Cu2+ acting as a bridging element between P and organic matter.

  15. Influence of phosphorus on Cu sorption kinetics: Stirred flow chamber experiments

    Energy Technology Data Exchange (ETDEWEB)

    Perez-Novo, C. [Area de Edafoloxia e Quimica Agricola, Departamento de Bioloxia Vexetal e Ciencia do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense (Spain); Fernandez-Calvino, D., E-mail: davidfc@uvigo.es [Area de Edafoloxia e Quimica Agricola, Departamento de Bioloxia Vexetal e Ciencia do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense (Spain); Bermudez-Couso, A.; Lopez-Periago, J.E.; Arias-Estevez, M. [Area de Edafoloxia e Quimica Agricola, Departamento de Bioloxia Vexetal e Ciencia do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense (Spain)

    2011-01-15

    A stirred flow reactor was used to study the influence of phosphorus on the adsorption and desorption kinetics of copper in two acid soils on granite and amphibolite. The presence of P was found to significantly increase Cu adsorption in both soils, albeit at different types of sites (mainly in slow adsorption sites in the soil on granite, and both in fast and slow adsorption sites in that on amphibolite). The increased Cu sorption at fast sites in the amphibolite soil was due to its high content in Fe oxyhydroxides, which bound P and released OH{sup -} as a result, thereby raising the pH and leading to a higher sorption capacity during fast reactions. On the other hand, the increased Cu sorption at slow adsorption sites was due to Cu{sup 2+} acting as a bridging element between P and organic matter.

  16. Structural analyses and geotectonic correlation of the Karasaki mylonites, western Shikoku

    Energy Technology Data Exchange (ETDEWEB)

    Yoshimura, Joji; Takagi, Hideo [Waseda Univ., Tokyo (Japan)

    1999-11-01

    The Karasaki mylonite unit crops out as a series of isolated klippes on the Sambagawa Belt, along its northern margin in western Shikoku. It mainly consists of strongly deformed amphibolites and subsidiary of pelitic and quartzo-feldspathic metamorphic rocks. This tectoric unit is distinguished on the basis of field occurrence, difference in metamorphic grade and in shear sence with respect to the underlying Sambagawa metamorphic rocks. The chemical composition of garnet and hornblende suggests that the protolith of the Karasaki mylonite is amphibolite facies metamorphic rocks, that have been devoid of Sambagawa metamorphism. Hornblendes from amphibolitic mylonites yield the K-Ar ages of 104{+-}5 Ma, 119{+-}6 Ma and 122{+-}6 Ma. Accordingly, the Karasaki mylonite is possibly correlative to the Higo, Oshima, Yorii, Nishidohira and Takanuki metamorphic rocks of the latest Early Cretaceous age, which are considered to constitute the Paleo-Ryoke Belt interleaved between the Ryoke and Sambagawa Belts. (author)

  17. Structural analyses and geotectonic correlation of the Karasaki mylonites, western Shikoku

    International Nuclear Information System (INIS)

    The Karasaki mylonite unit crops out as a series of isolated klippes on the Sambagawa Belt, along its northern margin in western Shikoku. It mainly consists of strongly deformed amphibolites and subsidiary of pelitic and quartzo-feldspathic metamorphic rocks. This tectoric unit is distinguished on the basis of field occurrence, difference in metamorphic grade and in shear sence with respect to the underlying Sambagawa metamorphic rocks. The chemical composition of garnet and hornblende suggests that the protolith of the Karasaki mylonite is amphibolite facies metamorphic rocks, that have been devoid of Sambagawa metamorphism. Hornblendes from amphibolitic mylonites yield the K-Ar ages of 104±5 Ma, 119±6 Ma and 122±6 Ma. Accordingly, the Karasaki mylonite is possibly correlative to the Higo, Oshima, Yorii, Nishidohira and Takanuki metamorphic rocks of the latest Early Cretaceous age, which are considered to constitute the Paleo-Ryoke Belt interleaved between the Ryoke and Sambagawa Belts. (author)

  18. Mixing of rocks of differing metamorphic histories within a subduction channel of upper mantle depth exposed on the Ohmachi Seamount, Izu-Bonin Arc

    Science.gov (United States)

    Ueda, H.; Usuki, T.; Imayama, T.; Hirauchi, K.

    2011-12-01

    Common occurrences of high-P/T metamorphic oceanic rocks in serpentinite melange suggest that deep portions of subduction channel could act as conduits both of subduction and exhumation transports. Metamorphic analysis of these rocks can provide information on modes of material transport along the subduction interfaces. Serpentinites exposed on the Ohmachi Seamount in Izu-Bonin arc contain minor amounts of metabasites, and their occurrence in sea floor and its least fractured nature imply to record primary structures and components of an intraoceanic subduction channel in upper mantle depths. Here we report metamorphic histories of some representative rocks. A garnet -zoisite amphibolite (R12) contains relics of eclogite-facies minerals. Its amphibole composition changed from hornblende presumably of non-eclogite stage I, via stage II when it merged to actinolite, to hornblende again in the final amphibolite stage III. The eclogite facies relic minerals co-occur with actinolitic hornblende of the stage II. This change suggests an anticlockwise path with cooling before decompression. A garnet-epidote amphibolite (D6-1) has glaucophane inclusions in garnet, suggesting a clockwise path with heating from blueschist to amphibolite facies before exhumation. Serpentinites show cooling from spinel peridotite via upper to lower amphibolite grades. Absence of high-pressure (e.g. Ti-clinohumite) and low-pressure (e.g. anthophyllite) minerals and occurrence of minor Tlc+Ol assemblage imply their medium pressure origins. Common deformation structures suggest that the serpentinites were juxtaposed with amphibolites being decompressed. Comparing rocks described above, the subduction channel exposed in the Ohmachi Seamount is regarded as a mixture of rocks of differing metamorphic histories. This implies that significant temporal or spatial variation in mode of material transport occur in a single subduction channel.

  19. First data on Sm-Nd systematization of Khanka Massif metamorphic rocks, Primor'e

    International Nuclear Information System (INIS)

    The age of the metamorphic rocks of the Khanka massif, Primor'e, is determined through the method of the Sm-Nd isotopic dating. The results of the isotopic studies on the amphibolites of the Nakhimov suite of the Khanka massif indicated that the rocks of this suite are not older than 1.7 billion years. The obtained age corresponds to the time of the amphibolite protolith formation, the source whereof is the moderately depleted mantle. The isotopic age of the amphibole and plagioclase mineral fractions constitutes 733 ± 25 mln years, which reflects the time of the Nakhimov suite rocks metamorphism

  20. Palaeoproterozoic tectonic evolution of the Alto Tererê Group, southernmost Amazonian Craton, based on field mapping, zircon dating and rock geochemistry

    Science.gov (United States)

    Lacerda Filho, J. V.; Fuck, R. A.; Ruiz, A. S.; Dantas, E. L.; Scandolara, J. E.; Rodrigues, J. B.; Nascimento, N. D. C.

    2016-01-01

    New geochemical and geochronological U-Pb and Sm-Nd data from amphibolites of the Alto Tererê Group, which are of Palaeoproterozoic age, are presented. The amphibolites are exposed in the central-eastern portion of the Rio Apa Block, southern Amazonian Craton, Mato Grosso do Sul, Brazil, and are composed of hornblende, plagioclase, quartz, biotite, cummingtonite and epidote. The amphibolites are subdivided into three lithofacies: (i) thinly banded amphibolites (metabasalts), (ii) coarse- and medium-grained amphibolites with relic subophitic texture (metagabbros), and (iii) amphibolites with relic cumulate texture (metapyroxenites). Chemical data also suggest the subdivision of the amphibolites into three different types. These rocks yield a U-Pb zircon age of 1768 ± 6 Ma and are therefore older than rocks of part of the Rio Apa Complex. Their Sm-Nd model ages range between 2.89 and 1.88 Ga, and their ɛNd (T) values range between -3.40 and + 3.74. Chemical analyses of these rocks indicate SiO2 concentrations between 45.23 and 50.65 wt.%, MgO concentrations between 4.34 and 8.01 wt.%, TiO2 concentrations between 0.91 and 1.74 wt.%, weakly fractionated rare-earth element (REE) patterns with mild depletion in heavy REEs, enrichment in large-ion lithophile elements (LILEs) and high-field-strength element (HFSEs), negative Nb, Ta and Co anomalies, positive Ba and Pb anomalies, low Ce concentrations, high Rb/Y ratios and low Th/La and Hf/Sm ratios. These features reflect metasomatism of the mantle wedge produced by sediments from the subducted plate. Various degrees of melting mark the evolution of the parent basic magmas, although subordinate crustal contamination may also have occurred. The geochemical signature of the amphibolites corresponds to that of tholeiitic basalts generated in an extensional back-arc-basin environment. The deposition in the basin apparently ceased during the first episode of compression and deformation at approximately 1.68 Ga, and the main

  1. Algoma-type Neoproterozoic BIFs and related marbles in the Seridó Belt (NE Brazil)

    DEFF Research Database (Denmark)

    Sial, Alcides N.; Campos, Marcel S.; Gaucher, Claudio;

    2015-01-01

    The Jucurutu Formation in the Seridó Belt, northeastern Brazil, encompasses fine-to coarse-grained amphibolite-facies marbles, locally with cross-bedding and stromatolites. Banded iron formations (BIF) at three localities in this belt comprise itabirites (actinolite- or cummingtonite-itabirite and...

  2. Geochemie a petrologie amfibolitů moldanubika

    Czech Academy of Sciences Publication Activity Database

    René, Miloš; Finger, F.; Mayer, A.

    2006-01-01

    Roč. 14, č. 4 (2006), s. 7-8. ISSN 1210-4612 R&D Projects: GA MŠk ME 845 Institutional research plan: CEZ:AV0Z30460519 Keywords : amphibolite * geochemistry * petrology Subject RIV: DB - Geology ; Mineralogy

  3. Comparison of Archean and Phanerozoic granulites: Southern India and North American Appalachians

    Science.gov (United States)

    Mcsween, Harry Y., Jr.; Kittleson, Roger C.

    1988-01-01

    Archean granulites at the southern end of the Dharwar craton of India and Phanerozoic granulites in the southern Appalachians of North America share an important characteristic: both show continuous transitions from amphibolite facies rocks to higher grade. This property is highly unusual for granulite terranes, which commonly are bounded by major shears or thrusts. These two terranes thus offer an ideal opportunity to compare petrogenetic models for deep crustal rocks formed in different time periods, which conventional wisdom suggests may have had different thermal profiles. The salient features of the Archean amphibolite-to-granulite transition in southern India have been recently summarized. The observed metamorphic progression reflects increasing temperature and pressure. Conditions for the Phanerozoic amphibolite-to-granulite transition in the southern Appalachians were documented. The following sequence of prograde reactions was observed: kyanite = sillimanite, muscovite = sillimanite + K-feldspar, partial melting of pelites, and hornblende = orthopyroxene + clinopyroxene + garnet. The mineral compositions of low-variance assemblages in mafic and intermediate rocks are almost identical for the two granulite facies assemblages. In light of their different fluid regimes and possible mechanisms for heat flow augmentation, it seems surprising that these Archean and Phanerozoic granulite terranes were apparently metamorphosed under such similar conditions of pressure and temperature. Comparison with other terrains containing continuous amphibolite-to-granulite facies transitions will be necessary before this problem can be addressed.

  4. Trace element behavior and P-T-t evolution during partial melting of exhumed eclogite in the North Qaidam UHPM belt (NW China): Implications for adakite genesis

    Science.gov (United States)

    Zhang, Guibin; Niu, Yaoling; Song, Shuguang; Zhang, Lifei; Tian, Zuolin; Christy, Andrew G.; Han, Lei

    2015-06-01

    We have studied trace element behavior and timing of decompression melting of UHP rocks during exhumation recorded in the magmatic products, i.e., the melt phase (leucosomes), cumulate (garnetite) and residue (amphibolitized eclogite) from a single outcrop in the south Dulan area, North Qaidam UHPM belt, NW China. Two distinct episodes of partial melting are recognized. First, Grt-free tonalitic-trondhjemitic leucosome melts with higher silica crystallized at 424.0 ± 2.7 Ma. Garnets grew in the leucosome melt but fractionated out to form garnetite cumulates along with Ti-rich phases (rutile and titanite), strengthening the adakitic signature of the leucosome. Later Grt-bearing leucosome melts with an age of 412.4 ± 2.9 Ma cross-cut boudins and layers of amphibolitized eclogite. Geochemical investigation of bulk-rocks and in situ minerals verifies the genetic relationship between the amphibolitized eclogite and the tonalitic-trondhjemitic melts. Zircons from the amphibolitized eclogite have older (> 700 Ma) protolith ages, with subsequent eclogite-facies metamorphism, retrograde granulite-facies overprinting and partial melting. Phase modeling and Zr-in-rutile thermometry calculations in combination with zircon geochronology reveal the evolution P-T-t path for the exhumation and the partial melting of the deeply subducted continental crust at the North Qaidam subduction zone in the Early Paleozoic.

  5. Similar quartz crystallographic textures in rocks of continental earth's crust (by neutron diffraction data): II. Quartz textures in monophase rocks

    International Nuclear Information System (INIS)

    The types of quartz textures found in a large collection of multiphase rocks from different regions of the earth are analyzed. Crystallographic textures of granulite, amphibolite, slate, and gneiss samples are measured, classified, and compared with the similar textures of monomineral rocks.

  6. Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt; Grupo Araxa em sua area tipo: um fragmento de crosta oceanica Neoproterozoica na faixa de dobramentos Brasilia

    Energy Technology Data Exchange (ETDEWEB)

    Seer, Hildor Jose [Centro Federal de Educacao Tecnologica de Araxa, (CEFET), MG (Brazil); Brod, Jose Affonso; Fuck, Reinhardt Adolfo; Pimentel, Marcio Martins; Boaventura, Geraldo Resende; Dardenne, Marcel Auguste [Brasilia Univ., DF (Brazil). Inst. de Geociencias

    2001-09-01

    This study reviews the geological characteristics and puts forward a new evolution model for the Araxa Group in its type-area, the southern segment of the Neo proterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxa Group is confined within a thrust sheet belonging to a syn formal regional fold, the Araxa Syn form, overlying two other thrust sheets made of the Ibia and Canastra Groups. The Araxa Group is described as a tectono stratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic meta sedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafic (serpentinite/ amphibole-talc schist). The basalts are similar to high Fe O tholeiites, with REE signatures that resemble E-MORB and {epsilon}{sub Nd(T)} =+ 1.1. The meta sedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and {epsilon}{sub Nd(T)} = -10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxa Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex. (author)

  7. Structural, metamorphic, and geochronologic constraints on the origin of the Condrey Mountain Schist, north central Klamath Mountains, northern California

    Energy Technology Data Exchange (ETDEWEB)

    Helper, M.A.

    1985-01-01

    The Condrey Mountain Schist (CMS) occupies a window through Late Triassic amphibolite facies melange in the north central Klamath Mountains in northern California and southwest Oregon. Transitional blueschist-greenschist facies assemblages are widespread in mafic schists in the structurally lowest levels of the window; structurally higher CMS near the window margins contains medium- to high-pressure greenschist facies parageneses. An /sup 40/Ar//sup 39/Ar crossite age indicates a late Middle Jurassic age of metamorphism. Pressure-temperature estimates for the overlying CMS greenschists suggest temperatures similar to those in the central part of the window, but at slightly lower pressures. Thrusting of the overlying amphibolites at 150-156 MA occurred while the amphibolites were above about 500C. Stretching lineations indicate a movement vector of about N45W. Comparisons of the sequence and timing of metamorphic and structural events, radiometric ages, and movement directions during thrusting indicate the CMS does not represent an inlier of Kalamath Western Jurassic Belt flysch but is instead an older, isolated thrust plate. Similarities with the age of metamorphism and plutonism in the overlying amphibolites suggest the two plates may be remnants of the same Middle Jurassic paired metamorphic belt.

  8. Geology and associated mineral occurrences of the Araxa Group, Mossamedes Region, Goias, Brazil

    International Nuclear Information System (INIS)

    In the region of Mossamedes, State of Goias, Brazil, the Precambrian metamorphic rocks of the Araxa group were mapped at the scale of 1:25,000, with emphasis on stratigraphic, structural, petrographic and economic aspects. These metamorphites represent a continous stratigraphic sequence which, from bottom to top can be subdivided into five informal lithostratigraphic units: 1) psamitic unit (quartzite, metaconglomerate); 2) psamitic-pelitic unit (quartzite, quartz schist, muscovite schist); 3) lower pelitic - volcanic unit (chlorite - biotite schist, fine grained blastoporphyritic gneiss, amphibolite and calc-schist); 4) upper pelitic - volcanic unit (garnet muscovite schist, biotite schist and gneiss, amphibolite, magnetite muscovite schist); 5) gneissic unit (epidote biotite gneiss, amphibolite). Three types of meta-intrusive rocks were found, besides basic dykes related to Mesozoic magmatism. Four phases of deformation affected the volcano-sedimentary sequence;D1, D2, D3 and D4, each of them developing distinct deformational features. Barrowian type metamorphism increases progressively from North to South from the biotite zone to the garnet zone (greenschist facies), reaching the staurolite-kyanite zone (amphibolite facies). The magmatism throughout the Group's evolution consists of mafic to felsic volcanic activity, mustly intermediary, as well as three intrusive events. Gold, copper and zinc minerals of economic interest occur within the studied area. The gold mineralizations are related to the pelitic-volcanic sequences. Copper occurs in several rocks from the pelitic-volcanic and gneissic sequences. (Author)

  9. Geotechnical variability of permafrozen glaciomarine clays in Sdr. Strømfjord in Greenland

    DEFF Research Database (Denmark)

    Foged, Niels Nielsen; Ingeman-Nielsen, Thomas; Belmonte, Louise Josefine

    2014-01-01

    This contribution presents the geotechnical properties of some permafrozen glaciomarine clays near to the Kangerlussuaq Airport at Sdr. Strømfjord in West Greenland.This fjord system was established by glacial erosion into the bedrock consisting of Nagssugtoqidian banded gneisses with amphibolitic...

  10. Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt

    International Nuclear Information System (INIS)

    This study reviews the geological characteristics and puts forward a new evolution model for the Araxa Group in its type-area, the southern segment of the Neo proterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxa Group is confined within a thrust sheet belonging to a syn formal regional fold, the Araxa Syn form, overlying two other thrust sheets made of the Ibia and Canastra Groups. The Araxa Group is described as a tectono stratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic meta sedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafic (serpentinite/ amphibole-talc schist). The basalts are similar to high Fe O tholeiites, with REE signatures that resemble E-MORB and εNd(T) =+ 1.1. The meta sedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and εNd(T) = -10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxa Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex. (author)

  11. Geochemistry and geochronology of mafic rocks from the Luobusa ophiolite, South Tibet

    Science.gov (United States)

    Zhang, Chang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Zhang, Liang-Liang; Ji, Wei-Qiang

    2016-02-01

    This study presents geochemical compositions of mafic rocks outcropped in the Luobusa ophiolite that locates at the eastern part of the Yarlung Zangbo Suture Zone (YZSZ). The studied mafic rocks include gabbroic/diabase dykes cutting the peridotites and foliated amphibolites embedded within the subophiolitic mélange. The diabases have low K2O contents and display N-MORB-like geochemical characteristics, i.e., with flat REE patterns and weak enrichment in LILE (e.g., Rb, Ba, Th and U). The gabbros show LREE-depleted patterns and variable enrichment in Rb and Ba. Foliated amphibolites mainly consist of hornblendes and plagioclases, with minor titanites. They commonly show LREE-depleted patterns, with strong enrichment in LILE. Three diabases have depleted Sr-Nd-Hf isotope compositions, with initial 87Sr/86Sr ratios of 0.703009-0.703502, εNd(t) of + 5.0 to + 8.5 and εHf (t) of ca + 14. Two gabbros have similar Nd-Hf isotopes with the diabases, but slightly higher initial 87Sr/86Sr ratios (i.e., 0.704820 and 0.704550). Compared to both diabases and gabbros, the amphibolites have higher initial 87Sr/86Sr ratios (i.e., 0.705131-0.705825), but more depleted Nd-Hf isotope compositions, with εNd(t) of + 9.1 to + 11.6 and εHf(t) of + 18.2 to + 21.9. Geochemical compositions of the diabase dykes indicate that they were formed in a mid-ocean ridge setting. Zircon SIMS U-Pb dating of the gabbroic dyke cutting the serpentinites yields an age of 128.4 ± 0.9 Ma, which is identical within uncertainty to the zircon U-Pb age of the amphibolite (i.e., 131.0 ± 1.2 Ma). Low U and Th contents of zircons in the amphibolite support their metamorphic origin. Titanites in the amphibolites have been dated by LA-ICPMS and give U-Pb ages of ~ 131-134 Ma, which are similar to the zircon U-Pb ages of the dated gabbro and amphibolite. Therefore, we suggest that the Luobusa ophiolite was generated at the Early Cretaceous and underwent the intra-oceanic emplacement immediately after its

  12. Unraveling the polymetamorphic history of garnet-bearing metabasites: Insights from the North Motagua Mélange (Guatemala Suture Zone)

    Science.gov (United States)

    Bonnet, G.; Flores, K. E.; Martin, C.; Harlow, G. E.

    2014-12-01

    The Guatemala Suture Zone is the fault-bound region in central Guatemala that contains the present North American-Caribbean plate boundary. This major composite geotectonic unit contains a variety of ophiolites, serpentinite mélanges, and metavolcano-sedimentary sequences along with high-grade schists, gneisses, low-grade metasediments and metagranites thrusted north and south of the active Motagua fault system (MFS). The North Motagua Mélange (NMM) outcrops north of the MFS and testifies the emplacement of exhumed subduction assemblages along a collisional tectonic setting. The NMM is composed of a serpentinite-matrix mélange that contains blocks of metabasites (subgreenschist facies metabasalt, grt-blueschist, eclogite, grt-amphibolite), vein-related rocks (jadeitite, omphacitite, albitite, mica-rock), and metatrondhjemites. Our new detailed petrographic and thermobarometric study on the garnet-bearing metabasites reveals a complex polymetamorphic history with multiple tectonic events. Eclogites show a classical clockwise PT path composed of (a) prograde blueschist/eclogite facies within garnet cores, (b) eclogite facies metamorphic peak at ~1.7 GPa and 620°C, (c) post-peak blueschist facies, (d) amphibolite facies overprint, and (e) late stage greenschist facies. Two types of garnet amphibolite blocks can be found, the first consist of (a) a relict eclogite facies peak at ~1.3 GPa and 550°C only preserved within anhedral garnet cores, and (b) surrounded by a post-peak amphibolite facies. In contrast, the second type displays a prograde amphibolite facies at 0.6-1.1 GPa and 400-650°C. The eclogites metamorphic peak suggests formation in a normal subduction zone at ~60 km depth, a subsequent exhumation to the middle section of the subduction channel (~35 km), and a later metamorphic reworking at lower P and higher T before its final exhumation. The first type of garnet amphibolite shows a similar trajectory as the eclogites but at warmer conditions. In

  13. Continental rift and oceanic protoliths of mafic-ultramafic rocks from the Kechros Complex, NE Rhodope (Greece): implications from petrography, major and trace-element systematics, and MELTS modeling

    Science.gov (United States)

    Baziotis, I.; Mposkos, E.; Asimow, P. D.

    2014-06-01

    The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe-Ti (LFT) eclogites have low-TiO2 content (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE-HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO2 loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.

  14. Zircon U-Pb And Biotite 40Ar/39Ar Ages Of Kohistan Lower Crustal Tonalite And Their Implications For The History Of Continental Collision

    Science.gov (United States)

    Nakajima, T.; Williams, I. S.; Hyodo, H.; Miyazaki, K.; Sano, S.; Kausar, A. B.

    2002-12-01

    The Kohistan block in northern Pakistan exposes a crustal cross section through an ancient oceanic island arc, comprising garnet pyroxenite, garnet granulite, banded amphibolite, norite gabbro, metasediment and metavolcanics. The Dasu Tonalite intrudes the lower crustal Kamila Amphibolite. The tonalite is foliated and folded concordantly with the host amphibolite, indicating syn-tectonic intrusion, and contains abundant magmatic epidotes, indicating high-P crystallization. The Dasu Tonalite is extremely poor in K2O (0.6-0.9 wt.% for SiO2 64-70%) and has a low initial 87Sr/86Sr (0.7037-0.7038, similar to the associated lower crustal amphibolite and granulite), consistent with juvenile granitic magma free of contamination by recycled upper crust. The Dasu Tonalite gave SHRIMP zircon U-Pb ages of 97.6+/-1.0 Ma and 98.0+/-1.1 Ma on two samples, and biotite 40Ar/39Ar ages of 69.7+/-0.7 Ma and 69.7+/-0.9 Ma. The euhedral shape and lack of overgrowth or resorption features in CL images of the zircons suggest a simple magmatic history starting at ca. 98 Ma with no evidence for a later major thermal event. The large discrepancy between the U-Pb and Ar-Ar ages might record the deep crustal residence time of the Dasu Tonalite. The tonalite magma was probably generated and crystallized at ca. 98 Ma, then remained in the lower crust at a temperature of about 700C (which is given by geothermometry of the intercalating Kamila amphibolite), cooling down to ca.300C at 69.7Ma. 69.7Ma is a cooling age during the process of exhumation of the Kohistan arc caused by the Indian collision.

  15. Characteristics of ophiolite-related metamorphic rocks in the Beysehir ophiolitic mélange (Central Taurides, Turkey), deduced from whole rock and mineral chemistry

    Science.gov (United States)

    Çelik, Ömer Faruk; Delaloye, Michel F.

    2006-04-01

    Small outcrops of the metamorphic rocks of the Beysehir ophiolite appear to the west of Gencek and to the south of Durak (South of Beysehir Lake) in the Central Tauride Belt in Turkey. Amphibolitic rocks in the ophiolitic mélange have an igneous origin. Protoliths of these rocks were probably alkali basalts, gabbros or some ultramafic cumulates, such as pyroxenite. The amphibolites of the Beysehir Ophiolite can be divided into four groups: (1) amphibole+garnet+plagioclase±epidote (as secondary minerals)±opaque such as ilmenite±accessory minerals such as sphene and apatite; (2) amphibole+pyroxene+plagioclase±epidote±accessory minerals such as sphene, apatite±chlorite, calcite (as secondary mineral); (3) amphibole±plagioclase±opaque±accessory minerals; (4) amphibole+plagioclase±epidote±biotite and muscovite±opaque±accessory minerals. These metamorphic rocks show mainly granoblastic, grano-nematoblastic, porphyroblastic and/or poikiloblastic textures. All amphiboles in the amphibolites are calcic and cluster in the range from magnesio-hastingsite, pargasite to actinolite. Amphibole compositions are characterized by SiO 2=(38.02-54.3%), Al 2O 3=(1.5-12.8), FeO=(10.03-14.67%), K 2O=(0.2-1.8%), MgO=(5.5-15.7), Mg*=(0.3-0.8). The amphibolites show an alkaline to subalkaline character. However, the primitive mantle normalized incompatible trace element diagram shows close similarity with the typical ocean island basalt (OIB) pattern. The Rock/Chondrite normalized REE diagram of the amphibolites also confirms their OIB signature. Tectonomagmatic discrimination diagrams based on the immobile trace elements suggest a mostly within-plate alkali basalt (WPB) environment. Beysehir ophiolitic mélange contains amphibolites from ophiolite-related metamorphic rocks, but the matrix of the Beysehir ophiolitic mélange is not metamorphosed. Blocks of metamorphic rocks and the ophiolitic rocks may have been incorporated into the ophiolitic mélange in an oceanic

  16. Petrografía y geoquímica de las anfibolitas del cerro La Cocha, Sierra Chica, Córdoba Petrography and geochemistry of the anphibolites from La Cocha Hill, Sierra Chica, Córdoba, Argentina

    Directory of Open Access Journals (Sweden)

    Patricia A Anzil

    2012-06-01

    Full Text Available En la Sierra Chica de Córdoba, a dos kilómetros al sur del Observatorio Astronómico de Bosque Alegre, se encuentran dos variedades litológicas de anfibolitas asociadas al cuerpo ultramáfico de La Cocha. Estas variedades incluyen anfibolitas sin piroxeno, compuestas por hornblenda, plagioclasa, titanita y minerales opacos, emplazadas en el núcleo de la estructura plegada del cuerpo ultramáfico. El segundo grupo son anfibolitas con dos piroxenos, compuestas por hornblenda, plagioclasa, ortopiroxeno (En, clinopiroxeno (Di y como accesorios minerales opacos, apatita, escasa titanita y circón. Estas anfibolitas se encuentran en el encajonante del cuerpo ultramáfico de La Cocha, asociadas a gneises granatíferos sillimaníticos. Con el objetivo de determinar el protolito, que por metamorfismo dieron origen a las anfibolitas, se realizó un estudio geoquímico de roca total. Las anfibolitas con dos piroxenos se clasifican dentro de la serie toleítica y, en base al diagrama de tierras raras se las podría asociar a un ambiente tectónico de generación de tipo N-MORB. Las anfibolitas sin piroxeno, también corresponderían a un ambiente asimilable a N-MORB, con mayor depresión en las tierras raras livianas, sugiriendo probablemente un ambiente de generación del fundido en un manto empobrecido.In the Sierra Chica of Córdoba (Argentina, two lithological varieties of amphibolites associated with the La Cocha ultramafic body, crops out two kilometer south of the Bosque Alegre Astronomical Observatory. These varieties include non-pyroxene amphibolite composed of hornblende, plagioclase, titanite and opaque minerals, situated in the core of the ultramafic body folded structure. The second group is pyroxene amphibolites comprising hornblende, plagioclase, orthopyroxene (En, clino-pyroxene (Di and accessories including opaque minerals, apatite, more rarely titanite and zircon. This group appears in the ultramafic country rock represented by garnet

  17. New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon

    Science.gov (United States)

    Moore, Diane E.; Blake, M.C., Jr.

    1989-01-01

    The early metamorphic history of high-grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high-grade glaucophane schists, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high-grade glaucophane schist and eclogite have replaced an earlier epidote-amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende-rich blocks. At least some of the hornblende-rich blocks may therefore be well-preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partititioning in the mixed-assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende-forming stage. -Authors

  18. Tectonic setting of blueschist and island-arc terranes of west-central Baja California, Mexico

    Energy Technology Data Exchange (ETDEWEB)

    Sedlock, R.L.

    1988-07-01

    Diverse oceanic rocks exposed in west-central Baja California (Mexico) are assigned to three shallowly dipping tectonic units. The lower plate is a subduction complex consisting of regionally metamorphosed blueschists and divided into structurally and petrologically distinct subterranes. The upper plate consists of Triassic-Lower Cretaceous arc rocks and overlapping mid-Cretaceous turbidites. A fault-bounded serpentinite-matrix melange between lower and upper plate rocks contains blueschist, eclogite, amphibolite, and variably serpentinized mafic and ultramafic blocks. Lower plate blueschists were metamorphosed in the late Early Cretaceous and uplifted along shallowly dipping normal faults during conditions of steady-state subduction. Exotic blocks of blueschist, eclogite, and amphibolite are derived from a cryptic terrane or terranes, record varied P-T-t histories, and were uplifted via tectonic emplacement or diapiric rise of serpentinite along the normal faults.

  19. Brazil Geologic Basic Survey Program - Limoeiro - Sheet SB.25-Y-C-V -Pernambuco State

    International Nuclear Information System (INIS)

    The Limoeiro map-sheet (SB.25-Y-C-V;1:100,000 scale), State of Pernambuco is delimited by the meridians 35000'W to 35030' W and parallels 7030' S to 8000' S. The sheet covers an area of about 3,000 km2. The basement rocks probable Archaean age consist of gneiss and migmatite. The basement rocks are overlain by Lower Proterozoic metasediments (schist and para gneiss), locally with flows (amphibolite), metamorphosed in the middle to high amphibolite facies. Geochemical surveys including stream sediment sampling and rock chip sampling were carried out. Ground geophysics included magnetometer, gravity and radiometric (scintillometer) surveys. A provisional metallogenetic map at 1:100,000 scale was prepared on which areas with potential for economic deposits of gold, apatite, barium copper, nickel, cobalt, zinc, niobium, iron, titanium and vanadium are shown. (author)

  20. Petrogenesis of the uraniferous albitites, BA, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Chaves, Alexandre de Oliveira, E-mail: alochaves@yahoo.com.br [Universidade Federal de Minas Gerais (CPMTC/IGC/UFMG), Belo Horizonte, MG (Brazil). Centro de Pesquisas Manoel Teixeira da Costa. Instituto de Geociencias

    2011-07-01

    After Chaves et al. (2007), the crystallization and coeval deformation of the Lagoa Real uraniferous sodic syenite (uraniferous albitite protolith) took place along paleoproterozoic shear zones developed under regional late-orogenic tectonic conditions. The interpretation of new major elements, Zr and Th lithogeochemical data points to a petrogenetic connection between alkali-gabbro (local amphibolite protolith) and sodic syenite by fractional crystallization through transitional alkaline series. This magmatic differentiation occurred either before or during shear process, which in turn led to albitite and amphibolite formation. The regional microclinegneiss, whose protolith is a syn-collisional potassic granite, represents the albitite host rock, and, apparently, it has no petrogenetic association to syenite (albitite protolith) in magmatic evolutive terms (author)

  1. Early mantle dynamics inferred from Nd-142 variations in Archean rocks from southwest Greenland

    DEFF Research Database (Denmark)

    Rizo, Hanika; Boyet, Maud; Blichert-Toft, Janne;

    2013-01-01

    The composition and evolution of the silicate Earth during Hadean/Eoarchean times are widely debated and largely unknown due to the sparse geological record preserved from Earth's infancy. The short-lived Sm-146-Nd-142 chronometer applied to 3.8-3.7 Ga old mantle-derived amphibolites from the Isu...... the compositional evolution and dynamic workings of Earth's primordial mantle. (C) 2013 Elsevier B.V. All rights reserved....... domains of the ISB, accompanied by their corresponding Sm-147-Nd-143 and Lu-176-Hf-176 systematics. The 3.8 Ga suite yields Nd-142 excesses comparable to those detected previously in 3.7 Ga old ISB amphibolites, indicating that Eoarchean mafic ISB Iavas originated from sources with similar differentiation...

  2. Petrogenesis of the uraniferous albitites, BA, Brazil

    International Nuclear Information System (INIS)

    After Chaves et al. (2007), the crystallization and coeval deformation of the Lagoa Real uraniferous sodic syenite (uraniferous albitite protolith) took place along paleoproterozoic shear zones developed under regional late-orogenic tectonic conditions. The interpretation of new major elements, Zr and Th lithogeochemical data points to a petrogenetic connection between alkali-gabbro (local amphibolite protolith) and sodic syenite by fractional crystallization through transitional alkaline series. This magmatic differentiation occurred either before or during shear process, which in turn led to albitite and amphibolite formation. The regional microclinegneiss, whose protolith is a syn-collisional potassic granite, represents the albitite host rock, and, apparently, it has no petrogenetic association to syenite (albitite protolith) in magmatic evolutive terms (author)

  3. Petrography and geochemistry of metamafic rocks intercalated in gneisses from Goiás Magmatic Arc, region of Indiara (GO

    Directory of Open Access Journals (Sweden)

    Guillermo Rafael Beltran Navarro

    2015-03-01

    Full Text Available In Indiara region (GO, dozens of metamafic rocks lenses of various dimensions (metric to kilometric occur, intercalated in gneisses of the Goiás Magmatic Arc and oriented according to the direction of the main foliation (Sn. These lenses consist of amphibolite (amphibole schists and amphibolites, have chemical compositions of sub-alkaline basalts to andesitic basalts and sub-alkaline tholeiitic affinity. The distribution of major and trace elements, as well as rare earth elements (REE, suggests that these rocks are derived from a source with similar geochemical signature. The analyzed rocks are enriched in large ion litophile elements (Cs, Rb, Ba, K, Th and U in relation to elements of high field strength (Nb, Ta, Zr, Hf and Y and in relation to REE, Sr, P and Ti, showing weak to medium negative anomalies of Nb, Ta and P and suggesting that these rocks were generated in a magmatic arc environment.

  4. Constraints on the tectonic evolution of the Namaqua Province: Pt. 2

    International Nuclear Information System (INIS)

    Palaeomagnetic samples were collected from high-grade metamorphic rocks to amphibolite facies rocks. A new palaeomagnetic site pole from the Port Edward Charnokite is reported. This paper also represents 40Ar/39Ar data for samples from the Namaqua Province and the Kheis Belt, which when combined with other published radiometric data for the region, are used to construct tentative cooling histories. When combined with the palaeomagnetic experiments, these curves are used to estimate the ages of the magnetization

  5. The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, southwest England.

    OpenAIRE

    Cook, C.A.; Holdsworth, R. E.; Styles, M.T.

    2002-01-01

    Upper mantle peridotites and associated oceanic rocks from the Lizard Complex, southwest England, preserve evidence for a multistage geological history. Steeply dipping pre-emplacement fabrics record high-temperature (900-1100 degreesC) shearing and exhumation of the mantle peridotites apparently formed during localized NE-SW rifting in a pull-apart basin setting (c. 400-390 Ma). Associated oceanic rocks (Landewednack amphibolites) preserve a pre-emplacement prograde brown amphibole-bearing m...

  6. The first data on paleomagnetism of Palaeoproterozoic rocks of the Serpovidny structure (the Kola region, northeastern Baltic Shield

    Directory of Open Access Journals (Sweden)

    Matyushkin A. V.

    2016-03-01

    Full Text Available The orientation of natural magnetization vector components in amphibolites and amphibole schists (magnetite up to 10 % coincides with that of the modern magnetic field vector. Different orientations have been discovered only in quartzitic gneiss (magnetite ≤ 2 % and regressively changed chlorite-amphibole schist. The palaeopole position determined for these rocks corresponds on the apparent pole wander path to ages of 1.95 Ga and ~1.80 Ga, respectively

  7. The effects of retrograde reactions and of diffusion on 39Ar-40Ar ages of micas

    DEFF Research Database (Denmark)

    Allaz, Julien; Engi, Martin; Berger, Alfons;

    2011-01-01

    Effects of metamorphic reactions occurring during decompression were explored to understand their influence on 39Ar-40Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) were studied. Collected samples reached lower amphibolite facies during the Barrovian...... retrograde chlorite formation. We conclude that even very minor chloritisation of biotite is apparently a more effective parameter than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is necessary to ensure that...

  8. Pressure-temperature evolution of Neoproterozoic metamorphism in the Welayati Formation (Kabul Block), Afghanistan

    Science.gov (United States)

    Collett, Stephen; Faryad, Shah Wali

    2015-11-01

    The Welayati Formation, consisting of alternating layers of mica-schist and quartzite with lenses of amphibolite, unconformably overlies the Neoarchean Sherdarwaza Formation of the Kabul Block that underwent Paleoproterozoic granulite-facies and Neoproterozoic amphibolite-facies metamorphic events. To analyze metamorphic history of the Welayati Formation and its relations to the underlying Sherdarwaza Formation, petrographic study and pressure-temperature (P-T) pseudosection modeling were applied to staurolite- and kyanite-bearing mica-schists, which crop out to the south of Kabul City. Prograde metamorphism, identified by inclusion trails and chemical zonation in garnet from the micaschists indicates that the rocks underwent burial from around 6.2 kbar at 525 °C to maximum pressure conditions of around 9.5 kbar at temperatures of around 650 °C. Decompression from peak pressures under isothermal or moderate heating conditions are indicated by formation of biotite and plagioclase porphyroblasts which cross-cut and overgrow the dominant foliation. The lack of sillimanite and/or andalusite suggests that cooling and further decompression occurred in the kyanite stability field. The results of this study indicate a single amphibolite-facies metamorphism that based on P-T conditions and age dating correlates well with the Neoproterozoic metamorphism in the underlying Sherdarwaza Formation. The rocks lack any paragenetic evidence for a preceding granulite-facies overprint or subsequent Paleozoic metamorphism. Owing to the position of the Kabul Block, within the India-Eurasia collision zone, partial replacement of the amphibolite-facies minerals in the micaschist could, in addition to retrogression of the Neoproterozoic metamorphism, relate to deformation associated with the Alpine orogeny.

  9. Petrology and U/Pb geochronology of the Santa Maria Ipalapa region in the southeastern part of the Xolapa Complex, Mexico: Constrains of the metamorphic evolution of the Xolapa Terrane

    Science.gov (United States)

    Gutiérrez Aguilar, F.; Victoria Morales, A.; Maldonado, R.

    2015-12-01

    The Xolapa Complex is a metamorphic-plutonic basement that forms a large belt with more than 600 km length and 50-100 km wide along the Pacific coast of southeastern Mexico. This Complex is constituted by a high grade sequence of meta-sedimentary and meta-igneous rocks, locally migmatisized, and which are intruded by strongly deformed plutonic rocks. Because of their representative characteristics, two samples were analyzed: 1) Para-schist: this rocks present a mineral assemblage composed of biotite, sillimanite, plagioclase, k-feldspar and garnet, and 2) Amphibolite: which are constituted by amphibole, plagioclase, biotite and garnet. The garnet porphydoblasts in the para-schist are subhedral, presents retrograde compositional zoning, with almandine and pyrope rich core (Alm74-75Sps7.-10.1Pyr12.1-12.5Grs3.8-3.9) and spessartine rich rim (Alm69-71Sps14-19Pyr7.9-9.6Grs3.6-3.7). The garnet in amphibolite, presents a prograde growth zoning with a slight increase in spessartine in the core (Alm59-60Grs24-25Pyr8.0-8.3Sps7.3-7.6), and low content of spessartine component toward the rim (Alm60-62Grs23-24Pyr8.8-9.6Sps5.4-5.5). In order to constrain the P-T evolution of the region, multiequilibria thermobarometry was applied to both samples, the para-schist unit presents P-T data from 706 (ºC) and 7.5 (kbar), in the other hand the garnet amphibolite unit shows P-T data from 734 (ºC) and 7 (kbar). This study provides new geochronological data (U/Pb in zircons) for the amphibolite facies metamorphism and for the migmatitic event in the region that contributes to the understanding of the tectonic evolution of southeastern Mexico.

  10. Campo Belo Metamorphic Complex: tectonic evolution of an Archean sialic crust of the southern São Francisco Craton in Minas Gerais (Brazil)

    OpenAIRE

    2001-01-01

    Systematic geological studies performed in the study area allowed the characterization of six lithodemic units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The mineral assemblage and the structural record of these lithodemic units indicate that the study area was affected by five tectonothermal events. The structural pattern of the first and oldest event occurred under granulite facies conditions and reveals essentially a sinistral kinematic pattern. The second e...

  11. The Murchison Greenstone Belt, South Africa: Accreted slivers with contrasting metamorphic conditions

    OpenAIRE

    Block, Sylvain; Moyen, Jean-François; Zeh, Armin; Poujol, Marc; Jaguin, Justine; Paquette, Jean-Louis

    2013-01-01

    This paper presents new petrological and geochronological data for the ∼3.09-2.92 Ga Murchison Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure-temperature-deformation (P-T-D) histories, and are separated from each other by relatively narrow, high-strain shear z...

  12. The first data on paleomagnetism of Palaeoproterozoic rocks of the Serpovidny structure (the Kola region, northeastern Baltic Shield)

    OpenAIRE

    Matyushkin A. V.; Balagansky V. V.

    2016-01-01

    The orientation of natural magnetization vector components in amphibolites and amphibole schists (magnetite up to 10 %) coincides with that of the modern magnetic field vector. Different orientations have been discovered only in quartzitic gneiss (magnetite ≤ 2 %) and regressively changed chlorite-amphibole schist. The palaeopole position determined for these rocks corresponds on the apparent pole wander path to ages of 1.95 Ga and ~1.80 Ga, respectively

  13. Detrital zircon SHRIMP U-Pb age study of the Cordillera Darwin Metamorphic Complex of Tierra del Fuego : sedimentary sources and implications for the evolution of the Pacific margin of Gondwana

    OpenAIRE

    Herve, F.; Fanning, C.M.; Pankhurst, R.J.; Mpodozis, C.; Klepeis, K.; Calderon, M.; S. N. Thomson

    2010-01-01

    The Cordillera Darwin Metamorphic Complex in the southernmost Andes includes a basement of probable Palaeozoic age, a mid-Jurassic and younger volcano-sedimentary cover, and a suite of Jurassic granites, all of which were jointly metamorphosed during the Cretaceous. Detrital zircon ages presented here show that some of the amphibolite-facies metamorphic rocks previously mapped as basement have a Jurassic protolith. Overall the detrital zircon age patterns for samples of the Cordillera Darwin ...

  14. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: A lead isotope study of an Archaean gold prospect in the Attu region, Nagssugtoqidian orogen, West Greenland

    OpenAIRE

    Stendal, Henrik; Frei, Robert; Stensgaard, Bo Møller

    2006-01-01

    This paper presents a lead isotope investigation of a gold prospect south of the village Attu in the northern part of the Nagssugtoqidian orogen in central West Greenland. The Attu gold prospect is a replacement gold occurrence, related to a shear/mylonite zone along a contact between orthogneissand amphibolite within the Nagssugtoqidian orogenic belt. The mineral occurrence is small, less than 0.5 m wide, and can be followed along strike for several hundred metres. The mineral assemblage is ...

  15. Mineralogy and the fluid inclusion data of the Bonče tourmaline-bearing pegmatite, the Selečka Mts., Republic of Macedonia

    OpenAIRE

    Šmajgl, Daniela; Strmić Palinkas, Sabina; Palinkas, Ladislav; Kampić, Stefica; Boev, Blazo; Váczi, Tamás

    2012-01-01

    The Bonče tourmaline-bearing pegmatite is situated approximately 150 km southern from Skopje, Republic of Macedonia, on the western slopes of the Selečka Mts. The Selečka Mts. belongs to the Eastern Pelagonian tectono-stratigraphic unit of the Dinaride-Hellenides. It comprises Precambrian ortho- and paragneisses, micaschists and amphibolites and hosts numerous pegmatites which differ according to their size, the mineralogical features, the internal structures and the fractionation degree. The...

  16. Rb-Sr measurements on metamorphic rocks from the Barro Alto Complex, Goias, Brazil

    International Nuclear Information System (INIS)

    The Barro Alto Complex comprises a highly deformed and metamorphosed association of plutonic, volcanic, and sedimentary rocks exposed in a 150 x 25 Km boomerang-like strip in Central Goias, Brazil. It is the southernmost tip of an extensive yet discontinuous belt of granulite and amphibolite facies metamorphic rocks which include the Niquelandia and Cana Brava complexes to the north. Two rock associations are distinguished within the granulite belt. The first one comprises a sequence of fine-grained mafic granulite, hypersthene-quartz-feldspar granulite, garnet quartzite, sillimanite-garnet-cordierite gneiss, calc-silicate rock, and magnetite-rich iron formation. The second association comprises medium-to coarse-grained mafic rocks. The medium-grade rocks of the western/northern portion (Barro Alto Complex) comprise both layered mafic rocks and a volcanic-sedimentary sequence, deformed and metamorphosed under amphibolite facies conditions. The fine-grained amphibolite form the basal part of the Juscelandia meta volcanic-sedimentary sequence. A geochronologic investigation by the Rb-Sr method has been carried out mainly on felsic rocks from the granulite belt and gneisses of the Juscelandia sequence. The analytical results for the Juscelandia sequence are presented. Isotope results for rocks from different outcrops along the gneiss layer near Juscelandia are also presented. In conclusion, Rb-Sr isotope measurements suggest that the Barro Alto rocks have undergone at least one important metamorphic event during Middle Proterozoic times, around 1300 Ma ago. During that event volcanic and sedimentary rocks of the Juscelandia sequence, as well as the underlying gabbro-anorthosite layered complex, underwent deformation and recrystallization under amphibolite facies conditions. (author)

  17. Evolução tectonoestratigráfica dos sistemas transcorrentes Carajás e cinzento, Cinturão Itacaiúnas, na borda leste do Craton Amazônico, Pará.

    OpenAIRE

    Pinheiro, R.V.L.; Holdsworth, R. E.

    2000-01-01

    TECTONOSTRA TIGRAPHIC EVOL UTION O F THE CARAJÁS AND CINZENTO STRIKE SLIP SYSTEMS, ITA C AI UNAS BELT, EAST OF THE AMAZONIAN CRATON, PARÁ STATE The Carajás-Cinzento fault system occurs within the Itacaiúnas Belt, Amazonian Craton, Brazil. The regional tectonostratigraphy can be subdivided into: (1) the Basement Assemblage - orthogneisses, migmatites, and granulites (the Xingu and Pium Complexes; Plaque Suite) and a later volcano-sedimentary sequence of ironstones, quartzites, amphibolites and...

  18. Can the Metamorphic Basement of Northwestern Guatemala be Correlated with the Chuacús Complex?

    Science.gov (United States)

    Cacao, N.; Martens, U.

    2007-05-01

    The Chuacús complex constitutes a northward concave metamorphic belt that stretches ca. 150 km south of the Cuilco-Chixoy-Polochic (CCP) fault system in central and central-eastern Guatemala. It represents the basement of the southern edge of the Maya block, being well exposed in the sierra de Chuacús and the sierra de Las Minas. It is composed of high-Al metapelites, amphibolites, quartzofeldspathic gneisses, and migmatites. In central Guatemala the Chuacús complex contains ubiquitous epidote-amphibolite mineral associations, and local relics of eclogite reveal a previous high-pressure metamorphic event. North of the CCP, in the Sierra de Los Cuchumatanes area of western Guatemala, metamorphic rocks have been considered the equivalent of the Chuacús complex and hence been given the name Western Chuacús group, These rocks, which were intruded by granitic rocks and later mylonitized, include chloritic schist and gneiss, biotite-garnet schist, migmatites, and amphibolites. No eclogitic relics have been found within metamorphic rocks in northwestern Guatemala. Petrographic analyses of garnet-biotite schist reveal abundant retrogression and the formation of abundant zeolite-bearing veins associated with intrusion. Although metamorphic conditions in the greenschist and amphibolite facies are similar to those in the sierra de Chuacús, the association with deformed intrusive granites is unique for western Guatemala. Hence a correlation with metasediments intruded by the Rabinal granite in the San Gabriel area of Baja Verapaz seems more feasible than a correlation with the Chuacús complex. This idea is supported by reintegration of the Cenozoic left-lateral displacement along the CCP, which would place the metamorphic basement of western Guatemala north of Baja Verapaz, adjacent to metasediments intruded by granites in the San Gabriel-Rabinal area.

  19. The Age and Geodynamic Evolution of the Metamorphic sole rocks from Izmir-Ankara-Erzıncan suture zone (Northern-Turkey)

    Science.gov (United States)

    Melih Çörtük, Rahmi; Faruk Çelik, Ömer; Özkan, Mutlu; Sherlock, Sarah C.; Marzoli, Andrea; Altıntaş, İsmail Emir; Topuz, Gültekin

    2016-04-01

    The İzmir-Ankara-Erzincan suture zone in northern Turkey is one of the major tectonic zones separating the Pontides to the North from the Anatolide-Tauride block and Kı rşehir Massif to the South. The accretionary complex of the İzmir-Ankara-Erzincan suture zone, near Artova, is composed mainly of peridotites with varying degree serpentinization, metamorphic rocks, basalt, sandstones, pelagic and neritic limestones. The metamorphic rocks are represented by amphibolite, garnet micaschit, calc-schist and marble. The metamorphic rocks were interpreted as the metamorphic sole rocks. Because; (i) They are tectonically located beneath the serpentinized peridotites. (ii) Foliation planes of both the amphibolites and mantle tectonites are parallel to each other. (iii) The metamorphic rocks are crosscut by non-metamorphic dolerite dikes which exhibite Nb and Ta depletion relative to Th enrichment on the N-MORB normalized multi-element spider diagram. The dolerite dikes display flat REE patterns (LaN/YbN=0.85-1.24). These geochemical signatures of the dolerite dikes are indicative of subduction component during their occurrences. Geochemical observations of the amphibolites suggest E-MORB- and OIB-like signatures (LaN/SmN= 1.39-3.14) and their protoliths are represented by basalt and alkali basaltic rocks. Amphiboles from the amphibolites are represented by calcic amphiboles (magnesio-hornblende, tchermakite and tremolite) and they yielded 40Ar-39Ar ages between 157.8 ± 3.6 Ma and 139 ± 11 Ma. These cooling ages were interpreted to be the intra-oceanic subduction/thrusting time of the İzmir-Ankara-Erzincan oceanic domain. This study was funded by TÜBİTAK (Project no: 112Y123).

  20. Geochemistry of the Puna Austral and Cordillera Oriental basement

    International Nuclear Information System (INIS)

    Major and trace elements, rare earths, and 143Nd/147Nd and, 147Sm/144Nd isotope ratios have been determined in the Puna Austral and Cordillera Oriental basement. The basement is formed by high temperature amphibolite facies rocks ranulites (750-550 degrees C) and green schists. They are represented by schists, paragneiss, orthogneiss, migmatites, few metabasites, marbles and chalcosilicatic banks. Hypotheses on the formation and evolution of the basement are presented

  1. Application of alternative methods for determination of rock quality designation (RQD) index: a case study from the Rožná I uranium mine, Strážek Moldanubicum, Bohemian Massif, Czech Republic

    OpenAIRE

    M. Vavro; Souček, K; Staš, L. (Lubomír); Waclawik, P. (Petr); Vavro, L. (Leona); P. Koníček; Ptáček, J.

    2015-01-01

    A comparison of rock quality designation (RQD) parameters obtained by drill core analysis and the RQD determined using alternative methods is presented using metamorphic rocks such as migmatized gneisses, migmatites, and amphibolites. Methods of borehole–wall imaging using high-resolution acoustic logging, optical televiewer, and simple video inspection as well as the structural analysis of exploration drift walls oriented subparallel to the analysed boreholes are used for alternatively e...

  2. The geology and geochemistry of the Lumwana Basement hosted copper-cobalt (uranium) deposits, NW Zambia

    OpenAIRE

    Bernau, Robin

    2007-01-01

    The Lumwana Cu±Co deposits Malundwe and Chimiwungo are examples of pre-Katangan mineralized basement that are located in the Domes Region of the Lufilian Arc, an arcuate North neo-Proterozoic fold belt, which hosts the Zambian and Congolese deposits that make up the Central African Copperbelt. The Lumwana deposits are situated within the Mwombezhi Dome; a Mesoproterozoic basement inlier consisting of highly sheared amphibolite grade schist to gneiss units that host the Cu±Co mineralization. K...

  3. A neoproterozoic age for the chromitite and gabbro of the Tapo Ultramafic Massif, Eastern Cordillera, Central Peru, and its tectonic implications

    OpenAIRE

    Tassinari, Colombo G.; Castroviejo Bolibar, Ricardo; Rodrigues, Jose F.; Acosta, Jorge; Pereira, Eurico

    2011-01-01

    The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites showa tholeiitic affiliation and a flat REE spider diagram, with a slight LREE depletion and a positive Eu anomaly suggesting ...

  4. Geología de las ultramafitas pre-andinas de Tapo y Acobamba, Tarma, Cordillera Oriental del Perú

    OpenAIRE

    Castroviejo, Ricardo; Feliciano Rodrigues, José; Acosta, Jorge; Pereira, Eurico; Romero, Darwin; Quispe, Jorge; Espí, José Antonio

    2009-01-01

    Ultramafic rocks occur scattered along a 300 km long NNW-SSE trending belt, parallel to the central Peruvian Andes in the Cordillera Oriental, from Tarma (Junín Dept.) to Huancapallac and Tingo María (Huánuco Dept.). The Tarma occurrences (Tapo and Acobamba) are dealt with here, as the first step of a broader research. The Tapo massif comprises strongly tectonised serpentinites with scarce peridotitic relics, amphibolites and podiform chromitites. It was overthrust on early Carbon...

  5. Barium-rich fluids and melts in a subduction environment (La Corea and Sierra del Convento mélanges, eastern Cuba)

    Science.gov (United States)

    Blanco-Quintero, Idael Francisco; Lázaro, C.; García-Casco, A.; Proenza, J. A.; Rojas-Agramonte, Y.

    2011-08-01

    Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del Convento and La Corea mélanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid-rock interaction produced crystallization of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning, characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)-2), with more moderate contributions of the celsian (BaAl-(KSi)-1) and paragonite (NaK-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the subduction to the volcanic arc environments.

  6. Investigations of garnets from polymetamorphic rocks of the Lapland Granulite Belt of the Kandalaksha Region

    Directory of Open Access Journals (Sweden)

    Miłosz A. Huber

    2012-01-01

    Full Text Available Introduction: The Lapland Granulite Belt is placed on the Kandalaksha region (Kola Peninsula, Russia. The rocks of this Belt are composed mainly of amphibolites and granulites.Materials and methods: The research were focused on the garnets from the amphibolite and granulite rocks of Lapland Granulite Belt. The petrological methods like polarizing microscopy (PM, SEM-EDS, XRD for powdered samples and single crystal diffraction were used together with IR and Mössbauer spectroscopy and REE analysis by ion–microprobe.Results: It was found that the garnets from studied amphibolite and granulite rocks could be classified to pyralspite group without hydrogarnets components, so they were formed in high metamorphic facies.Conclusions: The joint geological observations and results of the performed experiments suggest that the garnets were subject of a blastesy, i.e. there were formed in long lasting metamorphic processes of low dynamics, except of those garnets from tectonic zones, found in the vicinity of mineral veins.

  7. The age and origin of some Norwegian eclogites: a U-Pb ziron and REE study

    International Nuclear Information System (INIS)

    U-Pb zircon data from five different eclogites and one amphibolite, all occurring as boudinaged lenses within the 1.7-Ga old Basal Gneisses of western Norway, are reported along with eleven REE patterns on eclogites, amphibolite and country rocks. Based on the size, morphology and zoning patterns of zircon as well as the degree of U-Pb discordance, protoliths of both basaltic or gabbroic origin can be distinguished. Whereas zircons from eclogites formed from tholeiitic and calc-alkaline basaltic protoliths allow dating of the Caledonian eclogite-facies event at approx. 400 Ma, no precise information on their primary magnetic age is possible. On the other hand, eclogites formed from gabbroic precursors allowed the dating of both the primary magnetic age at approx. 1.5 Ga, and in one case at more than 1.76 Ga, as well as a Caledonian eclogitization. One amphibolite sample, probably of prograde metamorphic origin, yielded a Sveconorwegian primary age (approx. 950 Ma) and a Caledonian age of metamorphism. (Auth.)

  8. Regional Characterization of Tokyo Metropolitan area using a highly-dense seismic network (MeSO-net)

    Science.gov (United States)

    Hirata, Naoshi; Nakagawa, Shigeki; Sakai, Shin'ichi; Panayotopoulos, Yannis; Ishikawa, Masahiro; Ishibe, Takeo; Kimura, Hisanori; Honda, Ryou

    2015-04-01

    We have developed a dense seismic network, MeSO-net (Metropolitan Seismic Observation network), which consists of about 300 seismic stations, since 2007 in the greater Tokyo urban region(Hirata et al., 2009). Using MeSO-net data, we obtain P- and S- wave velocity tomograms (Nakagawa et al., 2010) and Qp, Qs tomograms (Panayotopoulos et al., 2014) which show a clear image of Philippine Sea Plate (PSP) and PAcific Plate (PAP). A depth to the top of PSP, 20 to 30 km beneath northern part of Tokyo bay, is about 10 km shallower than previous estimates based on the hypocenter distribution (Ishida, 1992). Based on elastic wave velocities of rocks and minerals, we constructed a petrologic model. The Vp steps in subducting Izu forearc crust occurs at a depth of 30km (blueschist or greenschist to garnet amphibolite transformation) and a depth of 50km (garnet amphibolite to eclogite transformation). Both temperatures are estimated to be 500 and 600 degree C, respectively. The high Vp/Vs anomaly (>1.9) implies large amounts of fluid H2O released by garnet amphibolite to eclogite dehydration reactions. This study is supported by MEXT Japan under the Special Project for Reducing Vulnerability for Urban Mega Earthquake Disasters.

  9. The mafic-ultramafic complex of Aniyapuram, Cauvery Suture Zone, southern India: Petrological and geochemical constraints for Neoarchean suprasubduction zone tectonics

    Science.gov (United States)

    Yellappa, T.; Venkatasivappa, V.; Koizumi, T.; Chetty, T. R. K.; Santosh, M.; Tsunogae, T.

    2014-12-01

    Several Precambrian mafic-ultramafic complexes occur along the Cauvery Suture Zone (CSZ) in Southern Granulite Terrain, India. Their origin, magmatic evolution and relationship with the associated high-grade rocks have not been resolved. The Aniyapuram Mafic-Ultramafic Complex (AMUC), the focus of the present study in southern part of the CSZ, is dominantly composed of peridotites, pyroxenites, gabbros, metagabbros/mafic granulites, hornblendites, amphibolites, plagiogranites, felsic granulites and ferruginous cherts. The rock types in the AMUC are structurally emplaced within hornblende gneiss (TTG) basement rocks and are highly deformed. The geochemical signature of the amphibolites indicates tholeiitic affinity for the protolith with magma generation in island arc-setting. N-MORB normalized pattern of the amphibolites show depletion in HFS-elements (P, Zr, Sm, Ti, and Y) and enrichment of LIL-elements (Rb, Ba, Th, Sr) with negative Nb anomalies suggesting involvement of subduction component in the depleted mantle source and formation in a supra-subduction zone tectonic setting. Our new results when correlated with the available age data suggest that the lithological association of AMUC represent the remnants of the Neoarchean oceanic lithosphere.

  10. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

    Science.gov (United States)

    Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

    2012-06-01

    Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

  11. Late Cretaceous to Paleocene metamorphism and magmatism in the Funeral Mountains metamorphic core complex, Death Valley, California

    Science.gov (United States)

    Mattinson, C.G.; Colgan, J.P.; Metcalf, J.R.; Miller, E.L.; Wooden, J.L.

    2007-01-01

    Amphibolite-facies Proterozoic metasedimentary rocks below the low-angle Ceno-zoic Boundary Canyon Detachment record deep crustal processes related to Meso-zoic crustal thickening and subsequent extension. A 91.5 ?? 1.4 Ma Th-Pb SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) monazite age from garnet-kyanite-staurolite schist constrains the age of prograde metamorphism in the lower plate. Between the Boundary Canyon Detachment and the structurally deeper, subparallel Monarch Spring fault, prograde metamorphic fabrics are overprinted by a pervasive greenschist-facies retrogression, high-strain subhorizontal mylonitic foliation, and a prominent WNW-ESE stretching lineation parallel to corrugations on the Boundary Canyon Detachment. Granitic pegmatite dikes are deformed, rotated into parallelism, and boudinaged within the mylonitic foliation. High-U zircons from one muscovite granite dike yield an 85.8 ?? 1.4 Ma age. Below the Monarch Spring fault, retrogression is minor, and amphibolite-facies mineral elongation lineations plunge gently north to northeast. Multiple generations of variably deformed dikes, sills, and leucosomal segregations indicate a more complex history of partial melting and intrusion compared to that above the Monarch Spring fault, but thermobarometry on garnet amphibolites above and below the Monarch Spring fault record similar peak conditions of 620-680 ??C and 7-9 kbar, indicating minor (magmatism suggests that both burial by thrusting and regional magmatic heating contributed to metamorphism and subsequent partial melting. ??2007 Geological Society of America. All rights reserved.

  12. Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: Evidence from U-Pb systematics in zircon

    Science.gov (United States)

    Mueller, P.A.; Wooden, J.L.; Mogk, D.W.; Nutman, A.P.; Williams, I.S.

    1996-01-01

    The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically complex assemblage of lithotectonic units that include: (1) a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant U-Pb ages of 3.5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this rock: (1) an apparently protracted period of high-grade metamorphism and/or intrusion of additional magmas at ??? 3.25 Ga; and (2) growth of hydrothermal zircon at ??? 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ??? 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on late Archean magmatism as well.

  13. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic terrains

    Directory of Open Access Journals (Sweden)

    Mazur, Stanislaw

    2006-12-01

    Full Text Available Structural analysis of the deeply eroded northern flank of the Palaeoproterozoic Nagssugtoqidian orogen shows marked regional variations in both the orientation and type of fabrics, as is characteristic of Precambrian high-grade terrains subjected to polyphase deformation. Here we investigate the relationship between strain, metamorphic grade, and the resulting structural patterns. The study area south of Aasiaat in West Greenland consists of amphibolite- togranulite-gradeArchaean orthogneisses and relatively thin supracrustal units. The regional foliation displays a WSW–ENE to SW–NE strike associated with steep to moderate dips towards the WNW or SSE. Lineation trends are WSW–ENE and generally plunge gently towards the WSW. Mesoscopic fold hinges are usually colinear with the regional lineation. A systematic change in the plunge of lineations occurs across the south-western part of the study area. Towards the south, the lineation plunge progressively increases, despite the generally uniform strike of foliation. This southward increase of lineation pitch is typically associated with the transition from L > S or L = S shape fabrics in rocks characterised by a low pitch, to S > L or S fabrics in the zone of moderate to high pitch. The structural patterns point to subdivision of the study area into a southern domain mostly characterised by S or S > L shape fabrics and a moderate to high angle of lineation pitch, and a northern domain showing L > S or L = S fabrics and low angles of lineation pitch. This subdivision corresponds well with the map scale boundary between granulite facies rocks in the south and amphibolite facies rocks farther north. The observed structural pattern may be explained by two alternative tectonic models: (1 northward indentation of the previously cooled granulite block into the rheologically weaker amphibolite domain, and (2 strain partitioning within a mid-crustal transpression zone. In model 2 the northern domain

  14. Geologic Map of the Kings Mountain and Grover Quadrangles, Cleveland and Gaston Counties, North Carolina, and Cherokee and York Counties, South Carolina

    Science.gov (United States)

    Horton, J. Wright, Jr.

    2008-01-01

    This geologic map of the Kings Mountain and Grover 7.5-min quadrangles, N.C.-S.C., straddles a regional geological boundary between the Inner Piedmont and Carolina terranes. The Kings Mountain sequence (informal name) on the western flank of the Carolina terrane in this area includes the Neoproterozoic Battleground and Blacksburg Formations. The Battleground Formation has a lower part consisting of metavolcanic rocks and interlayered schist and an upper part consisting of quartz-sericite phyllite and schist interlayered with quartz-pebble metaconglomerate, aluminous quartzite, micaceous quartzite, manganiferous rock, and metavolcanic rocks. The Blacks-burg Formation consists of phyllitic metasiltstone interlayered with thinner units of marble, laminated micaceous quartzite, hornblende gneiss, and amphibolite. Layered metamorphic rocks of the Inner Piedmont terrane include muscovite-biotite gneiss, muscovite schist, and amphibolite. The Kings Mountain sequence has been intruded by metatonalite and metatrondhjemite (Neoproterozoic), metagabbro and metadiorite (Paleozoic?), and the High Shoals Granite (Pennsylvanian). Layered metamorphic rocks of the Inner Piedmont in this area have been intruded by the Toluca Granite (Ordovician?), the Cherryville Granite and associated pegmatite (Mississippian), and spodumene pegmatite (Mississippian). Diabase dikes (early Jurassic) are locally present throughout the area. Ductile fault zones of regional scale include the Kings Mountain and Kings Creek shear zones. In this area, the Kings Mountain shear zone forms the boundary between the Inner Piedmont and Carolina terranes, and the Kings Creek shear zone separates the Battleground Formation from the Blacksburg Formation. Structural styles change across the Kings Mountain shear zone from steeply dipping layers, foliations, and folds on the southeast to gently and moderately dipping layers, foliations, and recumbent folds on the northwest. Mineral assemblages in the Kings Mountain

  15. Fluid induced metamorphism and strength of the middle to lower continental crust - field and textural examples from Bergen Arcs, Western Norway

    Science.gov (United States)

    Austrheim, H.; Putnis, A.; Putnis, C. V.

    2011-12-01

    Fluids may change the rheology of the lithosphere both by changing the deformation mechanism of minerals and by inducing metamorphic reactions. In the present account the influence of fluid-induced metamorphic reactions on the mineralogical and structural evolution of a thickened continental crust is described from anorthositic granulites in the Lindås Nappe, Bergen Arcs, Norway, where the Grenvillian age (~930 My) granulites (T 800°C, P≤10kbar) are transformed to Caledonian age (~420My) eclogite (~650°C and ≤ 20kbar) and amphibolite facies assemblages. The anorthosite complex ranges in composition from pure anorthosite via gabbroic anorthosite to gabbro with lenses of peridotite and pyroxenites which allow us to study the mineral reactions and assess relative rock strength in a wide range of compositions. The complex is locally banded with up to meter thick garnet-pyroxene rich layers alternating with plagioclase rich layers. In other localities the granulite facies structure is defined by oriented disc-shaped corona textures in a plagioclase rich matrix. The eclogites (garnet, omphacite, amphibole, kyanite, white micas ± plagioclase) and amphibolites (plagioclase, hornblende, kyanite, and white micas) are formed along fluid pathways such as fractures and shear zones. Breccias, where rotated blocks of granulites are surrounded by anastamosing eclogite- and amphibolite facies shear zones, outcrop over areas of km2. Pseudotachylytes are developed in the granulites while the hydrated rocks in the shear zone respond by ductile deformation. A hierachy of rock strength can be inferred from these field observations. Notably the relict granulites form rotated angular blocks within the shear zones suggesting that granulites, independent of composition, are stronger than hydrous eclogitites and amphibolites. The garnet pyroxenite layer forms rigid blocks in eclogites suggesting that the mafic parts of the granulite complex must have been stronger than the

  16. Revealing the significance and polyphase tectonothermal evolution of a major metamorphic unit in an orogen: the central Sanandaj-Sirjan zone, Zagros Mts., Iran

    Science.gov (United States)

    Shakerardakani, Farzaneh; Neubauer, Franz; Genser, Johann; Liu, Xiaoming; Dong, Yunpeng; Monfaredi, Behzad; Benroider, Manfred; Finger, Fritz; Waitzinger, Michael

    2016-04-01

    The Dorud-Azna region in the central Sanandaj-Sirjan metamorphic belt plays a key role in promoting the tectonic evolution of Zagros orogen, within the frame of the Arabia-Eurasia collision zone. From footwall to hangingwall, structural data combined with the U-Pb zircon and extensive 40Ar-39Ar mineral dating survey demonstrate three metamorphosed tectonic units, which include: (1) The Triassic June complex is metamorphosed within greenschist facies conditions, overlain by (2) the amphibolite-grade metamorphic Galeh-Doz orthogneiss, which is intruded by mafic dykes, and (3) the Amphibolite-Metagabbro unit. To the east, these units were intruded by the Jurassic Darijune gabbro. We present U-Pb detrital zircon ages of a garnet-micaschist from the Amphibolite-Metagabbro unit, which yield six distinctive age groups, including a previously unrecognized Late Grenvillian age population at ~0.93 to 0.99 Ga. We speculate that this unique Late Grenvillian group coupled with biogeographic evidence suggests either relationship with the South China craton or to the "Gondwana superfan". The laser ablation ICP-MS U-Pb zircon ages of 608 ± 18 Ma and 588 ± 41 Ma of the granitic Galeh-Doz orthogneiss reveals a Panafrican basement same as known from the Yazd block of Central Iran. Geochemistry and Sr-Nd isotopes of alkaline and subalkaline mafic dykes within the Galeh-Doz orthogneiss show OIB-type to MORB-type and indicate involvement of both depleted and enriched sources for its genesis. The new 40Ar-39Ar amphibole age of ca. 322.2 ± 3.9 Ma from the alkaline mafic dyke implies Carboniferous cooling age after intrusion. The metagabbros (including the Dare-Hedavand metagabbro with a 206Pb/238U age of 314.6 ± 3.7 Ma) and amphibolites with E-MORB geochemical signature of the Amphibolite-Metagabbro unit represent an Upper Paleozoic rift. The geochemical composition of the Triassic greenschist facies metamorphosed June complex, implying formation in a same, but younger tectonic

  17. Tectonic history of the central Sanandaj-Sirjan zone, Iran: Potentially Permian to Mesozoic polymetamorphism and implications for tectonics of the Sanandaj-Sirjan zone

    Science.gov (United States)

    Shakerardakani, Farzaneh; Neubauer, Franz; Genser, Johann; Masoudi, Fariborz; Mehrabi, Behzad; Monfaredi, Behzad; Friedl, Gertrude

    2015-04-01

    The determination of metamorphic conditions and of its age is critical to the understanding of the mountain belt formation as metamorphism is an expression of subduction or plate collision. In this study, we report the metamorphic evolution, preliminary Ar-Ar mineral ages and structures from two amphibolite-grade metamorphic units of the Dorud-Azna region in the central part of Sanandaj-Sirjan metamorphic zone and discuss the tectonic implications. The Sanandaj-Sirjan metamorphic zone is nearly parallel to the Main Zagros Reverse Fault and is located above the Neotethyan ophiolitic suture. Structural studies and our previous U-Pb zircon dating work demonstrated that the area comprise three metamorphosed tectonic units, which are from footwall to hangingwall: (1) The Triassic June complex is metamorphosed within greenschist facies conditions, overlain by (2) the amphibolite-grade metamorphic Panafrican Galeh-Doz orthogneiss, which is intruded by some mafic dykes, and (3) the Amphibolite-Metagabbro unit with Carboniferous metagabbro bodies. To the East, the Darijune gabbro intruded within the Permian Kuh-e-June Marble and the mentioned two other metamorphic units. The granitic Galeh-Doz orthogneiss displays two different P-T conditions. The best average estimates for the magmatic mineral assemblage (plagioclase core + amphibole core + K-feldspar + quartz) range between 675 and 710 °C and 3.7 and 4.2 kbar, whilst the temperature of 530 and 625 °C and pressure of 0.7 to 2.8 kbar is consistent with the first metamorphic mineral assemblage. Ar-Ar amphibole ages from the Galeh-Doz orthogneiss give plateau-like steps between 260 and 270 Ma. We interpret this age as the cooling age after an amphibolite facies-grade metamorphism. An amphibole from relatively well preserved dyke within the Galeh-Doz orthogneiss gives staircase pattern with an age of 261 ± 3 Ma in the first step considered similarly as a metamorphic overprint in metamorphic rocks, whereas plateau-like steps

  18. Eclogite Facies Relicts and Decompression Assemblages; Evidence for the Exhumation of a Large Coherent Metabasite Block From > 40 km Depth; Central Metamorphic Terrane, Eastern Klamath Mountains, Northern California

    Science.gov (United States)

    Barrow, W. M.; Fairhurst, R. J.; Metcalf, R. V.

    2007-12-01

    Recent exhumation models for eclogite terranes have focused on the exhumation of sialic rocks. Exhumed high pressure terranes are typically > 85% - 90% sialic material with only minor amounts of mafic and ultramafic rock. Most known metabasitic eclogites are blocks in mélange rather than large coherent bodies. The Central Metamorphic terrane (CMt) is a large (~300 km3) coherent, fault-bounded package of metabasites thought to represent a remnant of a downing plate subducted in an intra-oceanic convergent margin. Thermochronology indicates that the CMt was metamorphosed and later accreted to the base of the Trinity ophiolite along the Trinity fault during Early Permian extension (Hbl and Musc 40Ar/39Ar ages of 275 Ma - 294 Ma). Previous work suggested that the peak metamorphic temperatures and pressures were ~650°C and 0.4 to 0.8 GPa (Peacock and Norris, 1989) which is consistent with the amphibolite facies mineral assemblage. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures, however, suggest that CMt amphibolites record much deeper subduction burial with subsequent decompression exhumation. A decompression sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry (Watson et al., 2006) combined with experimental data for rutile stability in metabasites (Ernst and Lui, 1998) suggests that relict rutile crystals preserve early P-T conditions of ~600°C and > 1.3 GPa consistent with eclogite facies metamorphism. Transition from eclogite facies is further supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of garnet (Bhowmik and Roy, 2003) during decompression. Amphibole compositions vary significantly and reflect lower grade (low Na, Al, Ti actinolite) overprint of earlier amphibolite facies compositions (high Na, Al, Ti magnesio- hornblende). Application of the Al-Ti hornblende

  19. First pressure- and temperature estimates of the metamorphic sole of the Pinarbasi ophiolite, central Turkey

    Science.gov (United States)

    Peters, Kalijn; van Hinsbergen, Douwe; van Roermund, Herman; Brouwer, Fraukje; Drury, Martyn

    2014-05-01

    Ophiolites are interpreted as remnants of oceanic lithosphere. Many have a so-called supra-subduction zone (SSZ) geochemical signature, suggestive of formation at a spreading ridge overlying a subduction zone. Supra-subduction zone ophiolites frequently have a several-hundred-meter thick sequence of metamorphic rocks below their mantle section: the metamorphic sole. These dominantly mafic and generally heavily sheared metamorphic rocks have been shown to preserve an inverted metamorphic gradient with the highest pressures and temperatures at the top of the sole, decreasing downwards. Pressure estimates from rocks found at the top of metamorphic soles may be as much as 10-15 kbar with temperature estimates up to 875°C. The metamorphic grade varies from greenschist near the base, up to granulite facies at the top, with the bulk comprising of amphibolite facies rocks. At some locations a blueschist overprint of the amphibolite facies mineral assemblages has been described. The relative high pressures preserved in the metamorphic sole cannot simply result from overburden pressure of the currently overlying ophiolite, which is a long-standing problem. This raises the question of what process(es) can explain pressures up to 10-15 kbar in the top of metamorphic soles, in relation to the approximately synchronous formation of the SSZ oceanic lithosphere above the sole. One of the places to study the formation of SSZ ophiolites and their metamorphic soles is the Neotethyan Suture zone. Remnants of Neotethyan lithosphere are preserved as ophiolites that are discontinuously exposed from the Mediterranean region through the Himalaya to SE Asia. Supra-subduction zone ophiolites are particularly widespread in Turkey. The Pinarbaşi ophiolite is located in the SE of Central Anatolia, and overlies the Tauride fold-and-thrust belt that formed since the Late Cretaceous. It comprises mantle tectonites consisting of serpentinized harzburgite and dunite with remnants of gabbro to the

  20. Origin and implications of zoned amphiboles and other hydrous silicates during aqueous brine infiltration in the Bamble mega shearzone, S-Norway

    Science.gov (United States)

    Sorensen, B. E.; Larsen, R. B.

    2009-12-01

    This study addresses the metasomatic alteration of ortho-amphibolites, in the Froland area in the Bamble sector, South Norway. Potassic alteration is associated with the introduction of biotite and the formation of conspicuous rims on the amphiboles. Significant color variation from deep blue green to light green in the rims encouraged a chemical study of their evolution. Dark blue-green rims are ferrotschermakite/ferropargasite. Rims gradually becomes richer in Mg and depleted in Fe, K, Na, Al and Cl and, finally, terminates with Cl poor actinolite (XMg ≈ 0.9). Simultaneously biotite experiences metasomatic alteration that is strongly correlated with the coexisting calcic amphibole rims. Fluid inclusions combined with phase diagram calculations documents that high salinity brines with near constant salinities of c. 30 wt% solvents, infiltrated and metasomatised the amphibolites throughout cooling and uplift from 630 to 280°C. The compositional changes reflect the complex interaction between brine fluids and hydrous minerals during cooling and uplift. Accordingly, the brine fluids fully control the composition of Fe-Mg silicates by metasomatosis. Models that argue that the biotite Fe/Mg ratio together with the P, T and the fluid compositin control the Cl content of (Munoz and Swenson 1981), can not explain the Froland amphiboles and biotites. This is because the silicate and whole rock chemistries are changed by interaction with the brine fluids. The compositional zoning reflects interaction with a fluid having constant halogen contents during gradually changing PT-conditions. In intensively altered areas the original amphibolite mineralogy is entirely replaced. Here we observe the large-scale metasomatic processes in a small scale version, for example following this pattern toward the centre of a vein: Amphibolite with small amount of biotite (host rock): Zone1: biotite-plagioclase zone (K-enrichment). Zone2: amphibole-plagioclase (K-depletion). Zone3

  1. New structural and U-Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

    Science.gov (United States)

    Martha, Silviu O.; Dörr, Wolfgang; Gerdes, Axel; Petschick, Rainer; Schastok, Janina; Xypolias, Paraskevas; Zulauf, Gernold

    2016-06-01

    The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U-Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/-0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U-Pb ages and published K-Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/-0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late

  2. Development of concave-face boudin in Chhotanagpur Granite Gneiss Complex of Jasidih-Deoghar area, eastern India: Insight from finite element modeling

    Science.gov (United States)

    Samanta, Susanta Kumar; Deb, Indrasish

    2014-05-01

    With the help of 2D-finite element modeling the present study analyses the role of syntectonic migmatisation on the development of concave-face boudins within amphibolite dykes in Chhotanagpur Granite Gneiss Complex of Jasidih-Deoghar area, eastern India. Amphibolitic bands embedded in quartzofeldspathic gneiss show concave-face boudins with varied face geometries, resulted due to rheological changes during syntectonic migmatisation. Detailed study reveals that due to couple effect of H2O infiltration and potassium (K+)-metasomatism associated with the invasion of pegmatitic fluid, pyroxene converted to amphibole and later to biotite at the marginal part of the amphibolitic bands and especially, near the separation zone of boudin. In this study, three types of models are prepared to simulate three different patterns of syntectonic rheological changes that can best explain the features observed in the field. Type I is a symmetric rim model representing equal amount of rheological changes in all directions of a rectangular boudin object. Other two are asymmetric rim models (Type II and Type III) with different amount of rheological changes along length and width of the boudin block. The analysis also takes into account the effects of rate of syntectonic rheological changes (D). The study reveals that the pattern and rate of rheological changes have strong influences on the development of concave-face boudin. Type I model produces barrel-shaped fish-mouth boudin with extremely sharp corners, whereas Type III model produces more lensoid shape with relatively tighter fish mouth. For all types of model, U-shaped concave-face boudin develops at lower rate of rheological changes and the face geometry gradually transforms to V-shape with increasing the rate. The progressive change of face curvature (FC), exterior curvature (EC) and aspect ratio (AR) depends on the timing of rheological inversion during progressive deformation.

  3. High-pressure whiteschists from the Ti-N-Eggoleh area (Central Hoggar, Algeria): A record of Pan-African oceanic subduction

    Science.gov (United States)

    Adjerid, Zouhir; Godard, Gaston; Ouzegane, Khadidja

    2015-06-01

    The Ti-N-Eggoleh area (Sérouènout Terrane, Central Hoggar, Algeria) comprises mainly a high-pressure Neoproterozoic metamorphic formation consisting of talc-kyanite-quartz whiteschists, chlorite schists, marbles, sulphide ores, partially serpentinized peridotites and partially amphibolitized eclogites, and reminiscent of an ophiolitic mélange that was metamorphosed and strongly deformed under eclogite-facies conditions. Major and trace elements indicate that the whiteschists underwent intense hydrothermal alteration, with Mg enrichment and leaching of alkalis and Ca, prior to high-pressure metamorphism. The main talc-kyanite-quartz paragenesis is stable within a large multivariant P-T field extending from high-P amphibolite to eclogite facies; the Tschermak substitution in talc marginally constrains peak pressure conditions to P > 11 kbar and 600 eclogites record a similar P-T evolution: after the eclogite-facies metamorphic peak, they underwent partial amphibolitization and a subsequent high-temperature overprint under granulite-facies conditions that led to partial dehydration. The Ti-N-Eggoleh series is interpreted as the product of the thermal alteration of oceanic rocks that were subducted prior to the continental collision that formed the West Gondwana orogenic belt during the Neoproterozoic Pan-African orogeny. The decompression associated with the early phase of exhumation was followed by an important increase in temperature towards granulite-facies conditions, possibly determined by the intrusion of abundant mafic rocks in this region due to delamination of the lithospheric mantle. The Ti-N-Eggoleh area and its high-pressure meta-ophiolitic series apparently belong to the Sérouènout Terrane, which stretches along the eastern margin of the Western Gondwana orogenic belt and consists mainly of oceanic metasediments; they are possibly markers of an ancient, yet unidentified, subduction and suture zone.

  4. Metamorphic Sole and Accreted Units Along a subduction Interface: form Birth to Steady State (the Case of Western Turkey)

    Science.gov (United States)

    Plunder, A.; Agard, P.; Chopin, C.; Soret, M.

    2014-12-01

    In Western Turkey, obducted ophiolite, metamorphic sole and oceanic accretionnary complex units linked with the closure of the Neotethys are found along a 400 kilometre-long section (from north to south). We herein reappraise the metamorphic evolution of the sub-ophiolitic metamorphic units (both the metamorphic sole and the accretionnary units) of Western Turkey (i) to better characterize rock units exhumed along a cooling subduction interface, from birth to steady state (ii) to constrain the formation of metamorphic sole during the initiation of subduction (iii) and to track record of obducted ophiolite. On the basis of field and petrological observation three differents accretionnary units are reccognized with pressure-temperature estimates ranging from incipient metamorphism to blueschist-facies conditions providing information on plate coupling at different depths along the subduction interface. The upper part of the metamorphic sole was form in an amphibolite facies (garnet amphibolite - garnet clinopyroxene amphibolite). Different slices of metamorphic sole with different pressure-temperature conditions might be observed probably showing discrete timing of accretion to the upper plate. Part of the samples are characterized by a late blueschsit developpement. Both the blueschist overprint in the metamorphic sole and the high-pressure in oceanic unit were found only in the northern part of the field investigation. On the basis of the presented data, available radiometric and palaeogeographic data as well as recent themomechanical moddeling a tentative reconstruction of the subduction-zone evolution through time and the emplacement of a large-scale ophiolite is presented. Finally a comparison with ongoing work on the metamorphic sole of the Semail ophiolite of Oman is proposed with special highlights on the retrograde evolution in both settings.

  5. Petrology and Geochronology of High-Grade Metamorphic Rocks from Cedros Island, Baja California, Mexico

    Science.gov (United States)

    Gonzales, D.; Leech, M. L.

    2014-12-01

    High-grade metamorphic rocks exposed on Cedros Island, Baja California, Mexico, record the Mesozoic subduction history of western North America. Blocks of amphibolite, blueschist, and eclogite crop out in a serpentinite-matrix mélange on the southeast and southwestern parts of Cedros Island. Amphibolite blocks contain Amp + Ep + Ab + Chl ± Ms ± Grt ± Ttn ± Qz; blueschist blocks have the assemblage Na-Amp + Ms + Lw + Qz ± Ttn ± Grt ± Jd ± Chl; and eclogite blocks are comprised primarily of Omp + Grt with retrograde Na-Amp + Ms + Lw. Blueschists from Cedros have been dated using 40Ar/39Ar step-heating of white mica and sodic amphiboles that yield ages from 103 ± 4 Ma to 94.9 ± 1.1 Ma, respectively, that represent cooling during exhumation. Apatite fission-track dating gives ages from 32 ± 4 Ma to 22 ± 3 Ma that record exhumation through the upper crust. Related Mesozoic subduction zone rocks of the Franciscan Complex crop out in a serpentinite-matrix mélange along coastal northern California. The Franciscan rocks are older, yielding 40Ar/39Ar step-heating ages of hornblende from amphibolite ranging from 159 to 156 Ma and represent an older part of the subduction history of the oceanic Farallon plate along western North America. I will determine the prograde and peak metamorphic P-T conditions for these high-grade rocks using petrography, mineral chemistries, and isochemical phase diagram modeling with Perple_X to generate complete P-T paths. I will then supplement these data with Sm-Nd and Lu-Hf geochronology for these high-grade Cedros rocks to evaluate their subduction/exhumation history, and develop a tectonic model for these southernmost Franciscan-type rocks. Ultimately, I will compare my results to Franciscan rocks in northern California to better understand the Mesozoic subduction margin of western North America.

  6. Subduction-related metamorphism beneath ophiolites (Oman) and during early stages of continental collision (Himalaya)

    Science.gov (United States)

    Searle, Mike; Waters, David; Cowan, Robert; Cherry, Alan; Cooper, Charles

    2014-05-01

    Subduction-related metamorphism occurs beneath ophiolites (Oman), beneath island arcs (Kohistan) and during the early stages of continental collision (Kaghan, Tso Morari; Himalaya). Ophiolite obduction necessarily involves subduction of first oceanic, then continental crust to mantle depths beneath the ophiolite. In Oman an inverted pressure and temperature profile is exposed beneath the Semail ophiolite from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies meta-sediments, dominantly cherts, marbles and quartzites. Thermobarometry on Grt+Cpx-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at PT conditions of 770-900°C and 11-13 kbar, equivalent to depths of 30-40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle peridotites. Pressures are higher than can be accounted for by the thickness of the preserved ophiolite (15-20 km). Timing of peak metamorphism was synchronous with formation of the ophiolite gabbroic - trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96-95 Ma). During the later stages of obduction the continental margin was dragged down to depths of nearly 100 km and basaltic sills within calc-schists were converted to eclogites (20-25 kbar; 500-560oC; 79.1 Ma), then exhumed back up the same subduction channel. Apparent 'extensional' fabrics throughout the HP units are related to upward flow of deeply buried rocks in a wholly compressional environment. Eclogites in a similar structural position occur along the Himalaya in the northernmost exposures of Indian plate rocks. These eclogites formed either during the latest stage of ophiolite obduction or the earliest stage of continental collision.

  7. U-Pb zircon geochronology and Sm-Nd-Pb isotopic constraint for precambrian plutonic rocks in the northeastern part of Ryeongnam massif, Korea

    International Nuclear Information System (INIS)

    The Ryeongnam massif is composed of Precambrian gneisses, Paleozoic and Mesozoic sedimentary rocks and extensive Triassic-Jurassic plutonic rocks of felsic to mafic composition. In the northeast Ryeongnam massif, the oldest rocks belong to the Sobaegsan gneiss complex, which is composed of orthogneisses, paragneisses and mafic plutonic rocks. U-Pb zircon ages for the felsic and mafic intrusive bodies within the Sobaegsan gneiss complex are: the Icheon granite gneiss, 2357±43 and 2342±47 Ma; the Buncheon granite gneiss, 1963±5 Ma; the Pyeonghae granite gneiss, 1936±21 Ma; the Ogbang amphibolite, 1918±10 Ma; the Imwon leucogranite gneiss, 1826±20 Ma. The Hyeondong biotite schist, which is intruded by the Buncheon granite gneiss and the Ogbang amphibolite, yielded an age of 2271±44 Ma. The Nd-Sm-Pb isotopic data indicate that the felsic plutonic rocks are derived from an older Archean crust. The Nd TDM ages are Archean, and the εNd values are negative for the felsic rocks and positive for the amphibolite. Common Pb isotope compositions also indicate a crustal source for the felsic intrusives. The U-Pb ages of Precambrian rocks of the Ryeongnam massifs are similar to those in the Gyeonggi massif, and may have a similar crustal evolutionary history. The Precambrian rocks of South Korea could be related either to the North China block or to the South China block, as the isotope ages and patterns are not unique to either block. Similarly, a geologic correlation with Japan, although possible, is tenuous at present. (author)

  8. Response of detrital zircon and monazite, and their U-Pb isotopic systems, to regional metamorphism and host-rock partial melting, Cooma complex, southeastern Australia

    International Nuclear Information System (INIS)

    Progressive Early Silurian low-pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U-Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U-Pb ages survived through mid-amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of (211) crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U-Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ∼12 deg C/106 y from ∼ 730 to ∼170 deg C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600-500 Ma and 1 2-0.9 Ga old) indicate its derivation from a source remote from the Australian craton. Copyright (2001) Geological Society of Australia

  9. K/Ar hornblende ages from the higher Himalaya: implications for India-Asia collision and Himalayan metamorphosis

    International Nuclear Information System (INIS)

    Two amphibolite samples from the Higher Himalayan Crystalline (HHC) belt from the Suru Valley, Zanskar, have yielded Eocene K/Ar hornblende cooling ages between 40 and 45 Ma, thus indicating much older peak metamorphic conditions in northern parts of the Indian Plate. These ages are in conformity with almost identical ages from metamorphic complexes across the Nanga Parbat syntaxis in Pakistan and reveal a 65 to 70-Ma collision phase of the Indian indentor in the NW-Himalaya. (author). 21 refs., 2 figs

  10. Geochronology and evolution of the late-Archaean basement and Proterozoic rocks in the Alligator Rivers Uranium Field, Northern Territory, Australia

    International Nuclear Information System (INIS)

    U-Pb zircon and monazite studies, together with Rb-Sr and K-Ar total-rock and mineral studies, have been undertaken on various suites of amphibolite-grade gneisses and schists, granulites, intrusive granites, volcanic rocks, and dolerites in the Alligator Rivers Uranium Field. These studies cover all the major rock units of the region, and lead to the establishment of an overall chronology which is geologically consistent, and with which any petrogenetic hypothesis or model of mineralization is constrained

  11. Trepça ore belt and lead and zinc distribution in Badovc mineral deposit, Kosovo (SE Europe)

    OpenAIRE

    S. M. Hyseni; B. N. Durmishaj; B. L. Fetahaj; Large, D.

    2010-01-01

    The Trepça Belt of Pb-Zn-Ag mineralization is located the NNW-SSE trending Vardar zone. The Belt extends for ever 80 km, and supported five mines during period 1930-2010, and contains a number of the other Pb-Zn occurrences. The replacement and vein type mineralization is hosted primarily by Mesozoic carbonates, but also occasionally by amphibolites, and it display a clear structural control. Mineralization is spatially and genetically related to Neogene andesite-dacite extrusive and sub volc...

  12. Crust-mantle interaction beneath the Luxi Block, eastern North China Craton: Evidence from coexisting mantle- and crust-derived enclaves in a quartz monzonite pluton

    Science.gov (United States)

    Lan, Ting-Guang; Fan, Hong-Rui; Santosh, M.; Hu, Fang-Fang; Yang, Kui-Feng; Yang, Yue-Heng; Liu, Yongsheng

    2013-09-01

    The Laiwu quartz monzonite in the Luxi Block of eastern North China Craton (NCC) is characterized by the presence of abundant plagioclase amphibolite and gabbro-diorite enclaves. Here we present LA-ICPMS zircon U-Pb ages which show that the host quartz monzonite was emplaced at 129.8 ± 1.0 Ma, whereas the protolith of the plagioclase amphibolite enclaves formed during early Paleoproterozoic. The gabbro-diorite enclaves were produced simultaneously with or slightly earlier than the formation of the host quartz monzonite. Combined with the Archean and Paleoproterozoic zircons as well as the low εNd(0) values (- 18.4 to - 18.0) in the plagioclase amphibolite enclaves, the equilibrium temperature and pressure conditions (645-670 °C and 4.8-6.5 Kb) suggest that the plagioclase amphibolite enclaves are fragments of the middle crust. The gabbro-diorite enclaves mainly originated from an enriched lithospheric mantle metasomatized by melts/fluids derived from the continental crust, as indicated by their low SiO2 (54.4-54.7 wt.%) and high MgO (10.9-11.1 wt.%) contents as well as the negative εNd(t) values (- 13.5 to - 10.7) and enrichment of LILEs (e.g., Ba and Sr) and depletion of HFSEs (e.g., Nb, Ta, P and Ti). Compared with the ancient crustal rocks and the mafic plutons considered to have been derived from lithospheric mantle in the Luxi Block, the moderate εNd(t) (- 15.7 to - 15.1) and εHf(t) (- 20.7 to - 13.0) values of the quartz monzonite in our study suggest that both mantle- and crust-derived melts were involved in the magma generation. Thus we propose a model involving magma mixing between mantle- and crust-derived melts for the formation of the quartz monzonite. Since significant crust-mantle interaction is recorded not only in the quartz monzonite and its enclaves in the Luxi Block but also in the other granitoids widespread in the NCC, it is considered that large-scale crust-mantle interaction and magmatic underplating were associated with the Mesozoic

  13. Metamorphism of basic and pelitic rocks at Sulitjelma, Norway

    Science.gov (United States)

    Boyle, Alan P.

    1986-06-01

    The Sulitjelma area of the Scandinavian Caledonides consists of a variety of metasedimantary units with small basic intrusions, and a large ophiolitic complex of predominantly basic composition. All units underwent prograde greenschist facies to amphibolite facies regional metamorphism during the Scandinavian phase of the Caledonian orogeny. The resultant mineral assemblages and relationships are consistent with the presence of a miscibility gap in the actinolite-hornblende series under greenschist facies conditions; a garnet forming reaction in pelites involving the consumption of carbonate in order to produe the observed grossular content; some conflict between low-baric and medium-baric pressure estimates from equilibria involving Ca amphibole bearing assemblages in metabasites.

  14. Geology and geochronology of the metamorphic suite Colorado and his rock assemblages, southeastern Rondonia, Brazil: implications for the mesoproterozoic evolution of the southwestern Amazon Craton; Geologia e geocronologia da suite metamorfica Colorado e suas encaixantes, SE de Rondonia: implicacoes para evolucao mesoproterozoica do SW do Craton Amazonico

    Energy Technology Data Exchange (ETDEWEB)

    Rizzotto, Gilmar Jose [Centro de Pesquisas de Recursos Minerais (CPRM), Rio de Janeiro, RJ (Brazil). Servico Geologico do Brasil; Bettencourt, Jorge Silva; Teixeira, Wilson; Basei, Miguel Angelo Stipp [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica; Pacca, Igor Ivory Gil; D' Agrella Filho, Manoel Sousa [Sao Paulo Univ., SP (Brazil). Inst. de Astronomia, Geofisica e Ciencias Atmosfericas; Vasconcelos, Paulo [Queensland Univ., Brisbane, (Australia); Onoe, Artur Takashi [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas; Passarelli, Claudia Regina [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Programa de Pos-graduacao em Geoquimica e Geotectonica

    2002-12-15

    Combined geochronological (U-Pb, 40 Ar/ 39 Ar, and RB-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondonia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: porphyritic monzogranite associated with amphibolite (bimodal magmatism); interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation); muscovite-garnet leucogranite; and mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as underformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0,18). The porphyritic orthogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 {+-} 45 Ma, and 87 Sr/86 Sr, = 0.7040 {+-} 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age 1360 {+-}13 Ma. The 40 Ar/39 Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 {+-} 3 Ma (grain 1), and 1303 {+-} 2 Ma/1305 {+-} 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 {+-} 2 Ma. Hornblende from two amphibolite samples yielded 40 Ar/39 Ar plateau ages of 1313 {+-} 4 Ma; 1313 {+-} 6 Ma and 1312 {+-} 3 Ma (RO-18), and 1325 {+-} 3 Ma; 1326 {+-} 2 Ma; 1330 {+-} 3 Ma (RO-19). The weighted-mean age 1319 {+-} 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondonia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilandia Terrane (1215 - 1110 Ma

  15. Drainage water chemistry in geochemically contrasting catchments

    Czech Academy of Sciences Publication Activity Database

    Krám, Pavel; Myška, Oldřich; Čuřík, J.; Veselovský, F.; Hruška, Jakub

    Brno : Global change research centre, Academy of Sciences of the Czech Republic, v. v. i, 2013 - (Stojanov, R.; Žalud, Z.; Cudlín, P.; Farda, A.; Urban, O.; Trnka, M.), s. 173-177 ISBN 978-80-904351-8-6. [Global Change and Resilience. Brno (CZ), 22.05.2013-24.05.2013] R&D Projects: GA MŠk(CZ) ED1.1.00/02.0073 Institutional support: RVO:67179843 Keywords : granite * amphibolite * serpentinite * Norway spruce * soil water * stream water * acidification * weathering Subject RIV: EH - Ecology, Behaviour

  16. Structural and geochemical mapping of a Fe-mineralized quartz-mica rich unit in the Ringvassøya Greenstone Belt, West Troms Basement Complex

    OpenAIRE

    Elvenes, Hallgeir

    2015-01-01

    Ringvassøy is one of a chain of large coastal islands representing the Archaean to Paleoproterozoic West Troms Basement Complex (WTBC), west of the Caledonides. On Ringvassøy, a basement of mainly tonalitic gneiss is overlain by the Ringvassøy Greenstone Belt (RGB), which is metamorphosed up to middle amphibolite facies. Tonalitic gneiss in the west and southeast of the island has U–Pb zircon ages of 2.84–2.82 Ga, similar to U–Pb zircon ages of 2.85–2.83 Ga for metavolcanics in the RGB. Mafic...

  17. 3-D assessment of peak-metamorphic conditions by Raman spectroscopy of carbonaceous material: an example from the margin of the Lepontine dome (Swiss Central Alps)

    DEFF Research Database (Denmark)

    Wiederkehr, Michael; Bousquet, Romain; Ziemann, Martin; Berger, Alfons; Schmid, Stefan

    2011-01-01

    This study monitors regional changes in the crystallinity of carbonaceous matter (CM) by applying Micro-Raman spectroscopy to a total of 214 metasediment samples (largely so-called Bu¨ndnerschiefer) dominantly metamorphosed under blueschist- to amphibolite-facies conditions. They were collected...... refolded nappe contacts, both along and across strike within the northeastern margin of the Lepontine dome and adjacent areas. Further to the northeast, the isotemperature contours reflect temperatures reached during the Late Eocene subduction-related blueschist-facies event and/or during subsequent near...

  18. Geological setting and geochemical signatures of the mafic rocks from the Intra-Pontide Suture Zone: implications for the geodynamic reconstruction of the Mesozoic Neotethys

    Science.gov (United States)

    Sayit, Kaan; Marroni, Michele; Göncüoglu, M. Cemal; Pandolfi, Luca; Ellero, Alessandro; Ottria, Giuseppe; Frassi, Chiara

    2016-01-01

    A number of suture zones exist in Turkey, which is believed to represent the closure of Paleo and NeoTethyan oceanic basins. Regarding the development of the latter oceanic entity, namely Neotethys, the geodynamic evolution of the Intra-Pontide branch, the northernmost one of a number of oceanic basins remains enigmatic. The Intra-Pontide Suture Zone in Northwest Turkey includes several tectonic units most of which are characterized by the occurrence of mafic rocks with distinct geochemical signatures. In this paper, the mafic rocks collected from four of these units (the Domuz Dağ Unit, the Saka Unit, the Daday Unit and the Arkot Dağ Mèlange) have been studied in detail along two selected transects. The Domuz Dağ Unit is characterized by amphibolites, micaschists and marbles, which have been overprinted by low-grade metamorphism.The Saka Unit is in turn represented by an assemblage of slices of amphibolites, marbles and micaschists metamorphosed under upper amphibolite facies metamorphic conditions in the Late Jurassic time. In these units, the amphibolites and their retrograded counterparts display E-MORB-, OIB- BABB- and IAT-type signatures. The Daday Unit is characterized by metasedimentary and metamafic rocks metamorphosed under blueschist to sub-greenschist facies conditions. The metamafic rocks comprise actinolite-bearing schists and Na-amphibole-bearing varieties possibly derived from basaltic and gabbroic protoliths. They have a wide range of chemical compositions, displaying N-MORB-, E-MORB-, OIB- BABB- and IAT-type signatures. The Arkot Dağ Mèlange consists of a Late Santonian assemblage of slide-blocks mainly represented by basaltic lithologies showing affinities ranging from N-MORB- and IAT- to BABB-type magmas. The geochemical signature of the studied mafic rocks indicates that the tectonic units documented along the two studied transects of the Intra-Pontide Suture Zone have been derived from a supra-subduction zone. This hypothesis

  19. Seismicity, metamorphism and rheology of the lower continental crust

    Science.gov (United States)

    Austrheim, Håkon

    2014-05-01

    Seismological data document that both normal earthquakes and tremors occur in the lower continental crust. Pseudotachylytes (frictional melts and ultracommunited rocks) have been described from several high grade metamorphic terrains and may be the geological manifestation of this seismicity. The Grenville (c. 930Ma) granulite facies complex (T: 800 °C; P: ≤10kbar) of the Lindås Nappe in the Bergen Arcs, W-Norway underwent a fluid induced partial eclogite (T: 600-650 °C; P: 15-20 kbar) and amphibolite facies metamorphism during the Caledonian (c.400-430 Ma) continent collision. Pseudotachylyte fault and injection veins formed in the dry granulites at or close to the reaction fronts both in the eclogitized (western parts) and the amphibolitized (eastern parts) of the Nappe. They are locally recrystalized with the development of amphibolite and eclogite facies assemblages demonstrating that they formed pre or syn the Caledonian metamorphism. The pseudotachylytes transect lithologies ranging from peridotite to anorthosite and consequently the influence of the seismic energy release on a range of granulite facies minerals including garnet, pyroxenes, olivine, plagioclase, hornblende and scapolite can be observed. The seismic energy released promotes the Caledonian metamorphism and change the petrophysical properties of the lower crust in the following ways: The melting and the ultracommunition of the granulite facies minerals increased the reactive surface area and produce local pathways for fluid. S-rich scapolite, a common mineral in granulities play a key role in this process by releasing S and C to form sulfides and carbonates. Small sulfide grains impregnate the pseudotachylyte veins which may lead to an increased electrical conductivity of the deep crust. The pseudotachylyte veins impose inhomogeneities in the massive rocks through grain size reduction and lead to strain localization with development of amphibolite and eclogite facies shear zones. Formation

  20. Exhumation of an unusually large, ~3000 km3 coherent block of oceanic crust from >40 km depth

    Science.gov (United States)

    Barrow, Wendy; Metcalf, Rodney; Fairhurst, Robert

    2010-05-01

    The Central Metamorphic terrane (CMt) is an unusually large (~3000 km3) coherent block of mid-ocean ridge (MOR) metabasites; the first one of this scale reported with eclogite facies relicts, decompression assemblages, and thermobarometry indicating exhumation of the entire block from >40 km depth. The CMt is exposed in the eastern Klamath Mountains of northern California and is dominantly an amphibolite facies metabasite which represents remnant oceanic crust subducted in a mid-Paleozoic Pacific-type margin. Thermochronology indicates that the CMt was subsequently exhumed along the Trinity fault during an early Permian extensional event. Newly discovered relict textures with new thermobarometry results suggest the CMt metabasites record the retrograde segment of the P-T-deformation-time path during exhumation from hornblende eclogite facies P-T conditions. A decompression and cooling sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry combined with experimental data for rutile stability in metabasites suggests that relict rutile crystals preserve early P-T conditions of ~600° C and >1.3 GPa. Transition from eclogite facies is further supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of garnet or omphacite during decompression. The dominant mineral assemblages and metamorphic fabrics indicate dynamic recrystallization of metabasites during declining P-T conditions through amphibolite - epidote amphibolite facies. Exhumation via extension along the Trinity fault is suggested by the coplanar relationship between metabasite decompression-related deformation fabrics and the Trinity fault. We propose that subducted oceanic crust (CMt) was subsequently exhumed as a large coherent block from depths >40 km. This is significant because the conversion of mafic oceanic crust to eclogite produces the negative buoyancy (relative to mantle peridotite) that

  1. Trepça Ore Belt and Stan Terg mine – Geological overview and interpretation, Kosovo (SE Europe

    Directory of Open Access Journals (Sweden)

    Sylejman Hyseni

    2010-06-01

    Full Text Available The Trepça Belt of Pb-Zn-Ag mineralization is located within the NNW-SSE trending Vardar zone. The Beltextends for over 80 km, and supported five mines during the period 1930-2008. It contains a number of the otherPb-Zn occurrences too. The replacement and vein type mineralization is hosted primarily by Mesozoic carbonates,but also occasionally by amphibolites, and displays a clear structural control. Mineralization is spatially and geneticallyrelated to Neogene andesite-dacite extrusives and sub-volcanic intrusives. Only Stan Terg mine is presentedin this paper.

  2. Geology and geochronology of the metamorphic suite Colorado and his rock assemblages, southeastern Rondonia, Brazil: implications for the mesoproterozoic evolution of the southwestern Amazon Craton

    International Nuclear Information System (INIS)

    Combined geochronological (U-Pb, 40 Ar/ 39 Ar, and RB-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondonia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: porphyritic monzogranite associated with amphibolite (bimodal magmatism); interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation); muscovite-garnet leucogranite; and mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as underformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0,18). The porphyritic orthogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 ± 45 Ma, and 87 Sr/86 Sr, = 0.7040 ± 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age 1360 ±13 Ma. The 40 Ar/39 Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 ± 3 Ma (grain 1), and 1303 ± 2 Ma/1305 ± 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 ± 2 Ma. Hornblende from two amphibolite samples yielded 40 Ar/39 Ar plateau ages of 1313 ± 4 Ma; 1313 ± 6 Ma and 1312 ± 3 Ma (RO-18), and 1325 ± 3 Ma; 1326 ± 2 Ma; 1330 ± 3 Ma (RO-19). The weighted-mean age 1319 ± 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondonia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilandia Terrane (1215 - 1110 Ma). However, the unequivocal

  3. Retrograde metamorphism of the eclogite in North Qaidam, western China:Constraints by joint 40Ar/39Ar in vacuo crushing and stepped heating

    Institute of Scientific and Technical Information of China (English)

    Rongguo Hu; Jan Wijbrans; Fraukje Brouwer; Linghao Zhao; Min Wang; Huaning Qiu

    2015-01-01

    Two amphiboles and a syn-metamorphic quartz vein from the Yuka terrane, North Qaidam, western China, have been analyzed by joint 40Ar/39Ar crushing in vacuo and stepwise heating techniques. The crushing in vacuo results provide information to directly constrain the timing of fluid activity and the age of amphibolite-facies retrogression. The stepwise heating results could further be used to decipher the thermal history of the UHP rocks. Amphiboles from amphibolites and quartz vein within garnet-amphibolite lens analyzed by in vacuo crushing yield similarly shaped age spectra and exhibit rela-tively flat age plateaus for the last several steps. The characteristics of gas release patterns and geochronological data testify to the presence of significant excess 40Ar within the fluid inclusions. The age plateaux with weighted mean ages (WMA) ranges from 488 to 476 Ma for amphiboles and 403 Ma for quartz (2s). These data points constitute amphibole WMA yielding excellent isochrons with isochron ages of 469 and 463 Ma with initial 40Ar/36Ar ratios of 520 and 334, respectively. The isochron ages are interpreted to represent initial amphibolite-facies retrogression. The data points constituting the quartz age plateaux give an isochron age of 405 Ma with initial 40Ar/36Ar ratio of 295, recording a significant aqueous fluid flow episode during the early Devonian. Age spectra obtained by stepwise heating of amphibole residues remaining after crushing experiments are characterized by younger and relatively complex age spectra, which are probably influenced by the combined effects of resetting argon and/or mineral inclusions. Nevertheless, we note that the spectra shapes have features in common: excluding the last two steps, minimum apparent ages are found at temperatures of around 500 ?C, corresponding to 319 and 249 Ma, perhaps representing the time of isotopic resetting or resulting from release gas from mineral inclusions of, e.g., biotite or feldspar. Maximum apparent ages

  4. The Glória quartz-monzodiorite: isotopic and chemical evidence of arc-related magmatism in the central part of the Paleoproterozoic Mineiro belt, Minas Gerais State, Brazil

    OpenAIRE

    Ciro A. Ávila; Wilson Teixeira; Umberto G. Cordani; Héctor R. Barrueto; Ronaldo M. Pereira; Veridiana T.S. Martins; Liu Dunyi

    2006-01-01

    The Glória quartz-monzodiorite, one of the mafic plutons of the Paleoproterozoic Mineiro belt, is intrusive into banded gneisses, amphibolites, schists and phyllites of the Rio das Mortes greenstone belt, in the southern portion of the São Francisco Craton, State of Minas Gerais, Brazil. The Glória quartz-monzodiorite yields a SHRIMP U-Pb zircon age of 2188 ± 29 Ma, suggesting a tectonic relationship with the pre-collisional phase of the Mineiro belt. According to the Nd isotopic evide...

  5. Detailed petrographic-structural study of an outcrop of Crystalline Basement of Montevideo

    International Nuclear Information System (INIS)

    Preliminary data analysis of detailed outcrop Punta Virgilo, located on the S E coast of the department of Montevideo are presented. The investigated outcrop includes gneisses, amphibolite s and several generations of pegmatite and aplite dikes of Paleoproterozoic metamorphic basement, plus a set of dikes emplaced lamprófido exhumed once the area. Petrographic and microstructural studies of metamorphic units allowed to determine the conditions of metamorphism and deformation temperature between 520-720 ° C and pressure between 2 and 6 kbar (depth of 10 to 23 km)

  6. U/Pb zircon ages and model ages (Sm/Nd) of ortho gneisses and meta mafic enclaves of the Barro Vermelho area (state of Pernambuco, Brazil), Alto Moxoto terrain, Borborema province, northeastern Brazil

    International Nuclear Information System (INIS)

    The Barro Vermelho area is located in the border between municipalities of Custodia and Sertania - PE, Pajeu-Paraiba Fold Belt, Borborema Province. Geological mapping at 1/25.000 scale allowed to distinguish two metamorphic domains respectively built up of orthoderivated rocks and paraderivated ones. The orthoderivated domain is formed mainly by augen gneisses more or less migmatized of granitic, monzogranitic, granodioritic, tonalitic and quartz-dioritic composition inside of which are found metamafic enclaves of leucogabbros, gabbronorites, gabbros/diorites, and anorthosites, apart from banded amphibolites, with a small occurrence of Fe-Ti ore enclosed by some of these enclaves. In addition, inserted in the orthogneisses are found also others enclaves and intrusions (some apparently concordant and others certainly discordant in relation to the prominent foliation of the area, Sn) of metric to hectometric dimensions, built up of amphibolites/metadiorites, metaplite, calcissilicate rocks of mafic ultramafic protholiths, weakly deformed granites and diorites, and two hectometric bodies of olivine diabase to troctolite. Field relations and similarities in terms of composition, texture and lithogeochemistry allowed to place the lithotypes of the orthoderivated domain in the following groups, considering them in a relative sequence of events from the older to the newest ones: anorthositic-gabbros xenoliths; tonalite (protolith of the orthogneisse of equal composition); enclaves/ dikes of amphibolites/metadiorites, synplutonic in relation to tonalite and comagmatic to the xenoliths; granitic orthogneisses formed from migmatization of tonalite; enclaves/dikes of amphibolites/metadiorite, synplutonic in relation to migmatization of tonalites; granites and diorites late to post migmatization; and olivine diabase to troctolite post the last tectonic-metamorphic event recorded in the area. Concordia diagrams U/Pb with colinear regression of three zircon fractions to

  7. Geología de las ultramafitas pre-andinas de Tapo y Acobamba, Tarma, en la Cordillera Oriental de Perú

    OpenAIRE

    Castroviejo Bolibar, Ricardo; Rodrigues, Jose F.; Acosta, Jorge; Pereira, Eurico; Romero, Darwin; Quispe, Jorge; Espí Rodríguez, José Antonio

    2009-01-01

    Ultramafic rocks occur scattered along a 300 km long NNW-SSE trending belt, parallel to the central Peruvian Andes in the Cordillera Oriental, from Tarma (Junín Dept.) to Huancapallac and Tingo María (Huánuco Dept.). The Tarma occurrences (Tapo and Acobamba) are dealt with here, as the first step of a broader research. The Tapo massif comprises strongly tectonised serpentinites with scarce peridotitic relics, amphibolites and podiform chromitites. It was overthrust on early Carboniferous meta...

  8. Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Perú (Tarma province)

    OpenAIRE

    Willner, Arne P.; Colombo C. G Tassinari; Rodrigues, José Feliciano; Acosta, Jorge; Castroviejo Bolíbar, Ricardo; Rivera, Miguel

    2014-01-01

    High-pressure conditions of 11–13 kbar/500–540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm–Nd mineral-whole rock isochron at 465 ± 24 Ma dates fluid influx at peak temperatures of ~600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted...

  9. U-Pb zircon ages for the Luzhenguan Complex in northern part of the eastern Dabie orogen

    Institute of Scientific and Technical Information of China (English)

    JIANG; Laili; Wolfgang; Siebe; CHEN; Fukun; LIU; Yican

    2005-01-01

    The study presents U-Pb zircon ages for granitic gneiss and amphibolite from the Luzhenguan Complex (LZC) in northern part of the eastern Dabie Orogen and their geological significance. Two granitic gneisses and one amphibolite give protolith zircon U-Pb ages of 740 Ma and 725 Ma, respectively. The ages show that the LZC is composed mainly of Neoproterzoic acid and basic igneous rocks, suggesting that there is no Paleozoic active continental marginal rock association in the southern margin of the North China Block in northern part of the eastern Dabie Mountain. Based on the ages and combining the geological and geophysical analyses, the middle-low grade metamorphic rocks in the North Huiyang Belt (NHB) can be divided into three parts, which are the LZC composed mainly of the Neoproterzoic acid and basic igneous rocks, the Neoproterzoic Xianrechong Formation and Xiangyunzhai Formation and the Devonian Zhufoan Formation and Pangjialing Formation of the Foziling Group, and the suture zone between the Yongtze Block and the North China Block is inferred to be thrust under the NHB.

  10. Potassium, uranium and thorium contents in the basement rocks of the Camamu and Almada sedimentary basins; Teores de uranio, torio e potassio nas rochas do embasamento das bacias sedimentares de Camamu e Almada

    Energy Technology Data Exchange (ETDEWEB)

    Sapucaia, Najara S.; Argollo, Roberto M. de; Barbosa, Johildo S.F. [Universidade Federal da Bahia (UFBA), Salvador, BA (Brazil). Pos-graduacao em Geofisica

    2004-07-01

    The basement of the of Camamu and Almada basins is inserted in the granulitic region of the southeast Bahia. It is characterized, in bigger proportion, by the rocks of the Cinturao Itabuna represented by metatonalites calcium - alkaline of low potassium and matamonzonites with high-potassium geochemistry shoshonitics trend, associate to the basic granulites. In lesser proportion, one has: the rocks of the Jequie block, characterized by charnockites, charnoenderbites and enderbites with calcium-alkaline chemical and high-K contents and associated with amphibolites with low K-contents; the granite of Teolandia and the granodiorite of Moenda, representatives of the Ipiau Domain; and the neoproterozoics syenites and mafic dikes. On this context, the acid rocks of intermediate compositions, as the charnockites, the metamonzonites, the granitic rocks of the Ipiau band and the sienites, are the main lithologic units that show relevant concentrations of the U, Th and K elements. Already the metatonalites are more abundant in this basement ad show intermediate values of K, varying of 0,5 to 2.5 %, while the basic granulitos and the amphibolites show low K-contents as low as 0,02%. The Th concentrations in the basic metatonalites and ten granulite are below 10 ppm, arriving to below the determination limit of 0.4 ppm; the same thing occurs with U concentrations in these rocks. (author)

  11. The connection between crustal reworking and petrological diversity in the deep crust: clues from migmatites

    Science.gov (United States)

    Carvalho, Bruna B.; Sawyer, Edward W.; de Assis Janasi, Valdecir

    2016-04-01

    The deep levels of the continental crust have been extensively reworked as result of crustal differentiation. Migmatites are widespread in these high-grade metamorphic terrains, and provide valuable information on how processes such as partial melting, segregation of the melt from the residue and subsequent chemical exchanges lead to the petrological diversity found in the deep crust. This study investigates processes that transformed a largely uniform, metagranodiorite protolith into a very complex migmatite that contains three varieties of diatexites (grey, schlieren and homogenous diatexites) and several types of leucosomes. The Kinawa Migmatite is part of the Archean TTG crust in the São Francisco Craton (Brazil), which has been reworked in a shear zone environment at upper amphibolite facies conditions (data indicates that in the deep levels of the crust petrological diversity is produced by melt segregation, both during partial melting and crystallization, and by interaction of the anatectic melt with unmelted material in the source. During melting, segregation produced residuum plus anatectic melt and all intermediate stages, whereas during crystallization it resulted in crystal fractionation and generated diverse plagioclase-rich rocks and fractionated melts. Finally, crystals disaggregated from the amphibolites entrained and interact with anatectic melt producing leucosomes and diatexites with the compositional signature of contamination. [1] Carvalho, B.B; Sawyer, E.W.; Janasi, V.A. (2016). Crustal reworking in a shear zone: transformation of metagranite to migmatite. Journal of Metamorphic Geology DOI: 10.1111/jmg.12180

  12. Mineralkogical and Petrochemical Characterisitics and Genesis of Laoniugou Geneiss in Jiapigou Gold Mine,Jilin Province

    Institute of Scientific and Technical Information of China (English)

    孙晓明; 徐克勤; 等

    1992-01-01

    Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves.The major element contents show an obvious bimodal and trondhjemitic series evolutional trend.This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt.The contents of Rb,Sr and Ba are 7-21 ppm,153-363ppm and 201-1451 ppm respectively ,close to those of common Archaean grey gneisses.All the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6).The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks,belonging to typical TTG of Archaean high-grade metamorphic terrain .The gneiss is quite similar to the B-type Amitsoq gneiss of W.Greenland .The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.

  13. Geochronological evidence of Indosinian(high-pressure) metamorphic event and its tectonic significance in Taxkorgan area of the Western Kunlun Mountains,NW China

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    The CL images,LA-ICP-MS in situ trace elements analysis,and U-Pb dating for zircons indicate that the metamorphic ages of the sillimanite-garnet-biotite gneiss and the garnet-amphibole gneiss from eastern Taxkorgan of the Western Kunlun Mountains are 220±2 and 220±3 Ma respectively,and their protolith ages are younger than 253±2 and 480±8 Ma respectively.Two samples were collected at the same outcrops with HP mafic granulite and HP pelitic granulite.Mineral assemblage of the sillimanite-garnet-biotite gneiss(Grt+Sill+Per+Q) is consistent with that of HP pelitic granulite at early high amphibolite-granulite facies stage.Mineral assemblage of the garnet-amphibole gneiss(Grt+Amp+Pl+Q) is consistent with retro-metamorphic assemblage of HP mafic granulite at amphibolite facies stage.The dating results suggest that these HP granulites underwent peak metamorphism at 220±2 to 253±2 Ma.Thus,the Kangxiwar tectonic zone was probably formed by subduction and collision of the Paleo-Tethys Ocean during Indosinian.Protolith ages of the two samples,together with previously published U-Pb zircon dating age,suggest that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit,not a part of the Paleoproterozoic Bulunkuole Group.

  14. 1.57 Ga protolith age of the Neoproterozoic Forquilha eclogites, Borborema Province, NE-Brazil, constrained by U-Pb, Hf and Nd isotopes

    Science.gov (United States)

    Amaral, Wagner; Santos, Ticiano José; Ancelmi, Matheus Fernando; Fuck, Reinhardt Adolfo; Dantas, Elton Luiz; Matteini, Massimo; Moreto, Carolina Penteado

    2015-03-01

    The 30 km-long, N-S-trending Forquilha eclogite zone, occurs within a Paleoproterozoic block mainly composed of gneisses and migmatites, in the Ceará Central domain, Borborema Province, NE Brazil. The Forquilha eclogite zone contains lenses of high to ultra-high pressure metamafic rocks, found as granulites and amphibolites associated with kyanite-sillimanite gneisses. Three samples of clinopyroxene-garnet amphibolite yielded the U-Pb zircon ages of 1566 ± 9 Ma, 1547 ± 37 Ma and 1532 ± 24 Ma, interpreted as the timing of igneous crystallization of the mafic protolith. Additionally, zircon grains of a leucocratic layer of a metamafic rock and a retrograded eclogite provided the less precise U-Pb ages of 1613 ± 40 Ma and 1454 ± 120 Ma, respectively. Lu-Hf and Sm-Nd model ages provided TDM (Hf) between 1.55 and 1.81 Ga with positive ɛHf values of +7.50 to +10.48, and TDM (Nd) ranging between 1.57 and 1.92 Ga with positive ɛNd values of +1.84 to +4.36. It is believed that part of the rocks of the Forquilha eclogite zone were emplaced as mafic dikes in an extensional setting at ca. 1.57 Ga.

  15. Metamorphic conditions and CHIME monazite ages of Late Eocene to Late Oligocene high-temperature Mogok metamorphic rocks in central Myanmar

    Science.gov (United States)

    Maw Maw Win; Enami, Masaki; Kato, Takenori

    2016-03-01

    The high temperature (T)/pressure (P) regional Mogok metamorphic belt is situated in central Myanmar, and is mainly composed of pelitic gneisses, amphibolites, marbles, and calc-silicate rocks. The garnet-biotite-plagioclase-sillimanite-quartz assemblage and its partial system suggest equilibrium P/T conditions of 0.6-1.0 GPa/780-850 °C for the peak metamorphic stage, and 0.3-0.5 GPa/600-680 °C for the exhumation and hydration stage. Monazite grains show complex compositional zoning consisting of three segments-I, II, and III. Taking into consideration the monazite zoning and relative misfit curves, the calculated chemical Th-U-total Pb isochron method (CHIME) monazite age data (284 spot analyses) indicated four age components: 49.3 ± 2.6-49.9 ± 7.9, 37.8 ± 1.0-38.1 ± 1.7, 28.0 ± 0.8-28.8 ± 1.6, and 23.7 ± 1.3 Ma (2σ level). The ages of the Late Eocene and Late Oligocene epochs were interpreted as the peak metamorphic stage of upper-amphibolite and/or granulite facies and the postdated hydration stage, respectively.

  16. Well log responses in metamorphic rocks near Maribor

    Directory of Open Access Journals (Sweden)

    1995-12-01

    Full Text Available In the Stražun forest at Pobrežje near Maribor, (Eastern Slovenia six boreholes have been drilled from 860 to 1600m deep. The paper describes geological conditions in mentioned boreholes, as well as in wider surroundings of Maribor with stress on metamorphic rocks. Based on pétrographie analysis of the rocks cuttings and well logs the upper phyllitic part ant the lower Pohorje series of themetamorphic complex could be separated. The first one includes phyllites with phyllitic quartzites and silicate marmorized limestones. The Pohorje series is represented by two-mica gneiss and schist, mainly with inclusions of amphibolite and eclogite, and subordinately retrograde chlorite-amphibole schist. The welllog responses for particular lithological sequences of metamorphic complex have been distinguished on the basis of conventional electrologs and gamma ray measurements.The problem of lithological interpretation of well logs in these rocks is described. Two fields of well log responses are distinguishable, as separated by the degree of natural radioactivity. Apart from veined quartzite, all rocks fromthe phyllitic part of the metamorphic complex are highly radioactive. In the Pohorje series gneiss, schist and diaphtorite-phyllonite are highly radioactive, while amphibolite, eclogite and retrograde chlorite-amphibole schist have low radioactivity.Finally, typical well log responses in lithological sequences of the discussed metamorphic rocks are presented.

  17. Controls of Soil Spatial Variability in a Dry Tropical Forest

    Science.gov (United States)

    Pulla, Sandeep; Riotte, Jean; Suresh, H. S.; Dattaraja, H. S.; Sukumar, Raman

    2016-01-01

    We examined the roles of lithology, topography, vegetation and fire in generating local-scale (DBH), and spatial variation in fire frequency (times burnt during the 17 years preceding soil sampling) in a permanent 50-ha plot. Unlike classic catenas, lower elevation soils had lesser moisture, plant-available Ca, Cu, Mn, Mg, Zn, B, clay and total C. The distribution of plant-available Ca, Cu, Mn and Mg appeared to largely be determined by the whole-rock chemical composition differences between amphibolites and hornblende-biotite gneisses. Amphibolites were associated with summit positions, while gneisses dominated lower elevations, an observation that concurs with other studies in the region which suggest that hillslope-scale topography has been shaped by differential weathering of lithologies. Neither NO3−-N nor NH4+-N was explained by the basal area of trees belonging to Fabaceae, a family associated with N-fixing species, and no long-term effects of fire on soil parameters were detected. Local-scale lithological variation is an important first-order control over soil variability at the hillslope scale in this SDTF, by both direct influence on nutrient stocks and indirect influence via control of local relief. PMID:27100088

  18. Syn- and post-tectonic granite plutonism in the Sausar Fold Belt, central India: Age constraints and tectonic implications

    Science.gov (United States)

    Chattopadhyay, Anupam; Das, Kaushik; Hayasaka, Yasutaka; Sarkar, Arindam

    2015-08-01

    Sausar Fold Belt (SFB) in central India forms the southern part of the Central Indian Tectonic Zone (CITZ) - a crustal scale Proterozoic mobile belt dissecting the Indian craton, whose tectonothermal history and age is important for understanding the Proterozoic crustal history of the Indian craton. SFB comprises a gneissic basement (TBG: Tirodi Biotite Gneiss) overlain by a supracrustal sequence of quartzite-pelite-carbonate (SSG: Sausar Group). SSG and TBG are deformed and metamorphosed in greenschist to amphibolite facies. Two phases of granite intrusion are observed in the SSG - a syntectonic foliated granite and a post-tectonic massive granite, with clear structural relationship with the host rocks. Monazite chemical dating (U-Th-total Pb) of the foliated and massive granites yield Neoproterozoic (ca. 945-928 Ma) ages that contradict many earlier geochronological interpretations. Foliated granites and the immediately adjacent TBG show monazite grains with ca. 945 Ma mean age, interpreted as the timing of D2 deformation and amphibolite facies metamorphism of SSG. The post tectonic granites intruded these rocks around 928 Ma, and were largely undeformed. A terminal thermal overprint is found in some monazite grain rims at ca. 785 Ma age. The younger Sausar tectonothermal events have overprinted the adjacent high-grade granulites of Ramakona-Katangi Granulite (RKG) belt, and should not be considered as parts of the same tectonothermal event representing different depth sections only.

  19. Uranium and thorium migration under dislocative metamorphism

    International Nuclear Information System (INIS)

    Investigated were peculiarities of uranium and thorium behaviour in the process of dislocation metamorphism on the basis of regional fracture zones of early-proterozoic embedding of Ukrainian, Aldan and Baltic shields. The studied zones correspond to tectonite of green-shale and almandin-amphibolite facies of regional metamorphism according to mineral associations. The most peculiar feature of the tectonites of green-shale facies is uranium presence in migrationally able forms, which can be involved afterwards into the ore process by hydrothermal solutions. Adsorved forms of uranium on the crystal surface or separate grains and in the cracks, as well as microinclusions of uranium minerals, selectively timed to mineral structure defects prevail among easily mobile uranium compounds. Dissolved uranium is present, evidently in gas-liquid inclusions in minerals and pore waters. There forms of uranium presence are peculiar for epidote-chlorite mylonites, as well as cataclasites and diaphthorites related to them by blastomylonites of almandin-amphibolite facies. Wide range of manifestation of this process, caused by multikilometer extension of deep fracture zones permit to consider the formations of green-shale facies of dislocation metamorphism as one of the main uranium sources in deposit formation in different uranium-ore associations different age

  20. Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California

    Science.gov (United States)

    Barth, A.P.; Wooden, J.L.; Coleman, D.S.; Vogel, M.B.

    2009-01-01

    The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time. ?? 2009 by The University of Chicago.

  1. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: A lead isotope study of an Archaean gold prospect in the Attu region, Nagssugtoqidian orogen, West Greenland

    Directory of Open Access Journals (Sweden)

    Stendal, Henrik

    2006-12-01

    Full Text Available This paper presents a lead isotope investigation of a gold prospect south of the village Attu in the northern part of the Nagssugtoqidian orogen in central West Greenland. The Attu gold prospect is a replacement gold occurrence, related to a shear/mylonite zone along a contact between orthogneissand amphibolite within the Nagssugtoqidian orogenic belt. The mineral occurrence is small, less than 0.5 m wide, and can be followed along strike for several hundred metres. The mineral assemblage is pyrite, chalcopyrite, magnetite and gold. The host rocks to the gold prospect are granulite facies ‘brown gneisses’ and amphibolites. Pb-isotopic data on magnetite from the host rocks yield an isochron in a 207Pb/204Pb vs. 206Pb/204Pb diagram, giving a date of 3162 ± 43 Ma (MSWD = 0.5. This date is interpreted to represent the age of the rocks in question, and is older than dates obtained from rocks elsewhere within the Nagssugtoqidian orogen. Pb-isotopic data on cataclastic magnetite from the shear zone lie close to this isochron, indicating a similar origin. The Pb-isotopic compositions of the ore minerals are similar to those previously obtained from the close-by ~2650 Ma Rifkol granite, and suggest a genetic link between the emplacement of this granite and the formation of the ore minerals in the shear/mylonite zone. Consequently, the age of the gold mineralisation is interpreted tobe late Archaean.

  2. Thermobarometric and fluid expulsion history of subduction zones

    Energy Technology Data Exchange (ETDEWEB)

    Ernst, W.G. (Univ. of California, Los Angeles (United States))

    1990-06-10

    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite {r arrow} prehnite-pumpellyite {r arrow} glaucophane schist {r arrow} eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogites at depth. Inclusion studies suggest that two-phase immiscible volatiles are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses (e.g., Western Alps) have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Less common blueschist terranes (e.g., Franciscan belt of western California) preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain P{sub fluid} values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes).

  3. /sup 40/Ar//sup 39/Ar ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros range, Iran

    Energy Technology Data Exchange (ETDEWEB)

    Lanphere, M.A.; Pamic, J. (Geological Survey, Menlo Park, CA (USA); Zagreb Univ. (Yugoslavia). Geoloski Zavod)

    1983-07-20

    An ophiolite, considered to be an allochthonous fragment of Tethyan oceanic crust and mantle, crops out near Neyriz in the Zagros Range, Iran. /sup 40/Ar//sup 39/Ar ages ranging from 76.8+-23.8 Ma to 105+-23.3 Ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite. The most precise ages are 85.9+-3.8 Ma for a diabase and 83.6+-8.4 Ma for a plagiogranite. The weighted mean age of hornblende from the five samples is 87.5+-7.2 Ma which indicates that the igneous part of the Neyriz ophiolite formed during the early part of the Late Cretaceous. Pargasite from amphibolite below peridotite of the Neyriz ophiolite has a /sup 40/Ar//sup 39/Ar age of 94.9+-7.6 Ma. The pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite. Comparable ages have been measured on igneous rocks from the Samail ophiolite of Oman and on amphibolite below peridotite of the Samail ophiolite.

  4. 40Ar/39Ar ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros range, Iran

    International Nuclear Information System (INIS)

    An ophiolite, considered to be an allochthonous fragment of Tethyan oceanic crust and mantle, crops out near Neyriz in the Zagros Range, Iran. 40Ar/39Ar ages ranging from 76.8+-23.8 Ma to 105+-23.3 Ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite. The most precise ages are 85.9+-3.8 Ma for a diabase and 83.6+-8.4 Ma for a plagiogranite. The weighted mean age of hornblende from the five samples is 87.5+-7.2 Ma which indicates that the igneous part of the Neyriz ophiolite formed during the early part of the Late Cretaceous. Pargasite from amphibolite below peridotite of the Neyriz ophiolite has a 40Ar/39Ar age of 94.9+-7.6 Ma. The pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite. Comparable ages have been measured on igneous rocks from the Samail ophiolite of Oman and on amphibolite below peridotite of the Samail ophiolite. (orig.)

  5. Geology and mineral occurences of braquiantidinal do Lontra - GO

    International Nuclear Information System (INIS)

    This work involved the geological mapping (in the scale 1:60.000) of an area of 800 square kilometers in the nortwestern part of the state of Goias, near and east of the Araguaia river. Based on the stratigraphy, metamorphism, geochronology, magmatism and mineral deposits hypotheses on the geological evolution of the region are discussed. The area studied belongs to the Precambrian Araguaia Fold Belt. The oldest rocks identified are trondhjemitic gneisses and on these rocks was deposited a sedimentary sequence with minor volcanics of a geosynclinal type. The stratigraphic column of Abreu (1978) was adopted with minor modifications. The basement, of transamazonic age (2000 Ma), consists mostly of gneiss, migmatite, granite gneiss and amphibolite. The metasediments belongs to the lower unit (Estrondo Group) of the Supergroup Baixo Araguaia. The Estrondo Group, of brasilian age (600 Ma), consists in the area of the lowermost Morro do Campo Formation, mainly quartzite and amphibolite, which give the high relief of the brachyanticlines of Lontra and Ramal do Lontra.(author)

  6. Searching for giant, ancient impact structures on Earth: The Mesoarchaean Maniitsoq structure, West Greenland

    Science.gov (United States)

    Garde, Adam A.; McDonald, Iain; Dyck, Brendan; Keulen, Nynke

    2012-07-01

    A 100 km-scale, circular region in the Archaean North Atlantic Craton centred at 65°15'N, 51°50'W near Maniitsoq town in West Greenland comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes. The presently exposed features of the Maniitsoq structure were buried 20-25 km below the surface when this event occurred at c. 3 Ga, during waning convergent orogeny. These features include: a large aeromagnetic anomaly; a central 35×50 km2 large area of comminuted quartzo-feldspathic material; regional-scale circular deformation; widespread random fractures with featherlike textures; intense fracture cleavage; amphibolite-granite-matrix breccias unrelated to faulting or intrusions; formation and common fluidisation of microbreccias; abundant evidence of direct K-feldspar and plagioclase melting superimposed on already migmatised rocks; deformation of quartz by slip; formation of planar elements in quartz and plagioclase; and, emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite-facies conditions. The diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust. Nevertheless, the inferred scale, strain rates and temperatures necessary to create the Maniitsoq structure rule out a terrestrial origin of the structure.

  7. Petrology of metabasites in the south of Arousan, northeastern Isfahan province

    Directory of Open Access Journals (Sweden)

    Fereshteh Bayat

    2012-10-01

    Full Text Available Metagabbro, metadiabase and metabasalt of the Chah Palang and Me'raji mountains associated with Lower Paleozoic metamorphites are situated in the south of Arousan. Metabasites of these areas are relatively similar in terms of mineralogy and geochemical characteristics. Rock-forming minerals of the Me'raji metabasites are feldspar, amphibole, biotite, sphene, epidote, chlorite ± calcite. Metagabbro and metadiabase of the Chah Palang area are similar to the Me'raji metabasites in mineralogy and geochemistry. Volcanic rocks are overlain by metagabbros and consist of plagioclase, biotite, sphene, sanidine, chlorite, epidote and iron oxides. The rigid dykes, which are found in the volcanic units are associated with metagabbros and mineralogically are similar to the metavolcanics. Amphibolitic dykes are composed of amphibole, plagioclase and biotite with preferred orientation. Metabasites show limited range of differentiation. Me'raji metabasites are basalt and trachy-basalt in composition, whereas the Chah Palang ones present basalt and trachy-basalt composition. Similar to metabasites of the other parts of Iran, the studied basic rocks demonstrate alkaline to transitional chemical characteristic and are formed in an extensional environment by low-degree partial melting of a garnet-spinel peridotite. Metamorphic episodes have changed the studied rocks to amphibolite and greenschist, but the primitive igneous textures are preserved.

  8. Controls of Soil Spatial Variability in a Dry Tropical Forest.

    Science.gov (United States)

    Pulla, Sandeep; Riotte, Jean; Suresh, H S; Dattaraja, H S; Sukumar, Raman

    2016-01-01

    We examined the roles of lithology, topography, vegetation and fire in generating local-scale (DBH), and spatial variation in fire frequency (times burnt during the 17 years preceding soil sampling) in a permanent 50-ha plot. Unlike classic catenas, lower elevation soils had lesser moisture, plant-available Ca, Cu, Mn, Mg, Zn, B, clay and total C. The distribution of plant-available Ca, Cu, Mn and Mg appeared to largely be determined by the whole-rock chemical composition differences between amphibolites and hornblende-biotite gneisses. Amphibolites were associated with summit positions, while gneisses dominated lower elevations, an observation that concurs with other studies in the region which suggest that hillslope-scale topography has been shaped by differential weathering of lithologies. Neither NO3(-)-N nor NH4(+)-N was explained by the basal area of trees belonging to Fabaceae, a family associated with N-fixing species, and no long-term effects of fire on soil parameters were detected. Local-scale lithological variation is an important first-order control over soil variability at the hillslope scale in this SDTF, by both direct influence on nutrient stocks and indirect influence via control of local relief. PMID:27100088

  9. The Araxá Group in the South-Southwest Region of the Goiás State

    Directory of Open Access Journals (Sweden)

    Guillermo Rafael Beltran Navarro

    2013-06-01

    Full Text Available In the S – SW region of Goiás State, Brazil, the Araxá Group is constituted of a metasedimentary sequence containing schistoseand gneissic (garnet-biotite-quartz schists, feldspatic garnet-biotite-quartz schists, garnet-biotite-quartz paragneisses,with muscovite and locally amphibole, epidote, kyanite and staurolite. This sequence presents intercalations of metaultramaficrocks (serpentinite, actinolite schist, talc schist, chlorite schist, metamafic ones (amphibolite, amphibole schistcontaining or not garnet, garnet amphibolite and associated granitic bodies. Chemically, the Araxá Group metasedimentspresent peraluminous composition, showing enrichment in LILE, when compared with HFSE and REE, and displayingnegative anomalies of Nb, Ta, Sr, P and Ti. Their chemical composition is that of greywake and the chemical characteristicsof the sediments are generated in magmatic arcs. Isotopic data for Sm/Nd – model ages (TDM between 1,04 – 1,51 and1,76 – 2,26 Ga – and U/Pb (predominance of zircon with ages < 900 Ma suggest that these metasediments have Neoproterozoicrocks as the source rocks. Chemical and isotopic characteristics of the studied metasediments suggest that theirsource are rocks originated in magmatic arcs and that they were deposited in a fore arc basin developed in the margins ofNeoproterozoic island arcs.

  10. Current status of uranium exploration in Sri Lanka

    International Nuclear Information System (INIS)

    Apart from the few occurrences of Gondwana (Jurassic), Miocene and later sediments, most of Sri Lanka consists of Precambrian rocks of Archaean age. These rocks underwent metamorphism under amphibolite and granulite facies conditions about 200 Ma ago. Nine anomalous areas for uranium mineralization were identified after a preliminary geochemical survey of the whole island, except for the northwestern Miocene belt. Consistent low contents of uranium in stream sediment samples suggested that solution or hydromorphic dispersion of uranium is not a prominent mechanism and that most of the uranium dispersion is rather mechanical in nature in most of the country. Six of the above areas lie either within or close to the boundary between the Highland Series and the Vijayan Complex. The latter mainly consists of granitic gneisses, hornblende biotite gneisses, granitoids and migmatites formed under amphibolite facies conditions. Denser sampling (one sample per 1 km2) in Phase II of the programme in two areas, namely Maha Cya and Mala Oya, indicated that further exploration work would be worthwhile. A number of samples from these areas had uranium values greater than 500 ppm. Further, the composition of the amphiboles and pyroxenes from rocks of the Maha Cya area are comparable to those in rocks from known areas of uranium mineralization such as the Mary Kathleen uranium deposit in Australia. (author). 6 refs, 6 figs

  11. Old and juvenile source of Paleozoic and Mesozoic basaltic magmas in the Acatlán and Ayú complexes, Southern Mexico: Nd isotopic constraints

    Science.gov (United States)

    Keppie, J. Duncan; Dostal, Jaroslav; Shellnutt, J. Gregory

    2016-06-01

    The Neoproterozoic-Paleozoic Acatlán Complex and the Mesozoic Ayú Complex of southern Mexico consist of clastic rocks and rift-related igneous rocks inferred to have originated along a rifted passive margin in southwestern Mexico, either in an inactive (Neoproterozoic-Ordovician) or an active (Devonian-Carboniferous and Triassic-Early Jurassic) tectonic setting. The latter formed on the inner margin of a backarc basin. These passive margin rocks were partly underthrust beneath the Acatlán Complex, into which they were subsequently extruded: extrusion was synchronous with the backarc basin development. Thus, (i) the Neoproterozoic-Ordovician rocks underwent underthrusting, high-pressure metamorphism, and extrusion during the Late Devonian-Carboniferous (365-330 Ma); (ii) the Carboniferous rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Permian and Triassic; and (iii) the Triassic-Lower Jurassic rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Jurassic. Nd isotopic data from tholeiitic mafic rocks on either side of the HP extrusion zone reveal that both were underlain by similar peri-Rodinian subcontinental lithospheric mantle in the Neoproterozoic-Ordovician, which was supplemented in the Devonian-Carboniferous and Triassic-Early Jurassic by a juvenile depleted mantle source. The alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the Beniof zone, respectively.

  12. 内蒙古锡林浩特岩群岩石学特征及变质温压条件%Petrology and metamorphic temperature-pressure conditions of Xilinhot Group,Inner Mongolia,China

    Institute of Scientific and Technical Information of China (English)

    于洋; 葛梦春; 周文孝; 孙俊俊; 刘泽瑞

    2012-01-01

    Xilinhot Group,the supracrustal rock part of Xilin Gol complex,outcropped in the southeastern vicinity of Xilinhot of Inner Mongolia,is composed of gneiss with layers of amphibolite,magnetite quartzite and granofels.In this paper,we research the petrology of gneiss and amphibolite from Xilinhot Group.The amphibolites mainly contain amphibole+plagioclase assemblage;the sillimanite+K-feldspar assemblage appears in gneiss.The garnet from gneiss exhibits a pronounced compositional zoning resulting from progressive metamorphism.In such a case,the garnet domains that grow at peak metamorphic stage were chosen to calculate its formation temperatures.The p-T conditions of metabasic volcanic and metaclastic rocks penecontemporaneously formed in Xilinhot Group were appraised by garnet-biotite and hornblende-plagioclase geothermobarometry.The results suggest that the peak metamorphism of the Xilinhot Group had reached high amphibolite facies and its p-T ranges were 660-707 ℃and 0.5-0.6 GPa.Its metamorphic age is about 1000 Ma.The amphibolite facies metamorphism of Xilinhot Group might be result from the collision between Xilinhot microcontinent and other continental blocks.%锡林浩特岩群出露于内蒙古锡林浩特市东南部,指原锡林郭勒杂岩中表壳岩部分,为一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等的变质岩组合。选取岩群中片麻岩及斜长角闪岩进行岩石学及岩相学分析,其中斜长角闪岩主要矿物组合为角闪石+斜长石;片麻岩样品中见夕线石+钾长石矿物组合,石榴子石具明显进变质环带,所以在计算其形成条件时选取了生长于峰期变质阶段的特定部位。运用角闪石-斜长石、石榴子石-黑云母矿物温压计分别估算锡林浩特岩群中准同时形成的变质基性火山岩及变质碎屑岩的变质温压区间。综合二者计算结果,得锡林浩特岩群峰期变质温压条件为660~707℃,0.5~0.6GPa。变质达高角

  13. Thermal properties. Site descriptive modelling Forsmark - stage 2.2

    Energy Technology Data Exchange (ETDEWEB)

    Back, Paer-Erik; Wrafter, John; Sundberg, Jan [Geo Innova AB (Sweden); Rosen, L ars [Sweco Viak AB (Sweden)

    2007-09-15

    percentiles increase significantly as the scale of observation increases from 1 m to 5 m. Best estimates of the 0.1 percentile of thermal conductivity are: Domain RFM029: 2.30 W/(m*K) for the 1 m scale and 2.87 W/(m*K) for the 5 m scale. omain RFM045: 2.25 W/(m*K) for the 1 m scale and 2.33 W/(m*K) for the 5 m scale. The discretisation error of amphibolite is believed to be the largest uncertainty for the 1 m scale for domain RFM029. This error results in conservative estimates (believed to be too low) of the lower percentiles. For other cases, the uncertainties associated with the spatial structure of TRCs (lithology) and the spatial statistical thermal models of each TRC are believed to be the most important ones. Low-conductive rocks, mainly amphibolite and the tonalitic varieties of granodiorite to tonalite are decisive for the lower tail of the thermal conductivity distribution of a domain. The shape of the tail is therefore mainly determined by how these rock types are modelled. One of the most important uncertainties for the result for domain RFM045 is how amphibolite was modelled. Based on relatively limited data, the typical lengths of amphibolite rock bodies are modelled as being significantly longer than in domain RFM029. This may have resulted in a too heavy lower tail of the distribution of thermal conductivity for domain RFM045. In conclusion, the resulting thermal models are judged to represent the modelled rock domains, but may overemphasise the importance of the low-conductive amphibolite.

  14. Thermal properties. Site descriptive modelling Forsmark - stage 2.2

    International Nuclear Information System (INIS)

    percentiles increase significantly as the scale of observation increases from 1 m to 5 m. Best estimates of the 0.1 percentile of thermal conductivity are: Domain RFM029: 2.30 W/(m*K) for the 1 m scale and 2.87 W/(m*K) for the 5 m scale. omain RFM045: 2.25 W/(m*K) for the 1 m scale and 2.33 W/(m*K) for the 5 m scale. The discretisation error of amphibolite is believed to be the largest uncertainty for the 1 m scale for domain RFM029. This error results in conservative estimates (believed to be too low) of the lower percentiles. For other cases, the uncertainties associated with the spatial structure of TRCs (lithology) and the spatial statistical thermal models of each TRC are believed to be the most important ones. Low-conductive rocks, mainly amphibolite and the tonalitic varieties of granodiorite to tonalite are decisive for the lower tail of the thermal conductivity distribution of a domain. The shape of the tail is therefore mainly determined by how these rock types are modelled. One of the most important uncertainties for the result for domain RFM045 is how amphibolite was modelled. Based on relatively limited data, the typical lengths of amphibolite rock bodies are modelled as being significantly longer than in domain RFM029. This may have resulted in a too heavy lower tail of the distribution of thermal conductivity for domain RFM045. In conclusion, the resulting thermal models are judged to represent the modelled rock domains, but may overemphasise the importance of the low-conductive amphibolite

  15. Varena suite in the crystalline crust of the Southern Lithuania: implication to the genesis and mineralization

    Science.gov (United States)

    Kirkliauskaite, Vaida; Motuza, Gediminas; Skipityte, Raminta

    2016-04-01

    Keywords: Lithuania, Proterozoic, Varena suite, rare earth elements, metasomatosis. Crystalline crust in Southern Lithuania is covered by 200-500 m thick sedimentary cover and is investigated by potential field mapping and drilling. It is composed by amphibolites (metabasalts), biotite-quartz-plagioclase gneisses (metapsammites and porphyry metadacite and metaandesite) of Orosirian period. Each lithology is predominant in alternating bands extended NNE-SSW. Supracrustals are metamorphosed on the level of amphibolite facies and migmatized. Intrusive rocks are represented by coeval rare bodies of gabbro, peridotite and widespread Calymmian granitic plutons. Varena suite comprises olivine, enstatite, diopside, olivine-magnetite, magnetite, apatite-bearing, and presumably also dolomite rocks. They form integrated bodies few sq. km large, concentrated in the ˜300 km2area extended in N-S direction.Bodies of particular rocks from Varena suite fixed in the boreholes are few tenths up to few hundred meters thick, except apatite-bearing rocks, forming decimeter up to meter scale lenses and veins. Rocks of Varena suite are affected by strong metasomatic alteration. Olivine is substituted by serpentine, and phlogopite, pyroxenes with amphibols (hornblende, actinolite, tremolite, rarely richterite). The country rocks (amphibolites, metaporphyres) also affected by alkaline (mainly sodic) metasomatosis manifested by formation of scapolite, albite, clynopyroxene (often sodic), phlogopite, carbonates. The mineralization of REE (up to 2759-3100 ppm of La and Ce respectively), Th, U, P hosted by monazite, apatite, allanite is spatially related to the Varena suite and some metasomatized supracrustals. The views on the genesis of Varena suite are contradicting. By various authors they are regarded as skarns, presuming metasomatic origin, as layered intrusions or products of alkaline and carbonatitic magmatism. In this presentation the genetic model is reviewed based on

  16. Coupled mass transfer through a fluid phase and volume preservation during the hydration of granulite: An example from the Bergen Arcs, Norway

    Science.gov (United States)

    Centrella, Stephen; Austrheim, Håkon; Putnis, Andrew

    2015-11-01

    The Precambrian granulite facies rocks of Lindås Nappe, Bergen Arcs, Caledonides of W. Norway are partially hydrated at amphibolite and eclogite facies conditions. The Lindås Nappe outcrops over an area of ca. 1000 km2 where relict granulite facies lenses make up only ca. 10%. At Hillandsvatnet, garnetite displays sharp hydration fronts across which the granulite facies assemblage composed of garnet (55%) and clinopyroxene (45%) is replaced by an amphibolite facies mineralogy defined by chlorite, epidote and amphibole. The major element bulk composition does not change significantly across the hydration front, apart from the volatile components (loss on ignition, LOI) that increases from 0.17 wt.% in the granulite to 2.43 wt.% in the amphibolite. However the replacements of garnet and of clinopyroxene are pseudomorphic so that the grain shapes of the garnet and clinopyroxene are preserved even when they are completely replaced. The textural evolution during the replacement of garnet by pargasite, epidote and chlorite and of pyroxene by hornblende and quartz in our rock sample conforms to that expected by a coupled dissolution-precipitation mechanism. SEM and electron microprobe analysis coupled with the software XMapTools V 1.06.1 were used to quantify the local mass transfer required during the replacement processes. The element losses and gains in replacing the garnet are approximately balanced by the opposite gains and losses associated with the replacement of clinopyroxene. The coupling between dissolution and precipitation on both the grain and whole rock spatial scale preserves the volume of the rock throughout the hydration process. However, the hydration involves reduction of rock density and mass balance calculations, together with volume preservation (isovolumetric reaction) require a significant loss of the mass of the rock to the fluid phase. This suggests a mechanism for coupling between the local stress generated by hydration reactions and mass

  17. Metamorphism and gold mineralization of the Kenticha Katawicha area: Adola belt, southern Ethiopia

    Science.gov (United States)

    Tsige, Lulu

    2006-05-01

    I present geological and mineral chemistry results aiming at understanding the relationship between metamorphism and gold mineralization in the N-trending Neoproterozoic Kenticha-Katawicha area in the Adola belt of southern Ethiopia. The Kenticha-Katawicha area comprises low-grade metamorphic rocks including pelitic schists, marble, graphite schists and Fe-Mn quartzites (Kenticha marine metasedimentary rocks) and serpentinites and talc-tremolite-chlorite schists with podiform chromites (Kenticha ophiolite). Lenses of high-grade amphibolite schist are also locally present within the Kenticha-Katawicha area. These rocks are sandwiched between high-grade para- and ortho-gneisses, migmatites, schists and amphibolites. Post-orogenic granites intrude the metamorphic sequences. Mineral assemblages and textural data obtained from the high-grade rocks indicate that the dominant metamorphic condition is of upper amphibolite facies. Garnet-biotite (garnet-core-matrix-biotite) and amphibole-plagioclase geothermometry gave estimates for peak metamorphic temperatures of 630-650 °C and pressure of 7 kbar for the high-grade rocks. On the other hand, the low-grade rocks recrystallized in the greenschist facies conditions. The estimated peak T- P values for the high-grade rocks suggest a burial to ˜25 km depth and a clockwise P- T path is deduced from mineral thermometry and the textural and paragenetic relations. This combined with the geochemistry of the magmatic rocks and clockwise P- T is consistent with a collision setting in which rocks of the calc-alkaline, volcanic-arc, and oceanic fragments are assembled. In the study area, gold occurs in quartz veins and veinlets in lenses of biotite schists within ultramafic rocks that are confined to shear zones. Fluid inclusion studies of gold quartz veins and veinlets indicate a H 2O- and CO 2-rich fluid with low salinity (<5 wt% NaCl equivalent). The gold mineralization in the area is post-peak regional metamorphism and is

  18. XRD applied to the determination of pigments and composition of lithic materials and ceramics from archaeological pre-hispanic sites of the Rio de la Plata

    Energy Technology Data Exchange (ETDEWEB)

    Beovide, Laura [Department of Archeology, National Museum of Anthropology, Montevideo, (Uruguay); Pardo, Helena; Faccio, Ricardo; Mombru, Alvaro [Centro NanoMat, Polo Tecnologico de Pando, Facultad de Quimica, Universidad de la Republica, Pando, Canelones (Uruguay); Crystallography, Solid State and Materials Laboratory (Cryssmat-Lab), DETEMA, Facultad de Quimica, Universidad de la Republica, Gral, Montevideo (Uruguay); Piston, Mariela, E-mail: mpiston@fq.edu.u [Analytical Chemistry, Estrella Campos Department, Facultad de Quimica, Universidad de la Republica, Montevideo (Uruguay)

    2011-07-01

    Full text: The earliest records of human occupation on the lower basin of Santa Lucia River are dated ca. 4800 {sup 14}C years BP, in the area of one of the major tributaries of the Rio de la Plata on the Uruguayan coast. These societies were basically hunters and gatherers until ca. 3000 {sup 14}C years BP when they incorporated the horticulture. In this multidisciplinary work, two cases of application of XRD analysis of archaeological materials are presented to provide new perspectives in solving various problems related to the technological organization of these societies. In the first case, ceramics and pigments from an archaeological context prior to the hispanic-indian contact were analyzed. The X-ray powder diffraction patterns were obtained using a RIGAKU, Ultima IV with CBO monochromator, CuK{sub {alpha}} radiation was at 40 kV and 20 mA tube power at 0.02 deg/seg, operating in the range from 2{theta}=5.00 to 60.00 deg. According the comparison between the experimental recorded X-ray diffraction pattern to those stored in a X-ray powder diffraction database reveals that the piece of pottery is mainly constitute of quartz (SiO{sub 2}) and hematite (Fe{sub 2}O{sub 3}) while the mineral sample is probably composed mainly of quartz (SiO{sub 2}) and goethite (FeO{sub 3}.H{sub 2}O). The results allow a first approximation to know the inorganic pigments that were part of the decoration of the pottery and pigments used in the archaeological context. In the second case an amphibolite instrument from ca. 2700 {sup 14}C years BP related to a shell midden was analyzed and compared with amphibolites located 15 km of the archaeological site to assess if they were the raw materials for these instruments. Compositional XRD mineralogical analysis shows that the both samples seem to have similar mineral composition, which is mainly quartz (SiO{sub 2}) and a mixed sodium magnesium and calcium silicate (NaCa{sub 2}(Mg{sub 4}Ti)Si{sub 6}Al{sub 2}O{sub 23}(OH){sub 2}). This

  19. Obduction of western Anatolian ophiolites: from birth to steady state of a subduction zone

    Science.gov (United States)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Whitechurch, Hubert; Okay, Aral

    2015-04-01

    During Cretaceous times, the convergence between the Anatolide Tauride block (following the movement of Africa) and Eurasia lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. Obducted ophiolite and their sub-ophiolitic units can be found along a 400 kilometre-long north to south transect in western Anatolia. The aim of this contribution is twofold: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia, by constraining the formation of the metamorphic sole during the first stages of subduction and the unusual accretion of ocean-derived units along a subduction interface in an evolving, cooling thermal regime, and (2) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part is this sole is made of garnet and garnet clinopyroxene amphibolites, the lower part consisting in amphibolite or green-schist facies metapelites and metabasite suggestive of discrete accretion steps. In the northern part of the section the metamorphic sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis. This high-pressure overprint is lacking in the southern area. Using field and petrological observations, three units (namely and from top to bottom, OC1, OC2 and OC3) were distinguished in the accretionary complex with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows only low-grade metamorphism. Metamorphic conditions remains hard to establish in this unit made of a stack of hm-thick tectonic slices showing subtle differences in their metamorphic grade (from pristine pillow basalts and hydrothermalized lavas to lawsonite pumpellyite-lawsonite bearing basalts). In OC2, Fe-Mg carpholite-bearing layers

  20. Nappe-Bounding Shear Zones Initiated On Syn-Tectonic, Pegmatite-Filled Extensional Shear Fractures During Deep-Crustal Nappe Flow In A Large Hot Orogen

    Science.gov (United States)

    Culshaw, Nicholas; Gerbi, Christopher; Marsh, Jeffrey; Regan, Peter

    2014-05-01

    The Central Gneiss Belt (CGB) of the Proterozoic western Grenville Province is an extensive exposure of the mid-crustal levels (upper amphibolite facies, lesser granulites) of a large hot orogen. Numerical models give a credible prediction of structure and metamorphism accompanying CGB deep-crustal nappe flow and define a temporal framework based on four developmental phases: thickening, heating, nappe-flow and post convergence extensional spreading. These phases are diachronous in direction of orogen propagation and imply a spatial framework: externides (close to orogen-craton boundary) containing moderately inclined thickening and/or extensional structures, and internides containing thickening structures overprinted by sub-horizontal nappe flow structures, which may be locally overprinted by those due to extensional spreading. Although on average of granitoid composition, CGB nappes differ in rheology, varying from fertile and weak (unmetamorphosed before Grenville, meltable) to infertile and strong (metamorphosed at high grade before Grenville, unmeltable) or mixed fertile-infertile protoliths. Deformation style varies from diffuse in fertile nappes, weakened by pervasive melting, to localised in shear zones on boundaries or interiors of infertile nappes. Specifically, in terms of deformation phase and location within the orogen, shear zones occur as: thickening structures of externides, early thickening- and later overprinting nappe-flow structures of infertile internide nappes, and extension-related shear zones in externides and internides. Many of the nappe-flow shear zones of the internides are associated with pegmatites. One example has been recognized of a preserved progression from small-scale fracture arrays to regional shear zone. The sequence is present on a km-scale and initiates in the interior of a nappe of layered granulite with arrays of pegmatite filled extensional-shear fractures (mm to cm width) displaying amphibolized margins. The fracture

  1. U-Pb geochronological constraints on the timing of episodic regional metamorphism and rapid high-T exhumation of the Grand Forks complex, British Columbia

    Science.gov (United States)

    Cubley, J. F.; Pattison, D. R. M.; Tinkham, D. K.; Fanning, C. M.

    2013-01-01

    The Grand Forks complex (GFC) is a fault-bounded metamorphic core complex in the southern Omineca Belt of British Columbia, Canada. It experienced prograde metamorphism ranging from upper-amphibolite to granulite facies conditions during the Mesozoic to early Tertiary compressional stage of the Cordilleran orogeny. Peak metamorphism was followed by multi-stage exhumation in the Early Eocene. This study provides U-Pb monazite and zircon constraints on the timing of metamorphic episodes in the GFC and subsequent high-T, amphibolite facies decompression in the Early Eocene. Monazite LA-ICP-MS ages from metapelitic gneisses record episodic metamorphism from the Late Jurassic to Paleocene, with peak metamorphism occurring between ~ 59 and 50 Ma. Peak metamorphism was followed by rapid, near-isothermal decompression of the GFC between ~ 52 and 50 Ma, and leucosome crystallization at ~ 50 Ma. Thermodynamic modeling of metapelites in the system MnNCKFMASHPYCe predicts that monazite was not stable at peak metamorphic conditions, consistent with the dominant population of ~ 59 Ma ages representing growth along the prograde path, most likely at subsolidus conditions. Growth of widespread high-Y monazite rims (~ 50 Ma) is predicted along suprasolidus decompression and cooling paths. Zircon SHRIMP ages from igneous bodies in the GFC and hanging wall of the bounding Kettle River fault (KRF) suggest ductile deformation related to high-T decompression of the GFC was ongoing at 51 Ma but had ceased by 50 Ma, truncated by post-kinematic granitoids. This high-T deformation predates subsequent greenschist facies extension on the overlying KRF. A pre-KRF, hanging wall ductile shear zone is constrained to ~ 59-51 Ma. It deforms 59 Ma Ladybird suite leucogranites and may be related to high-T exhumation of the core complex. Rapid, > 100 °C/Ma cooling rates are required to accommodate high-T (amphibolite facies) exhumation of the GFC at 52-50 Ma followed by low-T (greenschist facies

  2. Mass transfer and trace element redistribution during hydration of granulites in the Bergen Arcs, Norway.

    Science.gov (United States)

    Centrella, Stephen; Austrheim, Håkon; Putnis, Andrew

    2016-04-01

    The Bergen Arcs located on the Western coast of Norway are characterized by Precambrian granulite facies rocks partially hydrated at amphibolite and eclogite facies conditions. Over an area of ca. 1000 km², relict of granulite facies lenses make up only ca. 10% of the observed outcrops. At Hilland Radöy, granulite displays sharp hydration fronts across which the granulite facies assemblage composed of garnet (55%) and clinopyroxene (45%) is replaced by an amphibolite facies mineralogy defined by chlorite, epidote and amphibole. The major element bulk composition does not change significantly across the hydration front, apart from the volatile components (loss on ignition, LOI) that increases from 0.17 wt.% in the granulite to 2.43 wt.% in the amphibolite (Centrella et al., 2015). The replacements of garnet and clinopyroxene are pseudomorphic indicating a perfect preservation of the parent crystal shape. The textural evolution during the replacement is consistent with the coupled dissolution-precipitation mechanism where garnet is replaced by chlorite, epidote and pargasite and clinopyroxene by hornblende and quartz. Based on the observations of an isovolumetric replacement, the mass loss during hydration was estimated at 13%. This study is based on the trace element redistribution during the hydration using the same samples as Centrella et al. (2015). The local mass transfer during the replacement process determined from the major element is also confirmed by the trace element redistribution. The LILE, HFSE and REE losses and gains in replacing the garnet are approximately balanced by the opposite gains and losses associated with the replacement of clinopyroxene. Because the hydration involves reduction of rock density, the volume preservation (isovolumetric reaction), together with the mass balance calculations, requires a significant loss of the mass of the rock to the fluid phase: 13% based on the major element redistribution and around 20% based on the REE

  3. XRD applied to the determination of pigments and composition of lithic materials and ceramics from archaeological pre-hispanic sites of the Rio de la Plata

    International Nuclear Information System (INIS)

    Full text: The earliest records of human occupation on the lower basin of Santa Lucia River are dated ca. 4800 14C years BP, in the area of one of the major tributaries of the Rio de la Plata on the Uruguayan coast. These societies were basically hunters and gatherers until ca. 3000 14C years BP when they incorporated the horticulture. In this multidisciplinary work, two cases of application of XRD analysis of archaeological materials are presented to provide new perspectives in solving various problems related to the technological organization of these societies. In the first case, ceramics and pigments from an archaeological context prior to the hispanic-indian contact were analyzed. The X-ray powder diffraction patterns were obtained using a RIGAKU, Ultima IV with CBO monochromator, CuKα radiation was at 40 kV and 20 mA tube power at 0.02 deg/seg, operating in the range from 2θ=5.00 to 60.00 deg. According the comparison between the experimental recorded X-ray diffraction pattern to those stored in a X-ray powder diffraction database reveals that the piece of pottery is mainly constitute of quartz (SiO2) and hematite (Fe2O3) while the mineral sample is probably composed mainly of quartz (SiO2) and goethite (FeO3.H2O). The results allow a first approximation to know the inorganic pigments that were part of the decoration of the pottery and pigments used in the archaeological context. In the second case an amphibolite instrument from ca. 2700 14C years BP related to a shell midden was analyzed and compared with amphibolites located 15 km of the archaeological site to assess if they were the raw materials for these instruments. Compositional XRD mineralogical analysis shows that the both samples seem to have similar mineral composition, which is mainly quartz (SiO2) and a mixed sodium magnesium and calcium silicate (NaCa2(Mg4Ti)Si6Al2O23(OH)2). This suggests that most likely the source of supply for the amphibolites instruments come from the sources of raw

  4. The role of cation exchange in the sorption of cadmium, copper and lead by soils saturated with magnesium

    International Nuclear Information System (INIS)

    The displacement of Ca2+, Mg2+, K+ and Al3+ from the A and Bw or Bt horizons of two soils developed over serpentinized amphibolites when equilibrated in Cu2+, Cd2+ or Pb2+ solutions was determined, together with the concomitant sorption of the heavy metal. The contributions of Mg2+ to the effective cation exchange capacities of the A and Bt horizons of the Endoleptic Luvisol were 57% and 94%, respectively, and its contributions to those of the A and Bw horizons of the Mollic Cambisol were 70% and 77%, respectively. In all four horizons, cation exchange, chiefly with Mg2+ and Ca2+, was the process chiefly responsible for sorption of Cd2+, Cu2+ and Pb2+. Al3+ and K+ were hardly implicated, especially in the case of Cd2+.

  5. A field trip guidebook to the type localities of Marland Billings' 1935 Paleozoic bedrock stratigraphy near Littleton, New Hampshire

    Science.gov (United States)

    Rankin, Douglas W.; Rankin, Mary B.

    2014-01-01

    Marland Billings' classic paper published in 1937 in the Geological Society of America Bulletin established a succession of six stratigraphic units in rocks of low metamorphic grade near Littleton, New Hampshire. The two youngest units are fossiliferous in the area, with ages established at the time as “middle” Silurian and Early Devonian. Billings and students mapped the same stratigraphic section in adjacent areas of progressively higher regional metamorphic grade. This work laid the foundation upon which a major part of subsequent work in New England has been directly or indirectly built. This guidebook was written for a field trip held in March 2013 to visit roadcuts that are as close as possible in March to the type localities or areas of Billings’ six-fold stratigraphic succession. Ten stops are in rocks of chlorite grade of Acadian(?) metamorphism; the final stop visits amphibolite of the Ammonoosuc Volcanics. Fieldwork by the authors over the past 20 years confirms Billings’ broad conclusions.

  6. Tectonic implications of new single zircon Pb-Pb evaporation data in the Lossogonoi and Longido ruby-districts, Mozambican metamorphic Belt of north-eastern Tanzania

    Science.gov (United States)

    Le Goff, Elisabeth; Deschamps, Yves; Guerrot, Catherine

    2010-01-01

    Three single zircon Pb-Pb evaporation dating studies were performed on felsic orthogneisses and migmatites from the Longido and Lossogonoi ruby districts, Mozambique Belt of north-eastern Tanzania, in order to better constrain the geological setting of gemstone mineralizations. Igneous emplacement ages of protoliths ranging between 2636 and 2448 Ma document for the first time the presence of a Neoarchean to Lower Paleoproterozoic (Siderian) basement reworked in the Late Neoproterozoic Mozambique Belt of north-eastern Tanzania. This ancient crust of unknown dimension is well documented farther south, but also in south-eastern Kenya. A shearing event under high-grade amphibolite facies conditions, postdating the Pan-African metamorphic peak at 640 Ma and following nappes emplacement is demonstrated at ca. 610 Ma from metamorphic zircons of Lossogonoi district. In Lossogonoi district, ruby crystallizes during this last stage of deformation.

  7. Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay, North Xinjiang

    Institute of Scientific and Technical Information of China (English)

    ZHANG Haixiang; NIU Hecai; Kentaro Terada; YU Xueyuan; Hiroaki Sato; Jun'ichi Ito

    2003-01-01

    Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.

  8. The age of mid Proterozoic phosphatic metasediments in Finland as indicated by radiometric U-Pb dates

    International Nuclear Information System (INIS)

    Two uranium-bearing phosphatic metasedimentary occurrences at Lampinsaari, Vihanti and Temo, Nilsiae, in Finland have been dated by isotopic U-Pb analyses of whole rock samples to be 1876+-2 Ma old. The dates reflect the time of the Svecokarelian regional metamorphism in high amphibolite facies. An analysis of a uranium thucolite from the deposit at Nuottijaervi, Paltamo, that was metamorphosed in intermediate greenschist facies gives a date of 1897+-7 Ma, suggesting that the phosphatic sedimentation had occurred before that time. An upper time limit for the sedimentation is set by a previous date of 2080+-45 Ma, which was obtained from a banded iron ore formation. The absence of thorium in the deposits suggests a marine environment of deposition. (Auth.)

  9. Gravity inferred subsurface structure of Gadwal Schist belt, Andhra Pradesh

    Indian Academy of Sciences (India)

    G Ramadass; I B Ramaprasada Rao; N Srinivasulu

    2001-03-01

    Detailed gravity data collected across the Gadwal schist belt in the state of Andhra Pradesh show an 8.4 mgal residual gravity anomaly associated with meta-sediments/volcanics of the linear NNW-SSE trending schist belt that shows metamorphism from green schist to amphibolite facies. This schist belt is flanked on either side by the peninsular gneissic complex. The elevation and slab Bouguer corrected residual gravity profile data were interpreted using 2-D prism models. The results indicate a synformal structure having a width of 1.8 km at the surface, tapering at a depth of about 2.6 km with a positive density contrast of 0.15 gm/cc with respect to the surrounding peninsular gneissic complex.

  10. A review of recent studies in the Pine Creek geosyncline with special reference to uranium

    International Nuclear Information System (INIS)

    The Lower Proterozoic metasediments of the Pine Creek Geosyncline form a chronostratigraphic sequence of mainly greenschistand amphibolite-grade shallow-marine to supratidal pelites, psammites, carbonate rocks, and volcaniclastics which in places rests unconformably on Archaean basement. Granite and later dolerite intrude the sequence, and are associated with the major orogenic 1800-m.y. event which regionally metamorphosed the sediments. Most mineralisation is stratabound, and can be related at least partly to volcanic activity. Uranium mineralisation is mainly confined to particular carbonate-rich horizons near basement. Specialist studies indicate that uranium was leached from its souce rock, and probably carried as carbonate complexes in highly saline fluids at between 100 and 350 deg. C. Precipitation took place by redox reactions in breccia zones in carbonate rocks; these zones were formed by carbonate solution or diapiric movement of evaporites which preceded the carbonate

  11. Granulites from Northwest Indian Shield: Their differences and similarities with Southern Indian granulite terrain

    Science.gov (United States)

    Sharma, R. S.

    1988-01-01

    Granulite facies suite in the NW Indian Shield is exposed at Sand Mata, Udaipur district, Rajasthan, as an oval-shaped massif within amphibolite facies rocks of the Banded Gneissic Complex (3.5 to 2.6 b.y. old) - a possible analogue of the Peninsular gneiss of Dharwar craton. On the basis of quantitative P-T estimates, combined with the textural evidence for the crystallization sequence of the Al-silicate polymorphs (kyanite to sillimanite to kyanite) in the pelitic granulite, the deduced P-T path for the Sand Mata granulites is the reverse of that characterizing the Plate tectonic collision zone. It, however, agrees with the P-T path inferred in the case of the southern Indian granulitic rocks.

  12. Boron in sillimanite.

    Science.gov (United States)

    Grew, E S; Hinthorne, J R

    1983-08-01

    Sillimanite in six granulite-facies, kornerupine-bearing rocks contains 0.035 to 0.43 percent B(2)O(3) and 0.02 to 0.23 percent MgO (by weight). Substitution of boron for silicon and magnesium for aluminum is coupled such that the ratio of magnesium to boron is about 0.5. Sillimanite incorporates more than 0.1 percent B(2)O(3) only at high temperatures in a boron-rich environment at very low partial pressures of water. In the amphibolite facies, the sillimanite boron contents are too low to appreciably affect the stability relations of sillimanite with kyanite and andalusite. PMID:17830955

  13. High grade metamorphism in the Bundelkhand massif and its implications on Mesoarchean crustal evolution in central India

    Science.gov (United States)

    Singh, S. P.; Dwivedi, S. B.

    2015-02-01

    The Bundelkhand Gneissic Complex (BnGC) in the central part of the Bundelkhand massif preserves a supracrustal unit which includes pelitic (garnet-cordierite-sillimanite gneiss, garnet-sillimanite gneiss, biotite gneiss and garnet-biotite gneiss) and mafic (hornblende-biotite gneiss and garnetiferous amphibolite) rocks. Granulite facies metamorphism of the complex initiated with breaking down of biotite to produce garnet and cordierite in the pelitic gneisses. Geothermobarometric calculations indicate metamorphic conditions of 720°C/6.2 kbar, followed by a retrograde (687°C/4.9 kbar) to very late retrograde stages of metamorphism (579°C/4.4 kbar) which is supported by the formation of late cordierite around garnet. The P-T conditions and textural relations of the garnet-cordierite-bearing gneiss suggest a retrograde cooling path of metamorphism.

  14. A Palaeoproterozoic multi-stage hydrothermal alteration system at Nalunaq gold deposit, South Greenland

    DEFF Research Database (Denmark)

    Bell, Robin-Marie; Kolb, Jochen; Waight, Tod Earle;

    2016-01-01

    Nalunaq is an orogenic, high gold grade deposit situated on the Nanortalik Peninsula, South Greenland. Mineralisation is hosted in shear zone-controlled quartz veins, located in fine- and medium-grained amphibolite. The deposit was the site of Greenland’s only operating metalliferous mine until its...... closure in 2014, having produced 10.67 t of gold. This study uses a combination of field investigation, petrography and U/Pb zircon and titanite geochronology to define a multistage hydrothermal alteration system at Nalunaq. A clinopyroxene-plagioclase-garnet(-sulphide) alteration zone (CPGZ) developed in...... the Nanortalik Peninsula, close to regional peak metamorphism and prior to gold-quartz vein formation. The ca. 1783–1762-Ma gold-quartz veins are hosted in reactivated shear zones with a hydrothermal alteration halo of biotite...

  15. Allanite behaviour during incipient melting in the southern Central Alps

    DEFF Research Database (Denmark)

    Gregory, C.J.; Rubatto, D.; Hermann, J.;

    2012-01-01

    in metamorphic allanite with low Th/U (Gabudianu et al., 2009). All of these effects have significant implications for allanite geochronology. This paper details the response of allanite to incipient melting and its performance as a chronometer by examining its occurrence, trace element composition, and age...... to amphibolite-grade rocks of the SSB (Burri et al., 2005). In situ melting of granitic gneisses occurred in the mid-crust at temperatures close to the wet granite solidus (~630 °C) and in some cases produced up to ~30 vol.% leucosomes (Burri et al., 2005). At outcrop, leucosomes are variably deformed...... of allanite was conducted over five separate sessions using the SHRIMP II and SHRIMP RG (Reverse Geometry) ion microprobes at the Research School of Earth Sciences, ANU. Analyses were performed on allanite in polished grain mounts with a 2.0–3.5 nA, 10 kV primary beam focused through a ~100 µm aperture...

  16. Unusual features caused by lightning impact in West Greenland

    DEFF Research Database (Denmark)

    Appel, P.; Abrahamsen, N.; Rasmussen, T.

    2006-01-01

    that a strong electric current indeed traversed the boulder. A few years later a second lightning impacted on a mountaintop close to the first impact. The second lightning left a trail on the rock surface covered by a thin layer of glass. The glass displays spectacular colours ranging from metallic blue, to red......   Two lightning impacts are described from an area near the Inland Ice in West Greenland. The first lightning blasted an outcrop of metacherts. It subsequently split into two branches, which traversed rock outcrops and boulders, leaving behind two white almost straight lines, 30 m and 14 m long...... respectively, where all lichens and plants were burned away. On the white lines the upper few millimetres of the traversed boulders were melted to a glass which subsequently peeled off by thermal expansion to leave a rough surface. Magnetic investigation of an amphibolite boulder found on the white line showed...

  17. High grade metamorphism in the Bundelkhand massif and its implications on Mesoarchean crustal evolution in central India

    Indian Academy of Sciences (India)

    S P Singh; S B Dwivedi

    2015-02-01

    The Bundelkhand Gneissic Complex (BnGC) in the central part of the Bundelkhand massif preserves a supracrustal unit which includes pelitic (garnet–cordierite–sillimanite gneiss, garnet–sillimanite gneiss, biotite gneiss and garnet–biotite gneiss) and mafic (hornblende–biotite gneiss and garnetiferous amphibolite) rocks. Granulite facies metamorphism of the complex initiated with breaking down of biotite to produce garnet and cordierite in the pelitic gneisses. Geothermobarometric calculations indicate metamorphic conditions of 720°C/6.2 kbar, followed by a retrograde (687°C/4.9 kbar) to very late retro-grade stages of metamorphism (579°C/4.4 kbar) which is supported by the formation of late cordierite around garnet. The P–T conditions and textural relations of the garnet–cordierite-bearing gneiss suggest a retrograde cooling path of metamorphism.

  18. Complex electric conductivity of rocks

    International Nuclear Information System (INIS)

    Laboratory measurements of complex conductivity were made on 28 drill-core samples from area MM1-Prospect 1 of the Carajas Mining District. The objective of this research was to help interpret field geophysical survey of the area using Induced Polarization and AFMAG methods. A petrographic study of the samples was done, using thin sections, polished sections and X-ray diffraction. Copper content, in the form of sulfides, was determined using atomic absorption. As a result of the petrographic study, the samples were classified in five distinct groups: granite, biotite schist, amphibolite and magnetite quartzite-iron formation. The grade of Cu was variable in the five groups, ranging from 50 ppm to 6000 ppm. In conclusion, these measurements show that the field Induced Polarization and AFMAG anomalies near these three drill holes (F1, F2 and F3) are due primarily to the magnetic iron formation, and secondarily due to associated low-grade chalcopyrite mineralization. (author)

  19. Mapping the Piute Mountains, CA with Thermal Infrared Multispectral Scanner (TIMS)

    Science.gov (United States)

    Hook, S. J.; Karlstrom, K. E.; Miller, C. F.; McCaffrey, K. J. W.

    1993-01-01

    Thermal Infrared Multispectral Scanner (TIMS) data were acquired in 1990 over the PiuteMountains, California to evaluate their usefulness for lithologic mapping in an area ofmetamorphosed, structurally complex, igneous and sedimentary rocks. The data were calibrated,atmospherically corrected, and emissivity variations extracted from them. There was an excellentvisual correlation between the units revealed in the TIMS data and the recent mapping in the easternside of the area. It was also possible to correct, improve and extend the recent map. For example,several areas of amphibolite were identified in the TIMS data that had been incorrectly mapped asgranodioritic gneiss, and the presence of a swarm of mafic dikes, of which only a few had previouslybeen identified, was revealed...

  20. On the metamorphic history of an Archaean granitoid greenstone terrane, East Pilbara, Western Australia, using the 40Ar/39Ar age spectrum technique

    International Nuclear Information System (INIS)

    Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably data updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840-2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism. (orig.)

  1. Approche métallogénique du "Greenstone Belt" de Bogoin (RCA). Sa minéralisation en or

    OpenAIRE

    Biandja, Jean

    1988-01-01

    La minéralisation aurifère primaire du secteur de Bogoin se présente sous forme d'un stockwerk quartzeux de direction N-S encaissé dans des roches vertes. Ces dernières appartiennent à une série métamorphique (limite faciès schistes verts-amphibolite) plissée en un synclinorium de direction N-S qui dessine un "doigt de gant" dans un vaste massif granito-gneissique. La foliation principale (SP) est plan axial de cette structure. L'ensemble est réputé archéen. Le synclinal s'ennoie vers le Sud ...

  2. The first occurrence of elbaite in Poland

    Directory of Open Access Journals (Sweden)

    Michał Sachanbiński

    2004-01-01

    Full Text Available An amphibolite-hosted quartzo-feldspar-mica pegmatite with schorl, green elbaite, spessartine,andalusite, spinel, hyalophane, zircon, columbite and beryl was found near Gilów in the E part of theGóry Sowie gneissic block (Lower Silesia, Poland. The black tourmaline crystals are chracterized by thechange of their composition from Mg- and Al-enriched schorl, typical of the Góry Sowie block, to Al- and,Al- and Li-enriched schorl, and to Fe-bearing elbaite. Light green tourmaline corresponds to (Fe,Mn--bearing elbaite. The crystallization sequence of the tourmaline varieties results from progressive change ofcomposition of pegmatite melts in the last metamorphic stage of a parent sedimentary protolith around370–380 Ma ago. The stage of Li-bearing tourmaline formation corresponds to crystallization of a phosphateassemblage with ferrisicklerite-sarcopside-graftonite lamellar intergrowths known from other pegmatites ofthe Góry Sowie block.

  3. Geochemistry and mineralogy of the radioactive minerals associated with some pegmatite veins of the Ukma-Nawahatu-Hursi sector, Purulia Dist., W.B., in the precambrian Chhotanagpur gneissic complex

    International Nuclear Information System (INIS)

    Some barite-bearing pegmatites in the Ukma-Nawahatu-Hursi sector (23° 25'-26'N, 86° 02'-04'E) of the Purulia dist., West Bengal, have association of radioactive minerals in the form of coarse grained pitchblack lumps and irregular patches. This radioactive belt of about 15 km length running through Ukma, Nawahatu and Hursi areas follows ENE-WSW trending shear zone. The barite-bearing pegmatites occur as lenses or lenticular veins hosted by garnetiferous sillimanite-biotite-quartz-schist or occasionally by migmatite. Near Nawahatu the radioactive barite-pegmatite vein occurs at or near the junction between footwall amphibolite and hangingwall garnetiferous schist. The pegmatite veins have mainly followed the schistosity of the host rock and dip 70°-80° towards south

  4. Charnockitic ortho gneisses and mafic granulites of Cerro Olivo complex, proterozoic basement of SE Uruguay, Part 1: Geology

    International Nuclear Information System (INIS)

    Charnockitic ortho gneisses and mafic granulite s exposed in the Cerro Bori Block, in the center of Punta del Este terrain, were the first document occurrence of granulitic rocks from SE sector of the Uruguayan Shield. We present here their main geological features, with the purpose to suggest some petrologic and structural interesting problems for a future lithogeochemical, mineral chemistry, stable isotopes and fluid inclusion studies about these rocks. We propose some speculation form field-based studies considering a cognate magmatic origin of both kinds of rocks, previous to a homogeneous granulitic metamorphism. Some structural evidences indicate that after their uplift, these rocks were located on over thickened crust, at great to medium deepness. A cataclasis during anatexis and amphibolite-facies mineral association stabilization are common phenomena. Other evidences suggest a polycyclic character for the regional geologic evolution

  5. Subsurface Geology of the Fenton Hill Hot Dry Rock Geothermal Energy Site

    Energy Technology Data Exchange (ETDEWEB)

    Levey, Schon S.

    2010-12-01

    The Precambrian rock penetrated by wells EE-2A and -3A belongs to one or more granitic to granodioritic plutons. The plutonic rock contains two major xenolith zones of amphibolite, locally surrounded by fine-grained mafic rock of hybrid igneous origin. The granodiorite is cut by numerous leucogranite dikes that diminish in abundance with depth. The most prominent structural feature is the main breccia zone, in which the rock is highly fractured and moderately altered. This zone is at least 75 m thick and is of uncertain but near-horizontal orientation. Fracture abundance decreases with increasing depth below the main breccia zone, and fractures tend to be associated with leucogranite dikes. This association suggests that at least some of the fractures making up the geothermal reservoir are of Precambrian age or have long-range orientations controlled by the presence of Precambrian-age granitic dikes.

  6. Pb-Sb and Pb-Sb-Bi sulfosalts and associated sulphides from Dubrava antimony deposit, Nizke Tatry Mts

    International Nuclear Information System (INIS)

    A sulfosalt study was carried out on an antimony deposit Dubrava in Western Carpathians, Slovakia. Quartz-sulphide veins are hosted by Variscan crystalline complex built by granitoid rocks with lenticular bodies of amphibolite gneisses and migmatites. Pb-Sb sulfosalts are constituents of Sb ores and occur together with stibnite, sphalerite, pyrite and other sulphides. Among the sulfosalts, identified by X-ray micro-analyses, the most abundant one is zinckenite. Other phases are chemically close to plagionite, robinsonite, heteromorphite, semseyite and boulangerite. Sulfosalts richter in Bi accompany later tetrahedrite, bournonite and chalcostibite. These include tintinaite (kobellite homologous series) and Bi zinckenite. Intermediate phases between stibnite and bismuthinite, referred to as horobetsuite, are also common. (authors)

  7. Contributions to the mineralogy, petrography and metallogeny of the Itataia phosphorus-uranium deposit, state of Ceara, Brazil

    International Nuclear Information System (INIS)

    The phosphorus-uranium mineralization of the Itataia deposit is unique. Most of the uranium is contained in the collophane (uraniferous fluorapatite) obtained through the transformation of metamorphic apatite of the country rocks (Itataia Group). A graphite-rich pelitic metasediment origin for the uranium and phosphorus is suggested. Initially, the rocks were subjected to isoclinal and homoclinal folding associated with overthrusting and regional metamorphism of the high amphibolite facies with some migmatization. The mineralization is epigenetic and dominated by sodium-rich metasomatic fluids. The mobilized quartz-feldspathic migmatites were transformed into collophane-albite-episyenites. The considerably thick markbles provided the conditions favoring an increase in the fluids pH which caused the precipitation of collophane concentrated in pockets and stockworks. (Author)

  8. Precise timing of the Early Paleozoic metamorphism and thrust deformation in the Eastern Kunlun Orogen

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    In Dulan County, Qinghai Province NW China, the arc volcanic sequences in the northern side of the Central Fault of the East Kunlun were metamorphosed progressively from upper greenschist facies in the south to epidote-amphibolite facies in the north. High-angle thrust deforma-tion was developed synchronously with the peak metamor-phim and superimposed with later low-angle striking-slip deformation. Zircon U-Pb dating yields a concordant age of (448 ± 4) Ma for the metavolcanics. Syn-kinematic horn-blende and muscovite separated from the high-angle thrust-ing belt give 40Ar-39Ar plateau age of (427 ± 4) Ma and 408 Ma, respectively. These results precisely constrain the timing of the closure of early Paleozoic volcanic basin (Proto-Tethys) over the eastern portion of the East Kunlun Orogen, and the thrust tectonic slice had a cool rate of ca. 9℃/Ma.

  9. Rb-Sr geochronology and geochemical characteristics of mafic dikes in the Nova Lacerda and Conquista D'Oeste region, Mato Grosso, SW Amazonian Craton; Geocronologia Rb-Sr e caracteristicas geoquimicas dos diques maficos da regiao de Nova Lacerda e Conquista D'Oeste (MT), porcao sudoeste do Craton Amazonico

    Energy Technology Data Exchange (ETDEWEB)

    Costa, Paulo Cesar Correa da; Matos, Joao Batista de [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Dept. de Recursos Minerais; Grupo de Pesquisas em Evolucao Crustal e Metalogenia Guapore, Cuiaba, MT (Brazil)], e-mail: pccorrea@ufmt.br, e-mail: jmatos@cpd.ufmt.br; Girardi, Vicente Antonio Vitorio [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica], e-mail: girardi@usp.br; Ruiz, Amarildo Salina [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Dept. de Geologia Geral; Grupo de Pesquisas em Evolucao Crustal e Metalogenia Guapore, Cuiaba, MT (Brazil)], e-mail: asruiz@rc.unesp.br

    2009-07-01

    In the Nova Lacerda and Conquista D'Oeste regions, Mato Grosso State, SW part of the Amazonian Craton, mafic dikes trending NNW intrude the Nova Lacerda Granite (1462{+-}12 Ma), within the Jauru Domain, in the Rondonia-San Ignacio Province (1.55 - 1.3 Ga). The mafic swarm comprises diabases, metadiabases and amphibolites. Metadiabases originated from uralitization of diabases. These rocks have tholeiitic affinity and predominant basaltic composition. Some samples are andesi-basalts. The ages of diabases and metabasites are 1380 {+-} 32 Ma and 1330 {+-} 120 Ma respectively. Geochemical data indicate that the compositional variation of diabases and metadiadases is due to fractional crystallization of evolved tholeiitic magmas. The origin of the basaltic magmas is related to a heterogeneous mantle source. (author)

  10. Rb-Sr geochronology and geochemical characteristics of mafic dikes in the Nova Lacerda and Conquista D'Oeste region, Mato Grosso, SW Amazonian Craton

    International Nuclear Information System (INIS)

    In the Nova Lacerda and Conquista D'Oeste regions, Mato Grosso State, SW part of the Amazonian Craton, mafic dikes trending NNW intrude the Nova Lacerda Granite (1462±12 Ma), within the Jauru Domain, in the Rondonia-San Ignacio Province (1.55 - 1.3 Ga). The mafic swarm comprises diabases, metadiabases and amphibolites. Metadiabases originated from uralitization of diabases. These rocks have tholeiitic affinity and predominant basaltic composition. Some samples are andesi-basalts. The ages of diabases and metabasites are 1380 ± 32 Ma and 1330 ± 120 Ma respectively. Geochemical data indicate that the compositional variation of diabases and metadiadases is due to fractional crystallization of evolved tholeiitic magmas. The origin of the basaltic magmas is related to a heterogeneous mantle source. (author)

  11. Significance of Airborne Gamma-ray spectrometric data of Umm bisilla Area, central Eastern Desert, Egypt

    International Nuclear Information System (INIS)

    Umm bisilla area, located in the Central Eastern Desert of Egypt, consists mainly of basement rocks. The present investigation of the airborne spectrometric data is to define the meaningful anomalies from the raw data by applying the significance factor techniques, by calculating the significant radioactive provinces. Determination of the gross structural pattern and broad variations in composition of the crystalline basement, to define the relationships between the tectonic features of the area as interpreted from aeromagnetic data, with the significant anomalies revealed from spectrometric data was carried out through the application of different magnetic techniques. Five significant uraniferous zones were detected associated with Umm Bisilla granite, amphibolite, and grey granite. The intersection of the structural lineaments interpreted from aeromagnetic data illustrated good correlation with the significant uranium anomalous zones interpreted from spectrometric data, and indicated that the concentration is structurally. 15 figs

  12. Geochemistry and source of iron-formation from Guanhaes group, Guanhaes district, Minas Gerais, Brazil

    International Nuclear Information System (INIS)

    The Guanhaes district is underlain by metavolcano-sedimentary rocks of the Guanhaes Group, emplaced over an older Archean basement and intruded by granitic bodies. The Guanhaes Group is composed of pelitic, mafic and ultramafic schists at the base; silicate and carbonate facies iron-formation, calcarious schists, calcsilicates rocks and quartzites at the median portion and para-gneisses (meta-graywacks) at the top. Geochemistry of iron-formation suggest a hydrothermal affinity comparable to the hydrothermal sediments flanking East Pacific Rise. Paragenetic studies indicates that the rocks were submited to two metamorphic processes: one of regional character (high-amphibolite facies) and one of themal character (pyroxene-hornfels facies). Chemical analysis, as X-ray and optic spectrography, atomic absorption and plasma spectrography are presented. (author)

  13. Hydrothermal alteration at the Roosevelt Hot Springs thermal area, Utah: characterization of rock types and alteration in Getty Oil Company well Utah state 52-21

    Energy Technology Data Exchange (ETDEWEB)

    Ballantyne, G.H.

    1978-11-01

    Getty Oil Company well 52-21 in the Roosevelt Hot Springs thermal area was drilled to 7500 feet in predominantly upper amphibolite facies metamorphic rocks. All lithologies in the drill hole are pervasively but weakly altered: the alteration assemblage is chlorite + sericite + clays with occasional traces of calcite, above 2300 feet, and chlorite + sericite + clays + calcite +- epidote below 2500 feet. A zone of increased alteration intensity from approximately 1800 feet to 2300 feet occurs within and adjacent to a dacite dike which cuts the metamorphic rocks. A second zone of stronger alteration extends from 6000 feet to the bottom of the drill hole. The drill hole which is located approximately 5000 feet south of the center of the silica apron known as the Opal Mound was apparently drilled beyond the influence of acid, high-sulfate brines such as have affected the upper portions of drill holes 72-16, 76-1 and University of Utah 1A and 1B.

  14. Petrography and geochemistry of rocks from the sor-rondane mountains, droning Maude land, eastern Antarctica

    International Nuclear Information System (INIS)

    Mamyu rock specimens, were collected from the sor-rondane mountains and Breid Bay area of Drojnning Maud land, eastern Antarctica, during the 2nd Pakistan Antarctic Expedition, 1992-93. Petrography and geochemical studies suggest that the rocks are essentially of igneous origin. The samples dredged from ocean bottom include olivine basalt, amygdaloidal volcanics, dacites and rhyodacites. A majority of these rocks are calc-alkaline and formed by the fraction of olivine, clinopyroxene and plagioclase +- titanomagnetite. Most of these rocks apparently formed in an island arc or continental margin set up. However, volcanics showing ocean floor basalt character are also present. A metamorphosed and deformed basement consisting of amphibolites, calc-silicate rocks and gneisses is intrude by under formed or only slightly deformed granites with a minor arkosic sandstone cover. The granites are chemically distinguished as I-type, originate at deeper crystal level by collisional/subduction related processes during organic environments. (author)

  15. Vesuvianite–wollastonite–grossular-bearing calc-silicate rock near Tatapani, Surguja district, Chhattisgarh

    Indian Academy of Sciences (India)

    S C Patel

    2007-04-01

    This paper reports the occurrence of vesuvianite + wollastonite + grossular + diopside + microcline + quartz assemblage in an enclave of calc-silicate rocks occurring within quartzofeldspathic gneiss near Tatapani in the western part of Chhotanagpur Gneissic Complex. The enclave contains phlogopite-absent and phlogopite-bearing calc-silicate rocks, the latter being much more abundant than the former. The above assemblage occurs in the phlogopite-absent rock. Phlogopite-bearing rock contains the assemblage phlogopite + salite + microcline + plagioclase + quartz. A strong schistosity is developed in both the calc-silicate rocks and the minerals are syntectonic with the major foliation-forming event in the area. The vesuvianite-bearing assemblage is formed by amphibolite facies regional metamorphism of a calcareous protolith at pressure > 4 kbar and XCO2 (fluid) > 0.15.

  16. 南秦岭勉-略地区某些阿尔卑斯型超基性侵入体的高温接触变质作用

    Institute of Scientific and Technical Information of China (English)

    夏林圻

    1975-01-01

    This paper gives a description of high-temperatUre contact metamorphism at the contact of some Alpine-type ultrabasic intrusions in the Mian-Luee region of the southern Qiuling Range. At the contact, because of the metamorphism, volcanic rocks and turfs have changed into amphibolites and pyroxene hornfelses; pelitic phyllites into bfotite-cordierite hornfelses; and limestones into marbles. Hornblende shows a variation in colour from green in the outer aureole to brown at the contact. The original hlgh-temperature contact effects have been obscured by later retrogressive metamorphism and tectonic deformation. In combination with some other experimental data on high-temperature and highpressure diagenesis, the author has come to the conclusion that Alpine-type uitrabasie intrusions in the Mian-Luee region are high-temperature intrusions of liquid magmas.

  17. Water - rock interaction in different rock environments

    International Nuclear Information System (INIS)

    The study assesses the groundwater geochemistry and geological environment of 44 study sites for radioactive waste disposal. Initially, the study sites were divided by rock type into 5 groups: (1) acid - intermediate rocks, (2) mafic - ultramafic rocks, (3) gabbros, amphibolites and gneisses that contain calc-silicate (skarn) rocks, (4) carbonates and (5) sandstones. Separate assessments are made of acid - intermediate plutonic rocks and of a subgroup that comprises migmatites, granite and mica gneiss. These all belong to the group of acid - intermediate rocks. Within the mafic -ultramafic rock group, a subgroup that comprises mafic - ultramafic plutonic rocks, serpentinites, mafic - ultramafic volcanic rocks and volcanic - sedimentary schists is also evaluated separately. Bedrock groundwaters are classified by their concentration of total dissolved solids as fresh, brackish, saline, strongly saline and brine-class groundwaters. (75 refs., 24 figs., 3 tabs.)

  18. A study based on trace elements of differentiated metabasic rocks from the Machado-MG region

    International Nuclear Information System (INIS)

    Large metabasic intrusive bodies occur in the Precambrian gneiss-migmatite basement around Poco Fundo - Campestre - Machado Triangle, south Minas Gerais. Separate occurrences consisting in each case of pyroxenite, metagabbro and amphibolite seem to be related to each other as is evident from their mineralogy, texture and trends of trace element concentration when plotted against their mg number (Mg/MgO + FeO mol.). Furthermore, their trace elements indicate that these rocks belong to a differentiated sequence resulting from separation of pyroxenes in situ. In spite of their high large ion lithophile element contents comparable to calc-alcaline rocks, the metabasic rocks show close resemblance to present-day mid-ocean ridge basalts when these are plotted in relevant variation diagrams. The tectonic implication of the observations is, however, not yet clear. (Author)

  19. Mineral Chemistry and Pressure and Temperature Estimates of Metasedimentary Rocks of the Araxá Group in the Region of Morrinhos, South of Goiás State

    Directory of Open Access Journals (Sweden)

    Guillermo Rafael Beltran Navarro

    2011-08-01

    Full Text Available In the Morrinhos region, Goiás state, Brazil, the rocks of the Araxá Group have mineral assemblages related to themetamorphic peak which occurred in this region and is typical of the amphibolite facies (kyanite zone. These rocks areassociated with the initial stage of the main ductile deformation that occurred in this area. Thermobarometric calculations,including the associations, garnet + plagioclase + biotite + muscovite; garnet + plagioclase +biotite + muscovite + kyanite;garnet + plagioclase + biotite + muscovite + amphibole, with quartz in excess, indicate that the metamorphic peak occurredat temperatures of approximately 610ºC and pressures in the order of 10 kbar. The P-T path is probably clockwise and is inagreement with the tectonic conditions observed in collisional settings.

  20. Rb-Sr geochronology from Barro Alto Complex, Goias: metamorphism evidence of high degree and continental collision around 1300 Ma ago in Central Brazil

    International Nuclear Information System (INIS)

    Rb-Sr geochronologic investigation carried out on rocks from the Barro Alto Complex, Goias, yielded iso chronic ages of 1266 +- 17 Ma, for felsic rocks from the granulite belt and 1330 +- 67 Ma, for gneisses belonging to the Juscelandia Sequence. Rb-Sr isotope measurements suggest that Barro Alto rocks have undergone an important metamorphic event during middle Proterozoic times, around 1300 Ma ago. During that event, volcanic and sedimentary rocks of Juscelandia Sequence, as well as the underlying gabbros-anorthosite layered complex, underwent deformation and recrystallization under amphibolite facies conditions. Deformation and metamorphism took place during the collision of two continental blocks, which resulted in a southeastward directed thrust complex, allowing the exposure of granulite slices from the middle-lower crust of the overthrusted block. (author)

  1. Archean to Paleoproterozoic polymetamorphic history of the Salma eclogite in Kola Peninsula, Russia

    Science.gov (United States)

    Imayama, Takeshi; Oh, Chang-Whan; Park, Chan-Soo; Yi, Keewook; Jung, Haemyeong

    2015-04-01

    One of the most important questions in the Earth Science is when and how plate tectonics operate in the Precambrian time. The tectonic and thermal evolution of the Precambrian eclogite is significant key for understanding the Precambrian geodynamic mechanisms. Eclogites in Kola Peninsula, Russia are some of the oldest eclogites of the world, but there has been much debate about the timing of eclogite-facies metamorphism: Archean (e.g. Volodichev et al. 2004; Mints et al., 2010) or Paleoproterozoic (e.g. Skublob et al., 2011, 2012). The controversy is mainly because of the lack of zircon dating coupled with the formation of garnet and omphacite. In this study, we present geochronological, petrographic, and geochemical data from the Salma eclogites in the Kola Peninsula, Russia to characterize subduction and collision processes in the Precambrian. Microstructural observations, P-T analyses, zircon inclusion analyses, and U-Pb zircon dating revealed multiple metamorphic stages that the Salma eclogite underwent. The amphibolite facies metamorphic event firstly occurred at 2.73-2.72 Ga during Archean. In the Paleoproterozoic period, the Salma eclogites underwent prograde stage of epidote-amphibolite facies metamorphism. The eclogite facies metamorphic event took place under the P-T condition of 16-18 kbar and 740-770 °C at 1.89-1.88 Ga, with a subsequent granulite facies metamorphism during decompression stage from 18 kbar to 9-12 kbar. Finally, later amphibolite facies metamorphism occurred at 8-10 kbar and 590-610 °C on cooling. The Archean metamorphic zircons that contain inclusions of Grt + Am + Bt + Pl + Qtz + Rt are unzoned grains with dark CL, and they are relatively enriched in HREE. In contrast, the 1.89-1.88 Ga sector or concentric zoned zircons with pale-grey CL include inclusions of Grt + Omp + Ca-Cpx + Am + Bt + Qtz + Rt, and they have the flat pattern of HREE due to the amounts of abundant garnet during the eclogite-facies metamorphism. Whole rock

  2. Geology of the Desert Hot Springs-Upper Coachella Valley Area, California (with a selected bibliography of the Coachella Valley, Salton Sea, and vicinity)

    Energy Technology Data Exchange (ETDEWEB)

    Proctor, Richard J.

    1968-01-01

    The Desert Hot Springs area is in the upper Coachella Valley at the junction of three natural geomorphic provinces of California--the Transverse Ranges, the Peninsular Ranges, and the Colorado Desert. The mapped area is about 100 miles east of Los Angeles and lies principally in north central Riverside County. The oldest rocks in the area are Precambrian(?) amphibolitic and migmatized paragneisses of the San Gorgonio igneous-metamorphic (Chuckwalla) complex. They are intruded by Cretaceous diorite porphyry, Cactus Granite, quartz monzonite, intrusive breccia, and basic plutonic rocks. Of probable late Paleozoic age are the metamorphic rocks of the San Jacinto Mountains which form spurs projecting into San Gorgonio Pass and Coachella Valley.

  3. Microstructures, deformation mechanisms and seismic properties of a Palaeoproterozoic shear zone: The Mertz shear zone, East-Antarctica

    Science.gov (United States)

    Lamarque, Gaëlle; Bascou, Jérôme; Maurice, Claire; Cottin, Jean-Yves; Riel, Nicolas; Ménot, René-Pierre

    2016-06-01

    The Mertz shear zone (MSZ) is a lithospheric scale structure that recorded mid-crustal deformation during the 1.7 Ga orogeny. We performed a microstructural and crystallographic preferred orientation (CPO) study of samples from both mylonites and tectonic boudins that constitute relics of the Terre Adélie Craton (TAC). The deformation is highly accommodated in the MSZ by anastomosed shear bands, which become more scattered elsewhere in the TAC. Most of the MSZ amphibolite-facies mylonites display similar CPO, thermal conditions, intensity of deformation and dominant shear strain. Preserved granulite-facies boudins show both coaxial and non-coaxial strains related to the previous 2.45 Ga event. This former deformation is more penetrative and less localized and shows a deformation gradient, later affected by a major phase of recrystallization during retrogression at 2.42 Ga. Both MSZ samples and granulite-facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted CPO of minerals (quartz, feldspar, biotite, amphibole and orthopyroxene). In particular, we highlight the development of an "uncommon" CPO in orthopyroxene from weakly deformed samples characterized by (010)-planes oriented parallel to the foliation plane, [001]-axes parallel to the stretching lineation and clustering of [100]-axes near the Y structural direction. Lastly, we computed the seismic properties of the amphibolite and granulite facies rocks in the MSZ area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the MSZ. Our results reveal that (i) the low content of amphibole and biotite in the rock formations of the TAC, and (ii) the interactions between the CPO of the different mineralogical phases, generate a seismically isotropic crust. Thus, the seismic anisotropy recorded by the seismic stations of the TAC, including the

  4. Collision in the Central Alps: 2. Exhumation of high-pressure fragments

    Science.gov (United States)

    Brouwer, F. M.; Burri, T.; Berger, A.; Engi, M.

    2003-04-01

    In the Central Alps high-pressure metamorphic rocks are confined to but a few tectonic units. In the Adula nappe pressures range from about 12 kbar in the north, to 20 kbar in the south [1]. The Southern Steep Belt (SSB) is a high-strain zone at the contact between rocks deriving from Apulia and Eurasia. The SSB contains a tectonic composite of ortho and paragneisses, with widespread bands and lenses of mafic and ultramafic composition. Many of the mafic fragments are garnet-amphibolites or eclogites, with a highly variable degree of retrogression. Our petrological studies indicate that the HP rocks in the SSB show extensive variation in metamorphic pressure. In mafic fragments, pressures retained by assemblages predating the amphibolite facies overprint range from 8 to 21 kbar, while pressure estimates for some peridotites are >30 kbar. Some HP fragments show evidence of substantial heating during decompression. New Lu-Hf and Sm-Nd geochronology, in conjunction with previously published data, indicates a spread in ages obtained from the high-pressure metamorphic assemblage. Thermal models based on simplified kinematics produce computed PTt histories that resemble those documented in individual HP fragments [2]. The SSB is interpreted to represent an exhumed part of a Tectonic Accretion Channel (TAC, cf. [3]), assembled of numerous, relatively small fragments which reflect a variety of paths. The different residence times and exhumation rates reflect a protracted history of subduction and extrusion, in which the fragments moved independently from their current neighbours. Combination of thermal modelling and field-based studies improve our conceptual thinking on the dynamics of exhumation of high-pressure rocks in a convergent orogen. [1] Heinrich (1986) J. Pet. 27: 123-154 [2] Roselle et al. (2002) Amer. J. Sci. 302: 381-409 [3] Engi et al. (2001) Geology 29: 1143-1146

  5. Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)

    Science.gov (United States)

    Beyssac, O.; Cox, S. C.; Vry, J.; Herman, F.

    2016-04-01

    The Southern Alps orogen of New Zealand results from late Cenozoic convergence between the Indo-Australian and Pacific plates and is one of the most active mountain belts in the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from low-greenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavorable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300 × 20 km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥ 640 °C locally in the direct vicinity of the Alpine Fault to less than 330 °C at the drainage divide 15-20 km southeast of the fault. Thermal field gradients exhibit a degree of similarity from the southernmost to the northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤ 50 °C) higher than those derived by traditional quantitative petrology using garnet-biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef-Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.

  6. The Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean: Further evidence from the Barleik subduction-related metamorphic complex in the West Junggar region, NW China

    Science.gov (United States)

    Liu, Bo; Han, Bao-Fu; Xu, Zhao; Ren, Rong; Zhang, Jin-Rui; Zhou, Jing; Su, Li; Li, Qiu-Li

    2016-06-01

    In this study, we present new evidence from the Barleik subduction-related metamorphic complex in the southern West Junggar region, northwestern China, for the Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean. The Barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic mélange and their protoliths are calc-alkaline andesite and alkali and tholeiitic basalts. The calc-alkaline andesite has a zircon U-Pb age of 502 ± 2 Ma, obtained from magmatic cores of zircon grains, and shares geochemical features similar to the 515-485 Ma intra-oceanic arc magmatic rocks in the West Junggar region. By contrast, the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid-ocean ridge basalts, respectively. Rutile and sodic-calcic amphibole from the amphibolite have a U-Pb age of 502 ± 25 Ma and a 40Ar/39Ar age of ∼504 Ma, respectively, which are in good agreement within errors with a 40Ar/39Ar age of 492 ± 4 Ma for phengite from the blueschist. These metamorphic ages of ∼500 Ma are interpreted to represent the timing of Pacific-type subduction-related metamorphism and are also compatible with ages of the oldest supra-subduction zone ophiolites (531-512 Ma) and intra-oceanic arc plutons (515-485 Ma) in the southern West Junggar region. Being one of the oldest subduction-related metamorphic complexes (509-490 Ma) in the southern Central Asian Orogenic Belt, the Barleik metamorphic complex, together with the oldest arc plutons, definitely indicate the initial intra-oceanic subduction in the southern Paleo-Asian Ocean at least in the Early Cambrian.

  7. K-Ar geochronology of mafic dyke swarms from the meridional part of Sao Francisco craton and implications on tectonic context

    International Nuclear Information System (INIS)

    The southern region of the Sao Francisco Craton is made up of gneissic-granitoid terranes (mainly of amphibolite facies) associated with supracrustals, which can be separed into two crustal provinces, the oldest formed during the Archean (3-2-2.6 Ga.), and the youngest in the Early Proterozoic (2.4-2.0 Ga.). Mafic dyke swarms inject the basement complexes in the area west of Belo Horizonte city, but not the Late proterozoic Bambui sedimentary cover. These dykes show NNW, NW, WNW, NNE and ENE trends and are of anorogenic character. Most dykes are tholeiitic in composition. Metamorphic recrystallization at greenschist to amphibolite facies as well as minor hidrothermal and/or deuteric transformations are characteristics in the majority of the these dykes. About sixty K/Ar determinations have been performed on plagioclases, amphiboles and whole rocks. They are interpretated combining the use of K/Ar diagrams and histogram, and according to the crustal evolution proposed for the craton. The available radiometric data suggest that the main period of mafic intrusions took place in the Early proterozoic as supported by the apparent ages on amphiboles. However, the beginning of the Middle Proterozoic (1.7-1.5 Ga.) probably corresponds to a period of tensional tectonics as well. On the other hand, most ages obtained on plagioclases and whole rocks, can be associated with Late Proterozoic processes of argon gain or loss. The results are tectonicaly associated with crustal rifting of the continental mass. This two radiometric groupings are characteristic for the evolution of the Early proterozoic crustal provine and of the Mid-Proterozoic intracratonic Espinhaco System respectively. The youngest Late Proterozoic apparent ages associated with the reflections of the contemporaneous evolution of the Braziliano marginal mobile belt which is also suggested by the partial resetting of the K/Ar ages of basement rocks within the eastern part of the Sao Francisco Craton. (author)

  8. Re-Os, Rb-Sr, and O isotopic systematics of the Archean Kolar schist belt, Karnataka, India

    Science.gov (United States)

    Walker, R.J.; Shirey, S.B.; Hanson, G.N.; Rajamani, V.; Horan, M.F.

    1989-01-01

    The Re-Os, Rb-Sr, and O isotopic compositions of mafic and ultramafic amphibolites, gold ores, and granitic gneisses of the circa 2700 Ma Kolar schist belt reveal at least two episodes of post-magmatic alteration that affected these systems. The Re-Os isotopic systematics of many of the rocks of the belt indicate that Os was introduced to the area via fluids that carried very radiogenic Os ( 187Os 186Os2.4Ga > 39). The source of the radiogenic Os was likely ancient crust. On an outcrop scale, this alteration is also characterized by relatively minor additions of excess 87Sr and ??18O values higher than magmatic. The Rb-Sr systematics of most of these rocks are consistent with closed-system behavior since a period between 2700 and 2400 Ma ago, indicating that the alteration event likely occurred no later than the early Proterozoic. In contrast to this late Archean or early Proterozoic alteration, samples of several komatiitic amphibolites have very 187Os-depleted compositions, indicating that open-system behavior also occurred at a much later time. This alteration may have been caused by surficial weathering or the interaction of the rocks with fluids bearing unradiogenic Os. Results suggest that the Re-Os system may have only limited utility for geochronologic applications in regions for which post-crystallization noble metal mineralization is evident (e.g., gold ores). In such regions, however, the system may have an important application in assessing the timing and the ultimate sources of noble metal additions. ?? 1989.

  9. METALLOGENY OF SOUTH TISIA - MOSLAVAČKA MT., PSUNJ, PAPUK AND KRNDIJA

    Directory of Open Access Journals (Sweden)

    Ivan Jurković

    2013-12-01

    Full Text Available Core of the Moslavačka Mt. is built of S-type granite and granodiorite, wrapped up in migmatites. Outer zone is represented with metamorphites of amphibolite facies. The Moslavačka Mt. is a product of regional metamorphism on Lower Paleozoic pelito-psammitic protholite. Numerous quarries exploited granite, amphibolite, gabbro, orthogneiss. Stronger concentrations of sillimanite, andalusite, cordierite, garnet, and tourmaline were observed in different types of rocks. The pegmatite phase generated veins and irregular boddies of metasomatic-injected pegmatites with garnet and tourmaline. Hydrothermal quartz veinlets with sulphides have only mineralogical significance. Migmatites and S-granites form central parts of the Slavonian Mts. Dating gave age of 314-333 Ma (Namurian-Westphalian. Metaclastites of lower metamorphic sequences contain palynomorphs from Silurian to the Lower Carboniferous. Graphitite deposits generated by regional metamorphism were exploited in the mines Brusnik, Sivornica and Brezovo Polje in the Psunj Mt. and in the mine Kapitol in the Papuk Mt. Numerous metasomatic injected pegmatites were exploited on the locations Veliki and Mali Debeljak, Točak, Bilo, Lom and quartz veins at Zavlaka and Motičina Gornja. Postcollisional I-granites gave a small volastonite skarn deposit in the Kiseljevački brook. Mineralogical occurrences of hematite, pyrite, asbestos and copper are genetically bounded with diabases. Talc-chlorite schists were exploited in the mine Koprivna (Psunj. Longlasting investigations of sedimentary uranium occurrences were negative. The best results were obtained in the Kaptol, Cipalovac and Ninkovac brooks. In the alluvium of numerous brooks more significant quantities of gold, monazite, rutile, zircon and scheelite were found (the paper is published in Croatian.

  10. Geochemistry and detrital zircon U-Pb and Hf isotopes of the paragneiss suite from the Quanji massif, SE Tarim Craton: Implications for Paleoproterozoic tectonics in NW China

    Science.gov (United States)

    Zhang, Lu; Wang, Qinyan; Chen, Nengsong; Sun, Min; Santosh, M.; Ba, Jin

    2014-12-01

    The Delingha paragneiss suite in the Quanji massif, southeastern Tarim Craton, is composed of mica schist, paragneiss, leptynite and quartzite, similar to the 'khondalite suites' described from elsewhere in the world. The mica schist is rich in Al2O3 (up to ∼26 wt%) and contains graphite and diagnostic minerals including sillimanite and garnet, with metamorphism under amphibolite-facies to locally granulite-facies conditions as manifested by association with amphibolite and granulite. The detrital zircon U-Pb ages and geochemical data indicate that the protolith materials of the Delingha paragneiss suite were mainly sourced from 2.20 to 2.45 Ga granites, felsic volcanic rocks and TTG, and were deposited at 2.17-1.92 Ga. The detrital zircon Hf and whole-rock Nd isotopes document important crustal growth at ∼2.5-2.7 Ga. The detrital zircon age spectra, the whole rock Nd and zircon Hf model ages, the low-maturity of the protolith, and short-distance transportation suggest that the detritus were derived from the underlying Delingha Complex and the lower Dakendaban sub-Group. The timing of magmatic activities in the source region, the depositional age and metamorphic histories of the Delingha paragneiss suite are all comparable to those recorded in the khondalite belt along northern margin of the Ordos Block in the North China Craton. Our study shows that the 2.2-2.45 Ga magmatic rocks were generated in arc or active continental margin settings, suggesting a prolonged subduction and accretion history prior to final amalgamation (∼2.5-1.8 Ga) to form the unified North China Craton and the assembly of the Tarim Craton in NW China.

  11. Metagabbro associated with the shear zone on Prins Karls Forland (Svalbard, Arctic)

    Science.gov (United States)

    Maraszewska, Maria; Manecki, Maciej; Czerny, Jerzy; Schneider, David; Myhre, Per Inge; Faehnrich, Karol; Barnes, Christopher

    2016-04-01

    Prins Karls Forland (PKF) is a N-S elongated island situated west of Spitsbergen in the Svalbard archipelago, High Arctic. The northern part of the island is dominated by siliciclastic metasediments regionally metamorphosed to greenshist facies assemblages during one distinct stage of tectonism. Amphibolite facies garnet-mica schists, mica schists, quartzites and carbonate-silicate rocks exhibiting evidence of at least two distinct, strong deformation episodes (including mylonitization) locally outcrop on the east coast of PKF, termed the Pinkie Unit. A ~1 km wide shear zone containing ductile to brittle structures and distinct outcrops of greenstones (metagabbros and greenschists), associated with magnetite ore, separates these two contrasting tectonic units. Ten samples of greenstones were collected on the slopes of Lauratzonfjellet and Boureefjellet for petrologic and geochemical analyses. Despite intense localized shearing, the metagabbros are undeformed and preserve coarse crystalline, magmatic texture, which is locally poikilitic. The primary magmatic assemblage consists of brown hornblende, plagioclase, biotite and opaque minerals, with accessory apatite and titanite. No relicts of pyroxenes are preserved. Formation of secondary uralite, sericite and chlorite is observed. Metamorphic assemblage consists of actinolite pseudomorhs after hornblende, epidote, and second generation biotite. Blue amphibole is observed in one sample from Boureefjellet; greenschists from Boureefjellet also contain fibrous blue amphibole, as well as garnets, actinolite, epidote and biotite. Some rocks sampled on Boureefjellet are more strongly deformed and exhibit probably two stages of metamorphism: amphibolite facies metamorphism resulting in blue amphibole-garnet assemblage followed by greenschist facies metamorphism resulting in actinolite-epidote-biotite paragenesis. Parallel and overlapping patterns on chondrite-normalized REE diagrams and spider diagrams indicate that these

  12. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

    Science.gov (United States)

    Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

    2006-01-01

    Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ???25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Lu??liang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

  13. Late Paleozoic evolution of the South Tien Shan: Insights from P-T estimates and allanite geochronology on retrogressed eclogites (Chatkal range, Kyrgyzstan)

    Science.gov (United States)

    Loury, Chloé; Rolland, Yann; Cenki-Tok, Bénédicte; Lanari, Pierre; Guillot, Stéphane

    2016-05-01

    In the South Tien Shan range (Kyrgyzstan), the Late Paleozoic geodynamic evolution remains debated especially to the west of the Talas-Fergana fault (TFF) fault where suture-related high-pressure (HP) rocks are scarce. We provide new petrological and geochronological data on garnet amphibolites from the Chatkal range, to the west of the TFF, northwest of the South Tien Shan suture. These rocks are retrogressed eclogites. We used a micro-mapping approach combined with forward modeling and empirical thermobarometry to decipher the P-T path of these amphibolitized eclogites. The metamorphic peak conditions culminated at 490 ± 50 °C and 18.5 ± 2 kbar and were followed by higher temperature retrogression (∼560 °C at 11-7 kbar). In order to constrain the age of the HP stage, we dated allanite crystals texturally coeval to the HP mineral assemblage. Allanite grains dated in situ with a U-Pb LA-ICPMS methodology yield an age of 301 ± 15 Ma. Compared with previously published data for the east of the TFF, these P-T constraints allow improving the understanding of the Late Paleozoic geodynamic evolution of the South Tien Shan. To the east of TFF, the Turkestan Ocean closed around 320 Ma with the collision of the Tarim Craton with the Kazakh microcontinent. To the west of TFF, the Turkestan Ocean closed around 300 Ma, when the Alai block collided with the Kazakh microcontinent. This later collision involved nappe-stacking and intense subvertical folding in the western South Tien Shan. This complex folding explains the S-shape of the suture to the west of the TFF that cannot be observed in the eastern part. These new data allow us to propose a distinct tectonic evolution of the two sides of the TFF, which suggests that this fault was a major transform fault before being a strike-slip intra-continental fault.

  14. Levels of potassium, uranium, thorium and rate of radiogenic heat production in the bedrock adjacent to Camamu and Almada sedimentary basins, Bahia, Brazil

    International Nuclear Information System (INIS)

    The bedrock adjacent to Camamu and Almada sedimentary basins is characterized mainly by rocks of granulite and amphibolite facies, with archaean and paleoproterozoic ages, which belong to orogen Itabuna-Salvador-Curaca. The units in major proportion in this context are the metatonalites associated with basic and metamonzonites belonging to Itabuna belt. In smaller area occur the Teolandia granite and the Moenda granodiorite associated with the Ipiau band amphibolites, the charnockites and charnoenderbites of Jequie bloc, the neoproterozoic sienites and the mafic dikes. The K, U and Th contents of the rocks vary from 0,02 to 6,33% for K, from -3. The coverage areas of such lithologies are, however, small compared with that of the metatonalitic granulites, metamonzonitic granulites and sienites where the rates vary from 0,10 to 1,44 μW m-3, 0,23 to 5,55 μW m-3 and 0,60 to 2,24 μW m-3, respectively. In this case, the heat production rates vary from 0,10 to 1,44 μW m-3. The basic granulites have the smaller rates, from 0,06 to 0,36 μW m-3. The observation of the lithologies in the margins of the two basins suggest that, in the bedrock under the younger sediments, may predominate the metatonalites, followed by the metamonzonites, with some significant participation of sienites in the Almada basin. In those lithologies, the volumetric heat production rates, with one standard deviation range, are 0,41 +- 0,30 μW m-3 for metatonalites, 0,71 +- 0,57 μW m-3 for metamonzonites and 1,20 +- 0,51 μW m-3 for sienites. (author)

  15. Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

    Science.gov (United States)

    Babarina, I. I.; Sibelev, O. S.; Stepanova, A. V.

    2014-07-01

    Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.

  16. A Palaeoproterozoic multi-stage hydrothermal alteration system at Nalunaq gold deposit, South Greenland

    Science.gov (United States)

    Bell, Robin-Marie; Kolb, Jochen; Waight, Tod Earle; Bagas, Leon; Thomsen, Tonny B.

    2016-07-01

    Nalunaq is an orogenic, high gold grade deposit situated on the Nanortalik Peninsula, South Greenland. Mineralisation is hosted in shear zone-controlled quartz veins, located in fine- and medium-grained amphibolite. The deposit was the site of Greenland's only operating metalliferous mine until its closure in 2014, having produced 10.67 t of gold. This study uses a combination of field investigation, petrography and U/Pb zircon and titanite geochronology to define a multi-stage hydrothermal alteration system at Nalunaq. A clinopyroxene-plagioclase-garnet(-sulphide) alteration zone (CPGZ) developed in the Nanortalik Peninsula, close to regional peak metamorphism and prior to gold-quartz vein formation. The ca. 1783-1762-Ma gold-quartz veins are hosted in reactivated shear zones with a hydrothermal alteration halo of biotite-arsenopyrite-sericite-actinolite-pyrrhotite(-chlorite-plagioclase-löllingite-tourmaline-titanite), which is best developed in areas of exceptionally high gold grades. Aplite dykes dated to ca. 1762 Ma cross-cut the gold-quartz veins, providing a minimum age for mineralisation. A hydrothermal calcite-titanite alteration assemblage is dated to ca. 1766 Ma; however, this alteration is highly isolated, and as a result, its field relationships are poorly constrained. The hydrothermal alteration and mineralisation is cut by several generations of ca. 1745-Ma biotite granodiorite accompanied by brittle deformation. A ca. 1745-Ma lower greenschist facies hydrothermal epidote-calcite-zoisite alteration assemblage with numerous accessory minerals forms halos surrounding the late-stage fractures. The contrasting hydrothermal alteration styles at Nalunaq indicate a complex history of exhumation from amphibolite facies conditions to lower greenschist facies conditions in an orogenic belt which resembles modern Phanerozoic orogens.

  17. Correlation of metabasic rocks from metamorphic soles of the Dinaridic and the Western Vardar zone ophiolites (Serbia: Three contrasting pressure-temperature-time paths

    Directory of Open Access Journals (Sweden)

    Srećković-Batoćanin Danica

    2012-01-01

    Full Text Available The field, petrological-mineralogical, geochemical and geochronological data of the metamorphic sole rocks recorded beneath the Fruška Gora, Povlen (Tejići, Stolovi and Banjska ophiolites in the Western Vardar Zone (WVZ and beneath the Zlatibor, Bistrica, Sjenički Ozren and Brezovica ophiolites in the Dinaridic ophiolite belt (DOB in Serbia are compared. The focus has been made on metabasic rocks formed in contact with the oceanic crust members: cumulate gabbro and basalts of SSZ-type with E-MORB and OIB-signature and more evolved tholeiitic basalts of MOR-affinity. Amphibole, the major phase formed from the mafic sole components, depending on pressure-temperature conditions exhibits compositional variations. According to mineral assemblages, estimated P-T conditions and ages, the potential P-T paths are given: high pressure - low temperature blueschist facies assemblage (7-9 kbar and ~400°C and <300-350°C and 4-8 kbar, recorded only in the metamorphic sole at the Fruška Gora (WVZ; high pressure - high temperature amphibolite to granulite facies (8-10 kbar and >700-850°C, recorded in both domains, the WVZ (Banjska and the DOB (Bistrica, Sjenički Ozren, Brezovica and medium pressure - medium temperature amphibolite facies assemblages (~3.5-7 kbar and >350-650°C recognized in the WZV (Tejići, Devovići and the DOB (Zlatibor. The peak metamorphic conditions point to depths of the oceanic lithosphere detachment and its initial cooling at 10-30 km, but the ages and tectonic setting of ophiolites remain poorly constrained. The summarized data may be used as an important key in geodynamic evolution of the Mesozoic Tethyan ophiolites. [Projekat Ministarstva nauke Republike Srbije, br. 176019 and br. 176016

  18. Geologic study of Kettle dome, northeast Washington. Final report

    International Nuclear Information System (INIS)

    This geologic study of Kettle dome, northeast Washington, encompasses an area of approximately 800 square miles (2048 sq km). The evaluation of uranium occurrences associated with the igneous and metamorphic rocks of the dome and the determination of the relationship between uranium mineralization and stratigraphic, structural, and metamorphic features of the dome are the principal objectives. Evaluation of the validity of a gneiss dome model is a specific objective. The principal sources of data are detailed geologic mapping, surface radiometric surveys, and chemical analyses of rock samples. Uranium mineralization is directly related to the presence of pegmatite dikes and sills in biotite gneiss and amphibolite. Other characteristics of the uranium occurrences include the associated migmatization and high-grade metamorphism of wallrock adjacent to the pegmatite and the abrupt decrease in uranium mineralization at the pegmatite-gneiss contact. Subtle chemical characteristics found in mineralized pegmatites include: (1) U increase as K2O increases, (2) U decreases as Na2O increases, and (3) U increases as CaO increases at CaO values above 3.8%. The concentration of uranium occurrences in biotite gneiss and amphibolite units results from the preferential intrusion of pegmitites into these well-foliated rocks. Structural zones of weakness along dome margins permit intrusive and migmatitic activity to affect higher structural levels of the dome complex. As a result, uranium mineralization is localized along dome margins. The uranium occurrences in the Kettle dome area are classified as pegmatitic. Sufficient geologic similarities exist between Kettle dome and the Rossing uranium deposit to propose the existence of economic uranium targets within Kettle dome

  19. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: 207Pb-206Pb dating of magnetite, monazite and allanite in the central and northern Nagssugtoqidian orogen, West Greenland

    Directory of Open Access Journals (Sweden)

    Frei, Robert

    2006-12-01

    Full Text Available Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Greenland, have been used to trace their source characteristics and the timing of metamorphism. Analyses of the magnetite define a Pb-Pb isochron age of 1726 ± 7 Ma. The magnetite is metamorphic in origin, and the 1726 Ma age is interpreted as a cooling age through the closing temperature of magnetite at ~600°C. Some of the amphibolites in this study come from the Naternaq supracrustal rocks in the northern Nagssugtoqidian orogen, which host the Naternaq sulphide deposit and may be part ofthe Nordre Strømfjord supracrustal suite, which was deposited at around 1950 Ma ago.Pb-isotopic signatures of magnetite from the Arfersiorfik quartz diorite in the central Nagssugtoqidian orogen are compatible with published whole-rock Pb-isotopic data from this suite; previous work has shown that it is a product of subduction-related calc-alkaline magmatism between 1920 and 1870 Ma. Intrusion of pegmatites occurred at around 1800 Ma in both the central and the northernparts of the orogen. Pegmatite ages have been determined by Pb stepwise leaching analyses of allanite and monazite, and source characteristics of Pb point to an origin of the pegmatites by melting of the surrounding late Archaean and Palaeoproterozoic country rocks. Hydrothermal activity took place after pegmatite emplacement and continued below the closure temperature of magnetite at 1800–1650 Ma. Because of the relatively inert and refractory nature of magnetite, Pb-isotopic measurements from this mineral may be of help to understand the metamorphic evolution of geologicallycomplex terrains.

  20. A new calibration of seismic velocities, anisotropy, fabrics, and elastic moduli of amphibole-rich rocks

    Science.gov (United States)

    Ji, Shaocheng; Shao, Tongbin; Michibayashi, Katsuyoshi; Long, Changxing; Wang, Qian; Kondo, Yosuke; Zhao, Weihua; Wang, Hongcai; Salisbury, Matthew H.

    2013-09-01

    large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase. We have measured P and S wave velocities (Vp and Vs) and anisotropy of 17 amphibole-rich rock samples containing 34-80 vol % amphibole at hydrostatic pressures (P) up to 650 MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean Vp-P and Vs-P coefficients, intrinsic Vp and Vs anisotropy, Poisson's ratios, the logarithmic ratio Rs/p, and elastic moduli of amphibole-rich rocks. The Vp values decrease with increasing SiO2 and Na2O + K2O contents but increase with increasing MgO and CaO contents. The maximum (≤0.38-0.40 km/s) and minimum S wave birefringence values occur generally in the propagation direction parallel to Y and normal to foliation, respectively. Amphibole plays a critical role in the formation of seismic anisotropy, whereas the presence of plagioclase, quartz, pyroxene, and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations (CPOs). The CPO variations cause different anisotropy patterns illustrated in the Flinn diagram of Vp(X)/Vp(Y)-Vp(Y)/Vp(Z) plots. The results make it possible to distinguish, in terms of seismic properties, the amphibolites from other categories of lithology such as granite-granodiorite, diorite, gabbro-diabase, felsic gneiss, mafic gneiss, eclogite, and peridotite within the Earth's crust. Hence, amphibole, aligned by dislocation creep, anisotropic growth, or rigid-body rotation, is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs, which contains rather little phyllosilicates such as mica or chlorite.

  1. Intermediate P/T-type regional metamorphism of the Isua Supracrustal Belt, southern west Greenland: The oldest Pacific-type orogenic belt?

    Science.gov (United States)

    Arai, Tatsuyuki; Omori, Soichi; Komiya, Tsuyoshi; Maruyama, Shigenori

    2015-11-01

    The 3.7-3.8 Ga Isua Supracrustal Belt (ISB), southwest Greenland, might be the oldest accretionary complex on Earth. Regional metamorphism of the ISB has a potential to constrain the tectonothermal history of the Earth during the Eoarchean. Chemical and modal analyses of metabasite in the study area (i.e., the northeast part of the ISB) show that the metamorphic grade increases from greenschist facies in the northern part of the study area to amphibolite facies in the southern part. To determine the precise metamorphic P-T ranges, isochemical phase diagrams of minerals of metabasite were made using Perple_X. A synthesis of the estimated metamorphic P-T ranges of the ISB indicates that both the metamorphic pressure and temperature increase systematically to the south in the study area from 3 kbar and 380 °C to 6 kbar and 560 °C. The monotonous metamorphic P-T change suggests that the northeast part of the ISB preserves regional metamorphism resulting from the subduction of an accretionary complex although the ISB experienced metamorphic overprints during the Neoarchean. Both the presence of the regional metamorphism and an accretionary complex having originating at subduction zone suggest that the ISB may be the oldest Pacific-type orogenic belt. The progressive metamorphism can be considered as a record of intermediate-P/T type geothermal gradient at the subduction zone in the Eoarchean. Intermediate-P/T type geothermal gradient is typical at the current zones of subducting young oceanic crust, such as in the case of the Philippine Sea Plate in the southwest part of Japan. Considering the fact that almost all metamorphisms in the Archean are greenschist-amphibolite facies, the intermediate-P/T type geothermal gradient at the ISB might have been worldwide in the Archean. This would indicate that the subduction of young micro-plates was common because of the vigorous convection of hot mantle in the Archean.

  2. Timing of Deformation in the Central Metasedimentary Belt Boundary Thrust Zone (CMBbtz), southern Ontario, Canada, from Electron Microprobe Dating of Monazite

    Science.gov (United States)

    Markley, M. J.; Dunn, S. R.; Peck, W. H.; Jercinovic, M. J.; Williams, M. L.

    2015-12-01

    In the Grenville Province of Southern Ontario, the Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a crustal-scale tectonic boundary between the older, granulite-facies Central Gneiss Belt to the NW and the younger, amphibolite-facies Central Metasedimentary Belt to the SE. Although there are a range of tectonic models for the CMBbtz, most workers agree it is a major tectonic boundary that accommodated ductile thrusting and crustal shortening during the Ottawan phase of the Grenville Orogeny (~1080-1020 Ma). Some studies suggest that ductile thrusting in the CMBbtz was roughly synchronous with synorogenic extensional collapse below an orogenic lid. Previous geochronological studies also provide evidence of earlier deformation and/or metamorphic events in the CMBbtz, although the relation between deformation in the CMBbtz to the Elzeviran (~1230 Ma) and Shawinigan (~1180 Ma) orogenies is unclear. Our study is the first to report in situ electron microprobe monazite (mnz) dates from amphibolite-grade ortho- and para-gneisses of the CMBbtz. Our results are broadly consistent with other chronometers. We present dates from 132 age-domains within 83 mnz grains in 14 samples. Although our data provide strong evidence for deformation and metamorphism along the length of the CMBbtz during the Ottawan (1080-1020 Ma), we also report two other clusters of ages: 1140-1110 Ma and 1230-1170 Ma. The latter cluster falls between the widely accepted ranges for the Elzeviran and Shawinigan orogenies. In addition, some individual outcrops, particularly those in Killaloe and Minden, show mnz ages spanning over 200 m.y., and the setting and compositions of individual monazite domains allow us to link mnz growth to episodes of garnet growth during multiple events. Together these data indicate an unexpectedly continuous and long-lived period of deformation and metamorphism in the CMBbtz.

  3. Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone

    Science.gov (United States)

    Rollinson, Hugh

    The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5km) lower volcanic formation and a thinner (2km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites - depleted in light REE; low-Ti komatiites - with flat REE patterns; high-Ti komatiitic basalts - with flat REE; low-Ti basalts - depleted in light REE; high-Ti basalts - with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a ``filter'' to komatiite eruption.

  4. Metamorphic Evolution of Garnet-bearing Epidote-Barroisite Schist from the Meratus Complex in South Kalimantan, Indonesia

    Directory of Open Access Journals (Sweden)

    Nugroho Imam Setiawan

    2015-10-01

    Full Text Available This paper presents metamorphic evolution of metamorphic rocks from the Meratus Complex in South Kalimantan, Indonesia. Eight varieties of metamorphic rocks samples from this location, which are garnet-bearing epidote-barroisite schist, epidote-barroisite schist, glaucophane-quartz schist, garnet-muscovite schist, actinolite-talc schist, epidote schist, muscovite schist, and serpentinite, were investigated in detail its petrological and mineralogical characteristics by using polarization microscope and electron probe micro analyzer (EPMA. Furthermore, the pressure-temperature path of garnet-bearing epidote-barroisite schist was estimated by using mineral parageneses, reaction textures, and mineral chemistries to assess the metamorphic history. The primary stage of this rock might be represented by the assemblage of glaucophane + epidote + titanite ± paragonite. The assemblage yields 1.7 - 1.0 GPa in assumed temperature of 300 - 550 °C, which is interpreted as maximum pressure limit of prograde stage. The peak P-T condition estimated on the basis of the equilibrium of garnet rim, barroisite, phengite, epidote, and quartz, yields 547 - 690 °C and 1.1 - 1.5 GPa on the albite epidote amphibolite-facies that correspond to the depth of 38 - 50 km. The retrograde stage was presented by changing mineral compositions of amphiboles from the Si-rich barroisite to the actinolite, which lies near 0.5 GPa at 350 °C. It could be concluded that metamorphic rocks from the Meratus Complex experienced low-temperature and high-pressure conditions (blueschist-facies prior to the peak metamorphism of the epidote amphibolite-facies. The subduction environments in Meratus Complex during Cretaceous should be responsible for this metamorphic condition.

  5. Timing of subduction and exhumation in a subduction channel: Evidence from slab melts from La Corea Mélange (eastern Cuba)

    Science.gov (United States)

    Blanco-Quintero, I. F.; Rojas-Agramonte, Y.; García-Casco, A.; Kröner, A.; Mertz, D. F.; Lázaro, C.; Blanco-Moreno, J.; Renne, P. R.

    2011-11-01

    High pressure igneous rocks (tonalites), generated by partial melting of subducted basaltic rocks accreted to the mantle wedge, are present in the La Corea serpentinite-matrix mélange (eastern Cuba) as centimeter- to meter-sized blocks and as concordant to crosscutting veins within high-pressure parent amphibolite blocks. The slab melts have adakitic signatures, in agreement with formation after partial melting of metabasite. Thermobarometric calculations indicate 620-680 °C and 13-15 kbar during crystallization of tonalites and down to 250-300 °C, 6 kbar during retrogression, indicating counter-clockwise P-T paths (hot subduction-cool exhumation). Free water required for melting of amphibolite at moderate temperature (700-750 °C) and moderate pressure (13-16 kbar) close to the wet basaltic solidus is inferred to have been provided after dehydration of sediments, altered basaltic crust and serpentinite of the subducting Proto-Caribbean lithosphere. Single zircon (SHRIMP) and phengite 40Ar/39Ar age data constrain the P-T-t evolution of the mélange from the timing of crystallization of melts at ~ 110-105 Ma to cooling at ~ 87-84 Ma, ca. 350 °C, ca. 9 kbar. These figures are consistent with subduction of an oblique ridge, shortly before 115 Ma. Furthermore, our data indicate very slow exhumation (ca. 1 mm/yr) in the subduction channel during the oceanic convergence stage (120-70 Ma) until final fast exhumation to the surface occurred at 70-65 Ma during a regional arc-platform collision event.

  6. Inferences on the Mesozoic evolution of the North Aegean from the isotopic record of the Chalkidiki block

    Science.gov (United States)

    Kydonakis, Konstantinos; Brun, Jean-Pierre; Poujol, Marc; Monié, Patrick; Chatzitheodoridis, Elias

    2016-07-01

    The Chalkidiki block is a major domain in the North Aegean that, contrary to other domains in the region, largely escaped thermal perturbations during Tertiary extension. As a result, the Chalkidiki block is an ideal candidate to glean information related to the timing of Mesozoic thermal events using appropriate geochronological techniques. We have undertaken a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study (U-Th-Pb on monazites and U-Pb on zircons) coupled with 40Ar/39Ar dating on nine samples from various structural levels within the thrust system of the Chalkidiki block. The eastern, and structurally lower part of the system revealed a complete isotopic reset of Carboniferous - Early Triassic monazites coeval with partial monazite destruction, REE-mobilisation and formation of apatite-allanite-epidote coronas at ~ 132 Ma, a reaction that is commonly observed in amphibolite-facies rocks. These coronas formed after crystallisation of garnet (i.e., at T > 580 °C) and, in all probability, either close to the peak-temperature conditions (~ 620 °C) on a prograde path or during retrogression between the peak-temperature and the low-temperature boundary of the amphibolite facies. Cooling of these rocks and arrival at mid-crustal levels occurred at 95-100 Ma. By contrast, the western, and structurally uppermost part of the system went through the same event by 120-125 Ma. Further structural considerations with respect to medium-temperature geochronology data imply that syn-metamorphic thrusting must have ceased by early Late Cretaceous. We emphasize that, with the sole exception of the Chalkidiki block, no pre-45 Ma medium-temperature geochronology data are preserved in other North Aegean domains, a feature that is clearly related to the extension-induced thermal perturbation of the region during the Tertiary.

  7. New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

    Science.gov (United States)

    Oliveira Chaves, Alexandre

    2013-09-01

    New evidence supported by petrography (including mineral chemistry), lithogeochemistry, U-Pb geochronology by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and physicochemical study of fluid and melt inclusions by LA-ICP-MS and microthermometry, point to an orogenic setting of Lagoa Real (Bahia-Brazil) involving uraniferous mineralization. Unlike the previous models in which uraniferous albitites represent Na-metasomatised 1.75 Ga anorogenic granitic rocks, it is understood here that they correspond to metamorphosed sodium-rich and quartz-free 1.9 Ga late-orogenic syenitic rocks (Na-metasyenites). These syenitic rocks are rich not only in albite, but also in U-rich titanite (source of uranium). The interpretation of geochemical data points to a petrogenetic connection between alkali-diorite (local amphibolite protolith) and sodic syenite by fractional crystallization through a transalkaline series. This magmatic differentiation occurred either before or during shear processes, which in turn led to albitite and amphibolite formation. The metamorphic reactions, which include intense recrystallization of magmatic minerals, led uraninite to precipitate at 1.87 Ga under Oxidation/Reduction control. A second population of uraninites was also generated by the reactivation of shear zones during the 0.6 Ga Brasiliano Orogeny. The geotectonic implications include the importance of the Orosirian event in the Paramirim Block during paleoproterozoic Săo Francisco Craton edification and the influence of the Brasiliano event in the Paramirim Block during the West-Gondwana assembly processes. The regional microcline-gneiss, whose protolith is a 2.0 Ga syn-collisional potassic granite, represents the albitite host rock. The microcilne-gneiss has no petrogenetic association to the syenite (albitite protolith) in magmatic evolutionary terms.

  8. Genesis of the metamorphic rock from southeastern Lhasa terrane and the Mesozoic-Cenozoic orogenesis%拉萨地体东南部变质岩的成因与中-新生代造山作用

    Institute of Scientific and Technical Information of China (English)

    董昕; 张泽明; 刘峰; 王伟; 于飞; 林彦蒿; 姜洪颖; 贺振宇

    2012-01-01

    temperature (HT) and medium pressure (MP) granulite-facics metamorphism, then amphibolite-facies retrograde metamorphism. The temperature and pressure conditions of the peak granulite-facies metamorphism are 830 ~900t and 0. 9 - 1. 3GPa. And the time of metamorphism is the Late Cretaceous of 89 -81 Ma. Secondly, Bayi belt generally experienced 1ow pressure (LP) amphibolite-faaes metamorphism, of which the time is Eocene of 55 ~49Ma. And the temperature and pressure conditions are 625 ~ 679℃ and 0. 4 ~ 0. 55GPa. Thirdly, Bujiu belt experienced MP amphibolite-facies metamorphism, of which the time is Oligocene of 36 ~26Ma. And the temperature and pressure conditions are 615 ~ 663t and 0. 5 - 0. 8GPa. This study demonstrates that the prctoliths of these metamorphic rocks is mainly composed of the Late Paleozoic sedimentary rocks and the Paleozoic to Cenozoie magmatic rocks. Moreover, the material sources of the metasedimentary rocks have the records of the tectono-thermal events related to the Grenville and Pan-African orogenesis, indicating a tectonic affinity to Gondwana supercontinent. We consider that the Late Cretaceous HT and MP metamorphism related to the Andean-type orogeny derived from the subduction of Neo-Tethyan oceanic lithosphere, the Eocene LP amphibolite-facies metamorphism formed during the collision orogeny between Indian and Eurasian continents and then the deep-subduction slab' s break-off of Neo-Tethyan, and the Oligocene MP amphibolite-facies metamorphism resulted from the crustal thickening caused by the subduction between India and Eurasia continents. Therefore, the high-grade metamorphic rocks located on the southeastern segment of Lhasa terrane not only reveals the middle and lower crust composition, but also the tectonic evolution for the hanging wall of the subduction/collision compound orogenic belt.

  9. Petrology, geochemistry, and metamorphic evolution of meta-sedimentary rocks in the Diancang Shan-Ailao Shan metamorphic complex, Southeastern Tibetan Plateau

    Science.gov (United States)

    Wang, Fang; Liu, Fulai; Liu, Pinghua; Shi, Jianrong; Cai, Jia

    2016-07-01

    Meta-sedimentary rocks are widely distributed within the Diancang Shan-Ailao Shan metamorphic complex in the Southeastern Tibetan Plateau. Detailed geochemical analyses show that all of them have similar geochemical features. They are enriched in light rare-earth elements (LREEs) and depleted in heavy rare-earth elements (HREEs), with moderately negative Eu anomalies (Eu/Eu∗ = 0.55-0.75). Major and trace element compositions for the meta-sedimentary rocks suggest that the protoliths were probably claystone, siltstone, and greywacke and deposited in an active continental margin. Garnet porphyroblasts in meta-sedimentary rocks have distinct compositional zonation from core to rim. The zonation of garnet in St-Ky-Grt-Bt-Ms schist indicates an increasing P-T trend during garnet growth. In contrast, garnets from (Sil)-Grt-Bt paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformations and P-T estimates using conventional geothermobarometers and pseudosection calculations, four metamorphic stages have been determined, including an early prograde metamorphic stage (M1), a peak amphibolite-granulite facies metamorphic stage (M2), a near-isothermal decompression stage (M3), and a late amphibolites-facies retrograde stage (M4). The relic assemblage of Ms + St ± Ky ± Bt ± Kfs + Qz preserved as inclusions in garnet porphyroblasts of the meta-sedimentary rocks belongs to prograde (M1) stage and records P-T conditions of 560-590 °C and 5.5-6.3 kb. Matrix mineral assemblages of Grt + Bt + Ky/Sil + Pl + Qz and Grt + Bt ± Sil + Pl ± Kfs + Qz formed at peak (M2) stage yield P-T conditions of 720-760 °C and 8.0-9.3 kb. M3 is characterized by decompression reactions, dehydration melting of assemblages that include hydrous minerals (e.g., biotite), and partial melting of felsic minerals. The retrograde assemblages is Grt + Bt + Sil + Pl + Qz formed at 650-760 °C and 5.0-7.3 kb. At the amphibolites-facies retrograde (M4) stage, fine

  10. Thermal properties Forsmark. Modelling stage 2.3 Complementary analysis and verification of the thermal bedrock model, stage 2

    International Nuclear Information System (INIS)

    This report present the results of thermal modelling work for the Forsmark area carried out during modelling stage 2.3. The work complements the main modelling efforts carried out during modelling stage 2.2. A revised spatial statistical description of the rock mass thermal conductivity for rock domain RFM045 is the main result of this work. Thermal modelling of domain RFM045 in Forsmark model stage 2.2 gave lower tail percentiles of thermal conductivity that were considered to be conservatively low due to the way amphibolite, the rock type with the lowest thermal conductivity, was modelled. New and previously available borehole data are used as the basis for revised stochastic geological simulations of domain RFM045. By defining two distinct thermal subdomains, these simulations have succeeded in capturing more of the lithological heterogeneity present. The resulting thermal model for rock domain RFM045 is, therefore, considered to be more realistic and reliable than that presented in model stage 2.2. The main conclusions of modelling efforts in model stage 2.3 are: - Thermal modelling indicates a mean thermal conductivity for domain RFM045 at the 5 m scale of 3.56 W/(mK). This is slightly higher than the value of 3.49 W/(mK) derived in model stage 2.2. - The variance decreases and the lower tail percentiles increase as the scale of observation increases from 1 to 5 m. Best estimates of the 0.1 percentile of thermal conductivity for domain RFM045 are 2.24 W/(mK) for the 1 m scale and 2.36 W/(mK) for the 5 m scale. This can be compared with corresponding values for domain RFM029 of 2.30 W/(mK) for the 1 m scale and 2.87 W/(mK)for the 5 m scale. - The reason for the pronounced lower tail in the thermal conductivity distribution for domain RFM045 is the presence of large bodies of the low-conductive amphibolite. - The modelling results for domain RFM029 presented in model stage 2.2 are still applicable. - As temperature increases, the thermal conductivity decreases

  11. Mineralogical Description of the Skarn from Mraconia Valley, Almaj Mountains, Romania

    Science.gov (United States)

    Anason, Maria Angela; Stefan, Marincea; Delia Georgeta, Dumitras

    2014-05-01

    The purpose of this paper is to update knowledge of the investegated area. The research are overlaps of the hidrographic basin of the Mraconia Valley, at north is bounded by the alignment Poiana Mraconia and Lugojistea; at east by the Satului Valley; souther limit is constitutes by the Ponicova Valley and the western limit is Cracul Radului-Cracul Urzicea. The skarn was described for the first time in the 1934 by the A. Streckeisan with the name Catramat Series; this series was a kata-mezonal character, which is the debris of an old canvas with Upper Carbonifer age. The crystalline schist of the Poiana Mraconia Series, are studied by Al. Codarcea, I. Bercia, E. Bercia (1934) and suffered a progressive metamorphism in the amphibolites with alamandine facies and the disten-alamandine-muscovite subfacies, together revealed the metapelithe paragenesis including disten, green hornblende, andezine and alamandine.The primar metamorphism were followed to the regressive metamorphism by the Assyntic orogene and Varisc cycles (I. Bercia, E. Bercia, 1975). Petrografically exceed the amphibolites paragnaice and the micacee paragnaice, with biotite and garnet, associated with the quartz-feldspar gneiss and feldspar quartzite, affected by the arthritic migmatization. Fine grain, sharp sistuozity, the muscovite are frequent and the pegmatite absence leading to the Poiana Mraconia crystalline differentiation by the Ielova crystalline, bud both are included in the Almaj complex. Mineralogical and petrographic study of the Mraconia skarns serves in predicting the relations between the magmatite form the upper basin of the valley (with the mineralization of the W and Mo), and the adjacent formations. This paper wants to emphasize the next petrographic types following: 1. the micacee paragnais are characterized by the existing of the quartz, plagioclase, microcline, muscovite, biotite, chlorite and epidote; 2. The micacee paragnais with garnet are definite by the paragenisis: quartz

  12. Continental Collision Zones are Primary Sites of net Continental Crustal Growth: Evidence From the Linzizong Volcanic Succession in Southern Tibet

    Science.gov (United States)

    Niu, Y.; Mo, X.; Dong, G.; Zhao, Z.; Hou, Z.; Zhou, S.; Ke, S.

    2007-12-01

    The Linzizong volcanics (ca. 65-45Ma) and the coeval batholiths (ca. 60-40Ma) of andesitic-to-rhyolitic composition are magmatic response to the India-Asia continental collision that began at ca. 70-65Ma and ended at ca. 45-40Ma with convergence continuing to present [1,2]. These syncollisional magmatic rocks are widely distributed along much of the >1500km long Gangdese Belt immediately north of the India-Asia suture (Yarlung-Zangbo) in southern Tibet [2-6]. Our study of the Linzizong volcanics from the Linzhou Basin (near Lhasa) encourages the proposal that syncollisional granitoid magmatism may in fact account for much of the net contribution to continental crust growth. The Linzizong volcanics in the Linzhou Basin show a first-order temporal change from the lower andesitic formation (64.4-60.6Ma), to the middle dacitic formation (ca. 54Ma), and to the upper rhyolitic formation (48.7-43.9Ma). The three formations show no systematic but overlapping Nd-Sr isotope variations. The isotopically depleted samples with ɛNd(t)>0 (up to + 8) indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India- Asia collision is the remaining part of the Tethyan ocean crust [6]. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanics (also the coeval batholiths) with inherited mantle isotopic signatures [6]. Ilmenite is abundant in amphibolite [7], and partial melting of amphibolite with ilmenite as a residual phase accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the elevated abundances of Ba, Rb, Th, U, K and Pb in the melt [8,9]. These give the syncolissional Linzizong volcanics characteristic "arc-like" geochemical signature. Residual

  13. Zircon U-Pb dating of metabasic rocks in the Zanhuang metamorphic complex and its geological significance%赞皇变基性岩中锆石的U-Pb定年及其地质意义

    Institute of Scientific and Technical Information of China (English)

    肖玲玲; 王国栋

    2011-01-01

    赞皇变质杂岩区位于阜平杂岩南部,地处华北克拉通中部造山带的中段,和中部带北段杂岩一样,是洞悉华北克拉通前寒武纪基底构造演化历史的一个重要窗口.研究区变基性岩可分为斜长角闪岩和角闪斜长片麻岩两种,二者均以似层状方式产于黑云斜长片麻岩或长英质片麻岩中,斜长角闪岩亦可呈透镜状,二者后期与围岩一起共同经历了高角闪岩相变质作用.斜长角闪岩和角闪斜长片麻岩中普遍存在变质锆石,锆石SIMS U-Pb原位定年获得的207Pb/206Pb谐和年龄表明,赞皇变基性岩记录了约18.5亿年(1842±21 Ma、1868±29 Ma)的一次较为广泛的变质作用事件,结合赞皇变质杂岩的构造背景和变质演化特征,推测该期变质作用事件与古元古代末期华北克拉通东部陆块和西部陆块间的俯冲碰撞作用有关.%The Zanhuang metamorphic complex is significant for understanding the Precambrian tectonics and evolution of the North China Craton. Metabasic rocks composed of amphibolite and amphibole-bearing plagio-clase gneiss as thin layers or lenses are widely distributed within felsic gneisses and biotite-plagioclase gneisses. These metabasic rocks together with country rocks underwent high-amphibolite facies metamorphism, and abundant metamorphic zircons were formed during this thermal event. SIMS U-Pb chronological analysis indicates that metamorphic zircons record peak high-amphibolite facies metamorphism at 1 868~1 842 Ma. These Paleo-proterozoic metamorphic ages are largely in accordance with metamorphic ages of ~ 1 850 Ma produced from high-pressure granulites in the northern segment of the Trans-North China Orogen, indicating that a significant subduction-collision event did occur in the Trans-North China Orogen between the eastern block and the western block during Paleoproterozoic ( ~ 1 850 Ma).

  14. Thermal properties Forsmark. Modelling stage 2.3 Complementary analysis and verification of the thermal bedrock model, stage 2.

    Energy Technology Data Exchange (ETDEWEB)

    Sundberg, Jan; Wrafter, John; Laendell, Maerta (Geo Innova AB (Sweden)); Back, Paer-Erik; Rosen, Lars (Sweco AB (Sweden))

    2008-11-15

    This report present the results of thermal modelling work for the Forsmark area carried out during modelling stage 2.3. The work complements the main modelling efforts carried out during modelling stage 2.2. A revised spatial statistical description of the rock mass thermal conductivity for rock domain RFM045 is the main result of this work. Thermal modelling of domain RFM045 in Forsmark model stage 2.2 gave lower tail percentiles of thermal conductivity that were considered to be conservatively low due to the way amphibolite, the rock type with the lowest thermal conductivity, was modelled. New and previously available borehole data are used as the basis for revised stochastic geological simulations of domain RFM045. By defining two distinct thermal subdomains, these simulations have succeeded in capturing more of the lithological heterogeneity present. The resulting thermal model for rock domain RFM045 is, therefore, considered to be more realistic and reliable than that presented in model stage 2.2. The main conclusions of modelling efforts in model stage 2.3 are: - Thermal modelling indicates a mean thermal conductivity for domain RFM045 at the 5 m scale of 3.56 W/(mK). This is slightly higher than the value of 3.49 W/(mK) derived in model stage 2.2. - The variance decreases and the lower tail percentiles increase as the scale of observation increases from 1 to 5 m. Best estimates of the 0.1 percentile of thermal conductivity for domain RFM045 are 2.24 W/(mK) for the 1 m scale and 2.36 W/(mK) for the 5 m scale. This can be compared with corresponding values for domain RFM029 of 2.30 W/(mK) for the 1 m scale and 2.87 W/(mK)for the 5 m scale. - The reason for the pronounced lower tail in the thermal conductivity distribution for domain RFM045 is the presence of large bodies of the low-conductive amphibolite. - The modelling results for domain RFM029 presented in model stage 2.2 are still applicable. - As temperature increases, the thermal conductivity decreases

  15. Comparing Carbon and Strontium Isotope Chemostratigraphy against U-Pb Detrital Zircon Analysis in Dating Marbles of the Uppermost Allochthon in North Norway

    Science.gov (United States)

    Verellen, Devon; Yaw Agyei-Dwarko, Nana; Steltenpohl, Mark; Andresen, Arild

    2015-04-01

    The basal parts of the Uppermost Allochthon between latitudes N67oN (Fauske) and N68.45oN (Ofoten) comprise a thick sequence of platformal marbles that overlie a fragmented ophiolite complex. This basement-cover package can be traced discontinuously from Ofoten more than 150 kilometers northward to connect with the Lyngen ophiolite, the largest ophiolite complex in Scandinavia. U-Pb zircon geochronology documents an Early Ordovician age (469 ±5 Ma) for magmatic crystallization of the Lyngen ophiolite and for the Ofoten mafic complex (474 ±0.7 Ma). A regional unconformity separates the Lyngen ophiolite from overlying rocks of the Balsfjord Group. Halysitid corals from the Balsfjord Group constrain an Upper Llandoverian age for the unconformity. In Ofoten, metasedimentary rocks of the Evenes Group nonconformably overlie the mafic complex and are lithologically correlated to parts of the Balsfjord Group. The basal unit of the Evenes Group, the Elvenes Conglomerate, contains clasts of plutonic igneous rocks clearly derived from the underlying mafic complex (Lillevik dike complex). Suites of multiple phases of felsic intrusions occur within overlying (Bogen/Niingen nappes) and underlying (Narvik nappe) allochthons but are absent in the Evenes Group. Carbon and strontium isotopes reported on these amphibolite-facies marbles have been interpreted to place chemostratigraphic ages that range from Neoproterozoic to Silurian, requiring the placement of hypothetical thrusts and normal faults to explain their vertical stacking although no faults are yet recognized based on field and structural studies. We present LA-ICPMS U-Pb isotope data on detrital zircons from a siliciclastic layer within one of the carbonate units of the Evenes Group that had previously been assigned a Neoproterozoic chemostratigraphic apparent age. Twenty-seven percent of the ages are younger than 600 Ma and define a prominent 470 Ma age population with the 5 youngest ones giving a concordia age of 460

  16. Paleoproterozoic igneous and metamorphic events in the Hongcheon area, southern margin of the Northern Gyeonggi Massif in the Korean Peninsula, and their links to the Paleoproterozoic collision in the North China Craton

    Science.gov (United States)

    Oh, Chang W.; Lee, Byung C.; Yengkhom, Kesorjit S.; Yi, Sang B.

    2014-05-01

    The Hongcheon area is located at the northern part of Gyeonggi Massif (GM) in the Korean Peninsula. The Hongcheon area is composed of the Paleoproterozoic Yongduri Gneiss Complex (YGC), Euiam Group (EG) and Euiam Gneiss Complex (EGC). Quartz-feldspathic gneisses in the northeastern part of the YGC record M2 peak metamorphic conditions of 790-840°C and 7.2-8.9 kbar, whereas granitic and garnet gneisses in the western part of the YGC record peak metamorphic conditions of 690-720°C and 6.1-6.9 kbar, and 640-660°C and 5.0-5.4 kbar, respectively. The M2 metamorphic conditions represent a regional low-P/T metamorphic event in which metamorphic grade increased towards east. SHRIMP zircon U-Pb age dating indicates that the M2 metamorphism occurred at ca. 1867-1883 Ma. The presence of relict kyanite in the gneisses within the YGC suggests that the M1 intermediate-P/T metamorphism (ca. 1925 Ma) occurred prior to the low-P/T metamorphic event. The YGC also records M3 metamorphic event related to Permo-Triassic continental collision between the North and South China Craton. Whole-rock geochemistry indicates that augen gneisses in the EGC were originally post-collision granitoids, and that amphibolites within these gneisses were originally within-plate mafic intrusions. These augen gneisses and amphibolites were emplaced between ca. 1864 and 1885 Ma, and metamorphosed during the Permo-Triassic event (ca. 246 and 265 Ma). The similarity in age between the Paleoproterozoic intrusion and the M2 low-P/T metamorphism indicates that the M2 metamorphism also occurred in a post-collision tectonic setting. The M1 intermediate-P/T metamorphism and post-collision events in the study area can be correlated to the 1.91-1.93 Ga collision related metamorphism and 1.84-1.88 Ga post-collision events in the North Korea and the Jiao-Liao-Ji collision belt in the North China Craton.

  17. Finite lattice distortion patterns in plastically deformed zircon grains

    Directory of Open Access Journals (Sweden)

    E. Kovaleva

    2014-07-01

    Full Text Available This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, Eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea-Verbano Zone (IVZ, Southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are: granitic orthogneisses, meta-lamprophyres and paragneisses, all of which are highly deformed. The investigated zircon grains ranging from 10 to 50 microns were studied in situ using a combination of scanning electron microscope (SEM techniques, including secondary electron (SE, backscattered electron (BSE, forward scattered electron (FSE, cathodoluminescence (CL imaging, and crystallographic orientation mapping by electron backscatter diffraction analysis (EBSD, as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: Type (I is defined by gradual bending of the zircon lattice with orientation changes of about 0.6° to 1.4° per μm without subgrain boundary formation. Type (II represents local gradual bending of the crystal lattice coupled with the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 7° to 40° within single grains. Type (III is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour in 2-D sections. The cumulative orientation variation within a single grain ranges from 3° to 10°. Types (I and (II predominate in granulite facies rocks, whereas type (III is restricted to the amphibolite facies

  18. Magmatism and metamorphism at the sheeted dyke-gabbro transition zone: new insight from beerbachite from ODP/IODP Hole 1256D and Oman ophiolite

    Science.gov (United States)

    Python, Marie; Abily, Bénédicte; France, Lydéric

    2014-05-01

    During IODP Expedition 335, two-pyroxenes bearing granulites (beerbachites) were extensively recovered as drilling cuttings at the gabbro-sheeted dyke transition zone of ODP Hole 1256D (East Pacific Rise, 6°44.163'N, 91°56.061'W). This lithology results from high-temperature metamorphism of previously hydrothermally altered diabases, basalts and/or gabbros; the heat source likely stems from the melt lens located at the top of the magmatic chambers imaged along present-day fast-spreading ridges. This lithology, associated with gabbroic bodies, characterises the transition zone between the sheeted dyke complex and the uppermost gabbroic section and represents the interface between magmatic and hydrothermal convecting systems in an oceanic crust formed at fast-spreading ridges. Samples acquired during IODP Exp. 335 show a particularly high degree of recrystallisation and are characterised by the absence of hydrous phases like amphibole, suggesting very high-T metamorphism. The Beerbachites mineral chemical characteristics are rather homogeneous compared to gabbros or dolerite from the sheeted dyke but pyroxenes Mg#, Ti, Al and Cr contents as well as the anorthite content of plagioclase are closer to gabbro than dolerite. This similarity may be explained by two hypothesis: either beerbachites in Hole 1256D are metamorphosed gabbros, or they underwent a melt-rock reaction process with the gabbros parental magma and were re-equilibrated at high temperature until their mineral composition become similar to that of gabbros. The gabbro-sheeted dyke transition zone in the Oman ophiolite is also outlined by the presence of high grade metamorphic rocks. Fine grained granulites and amphibolites that may be derived from the transformation of altered sheeted dyke diabases are in direct contact with fresh gabbroic and troctolitic bodies which are themselves cross-cut by dolerite dykes. The observation of textures show that high-T recrystallisation occurred in the fine grained

  19. U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

    Science.gov (United States)

    Dusel-Bacon, C.; Wooden, J.L.; Hopkins, M.J.

    2004-01-01

    New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376-353 Ma, 378-346 Ma, and 374-358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ?? 3 and 362 ?? 4 Ma) and associated orthoamphibolite (369 ?? 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust

  20. Orogenic gold mineralization at the Chah Bagh deposit, Muteh gold district, Iran

    Science.gov (United States)

    Kouhestani, Hossein; Rashidnejad-Omran, Nematollah; Rastad, Ebrahim; Mohajjel, Mohammad; Goldfarb, Richard J.; Ghaderi, Majid

    2014-09-01

    The Chah Bagh gold deposit, in the Muteh gold district, is located in the central part of the Sanandaj-Sirjan zone (SSZ), Iran. Gold mineralization at Chah Bagh is hosted by a Paleozoic sequence of rocks that is dominated by deformed schist, metarhyolite, and amphibolite that exhibits a greenschist- to lower amphibolite-facies metamorphism. Three deformation events are recognized in the Chah Bagh area, D1, D2, and D3. The major NW-trending (N280-N290) dextral strike-slip shear zone in the area was formed during D2 ductile events. Gold mineralization at Chah Bagh occurred over a prolonged deformation history, but is closely related to alteration, retrograde greenschist-facies assemblages, and ductile and brittle deformation during D2 and D3. The geometry of the Au-bearing quartz veins indicates that they are temporally related to the S2 foliation and therefore to the D2 flattening and shearing. Some veins, however, are spatially and temporally related to D3 brittle normal faults and are brecciated and boudinaged during the associated shear movement. The presence of deformed Au-bearing quartz veins, and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone, indicates continuous mineralization during the D2 and D3 episodes. The Au-hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones. This gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. The epigenetic Au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny. Our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for Chah Bagh deposit, similar to Qolqoleh and Kervian in the northwestern part of the

  1. Permo-Carboniferous granitoids with Jurassic high temperature metamorphism in Central Pontides, Northern Turkey

    Science.gov (United States)

    Gücer, Mehmet Ali; Arslan, Mehmet; Sherlock, Sarah; Heaman, Larry M.

    2016-04-01

    In the northern part of the Central Pontides (N Turkey) there are different metamorphic rocks exposed, notably the Devrekani metamorphic rocks. Here, upper amphibolite-lower granulite facies metamorphic rocks contain predominantly paragneiss, orthogneiss and metacarbonate, and to a lesser extent, amphibolite and quartzite, with cross-cutting aplite, pegmatite and granite veins. This is the first report of these rocks and includes new data on the petrochemistry, geochronology and metamorphic evolution of the Devrekani orthogneisses from the Central Pontides. The orthogneisses show five different mineral parageneses with the characteristic mineral assemblage quartz + K-feldspar + plagioclase + biotite ± hornblende ± opaque (± ilmenite and ± magnetite), and accessory minerals (zircon, sphene and apatite). These metamorphic rocks exhibit generally granoblastic, lepidogranoblastic and nematolepidogranoblastic with locally migmatitic and relic micrographic textures. They have well-developed centimeter-spaced gneissic banding and display gneissose structure with symmetric, asymmetric and irregular folds. The petrographic features, mineralogical assemblages and weak migmatization reflect high temperature conditions. Thermometric calculations in the orthogneisses indicate metamorphic temperatures reached 744 ± 33 °C. Field relations, petrography and petrochemistry suggest that the orthogneisses have predominantly granodioritic and some granitic protoliths, that show features of I-type, medium to high-potassic calc-alkaline volcanic arc granitoids. The orthogneisses have high contents of LILEs and low contents of HFSEs with negative Nb and Ti anomalies, which are typical of subduction-related magmas. The orthogneisses also show significant LREE enrichment relative to HREE with negative Eu anomalies (EuN/Eu* = 0.33-1.07) with LaN/LuN = 6.98-20.47 values. Based on U-Pb zircon dating data, the protoliths are related to Permo-Carboniferous (316-252 Ma) magmatism. It is

  2. Unraveling an antique subduction process from metamorphic basement around Medellín city, Central Cordillera of Colombian Andes

    Science.gov (United States)

    Bustamante, Andres; Juliani, Caetano

    2011-10-01

    In the surroundings of Caldas and El Retiro cities (Colombia) metamorphic rocks derived from basic and pelitic protoliths comprise the Caldas amphibole schist and the Ancón schist respectively. Subordinated metamorphosed granite bodies (La Miel gneiss) are associated to these units, and The El Retiro amphibolites, migmatites and granulites crops out eastwards of these units, separated by shear zones. The Caldas amphibole schist and the Ancón schist protoliths could have been formed in a distal marine reduced environment and amalgamated to the South American continent in an apparent Triassic subduction event. The El Retiro rocks are akin to a continental basement and possible include impure metasediments of continental margin, whose metamorphism originated granulite facies rocks and migmatites as a result of the anatexis of quartz-feldspathic rocks. The metamorphism was accompanied by intense deformation, which has juxtaposed both migmatites and granulite blocks. Afterward, heat and fluid circulation associated with the emplacement of minor igneous intrusions resulted in intense fluid-rock interaction, variations in the grain size of the minerals and, especially, intense retrograde metamorphic re-equilibrium. Thermobarometric estimations for the Caldas amphibole schist indicate metamorphism in the Barrovian amphibolite facies. The metamorphic path is counter-clockwise, but retrograde evolution could not be precisely defined. The pressures of the metamorphism in these rocks range from 6.3 to 13.5 kbar, with narrow temperature ranging from 550 to 630 °C. For the Ancón schist metapelites the P- T path is also counter-clockwise, with a temperature increase evidenced by the occurrence of sillimanite and the cooling by later kyanite. The progressive metamorphism event occurred at pressures of 7.6-7.2 kbar and temperatures of 645-635 °C for one sample and temperature between 500 and 600 °C under constant pressure of 6 kbar. The temperature estimated for these rocks

  3. Lithological model of the South China crust based on integrated geophysical data

    International Nuclear Information System (INIS)

    The structure and petrology of the earth's crust is critical to understand the growth and evolution of the continents. In this paper, we present the crustal lithological model along the 400-km-long seismic profile between Lianxian, near Hunan Province, and Gangkou Island, near Guangzhou City, South China. This model is based on an integrated geophysical data set including P-wave velocity (VP), S-wave velocity (VS), VP/VS ratio, mass density (ρ) and Lamé impedances (ρλ, ρµ), which are compared to those determined by laboratory measurements on a variety of crustal rock samples. The Bouguer gravity anomaly together with the seismic velocity enables us to constrain density. The heat flow and thermal field allow us to carry out corrections for temperature. Pressure correction is based on depth. We directly compare the property parameters determined from the South China seismic data with laboratory measurements made at the same conditions of pressure and temperature. Inversion of the available data for rock lithology indicates that there are substantial differences in the crustal lithology of the Yangtze and Cathaysia blocks. While the average lithology of the upper crust in both blocks is mainly characterized by granite–granodiorite and biotite gneiss, the granite–granodiorite layer is much thicker beneath the Cathaysia block. The middle crust in these two domains is not entirely similar, being granite–granodiorite and granite gneiss as the best fit for the Yangtze block, and granite gneiss and biotite gneiss for the Cathaysia block. The lower crust is composed of biotite gneiss, paragranulite and amphibolite for the Yangtze block, whereas biotite gneiss, paragranulite, diorite, mica quartz schist, amphibolite, green schist facies basalt and hornblende provide the best fit for the Cathaysia block. The results demonstrate that to the east of the Chenzhou–Linwu fault (CLF) that is the southern segment of the Jiangshan–Shaoxing fault, the lithology displays

  4. Metamorphic and Ar/Ar geochronology constraints on the Alakeci shear zone: Implications for the extensional exhumation history of the northern Kazdag Massif, NW Turkey

    Energy Technology Data Exchange (ETDEWEB)

    Bonev, N [Sofia University ' St. Kliment Ohridski' , Department of Geology and Paleontology, Sofia 1504 (Bulgaria); Beccaletto, L [BRGM Geological Survey/Geology of Sedimentary Basins, 45060 Orleans, Cedex 2 (France); Robyr, M [Geological Sciences Department, University of Texas, Austin, TX 787 12-0254 (United States); Monie, P [Geosciences, University of Montpellier 2, UMR 5243, 34095 Montpellier Cedex 5 (France)], E-mail: niki@gea.uni-sofia.bg

    2008-07-01

    The Kazdag Massif exposes a metamorphic dome in the Biga Peninsula of northwest Turkey. An extensional origin has been proposed for the dome, limited on both flanks by detachments and/or shear zones. The northern flank is bounded by the extensional Alakeci Shear Zone (ASZ), whose P-T-t path is still poorly known. We therefore focus on its metamorphic conditions and related temporal history to precise its tectono-metamorphic evolution. The local tectonostratigraphy in structurally ascending order comprises: (i) the high-grade metamorphic core rocks of the Kazdag Massif (gneisses and micaschists intercalated with amphibolites and marbles); (ii) the two kilometer-thick ASZ; (iii) the overlaying unmetamorphosed pre-Cenomanian accretionary Cetmi melange; and (iv) Neogene sedimentary and volcanic cover rocks. ASZ mylonites were derived from both the core rocks and the melange lithologies. From the north to the south the mylonitic fabrics in the ASZ depict a top-to-the N-NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdag Massif. The northward transition from ductile to brittle-ductile regime through the ASZ shows that the deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium within the mylonitic gneisses and schists contains Qtz + Fs + Ms + Bt + Grt {+-} St {+-} Sill, with late retrogressive chlorite after biotite and garnet. Four samples of ASZ rocks yielded pressures between 6.9-5.7 kbar and temperatures between 706-587 deg. C. Three samples from the mylonitic rocks supplied in situ isochron {sup 36}Ar/{sup 40}Ar mica ages between 31.2-24.2 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic and structural results support the extensional character of the ASZ, and sketch transition

  5. Levels of potassium, uranium, thorium and rate of radiogenic heat production in the bedrock adjacent to Camamu and Almada sedimentary basins, Bahia, Brazil; Teores de potassio, uranio, torio e taxa de producao de calor radiogenico no embasamento adjacente as bacias sedimentares de Camamu e Almada, Bahia, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Sapucaia, Najara Santos; Barbosa, Johildo Salomao Figueiredo [Instituto de Geociencias, Universidade Federal da Bahia, Salvador, BA (Brazil); Argollo, Roberto Max de, E-mail: nss@cpgg.ufba.br, E-mail: johildo@cpgg.ufba.br, E-mail: robmax@ufba.br [Laboratorio de Fisica Nuclear Aplicada, Instituto de Fisica, Universidade Federal da Bahia, Salvador, BA (Brazil)

    2005-10-15

    The bedrock adjacent to Camamu and Almada sedimentary basins is characterized mainly by rocks of granulite and amphibolite facies, with archaean and paleoproterozoic ages, which belong to orogen Itabuna-Salvador-Curaca. The units in major proportion in this context are the metatonalites associated with basic and metamonzonites belonging to Itabuna belt. In smaller area occur the Teolandia granite and the Moenda granodiorite associated with the Ipiau band amphibolites, the charnockites and charnoenderbites of Jequie bloc, the neoproterozoic sienites and the mafic dikes. The K, U and Th contents of the rocks vary from 0,02 to 6,33% for K, from < 0,2 to 9,10 ppm for U and from < 0,4 to 64,38 ppm for Th. These contents are higher in the charnockites, Moenda granodiorite, Teolandia granite and sienites, intermediate in the metatonalites and metamonzonites and lower in the basic granulites. The heat production rates are higher in the lithologies where K, U and Th are also higher, varying from 0,58 to 5,57 {mu}W m{sup -3}. The coverage areas of such lithologies are, however, small compared with that of the metatonalitic granulites, metamonzonitic granulites and sienites where the rates vary from 0,10 to 1,44 {mu}W m{sup -3}, 0,23 to 5,55 {mu}W m{sup -3} and 0,60 to 2,24 {mu}W m{sup -3}, respectively. In this case, the heat production rates vary from 0,10 to 1,44 {mu}W m{sup -3}. The basic granulites have the smaller rates, from 0,06 to 0,36 {mu}W m-3. The observation of the lithologies in the margins of the two basins suggest that, in the bedrock under the younger sediments, may predominate the metatonalites, followed by the metamonzonites, with some significant participation of sienites in the Almada basin. In those lithologies, the volumetric heat production rates, with one standard deviation range, are 0,41 +- 0,30 {mu}W m{sup -3} for metatonalites, 0,71 +- 0,57 {mu}W m{sup -3} for metamonzonites and 1,20 +- 0,51 {mu}W m{sup -3} for sienites. (author)

  6. Residual water in hydrous minerals as a kinetic factor for omphacite destabilization into symplectite in the eclogites of Vårdalsneset (WGR, Norway)

    Science.gov (United States)

    Martin, Céline; Duchêne, Stéphanie

    2015-09-01

    Symplectitic intergrowths of sodic plagioclase + diopside ± amphibole as replacement of omphacite are commonly found in eclogites. These symplectites are interpreted as the exhumation-related decompression of eclogite into the amphibolite facies. The role of aqueous fluid in symplectite development, which would act as a catalyst and favor open-system reaction, has been suggested but not yet clearly established. In the Vårdalsneset outcrop of the Western Gneiss Region (Norway), eclogites that were not amphibolitized either display a primary eclogitic dry paragenesis (garnet + omphacite + rutile ± quartz) or a paragenesis including phengite. In the last case, omphacite is partly transformed into symplectite. The two groups have been further distinguished from a combined petrological, geochemical, and thermochemical study. Group I samples have a fine-grained unaltered microstructure, with medium Al2O3 (14-16 wt.%), high Fe2O3 (13-16 wt.%) and TiO2 (1.4-2.4 wt.%). Group II samples have a coarse-grained microstructure and are characterized by the presence of symplectites and phengite. They display higher Al2O3 (17.5-23 wt.%) and lower Fe2O3 (5.5-8 wt.%) and TIO2 (0.2-0.5 wt.%) contents than Group I. The P-T estimates for samples from both Groups I and II lead to similar conditions for peak eclogite metamorphism: temperatures range from 590 to 720 °C and pressures from 15 to 25 kbar. Perple_X modeling indicates that for Group I eclogites, temperature range is similar to the temperature range of the water saturation curve, whereas for Group II eclogites, due to slightly different chemical composition, the water saturation curve is located at much higher temperatures (770-900 °C), so that OH- remains in residual phengite at peak eclogite temperature. With no residual hydrous phase in the eclogite assemblage, and although some structural water may persist in nominally anhydrous minerals, Group I eclogites were preserved without change during exhumation. In contrast

  7. Garnet pyroxenite from the Shackleton Range, Antarctica: Intrusion of plume-derived picritic melts in the continental lithosphere during Rodinia breakup?

    Science.gov (United States)

    Schmädicke, Esther; Will, Thomas M.; Mezger, Klaus

    2015-12-01

    Lenses of ultramafic rocks occur in supracrustal high-grade gneiss in the northern Haskard Highlands, Shackleton Range, East Antarctica. Olivine-bearing garnet pyroxenite is the dominant rock type that is associated with hornblendite and subordinate spinel peridotite and amphibolite. The high-pressure (23-25 kbar) garnet-olivine assemblage of the pyroxenite formed during Pan-African eclogite-facies metamorphism. Associated collisional tectonics led to the incorporation of the ultramafic and mafic rocks in upper crustal rocks of a subducting continental margin. The ultramafic-mafic rocks are tracers of a paleo-suture zone and are critical for reconstructing Gondwana amalgamation. Thus, it is important to infer the tectonic setting of the rocks prior to emplacement into their current position, i.e., were the rocks part of the oceanic crust, the sub-oceanic, or the sub-continental mantle? Major and trace elements together with Pb and Nd isotope data imply that the precursor rocks of the pyroxenites and hornblendites (the latter being retrogressed pyroxenite equivalents) formed as plume-related melts, with many characteristics typical for ocean-island tholeiitic magmas. Hence, pyroxenite and hornblendite are interpreted as metamorphic equivalents of picritic melts. They differ from most garnet pyroxenites worldwide in composition and genesis. The latter formed as high-pressure clinopyroxene-rich cumulates from basaltic melts. The volumetrically minor amphibolites, sharing many geochemical characteristics with pyroxenites and hornblendites, are also interpreted as metamorphic equivalents of plume-related melts. It is inferred that the picritic melts crystallized at medium- to high-pressure conditions in the upper continental mantle or in the transition zone between mantle and continental crust. The subordinate spinel peridotites are interpreted as fragments of the uppermost, depleted mantle. They are probably the wall rocks into which the picritic melts intruded. The Pb

  8. Sill-like bodies of high-pressure ultramafic cumulates in tectonic blocks of the Pekul'ney complex (central Chukotka): their composition and inner structure

    Science.gov (United States)

    Bazylev, B. A.; Ledneva, G. V.; Ishiwatari, A.; Kononkova, N. N.

    2012-04-01

    During the last decade petrology of high-pressure ultramafic-mafic cumulates originated in the lower crust of the relatively thick lithosphere in both subduction and extensional settings became a matter of keen interest owing to development of lower crust underplating and delamination hypothesis as well as proposals of high-pressure fractionation influence on composition of evolved magmas and volcanic rocks. Peculiar rocks of the deepest complexes are garnet ultramafic and mafic rocks that occur in the Pekul'ney complex. The latter includes several tectonic blocks that we found to be constituted by sill-like layered bodies and embedded them metamorphic rocks. These country metamorphic rocks are represented by lower crustal amphibolites and crystalline schists whose pike conditions of metamorphism correspond to high-pressure epidote-amphibolite facies field (610-680oC, 9-14 kbars). All varieties of ultramafic rocks of the Pekul'ney complex belong to a single cumulative suite. Various types of ultramafic rocks regularly and repeatedly intercalate; and their primary minerals display regular correlations consistent with trends of fractional crystallization. Peculiar features of the Pekul'ney complex ultramafic rocks are early hornblende crystallization (hornblende occur in peridotites and olivine pyroxenites), garnet crystallization in a wide interval of conditions (garnet presents in pyribolites along with clinopyroxene, ceylonite and hornblende), crystallization of igneous clinozoisite in the most differentiated assemblages (along with garnet, hornblende and clinopyroxene), and lack of plagioclase crystallization indicators. Most differentiated ultramafic rocks contain clinopyroxenes with Al2O3 contents up to 15 wt. %. A thickness of ultramafic sill-like bodies studied varies from 350 to 1100 meters in different blocks of the complex. An inner structure of bodies is determined by regular intercalation of regular cycles (dunites - peridotites and olivine

  9. U-Pb ages of detrital zircon from Pegasus Group, Stewart Island

    International Nuclear Information System (INIS)

    Stewart Island (New Zealand) is relatively unaffected by Cenozoic deformation related to the present-day plate boundary and has thus been a locale of investigations that focus on the relationship between Mesozoic continental margin magmatic rocks and the Western Province (WP) of NZ. Pegasus Group metasedimentary rocks represent the only candidate for WP equivalents on Stewart Island. We measured U-Pb and 207Pb / 206Pb ages of detrital zircons from the Pegasus Group in an effort to validate the correlation with the WP. On SI, Pegasus Group is exposed over a small area in the southern Tin Range where it forms amphibolite facies roof pendants and screens among plutons of mid Paleozoic to mid Cretaceous age. Mica schists predominate, with lesser amounts of psammite and metaquartzite and rare calcareous schist and amphibolite. Zircons were extracted from metaquarzite and 24 grains were chosen on the basis of morphology for geochronologic investigation. Sixteen single crystals were dated by U-Pb TIMS; 8 additional grains were studied by the total evaporation method described by Kober. Despite vigorous air abrasion prior to processing, the 16 grains measured by conventional U-Pb TIMS yielded slightly normally discordant ages that we interpret to reflect minor Pb-loss. The 8 grains studied by the total evaporation method yielded more than four analytically identical /sup 207Pb / 206Pb ages within each grain at progressively higher temperatures (1515-1580 degrees C) of evaporation. The results permit the following general conclusions: 1) the age range of 420-2700 Ma is grossly similar to that reported from the Greenland Group/Victoria Paragneiss (Ireland 1992) of the Buller Terrane, WP; 2) the zircon population is dominantly 530-680 Ma but a subsidiary population is 900-1100 Ma; 3) no zircons with ages that match the timing of the Ross Orogen (530-480 Ma) were identified; 4) two grains of 420 Ma and a 453 Ma are probably too young for the paleontogically-known Ordovician

  10. UHT granulite-facies metamorphism in Rogaland, S Norway, is polyphase in nature

    Science.gov (United States)

    Laurent, Antonin; Duchene, Stéphanie; Bingen, Bernard; Seydoux-Guillaume, Anne-Magali; Bosse, Valérie

    2016-04-01

    Propensity of metamorphic assemblages to remain metastable after melt extraction complicates singularly the petrologist's task to discriminate between a single granulite-facies P-T path and a polyphase one. Using an integrated petrological and in-situ geochronological approach in key rock-samples, we reconstruct the pressure-temperature-time path of Sveconorwegian metamorphism across a 30 km-wide metamorphic gradient ranging from upper amphibolite facies to ultra-high temperature (UHT) granulite-facies in Rogaland, S. Norway. Thermodynamic modelling of phase equilibria in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-Ti2O-O2 chemical system (PerpleX code) are carried out with an emphasis on moderately oxidized, spinel-bearing assemblages resulting from either garnet or sapphirine breakdown. Geochronological U-(Th)-Pb data acquired on both monazite (LA-ICP-MS) and zircon (SIMS) are complemented by minor- and trace-elements signatures of both minerals, to monitor REE distribution through time and to evaluate garnet apparition or demise. Coupling field, petrological and geochronological data lead to a polyphase metamorphic history, lasting about 100 My. The onset of regional granulite facies metamorphism at 1035 Ma is associated with the emplacement of large volumes of granitic magmas in the amphibolite to granulite facies transition zone. In the deeper part of the crustal section, localized sapphirine-bearing restitic lithologies testify to UHT temperatures (900‑920 °C). These conditions were reached at ca. 1010 Ma following a tight clockwise P-T path associated with minor exhumation (7 to 5.5 kbar) and subsequent cooling to 700 °C. A distinct thermal episode, initiated at ca. 950 Ma, reached UHT granulite-facies conditions with the intrusion of massif-type anorthosite plutons at ca. 930 Ma producing a 5-km wide aureole. The aureole is delimited by the presence of osumilite in high Fe-Al rocks yielding quantitative estimates of 900-950 °C at a maximum pressure of 5

  11. Mineral chemistry, P-T-t paths and exhumation processes of mafic granulites in Dinggye, Southern Tibet

    Institute of Scientific and Technical Information of China (English)

    LIU; Shuwen; ZHANG; Jinjiang; SHU; Guiming; LI; Qiugen

    2005-01-01

    The mafic granulites in Dinggye, as various scale lense-shaped enclaves within the high Himalayan crystalline rock series, occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System (STDS) and the Xainza-Dinggye normal fault system. The main lithological assemblage comprises garnet plagioclase pyroxenite, garnet two-pyroxene granulite, pyroxene garnet amphibolite and so on. The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution. The first metamorphic stage, the garnet+clinopyroxene+quart mineral assemblage (M1) was probably formed under eclogite facies, the second stage, the plagioclase+clinopyroxene symplectite mineral assemblage (M2) was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage, the third stage, the plagioclase+clinopyroxene+ hypersthene symplectite mineral assemblage (M3) was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage, plagioclase+hornblende mineral assemblage (M4) was formed by hydrolysis of earlier mineral assemblages during late uplifting. The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites, whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite. These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyroblasts (M1) could be formed at the eclogite facies, the early decompressive breakdown (M2) occurred at the high-pressures granulite facies of 1.35―1.48 GPa and 625―675℃, the M3 mineral assemblage recorded the granulite facies of 0.7―0.95 GPa and 775―900℃ and M4 plagioglase+hornblende retrograde mineral assemblage was produced under

  12. Triassic High-P Metamorphism of the central Qiangtang terrane, Tibet; constraints using mineral equilibria modelling and 40Ar/39Ar geochronology

    Science.gov (United States)

    Rajkumar, A.; Hui, L.; Clarke, G. L.; Aitchison, J. C.; Forster, M. A.

    2014-12-01

    The SE-trending Qiangtang metamorphic belt (QMB) stretches more than 500 km through the Qiangtang terrane in central Tibet and comprises tectonically disrupted blueschist and eclogite in lower-grade garnet-phengite-bearing schist and quartzite. These rocks record the closure of a paleo-Tethyan Triassic ocean that formerly separated Cathaysian and Gondwana components of Asia, now forming the northern and southern Qiangtang blocks. Eclogite is extensively recrystallized to high-P amphibolite and greenschist facies assemblages, formed during water ingression that accompanied terrane uplift. P-T pseudosections constructed in Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKMASHTO) in the context of petrography and mineral chemistry provides the ability to recover a dynamic PT history for the eclogite facies assemblages. Prograde (S1) assemblages for the Gemu Co eclogite are predicted to have formed at P≈21.5 kbars and T≈505°C and involved garnet, glaucophane, omphacite, rutile, lawsonite and chlorite, based on garnet composition and inferred pseudomorphs after lawsonite. Peak (S2) assemblages of garnet, barroisite, omphacite, rutile, epidote and quartz reflect P≈15 kbars and T≈570°C. Based on textural relations, post-peak stages can be divided into epidote-amphibolite and greenschist facies. The geothermal gradient for the prograde S1 assemblage and the peak S2 assemblage is 7.1 and 11.5°C/km respectively.40Ar/39Ar geochronology of phengitic mica using step heating in recrystallized eclogite components and surrounding garnet-mica schist components both yield maximum ages ranging 230-220 Ma. The congruency in ages of the deeply subducted high-pressure eclogites to the surrounding garnet phengite schists indicate they were the most probable source of fluids to extensively recrystallize most of the high-pressure eclogite components in the high-pressure belt. The P-T history of the high-P rocks of the QMB records the deep subduction of paleo-Tethyan oceanic crust to

  13. P- T- t evolution of eclogite/blueschist facies metamorphism in Alanya Massif: time and space relations with HP event in Bitlis Massif, Turkey

    Science.gov (United States)

    Çetinkaplan, Mete; Pourteau, Amaury; Candan, Osman; Koralay, O. Ersin; Oberhänsli, Roland; Okay, Aral I.; Chen, Fukun; Kozlu, Hüseyin; Şengün, Fırat

    2016-01-01

    The Alanya Massif, which is located to the south of central Taurides in Turkey, presents a typical nappe pile consisting of thrust sheets with contrasting metamorphic histories. In two thrust sheets, Sugözü and Gündoğmuş nappes, HP metamorphism under eclogite (550-567 °C/14-18 kbar) and blueschist facies (435-480 °C/11-13 kbar) conditions have been recognized, respectively. Whereas the rest of the Massif underwent MP metamorphism under greenschist to amphibolite facies (525-555 °C/6.5-7.5 kbar) conditions. Eclogite facies metamorphism in Sugözü nappe, which consists of homogeneous garnet-glaucophane-phengite schists with eclogite lenses is dated at 84.8 ± 0.8, 84.7 ± 1.5 and 82 ± 3 Ma (Santonian-Campanian) by 40Ar/39Ar phengite, U/Pb zircon and rutile dating methods, respectively. Similarly, phengites in Gündoğmuş nappe representing an accretionary complex yield 82-80 Ma (Campanian) ages for blueschist facies metamorphism. During the exhumation, the retrograde overprint of the HP units under greenschist-amphibolite facies conditions and tectonic juxtaposition with the Barrovian units occurred during Campanian (75-78 Ma). Petrological and geochronological data clearly indicate a similar Late Cretaceous tectonometamorphic evolution for both Alanya (84-75 Ma) and Bitlis (84-72 Ma) Massifs. They form part of a single continental sliver ( Alanya- Bitlis microcontinent), which was rifted from the southern part of the Anatolide-Tauride platform. The P- T- t coherence between two Massifs suggests that both Massifs have been derived from the closure of the same ocean ( Alanya- Bitlis Ocean) located to the south of the Anatolide-Tauride block by a northward subduction. The boundary separating the autochthonous Tauride platform to the north from both the Alanya and Bitlis Massifs to the south represents a suture zone, the Pamphylian- Alanya- Bitlis suture.

  14. Polyphase deformation of a Paleozoic metamorphosed subduction-accretionary complex in Beishan Orogen, southern Altaids

    Science.gov (United States)

    Song, Dongfang; Xiao, Wenjiao

    2014-05-01

    The Lebaquan Complex in central Beishan plays a significant role in understanding the subduction-accretion-collision processes of the Beishan orogenic collage, southern Altaids. This complex is a polyphase deformed upper greenschist to lower amphibolite facies metamorphic rock assemblages composed of metasedimentary sequence, gneissic plutons, metacherts, amphibolites and marbles, with multiple generations of syn-tectonic leucogranite-pegmatite and post-tectonic mafic dykes. Lithologic and geochemical characteristics show the Lebaquan Complex is a forearc-arc complex. Four stages of deformation can be recognized for the Lebaquan Complex. The D1 deformation is defined by pervasively developed foliations (S1) defined by alternative metamorphic layering and intrafolia isoclinal folds of quartz veins (F1). The D2 deformation is indicated by crenulation cleavage (S2) mainly developed in the schists, extensively-developed tight to open folds and asymmetric folds (F2) and pinch and swell structures. The D3 deformation is characterized by high-strain ductile shearing, which modified earlier structures. Small-scale asymmetric folds, σ-type porphyroclasts and S-C fabrics indicate dextral sense of shearing in east-west direction. The D4 deformation is represented by ductile to brittle structures including open/gentle folds, kink folds and small scale thrust faults which overprint earlier deformations. The overall deformation of this complex indicates a geodynamic setting change from initial north-south directed strong compression to later east-west directed transpression and finally extension. LA-ICP-MS zircon U-Pb dating was performed on key lithologies. Youngest age peak of detrital zircons from a garnet-mica-quartz schist is ~424 Ma, constraining the depositional age for the protolith of the metasedimentary sequence. A syn-deformation leucogranitic dyke that underwent D2 but did not experience D1 and a post-deformation mafic dyke that intrudes all the lithologies were

  15. Lithology, fracture intensity, and fracture filling of drill core from Chalk River research area, Ontario

    International Nuclear Information System (INIS)

    In 1977, 1978, and 1979, nine inclined cored boreholes, ranging in length from 113 to 704 m, were drilled in the Chalk River Research Area in order to define the geological subsurface characteristics of the rock mass at several selected test areas. A total of 2,458 metres of NQ-3 and HQ-3 core was obtained from the nine boreholes. Orthogneiss was the most predominant rock type intersected by the boreholes. Pyroxenite, amphibolite, metagabbro and dykes of diabase, pegmatite and aplite were also encountered. The crosscutting relationships and textures within the rocks indicate that the relative ages of the rock units, from youngest to oldest, are diabase; aplite and pegmatite dykes with no defined fabric; pyroxenite; meta-ferrogabbro; amphibolite; aplite and pegmatite dykes and pegmatite pods with a defined fabric; and orthogneiss. Textural characteristics and mineral assemblages indicate that the orthogneisses in the Chalk River Area are a product of regional, medium to high-grade metamorphism and belong to the upper amphibilite to granulite facies. A total of 35,597 fractures (an average of 14.5 fractures per metre) was observed in the core. Brecciated zones and open fractures were noted in the core from all of the boreholes, and major faults were identified in four of the nine boreholes. Nearly all of the fractures have a thickness between 0.4 and 1.2 mm and contain one or more types of filling. Chlorite and calcite are the most common types of filling. Epidote, hematite, clays, sulphides, talc, sericite, and rock fragments also occur in the fractures. The crosscutting relationships between fractures and the sequence of filling layers within the fractures indicate that several episodes of fracturing have occurred and that fractures containing more than one filling have probably been reactivated. A comparison of the geological logs from one of the boreholes with natural gamma, neutron-neutron and magnetic susceptibility logs indicates that certain rock types and

  16. Site investigation SFR. Overview Boremap mapping of drill cores from KFR04, KFR08, KFR09, KFR13, KFR35, KFR36, KFR54, KFR55, KFR7A, KFR7B and KFR7C

    Energy Technology Data Exchange (ETDEWEB)

    Petersson, Jesper; Andersson, Ulf B. (Vattenfall Power Consultant AB, Stockholm (Sweden))

    2011-01-15

    This report presents the results from a renewed geological overview mapping of 11 drill cores obtained during the construction of the final repository for low and middle level radioactive operational waste (SFR) during the 80's. Drill cores from KFR04, KFR08, KFR09, KFR13, KFR35, KFR36, KFR54, KFR55, KFR7A, KFR7B and KFR7C, with a total length of 837 m, was selected primarily because of their distinctly crosscutting relationship with inferred deformation zones in the area. The main purpose for this geological mapping is calibration with the original mappings, which in turn aims to facilitate geological single-hole interpretation. The mapping was generally focused on the location and infilling mineralogy of broken and unbroken fractures, as well as crush zones, breccias and sealed networks. Also the overview lithology, alterations and ductile shear zones were documented. All boreholes selected for renewed mapping are located in a ductile, high-strain belt, which defines the northeastern margin of a structurally more homogeneous tectonic lens. The main component of the high-strain belt is felsic to intermediate rocks of inferred volcanic origin. The predominant rock in the selected drill cores is, however, a fine- to finely medium-grained metagranite, which clearly appears to be a high-strain variety of the typically medium-grained metagranite-granodiorite that prevails the tectonic lens. It is obvious that varieties of this high-strain rock previously was inferred to be meta volcanic rocks. Other volumetrically important rock types in the drill cores are pegmatitic granite, finely medium-grained granite and metagranodiorite-tonalite, aplitic metagranite, amphibolites and slightly coarser metagabbros. Virtually all rocks in the borehole have experienced Svecofennian metamorphism under amphibolite facies conditions. Excluding fractures within crush zones and sealed networks, there is a predominance of broken fractures in most of the drill cores. The total

  17. Pegmatite geology of the Shelby district, North Carolina

    Science.gov (United States)

    Griffitts, Wallace R.

    1957-01-01

    The Shelby district is divided into a northwestern and a southeastern province. The rocks in the southeastern province include various units in the Battleground schist formation and the Yorkville granodiorite. Those in the northwestern province include the Carolina gneiss, with its Shelby gneiss member, and the Toluca quartz monzonite. The Cherryville quartz monzonite forms a batholith that is just west of the boundary between the two provinces. Pegmatites related to both the Toluca and the Cherryville quartz monzonites lie in the Carolina gneiss and many dikes of pegmatite that are related to the Cherryville quartz monzonite are in the tin-spodumene belt that lies along the boundary between provinces. The rocks of the southeastern province have been bent into steep isoclinal folds; those of the northwestern province were bent into open folds and gently-dipping isoclinal folds. The rocks to the southeast have been metamorphosed in the epidote-amphibolite facies whereas the rocks to the northwest represent the amphibolite or granulite facies. The pegmatites related to the Toluca quartz monzonite form sills, dikes, and concordant lenses in the Carolina gneiss, as well as dikes in the Toluca quartz monzonite. The bodies are unzoned and consist mainly of gneissic microcline-plagioclase-quartz pegmatite. The pegmatites related to the Cherryville quartz monzonite form dikes and disconformable lenses in the Carolina gneiss and the Toluca quartz monzonite. These pegmatites range widely in composition and many are zoned. The dikes west of the Cherryville batholith are rich in muscovite and plagioclase and may contain no microcline or only a moderate amount of microcline. Quartz cores and microcline-rich intermediate zones are common. Similar pegmatite forms dikes along the west edge of the tin-spodumene belt. The tin-spodumene belt containes albite-microcline-spodumene-quartz pegmatite. These dikes of albitic pegmatite are largest and most nearly parallel to one another

  18. Obduction of Tethyan-type ophiolites-A case-study from the Thetford-Mines ophiolitic Complex, Quebec Appalachians, Canada

    Science.gov (United States)

    Tremblay, Alain; Ruffet, Gilles; Bédard, Jean H.

    2011-07-01

    Oceanic plagiogranites and peridotite-hosted granites (PHG) of the Thetford-Mines ophiolite (TMO) yield mean U-Pb zircon ages of 479.2 ± 1.6 Ma and 469.5 ± 2.8 Ma, and are interpreted to be the products of fractionation of basaltic magma and melting of the Laurentian margin, respectively. The existing 40Ar/39Ar age for the ophiolitic metamorphic sole of the TMO is a hornblende isochron age of 477 ± 5 Ma for amphibolite facies rocks. Sites previously sampled for U-Pb and 39Ar-40Ar analyses were resampled for more accurate 39Ar-40Ar dating, in order to better constrain the thermo-tectonic evolution of the ophiolite and underlying Laurentian margin rocks on which they were overthrust, and quantify the time required for obduction of Tethyan-type ophiolites in the Canadian Appalachians. Amphiboles from TMO gabbro and plagiogranite yield 39Ar-40Ar plateau ages of 477-475 Ma. In contrast, muscovites from PHG yield cooling ages of 466-465 Ma, and show evidence for thermal resetting at ca. 460 Ma for both a granitic mylonite and a crosscutting pegmatite facies. The amphibolitic metamorphic sole yields 39Ar-40Ar ages of ca. 471 Ma in amphibole, and high-temperature ages of ca. 466 Ma in muscovite, with evidence for recrystallization to ages as young as 457 Ma. Muscovites from the sub-ophiolitic Laurentian metasedimentary basement, and micaschist clasts in supraophiolitic debris flows of the TMO, yield similar pseudo-plateau and high-temperature ages of ca. 463-464 Ma. The U-Pb and 39Ar-40Ar data suggest that the obduction of peri-Laurentian oceanic crust lasted approximately 5 to 10 m.y. Since both facies formed approximately m.y. after the ophiolitic crust, this supports models whereby the still-hot ophiolitic mantle provided the heat needed to generate the PHG and the metamorphic sole between ca. 475 and 470 Ma. Muscovite ages of PHG record cooling below ca. 450-500 °C by ca. 465 Ma, but 39Ar-40Ar ages from both a mylonitized PHG facies and micaschist clasts suggest

  19. Strain localization and fluid infiltration in the mantle wedge during subduction initiation: Evidence from the base of the New Caledonia ophiolite

    Science.gov (United States)

    Soret, M.; Agard, P.; Dubacq, B.; Vitale-Brovarone, A.; Monié, P.; Chauvet, A.; Whitechurch, H.; Villemant, B.

    2016-02-01

    Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle-crust transition of the New Caledonia obducted ophiolite and ~ 50 to 100 m above the basal thrust contact of the ophiolite. These m- to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40Ar/39Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction. Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700-800 °C and 3-5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40Ar/39Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650-750 °C); (3) the last, low temperature (< 600 °C) metasomatic stage results in the formation of deformed veinlets containing talc

  20. Zircon U-Pb geochronology and geochemistry of low-grade metamorphosed volcanic rocks from the Dantazi Complex: Implications for the evolution of the North China Craton

    Science.gov (United States)

    Ge, Songsheng; Zhai, Mingguo; Li, Tiesheng; Peng, Peng; Santosh, M.; Shan, Houxiang; Zuo, Pengfei

    2015-11-01

    The late Neoarchean witnessed the cratonization of the North China Craton (NCC) through amalgamation of several micro-blocks to form a coherent basement. The Archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at ∼2500 Ma. Recently, a suite of low-grade metamorphosed (greenschist to lower amphibolite facies) volcanic rocks was discovered in the late Neoarchean Dantazi Complex in northern Hebei province. These meta-volcanic rocks consist of bimodal basalt-andesite and trachyte-dacite with a SiO2 gap between 54.4 wt.% and 60.7 wt.%. Here we report SHRIMP zircon U-Pb ages of 2490 ± 19 Ma (MSWD = 2.0) and 2502 ± 8 Ma (MSWD = 0.83) from the meta-mafic and meta-felsic volcanics, respectively, representing the timing of igneous activity. All the meta-mafic volcanic rocks display coherent trace element and REE patterns which are characterized by enriched LILE and LREE but depleted HFSE and HREE ((La/Yb)N = 6.29-15.10). Combining these trace element features with the positive zircon εHf(t) values (+1.3 to +6.6), we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle. In the primitive mantle-normalized diagram, the felsic rocks display uniform patterns enriched in LILE but depleted in Nb and Ta, similar to those of lower crust. Furthermore, their strongly fractionated REE ((La/Yb)N = 15.24-61.20), lower HREE concentrations (Yb = 0.47-1.65 ppm) and positive zircon εHf(t) values (+1.6 to +5.3) suggest that they were derived from partial melting of the lower crust with garnet in the residue. This coeval occurrence of metasomatized mantle-derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late Neoarchean to earliest Paleoproterozoic following the cratonization of the NCC. Our new data, combined with previous

  1. Orogenic-type copper-gold-arsenic-(bismuth) mineralization at Flatschach (Eastern Alps), Austria

    Science.gov (United States)

    Raith, Johann G.; Leitner, Thomas; Paar, Werner H.

    2015-10-01

    Structurally controlled Cu-Au mineralization in the historic Flatschach mining district (Styria, Austria) occurs in a NE-SW to NNE-WSW oriented vein system as multiple steep-dipping calcite-(dolomite)-quartz veins in amphibolite facies metamorphic rocks (banded gneisses/amphibolites, orthogneisses, metagranitoids) of the poly-metamorphosed Austroalpine Silvretta-Seckau nappe. Vein formation postdated ductile deformation events and Eoalpine (Late Cretaceous) peak metamorphism but predated Early to Middle Miocene sediment deposition in the Fohnsdorf pull-apart basin; coal-bearing sediments cover the metamorphic basement plus the mineralized veins at the northern edge of the basin. Three gold-bearing ore stages consist of a stage 1 primary hydrothermal (mesothermal?) ore assemblage dominated by chalcopyrite, pyrite and arsenopyrite. Associated minor minerals include alloclasite, enargite, bornite, sphalerite, galena, bismuth and matildite. Gold in this stage is spatially associated with chalcopyrite occurring as inclusions, along re-healed micro-fractures or along grain boundaries of chalcopyrite with pyrite or arsenopyrite. Sericite-carbonate alteration is developed around the veins. Stage 2 ore minerals formed by the replacement of stage 1 sulfides and include digenite, anilite, "blue-remaining covellite" (spionkopite, yarrowite), bismuth, and the rare copper arsenides domeykite and koutekite. Gold in stage 2 is angular to rounded in shape and occurs primarily in the carbonate (calcite, Fe-dolomite) gangue and less commonly together with digenite, domeykite/koutekite and bismuth. Stage 3 is a strongly oxidized assemblage that includes hematite, cuprite, and various secondary Cu- and Fe-hydroxides and -carbonates. It formed during supergene weathering. Stage 1 and 2 gold consists mostly of electrum (gold fineness 640-860; mean = 725; n = 46), and rare near pure gold (fineness 930-940; n = 6). Gold in stage 3 is Ag-rich electrum (fineness 350-490, n = 12), and has a

  2. Geology and geochemistry of the Macheng Algoma-type banded iron-formation, North China Craton: Constraints on mineralization events and genesis of high-grade iron ores

    Science.gov (United States)

    Wu, Huaying; Niu, Xianglong; Zhang, Lianchang; Pirajno, Franco; Luo, Huabao; Qin, Feng; Cui, Minli; Wang, Changle; Qi, Min

    2015-12-01

    The Macheng iron deposit is located in the eastern Hebei province of the North China Craton (NCC). It is hosted in Neoarchean metamorphic rocks of Baimiaozi formation in the Dantazi Group, consisting of biotite-leptynite, plagioclase-gneiss, plagioclase-amphibolite, migmatite, migmatitic granite and quartz schist. Geochemical analyses of the host biotite leptynite and plagioclase amphibolites show that their protoliths are both volcanics, inferred to be trachytic basalt and basaltic andesite, respectively. Based on the geochemical signature of the host rocks, together with geology of the iron deposit, it is inferred that the Macheng BIF is an Algoma-type iron exhalative formation, formed in an arc-related basin in the Neoarchean. Post-Archean Australian Shale (PAAS)-normalized rare earth elements (REEs) plus yttrium (Y) concentrations of different BIF ores with gneissic, striated and banded structure in the Macheng deposit, show similar patterns with depletions in light rare earth elements (LREEs) and middle rare earth elements (MREEs) relative to heavy rare earth elements (HREEs) and with apparently positive La, Y and Eu anomalies. Y/Ho ratios of the gneissic, striated and banded BIF ores vary from 37 to 56. These geochemical features of the BIF ores reveal their affinity with the sea water and the presence of a high-temperature hydrothermal component, indicating that both the seawater and high temperature hydrothermal fluids derived from alteration of oceanic basalts and komatiites may contribute to formation of the Macheng BIF. Geological, mineralogical and geochemical studies of the Macheng deposit recognized two kinds of high-grade iron ores. One is massive oxidized high-grade ore (Fe2O3T = 74.37-86.20 wt.%), mainly consisting of hematite with some magnetite, which shows geochemical characteristics of the gneissic, striated and banded BIF ores. The other type is magnetite high-grade ore, also massive and consisting of magnetite, with distinct characteristics

  3. Trace element composition of rutile and Zr-in-rutile thermometry in meta-ophiolitic rocks from the Kazdağ Massif, NW Turkey

    Science.gov (United States)

    Şengün, Fırat; Zack, Thomas

    2016-08-01

    In northwest Turkey, ophiolitic meta-gabbros are exposed on the Kazdağ Massif located in the southern part of the Biga Peninsula. Trace element composition of rutile and Zr-in-rutile temperatures were determined for meta-gabbros from the Kazdağ Massif. The Zr content of all rutiles range from 176 to 428 ppm and rutile grains usually have a homogeneous Zr distribution. The rutile grains from studied samples in the Kazdağ Massif are dominated by subchondritic Nb/Ta (11-19) and Zr/Hf ratios (20-33). Nb/Ta and Zr/Hf show positive correlation, which is probably produced by silicate fractionation. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents. The core of rutile grains are generally characterized by low Nb/Ta ratios of 17-18 whereas the rims exhibit relatively high Nb/Ta ratios of 19-23. Trace element analyses in rutile suggest that these rutile grains were grown from metamorphic fluids. The P-T conditions of meta-gabbros were estimated by both Fe-Mg exchange and Zr-in-rutile thermometers, as well as by the Grt-Hb-Plg-Q geothermobarometer. The temperature range of 639 to 662 °C calculated at 9 kbar using the Zr-in-rutile thermometer is comparable with temperature estimates of the Fe-Mg exchange thermometer, which records amphibolite-facies metamorphism of intermediate P-T conditions. The P-T conditions of meta-ophiolitic rocks suggest that they occur as a different separate higher-pressure tectonic slice in the Kazdağ metamorphic sequence. Amphibolite-facies metamorphism resulted from northward subduction of the İzmir-Ankara branch of the Neo-Tethyan Ocean under the Sakarya Zone. Metamorphism was followed by internal imbrication of the Kazdağ metamorphic sequence resulting from southerly directed compression during the collision.

  4. Trace element composition of rutile and Zr-in-rutile thermometry in meta-ophiolitic rocks from the Kazdağ Massif, NW Turkey

    Science.gov (United States)

    Şengün, Fırat; Zack, Thomas

    2016-02-01

    In northwest Turkey, ophiolitic meta-gabbros are exposed on the Kazdağ Massif located in the southern part of the Biga Peninsula. Trace element composition of rutile and Zr-in-rutile temperatures were determined for meta-gabbros from the Kazdağ Massif. The Zr content of all rutiles range from 176 to 428 ppm and rutile grains usually have a homogeneous Zr distribution. The rutile grains from studied samples in the Kazdağ Massif are dominated by subchondritic Nb/Ta (11-19) and Zr/Hf ratios (20-33). Nb/Ta and Zr/Hf show positive correlation, which is probably produced by silicate fractionation. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents. The core of rutile grains are generally characterized by low Nb/Ta ratios of 17-18 whereas the rims exhibit relatively high Nb/Ta ratios of 19-23. Trace element analyses in rutile suggest that these rutile grains were grown from metamorphic fluids. The P-T conditions of meta-gabbros were estimated by both Fe-Mg exchange and Zr-in-rutile thermometers, as well as by the Grt-Hb-Plg-Q geothermobarometer. The temperature range of 639 to 662 °C calculated at 9 kbar using the Zr-in-rutile thermometer is comparable with temperature estimates of the Fe-Mg exchange thermometer, which records amphibolite-facies metamorphism of intermediate P-T conditions. The P-T conditions of meta-ophiolitic rocks suggest that they occur as a different separate higher-pressure tectonic slice in the Kazdağ metamorphic sequence. Amphibolite-facies metamorphism resulted from northward subduction of the İzmir-Ankara branch of the Neo-Tethyan Ocean under the Sakarya Zone. Metamorphism was followed by internal imbrication of the Kazdağ metamorphic sequence resulting from southerly directed compression during the collision.

  5. Geochemistry and petrogenesis of mafic-ultramafic suites of the Irindina Province, Northern Territory, Australia: Implications for the Neoproterozoic to Devonian evolution of central Australia

    Science.gov (United States)

    Wallace, Madeline L.; Jowitt, Simon M.; Saleem, Ahmad

    2015-10-01

    Petrological and geochemical data for magmatic mafic-ultramafic suites of the Irindina and Aileron provinces of the Eastern Arunta region, Northern Territory, Australia constrain the petrogenesis and tectonic setting of magmatic events covering ~ 500 million years. Six geochemically distinct magmatic suites, here named A-F, have been identified and provide evidence of the tectonic history of this region and also are linked to two mineralisation-related magmatic events: the Lloyd Gabbro (Ni-Cu-PGE mineralisation) and the Riddoch Amphibolite (Cyprus-style Cu-Co volcanogenic massive sulphide mineralisation). The whole-rock geochemistry of Suites A and F is indicative of melts derived from a range of mantle depths (garnet to spinel lherzolite) and source enrichment. Suite D is likely related to the ~ 1070 Ma Warakurna/Giles event of central Australia, including the Alcurra (Musgrave) and Stuart (Arunta) dyke swarms, and likely formed through either: a) melting of subduction modified, sub-continental lithospheric mantle (SCLM) by an upwelling mantle plume; or b) a combination of intra-plate tectonic processes involving a long-lived thermal anomaly, lithospheric-scale architecture that focussed magmatism, and large-scale tectonism. Suite F represents more alkaline magmas, derived from a deeper source, but most likely formed during the same Warakurna LIP event (possibly contemporaneously) as Suite D. Suite E (the Riddoch Amphibolite) was most likely emplaced in a back-arc basin (BAB) setting at ~ 600 Ma, coincident with Delamerian subduction and BAB formation along the eastern Proterozoic margin of Australia from Queensland to the eastern Arunta and possibly further south. Subsequent destabilisation of the SCLM underneath the North Australian Craton generated the ~ 510 Ma Kalkarindji LIP in the form of Suite B intrusions that assimilated some of the older Suite E (Riddoch) material. This event is locally known as the ~ 506 Ma Stanovos Igneous Suite and represents the most

  6. Asbestos quantification in track ballast, a complex analytical problem

    Science.gov (United States)

    Cavallo, Alessandro

    2016-04-01

    Track ballast forms the trackbeb upon which railroad ties are laid. It is used to bear the load from the railroad ties, to facilitate water drainage, and also to keep down vegetation. It is typically made of angular crushed stone, with a grain size between 30 and 60 mm, with good mechanical properties (high compressive strength, freeze - thaw resistance, resistance to fragmentation). The most common rock types are represented by basalts, porphyries, orthogneisses, some carbonatic rocks and "green stones" (serpentinites, prasinites, amphibolites, metagabbros). Especially "green stones" may contain traces, and sometimes appreciable amounts of asbestiform minerals (chrysotile and/or fibrous amphiboles, generally tremolite - actinolite). In Italy, the chrysotile asbestos mine in Balangero (Turin) produced over 5 Mt railroad ballast (crushed serpentinites), which was used for the railways in northern and central Italy, from 1930 up to 1990. In addition to Balangero, several other serpentinite and prasinite quarries (e.g. Emilia Romagna) provided the railways ballast up to the year 2000. The legal threshold for asbestos content in track ballast is established in 1000 ppm: if the value is below this threshold, the material can be reused, otherwise it must be disposed of as hazardous waste, with very high costs. The quantitative asbestos determination in rocks is a very complex analytical issue: although techniques like TEM-SAED and micro-Raman are very effective in the identification of asbestos minerals, a quantitative determination on bulk materials is almost impossible or really expensive and time consuming. Another problem is represented by the discrimination of asbestiform minerals (e.g. chrysotile, asbestiform amphiboles) from the common acicular - pseudo-fibrous varieties (lamellar serpentine minerals, prismatic/acicular amphiboles). In this work, more than 200 samples from the main Italian rail yards were characterized by a combined use of XRD and a special SEM

  7. Rehydration Metamorphism of the Iratsu Eclogite Mass in the Sambagawa Belt, Japan

    Science.gov (United States)

    Kuwatani, T.; Toriumi, M.

    2008-12-01

    The Iratsu eclogite mass in the Sambagawa metamorphic belt, central Shikoku, Japan, underwent pervasive rehydration metamorphism so that most of all retrogressed to amphibolite. The Iratsu eclogite mass is originated from gabbro and basalt, and is interpreted as a relic of a fossil subducted slab. Gradual stages of rehydration retrogression from eclogite to low-grade greenschist are preserved by disequilibrium textures of incomplete reactions. Hence, the Iratsu eclogite mass provide an exceptional opportunity to study the process of rehydration reactions and exhumation of a subducted oceanic crust. The petrological investigations showed that the retrogressed eclogites can be classified into two types by reaction textures during rehydration stage: One is the perfectly equilibrated type, which is a rock almost perfectly retrogressed into well-foliated epidote-amphibolite consists of amphibole + epidote + chlorite + plagioclase + quartz +/- garnet, mainly sampled from the rim of the Iratsu body. The other is the locally equilibrated type, which is a rock partially retrogressed rock often preserved garnet and omphacite and weakly foliated sampled from the central part of the Iratsu body. Garnet grains are rimmed by amphibole (pargasite) + epidote layers, whereas omphacite grains broke into symplectites of amphibole (hornblende) + plagioclase. Such clear distinction of the mineral assemblages and mineral compositions between two parts around the dissolving minerals demonstrates that the system was not equilibrated as a whole but locally equilibrated in each part around the dissolving minerals. Partial pseudomorphs, which are frequently observed in rocks of the locally equilibrated type, are the most reliable evidence of a reaction, since the replaced mineral and the solid products can be directly observed. In this study, we developed the new methodology based on a simple mass-balance relation, by improving Gresens 1967 and Godard & Mabit 1998. Our method can easily

  8. 40Ar/ 39Ar mineral ages within metamorphic clasts from the Kuma Group (Eocene), central Shikoku, Japan: Implications for tectonic development of the Sambagawa accretionary prism

    Science.gov (United States)

    Takasu, A.; Dallmeyer, R. D.

    1992-06-01

    The Sambagawa metamorphic belt exposed in central Shikoku records a high- P/T metamorphism. It is comprised by the Oboke nappe and the structurally overlying, internally imbricated Besshi nappe complex. The Besshi nappe complex locally is unconformably overlain by the Kuma Group (Eocene) which consists of conglomerates and subordinate sandstones and mudstones. Clasts within Kuma conglomerates exposed in the Kamegamori district include low-grade schists (derived from the unconformably underlying Sambagawa complex) and high-grade metamorphic rocks of uncertain provenance (including garnet-amphibolite and oligoclase-bearing pelitic schist). 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope correlation ages recorded by hornblende from two amphibolite clasts are 131.1 ± 4.9 Ma and 156.8 ± 4.3 Ma. These are interpreted to date post-metamorphic cooling through temperatures required for intracrystalline retention of argon, and are older than cooling ages previously reported from any presently exposed segments of the Sambagawa terrane. Plateau ages of 108.8 ± 0.7 Ma and 115.7 ± 0.6 Ma are recorded by muscovite from two clasts of schist. Muscovite within a proximal basement exposure of unconformably underlying Sambagawa pelitic schist records a plateau age of 78.7 ± 0.5 Ma. Combined with isotopic ages previously reported for the Sambagawa terrane, 40Ar/ 39Ar data from clasts in the Kuma Group suggest that metamorphic culmination within a high-grade source terrane occured at c. 185-145 Ma. Sectors of this complex cooled through c. 500°C at c. 150 Ma. Other portions cooled through c. 500°C at c. 130 Ma. Contrasts in the cooling ages likely reflect internal imbrication within the source terrane. The Besshi nappe complex reached peak metamorphic conditions at c. 100-90 Ma and experienced relatively rapid uplift and cooling at c. 85-75 Ma. The Besshi nappe complex was structurally emplaced onto the Oboke nappe which attained peak metamorphic conditions at c. 75 Ma. Subsequently, the

  9. Landsat TM data processing for lithological discrimination in the Caraculo area (Namibe Province, SW Angola)

    Science.gov (United States)

    Alberti, A.; Alessandro, V.; Pieruccini, U.; Pranzini, E.

    1993-10-01

    Landsat TM data were used for lithological discrimination and mapping in the little-known, semiarid 900 km 2 area around Caraculo station and the middle course of the Rio Giraul (Namibe Province, SW Angola) following two main procedures. The first of these was based on visual evaluation of three-band composites, band-ratio composites and Principal Component Analysis. The second method relied on the extraction of spectral signatures, and their use to obtain automatic classifications. Satisfactory results were reached with the first procedure, thus allowing - with limited support of ground information — the draft of a lithological map, while the second method was not systematically efficient, even for confirmation of data acquired with the first procedure. Image interpretation suggests that an extensive but hithertoun differentiated metasedimentary complex consisting of a heterogeneous supracrustal sequence should be subdivided into at least two units. Field observations proved that one of these is marked by a notable frequency of marbles and the other is characterized by a widespread occurrence of amphibolitic bodies. Moreover, a belt of undetermined (thermally metamorphosed ?) metamorphic rocks is interposed between them. The distinction of so far unidentified units, though restricted to interpretation of processed Landsat TM data, has significant geological implications also in the regional context and will be helpful in guiding future work with conventional geological methods.

  10. Phase boundary mobility in naturally deformed, high-grade quartzofeldspathic rocks: evidence for diffusional creep

    Science.gov (United States)

    Gower, Robert J. W.; Simpson, Carol

    1992-03-01

    Grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern Grenville Province, Ontario, Canada, suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep. In these rocks, quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions. In the naturally deformed rocks, however, phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation (i.e. in directions of inferred finite extension) which implies that phase boundary motion and cusp formation occurred during deformation. Optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep. However, the morphology of quartz-feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone. We propose that phase boundary motion resulted from a diffusion-assisted process involving dissolution at foliation-parallel quartz-feldspar phase boundaries, mass transfer over length scales of the order of feldspar domain size (≈200 μm or greater) and precipitation at quartz-feldspar phase boundary cusps. This study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks. It also has important implications for the natural rheological behavior of the mid- and lower-continental crust.

  11. Ultrasonic P and S wave Velocity Measurements at Mid-to-Lower Crustal Conditions of Pressure and Temperature in a Piston Cylinder Apparatus

    Science.gov (United States)

    Ishikawa, M.; Arima, M.

    2007-12-01

    In order to interpret seismic structures in terms of rock type, temperature anomaly, degree of partial melting and distribution of fluids, we have carried out research on the elastic properties of the crustal rocks using ultrasonic measurements. We have developed techniques to perform ultrasonic velocity measurements at mid-to-lower crustal conditions of pressure and temperature. These techniques are now been applied to study the rock physics of exposed deep crustal sections and crustal xenoliths, including gabbro, tonalite, granite, anorthosite, granulite and amphibolite, which were collected from the Tanzawa Mountain of central Japan, Kohistan area of Pakistan, Ichinomegata of NE Japan, Takashima and Kurose of SW Japan, and granulite-facies complex of East Antarctica. Compressional (P) and shear (S) wave velocities for these rock specimens are measured in piston cylinder apparatus. In order to compare directly to seismic velocities at the deep island arc pressures and temperatures, we developed ultrasonic velocity measurements using buffer rod technique. Pt buffer rod is used to isolate the piezoelectric transducer from the high-temperature condition. Travel times through the rock sample were determined with the pulse reflection technique. We are developing a method for simultaneous P-wave and S-wave velocity measurements using dual-mode piezoelectric transducer which generates P-waves and S-waves simultaneously. Using these techniques, we can determine Vp/Vs ratio and Poisson's ratio precisely.

  12. Uranium occurence in California near Bucaramanga (Columbia)

    International Nuclear Information System (INIS)

    The mining district of California, Bucaramanga, is on the west side of the Cordillera Oriental in the Santander massif region. The oldest rocks of the area form a complex of metamorphites and migmatites of the predevonic age. Amphibolite various types of paragneiss and orthogneiss are represented. Several stages of metamorphism can be documented in some rocks, as well as double anatexis. Triassic to jurassic quarz diorites and leukogranites show wide distribution. Porphyric rocks of granodioritic to granitic composition, to which the uranium mineralization is mainly bonded, intruded into the sediments of the lower cretaceous. Atomic absorption spectral analyses were carried out for the elements Cu, Zn and Li, as well as the uranium contents of some samples using fluorimetry. Uranium is primarily bonded to pitch blende and coffinite. The latter mostly occur in fine distribution grown in quarz and belong to the most recent mineralization phase. Autunite, meta-autunite, torbernite, meta-torbernite, zeunerite, meta-zeunerite and meta uranocircite detected as secondary uranium minerals. (orig./HP)

  13. Re-Os system of black schist from the Mesoproterozoic Bayan Obo Group, Central Inner Mongolia, China and its geological implications

    Science.gov (United States)

    Liu, Yifei; Bagas, Leon; Nie, Fengjun; Jiang, Sihong; Li, Chao

    2016-09-01

    The Mesoproterozoic Bayan Obo Group in central Inner Mongolia, China, represents a sedimentary sequence deposited in the Bayan Obo Rift Basin. New Re-Os dates are reported here for the black shale (schist) from the group. Re-Os geochronology of the black schist provides an age of 1447 ± 42 Ma, which is consistent with the previous work from the literature and provides a direct and new constraint for the age of the group. The dating indicates that the Re-Os isotopes for the black schist have remained undisturbed during lower amphibolite facies metamorphism. The ca. 250 Ma age for gold-bearing sulfides also indicates that the Re-Os isotopes for the schist have not been affected by hydrothermal overprinting of reduced fluid. The initial 187Os/188Os value of 0.93 ± 0.14 for the schist is interpreted to represent the Os isotopic composition of the seawater during deposition of the group, which indicates that the Os composition of Mesoprotozoic seawater fluctuated during the Mesoproterozoic, and was temporary very radiogenic similar to present day seawater. The Re-Os data suggests that rifting of the Bayan Obo Basin peaked during ca. 1447 Ma, and the Bilute Formation in the Bayan Obo Group may have been deposited in the Bayan Obo Sea in a restricted or semi-restricted depositional environment.

  14. The effect of a corrugated breakaway on drainage configuration: The Miocene Shadow Valley supradetachment basin

    Energy Technology Data Exchange (ETDEWEB)

    Friedmann, S.J.; Davis, G.A.; Burbank, D.; Brudos, T. (Univ. of Southern California, Los Angeles, CA (United States). Dept. of Geological Sciences)

    1993-04-01

    The late Miocene Shadow Valley basin formed above the Kingston Range/Halloran Hills detachment system due to regional extension. The fault's original geometry is corrugated (curviplanar), and cuts earlier structures of the southern Cordillera thrust belt. Synformal corrugations cut the entire belt and locally into the craton. Small, proximally sourced basins, which are involved in the deformation at the breakaway fault, form in these synformal corrugations. The footwall thrusts are not affected by Tertiary transverse folding and their traces continue unwarped across areas where they are cut out by the detachment. This indicates that the curviplanar nature of the detachment surface is primary. Paleodrainage data in the form of paleocurrents, clast counts, and three-dimensional mapping of facies indicate long-lived westward transport derived from the footwall. In particular, clasts are sourced from the footwall east of the forward-most Mesozoic thrust. Source lithologies include cratonal amphibolitic gneisses, Tapeats Sandstone, and Cretaceous Delfonte Volcanics. These lithologies are only exposed at or near the head of the synformal detachment corrugations. In addition, large fans bearing these clasts have paleocurrent and facies trends in alignment with the corrugations. Fans are composed predominantly of debris flows, often with clast diameters greater than one meter. These data suggest that corrugations create large catchments, probably by knickpoint incision at the breakaway. The catchments shunt footwall material into the basin; high rates of sediment supply are inferred from the abundance of mass wasting deposits, the coarseness of fan material, and radiometric constraints.

  15. New insights into the history and origin of the southern Maya block, SE Mexico: U-Pb-SHRIMP zircon geochronology from metamorphic rocks of the Chiapas massif

    Science.gov (United States)

    Weber, Bodo; Iriondo, Alexander; Premo, Wayne R.; Hecht, Lutz; Schaaf, Peter

    2007-01-01

    The histories of the pre-Mesozoic landmasses in southern México and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea. The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, México, were dated by U–Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (∼ 272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252–254 Ma) medium- to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0–1.2 or 1.4–1.5 Ga sources, indicating provenance from South American Sunsás and Rondonian-San Ignacio provinces.

  16. Rb-Sr geochronology of the rocks of the Himalayas, Eastern Nepal, (1)

    International Nuclear Information System (INIS)

    Rb-Sr isotopic measurements were carried out for whole rock and small sliced rock from the Himalayan gneiss which constitutes the metamorphosed basement of the Tethyan sediments. The results of the measurements on whole rock from the Barun migmatite, the Barun gneiss and the Irkhua gneiss indicate that complete Sr isotopic redistribution occurred about 520 m.y. ago. This age is interpreted as the time of the regional metamorphism. The analytical results of the small sliced slabs of the Himalayan gneiss indicate that Sr isotopic redistribution occurred among the sliced slabs 33.3 +- 13.2 m.y. ago. This age is interpreted as the time of the metamorphism in the sillimanite-amphibolite facies. The high initial 87Sr/86Sr ratio of 0.7372 +- 0.0031 from the Barun migmatite of the Himalayan gneiss suggests the Precambrian Origin of the source rocks. The analytical results on the sliced slab of the Barun migmatite indicate that the original age of the rocks is about 800 m.y. The rock is interpreted as the remobilized Precambrian crustal rock. The high initial 87Sr/86Sr ratio of the Barun gneiss and the Irkhua gneiss of the Himalayan gneiss (0.7234 +- 0.0013) indicates also the Precambrian origin of the source rocks. (author)

  17. Geology and industrial mineral resources of the Macon-Gordon Kaolin District, Georgia

    Science.gov (United States)

    Buie, Bennett Frank; Hetrick, J.H.; Patterson, S.H.; Neeley, C.L.

    1979-01-01

    The Macon-Gordon kaolin district is about 80 miles (130 km) southeast of Atlanta, Georgia. It extends across the boundary between, and includes parts of, the Piedmont and Atlantic Coastal Plain physiographic provinces. The rocks in the Piedmont are mainly intensely folded sericite schist and granite gneiss containing irregular masses of amphibolite and feldspathic biotite gneiss and scattered igneous intrusive rocks. Most of the crystalline rocks are thought to be of Paleozoic age, but some of the intrusive rocks may be younger. The crystalline rocks are cut by a major unconformity and are overlain by sedimentary formations ranging in age from Cretaceous to Miocene. The valuable kaolin deposits occur in the Cretaceous beds, undivided, and in the Huber Formation which is of Paleocene to middle Eocene age. The resources of kaolin in the district are estimated in millions of metric tons as follows: reserves, 100; subeconomic resources, 700 to 900; undiscovered resources, probably 700 to 1,000. In addition to kaolin, the leading mineral commodity mined in the district, crushed stone and sand are now being produced, and fuller's earth and a minor amount of limestone were formerly produced. The crushed stone is quarried from igneous rocks in the Piedmont province. The sand is washed from the Cretaceous beds, undivided. The fuller's earth was mined from the Twiggs Clay Member of the Barnwell Formation, and limestone was dug from the Tivola Limestone.

  18. Complete Alpine reworking of the northern Menderes Massif, western Turkey

    Science.gov (United States)

    Cenki-Tok, B.; Expert, M.; Işık, V.; Candan, O.; Monié, P.; Bruguier, O.

    2016-07-01

    This study focuses on the petrology, geochronology and thermochronology of metamorphic rocks within the northern Menderes Massif in western Turkey. Metasediments belonging to the cover series of the Massif record pervasive amphibolite-facies metamorphism culminating at ca. 625-670 °C and 7-9 kbars. U-Th-Pb in situ ages on monazite and allanite from these metapelites record crustal thickening and nappe stacking associated with the internal imbrication of the Anatolide-Taurides platform during the Eocene. In addition, new 39Ar/40Ar single muscovite grain analyses on deformed rocks were performed in three localities within the northern Menderes Massif and ages range from 19.8 to 25.5 Ma. These mylonites may be related to both well-known detachments, Simav to the north and Alaşehir to the south, which accommodate Oligo-Miocene exhumation of the Menderes core complex. U-Th-Pb data on monazite grains (22.2 ± 0.2 Ma) from migmatites emplaced within the Simav detachment confirm these ages.

  19. High-pressure granulite from Western Kunlun,northwestern China:Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were ob-tained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic con-dition of these high-pressure granulites is about 760―820℃,1.0―1.2 GPa and the retrograde meta-morphic condition is about 620―720℃,0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening,subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important im-plications for the research on Tethys and Paleo-Asian tectonic zone.

  20. 3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

    Science.gov (United States)

    Kampmann, Tobias C.; Stephens, Michael B.; Weihed, Pär

    2016-06-01

    Altered and mineralized rocks at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic Bergslagen ore district in the south-western part of the Fennoscandian Shield, have been metamorphosed at low-pressure, amphibolite-facies conditions and affected by ductile deformation. Using combined surface mapping of lithology and structure, drill core logging and microstructural work, the polyphase (D1 and D2) ductile deformation is demonstrated and a 3D model for the deposit created. Mineral associations include quartz, biotite, cordierite, anthophyllite, and minor almandine, andalusite and chlorite in silicate-rich altered rock, calcite or dolomite in marble and tremolite-actinolite or diopside-hedenbergite in skarn. The silicate minerals show varying growth patterns during the different phases of the tectonothermal evolution, with considerable static grain growth occurring between D1 and D2, and even after D2. F2 sheath folding along axes that plunge steeply to the SSE, parallel to a mineral stretching lineation and the dip direction of the S2 foliation, is suggested as a key deformation mechanism forming steeply plunging, cone- to rod-shaped mineralized bodies. This contrasts with a previous structural model invoking fold interference. A major shear zone with talc-chlorite-(quartz-biotite) mineral association separates the northern and southern structural domains at the deposit and bounds the polymetallic massive sulphides to the north.

  1. K-Ar geochronology of the Kulu-Mandi Belt, NW Himalaya, India

    International Nuclear Information System (INIS)

    The K-Ar dates of micas and whole rock amphibolites from the Kulu-Mandi Belt define two distinct groups, (1) 20 to 75 m. y., and (2) 277 to 366 m. y. Our data together with the other available K-Ar and Rb-Sr mineral and whole rock data, enable us to confirm three major events in the Himalaya, the Late Precambrian-Cambrian Assyntian (Cadomian) Orogenic cycle, the Late Palaeozoic Hercynian Magmatic-Epeirogenic cycle and the Late Cretaceous-Teritiary Himalayan Orogenic cycle. The mineral dating is significant for delineating different phases of the last i.e. the Himalayan Oregeny. The radiometric data, so far to hand, indicate that the main activity of the young, Himalayan metamorphism was probably around 50 to 70 m. y. (Late Cretaceous-Eocene) and this was followed by a major uplift during the 10 to 25 m. y. (Mid. Miocene) time, which was responsible for thrusting and formation of nappe structures in the Himalaya. (orig.)

  2. Genesis of the Hongzhen metamorphic core complex and its tectonic implications

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    The Hongzhen metamorphic core complex is situated in the Yangtze plate to the east of the Dabie oro- genic belt. Its ductile detachment zone in the foot wall overprints on the metamorphic complex of the Proterozoic Dongling Group. The present profile of the ductile shear zone with consistent SW-dipping mineral elongation lineation shows antiform and reversed S-shape from northeast to southwest respectively. Exposure structures, microstructures and quartz C-axis fabric all indicate top-to-SW movement for the ductile shear zone. Recrystallisation types of quartz and feldspar in the mylonites demonstrate that the shear zone was developed under the amphibolite facies condition and at mid-crust levels. The metamorphic core complex formed in the Early Cretaceous with a muscovite plateau age of 124.8±1.2 Ma. Regional NE-SW extension along a SW-dipping, gentle detachment zone was responsible for formation of the core complex. Intrusion of the Hongzhen granite with a biotite plateau age of 124.8±1.2 Ma rendered the ductile shear zone curved, uplifted and final localization of the core complex. The Hongzhen metamorphic core complex suggests that the Early Cretaceous magma- tism in this region took place under the condition of regional extension and the eastern Yangtze plate also experienced lithospheric thinning.

  3. Strontium geochemistry and carbon and oxygen isotopic compositions of Lower Proterozoic dolomite and calcite marbles from the Marmorilik Formation, West Greenland

    International Nuclear Information System (INIS)

    The Marmorilik Formation, Rinkian mobile belt, West Greenland, is a large, Lower Proterozoic carbonate-rock sequence, deformed and metamorphosed under greenschist to amphibolite facies conditions. The pre-deformation thickness of the sequence is at least 2000 m, with about 1400 m of dolomite marble and 350 m of calcite marble. Strontium contents of forty-two dolomite and calcite marbles range from 30 to 100 ppm and 300 to 800 ppm, respectively, whereas samples with calcite of secondary origin have strontium contents between 80 ppm and 200 ppm. Carbon and oxygen isotope ratios were determined for forty calcite and dolomite marbles as -0.2+-1.0 per 1000 delta13C and -9.9+-1.5 per 1000 delta18O (vs. PDB) and are compatible with the isotopic compositions of unmetamorphosed carbonates of similar age. Calcite from eight calciumsilicate rocks, breccias and calcite veins is significantly more negative in delta13C and delta18O. Five 13C analyses of graphite in marble range from -9.6 to -14 per 1000. Possible post-depositional changes in the strontium content and carbon and oxygen isotope compositions are discussed. It is concluded that (a) the calcite marbles are not dedolomites and are therefore of primary origin, (b) the delta 13C and delta 18O values of the marbles are primary or diagenetic (i.e., pre-metamorphic), and (c) the isotopic composition of the graphite is compatible with, though not necessarily evidence for, a biogenic origin. (Auth.)

  4. Precambrian metamorphic conditions and crustal evolution, northeastern Alberta, Canada

    International Nuclear Information System (INIS)

    A complex of Precambrian polymetamorphic gneisses and granitoids of the Churchill structural province, northeastern Alberta, Canada has been examined structurally, petrographically, chemically and geochronologically. An Archean basement gneiss complex is indicated by Rb-Sr dating of pegmatites which cut both gneisses and granitoids (2470 +- 26 Ma with an initial 87Sr/86Sr ratio of 0.7030 +- 0.0008). A high pressure granulite facies (M1) mineral assemblage and older structures (D1) are assigned to the Archean. A moderate-pressure granulite facies (Msub(2.1)), a low-pressure amphibolite facies (Msub(2.2)), a greenschist facies (Msub(2.3)), and younger structures (D2) are of Aphebian age. Formation of granitoids by anatexis of the pre-existing Archean basement complex during Msub(2.1) is indicated by their Aphebian ages (ca. 1900 Ma) and high initial 87Sr/86Sr ratios (0.7100 +-0.0018). The path of retrograde metamorphism is linked with relatively slow rates of uplift and cooling. Late Aphebian sediments attained low-grade greenschist facies metamorphism only and are younger than the other metamorphic rocks. The tectonic evolution of this Precambrian mobile belt during the Aphebian contrasts with the stable Archean cratonic block in the Slave province to the north. (Auth.)

  5. Brazil Geological Basic Survey Program - Lima Duarte - Sheet SF.23-X-C-VI - Minas Gerais State

    International Nuclear Information System (INIS)

    The present report refers to the Lima Duarte sheet (SF.23-X-C-VI) systematic geological mapping, on the 1:100.000 scale. The surveyed area, localized in the Zona da Mata, Juiz de Fora micro-region, in South Minas Gerais, is dominantly composed by metamorphic rocks of the granulite and amphibolite facies and presents important diphtheritic process. An analysis of the Crustal Evolution Patterns based mostly on geological mapping, and gravimetric, air magneto metric and geochronologic data is given in the Chapter 6, Part II, of the text. Geophysical information is in the Chapter 5, Part II. Seventy two samples were analysed for oxides, trace-elements and REE, to provide litho environment and metallogenesis definition subsidies. Were studied 174 petrographic thin section, and 48 samples of quartzite and schist residual materials were analysed for heavy metals. Seven hundred and fifty outcrops were described. A geochemical survey, based on 81 pan concentrated samples and 277 stream sediments was carried out throughout the Sheet. The anomalies found in the stream sediments reflect the geochemical signature of the analysed elements for the litho types of the investigated terrains. (author)

  6. Petrological and Mineralogical Study of Enclaves in Plutons in the Typical Mining Districts of Tongling,Anhui and Its Bearing on the Process of Magmatism—Metallogeny

    Institute of Scientific and Technical Information of China (English)

    杜杨松

    1999-01-01

    Two types of enclaves occur in magmatic plutons in Tongling,Anhui.Enclaves of the first type are residuals of metamorphic rocks of high amphibolite facies,and those of the other type are magmatic rocks ranging from monzonitic to dioritic in composition. A combined petrological and mineralogical study has been carried out on the two types of enclaves in order to estimate their forming conditions and analyze their relations to their hosts.so as to have an insight into the material sources of magmatic rocks and associated mineral deposits and give a clue to better understanding the mechanism of magmatism-metallogeny.This leads us to propose a new metallogenic model for strats-bound skarn-type ore deposits associated with a syntectic type of magmatic rocks.The new model can be simply summarized as partial melting of old metamorphic basement rocks at depth and accumulating,differentiating and positioning of magmas to form deep-level and shallow-level magma chambers,follower by mixing of different magmas associated with their crypto-explosion,migration of gas-bearing ore fluids and precipitation of metals in fluids within the magmas.

  7. Episodic construction of the Tatra granitoid intrusion (Central Western Carpathians, Poland/Slovakia): consequences for the geodynamics of Variscan collision and Rheic Ocean closure

    Science.gov (United States)

    Gawęda, Aleksandra; Burda, Jolanta; Klötzli, Urs; Golonka, Jan; Szopa, Krzysztof

    2016-06-01

    The Tatra granitoid pluton (Central Western Carpathians, Poland/Slovakia) is an example of composite polygenetic intrusion, comprising many magmatic pulses varying compositionally from diorite to granite. The U-Pb LA-MC-ICP-MS zircon dating of successive magma batches indicates the presence of magmatic episodes at 370-368, 365, 360, 355 and 350-340 Ma, all together covering a time span of 30 Ma of magmatic activity. The partial resorption and recycling of former granitoid material ("petrological cannibalism") was a result of the incremental growth of the pluton and temperature in the range of 750-850 °C. The long-lasting granitoid magmatism was connected to the prolonged subduction of oceanic crust and collision of the Proto-Carpathian Terrane with a volcanic arc and finally with Laurussia, closing the Rheic Ocean. The differences in granitoid composition are the results of different depths of crustal melting. More felsic magmas were generated in the outer zone of the volcanic arc, whilst more mafic magmas were formed in the inner part of the supra-subduction zone. The source rocks of the granitoid magmas covered the compositional range of metapelite-amphibolite and were from both lower and upper crust. The presence of the inherited zircon cores suggests that the collision and granitoid magmatism involved crust of Cadomian consolidation age (c. 530 and 518 Ma) forming the Proto-Carpathian Terrane, crust of Avalonian affinity (462, 426 Ma) and melted metasedimentary rocks of volcanic arc provenance.

  8. Quartz deformation mechanisms during Barrovian metamorphism: Implications from crystallographic orientation of different generations of quartz in pelites

    Science.gov (United States)

    Rahimi-Chakdel, A.; Boyle, A. P.; Prior, D. J.

    2006-12-01

    The behaviour of quartz during metamorphism is studied based on two case studies from the Barrovian terrains of Sulitjelma in arctic Scandinavia and Loch Tay in the Central Highlands Dalradian of Scotland. Both terrains preserve evidence for metamorphism in pelites involving nucleation and growth of garnet at different times in the deformation history. Data are presented on the size, shape and crystallographic orientation of quartz preserved as inclusions in garnet and as grains in the surrounding matrix. While quartz-grains remain small and dispersed between mica grains, deformation appears to be dominated by grain-boundary sliding accommodated by dissolution-precipitation. At amphibolite facies, textural coarsening occurs by dissolution of small quartz grains and growth of larger quartz grains, coupled with segregation of quartz from mica. As a result, quartz deforms by dislocation creep, developing crystallographic preferred orientations (CPO) consistent with both coaxial and non-coaxial strain. Quartz CPOs with axes lying parallel to foliation and stretching direction are commonly developed, and best explained by mechanical rotation of inequant (detrital?) quartz grains. There is no evidence for selective entrapment of quartz inclusions in garnet on the basis of quartz crystallographic orientation.

  9. Petrography and zircon U-Pb isotopic study of the Bayanwulashan Complex: Constrains on the Paleoproterozoic evolution of the Alxa Block, westernmost North China Craton

    Science.gov (United States)

    Wu, Sujuan; Hu, Jianmin; Ren, Minghua; Gong, Wangbin; Liu, Yang; Yan, Jiyuan

    2014-11-01

    The Bayanwulashan Metamorphic Complex (BMC) exposes along the eastern margin of the Alxa Block, the westernmost part of the North China Craton (NCC). BMC is principally composed of metamorphic rocks with amphibole plagiogneiss, biotite plagioclase gneiss and granitic gneiss. Our research has been focused on the petrography and zircon U-Pb geochronology of the BMC to better understand the evolution of the Alxa Block and its relationship with the NCC. Evidences from field geology, petrography, and mineral chemistry indicate that two distinct metamorphic assemblages, the amphibolite and greenschist facies, had overprinted the preexisting granitic gneiss and suggest that the BMC experienced retrograde metamorphic episodes. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of BMC were at ca. 2.30-2.24 Ga and the two metamorphic events were at ca. 1.95-1.91 Ga and ca. 1.88-1.85 Ga respectively. These ages indicate that BMC initially intruded during Paleoproterozoic, not as previously suggested at Archean period. The Early Paleoproterozoic metamorphic records and the magmatic thermochronological data in BMC exhibit different evolution paths between the Alxa Block and the NCC. The Alxa Block was most likely an independent Early Paleoproterozoic terrain. Following different amalgamation processes, The Alxa Block combined with Western Block at ca. 1.95 Ga and then united with NCC at ca. 1.85 Ga.

  10. Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

    Institute of Scientific and Technical Information of China (English)

    蔡金郎; 魏光华; 王庆树

    2002-01-01

    The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammiticschists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist andmetabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz + tourmaline ±titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ±garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consistsof muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist orthose of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivinewas subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed promi-nently on the psammitic schists, occasionally on metabasitic rocks, but not on migmatitic rocks. The basement experiencedmetamorphism up to temperature 606-778C and pressure 4.8-6.1 kbar (0.48-0.61 GPa), equivalent to amphibolite-granulite facies. The peak of the metamorphism is marked by a migmatization which occurred at several localities alongthe studied route 587-535 Ma ago. The basement also recorded a retrograde metamorphism of greenschist facies, duringwhich biotite, garnet, amphibole, and pyroxene were partly altered to chlorite.

  11. Integrated evaluation of the geology, aero gamma spectrometry and aero magnetometry of the Sul-Riograndense Shield, southernmost Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Hartmann, Leo A.; Savian, Jairo F., E-mail: leo.hartmann@ufrgs.br [Universidade Federal do Rio Grande do Sul (UFRS), Porto Alegre, RS (Brazil). Instituto de Geociencias; Lopes, William R. [Servico Geologico do Brasil (CPRM), Porto Alegre, RS (Brazil). Gerencia de Geologia e Mineracao

    2016-03-15

    An integrated evaluation of geology, aero gamma spectrometry and aero magnetometry of the Sul-Riograndense Shield is permitted by the advanced stage of understanding of the geology and geochronology of the southern Brazilian Shield and a 2010 airborne geophysical survey. Gamma rays are registered from the rocks near the surface and thus describe the distribution of major units in the shield, such as the Pelotas batholith, the juvenile São Gabriel terrane, the granulite-amphibolite facies Taquarembo terrane and the numerous granite intrusions in the foreland. Major structures are also observed, e.g., the Dorsal de Cangucu shear. Magnetic signals register near surface crustal compositions (analytic signal) and total crust composition (total magnetic signal), so their variation as measured indicates either shallow or whole crustal structures. The Cacapava shear is outstanding on the images as is the magnetic low along the N-S central portion of the shield. These integrated observations lead to the deepening of the understanding of the largest and even detailed structures of the Sul-Riograndense Shield, some to be correlated to field geology in future studies. Most significant is the presence of different provinces and their limits depending on the method used for data acquisition - geology, aero gamma spectrometry or aero magnetometry. (author)

  12. Progade PT path, prograde fluid flow, metasomatism and hydrous melting in the Osor high-grade HT-LP complex (Catalan Coastal Ranges-CCR, NE Iberia).

    Science.gov (United States)

    Reche, Joan; Martínez, Francisco; Leoz, Gisela

    2015-04-01

    Fast thermal pulses related to HT-LP metamorphism may imply dehydration reaction overstepping, higher than normal fluid production rates, quick local increases in Pfluid and common situations of Pfluid >> Plitostatic and surpassing locally the tensile stresses. This ambient would be favorable to transient hydrofracturing and fluid flow even if the ongoing HT-LP event develops on dominantly ductile crustal levels. In inner zones where temperatures are high enough, hydrous melting and melt migration would be favored as well. Such movement of fluids and melts would tend to be sustained if non-hydrostatic stresses are active during heating, and would be favored in high strain domains such high-T shear zones or along foliation planes. In such scenario, local metasomatic processes and mass-transfer phenomena are expected to occur along these high strain zones and so distributed along tectonic anisotropies. A variety of features found in high T Garnet - biotite-sillimanite±cordierite±plagioclase±K-feldspar±quartz metapelitic gneisses from the Osor Complex (Guilleries massif, CCR), testify from this kind of processes operating in the lower crustal section, at the amphibolite to granulite transition zone during a prograde Variscan HT-LP thermal pulse. Such features include: syn-D2 quartz veining, leucogranitoid (leucotonalite, trondhjemitic) lenses sub parallel to S2 dominant foliation, fibrolite-rich foliation planes and prograde sub-idiomorphic garnet developing preferentially near fluid migration channels (quartz veins) or near melt lenses.

  13. Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan.

    Science.gov (United States)

    Attoh

    2000-05-01

    It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleoproterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamorphism. Heat production was anomalously high ( approximately 10.6 µWm-3) where andalusite-bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels ( approximately 550 degrees -600 degrees C at approximately 3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600 degrees -650 degrees C at 6-7 kbar, heat production in the Archean basement was lower ( approximately 3.7 µWm-3). The effect of heat production rates on the metamorphic pressure-temperature paths was tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite-bearing assemblages formed, was significantly sillimanite or kyanite stability fields; and (2) differences between PTmax estimates for the metamorphic highs based on thermobarometry can be reproduced if thermal history involved significant crustal thickening as well as moderate unroofing rates. PMID:10769161

  14. Light stable isotope study of the Roosevelt Hot Springs thermal area, Southwestern Utah

    Energy Technology Data Exchange (ETDEWEB)

    Rohrs D.T.; Bowman, J.R.

    1980-05-01

    The isotopic composition of hydrogen, oxygen, and carbon has been determined for regional cold springs, thermal fluids, and rocks and minerals from the Roosevelt Hot Springs thermal area. The geothermal system has developed within plutonic granitic rocks and amphibolite facies gneiss, relying upon fracture-controlled permeability for the migration of the thermal fluids. Probably originating as meteoric waters in the upper elevations of the Mineral Mountains, the thermal waters sampled in the production wells display an oxygen isotopic shift of at least +1.2. Depletions of delta /sup 18/O in wole rock, K-feldspar, and biotite have a positive correlation with alteration intensity. W/R mass ratios, calculated from the isotopic shifts of rock and water, range up to 3.0 in a producing horizon of one well, although the K-feldspar has experienced only 30% exchange with the thermal waters. While veinlet quartz has equilibrated with the thermal waters, the /sup 18/O values of K-mica clay, an alteration product of plagioclase, mimic the isotopic composition of K-feldspar and whole rock. This suggests that locally small W/R ratios enable plagioclase to influence its alteration products by isotopic exchange.

  15. Alligator Rivers Analogue project. Geophysics, petrophysics and structure

    International Nuclear Information System (INIS)

    The geophysical and geological field work at Koongarra (including borehole core logging) showed that the site itself is a folded, faulted, variably fractured Precambrian psammitic and pelitic schist sequence with a quasi-horizontal weathered zone superimposed on the steeply dipping rock fabric. The site is flanked by a high resistivity younger sandstone unit to the northwest and by a magnetic amphibolite/ferricrete sequence to the far southeast. The data interpretations elicited the essential structural and broad lithological elements. Gravity, magnetic and electrical laboratory and field studies confirmed a broad folded fractured sequence of dipping layered host rocks weathered in their upper parts and trending in a southwest-northeast direction. Qualitatively interpreted anomalies indicated the trend of the main groundwater movement to the south where dolomites are thought to act as a sink. These drainage features have SP, resistivity and radiometric expression. The roles of the Kombolgie Sandstone as a source of water and the Koongarra Fault as a barrier or otherwise were not established owing to the lack of sufficient samples for testing and also on account of the difficulty of geophysical access over the site's rugged escarpment. 40 refs., 13 tabs., 69 figs

  16. Extensional deformation of post ultrahigh-pressure metamorphism and exhumation process of ultrahigh-pressure metamorphic rocks in the Dabie massif, China

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the post-Indosinian continent-continent oblique collision between the Sino-Korean and Yangtze cratons and ultrahigh-pressure metamorphism (UHPM). The configuration is characterized by a regional tectonic pattern similar to metamorphic core complexes and by the development of multi-layered detachment zones. On the basis of the identification of compressional and extensional fabrics, it is indicated that the exhumation and uplift of ultrahigh-pressure (UHP) metamorphic rocks from the mantle depth to the surface can be divided into at least three different decompression retrogressive metamorphism and tectonic deformation stages, in which the subhorizontal crustal-scale extensional flow in the middle-lower crust under amphibolite facies conditions is an important geodynamic process in the exhumation of UHP metamorphic rocks. Moreover, the extensional flow is probably driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.

  17. Tectonic evolution of the continental crust of South America and its importance in the characterization of uraniferous provinces

    International Nuclear Information System (INIS)

    The tectonic evolution of the South American Continent and its relationship with uranium mineralization is discussed. During the Phanerozoic at least three phases are identified as related to the Andean chain, namely, in the lower Palaeozoic, in the upper Palaeozoic and in the Meso-Cenozoic. Recent systematic age dating of the Precambrian indicates the period of 450-700 million years (m.y.) (Brazilian Cycle) as one of the most important tectonic events in South America. Another age-dating cluster corresponds to the 1700-2100 m.y. interval (Transamazonic Cycle). An even older event within the Archean is identified with datings older than 2600 m.y. in Venezuela (Estado Bolivar), Surinam and Brazil (Bahia, Santa Catarina, Goias). All the Brazilian uranium deposits related to the Brazilian platform, such as Amorinopolis, are located on the eastern border of the platform where the Brazilian tectonic cycle is dominant. The uranium source rocks are of alkaline granitic nature. Other deposits (Itataia, Campos Belos) are associated with polycyclic rocks belonging to the basement of the Brazilian Cycle but were affected by the 450-700 m.y. tectonic event; these amphibolitic facies rocks show alkaline metamorphism and magmatization processes which indicate large geochemical mobility during which important uranium mobilization has taken place. Finally, the Pocos de Caldas deposit is excellent evidence of the important relationship of tectonic reactivations and uranium enrichments within the Brazilian platform. (author)

  18. Geochemical and geochronological constrains from the Heilongjiang Complex, NE China and its tectonic implication

    Science.gov (United States)

    Zhu, Yanlin; Zhao, Guochun

    2016-04-01

    The Heilongjiang complex, a sequence of high-pressure metamorphic rocks belt, is located along the suture zone that separates the Jiamusi and Songliao-Zhangguangcai Range blocks in NE China. The complex consists of mafic-ultramafic rocks, mainly has been metamorphosed to blueschists and greenschists and mica schists. Controversy has long surrounded the Helongjiang complex relating to when and how this complex has formed. The lithological association and geochemical features of the mafic rocks indicate that they were mostly generated in a rifting and oceanic islands settings with OIB and E-MORB affitnities. Magmatic zircons from several mafic samples indicate that they yielded protolithic ages ranging from 275 Ma to 140 Ma. The micaschists outcrop extensively in the Heilongjiang complex interlayered with blueschists, greenschists and amphibolites, but few study were focused on them. Our results about the U-Pb and Hf isotope compositions of the detrital zircons derived from these meta-sedimentary rocks reveal that the youngest concordant 206Pb/238U age is ~170Ma, suggesting that the latest depositional age of the mice-schists happened later than ~170Ma. These data suggest that the rifting between the Jiamusi and Songliao-Zhuangguangcai Range Blocks happened during Permian to Triassic time and this rift further developed into an ocean. The closure time of the ocean between the two blocks is not earlier than 140 Ma. Key words: Heilongjiang Complex, mafic rocks, micaschists Acknowledgements: NSFC (41190070, 41190075) and Hong Kong RGC GRF (HKU7063/13P).

  19. Integrated evaluation of the geology, aero gamma spectrometry and aero magnetometry of the Sul-Riograndense Shield, southernmost Brazil

    International Nuclear Information System (INIS)

    An integrated evaluation of geology, aero gamma spectrometry and aero magnetometry of the Sul-Riograndense Shield is permitted by the advanced stage of understanding of the geology and geochronology of the southern Brazilian Shield and a 2010 airborne geophysical survey. Gamma rays are registered from the rocks near the surface and thus describe the distribution of major units in the shield, such as the Pelotas batholith, the juvenile São Gabriel terrane, the granulite-amphibolite facies Taquarembo terrane and the numerous granite intrusions in the foreland. Major structures are also observed, e.g., the Dorsal de Cangucu shear. Magnetic signals register near surface crustal compositions (analytic signal) and total crust composition (total magnetic signal), so their variation as measured indicates either shallow or whole crustal structures. The Cacapava shear is outstanding on the images as is the magnetic low along the N-S central portion of the shield. These integrated observations lead to the deepening of the understanding of the largest and even detailed structures of the Sul-Riograndense Shield, some to be correlated to field geology in future studies. Most significant is the presence of different provinces and their limits depending on the method used for data acquisition - geology, aero gamma spectrometry or aero magnetometry. (author)

  20. Systematics and problems in isotope work on eclogites

    International Nuclear Information System (INIS)

    In the complex field of metamorphic petrology, the interpretation of isotopic data may become difficult. As exchange distances are extremely short, equilibration depends upon the inter-relationship of a number of different parameters, including temperature, fluid transport and tectonic deformation. Since under normal metamorphic conditions increasing temperature and fluid pervasion facilitate exchange reactions, answers are rather more conclusive in the amphibolite facies than for the greenschist facies. In granulite-facies terrains, fluids being very scarce, equilibration is rendered somewhat more difficult. Under eclogite-facies conditions, disequilibrium amongst the major and trace elements is common and therefore isotopic systems very often do not reach equilibrium. Minerals rarely reach equilibrium in low-T eclogites in contrast to high-T eclogites. Sm-Nd systematics on minerals from high-T eclogites are therefore a very good tool for determining the age of the eclogite paragenesis. U-Pb systematics on zircons from eclogites is also very promising, since, despite the difficult analytical task, the results are easier to interpret than in acidic rocks and can accurately provide both the age of the protolith and the age of high-P metamorphism. H and O isotopes represent a powerful tool to reconstruct crystallization conditions whilst C isotopes help in the interpretation of carbonate eclogites. (Auth.)

  1. Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

    Science.gov (United States)

    Möller, Charlotte; Andersson, Jenny; Dyck, Brendan; Antal Lundin, Ildiko

    2015-06-01

    We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, and stalled at 35-40 km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High-temperature mylonitic gneiss with remnants of kyanite eclogite (P > 15 kbar) composes a basal shear zone 50 km long and eclogite and stromatic orthogneiss constrain the time of eclogitization at 988 ± 6 Ma and 978 ± 7 Ma. Migmatization, concomitant deformation, and exhumation are dated at 976 ± 6 Ma, and crystallization of post-kinematic melt at 956 ± 7 Ma. Orthogneiss protoliths are dated at 1733 ± 11 and 1677 ± 10 Ma (stromatic gneiss) and 1388 ± 7 Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia.

  2. Zircon Uranium-Lead dating, petrography and geochemistry for sub volcanic bodies in southwest of Birjand (Rich area)

    International Nuclear Information System (INIS)

    Rich area's sub volcanic bodies in southwest of Birjand have intruded into Upper Cretaceous ophiolite melange. These rocks are in diorite porphyry to monzonite porphyry range and their main texture is porphyry with micro granular groudmass. Plagioclase and green hornblende phenocrysts are made about 70% of these rocks. Plagioclases are andesine with albitic and carlsbad twining and distinct zoning which altered to calcium carbonate in central parts. Hornblende phenocrysts have altered partly to biotite so show a weakly potassic alteration. The average ratio of Ce/Yb in these rocks are 25.40 and shows they belong to enrich magmatic arc. There is a negative anomaly in high field strength elements such as P, Nb and Ti in these rocks which is a geochemical characteristic of continental margin magmatic arc. High ratio of LREE / HREE, Sr/Y (average is 38.14,) and the amount of SiO2, with the absent of Eu negative anomaly show that these rocks are very similar to high SiO2 adakites. High ratio of Sr/Y can be interpreted as high depth melting in plagioclase unstability and garnet stability field. The source of this magma can be an eclogite or garnet amphibolites which is derived from subducted metamorphosed oceanic plate in east of Iran. Dating with zircon uranium - lead show that this magmatic event occurred about 39.1 ± 0.7 Ma (Late Eocene).

  3. Oxygen and hydrogen isotope compositions of eclogites and associated rocks from the Eastern Sesia zone (Western Alps, Italy)

    Science.gov (United States)

    Desmons, J.; O'Neil, J.R.

    1978-01-01

    Oxygen and hydrogen isotope analyses have been made of mineral separates from eclogites, glaucophanites and glaucophane schists from the eastern Sesia zone (Italian Western Alps). Regularities in (1) hydrogen isotope compositions, (2) order of 18O enrichment among coexisting minerals, and (3) ?? 18O (quartz-rutile) and ?? 18O (quartz-phengite) imply attainment of a high degree of isotopic equilibrium. However, some scattering of ??18O values of individual minerals indicates that the eclogitic assemblage did not form in the presence of a thoroughly pervasive fluid. Minerals from an eclogitic lens enclosed in marble have ??18O values distinctly different from those measured in the other rocks. The ??18O values are high in comparison with other type C eclogites of the world, and it is proposed that the fluid present during the high pressure metamorphism has to a large extent been inherited from the precursor rocks of amphibolite facies. An average formation temperature of 540 ?? C is inferred from the oxygen isotope fractionations between quartz and rutile and between quartz and white mica. This temperature is in accordance with petrologic considerations and implies subduction of the precursor rocks into the upper mantle to achieve the high pressures required. ?? 1978 Springer-Verlag.

  4. Geochemistry and evolution of MORB-type eclogites from the Muenchberg Massif, southern Germany

    International Nuclear Information System (INIS)

    In the Muenchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal Sm-Nd isochroms (clinopyroxene-amphibole-garnet) and one Rb-Sr isochron (clinopyroxene-amphibole-mica) yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher (Nd of 8.7 ± 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high 87Sr/86Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock Sm-Nd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses. (orig.)

  5. Garnet-sillimanite bearing gneisses from Darjeeling, eastern Himalaya: Textural relationship and P–T conditions

    Indian Academy of Sciences (India)

    Divya Prakash; Suparna Tewari

    2015-08-01

    The area around Darjeeling consists of medium grade metamorphic rocks and provides a classic example of inverted Himalayan metamorphism. The area under investigation shows upper amphibolite facies metamorphism (sillimanite-muscovite subfacies), rocks are intimately associated with the migmatites and granites. The presence of quartzite, calc-silicate rocks, graphitic schist and abundance of aluminous minerals like kyanite or sillimanite in these rocks indicate their metasedimentary character. Granetsillimanite bearing gneisses occupy most of the area of Darjeeling but not persistent throughout. Textural relationship suggests sequential growth of progressively higher-grade metamorphic minerals during D1 and D2 deformation. The relative XMg in the minerals varies in the order: biotite>staurolite>garnet, and the XMn decreases in the order: garnet>staurolite>biotite. The P–T evolution of these garnetsillimanite gneiss has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and Perple_X software in the KFMASH model system, the combination of these three approaches demonstrates that the Darjeeling gneisses experienced peak pressure and temperature at 7.0 ± 0.3 kbar and 700 ± 30°C. The observation in this study has important bearing on the inverted metamorphism in the Himalayan metamorphic belt.

  6. Geochemical characteristics of Mesoproterozoic metabasite dykes from the Chhotanagpur Gneissic Terrain, eastern India: Implications for their emplacement in a plate margin tectonic environment

    Indian Academy of Sciences (India)

    Rajesh K Srivastava; Anup K Sinha; Suresh Kumar

    2012-04-01

    A number of mafic intrusive bodies (mostly dykes) are exposed in the Chhotanagpur Gneissic Terrain (CGT). Most dykes trend in ENE–WSW to E–W following major structural trends of the region. These metabasite dykes show granoblastic to grano-nematoblastic textures and contain hornblende, plagioclase, chlorite, quartz and epidote which suggest their metamorphism under amphibolite grade P–T conditions. Although no radiometric age is available for the metabasite dykes, field relationships with host rock and available geochronology on granitoids suggest their emplacement during Mesoproterozoic. Geochemical characteristics of these dykes classify them as low-K tholeiite to medium-K calcalkaline type. At least two types of metabasite dykes are recognized on the basis of their HFSE contents; one group shows entirely calc-alkaline nature, whereas the other group has rocks of tholeiite-calc-alkaline series. High Mg#observed in a number of samples indicates their derivation from primary melt. Multielement spidergrams and rare-earth element patterns observed in these samples also corroborate their derivation from different magma batches. Trace element patterns observed for Nb–Ta, Hf–Zr, Sr and Y suggesting involvement of subduction related processes in the genesis of CGT metabasite dykes. Perceived geochemical characteristics suggest that metamorphism did not affect much on the chemistry of metabasites but source region, responsible for the generation of CGT metabasites, was possibly modified during subduction process. This study suggests that magma generated in a destructive plate setting fed the Mesoproterozoic mafic dykes of the CGT.

  7. The history of crustal uplift and metamorphic evolution of Panzhihua-Xichang micro-palaeoland, SW China:Constraints on Sm-Nd, 40Ar/39Ar and FT ages of granulites

    Institute of Scientific and Technical Information of China (English)

    XU Shijin; LIU Wenzhong; WANG Rucheng; YU Hangbo; LI Daming; WAN Jinglin; FANG Zhong

    2004-01-01

    Panzhihua-Xichang (Panxi) micro-palaeoland is the oldest terrane on the western margin of the Yangtze Block. Some intermediate-basic granulites are considered to be the crystalline basement of lower crust in the terrane. Granulite-facies metamorphism of the granulites was developed in the period from 1186 Ma to 1128 Ma. The origin of granulites was related to the collision orogenic process occurring when the micro-palaeolands merged to form the Rodinia Supercontinent. Amphibolite-facies retrogressive metamorphism of granulites took place in the period from 877 Ma to 825 Ma. This period was consistent with the breakup time of the Rodinia Supercontinent. 40Ar/39Ar ages and fission track (FT) ages of granulites in the Panxi micro-palaeoland show that the vertical movement history of crustal rocks was a slow uplift process of the rigid terrane in the time from Neoproterozoic to Mesozoic. The subduction of India Plate towards Euroasia Plate resulted in the rapid uplift of the Qinghai-Tibetan Block in Cenozoic.Meanwhile, the Qinghai-Tibetan Block moved towards east. Consequently the Panxi terrane was uplifted rapidly. As a result of the collision orogeny between the Qinghai-Tibetan Block and the Panxi terrane, the granulite-facies crystalline basement in this region was exhumed and exposed to the surface.

  8. Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

    Science.gov (United States)

    Engvik, A.K.; Elevevold, S.; Jacobs, J.; Tveten, E.; de Azevedo, S.; Njange, F.

    2007-01-01

    Granulite-facies metamorphism is extensively reported in Late Neoproterozoic/Early Palaeozoic time during formation of the East-African-Antarctic orogen (EAAO). Metamorphic data acquired from the Pan-African orogen of central Dronning Maud Land (cDML) are compared with data from northern Mozambique. The metamorphic rocks of cDML are characterised by Opx±Grt-bearing gneisses and Sil+Kfs-bearing metapelites which indicate medium-P granulite-facies metamorphism. Peak conditions, which are estimated to 800-900ºC at pressures up to 1.0 GPa, were followed by near-isothermal decompression during late Pan-African extension and exhumation. Granulite-facies lithologies are widespread in northern Mozambique, and Grt+Cpx-bearing assemblages show that high-P granulite-facies conditions with PT reaching 1.55 GPa and 900ºC were reached during the Pan-African orogeny. Garnet is replaced by symplectites of Pl+Opx+Mag indicating isothermal decompression, and the subsequent formation of Pl+amphibole-coronas suggests cooling into amphibolite facies. It is concluded that high-T metamorphism was pervasive in EAAO in Late Neoproterozoic/Early Paleozoic time, strongly overprinting evidences of earlier metamorphic assemblages.

  9. Bedrock Model of the Syyry area

    International Nuclear Information System (INIS)

    Preliminary site investigations implemented in accordance with the research programme drawn up by Teollisuuden Voima Oy (TVO) were carried out at Syyry (in Finland) in 1987-1992. Models of the site were compiled and used for describing the rock types, fracturing, fracture structures and geohydrological conditions, the main emphasis being on the examination of the bedrock fracturing and related hydraulic conductivity. Three-dimensional models were used for the classification of the various properties of the bedrock structures. The descriptive models were gathered into a computer system to facilitate illustration and storage. The main rock type at Syyry is tonalite. A mica gneiss formation SE of the investigation site dips towards the NW and delimits the tonalite as far as the central part of the investigation site. The miga gneiss has a heterogeneous composition and includes intermediate layers consisting of quartz feldspar schist and amphibolite. There are mafic formations in the vicinity of the investigation site. The intrusive rocks have been deformed during three plastic and three mainly brittle deformation stages. (47 refs., 61 figs.)

  10. An isotopic study of granitoids in the Litchfield Block, Northern Territory

    International Nuclear Information System (INIS)

    The Litchfield Block contains a variety of granitoids, gneissic rocks, and migmatites that are intrusive into metasediments of probable Early Proterozoic age at the western margin of the Pine Creek Inlier. Isotopic data, which include U-Pb measurements on cogenetic zircon and xenotime and Rb-Sr total-rock measurements on the least metamorphosed granodiorites, show that these rocks crystallised from mantle-derived melts between 1840 Ma and 1850 Ma ago. Radiogenic Pb was lost from zircon in the early Palaeozoic (about 435 Ma), but xenotime remained a closed system until recent time. Many granitoids in the Litchfield Block have a gneissic fabric imposed during greenschist to amphibolite-grade regional metamorphism. This disturbance allowed partial isotopic re-equilibration of Rb-Sr total-rock systems, at about 1770 +- 16 Ma. As this igneous and metamorphic evolution is mirrored in other parts of the Pine Creek Inlier, the Halls Creek Inlier, and many other orogenic belts in northern Australia, its recognition and isotopic definition in the Litchfield Block further emphasise the magnitude and chronological integrity of this early Proterozoic tectonic event

  11. Isukasia area: Regional geological setting (includes excursion guide)

    Science.gov (United States)

    Nutman, A. P.; Rosing, M.

    1986-01-01

    A brief account of the geology of the Isukasis area is given and is biased toward the main theme of the itinerary for the area: What has been established about the protoliths of the early Archean rocks of the area - the Isua supracrustal belt and the Amitsoq gneisses? The area's long and complex tectonometamorphic history of events can be divided into episodes using a combination of dike chronology, isotopic, and petrological studies. The earliest dikes, the ca 3700 Ma Inaluk dikes, intrude the earliest (tonalitic) components of the Amitsoq gneisses but are themselves cut up by the injection of the younger (granitic and pegmatitic) phases of the Amitsoq gneisses of the area. The areas of low late Archean deformation, strongly deformed early Archean mafic rocks have coarse grained metamorphic segregations and are cut by virtually undeformed mid-Archean Tarssartoq (Ameralik) dikes devoid of metamorphic segregations. The shows that the area was affected by regional amphibolite facies metamorphism in the early Archean. Late Archean and Proterozoic metamorphic imprints are marked to very strong in the area. Much of the early Archean gneiss complex was already highly deformed when the mid-Archean Tarssartoq dikes were intruded.

  12. From Mesoproterozoic magmatism to collisional Cretaceous anatexis: Tectonomagmatic history of the Pelagonian Zone, Greece

    Science.gov (United States)

    Schenker, Filippo Luca; Burg, Jean-Pierre; Kostopoulos, Dimitrios; Moulas, Evangelos; Larionov, Alexander; Quadt, Albrecht

    2014-08-01

    The magmatic history of the Pelagonian Zone, in northern Greece, is constrained with secondary ion mass spectrometer (SIMS) U-Pb dating on zircons of various granitoids whose structural positions were defined with respect to the regional main foliation. Ages pertain to four groups: (i) Mesoproterozoic (circa 1430 Ma) crystallization of granites inferred from inherited magmatic zircon cores that have been partially molten during the (ii) Neoproterozoic at circa 685 Ma (metamorphic zircon rims) and subsequently intruded by a Neoproterozoic leucogranite (circa 600 Ma). (iii) Late- or post-Variscan calc-alkaline granitoids (315-301 Ma) were in turn intruded by a subvolcanic dike at about 280 Ma. In the Early Permian the ɛNd(t) in magmas decreased from -7.3 to -1.3, hinting to mantle-derived melts produced during extension. Rifting is further heralded by two acidic and one mafic dike containing Lower-Middle Triassic zircons (246-242 Ma). (iv) Early Cretaceous anatectic melts at 117 ± 8 Ma formed during regional metamorphism. This age is the first report of in situ anatexis in the Pelagonian Zone. Cretaceous anatexis developed during the Mesozoic collision of Pelagonia with the Eurasian margin. Major- and trace-element geochemistry of amphibolites further attests for the complex pre-Alpine tectonic history with Neoproterozoic calc-alkaline and back-arc geochemical signature and Triassic alkali-magmatism.

  13. The role of phosphorus, magnesium and potassium availability in soil fungal exploration of mineral nutrient sources in Norway spruce forests.

    Science.gov (United States)

    Rosenstock, Nicholas P; Berner, Christoffer; Smits, Mark M; Krám, Pavel; Wallander, Håkan

    2016-07-01

    We investigated fungal growth and community composition in buried meshbags, amended with apatite, biotite or hornblende, in Norway spruce (Picea abies) forests of varying nutrient status. Norway spruce needles and soil collected from forests overlying serpentinite had low levels of potassium and phosphorus, those from granite had low levels of magnesium, whereas those from amphibolite had comparably high levels of these nutrients. We assayed the fungal colonization of meshbags by measuring ergosterol content and fungal community with 454 sequencing of the internal transcribed spacer region. In addition, we measured fine root density. Fungal biomass was increased by apatite amendment across all plots and particularly on the K- and P-deficient serpentinite plots, whereas hornblende and biotite had no effect on fungal biomass on any plots. Fungal community (total fungal and ectomycorrhizal) composition was affected strongly by sampling location and soil depth, whereas mineral amendments had no effect on community composition. Fine root biomass was significantly correlated with fungal biomass. Ectomycorrhizal communities may respond to increased host-tree phosphorus demand by increased colonization of phosphorus-containing minerals, but this does not appear to translate to a shift in ectomycorrhizal community composition. This growth response to nutrient demand does not appear to exist for potassium or magnesium limitation. PMID:26996085

  14. Microstructures and rheology of the shear zones in granite Marmarajá, Lavalleja Province, Uruguay

    International Nuclear Information System (INIS)

    The study area (coordinates x : 567 , x : 577.7 , y: 6216 : and ' : 6225 km ) is located near the town of Marschallin (Department of Lavalleja). It is represented mostly by granite, deformed granites and quartzite mylonites , whereas amphibolites and volcanic breccias are of small size . The Marmarajá (biotite - monzogranite) batholith, considered to post- orogenic tardi occupies about 80% of the study area , and is fragmented into three sectors per kilometer mylonitic belts by the SW- NE direction. The deformed granite is located west and east of the study area forming an extensive parallel on both sides of the mylonite belt. The mylonites are in topographic low along which the major waterways of the narrow belts direction N50E and dips 40 ° -50 ° to the area SE with thicknesses of up to 1km and lengths of tens of kilometers continuously , north and south of the area study. These belts have similar directions mylonitic the Sierra megatranscurrencia whale and may be contemporaneous to it. In turn, the kinematic indicators suggest sinistral sense justifying further similarity to the previous one. Major fractures have three orientations: N15E ; Vertical to subvertical N64E and N45W ( approx. 80 °). Based on studies of the lithologies petrographic areas of low deformation and is relieved areas of moderate to high strain, each having typical microstructures of ductile deformation (greater than 400 ° C )

  15. Superposition de la tectonique éburnéenne et panafricaine dans les granitoïdes de la bordure nord du craton ouest africain, boutonniére de Zenaga, Anti-Atlas central, Maroc(Pan-african overprint on Eburnian granitoids at the northern boundary of the West African Craton, Zenaga Inlier, central Anti-Atlas, Morocco)

    Science.gov (United States)

    Ennih, N.; Laduron, D.; Greiling, R. O.; Errami, E.; de Wall, H.; Boutaleb, M.

    2001-05-01

    The Zenaga Inlier shows a comprehensive record of the Eburnian and Pan-African Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the Pan-African shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas.

  16. Miocene magmatism in the Western Nyainqentanglha mountains of southern Tibet: An exhumed bright spot?

    Science.gov (United States)

    Weller, O. M.; St-Onge, M. R.; Rayner, N.; Searle, M. P.; Waters, D. J.

    2016-02-01

    The Western Nyainqentanglha (WNT) mountain range of south-central Tibet predominantly comprises granitoids that intrude into metasedimentary strata, and was exhumed from ~ 15 to 20 km depth in the footwall of the Yangbajain graben during the late Neogene. The range provides a rare exposure of deeper crustal levels of Tibet, which can be used to gain insight into processes that may be occurring beneath the plateau. Field, petrological, thermobarometric and U-Pb geochronological analyses are applied to determine the magmatic and metamorphic history of the WNT, revealing three tectonothermal events: (1) 213-201 Ma magmatism and amphibolite-facies metamorphism associated with north-south Lhasa terrane accretion, (2) 140-52 Ma magmatism resulting from subduction of Neotethys preceding India-Asia collision, and (3) 25-8 Ma magmatism that we suggest to have resulted from partial melting of the thickened Tibetan plateau crust. The latter is correlated with seismic bright spots imaged in the region at ~ 15-18 km depth, indicating that restricted partial melting at mid-crustal levels may have been ongoing since 25 Ma, in accordance with observations from xenolith data and predictions made by thermal modelling of thickened crust.

  17. Environmental geophysics: Buildings E5485, E5487, and E5489 decommissioning - the open-quotes Ghost Townclose quotes complex, Aberdeen Proving Ground, Maryland

    International Nuclear Information System (INIS)

    Buildings E5485, E5487, and E5489, referred to informally as the open-quotes Ghost Townclose quotes complex, are potentially contaminated sites in the Edgewood section of Aberdeen Proving Ground. Noninvasive geophysical surveys, including magnetics, EM-31, EM-61, and ground-penetrating radar, were conducted to assist a sampling and monitoring program prior to decommissioning and dismantling of the buildings. The buildings are located on a marginal wetland bordering the west branch of Canal Creek. The dominant geophysical signature in the open-quotes Ghost Town close quotes complex is a pattern of northeast-southwest and northwest-southeast anomalies that appear to be associated with a trench/pipe/sewer system, documented by the presence of a manhole. Combinations of anomalies suggest that line sources include nonmetallic and ferromagnetic materials in trenches. On the basis of anomaly associations, the sewer lines probably rest in a trench, back-filled with conductive, amphibolitic, crushed rock. Where the sewer lines connect manholes or junctions with other lines, ferromagnetic materials are present. Isolated, unidentified magnetic anomalies litter the area around Building E5487, particularly to the north. Three small magnetic sources are located east of Building E5487

  18. The Kan River Gneiss terrane of central Côte D'Ivoire: mylonitic remnants of an ancient magmatic arc?

    Science.gov (United States)

    Mortimer, J.

    1992-11-01

    The Kan River Gneisses (KRG) which crop out to the east of the Birrimian Fetekro supracrustal belt in central Cote d'Ivoire are described. They comprise a TTG intrusive suite, part of which has been metamorphosed to amphibolite facies and extensively deformed by strike-slip deformation. These gneisses are considered to have been originally linked to the better known Dabakala Gneiss to the north. The eastern and western boundaries of the KRG are defined by major crustal shear zones which separate discrete lithostratigraphic terranes. To the east, the Comoe flysch basin lies beyond the Dimbokro strike-slip shear zone. The Boni Andokro shear zone marks the boundary between the KRG and the Toumodi Volcanic Group (TVG) to the west, where there is evidence of shortening and transcurrent movement. The KRG are geochemically similar to modern volcanic arc granites. They are clearly petrologically distinct from the calc-alkaline intrusive granitoids which intrude the TVG, and which geochemically resemble modern volcanic arc and syn-collisional granites. The KRG terrane resembles a deformed magmatic arc. In view of the nature of its eastern and western boundaries, its relations with neighbouring terranes are considered suspect.

  19. Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton

    International Nuclear Information System (INIS)

    Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite- gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3.285 ± 0.17 Ga. εNd = 0.82±0.78 for the Honnavalli meta-anorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033 Ga, εNd = -2.2±0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5 Ga. Our results set a younger age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dharwar craton. (author)

  20. The Pangidi Anorthosite Complex, Eastern Ghats Granulite Belt, India: Mesoproterozoic Sm-Nd isochron age and evidence for significant crustal contamination

    International Nuclear Information System (INIS)

    The Pangidi Anorthosite Complex (PAC) is a small, magmatically layered body emplaced into high-grade supracrustal rocks and metamorphosed together with the host rocks in the southern sector of the Eastern Ghats Granulite Belt, South India. It is dominated by coarse-grained anorthosite and leuconorite, minor leuco-gabbronorite and ultramafics with chromitites. The anorthositic rocks contain plagioclase (An55-70) + orthopyroxene (En50-60) + augite + amphibole + ilmenite + magnetite with accessory olivine, biotite, apatite and rarely coronal garnet related to metamorphic reconstitution. Despite a less calcic plagioclase composition in the anorthosites, the major and trace element distributions are akin to comparable litho types (at similar SiO2 wt %) of the Kondapalli layered anorthosite complex in close proximity. However, the PAC shows distinct effects of metamorphism and significant modal volume of secondary hydrous mineral phases unlike the latter. A five point whole-rock Sm-Nd isochron gives 1739 ± 220 million years (Ma) (2δ) age for the complex, which constraints the younger limit to its intrusion and probably metamorphism under amphibolite to granulite facies conditions. The PAC is characterized by strikingly low εNd(at1750 Ma) of -14.4± 3.7, indicating the importance of crustal contamination in its genesis possibly involving significantly older (Late Archaean) crustal components. (author)

  1. Ground Magnetic Data Interpretation of Ijebu-Jesa Area, Southwestern Nigeria, Using Total Component

    Directory of Open Access Journals (Sweden)

    Kayode John Stephen

    2010-12-01

    Full Text Available The ground magnetic studies of Ijebu-jesa town in Oriade local government area of Osun State Southwestern Nigeria were carried out to study the subsurface geology of the area. The Total Component Magnetic Intensities data was interpreted. The qualitative interpretation of the ground magnetic data showed varying magnetic intensities over the different rock types in this area from the approximation of the geologic boundaries were delineated. Also the quantitative interpretation resulted in generation of geomagnetic section using linear trends gives magnetic intensity values that vary between about -250 nT to about 300 nT. Depth to the basement rock was estimated using slope techniques this allow s the Lateral and Depth estimates of different rock types to be determined. Also basement structures such as bedrock topography; bedrock depression; rock boundaries, contact zone, fractures/faults were delineated which serve as deposits centers’ for mineral resources. The trend analysis was used to produce a residual magnetic map which allows better mapping of Iwaraja fault as seen on Amphibolites schist.

  2. Review of tungsten mineralisation in greenstone belts in the eastern and north-eastern Transvaal

    International Nuclear Information System (INIS)

    Tungsten (scheelite) mineralisation is known to occur, occasionally as noteworthy deposits, in the Swazian greenstone belts in the Eastern and North-Eastern Transvaal. An outline and description are given of all known scheelite occurrence in the Sutherland and Murchison Ranges and possibilities of finding such mineralisation in the Barberton greenstone belt are described. Scheelite deposits in these environments: (i) are commonly associated with gold and/or antimony mineralisation; (ii) although found in a variety of host types, commonly occur in amphibolitic and siliceous carbonate rocks; (iii) are found in host rocks belonging to widely separated stratigraphic units in the greenstone belt succession; (iv) display no obviously consistent genetic associations; (v) are typically weakly mineralised. Attention is drawn to the recently postulated theory that tungsten, and spatially related antimony and mercury, mineralisation in the Murchison Range owes its origin to syngenetic processes of concentration, the implications of which, may prove to be important to future investigations. All indications are, however, that the mineralisation is not characterised by any one particular mode of occurrence and that it may not be all of the same age and origin. All known scheelite deposits in the region under review have thus far proved to be of little economic importance

  3. Phosphorus - uranium mineralization of the Mandacaru Farm, Iraucuba, state of Ceara, Brazil

    International Nuclear Information System (INIS)

    The phosphorus-uranium mineralization of the Mandacaru Farm (Iraucuba - state of Ceara) is located in the phosphorus-uranium Province of north-central Ceara. The area is a mobile belt placed between the Sao Luiz and the Sao Francisco cratons, related to the Northeast Folding Region. It is represented by lithologies from the Fundamental Complex (Transamazonico Cycle) and the Ceara Group ectinict series (Brasiliano Cycle). All the rocks are cut by Eo-Cambrian acidic dikes and Jurassic basic dikes. The plastic tectonics acted over the regional rocks through four folding phases; the first two being of isoclinal recumbent type, and the last two subvertical open folds. The fissural tectonics affected the area in the form of overthrust faults and transcurrent faults. The fracturing system was reactivated as normal faults during the Brasiliano Cycle. The phosphorus-uranium mineralization appears in the form of uraniferous collophane/apatite forming disseminations i gnaisses, calc-silicated rocks and amphibolites; stockwork structures in marbles; and occurring as matrix in breccias and cataclasites. This mineralization occurs in fractured ad faulted areas, associated to diaphthoresis, sodic metasomatism and episyenitization processes. (Author)

  4. Tectonic history of subduction zones inferred from retrograde blueschist P-T paths

    Energy Technology Data Exchange (ETDEWEB)

    Ernst, W.G. (Univ. of California, Los Angeles (USA))

    1988-12-01

    Many Phanerozoic convergent plate junctions are marked by discontinuous blueschist belts, reflecting relatively high-pressure (P) prograde trajectories. Common blueschist paragneisses, such as those of the western Alps, exhibit widespread overprinting by greenschist and/or epidote-amphibolite facies assemblages. For this type of high-P belt, retrograde metamorphism involved fairly rapid, nearly isothermal decompression; some terranes underwent continued heating during early stages of pressure release. Uplift probably occurred as a consequence of the entrance of an island arc, oceanic plateau, or segment of continental crust into the subduction zone (collision), resulting in marked deceleration or cessation of underflow and buoyant, approximately adiabatic rise of the stranded, recrystallized subduction complex. Other high-P belts, such as the Franciscan of western California, preserve metamorphic aragonite and lack a low-P overprint; retrogression approximately retraced the prograde P-T (temperature) path, or for early formed high-grade blocks, occurred at even higher P/T ratios. Parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration. Upward motion took place as tectonically imbricated slices, as laminar return flow in melange zones, and perhaps partly a lateral spreading/extension of the underplated accretionary prism. Retrograde P-T trajectories of high-P belts therefore provide important constraints on the tectonic evolution of convergent plate junctions.

  5. Deformation inside a paleosubduction channel - Insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the Tauern Window, Austria

    Science.gov (United States)

    Keppler, Ruth; Stipp, Michael; Behrmann, Jan H.; Ullemeyer, Klaus; Heidelbach, Florian

    2016-01-01

    The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed to 600 °C and 20-25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units.

  6. Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long-wavelength magnetic anomalies

    Science.gov (United States)

    Kelso, Paul R.; Banerjee, Subir K.; Teyssier, Christian

    1993-01-01

    Rock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudosingle-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to the thermal demagnetization, with nearly 50 percent of the magnetization remaining after 400 C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanant magnetization along the present-day field is likely to dominate.

  7. Uraniferous pegmatites of the Sharlot Lake area, Ontario

    International Nuclear Information System (INIS)

    Reconnaissance airborne gamma ray spectrometric data covering the southern Grenville Province clearly indicate four anomalous zones, namely Mont Laurier and Huddersfield Township in western Quebec, and the Bancroft and Sharbot Lake areas in eastern Ontario. These surveys show that the Sharbot Lake area has a distinctly higher average eU/eTh ratio compared to the other Grenville pegmatite districts. Detailed surveys in the Sharbot Lake area have been particularly useful in providing a comprehensive picture of the pegmatite distribution. The majority of the pegmatites occur as conformable to semiconformable sill-like bodies ranging from bands less than 1 metre wide to bodies exceeding 500 by 50 metres. They are generally white to pale pink, massive to locally foliated and coarse grained. The principal radioactive mineral is uraninite commonly associated with biotite. Other radioactive phases include allanite, monazite, thorite, uranothorite, zircon and apatite. Average equivalent uranium concentrations measured by in situ gamma ray spectrometry range from a low of 3 ppm (averaging 36 ppm eTh) for pegmatites hosted by pink leucogranite gneiss to amphibolite-hosted pegmatite with an averge of 41 ppm (averaging 24 ppm eTh). Locally, equivalent uranium concentrations exceeding 5000 ppm may be found. Field evidence suggests that the pegmatites may have been developed by partial melting of Grenville Supergroup paragneisses. In places the pegmatites show evidence of mobilization and emplacement into adjacent granite gneisses and metasediments

  8. Field Features And Mode Of Emplacement Of Pegmatites Of Keffi Area North Central Nigeria

    Directory of Open Access Journals (Sweden)

    Tanko

    2015-04-01

    Full Text Available Abstract The Keffi area of North Central Nigeria hosts numerous pegmatite bodies which are related to the surrounding granitic intrusions islocated about 45 km east of the Federal Capital Territory Abuja Nigeria. Petrological investigation of the pegmatites and surrounding host rocks aimed at characterising and understanding field relations and mode of emplacement of the rocks with a view to assess their mineralisation potentials were carried out. From the field observations the pegmatites were characterised into 1 Pelitic schist-amphibolite hosted pegmatites and 2 Granitoids orthogneisses hosted pegmatites and the granites into 1 the Bakin Ayini biotite granites 2 the Angwan Madugu biotite-muscovite granites and 3 the Sabongida biotite-muscovite granites. It is clear that those discordantly emplaced in pelitic schists varied in shape and size with length and width ranging from 400-2000m and 2-20m respectively some are huge isolated sill-like and flat-lying whilst those hosted in orthogneisses are narrow ranging in length 40-1000m and width 1-4m crosscutting and vertically oriented along shear zones which suggest passive emplacement

  9. PIXE analysis of museum soapstone sculptures from Esie, south west Nigeria

    International Nuclear Information System (INIS)

    The PIXE technique was employed for the study of Esie museum stone sculptures using 2.55 MeV protons from the 3 MeV tandem accelerator (NEC 3 UDH) in Lund, coupled with the geological and archaeological findings. The aim is to elucidate and decipher the prodigious but rather enigmatic and bewildering stone sculptures. PIXE results show that the composition of the stone sculptures are 40.69% talc-tremolite schist, 30.88% talc-chlorite schist, 15.20% talc-tremolite-anthophyllite schist and 13.24% talc-amphibolite schist. Thus the composition of Esie sculptures are found to be the same with the locally available talc-schists present around Esie. The geological evidence (mineralogical results) corroborated this as there was no textural or mineralogical difference between the talc-bearing country rock (outcrop) in Esie and the museum soapstone samples studied. Consequently, there is a very high probability that the sculptures were carved using the locally available talc-schists. (author). 7 refs, 4 figs, 2 tabs

  10. Structural development of the Red Hill portion of the Feather River ultramafic complex, Pulmas County, California

    Energy Technology Data Exchange (ETDEWEB)

    Weisenberg, C.W.

    1979-01-01

    The Feather River Ultramafic Complex is a partially serpentinized body of metamorphosed alpine peridotite and gabbro that lies along the northern part of the Melones fault zone, a NNW trending belt in the Northern Sierra Nevada. The complex was studied in the area of Red Hill, near the canyon of the North Fork, Feather River. The complex is separated from the Calaveras Terrain and Arlington Formation country rocks by steep faults; the Melones Fault on the east and the Rich Bar Fault on the west. Units recognized within the complex include Rich Bar metamorphic rocks, peridotite, metaperidotite, tremolite-olivine schist, hornblende schist, and layered metagabbro. The Rich Bar metamorphic rocks are tectonic slices of amphibolite grade hornblende schist, mica schist, and quartzite found along the Rich Bar Fault. The complex shows evidence of 4 major events. E-1 (Pennsylvania-Permian) was formation of the peridotite-gabbro complex. E-2 (Permo-Triassic) consisted of pervasive shearing parallel to the Rich Bar Fault associated with initial emplacement within the Sierra Nevada. E-3 is believed to be compression and metamorphism (serpentinization) associated with the Nevadan orogeny. E-4 was associated with intrusion of nearby plutons. The regional association of the complex with late paleozoic arc volcanics of the Taylorsville area suggest formation near or under an island arc. Metamorphism during emplacement indicates association with the arc at that time. Left-lateral shear during emplacement along the Rich Bar Fault indicates NW directed thrusting when the layering in metagabbro is rotated to horizontal.

  11. Petrology and geochemistry of the Eastern Loma de Cabrera Batholith, Dominican Republic

    Energy Technology Data Exchange (ETDEWEB)

    Cribb, J.W.; Lewis, J.F.

    1985-01-01

    The Eastern Loma de Cabrera Batholith, located in the NW Cordillera Central, Dominican Republic, is a heterogeneous intrusive complex composed of a zoned ultramafic-mafic core surrounded by tonalite and diorite. The batholith intrudes metasbasaltic rocks of the Duarte Complex of early Cretaceous age. The ultramafic-mafic core consists of peridotite, olivine-pyroxenite, pyroxenite, and augite-hypersthene gabbro-norite. Pyroxenites and gabbro-norites exhibit large scale interlayering and small scale layering involving a regular variation in the proportions of ortho- and clinopyroxene. Tonalities and diorites are mafic to leucocratic, some being porphyritic. Petrographic types include hornblende, hornblende-pyroxene, hornblende-biotite, and muscovite-biotite types. Aplites are abundant. Intrusive relations suggest that ultramafic-mafic complex is the oldest intrusive phase, and was partially amphibolitized during later intrusion of the felsic rocks. Ultramafic-mafic rocks contain 43-54% SiO/sub 2/ and MgO ranges from 8-45%. Trace and REE in these rocks are relatively depleted. Tonalitic rocks range in SiO/sub 2/ from 53-76%, with K/sub 2/O varying from 0.15-2.9%. In addition, they are LREE enriched. A small Eu anomaly is best explained by fractionation of plagioclase and hornblende. Trends shown by Rb-Sr data suggest that fractional crystallization of hornblende and plagioclase, that is high level fractionation, is the important factor in controlling chemical variation in the tonalites.

  12. K-Ar ages of allochthonous mafic and ultramafic complexes and their metamorphic aureoles, Western Brooks Range, Alaska

    Energy Technology Data Exchange (ETDEWEB)

    Boak, J.L.; Turner, D.L.; Wallace, W.K.; Moore, T.E.

    1985-04-01

    New K-Ar ages from allochthonous mafic and ultramafic complexes of the western Brooks Range (Brooks Range ophiolite) show that igneous rocks yielded ages nearly identical to those of underlying metamorphic aureole rocks. Dated rocks of the Misheguk igneous sequence from Tumit Creek consist of (1) hornblende gabbro with minor greenschist and lower grade alteration, hornblende age 147.2 +/- 4.4 Ma; and (2) hornblende-bearing diorite, also slightly altered, age 155.8 +/- 4.7 Ma. Both samples come from presumed higher levels of the Misheguk sequence. Dated samples of metamorphic aureole rocks come from outcrops near Kismilot Creek and lie structurally beneath the Iyikrok Mountain peridotite body. The rocks consist of amphibolite and garnet-bearing biotite-hornblende gneiss considered to be metamorphosed Copter igneous sequence and related sedimentary rocks. Hornblende ages are 154.2 +/- 4.6 Ma and 153.2 +/- 4.6 Ma. metamorphism is clearly related to the structurally overlying perioditite, as the degree of alteration decreases downward. The authors suggest that the K-Ar ages of these rocks represent the effects of thermal metamorphism post-dating igneous crystallization, and are related to tectonic emplacement of the complex. Earlier K-Ar data on igneous rocks give similar ages and have been interpreted as reflecting tectonothermal events. The age of igneous crystallization of the mafic and ultramafic rocks of the Misheguk igneous sequence remains uncertain.

  13. Integrated evaluation of the geology, aerogammaspectrometry and aeromagnetometry of the Sul-Riograndense Shield, southernmost Brazil.

    Science.gov (United States)

    Hartmann, Léo A; Lopes, William R; Savian, Jairo F

    2016-03-01

    An integrated evaluation of geology, aerogammaspectrometry and aeromagnetometry of the Sul-Riogran-dense Shield is permitted by the advanced stage of understanding of the geology and geochronology of the southern Brazilian Shield and a 2010 airborne geophysical survey. Gamma rays are registered from the rocks near the surface and thus describe the distribution of major units in the shield, such as the Pelotas batholith, the juvenile São Gabriel terrane, the granulite-amphibolite facies Taquarembó terrane and the numerous granite intrusions in the foreland. Major structures are also observed, e.g., the Dorsal de Canguçu shear. Magnetic signals register near surface crustal compositions (analytic signal) and total crust composition (total magnetic signal), so their variation as measured indicates either shallow or whole crustal structures. The Caçapava shear is outstanding on the images as is the magnetic low along the N-S central portion of the shield. These integrated observations lead to the deepening of the understanding of the largest and even detailed structures of the Sul-Riograndense Shield, some to be correlated to field geology in future studies. Most significant is the presence of different provinces and their limits depending on the method used for data acquisition - geology, aerogammaspectrometry or aeromagnetometry. PMID:26840006

  14. 40Ar/ 39Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

    Science.gov (United States)

    Liu, Yongjiang; Genser, Johann; Neubauer, Franz; Jin, Wei; Ge, Xiaohong; Handler, Robert; Takasu, Akira

    2005-04-01

    40Ar/ 39Ar dating and estimates of regional metamorphic P- T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian-Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low-middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212-242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104-172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.

  15. Fluid inclusions hidden in coesite-bearing zircons in ultrahigh-pressure metamorphic rocks from southwestern Sulu terrane in eastern China

    Institute of Scientific and Technical Information of China (English)

    LIU Fulai; XU Zhiqin

    2004-01-01

    Primary fluid inclusions, together with coesite mineral inclusions, are identified in the same zircon domains by laser Raman spectroscopy, cathodoluminescence (CL) image and micro-texture analysis in paragneiss and eclogite from the main drilling hole of Chinese Continental Scientific Drilling Project in southwestern Sulu terrane. Most fluid inclusions are characterized by CO2 (gas)-H2O (liquid) two-phase, a few by H2O one-phase liquid inclusions. These features indicate that the eclogite and its country-rocks may be located in the "wet system" rather than in the "dry system" during UHP metamorphism. SHRIMP U-Pb dating indicates that the timing of trapping the fluid and coesite inclusions in metamorphic zircon domains is about 233.7 ± 4.3 Ma, which may represent the age of zircon growth in the stage of pressure decrease but temperature increase during the retrograde period of UHP metamorphism thus indicating the fluid activity still under the UHP conditions. The zircons further overgrew at about 213.2 ± 5.2 Ma in response to amphibolite-facies retrogression. Therefore, fluid activity in the Sulu UHP metamorphic rocks principally occurred during the exhumation of UHP slab in the Middle to Late Triassic. The present results not only provide insight into the fluid property and fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane, but also present a new means to exactly identify the primary fluid inclusions preserved in zircons from the UHP metamorphic rocks.

  16. Late Proterozoic Colisional Orogen and Geosuture in Southeastern China:Petrological Evidence

    Institute of Scientific and Technical Information of China (English)

    周新民; 朱云鹤

    1993-01-01

    The Jiangshan-Shaoxing fracture belt(JSFB)is a Late Proterozoic geosuture due to island arc-continent collision in South China,The Cathaysian Block(CT),lying on the southeast side of JSFB,is composed of green schist-amphibolite complexes in the form of a series of tectonic flakes. On the northwest side of JSFB,which is located in the border area of Zheijiang,Jiangxi and Ahhhi provinces(abbreviated as ZJP-JXP-AHP),are distrbuted and ophiolite suite and other rocks,constituting the Jiangnan ancient island arc(JN)on the southeast margin of the Yangtze Block(YZ).The collision between JN and CT at-0.9Ga ago led to the folding of JN.followed by the intrusion(-0.9-0.8Ga ago)of many dioritic and ultramafic stitching plutons along the fracture belt.As a result,the basic Precambrian tectonic framework of southeastern China was shaped.

  17. Southern complex: geology, geochemistry, mineralogy, and mineral chemistry of selected uranium- and thorium-rich granites

    International Nuclear Information System (INIS)

    Four major rock groups are defined in the Southern Complex: the Bell Creek Granite (BCG), the Clotted Granitoids (CGR), the Albite Granite (AGR), and the Migmatite Complex. Metatexites of the Migmatite Complex are the oldest rocks and include paleosome of a metasedimentary and metavolcanic protolith represented by Banded Iron Formation, Banded Amphibolite, and Banded Gneisses, and interlayered or crosscutting leucogranites. The CGR span the range from metatexite to diatexite and represent in-situ partial melting of metapelitic layers in the protolith during intrusion of the BCG. The BCG cuts the migmatites, is locally cut by the CGR, and was derived by partial melting of a dominantly metasedimentary protolith at some depth below the presently exposed migmatites during a regional tectonothermal event. The Albite Granite is a 2km diameter, muscovite-fluorite-columbite-bearing intrusive stock that cuts all other major units. The thorium history of the BCG is a function of the history of monazite. The thorium history of the CGR is also dominated by monazite but the thorium content of this unit cannot be entirely accounted for by original restite monazite. The uranium history of the BCG and CGR was dominated by magmatic differentiation and post magmatic, metamorphic and supergene redistributions and is largely independent of the thorium history. The thorium and uranium history of the AGR was dominated by magmatic/deuteric processes unlike the BCG and CGR

  18. The nature and location of the suture zone in the Rokelide orogen, Sierra Leone: Geochemical evidence

    Science.gov (United States)

    Lytwyn, Jennifer; Burke, Kevin; Culver, Stephen

    2006-12-01

    The boundaries of the West African Craton mark the location of a continuous suture zone that records Neoproterozoic to Early Cambrian oceanic closure. The western part of the circum-West African suture zone extends through the line of outcrop of the Mauritanide, Bassaride and Rokelide mountain belts. Our geochemical analyses are consistent with the idea that igneous and metamorphic rocks of the Rokelide and Southern Mauritanide mountain belts of West Africa occupy a suture zone that records the closing of a Neoproterozoic to Early Cambrian ocean basin during the Pan-African orogeny and final assembly of Gondwana. The closing of that basin was marked by the collision between Archean rocks of the Leo massif of the West African Craton and reactivated Archean and Paleoproterozoic rocks that now outcrop nearer to the coast of Africa in Sierra Leone and Liberia. Within the Rokelides, the geochemistry of the Kasewe Hills volcanic rocks and Marampa amphibolite indicate that remnants of an arc system are caught up in the suture zone. The geochemistry of Guingan schists that outcrop along strike of the Rokelides is compatible with the idea that the metamorphosed equivalents of the Marampa and Kasewe Hills arc volcanic rocks extend through the Bassarides and into the Southern Mauritanides.

  19. Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

    Directory of Open Access Journals (Sweden)

    Reddy Steven M

    2006-12-01

    Full Text Available Abstract The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (207Pb/206Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent 207Pb/206Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.

  20. Geophysical investigations of the Romuvaara area, Finland

    International Nuclear Information System (INIS)

    In the study area of Romuvaara, investigations have been carried out during 1987 - 90 with the aim of finding out whether the polyphasically deformed Precambrian gneiss complex is suitable for the final disposal of spent nuclear fuel. The bedrock has been studied by geological, geophysical, geohydrological and geochemical methods. Airborne, ground and borehole geophysical surveys were used in studying the rock type distribution, fracturing and hydraulic conductivity of the bedrock to a depth of one kilometre. Airborne surveys (magnetic, radiometric and two electromagnetic methods) and ground surveys (VLF and VLF-R, magnetic and soil radar methods) were useful in distinguishing the metadiabases, amphibolites and granodiorites from the less magnetized migmatites. The electromagnetic and seismic refraction surveys were used in locating crushed and fractured zones. The rock type distribution was studied by single-hole logging of susceptibility, natural γ radiation and radiometric γ-γ -density. Electrical and acoustic logging served the mapping of fractures and the interpretation of water injection tests. The flow conditions in the boreholes were studied by fluid logging and tube-wave sounding. The rock volume surrounding the boreholes was mapped by borehole radar with a frequency of 22 MHz. The upper parts of the boreholes were also studied by vertical radar profiling (VRP). Larger volumes of rock were mapped by vertical seismic profiling (VSP) using 4 - 5 transmitter shotholes per borehole

  1. The geology of Pedra Verde region, Ceara, Brazil

    International Nuclear Information System (INIS)

    The upper Proterozoic Mambira Formation (≥ 1.1000 m.y.) consists of > 1000 m of micaschists with subordinate quartzites and limestones, its upper part, the ''Pedra Verde Phyllite'', is composed of phyelitic slump breccias, with abundance of pyrite. The Mambira formation crops out in the west flank of the tope Anticline, formed by ∼ 600 m of quartzites with sillimanite and/or andalusite (Sao Joaquim Formation), underlain by granitoid gneisses, representing the amphibolite facies. The contact between the Sao Joaquim and Mabira Formations appears to be tectonic. The folding and the metamorphism of the latter date from the Brazilian Cycle, which also affected the gneisses, causing homogeneization of the whole-rock K/Ar ages around 500 m.y. After erosion of some 10.000 m of sedimentary re-activation of the faults, which may already have formed during the deposition of the Mambira Formation resulted in the development of the Ubari Graben. The above formations are unconformably overlain by 300-350 m of subhorizontal Shallow-Marine sandstones belonging to the Serra Grande Formation, of upper silurian to lower devonian ages

  2. Middle cretaceous crustal anatexis associated to contractional deformation on Eden's shear zone

    International Nuclear Information System (INIS)

    The Puerto Eden's igneous and metamorphic complex (PEIMC) is composed by amphibolite facies schists, melanocratic and leucocratic diatexites (with biotite in schlieren structure), and orthomylonites intruded by schlieren bearing porphyritic biotite monzogranite, tabular garnet - tourmaline and white mica - garnet leucogranites, pegmatitic felsic dikes, andesitic dikes, and biotite hornblende granodiorites belonging to the South Patagonian Batholith (SPB). This locality represent the westernmost outcrop of the Eastern Andean Metamorphic Complex (EAMC), situated at 49o8min. 20.seg S - 74o 23min.20seg. W, on the eastern margin of the SPB. A common relationship exists in convergent orogenic belts between a shear zone system, high-grade metamorphic rocks, and granites, which suggests a feedback relationship between crustal anatexis and contractional deformation that helps granite extraction and focusses granite ascent (Solar et al, 1998). The aim of this study is to constraint the temporal relationship between the magmatic and deformational evolution in Puerto Eden, associated to one or more events of crustal anatexis recorded. This work is based on geochemistry and K-Ar radiometric age data set previously presented as part of first author's MSc thesis related to the petrogenesis of PEIMC (au)

  3. Investigation of uranium potential of Precambrian metasedimentary rocks, central Laramie Range, Wyoming. Final report

    International Nuclear Information System (INIS)

    This study is directed toward evaluation of potential uraniferous quartz-pebble conglomerate occurrences or strata-bound uranium occurrences in the Laramie Range. Geologic investigations pursuant to that evaluation provide that metasediments in the central Laramie Range constitute an Archean greenstone belt which overlies remobilized granite-gneiss basement terrane of the Archean Wyoming Province. This greenstone belt is formed of branching arcuate, synformal keels of supracrustal rocks infolded and enveloped by granitic domes. The stratigraphic succession of the belt is a triad formed of lower mafic-ultramafic rocks including komatiites, middle mafic volcanics including pillow basalts, and an upper metasedimentary sequence composed of boulder paraconglomerates, graywackes, marbles, quartzites, pelites, thin quartz-pebble conglomerates and thin, banded iron formation. Because the metasedimentary sequence does not contain a thick, mature clastic wedge, a thick fluvial section, basal quartz-pebble conglomerates, or significant geochemical or radiometric anomalies and because the sequence is structurally complex and regionally of amphibolite facies metamorphic rank, the potential for uraniferous fossil placers is considered nil. Occurrences of other types may be indicated by radiometric and geochemical anomalies discovered over sheared zones within the granite-gneiss terrane and small radiometric anomalies over a coarse-grained phase of granite located north of the greenstone belt

  4. A petrogenetic study of the garnet pyroxenite enclaves in spinel peridotite, North Dabieshan, China

    Institute of Scientific and Technical Information of China (English)

    游振东[1; 钟增球[2; 索书田[3; 郑曙[4

    2000-01-01

    Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prpas-as), sodium augite (Jd 10-25) with a small amount of ilmenite. There are two stages of retro-metamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and Iherzolite. Due to intensive serpentinitization, only 5%-40% of the relic olivine (Fo92-93) are preserved. The orthopyroxenes are Mg-rich (En87-93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe-Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988) KD= 4.06 - 5.28; T= 793-919℃, P= 1.5 GPa are estimated for

  5. 3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

    Science.gov (United States)

    Kampmann, Tobias C.; Stephens, Michael B.; Weihed, Pär

    2016-01-01

    Altered and mineralized rocks at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic Bergslagen ore district in the south-western part of the Fennoscandian Shield, have been metamorphosed at low-pressure, amphibolite-facies conditions and affected by ductile deformation. Using combined surface mapping of lithology and structure, drill core logging and microstructural work, the polyphase (D1 and D2) ductile deformation is demonstrated and a 3D model for the deposit created. Mineral associations include quartz, biotite, cordierite, anthophyllite, and minor almandine, andalusite and chlorite in silicate-rich altered rock, calcite or dolomite in marble and tremolite-actinolite or diopside-hedenbergite in skarn. The silicate minerals show varying growth patterns during the different phases of the tectonothermal evolution, with considerable static grain growth occurring between D1 and D2, and even after D2. F2 sheath folding along axes that plunge steeply to the SSE, parallel to a mineral stretching lineation and the dip direction of the S2 foliation, is suggested as a key deformation mechanism forming steeply plunging, cone- to rod-shaped mineralized bodies. This contrasts with a previous structural model invoking fold interference. A major shear zone with talc-chlorite-(quartz-biotite) mineral association separates the northern and southern structural domains at the deposit and bounds the polymetallic massive sulphides to the north.

  6. Eclogites of the Dabie Region: Retrograde Metamorphism and Fluid Evolution

    Institute of Scientific and Technical Information of China (English)

    顾连兴; 杜建国; 翟建平; 赵成浩; 范建国; 张文兰

    2002-01-01

    Based upon fluid effects, retrograde metamorphism of eclogites in the Dabie region can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stage is marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and is thought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely to have occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyanite porphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals such as phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such as amphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with lower amphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. The product of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferential shearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanying vein rutile was precipitated from fluids of this stage after local transport and concentration, and may hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins can be used as an exploration indicator for dissemination-type rutile deposits.

  7. Investigating Elastic Anisotropy of the Leech River Complex, Vancouver Island using finite-frequency sensitivity kernels

    Science.gov (United States)

    Matharu, G.; Bostock, M. G.; Christensen, N. I.; Tromp, J.; Peter, D. B.

    2012-12-01

    The Leech River Complex (LRC) of southern Vancouver Island is part of a once continuous belt of Cretaceous sandstone, mudstone and volcanics that formed an accretionary wedge along the northwestern margin of North America. Metamorphism at 50 Ma to prehnite-pumpellyite, greenschist, amphibolite and blueschist facies produced pervasive foliations with strong phyllosilicate lattice preferred orientations. Laboratory measurements and in-situ S-wave splitting analysis of tectonic tremor wavetrains indicate that this fabric produces substantial S-wave anisotropy of up to 30%. In this study we seek to gain further understanding on the nature of anisotropy within the LRC using high signal to noise ratio low frequency earthquake (LFE) templates and 3-D simulations from the spectral element method (SEM). The LFEs are characterized by impulsive, double couple, point sources and lie along a surface between 27 and 37 km depth that is inferred to be the plate boundary, immediately underlying the LRC. The SEM modelling employs a regional mesh that incorporates realistic topography, bathymetry and a 3-D tomographic P-wave velocity model of southern Vancouver Island. It allows us to readily simulate wave propagation in general anisotropic media with up to 21 independent elastic constants. We will investigate the orientation and distribution of anisotropy within the LRC by employing sensitivity kernels determined using adjoint methods in conjunction with SEM.

  8. Contribution for geochronological evolution study of the Pianco-Alto Brigida fold belt system

    International Nuclear Information System (INIS)

    The Proterozoic Painco-Alto Fold Belt is situated in the central portion of the Borborema Province and it probably is just a segment of a longer structural development encompassed between the Patos (N) and Pernambuco (S) lineaments. The geochronological study was carried out along a cross section in the central part of the belt (Paraiba State) where biotite-muscovite quartz schists are the predominating rock types, including intercalations of bi-modal volcanics, marbles and quartzites. These rocks were metamorphosed in the amphibolite facies, and they display a complex history of folding. Zircons of the acid volcanics (meta-rhyolytes where analysed through U/Pb method and they plot in a discordia diagram with superior intercept indicating ages around 1100 Ma. Whole rocks Rb/Sr analyses on the same meta-volcanics are indicating isochrons of 950Ma. These data are being respectively interpreted as ages of sedimentation (and volcanism) and regional metamorphism associated to the main (D sub(2)) phase of folding. One of the main purpose of this paper is to stress the importance of the ages around 950Ma, in the central domain of the Borborema Province, as result of regional folding and metamorphism. Some other occurence of ages in the 1000-900Ma range will be discussed as support for this interpretation from now on adopted. (author)

  9. Polyphase deformation and garnet growth in politic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast–matrix relationship

    Indian Academy of Sciences (India)

    A Chattopadhyay; N Ghosh

    2007-10-01

    Polyphase deformation and metamorphism of pelitic schists of Chorbaoli Formation of Sausar Group in and around Ramtek area,Nagpur district,Maharashtra,India has led to the development of garnet and staurolite porphyroblasts in a predominantly quartz –mica matrix.Microstructural study of oriented thin sections of these rocks shows that garnet and staurolite have different growth histories and these porphyroblasts share a complex relationship with the matrix.Garnet shows at least two phases of growth –first intertectonic between D1 and D2 (pre-D2 phase)and then syn-tectonic to post-tectonic with respect to D2 deformation.Growth of later phase of garnet on the earlier (pre-D2 garnet grains has led to the discordance of quartz inclusion trails between core and rim portion of the same garnet grain.Staurolite develops only syn-D2 and shows close association with garnet of the later phase.The peak metamorphic temperature thus coincided with D2 deformation,which developed the dominant crenulation schistosity (S2 ,regionally persistent in the terrain.The metamorphic grade reached up to middle amphibolite facies in the study area, which is higher than the adjoining southern parts of Sausar Fold Belt.

  10. Contribution of metapelitic sediments to the composition, heat production, and seismic velocity of the lower crust of southern New Mexico, USA

    Energy Technology Data Exchange (ETDEWEB)

    Reid, M.R.; Hart, S.R. (Massachusetts Inst. of Tech., Cambridge, MA (USA). Dept. of Earth, Atmospheric, and Planetary Sciences); Padovani, E.R.; Wandless, G.A. (Geological Survey, Reston, VA (USA))

    1989-11-01

    Granulite xenoliths erupted at Kilbourne Hole maar were recently extracted from the lower crust of southern New Mexico. Garnet- and silimanite-bearing quartzofeldspathic xenoliths had pelitic protoliths and were probably emplaced in the lower crust by tectonic underplating at a lower Proterozoic subduction zone. Thus the Kilbourne Hole metapelitic xenoliths illustrate the potential role of tectonosedimentary processes at convergent margins in determining the ultimate composition of the crust. Average P-wave velocities for metapelitic xenoliths from Kilbourne Hole are {approx equal} 7 km/s at 6 kbar, like those of mafic metagabbros and anorthosites. However, in contrast to mafic lithologies, the major element composition of the representative pelitic paragneiss (RPP) described in this paper is relatively siliceous and like that of average upper crust. Except for depletions of U and Cs, the trace element characteristics of the RPP are like those of pelitic sediments and are 3-10 times higher than those typically estimated for the lower crust. The heat production of the RPP is high (1.0 {mu}W/m{sup 3}) as are those of many granulite- and amphibolite-grade metapelites. In general, portions of the lower crust in which sediments are present may be high in light ion lithophile and rare earth element abundances, heat production, {delta}{sup 18}O, and {sup 87}Sr/{sup 86}Sr. Moreover, the high Pb contents and unradiogenic Pb isotope signatures of metapelites provide an important reservoir for unradiogenic Pb in the earth as a whole. (orig.).

  11. Contribution of metapelitic sediments to the composition, heat production, and seismic velocity of the lower crust of southern New Mexico, U.S.A.

    Science.gov (United States)

    Reid, M.R.; Hart, S.R.; Padovani, E.R.; Wandless, G.A.

    1989-01-01

    Granulite xenoliths erupted at Kilbourne Hole maar were recently extracted from the lower crust of southern New Mexico. Garnet- and sillimanite-bearing quartzofeldspathic xenoliths had pelitic protoliths and were probably emplaced in the lower crust by tectonic underplating at a lower Proterozoic subduction zone. Thus the Kilbourne Hole metapelitic xenoliths illustrate the potential role of tectonosedimentary processes at convergent margins in determining the ultimate composition of the crust. Average P-wave velocities for metapelitic xenoliths from Kilbourne Hole are ??? 7 km/s at 6 kbar, like those of mafic metagabbros and anorthosites. However, in contrast to mafic lithologies, the major element composition of the representative pelitic paragneiss (RPP) described in this paper is relatively siliceous and like that of average upper crust. Except for depletions of U and Cs, the trace element characteristics of the RPP are like those of pelitic sediments and are 3-10 times higher than those typically estimated for the lower crust. The heat production of the RPP is high (1.0 ??W/m3) as are those of many granulite- and amphibolite-grade metapelites. In general, portions of the lower crust in which sediments are present may be high in light ion lithophile and rare earth element abundances, heat production, ??18O, and 87Sr 86Sr. Moreover, the high Pb contents and unradiogenic Pb isotope signatures of metapelites provide an important reservoir for unradiogenic Pb in the earth as a whole. ?? 1989.

  12. TL studies of calcareous rocks of Danta area, North Gujarat

    International Nuclear Information System (INIS)

    The lithounits exposed around Danta in Banaskantha district of North Gujarat belong to Ajabgarh Group, the upper division of the Delhi super group. These rocks are intruded by syn to late kinematic basic rocks and by Erinpura granites of post Delhi age. The Ajabgarh group consists of pelitic and calcareous components. Mineralogically the pelitic rocks comprise cordierite, almandine garnet, k-feldspar, sillimanite, quartz and mica in variable proportions. The calcareous rocks are seen to contain dominantly calcite, scapolite, forsterite, sphene, k-feldspar. These mineral assemblages correspond to upper Amphibolite to lower Granulite facies of regional metamorphism. The chemistry of the calcareous rocks show predominance of CaO over MgO. The glow curves obtained from virgin samples (NTL) as well as artificial beta irradiated indicate glow peaks at 140oC, 290oC, 310oC and 390oC. The TL glow peak temperatures are in general agreement with those reported by Borsi and Rinaldi and Medlin. The pronounced peak at 390oC and 290oC are suggestive of their high irradiation sensitivity and also probably reflect variation in the Mn content of the rocks. (author). 9 refs., 16 tabs., 2 figs

  13. Bedrock Model of the Syyry area. Summary report

    Energy Technology Data Exchange (ETDEWEB)

    Saksa, P. [ed.; Ahokas, H. [Fintact Ky, Helsinki (Finland); Kuivamaeki, A.; Kurimo, M.; Paananen, M. [Geological Survey of Finland, Espoo (Finland); Anttila, P. [IVO International Oy, Vantaa (Finland); Front, K.; Pitkaenen, P.; Hassinen, P.; Ylinen, A. [Technical Research Centre of Finland, Espoo (Finland). Road, Traffic and Geotechnical Lab.

    1993-09-01

    Preliminary site investigations implemented in accordance with the research programme drawn up by Teollisuuden Voima Oy (TVO) were carried out at Syyry (in Finland) in 1987-1992. Models of the site were compiled and used for describing the rock types, fracturing, fracture structures and geohydrological conditions, the main emphasis being on the examination of the bedrock fracturing and related hydraulic conductivity. Three-dimensional models were used for the classification of the various properties of the bedrock structures. The descriptive models were gathered into a computer system to facilitate illustration and storage. The main rock type at Syyry is tonalite. A mica gneiss formation SE of the investigation site dips towards the NW and delimits the tonalite as far as the central part of the investigation site. The miga gneiss has a heterogeneous composition and includes intermediate layers consisting of quartz feldspar schist and amphibolite. There are mafic formations in the vicinity of the investigation site. The intrusive rocks have been deformed during three plastic and three mainly brittle deformation stages. (47 refs., 61 figs.).

  14. Metamorphism, metasomatism and mineralization at Lagoa Real, Bahia, Brazil

    International Nuclear Information System (INIS)

    Uranium deposits cumulatively in the 100,000 tonne U3 O8 range occur within ductile shear zones transecting Archean basement gneisses of the Sao Francisco Craton, at the Lagoa Real region of south-central Bahia, Brasil. The gneisses, dated at 2.6-3.0Ga, are at amphibolite and granulite facies and overlie to the west, the Proterozoic Espinhaco metasedimentary sequence along a thrust fault. Petrography and mineral chemistry show that in the zones of alteration/mineralization, the original K-feldspar + quartz + albite/oligoclase + hastingsite assemblage, is replaced by albite + aegirine - angite + andradite + hematite assemblages, with or without uraninite. This information along with oxygen isotope, whole rock geochemistry and fluid inclusion studies indicate that the alteration process involves removal of Si, K, Rb, Ba and addition of Na under oxidizing conditions. V, Pb and Sr were introduced along with U via interaction with saline SO2 - rich, isotopically light fluids under varying water/rock ratios and at temperatures of 500 - 5500C. 87Sr/86Sr systematics suggest that it is unlikely that Sr, and by extension uranium, were introduced by fluids originating from the basement gneisses. Geological constraints and the general alteration pattern are consistent with the release of the mineralizing fluids in response to the overloading of the basement rocks onto the Sedimentary Espinhaco via a thrust mechanism. (Author)

  15. Zircon U-Pb ages and geochemistry of igneous and metamorphic rocks in the northern Prince Charles Mountains, Antarctica

    International Nuclear Information System (INIS)

    High-grade metamorphic and felsic igneous rocks from the northern Prince Charles Mountains, East Antarctica, have been characterised geochemically and dated from SHRIMP zircon geochronological data. Around 980 Ma ago, voluminous magmas representing a combination of mantle-derived and intracrustal melts, including orthopyroxene- quartz monzonite ('charnockite') on Loewe Massif and granitic and syenitic intrusions on Mount Collins, were emplaced during a regional high-grade tectonothermal event. Garnet leucogneiss sheets on Mount McCanhy, the products of local partial melting, were also emplaced at about this time. The geology of Fisher Massif is exceptional in that a ca 1280-Ma metavolcanic sequence and coeval granodiorite have been metamorphosed only up to the lower amphibolite facies, and intruded by a ca 1020-Ma biotite granite. None of the analysed samples shows, in its isotopic systematics, the effects of 500-Ma events, prominent elsewhere in East Antarctica. Rare inherited components, 1850-1900 Ma old, were found in some samples. A paragneiss on Mount Meredith yielded 2500-2800-Ma and 1800-2100-Ma detrital zircon populations. (authors)

  16. Kyanite from the Deep Freeze Range, Terra Nova Bay, Antarctica

    International Nuclear Information System (INIS)

    During GANOVEX VII in 1992, kyanite was discovered in quartz veins on the southwest flank of Mt Levick, in the Deep Freeze Range, Terra Nova Bay, Antarctica. The quartz veins cut an isoclinally (D1) folded sequence of low-grade (Mu-Bt-Crd±And±St) pelitic schist with associated para-amphibolites, calc-silicates, and quartzites (Priestley Formation), which forms the western, steeply dipping to overturned limb of a D2, kilometric fold. The schists grade northeastwards into higher grade schists (Kfs-Sil-Crd) of the low-angle upper limb of the D2 fold, and thus the regional metamorphism postdates the fold. D3 southeast-verging folds lie on the upper limb. The kyanite crystals (up to 3.5 cm long) occur with paragonitic muscovite and minor plagioclase (An36. The quartz veins and saddle reefs are cleaved and boudinaged, but the kyanite shows only mild deformation suggesting late tectonic growth. There is no indication that the host schists entered the stability field of kyanite. The change in P-T conditions that promoted the growth of kyanite appears to have been transient and temporally insufficient to allow the country rocks to react. It is suggested that the action of the nearby Boomerang Thrust bringing older gneiss over the Priestley Formation schists could have generated the D3 folds and provided the necessary overpressure conditions for the kyanite to grow from the quartz vein fluids. (author). 23 refs., 5 figs., 1 tab

  17. Low- to high-grade metamorphic transition in the Southern part of Karnataka Nucleus, India

    Science.gov (United States)

    Naqvi, S. M.

    The southern part of Karnataka Nucleus has a strong imprint of 2.6 Ga metamorphism. This has affected the schist belts of Karnataka Nucleus from greenschist to upper amphibolite facies. The higher grades of metamorphism are in the Holenarasipur, Nuggihalli, Krishnarajpet, Hadnur and Melkote schist belts. In the high grade transition zone, around Sargur only keels of schist belts are preserved and occur as highly dismembered, disconnected belts with the top and bottom of the stratigraphic column obliterated due to high grade metamorphism and accompanying migmatization. Absence of high-grade metamorphic minerals in the sediments of the Dharwar schist belts supports the contention that high grade metamorphism post-dated the Dharwar sedimentation and occurred around 2.6 Ga ago. Sargur type metamorphism occurred at upper crustal levels and charnockite type metamorphism occurred in lower crustal levels. The P-T conditions for the mineral assemblage in metapelites of Sargur Group indicate burial depths up to at least 15 km suggesting that they were subducted and later obducted during the development of Early Proterozoic Mobile Belt along the southern border of the Karnataka Nucleus.

  18. Regional geology of the Pine Creek Geosyncline

    International Nuclear Information System (INIS)

    The Pine Creek Geosyncline comprises about 14km of chronostratigraphic mainly pelitic and psammitic Lower Proterozoic sediments with interlayered tuff units, resting on granitic late Archaean complexes exposed as three small domes. Sedimentation took place in one basin, and most stratigraphic units are represented throughout the basin. The sediments were regionally deformed and metamorphosed at 1800Ma. Tightly folded greenschist facies strata in the centre grade into isoclinally deformed amphibolite facies metamorphics in the west and northeast. Pre and post-orogenic continental tholeiites, and post-orogenic granite diapirs intrude the Lower Proterozoic metasediments, and the granites are surrounded by hornfels zones up to 10km wide in the greenschist facies terrane. Cover rocks of Carpentarian (Middle Proterozoic) and younger ages rest on all these rocks unconformably and conceal the original basin margins. The Lower Proterozoic metasediments are mainly pelites (about 75 percent) which are commonly carbonaceous, lesser psammites and carbonates (about 10 percent each), and minor rudites (about 5 percent). Volcanic rocks make up about 10 percent of the total sequence. The environment of deposition ranges from shallow-marine to supratidal and fluviatile for most of the sequence, and to flysch in the topmost part. Poor exposure and deep weathering over much of the area hampers correlation of rock units; the correlation preferred by the authors is presented, and possible alternatives are discussed. Regional geological observations pertinent to uranium ore genesis are described. (author)

  19. The link between strength of lattice preferred orientation, second phase content and grain boundary migration: A case study from the Alpine Fault zone, New Zealand

    Science.gov (United States)

    Little, Timothy A.; Prior, David J.; Toy, Virginia G.; Lindroos, Zoe Reid

    2015-12-01

    We analyse the microstructure and quartz LPOs of 36 layers of varying composition from a several-meter thick sequence of amphibolite-facies metacherts and related mica-garnet-plagioclase-quartz bearing schists from the central Southern Alps in the mylonite zone related to the Alpine Fault. Quartz contents vary from ∼10 to 100% and all of the LPO fabric skeletons are similar, featuring an asymmetric single girdle of [c]-axes inclined ∼30° away from the ZY plane. LPO strength is typically low at quartz contents phase-boundary density, high. This may have promoted grain-size sensitive creep and dislocation glide in mica. Dislocation creep in the interspersed quartz grains was correspondingly reduced, and weaker quartz LPOs were produced. In highly quartzose layers, quartz grain boundaries experienced little drag or pinning from impurity phases and were able to migrate quickly into higher strain-energy grains. Preferential consumption of poorly oriented grains strengthened quartz LPOs, geometrically softened the dislocation creep process in these quartzose layers, and contributed to grain coarsening. The lack of evidence for instabilities in the thinly layered (sensitive flow and dislocation creep, in the layers were able to accommodate a nearly homogeneous deformation between the different composition layers.

  20. Textural and Rb-Sr isotopic evidence for late Paleozoic mylonitization within the Honey Hill fault zone southeastern Connecticut

    International Nuclear Information System (INIS)

    A petrographic and Rb-Sr isotopic study of rocks within and near the Honey Hill fault zone places important constraints on its history of movement. Rb-Sr apparent ages for micas and plagioclase from these rocks have been reset and range from Permian to Triassic, considerably younger than the minimum stratigraphic age (Ordovician) of the rocks studied or of Acadian (Devonian) regional metamorphism. Permian Rb-Sr ages of dynamically recrystallized muscovite date the development of mylonite fabric. An older age is precluded by the excellent preservation of unrecovered quartz, which indicates that these rocks did not experience temperatures high enough to anneal quartz or thermally reset Rb-Sr isotopic systems in muscovite since the time of mylonitization. Metamorphic mineral assemblages and mineral apparent ages in rocks north of the fault zone indicate recrystallization under similar upper greenschist-lower amphibolite grade conditions during Permian to Triassic time. Collectively these results indicate that the Honey Hill fault zone was active during the Late Paleozoic and that ductile deformation and metamorphism associated with the Alleghanian orogeny extend well into southern Connecticut. An Alleghanian age for mylonitization within the Honey Hill fault zone suggests it should be considered as a possible site for the major Late Paleozoic strike-slip displacements inferred from paleomagnetic studies for parts of coastal New England and maritime Canada

  1. The Glória quartz-monzodiorite: isotopic and chemical evidence of arc-related magmatism in the central part of the Paleoproterozoic Mineiro belt, Minas Gerais State, Brazil.

    Science.gov (United States)

    Avila, Ciro A; Teixeira, Wilson; Cordani, Umberto G; Barrueto, Héctor R; Pereira, Ronaldo M; Martins, Veridiana T S; Dunyi, Liu

    2006-09-01

    The Glória quartz-monzodiorite, one of the mafic plutons of the Paleoproterozoic Mineiro belt, is intrusive into banded gneisses, amphibolites, schists and phyllites of the Rio das Mortes greenstone belt, in the southern portion of the São Francisco Craton, State of Minas Gerais, Brazil. The Glória quartz-monzodiorite yields a SHRIMP U-Pb zircon age of 2188 +/- 29 Ma, suggesting a tectonic relationship with the pre-collisional phase of the Mineiro belt. According to the Nd isotopic evidence (epsilonNd(T) = -3.4; T DM = 2.68 Ga) the original magmas was formed by a mixture among Archean crustal material and Paleoproterozoic juvenile magma. The Glória quartz-monzodiorite shows metaluminous and calc-alkaline tendency with intermediate K content, comparable to that of volcanic-arc rocks. The primary mineralogical assemblage was partly modified by metamorphism, dated between 2131-2121 Ma in nearby coeval plutons. Such metamorphism is significantly older than the reported metamorphic episodes of the Mineiro belt in the Quadrilátero Ferrífero region (2059-2041 Ma) in the eastern portion of the study area. This evidence, together with chemical and isotopic data from other mafic and felsic plutons coeval with the Glória quartz-monzodiorite, indicate a tectonic and magmatic migration within the Mineiro belt from west to east. PMID:16936942

  2. Temporal and Hf isotope geochemical evolution of southern Finnish Lapland from 2.77 Ga to 1.76 Ga

    Directory of Open Access Journals (Sweden)

    Laura S. Lauri

    2012-12-01

    Full Text Available The southern Finnish Lapland area in the central part of the Fennoscandian shield is a geologically complex zone comprising several Archean blocks and Paleoproterozoic supracrustal belts all of which are intruded by voluminous Paleoproterozoic granites (the central Lapland granitoid complex, CLGC. New in-situ single crystal zircon U–Pb age determinations coupled with Lu–Hf isotope data from the same zircons were acquired from five granitoid rocks and one amphibolitic rock sample from the southern Lapland area. The samples represent at least four distinct magmatic events (at ca. 2.77 Ga, 2.12Ga, 1.81 Ga, and 1.76 Ga. The 2.77 Ga and the 1.81-1.76 Ga events have initial Hf isotope signatures implying that local Archean rocks represent the source for the younger granites. The 2.12 Ga event has a slightly more juvenile Hf isotope composition suggesting either that the source for the 2.12 Ga granites represents a different Archean block or that the source is composed of mixed Archean and Paleoproterozoic components. The Neoarchean source for the Paleoproterozoic granites may be traced through the CLGC all the way to the Jokkmokk area in Sweden and possibly to the Lofoten area in Norway

  3. Late-Proterozoic to Paleozoic history of the peri-Gondwana Calabria-Peloritani Terrane inferred from a review of zircon chronology.

    Science.gov (United States)

    Fornelli, Annamaria; Micheletti, Francesca; Piccarreta, Giuseppe

    2016-01-01

    U-Pb analyses of zircon from ten samples of augen gneisses, eight mafic and intermediate metaigneous rocks and six metasediments from some tectonic domains along the Calabria-Peloritani Terrane (Southern Italy) contribute to knowledge of peri-Gondwanan evolution from Late-Proterozoic to Paleozoic times. All samples were equilibrated under amphibolite to granulite facies metamorphism during the Variscan orogeny. The zircon grains of all considered samples preserve a Proterozoic memory suggestive of detrital, metamorphic and igneous origin. The available data fit a frame involving: (1) Neoproterozoic detrital input from cratonic areas of Gondwana; (2) Pan-African/Cadomian assemblage of blocks derived from East and West African Craton; (3) metamorphism and bimodal magmatism between 535 and 579 Ma, within an active margin setting; (4) rifting and opening of Ordovician basins fed by detrital input from the assembled Cadomian blocks. The Paleozoic basins evolved through sedimentation, metamorphism and magmatism during the Variscan orogeny involving Palaeozoic and pre-Paleozoic blocks. The Proterozoic zircon records decidedly decrease in the high grade metamorphic rocks affected by Variscan pervasive partial melting. PMID:27026906

  4. Petrology and physical conditions of metamorphism of calcsilicate rocks from low- to high-grade transition area, Dharmapuri District, Tamil Nadu

    Science.gov (United States)

    Narayana, B. L.; Natarajan, R.; Govil, P. K.

    1988-01-01

    Calc-silicate rocks comprising quartz, plagioclase, diopside, sphene, scapolite, grossularite-andradite and wollastonite occur as lensoid enclaves within the greasy migmatitic and charnockitic gneisses of the Archaean amphibolite- to granulite-facies transition zone in Dharmapuri district, Tamil Nadu. The calc-silicate rocks are characterized by the absence of K-feldspar and primary calcite, presence of large modal quartz and plagioclase and formation of secondary garnet and zoisite rims around scapolite and wollastonite. The mineral distributions suggest compositional layering. The chemical composition and mineralogy of the calc-silicate rocks indicate that they were derived from impure silica-rich calcareous sediments whose composition is similar to that of pelite-limestone mixtures. From the mineral assemblages the temperature, pressure and fluid composition during metamorphism were estimated. The observed mineral reaction sequences require a range of X sub CO2 values demonstrating that an initially CO2-rich metamorphic fluid evolved with time towards considerably more H2O-rich compositions. These variations in fluid composition suggest that there were sources of water-rich fluids external to the calc-silicate rocks and that mixing of these fluids with those of calc-silicate rocks was important in controlling fluid composition in calc-silicate rocks and some adjacent rock types as well.

  5. Brazil Geological Basic Survey Program - Ponte Nova - Sheet SF.23-X-B-II - Minas Gerais State

    International Nuclear Information System (INIS)

    The present report refers to the Ponte Nova Sheet (SF.23-X-B-II) systematic geological mapping, on the 1:100.000 scale. The Sheet covers the Zona da Mata region, Minas Gerais State, in the Mantiqueira Geotectonic Province, to the eastern part of Sao Francisco Geotectonic Province, as defined in the project. The high grade metamorphic rocks to low amphibolite, occurring in the area were affected by a marked low angle shearing transposition, and show diphtheritic effects. Archaean to Proterozoic ages are attributed to the metamorphites mostly by comparison to similar types of the region. Three deformed events were registered in the region. Analysis of the crustal evolution pattern based on geological mapping, laboratorial analyses, gravimetric and air magnetometry data, and available geochronologic data is given in the 6. Chapter, Part II, in the text. Major element oxides, trace-elements, and rare-earths elements were analysed to establish parameters for the rocks environment elucidation. Geochemical survey was carried out with base on pan concentrated and stream sediments distributed throughout the Sheet. Gneisses quarries (industrial rocks) in full exploration activity have been registered, as well as sand and clay deposits employed in construction industry. Metallogenetic/Provisional analysis points out the area as a favorable one for gold prospection. (author)

  6. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: The Nordre Strømfjord shear zone and the Arfersiorfik quartz diorite in Arfersiorfik, the Nagssugtoqidian orogen, West Greenland

    Directory of Open Access Journals (Sweden)

    Stensgaard, Bo Møller

    2006-12-01

    Full Text Available The Nordre Strømfjord shear zone in the fjord Arfersiorfik, central West Greenland, consists of alternating panels of supracrustal rocks and orthogneisses which together form a vertical zone up to 7 km wide with sinistral transcurrent, ductile deformation, which occurred under middle amphibolite facies conditions. The pelitic and metavolcanic schists and paragneisses are all highly deformed, while the orthogneisses appear more variably deformed, with increasing deformation evident towards the supracrustal units. The c. 1.92 Ga Arfersiorfik quartz diorite is traceable for a distance of at least 35 km from the Inland Ice towards the west-south-west. Towards its northern contact with an intensely deformed schist unit it shows a similar pattern of increasing strain, which is accompanied by chemical and mineralogical changes. The metasomatic changes associated with the shear zone deformation are superimposed on a wide range of original chemical compositions, which reflect magmatic olivine and/or pyroxene as well as hornblende fractionation trends. The chemistry of the Arfersiorfik quartz diorite suite as a whole is comparable to that of Phanerozoic plutonic and volcanic rocks of calc-alkaline affinity.

  7. Structural and magmatic evolution in the Loimaa area, southwestern Finland

    Directory of Open Access Journals (Sweden)

    Nironen, M.

    1999-06-01

    Full Text Available Within the Loimaa area there is a junction of the general E-W structural trend of southern Finland and a NW-N-NE curving trend. The structure of the area is dominated by ductile D, and D4 deformations with E-W and N-S axial traces, respectively. The typical semicircular structures in the study area are interpreted as F3-F4 fold interference structures. The predominant plutonic rocks in the Loimaa area are penetratively foliated tonalites and granodiorites which probably intruded during D2 deformation. Peak regional metamorphism at upper amphibolite facies and emplacement of the Pöytyä Granodiorite ca. 1870 Ma ago occurred during D, deformation. The ductile style of D4 deformation in the Loimaa area is probably related to the high-grade metamorphism at 1850-1810 Ma in the late Svecofennian granite-migmatite (LSGM zone immediately south of the study area. The Oripää Granite was emplaced during D4 deformation. The structural evolution in the Loimaa area may be correlated with the evolution further to the northwest (Pori area and north (Tampere-Vammala area whereas correlation to the south and west is problematic. A transpressional model presented for the LSGM zone is not applicable to the Loimaa area.

  8. Post-collisional multistage magmatism in the Ribeira mobile belt: geochemical and isotopic study of the Varzea Alegre intrusive complex, Espirito Santo, Brazil

    International Nuclear Information System (INIS)

    The Varzea Alegre Intrusive Complex (VAIC) corresponds to a post-collisional (late orogenic) pluton, related to the Brasiliano cycle, situated in the central part of the Espirito Santo State. It intrudes amphibolite to granulite facies metamorphic rocks of the Ribeira Belt. Two distinct domains were recognised in this zoned pluton: an inner domain with opx-gabbro, monzogabbro, diorite, quartz-diorite and megaporphyritic granite, and an outer one comprising an irregular and large ring of charnockitic rocks. Geochemical data from the former reveal medium to high-K calc-alkalic rocks, enriched in incompatible elements, mainly Ba, Sr, La, Ce and Pb and partially depleted in HFS elements. The charnockitic rocks show a high-K alkali-calcic signature; they are rich in Ba, K and some HFS elements, such a Zr, P and Nb. The incompatible element enrichment detected in the rocks of the VAIC has been reported for several intrusions from this part of the Ribeira Belt. A Rb-Sr isochronic age of 508± 12 Ma was determined for the megaporphyritic granite. The TDM model age varies from 1.3 Ga (opx-gabbro) to ca. 1.67 Ga (charnockitic rocks), which can be related to an important Mesoproterozoic crustal event. The calculated TCHUR model ages of the cogenetic opx-gabbros and intermediary rocks is ca. 1.0 Ga, interpreted as the time when the basic magma was extracted from the source. (author)

  9. Ion microprobe (SHRIMP dates complex granulite from Santa Catarina, southern Brazil

    Directory of Open Access Journals (Sweden)

    HARTMANN LÉO A.

    2000-01-01

    Full Text Available Complex polymetamorphic granulites have been dated in the Santa Catarina granulite complex of southern Brazil through SHRIMP study of zircon. This complex is dominated by intermediate-acid plutonic rocks and contains small volumes of mafic and ultramafic rocks, and minor quartzite and banded iron formation. Porphyroblasts of orthopyroxene, clinopyroxene and plagioclase in mafic and acid rocks are interpreted as magmatic remnants in a volumetrically dominant granoblastic aggregate (M1 of the same minerals and hornblende. Hornblende formed during a later M2 metamorphic event constitutes rims around pyroxene, but the hornblende is also rimmed by granoblastic simplectites of orthopyroxene, clinopyroxene, hornblende and plagioclase in a second granulite facies event (M3. Chlorite and epidote occur in shear zones (M4. This granulite terrain is part of a Neoproterozoic craton, because it was little affected by the Brasiliano Cycle. The two granulite-facies events (M1 and M3 are dated by U/Pb zircon SHRIMP at about 2.68 and 2.17 Ga, while the magmatic protoliths formed at about 2.72 Ga. The amphibolite facies event (M2 probably occurred close to the 2.17 Ga granulitic metamorphism.

  10. Proterozoic-Cambrian detrital zircon and monazite ages from the Anakie lnlier, central Queensland: Grenville and Pacific-Gondwana signatures

    International Nuclear Information System (INIS)

    The Anakie Metamorphic Group is a complexly deformed, dominantly metasedimentary succession in central Queensland. Metamorphic cooling is constrained to ca 500 Ma by previously published K-Ar ages. Detrital-zircon SHRIMP U-Pb ages from three samples of greenschist facies quartz-rich psammites (Bathampton Metamorphics), west of Clermont, are predominantly in the age range 1300-1000Ma (65-75%). They show that a Grenville-aged orogenic belt must have existed in northeastern Australia, which is consistent with the discovery of a potential Grenville source farther north. The youngest detrital zircons in these samples are ca 580 Ma, indicating that deposition may have been as old as latest Neoproterozoic. Two samples have been analysed from amphibolite facies pelitic schist from the western part of the inlier (Wynyard Metamorphics). One sample contains detrital monazite with two age components of ca 580-570 Ma and ca 540 Ma. The other sample only has detrital zircons with the youngest component between 510Ma and 700Ma (Pacific-Gondwana component), which is consistent with a Middle Cambrian age for these rocks. These zircons were probably derived from igneous activity associated with rifting events along the Gondwanan passive margin. These constraints confirm correlation of the Anakie Metamorphic Group with latest Neoproterozoic - Cambrian units in the Adelaide Fold Belt of South Australia and the Wonominta Block of western New South Wales. Copyright (2001) Geological Society of Australia

  11. Isotopic mapping of age provinces in Precambrian high-grade terrains: Sri Lanka

    Energy Technology Data Exchange (ETDEWEB)

    Milisenda, C.C.; Liew, T.C.; Hofmann, A.W.; Kroener, A.

    1988-09-01

    Nd model ages of amphibolite- and granulite-grade rocks in Sri Lanka form a simple region pattern that broadly correlates with mappable geological units, and is in effect an isotopic map of the island's basement. The granulite-grade units of the Highland Group and Southwest Group have model ages of 2.2-3.0 Ga indicating derivation mainly from late Archean sources. They are bounded to the east and west by late Proterozoic gneisses of the Vijayan Complex with model ages of 1.1-2.0 Ga. The isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the Vijayan gneisses are retrograde equivalents of the Highland granulites. Sri Lanka is not a direct continuation of the Archean Dharwar Craton of southern India. Identification of Vijayan-type juvenile crustal terrains in other Gondwana fragments may play a key role in determining the precise attachment of southern India-Sri Lanka in eastern Gondwana.

  12. Sm-Nd Ages of Two Meta-Anorthosite Complexes Around Holenarsipur: Constraints on the Antiquity of Archean Supracrustal Rocks of the Dharwar Craton

    Indian Academy of Sciences (India)

    Y J Bhaskar Rao; Anil Kumar; A B Vrevsky; R Srinivasan; G V Anantha Iyer

    2000-03-01

    Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar carton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c.2.5Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3.285 ± 0.17 Ga, Nd = 0.82 ± 0.78 for the Honnavalli meta-anorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033 Ga, Nd = -2.2+-0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c.3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scake about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5Ga. Our results set a younger age limit at c.3.29Ga for the supracrustal rocks of the HSB in the Dharwar craton.

  13. Multiple sources of magmatism. granitoids from southeast Kohistan, NW Himalayas, Pakistan

    International Nuclear Information System (INIS)

    The Kohistan island arc terrane in the northwestern Himalayas of N. Pakistan is sandwiched between the Indian and Karakoram plated. The base of the arc is occupied by a major stratiform ultramafic-gabbroic complex (the Sapat-Babusar complex), which overrides the crust of the Indian plate along the Indus suture (i.e., the Main Mantle Thrust; MMT). It was intruded into the base of a thick pile of metavolcanics (the Kamila belt), which comprise a tectonic collage of MORB-type tholeiitic basalts, island-arc tholeiites and calc-alkaline andesites. The Chilas complex, comprising ultramafic and gabbronorite rocks, is also intrusive into the Kamila belt, it is emplaced onto the top rather than the base of the Kamila belt. A sizeable proportion of granitoid rocks are present in the south-eastern part of Kohistan, which intruded the Kamila amphibolites. These are predominantly dioritic in composition, but include gabbros, granodiorites, granites and trondhjemites. The granitoids occur in two types. (1) large sheet-like lenticular masses, and (2) minor intrusives in the form of veints, sills or dykes. Three large sheets like bodies are mapped. All these bodies are composite, comprising gabbros, diorite/tonalite, granodiorite and granite. The minor intrusion of granitic and trondhjemitic composition are abundantly present in the form of veins, sills and dykes; and are characterized by variation in distribution. Strong shearing transformed the rocks into blastomylonite gneisses. The mineral assemblage consists of quartz, plagioclase, emphibole, epidote, chlorite, biotite, muscovite, sphene, magnetite and apatite. (author)

  14. Multiple sources of magmatism: granitoids from southeast kohistan, nw himalayas Pakistan

    International Nuclear Information System (INIS)

    The Kohistan island arc terrane in the northwestern Himalayas of N. Pakistan is sandwiched between the Indian and Karakoram plates. The base of the arc is occupied by a major stratiform ultramafic-gabbroic complex (the Sapat-Babusar complex). which overrides the crust of the Indian plate along the Indus suture (i. e., the Main Mantle Thrust; MMT). It was intruded into the base of a thick pile of metavolcanics (the Kamila belt), which comprise a tectonic collage of MORB-type tholeiitic basalts, island-arc tholeiites and calc-alkaline andesites. The Chilas complex, comprising ultramafic and gabbronorite rocks, is also intrusive into the Kamila belt. It is emplaced onto the top rather than the base of the Kamila belt. A sizeable proportion of granitoid rocks are present in the south-eastern part of Kohistan. Which intruded the Kamila amphibolites. These are predominantly dioritic in composition but include gabbros, granodiorites, granites and trondhjemites. The granitoids occur in two types: (I) large sheet-like lenticular masses, and (2) minor intrusives in the form of veins sills or dykes. Three large sheets like bodies are mapped. All these bodies are composite, comprising gabbros, diorite/tonalite. granodiorite and granite. The minor intrusions of granitic and trondhjemitic composition are abundantly present in the form of veins, sills and dykes and are characterized by variation in distribution. Strong shearing transformed the rocks into blastomylonite gneisses. The mineral assemblage consists of quartz, plagioclase, Amphibole, epidote, chlorite, biotite, muscovite, sphene, magnetite and apatite. (author)

  15. Calc-sodic zoning in rocks from Lagoa Real uranium Province, state of Bahia, Brazil

    International Nuclear Information System (INIS)

    Preliminary studies of the lithogeochemistry, carried out on various deposits from the Uranium Province of Lagoa Real, show that in relation to the complex and multiphased processes of metasomatism, the sodic metasomatites are younger and were superposed on the potassic metasomatites. Such metasomatites developed in zones of extended and rejuvenated faults, after the transformation of the gnaisses, granitic rocks, amphibolites and ferruginous quartzites into various tectonites (mylonites, blastomylonites and protomylonites). The calcic metasomatism, subsequent to those phases, took place during the course of tectonothermal reactivations marking the termination of the principal metasomatic processes which aftected the rocks of Lagoa Real. The uranium deposit 'Jazida Laranjeiras', centrally situated in the Uranium Province of Lagoa Real is the only deposit in which the uranium mineralization is not intimately associated with the calcic metasomatism. Here, the ratio Na2O/CaO in the albitites is in the vicinity of 7,5 whereas in the other deposits this ratio decreases to values of about 1,5. The authors suggest calcic zoning (in the post-albitite phase) along the sigmoidal structure which localizes the uranium deposits, with an increase of the CaO content in the extreme parts of the structure - deposit 'Jazida Cachoeira' in the north and anomaly No. 02/12 in the south. Also suggested is the vertical zoning of calcium, whereby the intensity of the metasomatic phenomenon is increased towards the hanging wall side of the mieral parcel. (Author)

  16. Preliminary geochronological data of the Morro Agudo de Goias Dike Swarm

    International Nuclear Information System (INIS)

    The Morro Agudo Dyke Swarm consists of mafic and ultramafic dykes and ultramafic dykes and stocks that intrude Archean granite-gneiss terranes of the Goias Massif. The dyke swarm was subdivided into five rock groups: Group I-basaltic andesite in dykes; Group II-metadiabase, diabase, metagabbro and amphibolite in dykes: Group III-metadiabase associated with-meta ultramafic rocks in the same dyke; Group IV-meta ultramafic rocks in dykes; Group V-meta ultramafic rocks in stocks. Four basaltic andesite dyke samples were dated using the K-Ar whole-rock method, yielding 2,412 ± 30 Ma, 2,403 ± 48 Ma, 2,254 ± Ma and 2,006 ±45 Ma ages. A Sm-Nd isochronic diagram for Group III mafic-ultramafic dyke yielded a 2,331 ± 101 Ma age with 0.50976 initial ratio. It is believed that this age may be extensive to the other mafic-ultramafic dykes of the dyke swarm. (author)

  17. Reworked old crust-derived shoshonitic magma: The Guarany pluton, Northeastern Brazil

    Science.gov (United States)

    Ferreira, Valderez P.; Sial, Alcides N.; Pimentel, Marcio M.; Armstrong, Richard; Guimarães, Ignez P.; da Silva Filho, Adejardo F.; de Lima, Mariucha Maria C.; da Silva, Thyego R.

    2015-09-01

    The 572 Ma Guarany stock consists of magmatic epidote-bearing hornblende monzodiorite to biotite granite that intruded Paleoproterozoic orthogneisses about 10 km inland from the coast in northeastern Brazil. Co-magmatic diorite enclaves and dikes are abundant throughout the pluton. The monzodiorite-granite pluton and diorite enclaves are shoshonitic and display continuous trends in variation diagrams. They display chemical and isotopic characteristics of crustal melts, such as enrichment in incompatible elements, high back-calculated initial 87Sr/86Sr ratios (avg. 0.71253), negative εNd (0.57Ga) values (avg. - 14.58), as well as high and variable (+ 9.1 to + 11.1‰VSMOW) δ18O (zircon) values. Correlations between O-isotope and whole-rock silica contents, as well as initial 87Sr/86Sr ratios with 1/Sr concentrations, suggest hybridization of a lower continental crustal melt with more felsic crustal rocks, concomitant with fractional crystallization. Amphibole chemistry and whole rock Zr, TiO2 and P2O5 contents suggest magma solidification at a pressure ~ 7 kbar and near liquidus temperature ~ 900 °C. The parental magma was likely formed by partial melting of old (tDM = 2.0 Ga) amphibolitic lower continental crustal rocks, in a post-collisional setting, probably triggered by underplating of mantle-derived mafic magma during the period of relaxation after collision.

  18. Change from calc-alkaline to adakitic magmatism recorded in the Early Cretaceous Darran Complex, Fiordland, New Zealand

    International Nuclear Information System (INIS)

    The Early Cretaceous Darran Complex of the Median Tectonic Zone near Milford, South Island, New Zealand, consists dominantly of biotite - two-pyroxene diorites cut by biotite-hornblende microdiorite and quartz monzodiorite dikes. The host diorite gives a 138 ± 2.9 Ma SHRIMP age on zircons, which is interpreted to be the age of igneous crystallisation. The calc-alkaline geochemistry of the host diorites and the microdiorites is attributed to melting of a mantle wedge source fluxed by slab-derived fluids. Ductile deformation (D1) of the host diorite and the microdiorite dikes took place at mid-upper amphibolite facies conditions, with extension lineations indicating a top to the NNE sense of shear. A SHRIMP age of 136 ± 1.9 Ma on zircons from a quartz monzodiorite dike injected along D1 shears is statistically indistinguishable from that of the host diorite. This suggests that D1 was synmagmatic and that subduction, during or shortly after magma emplacement, was oblique to the Gondwana margin at c. 138 Ma. The quartz monzodiorite dikes are enriched in Na2O, Al2O3, and Sr, depleted in Y, and have a distinctly adakitic geochemistry. The change in chemistry from calc-alkaline magmas to alkali-calcic adakitic magmas reflects the melting of a mafic, garnet-bearing, essentially plagioclase-free source in the root of a volcanic arc system. (auth)

  19. Sr isotopic features of the ophiolitic rocks in the Kamuikotan Zone, Hokkaido

    International Nuclear Information System (INIS)

    Rb and Sr contents and Sr isotopic ratios were determined for the ophiolitic rocks in the Kamuikotan Zone, Hokkaido. The initial ratios of these rocks are as follows; Sorachi Group basic rocks (0.7019 - 0.7045), ultrabasic rocks (dunite, harzburgite and serpentinite) (0.7069 - 0.7113), olivine clinopyroxenites (0.7037 - 0.7103), microdiorites (0.7039 - 0.7054), rodingite (0.7038), quartz albitite (0.7048), amphibolitic rocks (0.7032 - 0.7053), and trondhjemites (0.7028 - 0.7052). The genertical relationship between the Sorachi Group basic rocks and the associated ultrabasic rocks can be explained by considering them as the complementary products formed by partial melting of certain lherzolitic mantle material, which is composed of various minerals with different Sr contents and Sr isotopic ratios. It seems plausible to consider that the dike rocks such as olivine clinopyroxenite, microdiorite and gabbroic pegmatite were formed from the liquids trapped within the hot residual mantle diapir. (author)

  20. A Neoproterozoic age for the chromitite and gabbro of the Tapo ultramafic Massif, Eastern Cordillera, Central Peru and its tectonic implications

    Science.gov (United States)

    Tassinari, Colombo C. G.; Castroviejo, Ricardo; Rodrigues, Jose F.; Acosta, Jorge; Pereira, Eurico

    2011-12-01

    The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites show a tholeiitic affiliation and a flat REE spider diagram, with a slight LREE depletion and a positive Eu anomaly suggesting magmatic accumulation of plagioclase, in an ocean ridge or ocean island environment. Sm-Nd isotopic analyses were performed on chromite as well as on whole rock from the gabbro. All samples yielded an Sm-Nd isochrone age of 718 ± 47 Ma with an initial 143Nd/ 144Nd of 0.51213 ± 0.00005. The ɛNd (718 Ma) values calculated for both chromite and gabbro are in close agreement, around 8.0, implying that they were formed at the same time from the same mantelic magma source. Furthermore a K-Ar age on amphibole of 448 ± 26 Ma was obtained, interpreted as the cooling age of a younger orogenic event. These rocks represent slices of oceanic crust (from a dismembered ophiolitic complex), metamorphosed and later overthrust on upper Palaeozoic continental formations.

  1. Contrasting Ordovician high- and low-pressure metamorphism related to a microcontinent-arc collision in the Eastern Cordillera of Perú (Tarma province)

    Science.gov (United States)

    Willner, Arne P.; Tassinari, Colombo C. G.; Rodrigues, José F.; Acosta, Jorge; Castroviejo, Ricardo; Rivera, Miguel

    2014-10-01

    High-pressure conditions of 11-13 kbar/500-540 °C during maximum burial were derived for garnet amphibolite in the Tapo Ultramafic Massif in the Eastern Cordillera of Peru using a PT pseudosection approach. A Sm-Nd mineral-whole rock isochron at 465 ± 24 Ma dates fluid influx at peak temperatures of ˜600 °C and the peak of high pressure metamorphism in a rodingite of this ultramafic complex. The Tapo Ultramafic Complex is interpreted as a relic of oceanic crust which was subducted and exhumed in a collision zone along a suture. It was buried under a metamorphic geotherm of 12-13 °C/km during collision of the Paracas microcontinent with an Ordovician arc in the Peruvian Eastern Cordillera. The Ordovician arc is represented by the western Marañon Complex. Here, low PT conditions at 2.4-2.6 kbar, 300-330 °C were estimated for a phyllite-greenschist assemblage representing a contrasting metamorphic geotherm of 32-40 °C/km characteristic for a magmatic arc environment.

  2. Origins of felsic magmas in Japanese subduction zone: Geochemical characterizations of tephra from caldera-forming eruptions <5 Ma

    Science.gov (United States)

    Kimura, Jun-Ichi; Nagahashi, Yoshitaka; Satoguchi, Yasufumi; Chang, Qing

    2015-07-01

    Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb-208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera-forming eruptions are ultimately the effect of a mantle-driven cause.

  3. Concentrations of 222Rn in groundwaters flowing through different crystalline rocks: An example from Sleza Massif (SW Poland)

    International Nuclear Information System (INIS)

    Sleza Massif is situated in the south-western part of Poland, about 30 km SW of Wroclaw, the capital city of Lower Silesia. The geological setting of the research area is typical of the Sudety Mountains. Different types of crystalline rocks, of both igneous and metamorphic origin, occur over an area of 25 km2. On the surface of this relatively small area, Lower Carboniferous to Lower Permian Strzegom-Sobotka Granite and Devonian Sleza Ophiolite are uncovered. The result is the occurrence of granites, gabbros, amphibolites and serpentinites, directly neighbouring on each other. The author selected this area for determining the influence of rock type on the concentration of 222Rn dissolved in groundwater flowing through crystalline rocks. The first stage of the research consisted of determining typical values of 222Rn concentration in groundwater flowing through different types of rocks and describing the scale of seasonal changes in 222Rn concentration. In the next stage of the research, an attempt to apply 222Rn as one of the isotopic hydrogeochemical tracers of the flow pathway of fissure groundwater will be undertaken. The results show that the highest values of 222Rn concentration (reaching 229 Bq/L) were observed in groundwater flowing out of springs located within granite, whereas the lowest one (1.1 Bq/L) was noted in a spring located within serpentinite. The average 222Rn concentrations obtained in groundwater flowing out of two springs within granite were 170 and 103 Bq/L, whereas the average values in two springs located within amphibolites reached 7.3 and 8.2 Bq/L. The average 222Rn concentrations in the springs flowing out of gabbro and serpentinite amounted to 7.6 and 1.2 Bq/L respectively. The 222Rn concentration in the groundwater flowing out of the spring located within serpentinites was stable during the whole year -- likewise for the discharge of the spring. 222Rn concentrations between 1.1 ± 0.2 and 1.4 ± 0.2 Bq/L were measured. On the other

  4. Paragenesis and chemical characteristics of the celsian-hyalophane-K-feldspar series and associated Ba-Cr micas in barite-bearing strata of the Mesoarchaean Ghattihosahalli Belt, Western Dharwar Craton, South India

    Science.gov (United States)

    Raith, Michael M.; Devaraju, Tadasore C.; Spiering, Beate

    2014-04-01

    The upper greenschist - lower amphibolite facies, argillaceous to chemical-exhalative metasedimentary sequence of the Mesoarchaean Ghattihosahalli Schist Belt (GHSB), southern India, has been examined with a special focus on the paragenesis and solid solution characteristics of barian feldspars and associated dioctahedral Ba-Cr-bearing micas. Barian feldspars occur as untwinned porphyroblasts in a recrystallized finely banded matrix of barite, quartz and minor white mica. Idioblastic celsian (Cls98-76Or2-20Ab1-8) and hyalophane (Cls55-39Or35-51Ab10) predate the greenschist-facies foliation, whereas xenoblastic hyalophane (Cls44-35Or45-59Ab8-17) and mantles on celsian (Cls45-35Or42-60Ab13-5) as well as xenoblastic barian K-feldspar (Cls6Or90Ab2) postdate the last fabric-defining event. The preservation of extremely complex zoning patterns down to the micron-scale shows that diffusional homogenization did not operate at fluid-present low to medium-grade conditions (350-550 °C, 3-5 kb). Microstructures indicate that at these conditions barian feldspars deform exclusively by brittle fracturing and do not undergo recrystallization. Barian feldspar compositions confirm the positive correlation of Na-content with temperature and the existence of a narrow asymmetric compositional gap (Cls90-85↔Cls55, ~350 °C) which probably closes at lower amphibolite facies conditions (Xc ~Cls75; Tc ~550 °C). White micas are solid solutions of the end-members muscovite, ganterite (Ba0.5 K0.5)Al2(Al1.5Si2.5)O10(OH)2, paragonite, celadonite with a significant substitution of [VI]Al by Cr. Zoning is a common feature with cores being enriched in Ba. The data document extensive Ba substitution for K from muscovite to ganterite, exclusively controlled by the coupled substitution [XII]K + [IV]Si ↔ [XII]Ba + [IV]Al and strongly dependent on bulk composition. The extent of solid solution from (Ms+Gnt) towards paragonite and celadonite end-members is controlled by the miscibility gap in the

  5. GEOLOGICAL CHARACTERISTICS AND AGES ASSURANCE OF THE MUZHAERTE GROUP COMPLEX OF PALAEOPROTE ROZOIC IN WESTERN TIANSHAN MOUNTAINS%西天山古元古代木札尔特岩群地质特征及时代厘定

    Institute of Scientific and Technical Information of China (English)

    于海峰; 王福君; 潘明臣; 梁有为; 郭洪方; 王志军

    2011-01-01

    西天山木札尔特岩群发育于塔里木原始古陆台内毗邻陆缘活动带,为一套角闪岩相中深变质岩系,主要岩石组合为变粒岩-浅粒岩-片麻岩-斜长角闪岩-大理岩等,局部受韧性变形改造形成各类糜棱岩系,原岩为中基性火山熔岩-火山碎屑岩-火山碎屑沉积岩夹碳酸盐岩建造.由于缺少古生物化石,其地层时代主要依据区域地层对比和同位素年代学数据进行确定.笔者应用钐钕全岩等时线定年法,在该岩群斜长角闪岩中获得(1966±93)Ma的同位素年龄,这是迄今为止,西天山范围内该岩群内获得的最古老同位素年龄,代表了其成岩年龄.据国际地层表(2000)关于古元古界造山系2050~1800 Ma的划分方案,笔者最终将西天山木札尔特岩群成岩时代厘定为古元古代造山纪.%The Muzhaerte Group Complex in western Tianshan Mountains formed in the mobile belt of the Tarim proto-contcnent margin.lt's main rock group is leptynite-leptite-gneiss-amphibolite-marble etc.which passed with hornblend-phase regional metamorphism and all kinds af mylonite which passed with ductile shear deformation in the strain localization region. The protolish of Muzhaerte Group Complex is intermediate-busic pyroclastic lava-pyrolastic rock-pyroclastic sedimentary rock via carbonate rock formation. Because of the lack of extinct animals and plants fossil,it's diagonesis age has been one of the controversial issues in the geological community for a long time. What's more, the diagonesis age assurance of Muzhaerte Group Complex mainly based on regional stratigraphic correlation and isotopic age determination. The isotopic age of ((1966±93) Ma) had been obtained from amphibolite of Muzhaerte Group Complex through Sm-Nd total-rock isochron method by writer,and was stand for Muzhaerte Group Complex time which is the earliest isotope age founded in this geologic body in Western Tianshan by now.According to the project of 2050~1800 Ma

  6. Significance of orthogonal flow in the Funeral Mountains metamorphic core complex, Death Valley, California: Insights from geochronology and microstructural analysis

    Science.gov (United States)

    Sauer, K. M.; Wells, M. L.; Hoisch, T. D.

    2013-12-01

    The Funeral Mountains metamorphic core complex (FMMCC) in Death Valley, California, exposes middle to lower crustal rocks of the Sevier-Laramide orogen in the footwall of the Boundary Canyon detachment (BCD). Monarch Canyon, located in the northwest section of the Funeral Mountains, exposes the structurally deepest rocks in the FMMCC. These Mesoproterozoic to Neoproterozoic metasedimentary rocks record upper amphibolite facies metamorphism with migmatites developed at the deepest levels. The Monarch Spring fault (MSF) juxtaposes migmatitic paragneisses below against pelitic schists, calcsilicate schists, and marbles above, and represents a deformed anatectic front. In the footwall of the BCD above the MSF, distributed ductile deformation and stratigraphically localized high-strain zones, termed intracore shear zones, are responsible for attenuation and local stratigraphic omission during top-northwest non-coaxial deformation. The relative contributions of Late Cretaceous-early Tertiary and Miocene extensional strains which manifest in the top-northwest fabrics remains unclear, and is being addressed by ongoing and combined thermochronologic, microstructural, and EBSD studies. Our working hypothesis is a polystage extensional history in the FMMCC, with Late Cretaceous extensional intracore shear zones locally reactivated during the Miocene. Below the MSF, migmatitic paragneisses lack similar greenschist to lower amphibolite facies top-northwest fabrics. These rocks instead exhibit heterogeneous strain and a weak to moderately developed northeast-trending mineral lineation, and a local, strong fabric asymmetry indicative of a top-southwest sense of shear. We propose that the anatectic front is an apparent zone of structural decoupling between top-southwest shear below and top-northwest shear above the MSF. Structural and geochronologic studies are currently underway to establish whether the orthogonally directed flow above and below the anatectic front were coeval or

  7. Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology

    Science.gov (United States)

    Mulcahy, S. R.; Vervoort, J. D.

    2013-12-01

    Lawsonite [CaAl2Si2O7(OH)2 H2O] is a critical index mineral for high- to ultrahigh-pressure metamorphism associated with subduction. Lawsonite is an important carrier of water into the mantle, a likely contributor to subduction zone seismicity, and a bearer of trace elements that link metamorphism to arc magmatism. Due to its limited pressure-temperature stability, lawsonite can serve as a powerful petrogenetic indicator of specific metamorphic events. Lu-Hf dating of lawsonite, therefore provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. Broad application of lawsonite Lu-Hf geochronology requires constraining the role of pressure-temperature path, lawsonite forming reactions, and the Lu and Hf systematics within lawsonite and other blueschist facies minerals. We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in melange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet. We sampled exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist from the Berkeley Hills and Tiburon Peninsula of California. We collected four samples from coherent lawsonite blueschist across the lawsonite-pumpellyite-epidote isograds in Ward Creek, near Cazadero California. High-grade blocks give ages similar to existing Franciscan geochronology: multi-stage garnet in hornblendite gives the following ages: 171×1.3 Ma (MSWD 2.8) for the core and 159.4×0.9 Ma (MSWD 2.0) for the corresponding rim; 166

  8. Bedrock geology Forsmark. Modelling stage 2.3. Description of the bedrock geological map at the ground surface

    Energy Technology Data Exchange (ETDEWEB)

    Stephens, Michael B.; Bergman, Torbjoern (Geological Survey of Sweden, Uppsala (Sweden)); Isaksson, Hans (GeoVista AB, Luleaa (Sweden)); Petersson, Jesper (SwedPower AB, Stockholm (Sweden))

    2008-12-15

    A description of the bedrock geological map of the ground surface at the Forsmark site is presented here. This map is essentially a 2D model for the distribution of different types of rock unit on this surface. Besides showing the distribution of these rock units, the bedrock geological map also displays the distribution of some deformation zones that intersect the ground surface. It also presents information bearing on the position and form of outcrops, the location and projection of boreholes drilled during the site investigation programme, subordinate rock types, the occurrence of abandoned mines or exploration prospects, measurements of ductile structures in outcrops, inferred form lines, key minerals, and the occurrence of mylonite and cataclastic rock. Bedrock data from outcrops and excavations, airborne and ground magnetic data and information from the uppermost part of boreholes have all been used in the construction of the geological map. The description has also made use of complementary analytical data bearing on the composition and age of the rocks as well gamma-ray spectrometry and gravity data. Uncertainty in the position of the boundaries between rock units over the mapped area are addressed in a qualitative manner. Four model versions of the bedrock geological map have been delivered to SKB's GIS database (bedrock geological map, Forsmark, versions 1.1, 1.2, 2.2 and 2.3) at different times during the site investigation programme. The Forsmark area is situated along the coast of the Baltic Sea in northern Uppland, Sweden, in a region where the overall level of ductile strain in the bedrock is high. This high-strain region extends several tens of kilometres across the WNW-ENE to NW-SE strike of the rocks in this part of the Fennoscandian Shield. At Forsmark, the coastal region is composed partly of high-strain belts, which formed under amphibolite-facies metamorphic conditions, and partly of tectonic lenses, where the bedrock is also affected by

  9. Metamorphic and tectonic evolution of Ceuta peninsula (Internal Rif): new interpretation in the framework of arc and back arc evolution

    Science.gov (United States)

    Homonnay, Emmanuelle; Lardeaux, Jean-Marc; Corsini, Michel; Cenki-Tok, Bénédicte; Bosch, Delphine; Munch, Philippe; Romagny, Adrien; Ouazzani-Touhami, Mohamed

    2016-04-01

    In the last twenty years, various geophysical investigations have established that the Western Mediterranean opened in a subduction context as a back arc domain. In the Alboran basin the dip of the subduction plane is eastwards or southeastwards depending of considered models. If the geological records of back-arc opening are well-known, the arc-related tectonic and petrologic evolutions are still poorly documented. In order to decipher these markers, we focalised structural, petrological and thermo-chronological studies on the Ceuta peninsula located in the Rif belt, on the western part of the Gibraltar arc to the North of Morocco. The present-day tectonic pile is constituted by: (1) the upper Ceuta unit, composed of High Pressure and High Temperature metapelites retromorphosed under Amphibolite-facies condition, with Ultra-High Pressure relicts, and pyrigarnite and spinel bearing peridotites boudins at its base, (2) the lower Monte Hacho unit, with orthogneisses metamorphosed under Amphibolite-facies conditions. Structural analysis indicates a polyphase tectonic evolution: (1) an earlier deformation phase only observed in the UHP metapelites and characterized by a steep S1 foliation plane, (2) a main deformation phase associated to a pervasive gently dipping S2 foliation plane bearing a L2 stretching lineation and synschistose folds whose axes are parallel to L2 and (3) a late deformation phase which developed S3 foliation plane and L3 stretching lineation coeval with development of narrow normal ductile shear zones. A zone of increasing deformation, several dozen meters wide, is identified as a major ductile shear zone involving the peridotitic lenses at the base of the metapelites of the Ceuta unit and overlaying this upper unit on top of the orthogneisses of the Monte Hacho lower unit. The attitude of mylonitic foliation and stretching and mineral lineations as well as the numerous shear sense indicators observed in the shear zone are consistent with a

  10. Redefinition and Significance of Metamorphism Xilinhot Group in Xilinhot Area, Inner Mongolia, China%内蒙古锡林浩特地区中元古代锡林浩特岩群的厘定及其意义

    Institute of Scientific and Technical Information of China (English)

    周文孝; 葛梦春

    2013-01-01

    Based on large scale geologic mapping and measuring section,the Xilin Col complex is divided into three parts:supracrustal rock,Late Proterozoic basic-uhrabasic rock and Early Paleozoic acid intrusive rocks.According to lithologic association and characteristics of deformation and metamorphism,biotite (garnet) plagioclase gneiss intercalated with several layers barred amphibolite,with thin layer magnetite quartzite on its top; medium-coarse biotite plagioclase gneiss and fine-grained felsic gneiss; lithologic association of migmatization banding biotite (hornblende) plagioclase gneiss and garnet monzonitic gneiss intercalated with barred amphibolite; sillimanite biotite plagioclase gneiss,barred biotite plagioclase gneiss,intercalated with garnet bearing biotite quartz schist.Comparison study on the different lithologic association,metamorphic characteristics and depositional environment of Baoyintu Group,Ailegemiao Formatio,Bainaimiao Group and others.This paper proposes to redefine these supercrustal rocks named "Xilinhot Group",which compose the Precambrian metamorphic basement of Xilin Col micro block.It's important to study the formation and development of Xilin Col micro block and the evolution of structural relationship between Siberia plate and North China plate.%通过大比例尺填图和剖面研究,将内蒙古锡林浩特地区的“锡林郭勒杂岩”解体分为3大部分:一套表壳岩、晚元古代基性-超基性侵入岩和早古生代酸性侵入岩.在此基础上根据变质岩的岩性组合和变形变质特点,可将其中的表壳岩化分为4个岩性段:黑云(石榴石)斜长片麻岩,间夹多层条纹状斜长角闪岩,在其顶部多见薄层含磁铁石英岩;中粗粒黑云斜长片麻岩和细粒长英质片麻岩;混合岩化条带状黑云(角闪)斜长片麻岩,含石榴石黑云二长片麻岩夹条纹状斜长角闪岩组合;夕线石黑云斜长片麻岩,条纹状黑云斜长片麻岩,间夹含石榴石黑云母

  11. The Analysis of Metamorphism-deformation Sequence for the Paired Metamorphic Belt in West Tianshan Orogen:An Example from Chahanwusu Area,Zhaosu County,Xinjiang%西天山造山带双变质带变质变形序列分析--以新疆昭苏县察汗乌苏一带为例

    Institute of Scientific and Technical Information of China (English)

    程培起

    2015-01-01

    西天山造山带中双变质带以新疆昭苏察汗乌苏一带较为典型,根据变质作用的不同,划分出绿片岩相绢云母绿泥石带、绿片岩相蓝闪石带、角闪岩相黑云母普通角闪石带、角闪岩相矽线石铁铝榴石带等变质相带;按其变质作用以及构造变形程度分为上部层次脆性性构造层、中上部层次、深部层次3种构造层次。根据之间的相互叠加关系,确立了造山带内的岩石主要存在着3期以上构造变形特征;在主要的3期构造变形过程中见有相应的变质矿物,并根据生成的矿物组合与变形之间的相互关系,通过变质矿物先后生成关系与岩石的结构、构造建立了变质与变形序列关系,确立了变质与变形非简单一一对应,而是具有渗透叠加和循环往复的特点。%The paired metamorphic belt of West Tianshan Orogen is more typical in Chahanwusu Area,Xinjiang. According to the difference of metamorphism,this paired metamorphic belt can be divided into several metamorphic facies zone,including sericite-chlorite zone with greeanschist-facies,glaucophane zone with greenschist facies,biotite-hornblende zone with amphibolite facies belt and sillimanite-almandite zone with amphibolite facies. Based on the degree of metamorphism and tectonic deformation,this paired metamorphic belt can be divided into three kinds of structur-al levels:the brittle structural layer in the upper level,the structural layers in the middle-upper level and the ones in the lower level. After analyzing the superimposed relationship among these structural levels,more than threetimes of tectonic deformation characteristics have been distin-guished in the rocks of West Tianshan Orogen. And the corresponding metamorphic minerals for these three tectonic stages in the process of deformation can be observed. And then,the interre-lationship between generated mineral association and deformation has been discussed,the rela

  12. Kyanite-garnet gneisses of the Kåfjord Nappe - North Norwegian Caledonides: P-T conditions and monazite Th-U-Pb dating

    Science.gov (United States)

    Ziemniak, Grzegorz; Kośmińska, Karolina; Majka, Jarosław; Janák, Marian; Manecki, Maciej

    2016-04-01

    The Kåfjord Nappe is the part of the Skibotn Nappe Complex traditionally ascribed to the Upper Allochthon of the North Norwegian Caledonides. Pressure-temperature (P-T) conditions and metamorphic age of the Kåfjord Nappe are not well constrained, geochronological data are limited to a single Rb-Sr age of c. 440 Ma (Dangla et al. 1978). Metamorphic evolution of kyanite-garnet gneisses of the Kåfjord Nappe is presented here. The kyanite-garnet gneisses are associated with a few meters thick amphibolite lenses. The gneisses mainly consist of quartz, plagioclase, biotite, muscovite, garnet, kyanite, and rutile. Retrograde minerals are represented by sillimanite and chlorite. Garnet occurs as two textural types. Garnet-I forms euhedral porphyroblasts with multiple small inclusions. Profiles through garnet-I show chemical zonation in all components. The composition varies from Alm64-68Prp11-16Grs13-18Sps2-8 in the core to Alm68-70Prp17-18Grs10-13Sps1-3 in the rim. Garnet-II is subhedral to anhedral, its core is inclusion-rich, whereas rim contains only single inclusions. Chemical composition of garnet-II is similar to that of the garnet-I rim. P-T conditions have been estimated using the garnet-biotite-muscovite-plagioclase (GBPM) geothermobarometer (Holdaway, 2001; Wu, 2014). Calculated peak P-T metamorphic conditions are 610-625 °C and 7.6-8.2 kbar corresponding to the amphibolite facies conditions. Phase equilibrium modelling in the NCKFMMnASH system yields peak metamorphic conditions of c. 620 °C at 8 kbar. Growth conditions of garnet-I core modelled in the NCKFMMnASH system are c. 570 °C at 9.7 kbar. Chemical Th-U-total Pb monazite dating has been performed. Preliminary dating results from the kyanite-garnet gneiss of the Kåfjord Nappe yield an array of dates from 468 Ma to 404 Ma. There is a correlation between an increase of yttrium content and decrease of monazite single dates. Compositional maps confirm an increase of yttrium towards the rim of the

  13. Extreme extension across Seram and Ambon, eastern Indonesia: Evidence for Banda slab rollback

    Directory of Open Access Journals (Sweden)

    J. M. Pownall

    2013-04-01

    Full Text Available The island of Seram, which lies in the northern part of the 180°-curved Banda Arc, has previously been interpreted as a fold-and-thrust belt formed during arc-continent collision, which incorporates ophiolites intruded by granites thought to have been produced by anatexis within a metamorphic "sole". However, new geological mapping and a re-examination of the field relations cause us to question this model. We instead propose that there is evidence for recent N–S extension that has caused the high-temperature exhumation of hot mantle peridotites, granites, and granulites (the "Kobipoto Complex" beneath low-angle lithospheric detachment faults. Greenschist- to lower-amphibolite facies metapelites and amphibolites of the Tehoru Formation, which comprise the hanging wall above the detachment faults, were overprinted by sillimanite-grade metamorphism, migmatisation and limited localised diatexis to form the Taunusa Complex. Highly aluminous metapelitic garnet + cordierite + sillimanite + spinel + corundum + quartz granulites exposed in the Kobipoto Mountains (central Seram are intimately associated with the peridotites. Spinel + quartz inclusions in garnet, which indicate that peak metamorphic temperatures for the granulites likely approached 900 °C, confirm that peridotite was juxtaposed against the crust at typical lithospheric mantle temperatures and could not have been part of a cooled ophiolite. Some granulites experienced slight metatexis, but the majority underwent more advanced in situ anatexis to produce widespread granitic diatexites characterised by abundant cordierite and garnet xenocrysts and numerous restitic sillimanite + spinel "clots". These Mio-Pliocene "cordierite granites", which are present throughout Ambon, western Seram, and the Kobipoto Mountains in direct association with peridotites, demonstrate that the extreme extension required to have driven Kobipoto Complex exhumation must have occurred along much of the northern

  14. Status and targets in the Collisional Orogeny in the Scandinavian Caledonides project

    Science.gov (United States)

    Almqvist, Bjarne; Juhlin, Christopher; Lorenz, Henning; Gee, David; Pascal, Christophe; Tsang, Chin-fu; Pedersen, Karsten; Roberts, Nick; Rosberg, Jan-Erik

    2015-04-01

    The COSC project is a multidisciplinary international project with the aim to provide a deeper understanding of mountain belt dynamics in the Scandinavian Caledonides. Scientific investigations include a range of topics, from understanding the ancient orogeny to the present-day hydrological cycle. Six working groups comprise the project and include 1) tectonics, 2) geophysics, 3) geothermics, 4) hydrology, 5) microbiology and 6) drilling management and technology. This presentation provides an overview of the scientific goals of COSC and the first phase of drilling activities, which took place from April 28 until August 26, 2014 (COSC-1). COSC investigations and drilling activities are focused in central Scandinavia, near Åre (Sweden), where rocks from the mid to lower crust of the orogen are exposed. Rock units of interest include granulite facies migmatites (locally ultra-high pressure), gneisses and amphibolites in the middle allochthon (Seve nappe) that overlie greenschist facies metasedimentary rocks in the lower allochthons (Särv and Jämtlandian nappes). The base of the lower allochthon marks the contact with the autochthonos Precambrian basement. Scientific drilling is a central part of the COSC project, and is motivated from a tectonic viewpoint to obtain (i) better understanding of the exhumation and emplacement of the hot middle allochthon, which may enable comparison with exhumation processes in the Himalaya-Tibet orogen, (ii) a broad understanding of orogeny and deformation in the middle to deep crust and upper mantle of mountain belts, and (iii) constraints on the abundant geophysical data that has been collected in the area. Drilling is divided into two phases, firstly to drill through the high grade Seve nappe into the underlying allochthon. The second phase (COSC-2) will target the tectonostratigraphic units below COSC-1 including the Caledonian décollement and the autochthonos Precambrian basement, and investigate the involvement of the

  15. [Cancer risk in asbestos-cement industry workers in Poland].

    Science.gov (United States)

    Szeszenia-Dabrowska, N; Wilczyńska, U; Szymczak, W

    1997-01-01

    A cohort study was carried out in order to evaluate the cancer risk in the asbestos-cement industry workers. The cohort consisted of workers employed in four asbestos-cement plants. One of those plants was established in 1924, the other three in the 1960s and 1970s. Currently only two of these plants continue their production. The plants used mainly chrysotile asbestos as well as crocidolite and amosite. Amphibolite asbestos was used before the mid-nineteen eighties in production of pressure pipes utilising about 15% of the total quantity of asbestos used. The measurements of the asbestos fibre concentration at work-sites have been taken occasionally since the mid 1980s, thus, the determination of a cumulative dose for individual persons in the cohort and the evaluation of the dose-effect relationship were not feasible. It could only be supposed that the concentrations at the preparatory work-site during first years of the plants' operation accounted for several tens fibres/cm3 in the production that employed the dry method. The cohort consisted of workers employed in the plant for at least three months between beginning of the plant during the post-war period, and 1980, that is during the period when amphibolite asbestos was in use. The retrospective observation was completed on 31 December 1991. The analysis of the death risk by causes was based on a standardized mortality ratios (SMRs) calculated using the person-years method. Statistical significance of SMRs was assessed by means of Poisson distribution one-sided test. The general population of Poland was used as the reference population to estimate the death risk. The cohort comprised 4,712 persons (3,563 males and 1,149 females). Of this number 4,500 persons (3,405 males and 1,095 females) were followed. The cohort availability were 95.5%. Male mortality, both total (473 deaths; SMR = 83) and due to malignant neoplasms (108 deaths; SMR = 86) was lower than in the general population. An excess of deaths from

  16. Bedrock geology Forsmark. Modelling stage 2.3. Description of the bedrock geological map at the ground surface

    International Nuclear Information System (INIS)

    A description of the bedrock geological map of the ground surface at the Forsmark site is presented here. This map is essentially a 2D model for the distribution of different types of rock unit on this surface. Besides showing the distribution of these rock units, the bedrock geological map also displays the distribution of some deformation zones that intersect the ground surface. It also presents information bearing on the position and form of outcrops, the location and projection of boreholes drilled during the site investigation programme, subordinate rock types, the occurrence of abandoned mines or exploration prospects, measurements of ductile structures in outcrops, inferred form lines, key minerals, and the occurrence of mylonite and cataclastic rock. Bedrock data from outcrops and excavations, airborne and ground magnetic data and information from the uppermost part of boreholes have all been used in the construction of the geological map. The description has also made use of complementary analytical data bearing on the composition and age of the rocks as well gamma-ray spectrometry and gravity data. Uncertainty in the position of the boundaries between rock units over the mapped area are addressed in a qualitative manner. Four model versions of the bedrock geological map have been delivered to SKB's GIS database (bedrock geological map, Forsmark, versions 1.1, 1.2, 2.2 and 2.3) at different times during the site investigation programme. The Forsmark area is situated along the coast of the Baltic Sea in northern Uppland, Sweden, in a region where the overall level of ductile strain in the bedrock is high. This high-strain region extends several tens of kilometres across the WNW-ENE to NW-SE strike of the rocks in this part of the Fennoscandian Shield. At Forsmark, the coastal region is composed partly of high-strain belts, which formed under amphibolite-facies metamorphic conditions, and partly of tectonic lenses, where the bedrock is also affected by

  17. Late Paleozoic tectonomagmatic evolution of the western southern Tian Shan, Tajikistan

    Science.gov (United States)

    Worthington, James R.; Kapp, Paul; Minaev, Vladislav; Chapman, James B.; Oimahmadov, Ilhomjon; Gadoev, Mustafo

    2015-04-01

    The 2500-km-long Tian Shan orogenic belt constitutes a dominantly Paleozoic amalgmation of Eurasia that has been overprinted by the Mesozoic and Cenozoic Cimmerian and Indo-Eurasian collisions. This southernmost unit of the Central Asian Orogenic System (CAOS) is divided N-S by discontinuous suture zones that reflect its complex assemblage and E-W by the Talas-Fergana dextral (modern kinematics) fault zone. The western southern Tian Shan in Tajikistan/Uzbekistan is poorly studied compared to the rest of the orogen in Kyrgyzstan/China, but a dominant signal of late Paleozoic magmatism synchronous to widespread magmatism documented along strike provides an intriguing opportunity to investigate regional tectonic processes at this time. The late Carboniferous-early Permian Gissar batholith is the southern Tian Shan's southernmost lithotectonic unit. Zircon U-Pb weighted-mean crystallization ages for Gissar granitoids range from ~310-290 Ma, are youngest in the east, and define a primary stage of arc magmatism related to closure of the Turkestan ocean. A ~280 Ma crystallization age was obtained for a Ne syenite, which corresponds to small, 'post-collisional,' alkaline intrusions in 1:200,000 Soviet geologic maps. Zircon ɛHf in Gissar granitoids generally decreases with decreasing zircon U-Pb age from +5'10. Zircon ɛHf in the young Ne syenite is +1-+6. Taken together, these trends indicate a progressive shift from juvenile to intermediate magmatism over 20 Myr, followed by a marked return to juvenile magmatism within 10 Myr. The Garm 'metamorphic' massif is situated within the eastern Gissar batholith and is derived from greater depths than the rest of the batholith, as indicated by its defining features: (i) Discontinuous outcrops of Bt+Grt quartzofeldspathic gneisses/schists; and (ii) Presence of igneous garnet in granitoids. Zircons from the Garm quartzofeldspathic gneisses/schists exhibit pronounced Pb-loss discordia that are consistent with ~amphibolite

  18. Phengite-hosted LILE enrichment in eclogite and related rocks: Implications for fluid-mediated mass transfer in subduction zones and arc magma genesis

    Science.gov (United States)

    Sorensen, Sorena S.; Grossman, J.N.; Perfit, M.R.

    1997-01-01

    Geochemical differences between island arc basalts (LAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ?? garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an LAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to melasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ???3??25-3??55 Si per 11 oxygens, and ???0??25-0??50 Mgper 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to ???0??12. Ba contents of phengite range to over 1 wt % (0??027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged KlBa ratios for phengite and host samples define an array which describes other subsamples of

  19. The Carbonate-Hosted Willemite Deposits in the Zambesi Metamorphic Belt (Zambia): a "Franklin-Type" Mineralization?

    Science.gov (United States)

    Boni, M.; Terracciano, R.

    2009-05-01

    associated with willemite in several hypogene nonsulfide zinc deposits. The typical occurrences are in the Franklin Marbles of the Middle Proterozoic Grenvillian basement in North America, with the best example represented by the Franklin-Sterling Hill deposit. In the latter the franklinite-gahnite-willemite ore association is considered as having been originated by amphibolite-granulite facies metamorphism from a previous zinc sulfide/nonsulfide mineralization. Franklinite (as well as genthelvite) has been reported from the Gamsberg Zn-Pb deposit (Namaqua province, South Africa), which has been also metamorphosed to amphibolite facies. The origin of the franklinite-gahnite protore in the Lusaka area is still unclear. However, the existing mineralization could represent a metamorphosed occurrence derived from primary sulfide concentrations (now completely disappeared), as the Nampundwe massive sulfide deposit occurring SW of Lusaka. Precise age constraints are currently lacking for these willemite deposits. Since no major tectonic deformation is affecting the ores, and there is no record of any geological cover, their only possible temporal constraint may involve the emplacement age of the Hook granite, dated at 559±18 Ma. This intrusion is supposed to be coeval with the Mwembeshi shear zone, whose lineaments are also controlling the Star Zinc deposit.

  20. Crust Formation and Stabilization of the Western Archean Kaapvaal Craton: Evidence from U-Pb Geochronology of Basement Blocks and Deep Crustal Xenoliths from the Kimberley Region, South Africa

    Science.gov (United States)

    Schmitz, M. D.; Bowring, S. A.

    2001-05-01

    The kimberlites of the Kimberley region of South Africa have yielded one of the most abundantly sampled and studied suites of lithospheric mantle xenoliths in the world, providing a detailed picture of the composition and thermal evolution of the continental mantle beneath the western Kaapvaal craton. Surprisingly however, little published data exist regarding the nature of the basement and deeper crustal rocks in the western craton, with which to contrast the evolution of the crustal and mantle portions of this Archean cratonic region. Crustal xenoliths collected in the various mine dumps around Kimberley are predominantly large blocks of near-surface basement lithologies, including deformed granitic to tonalitic gneisses and amphibolites, weakly deformed pegmatoids, and non-deformed biotite granite. U-Pb zircon geochronological data for a number of xenoliths have been used to develop a preliminary framework for the age and evolution of the Archean crust of the Kimberley region. The youngest component of the Kimberley basement is a non-deformed sample of biotite granite with an age of 2724+/-2 Ma. A major episode of metamorphism and crustal anatexis is recorded by 2928+/-2 Ga metamorphic zircon growth in amphibolitic and tonalitic components of banded gneisses, and igneous zircons of identical age in weakly deformed cross-cutting pegmatoids. Zircons from these same pegmatoids also have inherited cores which yield 207Pb/206Pb dates as old as 3265 Ma. These inherited zircons, as well as cores of zircons from a foliated granodioritic xenolith with 207Pb/206Pb dates as old as 3184 Ma, indicate the antiquity of the oldest crustal components of the Kimberley basement. These data are consistent with cursory SHRIMP U-Pb zircon geochronological information reported for lithologies collected in situ in the diamond mine walls of Kimberley. Two important implications of this data are considered: first, we interpret the major metamorphism and crustal anatexis at 2.93 Ga as

  1. The Pootlass High Strain Zone, Bella Coola BC: Timing, Kinematics and Significance

    Science.gov (United States)

    Demerse, D.; Kennedy, L. A.; Ullrich, T.; Mortensen, J. K.

    2007-12-01

    The Pootlass High Strain Zone (PHSZ) is a corridor of brittle and ductile deformation, at least 2 km wide and up to 30 km long, located in the Coast Belt just northeast of the boundary between the Intermontane and Insular superterranes, which is demarked by the Coast Shear Zone at this latitude (Rusmore et al., 2001). In this paper, we report recent observations from field mapping, and new geochronological and petrological data, from which we place the PHSZ into a regional tectonic framework. The PHSZ is comprised of volcanic and sedimentary rocks of the lower to mid-Jurassic Hazelton Formation, and Jurassic to Eocene plutonic rocks varying in composition from granodiorite to diorite and tonalite. The field area has undergone at least three phases of deformation; a folding event that resulted in tight, southwest-verging folds correlated with the regional Late Cretaceous Coast Belt thrust and fold system, intense ductile, sinistral shear, and brittle dextral shear. The PHSZ is defined by a steep foliation, with a very well developed subhorizontal stretching lineation, and southwest-verging folds with the fold axes parallel to the stretching lineation. Many of the intrusive rocks occur as syn-kinematic mafic and felsic sheeted intrusions that are tightly folded and display sinistral kinematics. L-tectonites within the deformed plutonic rocks attest to the weakness of the rocks during deformation and support syn-kinematic magmatism. Zones of highest strain are found within the mafic and felsic sheeted intrusions. Predominantly sinistral ductile shear in the PHSZ is overprinted by brittle, dextral faults. Along strike to the northwest, the high strain zones displays both dextral and sinistral ductile shear components. Metasedimentary rocks in the PHSZ reached a maximum of amphibolite facies metamorphism as indicated by the abundance of hornblende. Greenschist facies metamorphism is dominant however, with rocks composed mainly of quartz, chlorite, muscovite and garnet

  2. Shear Zone Development and Rheology in the Deep Orogenic Crust

    Science.gov (United States)

    Marsh, J. H.; Johnson, S. E.; Gerbi, C. C.; Culshaw, N. G.

    2008-12-01

    Within the Central Gneiss Belt (CGB) of the southwestern Grenville Province, Ontario, Canada, a number of allocthonous lithotectonic domains are juxtaposed along crustal-scale shear zones. Extensive exposure of variably reworked granulites of the interior Parry Sound domain (iPSD) has enabled investigation of the structural and petrologic character of domain-bounding shear zones within the deep orogenic crust. Recent detailed mapping and structural data collected along the southwestern margin of the iPSD is consistent with the suggestion of Culshaw et al. (in prep) that spaced outcrop-scale shear zones have coalesced and progressively reworked layered granulites into a transposed amphibolite-facies tectonite. The tectonites comprise the Twelve Mile Bay Shear Zone (TMBSZ), which separates the iPSD from para-autocthonous rocks to the south. This study investigates the grain- and outcrop-scale mechanisms involved in shear zone development and attempts to quantify the associated changes in rock rheology. Northwest of TMBSZ, samples collected across individual outcrop-scale shear zones (i.e., across large strain gradients) have distinct differences in mineralogy and microstructure. In mafic layers the original granulite texture and cpx + opx + pl + hbl +/- grt assemblage is commonly retained away from the shear zones within unsheared "panels". With proximity to the shear zones pyroxenes and garnet are progressively consumed in hydration reactions producing hornblende and biotite, which define a new planar foliation within the highly attenuated and deflected layering. Felsic layers generally have only minor mineralogical changes across the zones, but develop an increasingly intense and recrystallized structural fabric into the sheared margin. The shear zones are commonly cored by variably deformed pegmatite dikes that were emplaced prior to, or during the early stages of shearing. Evidence for incipient shear zone formation along mineralized fracture sets that cut

  3. A multi-method study of metamorphism and fluid flow through the May Lake Interpluton Screen, Yosemite National Park, CA

    Science.gov (United States)

    Gervais, S. M.; Metzger, E. P.

    2012-12-01

    The May Lake interpluton screen is a 4 km long, approximately 0.5 km wide swath of metamorphic rock cropping out between plutons of the 102 Ma Yosemite Valley Intrusive Suite (YVIS) and the 93 Ma Tuolumne Intrusive Suite (TIS) in Yosemite National Park, CA. The screen is predominantly quartzite and metapelite with interspersed calc-silicate and marble layers, but the southwestern-most section of the screen consists of a thick marble unit that is stratigraphically basal. The metasedimentary rocks show extensive structural deformation in the form of folds, faults, and boudinaged layers. Metasomatic alteration and recrystallization due to contact metamorphism and fluid infiltration resulted from the emplacement of the Cretaceous plutons. Mineral assemblages are consistent with metamorphism to hornfels/amphibolite grade, with sillimanite present in the quartzite near the northern contact with the YVIS. Whole rock geochemical data on three metapelite samples is being used to create pseudosection P-T diagrams in order to further assess peak metamorphic conditions. Preliminary microprobe data from calc-silicate minerals show a general trend of more aluminous calcic garnet in the northeast section of the screen and more Fe-rich calcic garnet in the central and southwest regions. A 2 m thick, ~4 m long pegmatitic skarn within the granodiorite adjacent to the southeast-central border of the screen has garnet megacrysts up to 5 cm in diameter and bladed clinozoisite 7 cm in length. Microprobe analysis of a 3 cm garnet reveals growth zoning with oscillation in Fe, Ca, and Al between zones. Endoskarn within the granodiorite shows alteration of hornblende to pyroxene as aluminum-rich aqueous fluid was released from the granitic intrusion and calcium-rich fluid infiltrated from the calcareous metasediments. Results from stable isotope data suggest that rock permeability and preexisting structures were the strongest factors controlling fluid infiltration through the contact

  4. Rb-Sr and Sm-Nd isotopic compositions and Petrogenesis of ore-related intrusive rocks of gold-rich porphyry copper Maherabad prospect area (North of Hanich), east of Iran

    International Nuclear Information System (INIS)

    %) of a basaltic garnet-bearing (lower than 10%) amphibolite to amphibolite lacking plagioclase as a residual or source mineral can explain most of the moderate to low Y and Yb contents, low (La/Yb)N, high Sr/Y ratios and lack of negative anomaly of Eu in the rocks of the district. The geochemical signature of the adakites within the granitoid rocks represents a characteristic guide, for further exploration for copper porphyry-type ore deposit in Eastern Iran.

  5. Geology and Geochemistry of the Early Proterozoic Kortejärvi and Laivajoki Carbonatites, Central Fennoscandian Shield, Finland

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    Nykänen, J.

    1997-12-01

    Full Text Available This paper provides for the first time extensive petrological, mineralogical and geochemical data on the early Proterozoic Kortejärvi and Laivajoki carbonatites, northern Finland, which form metamorphosed and highly strained bodies 2 and 4 km long within a Svecokarelian shear zone in central Fennoscandian Shield. They are not exposed, but have been penetrated by a couple of deep drill holes. In terms of modal mineralogy, both intrusions contain calcite carbonatite and dolomite-calcite carbonatite as their main rock types, but Kortejärvi also contains dolomite carbonatite and calcite-dolomite carbonatite, some glimmerite and olivine-magnetite rock and Laivajärvi tremolite-calcite carbonatite, tremolite-dolomite carbonatite, serpentine-talc-dolomite rock and glimmerite. The main country rock is an amphibolite which is not fenitized. No alkaline rocks have been detected in these intrusions. Calcite is most common mineral in both occurrences. Other carbonate minerals include dolomite with minor ankerite and occassional siderite. In addition to low-Ti phlogopite, tetraferriphlogopite is also encountered. Fresh olivine is rare, and its alteration products include titaniferous clinohumite. The amphiboles are mainly calcic amphiboles, including actinolite, tremolite and edenite. The only sodic-calcic amphibole is accessory richterite. Other essential minerals are Ti-poor magnetite with ilmenite exsolutions, fluorapatite (3.95-4.89 wt. % F, monazite, and allanite-(Ce. Geochemically, the Kortejärvi rocks are mostly magnesiocarbonatites, whereas those of Laivajärvi, due to their higher magnetite content, are ferrocarbonatites. Of the trace elements, Nb is much lower (8-30 ppm in proper carbonatites than the average for carbonatites and U and Th (<0.9 ppm and<2.4 pm, respectively lower than average. Sr is typical, but not high (1830-3480 ppm, and Ba is rather low (27-348 ppm. The REEs are hosted by allanite and monazite and their concentrations in the

  6. Multi-stage barites in partially melted UHP eclogite: implications for fluid/melt activities during deep continental subduction in the Sulu orogenic belt

    Science.gov (United States)

    Wang, Songjie; Wang, Lu

    2015-04-01

    Barite (BaSO4) is well-known from deep-sea sedimentary environments but has received less attention to its presence in high-grade metamorphic rocks. Recently, barite in ultrahigh pressure (UHP) eclogite has drawn increasing attention from geologists, especially in the Dabie-Sulu orogen, since it is an important indicator for high-salinity fluid events, thus aiding in further understanding HP-UHP fluid / melt evolution. However, its formation time and mechanism in UHP eclogite are still controversial, with three representative viewpoints: (1) Liu et al. (2000) found barite-anhydrite-coesite inclusions in zircon and interpreted them to have formed by UHP metamorphic fluids; (2) Zeng et al. (2007) recognized isolated barite within K-feldspar (Kfs) and Quartz (Qz) surrounded by radial cracks in omphacite, and interpreted Kfs+Qz to be reaction products of potassium-rich fluid/melt and coesite, with the barite formed by prograde metamorphic fluids; (3) Gao et al. (2012) and Chen et al. (2014) found barite-bearing Multiphase Solid (MS) inclusions within garnet and omphacite and assumed that the barite formed by phengite breakdown possibly caused by eclogite partial melting during exhumation, though no direct evidence were proposed. The controversy above is mainly due to the lack of direct formation evidence and absence of a clear link with the metamorphic evolution of UHP eclogite along the subduction-exhumation path. We report detailed petrological and micro-structural analyses revealing four types of barites clearly linked with (1) the prograde, (2) earlier stage of partial melting and (3) later stage of crystallization differentiation, as well as (4) high-grade amphibolite-facies retrogression of a deeply subducted and partially melted intergranular coesite-bearing eclogite from Yangkou Bay, Sulu Orogen. Round barite inclusions (type-I) within UHP-stage garnet and omphacite are formed by internally buffered fluids from mineral dehydration during prograde metamorphism

  7. Imaging Water in Deformed Quartzites: Examples from Caledonian and Himalayan Shear Zones

    Science.gov (United States)

    Kronenberg, Andreas; Ashley, Kyle; Hasnan, Hasnor; Holyoke, Caleb; Jezek, Lynna; Law, Richard; Thomas, Jay

    2016-04-01

    amphibolite conditions by regime III GBM creep show varying trends with structural level. Water contents increase toward the Lhotse detachment of the Rongbuk valley, reaching 11,350 (± 1095) ppm, whereas they decrease toward the Main Central Thrust exposed in the western part of the Sutlej valley to values as low as 170 (± 25) ppm. Maps of intragranular water content correspond to populations of fluid inclusions, which depend on the history of deformation and dynamic recrystallization. Increases in water content require the introduction of secondary fluid inclusions, generally by brittle microcracking followed by crack healing and processes of inclusion redistribution documented in milky quartz experiments. Decreases in water content result from dynamic recrystallization, as mobile grain boundaries sweep through wet porphyroclasts, leaving behind dry recrystallized grains. Intragranular water contents throughout greenschist mylonites of the Moine thrust are comparable to those of quartz weakened by water in laboratory experiments. However, water contents of upper amphibolite mylonites of the Main Central Thrust are far below those required for water weakening at experimental strain rates and offer challenges to our understanding of quartz rheology.

  8. Geología de la parte sur de la Sierra de San Luis y granitoides asociados, Argentina

    Directory of Open Access Journals (Sweden)

    Llambías, E. J.

    1992-12-01

    Full Text Available The lower Palaeozoic basement of the southern part of Sierra de San Luis, Argentina, is made up of metamorphic rocks, granitoids and ultramafic rocks. The metamorphic grade ranges from the lower limit of greenschist facies through the upper limit of amphibolite facies, in apparent transition. The lower grade metamorphic rocks include slates, metavolcanics, metaconglomerates, quartzites and phyllites. Rocks with intermediate metamorphism are biotite-quartz-oligoclase schists with abundant pegmatoid veins. The higher grade metamorphism is represented by gneisses, amphibolites and migmatites, to which mafic and ultramafic bodies are associated. Three deformation phases have been recognized, being the last one (D3, of ordovician age, responsible of the most marked structures.The granitoids were grouped into pre-, syn- and post-kynematic bodies respect to D3. The pre-kynematic granitoids are located within the belt of lower grade metamorphic rocks and are composed of tonalites, granodiorites and monzogranites, strongly deformed. Their age is unknown. The syn-kynematic bodies are mainly garnet-moscovite-bearing leucogranodiorites. They show low thermal and rheological contrasts respect to the country rocks, and the age is 454 ± 21 m.a. The post-kynematic granitoids are mainly monzogranites with K-feldspar megacrysts and abundant sphene. They are subcircular and discordant, and are associated with an extensional regime. Their age is comprised between 423 and 320 m.a., and corresponds to the end of the Famatinian cycle.El basamento cristalino de la parte sur de la Sierra de San Luis, de edad Paleozoico Inferior, está constituido por rocas metamórficas, granitoides y rocas ultramáficas. El grado de metamorfismo varía desde la parte baja de esquistos verdes hasta la parte alta de la facies anfibolita, siendo sus relaciones de aparente transicionalidad. Las rocas con menor grado metamórfico consisten en pizarras, metavolcanitas, metaconglomerados

  9. Isotope age of the rare metal pegmatite formation in the Kolmozero-Voron'ya greenstone belt (Kola region of the Fennoscandian shield): U-Pb (TIMS) microlite and tourmaline dating

    Science.gov (United States)

    Kudryashov, Nikolay; Lyalina, Ludmila; Mokrushin, Artem; Zozulya, Dmitry; Groshev, Nikolay; Steshenko, Ekaterina; Kunakkuzin, Evgeniy

    2016-04-01

    The Kolmozero-Voron'ya greenstone belt is located in the central suture zone, which separates the Murmansk block from the Central-Kola and the Keivy blocks. The belt is represented by volcano-sedimentary rocks of Archaean age of 2.9-2.5 Ga. Rare metal pegmatites (Li, Cs with accessory Nb, Ta, and Be) occur among amphibolite and gabbroid intrusions in the northwestern and southeastern parts of the belt. According to the Rb-Sr data, the age of pegmatites was considered to be 2.7 Ga. Until recently there was no generally accepted point of view on the origin of pegmatites. Now we have isotopic data for a range of rock complexes that could pretend to be parental granites for the rare metal pegmatites. These are granodiorites with the zircon age of 2733±Ma, and microcline and tourmaline granites, which Pb-Pb isochronal age on tourmaline from the tourmaline granite located near the deposit is estimated to be 2520±70 Ma. The pegmatite field of the Vasin Myl'k deposit with the lepidolite--albite--microcline--spodumene--pollucite association is located among amphibolites in the northwestern part of the belt. The deposit is represented by subparallel low-angle zoned veins up to 220 m long and 5 m thick dipping in the southeastern direction at an angle of 10° too 30°. The minerals of the columbite--tonalite group from Vasin Myl'k deposit include microlite, simpsonite, and torolite, and are the oldest among different minerals represented by several generations in pegmatites under consideration. Zircons from the pegmatites are mostly represented by crystals with the structure affected by the action of fluids that put certain restrictions on its use as a geochronometer of the crystallization process. Microlite from the pegmatite taken from the dump of a prospecting drill hole was used for U--Pb (TIMS). The mineral is represented by 0.5--1.0 mm long euhedral octahedral crystals. It is brown in color, and transparent. The microlite crystals were preliminarily cleaned from

  10. Towards untangling the changing tectonic and climatic influence on deposition on the Surveyor Fan, Gulf of Alaska: A single grain geochemical and geochronological study

    Science.gov (United States)

    Huber, Barbara; Bahlburg, Heinrich; Drewer, Christian

    2016-04-01

    resemble published REE patterns of zircons from the Sanak-Baranof plutonic belt in the CMC. Microprobe analyses of 450 hornblende grains of the same samples and additional analyses of samples from sites U1419 and U1420 show an overall dominance of magnesiohornblende and varying amounts of actinolite, kaersutite and tschermakite. Similar hornblende compositions have been published for a belt of metamafic rocks in the CMC. Microprobe data of garnets from these samples indicate derivation from granites or gneisses and amphibolites of metamorphic conditions transitional between amphibolite and granulite facies. This also matches with published information about lithologies in the area of the CMC. The Chugach Metamorphic Complex, via the Bering glacier, seems to be one of the main long time sources of the Surveyor Fan sediments. Shipboard clast and other data indicate input also from the eastward lying Seward glacier and longshore transfer into the Surveyor Fan system. Changing amphibole compositions with time of deposition and zircons with ages older than ca. 50 Ma point to changes in source terranes which will be constrained by future analysis.

  11. MULTIDISCIPLINARY APPROACH TO STUDY MIGMATITES: ORIGIN AND TECTONIC HISTORY OF THE NASON RIDGE MIGMATITIC GNEISS, WENATCHEE BLOCK, CASCADES CRYSTALLINE CORE, WA, USA

    Directory of Open Access Journals (Sweden)

    Stowell Harold H.

    2008-12-01

    Full Text Available The Nason Ridge Migmatitic Gneiss of the Cascades Core is a migmatitic unit comprising concordant pelitic schist and gneiss, amphibolite, and tonalite gneiss, and cross cutting tonalite, quartz-rich granitoid, and pegmatite. There are several generations of 'igneous' lithologies (leucosomes = tonalite, quartz-rich granitoid, and pegmatite some of which are concordant; others clearly crosscut the strongly deformed host rocks. The host rocks are interpreted to be Chiwaukum Schist with metasedimentary (pelitic schist and some gneiss and metavolcanic(amphibolites origins. Metamorphic fabric in the Nason Ridge Migmatitic Gneiss is characterized by preferred orientation of platy minerals (continuous schistosity, compositional layering, mineral lineations (elongate grains and grain aggregates, and non-coaxial deformational features (asymmetric augen, grain offsets,rotated porphyroblasts, etc.. Compositional layering is characterized by quartz-plagioclase lenses and patches (mm to cm scale and by large variations in biotite content. This composite fabric is faulted and folded by mesoscopic structures. The most strongly foliated leucosomes (gneissic tonalites are generally concordant with the regional trend of foliation, while weakly foliated leucosomes (tonalites and pegmatite veins crosscut host rock and tonalite gneisses. Thin melanosome layers (biotiteand amphibole schist are developed locally around quartz - plagioclase lenses and patches. Metamorphism in the Nason Ridge Migmatitic Gneiss and the nearby Chiwaukum Schist likely peaked after intrusion of the Mt. Stuart Batholith ca. 91-94 Ma. Peak temperatures and pressures for the Nason Ridge Migmatitic Gneiss in the Wenatchee Ridge and Pacific Crest areas were 650 - 720 °C and 6 - 9 kbar with a pressure increase of £ 2.0 kbar during metamorphism. Thermodynamic modeling indicates that hydrous partial melting would begin at ca. 660 °C and is relatively pressure independent. Field

  12. First evidence of the Ellesmerian metamorphism on Svalbard

    Science.gov (United States)

    Kośmińska, Karolina; Majka, Jarosław; Manecki, Maciej; Schneider, David A.

    2016-04-01

    metapelites were formed under amphibolite facies conditions at c. 7-9 kbar and 550-650 °C (Kośmińska et al., 2015b). Monazite dating was performed on samples from these three metamorphic zones. The chemical zonation of monazite allows the identification of several monazite populations, which likely developed during different stages of Barrovian metamorphism. The geochronology demonstrate protracted monazite growth from the early prograde stage at c. 370 Ma to the peak conditions at c. 355 Ma. Thus it is evident that the Ellesmerian event was not limited to the relatively cold deformation as previously thought. The amphibolite facies metamorphism of c. 370-355 Ma that was documented in our study sheds new light on understanding of the character of this tectonothermal event. This project is financed by NCN research project No 2013/11/N/ST10/00357 and partially funded by AGH research grant no 11.11.140.319. References: Faehnrich et al., 2016. A tectonic window into the crystalline basement of Prins Karls Forland, Spitsbergen. EGU General Assembly 2016. Kośmińska et al., 2015b. Metamorphic evolution of the Pinkie unit metapelites from Svalbard (High Arctic): P-T-t study including Quartz-in-garnet barometry (QuiG). GSA 2015: Annual Meeting, Baltimore. Kośmińska et al., 2015a. Detrital zircon U-Pb geochronology of metasediments from southwestern Svalbard's Caledonian Province. EGU General Assembly 2015. Piepjohn et al., 2015. Tectonic map of the Ellesmerian and Eurekan deformation belts on Svalbard, North Greenland, and the Queen Elizabeth Islands (Canadian Arctic). Arktos, DOI 10.1007/s41063-015-0015-7.

  13. The southern Araçuaí belt and the Dom Silvério Group: geologic architecture and tectonic significance

    Directory of Open Access Journals (Sweden)

    Guilherme G. Peres

    2004-12-01

    Full Text Available The Araçuaí belt corresponds to the external portion of the western half of the Araçuaí-West Congo Neopro-terozoic orogen. TheAraçuaí belt fringes the São Francisco craton to east and is separated from the crystalline core of the orogen by the Abre Campo geophysical discontinuity. The southern Araçuaí belt involves four major lithologic units: the Archean and Paleoproterozoic gneisses of the Mantiqueira Complex, the Pedra Dourada Charnockite, the Paleoproterozoic Borrachudos Granitoid, and the metavolcanosedimentary rocks of the Dom Silvério Group. The Dom Silvério Group occurs in a NNE-SSW striking belt and consists of a thick package of metapelitic rocks with intercalations of quartzites, amphibolites, meta-ultramafics, banded iron formations, gondites and marbles. All units of the southern Araçuaí belt underwent four syn-metamorphic phases of deformation in the course of the Brasiliano event. The first phase, synchronous to a regional amphibolite facies metamorphism, was associated to a general tectonic transport towards north along the left-lateral Dom Silvério shear zone and its low angle segment. The second and third phases represent progressive stages of a west directed shortening, which led to the development of local thrusts and pervasive folds in several scales. The fourth phase is extensional and reflects the collapse of the orogen.A Faixa Araçuaí corresponde à porção ocidental externa do orógeno neoproterozóico Araçuaí-Congo Ocidental. Margeia a borda leste do Cráton do São Francisco e é separada do núcleo cristalino do orógeno pela descontinuidade geofísica de Abre Campo. A porção meridional da Faixa Araçuaí envolve quatro unidades litológicas principais: os ortognaisses arqueanos e paleoproterozóicos do Complexo Mantiqueira, os charnoquitos Pedra Dourada, os granitóides paleoproterozóicos da Suíte Borrachudos e as rochas metavulcanossedimentares do Grupo Dom Silvério. O Grupo Dom Silv

  14. Structural analysis and implicit 3D modelling of high-grade host rocks to the Venetia kimberlite diatremes, Central Zone, Limpopo Belt, South Africa

    Science.gov (United States)

    Basson, I. J.; Creus, P. K.; Anthonissen, C. J.; Stoch, B.; Ekkerd, J.

    2016-05-01

    The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.

  15. Geology of the Grove Mountains in East Antarctica--New evidence for the final suture of Gondwana Land

    Institute of Scientific and Technical Information of China (English)

    刘小汉; 赵越; 刘晓春; 俞良军

    2003-01-01

    Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D1 is related to the event before Pan-African age, D2 corresponds to the regional granulite peak metamorphism, whereas D3 reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800℃, 9.3×105 Pa; M2 800-810℃, 6.4×105 Pa ; and M3 650℃ have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.

  16. Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

    Science.gov (United States)

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.

    2013-01-01

    We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

  17. Electron microprobe Th-U-Pb monazite dating and metamorphic evolution of the Acaiaca Granulite Complex, Minas Gerais, Brazil

    International Nuclear Information System (INIS)

    The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulite as well as lower grade gneisses and mylonite. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometry calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulite based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 deg C and pressures between of 5.0 and 9.9 kbar and a retro metamorphic path characterized by near-isobaric cooling. Part of the granulite was affected by anatexis. The melting of felsic granulite resulted in the generation of pegmatites and two aluminous lithotypes. These are: 1) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and 2) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronological related to the lower grade Mantiqueira Complex. (author)

  18. Dehydration and partial melting of tremolitic amphibole coexisting with zoisite, quartz, anorthite, diopside, and water in the system H2O-CaO-MgO-Al2O3-SiO2

    Science.gov (United States)

    Quirion, Diane M.; Jenkins, David M.

    The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water has been experimentally investigated in the chemical system H2O-CaO- MgO-Al2O3-SiO2 over the range of 0.4-0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite + quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz, and water were obtained at 600, 650, and 700°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5+/-0.08 at the highest temperature. Thermodynamic analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S0 (1 bar, 298 K) of 575.4+/-1.6 J/K.mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several experiments in the range of 1.0-1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770°C.

  19. In-situ ground gamma spectrometry — an effective tool for geological mapping (the Male Karpaty Mts., Slovakia

    Directory of Open Access Journals (Sweden)

    Mojzeš Andrej

    2016-06-01

    Full Text Available This contribution presents the results of profile in-situ gamma spectrometry measurements that sought to determine the content of natural radionuclides 40K, 238U and 232Th in a near surface horizon of rocks, their weathering cover and soils in the area of the Malé Karpaty Mts. It is widely established that the exploration of radioactivity of bedrocks and cover rocks can be a very effective and useful tool for both geological mapping, for identifying deposits of mineral resources, and even addressing the issues of structural and tectonic geology. This assertion is equally confirmed by the ground gamma spectrometry measurements carried out as part of this case study on larger scales, seeking more detailed geological structure solutions. The results obtained provide a welcome addition to an already existing database, which monitors the content of naturally occurring radionuclides individually for every rock lithotype of the Western Carpathians, by elaborating on the data collected by previous research and by updating this database for any future needs. The presented results confirmed the low to medium radioactivity levels of rocks and soils in the studied area. The highest values were detected in granitoids and metamorfic phyllitic rocks of the Malé Karpaty Mts. core; the lowest values were detected in carbonates, arenaceous sediments and, above all, amphibolite bodies. In this way, the presented results of the interpreted profile (P5 confirm the model of local geological structure as represented on the most up-to-date edition of the geological map of the Male Karpaty Mts. (Polak et al. 2011.

  20. In-situ ground gamma spectrometry — an effective tool for geological mapping (the Male Karpaty Mts., Slovakia)

    Science.gov (United States)

    Mojzeš, Andrej; Porubčanová, Barbara

    2016-06-01

    This contribution presents the results of profile in-situ gamma spectrometry measurements that sought to determine the content of natural radionuclides 40K, 238U and 232Th in a near surface horizon of rocks, their weathering cover and soils in the area of the Malé Karpaty Mts. It is widely established that the exploration of radioactivity of bedrocks and cover rocks can be a very effective and useful tool for both geological mapping, for identifying deposits of mineral resources, and even addressing the issues of structural and tectonic geology. This assertion is equally confirmed by the ground gamma spectrometry measurements carried out as part of this case study on larger scales, seeking more detailed geological structure solutions. The results obtained provide a welcome addition to an already existing database, which monitors the content of naturally occurring radionuclides individually for every rock lithotype of the Western Carpathians, by elaborating on the data collected by previous research and by updating this database for any future needs. The presented results confirmed the low to medium radioactivity levels of rocks and soils in the studied area. The highest values were detected in granitoids and metamorfic phyllitic rocks of the Malé Karpaty Mts. core; the lowest values were detected in carbonates, arenaceous sediments and, above all, amphibolite bodies. In this way, the presented results of the interpreted profile (P5) confirm the model of local geological structure as represented on the most up-to-date edition of the geological map of the Male Karpaty Mts. (Polak et al. 2011).

  1. Bimodal tholeiitic-dacitic magmatism and the Early Precambrian crust

    Science.gov (United States)

    Barker, F.; Peterman, Z.E.

    1974-01-01

    Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite-compositionally similar to the 1.8-b.y.-old Twilight Gneiss - and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago. ?? 1974.

  2. Cretaceous metamorphism, magmatism and shearing in the Waipuna Valley, directly south of the Reefton Goldfield

    International Nuclear Information System (INIS)

    Rocks in the Waipuna Valley in Westland show that the area immediately south of the Reefton Goldfield has been dramatically affected by Cretaceous tectonism. The main lithology in the Waipuna Valley is metasedimentary Greenland Group that is here subdivided into four types (I, II, III and IV). Type I contains abundant detrital grains with a weak Palaeozoic greenschist facies overprint and is compositionally indistinguishable from unmineralised Greenland Group within the goldfield. Type II contains detrital grains, but also a weak amphibolite facies foliation or, where proximal to granitoid intrusions, a hornfelsic texture. Type III contains no detrital components and records polyphase recrystallisation. U-Th-Pb monazite dating indicates that metamorphism in the Type III rocks occurred, at least in part, at 108.1 ± 1.2 Ma. Type IV Greenland Group has partially melted. Zircon from granitoids intruding Greenland Group in the Waipuna Valley indicate Cretaceous (c. 111 Ma) emplacement. Greenland Group Types III and IV, and the granitic plutons, have been variably sheared with quartz textures indicating progressive cooling during deformation. A mylonite zone occurs just above outcrop of the Type IV rocks. Field and age relationships are interpreted to show that the Waipuna Valley exposes portions of an upper plate and lower plate to a Cretaceous ductile shear zone that is here named the Waipuna Shear Zone. The overall structure is similar to the nearby Cretaceous extensional Paparoa Metamorphic Core Complex. The Reefton Goldfield lies in the upper plate to the Waipuna Shear Zone, not far above the mylonite zone. Cretaceous metamorphism and shearing of Greenland Group could have aided formation and then transport of gold-bearing fluids into structurally higher rocks. (author).

  3. Geology of the Alligator Rivers Uranium Field

    International Nuclear Information System (INIS)

    The uranium deposits of Ranger 1, Koongarra, Jabiluka One and Two, and Nabarlek are in the Alligator Rivers Uranium Field, the northeastern part of the Pine Creek Geosyncline. Lower Proterozoic metasediments, which were metamorphosed mainly to amphibolite-grade and multiply isoclinally folded at about 1800 Ma, host much of the uranium and overlie or grade into the Archaean to Lower Proterozoic granitoid Nanambu Complex. In the northeast of the Field the metasediments grade into schist and gneiss forming the outer parts of the Lower Proterozoic Nimbuwah Complex; the inner parts of this Complex contain granodioritic and tonalitic migmatite and granitoid rocks which were emplaced before the 1800 Ma event. The metasediments are intruded by pre-orogenic and post-orogenic tholeiitic dolerite, by synorogenic granite, and by later minor phonolite and dolerite dykes. All but the minor dykes are overlain with marked unconformity by Carpentarian (Middle Proterozoic) sandstone with basalt flows, which conceals older rocks over most of the southeastern half of the area. The pre-Carpentarian (pre-Middle Proterozoic) rocks are deeply weathered and lateritised and are covered extensively by Mesozoic and Cainozoic sediment. The uranium is mainly contained in the lower member of the Cahill Formation, comprising mica quartz schist, magnesite and carbonaceous schist, which is chloritised around the uranium occurrences and along faults, shears and some stratigraphic breaks. The ore zones are located in breccia. The stratabound nature of the ore suggests that it has formed partly syngenetically; however, epigenetic processes appear essential for the development of such high-grade deposits. (author)

  4. Discovery of the Jabiluka uranium deposits, East Alligator River Region, Northern Territory of Australia

    International Nuclear Information System (INIS)

    The Jabiluka One and Two uranium deposits occur in Lower Proterozoic metasediments of the Cahill Formation. The observed part of the Cahill Formation exhibits four horizons which are favourable hosts for uranium mineralization. The host rocks are mainly chlorite and/or graphite schists and their brecciated equivalents which have undergone initial regional prograde metamorphism to amphibolite facies, then retrograde metamorphism to greenschist facies. Mineralization consists of uraninite, mainly filling open spaces and to a lesser extent in disseminated form. Chlorite alteration is intimately associated with the uranium mineralization. A portion of the Jabiluka Two deposit contains economic concentrations of gold. Although the deposits are generally stratabound, structural preparation appears to be the most significant ore control on a local scale. The Jabiluka case history illustrates an effective philosophy which was successful in exploration for stratabound uranium deposits in the East Alligator River Region. This philosophy encompassed the following points: (a) The value of regional appraisals in selection of a property; (b) The recognition of the detection limits of airborne radiometric surveys; (c) The importance of ground prospecting for low-order point-source radiometric anomalies which cannot be detected by airborne survey; (d) The importance of evaluating all anomalies and the flexibility to change priorities as further exploration results are obtained; (e) The necessity of establishing the controls on the mineralization before proceeding with further exploration; (f) The necessity of exploring extensions of favourable lithologies to test for periodicity of mineralization even where cover precludes surface expression; and (g) The desirability for modification of exploration techniques on different types of anomalies

  5. Geochemistry of Archean metasedimentary rocks of the Aravalli craton, NW India: Implications for provenance, paleoweathering and supercontinent reconstruction

    Science.gov (United States)

    Ahmad, Iftikhar; Mondal, M. E. A.; Satyanarayanan, M.

    2016-08-01

    Basement complex of the Aravalli craton (NW India) known as the Banded Gneissic Complex (BGC) is classified into two domains viz. Archean BGC-I and Proterozoic BGC-II. We present first comprehensive geochemical study of the Archean metasedimentary rocks occurring within the BGC-I. These rocks occur associated with intrusive amphibolites in a linear belt within the basement gneisses. The association is only concentrated on the western margin of the BGC-I. The samples are highly mature (MSm) to very immature (MSi), along with highly variable geochemistry. Their major (SiO2/Al2O3, Na2O/K2O and Al2O3/TiO2) and trace (Th/Sc, Cr/Th, Th/Co, La/Sc, Zr/Sc) element ratios, and rare earth element (REE) patterns are consistent with derivation of detritus from the basement gneisses and its mafic enclaves, with major contribution from the former. Variable mixing between the two end members and closed system recycling (cannibalism) resulted in the compositional heterogeneity. Chemical index of alteration (CIA) of the samples indicate low to moderate weathering of the source terrain in a sub-tropical environment. In A-CN-K ternary diagram, some samples deceptively appear to have undergone post-depositional K-metasomatism. Nevertheless, their petrography and geochemistry (low K2O and Rb) preclude the post-depositional alteration. We propose non-preferential leaching of elements during cannibalism as the cause of the deceptive K-metasomatism as well as enigmatic low CIA values of some highly mature samples. The Archean metasedimentary rocks were deposited on stable basement gneisses, making the BGC-I a plausible participant in the Archean Ur supercontinent.

  6. A crustal model of the ultrahigh-pressure Dabie Shan orogenic belt, China, derived from deep seismic refraction profiling

    Science.gov (United States)

    Wang, Chun-Yong; Zeng, Rong-Sheng; Mooney, W.D.; Hacker, B.R.

    2000-01-01

    We present a new crustal cross section through the east-west trending ultrahigh-pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400-km-long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35-km-thick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0±0.2 km/s, 6.5±0.1 km/s, and 6.8±0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the northern margin of the orogen, and thus the present-day root beneath the orogen is only 6.5 km thick. The upper mantle velocity is 8.0±0.1 km/s. Modeling of shear wave data indicate that Poisson's ratio increases from 0.24±0.02 in the upper crust to 0.27±0.03 in the lower crust. This result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant component in the middle or lower crust. Much of the Triassic structure associated with the formation of the UHP rocks of the Dabie Shan has been obscured by post-Triassic igneous activity, extension and large-offset strike-slip faulting. Nevertheless, we can identify a high-velocity (6.3 km/s) zone in the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh-pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike-slip fault.

  7. Karnali and Jajarkot Klippen in Western Nepal Himalaya Inconsistent with Tectonic Wedging Model Predictions

    Science.gov (United States)

    Soucy La Roche, R.; Godin, L.; Cottle, J. M.; Kellett, D.

    2015-12-01

    The Himalayan metamorphic core, exposed between two opposite sense shear zones, is locally preserved in a series of foreland klippen. The upper shear zone, the South Tibetan Detachment (STD), is a key element in many competing tectonic models. One of these models, tectonic wedging, requires that the STD merges with the reverse-sense basal shear zone towards the foreland. We tested this hypothesis in two foreland klippen in western Nepal. The Karnali klippe is a doubly-plunging synform underlain by a folded reverse-sense shear zone. It comprises amphibolite metamorphic facies rocks overlain by greenschist to subgreenschist facies sedimentary rocks. The contact is marked by a folded ca. 1 km thick normal-sense shear zone, which we correlate with the STD. Quartz and calcite recrystallization textures and quartz crystallographic preferred orientations suggest an abrupt decrease in temperature of deformation from ~750 °C in the footwall to 580 and 475 °C at the base and top of the shear zone, respectively, and to 150-200 °C in the hanging wall. In-situ monazite petrochronology indicates prograde metamorphism between 36 and 30 Ma in the immediate footwall of the STD, followed by tectonic exhumation from 28 to <24 Ma, possibly starting as early as 30 Ma. Preliminary muscovite 40Ar/39Ar ages suggest that deformation along the STD ceased by ca. 18 Ma. Field data from the adjacent Jajarkot klippe indicate a similar first order structural architecture, although protoliths, metamorphic grade and deformation temperature differ significantly. Transport-parallel exposure of the STD in this area implies a minimum slip of 165 km. The presence of the STD on both flanks of the Karnali and the Jajarkot klippen is inconsistent with predictions that the STD merges at depth with the basal shear zone in the Karnali klippe and north of the Jajarkot klippe. Our observations are consequently not compatible with the tectonic wedging model proposed for western Nepal.

  8. Post-collisional potassic granitoids from the southern and northwestern parts of the Late Neoproterozoic East African Orogen: a review

    Science.gov (United States)

    Küster, Dirk; Harms, Ulrich

    1998-12-01

    Potassic metaluminous granitoids with enrichments of HFS elements constitute part of widespread post-collisional magmatism related to the Late Neoproterozoic Pan-African orogeny in northeastern Africa (Sudan, Ethiopia, Somalia) and Madagascar. The plutons were emplaced between 580 and 470 Ma and comprise both subsolvus and hypersolvus biotite-granite, biotite-hornblende-granite, quartz-monzonite and quartz-syenite. Pyroxene-bearing granitoids are subordinate. Basic dikes and enclaves of monzodioritic composition are locally associated with the granitoid plutons. Granitoids emplaced in pre-Neoproterozoic crust have Sr i-ratios between 0.7060 and 0.7236 and ɛNd( t) values between -15.8 and -5.6 while those emplaced in, or close to the contact with, juvenile Neoproterozoic crust have lower Sr i-ratios (0.7036-0.7075) and positive ɛNd( t) values (4.6). However, it is unlikely that the potassic granitoids represent products of crustal melting alone. The association with basic magmas derived from subduction-modified enriched mantle sources strongly suggests that the granitoids represent hybrid magmas produced by interaction and mixing of mantle and crust derived melts in the lower crust. The most intense period of this potassic granitoid magmatism occurred between 585 and 540 Ma, largely coeval with HT granulite facies metamorphism in Madagascar and with amphibolite facies retrogression in northeastern Africa (Somalia, Sudan). Granitoid magmatism and high-grade metamorphism are probably both related to post-collisional lithospheric thinning, magmatic underplating and crustal relaxation. However, the emplacement of potassic granites continued until about 470 Ma and implies several magmatic pulses associated with different phases of crustal uplift and cooling. The potassic metaluminous granites are temporally and spatially associated with post-collisional high-K calc-alkaline granites with which they share many petrographical, geochemical and isotopical similarities

  9. 2.9, 2.36, and 1.96 Ga zircons in orthogneiss south of the Red River shear zone in Viet Nam: evidence from SHRIMP U-Pb dating and tectonothermal implications

    Science.gov (United States)

    Nam, Tran Ngoc; Toriumi, Mitsuhiro; Sano, Yuji; Terada, Kentaro; Thang, Ta Trong

    2003-05-01

    Orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the Day Nui Con Voi-Red River shear zone in northern Viet Nam. The orthogneiss complex has given some radiogenic dates of Early Proterozoic and Late Archean, which are the oldest ages ever registered for the Southeast Asian continent. Zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high-grade tectonothermal events in the complex. Sixty-five SHRIMP U-Th-Pb analyses of these zircons define three age groups of 2.84-2.91, 2.36, and 1.96 Ga. The age groups correspond to three periods of zircon generation. The oldest ˜2.9 Ga cores indicate a minimum age for the protolith of the orthogneiss complex. Two younger generations (including ˜2.36 Ga outer-cores and ˜1.96 Ga rims) probably grew during later high-grade tectono-metamorphic events, which were previously suggested by K-Ar and 40Ar/ 39Ar cooling ages of ˜2.0 Ga for synkinematic hornblendes. An early thermal history of the orthogneiss complex has been constrained, including a primary magma-crystallization stage starting at ˜2.9 Ga, followed by two Early Proterozoic (˜2.36 and ˜1.96 Ga) high-grade tectonothermal events. The ca. 2.9 Ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of Archean rocks in Indochina, and clearly indicates that the crustal evolution of northern Viet Nam started as early as Late Archean time.

  10. Electron microprobe Th-U-Pb monazite dating and metamorphic evolution of the Acaiaca Granulite Complex, Minas Gerais, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Medeiros Junior, Edgar Batista; Marques, Rodson Abreu, E-mail: edgarjr@ymail.com, E-mail: rodson.marques@ufes.br [Universidade Federal do Espirito Santo (UFES), Alegre, ES (Brazil). Departamento de Geologia; Jordt-Evangelista, Hanna; Queiroga, Glaucia Nascimento, E-mail: hanna@degeo.ufop.br, E-mail: glauciaqueiroga@yahoo.com.br [Universidade Federal de Ouro Preto (UFOP), Ouro Preto, MG (Brazil). Escola de Minas. Departamento de Geologia; Schulz, Bernhard, E-mail: bernhard.schulz@mineral.tu-freiberg.de [TU Bergakademie - Institute of Mineralogy, Freiberg - Saxony (Germany)

    2016-01-15

    The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulite as well as lower grade gneisses and mylonite. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometry calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulite based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 deg C and pressures between of 5.0 and 9.9 kbar and a retro metamorphic path characterized by near-isobaric cooling. Part of the granulite was affected by anatexis. The melting of felsic granulite resulted in the generation of pegmatites and two aluminous lithotypes. These are: 1) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and 2) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronological related to the lower grade Mantiqueira Complex. (author)

  11. Disjunctive Grade Variation from Greenschist to Granulite Facies, Siyom Valley, Eastern Arunachal Pradesh, India

    Science.gov (United States)

    Clarke, G. L.; Bhowmik, S. K.; Aitchison, J. C.; Ireland, T. R.

    2014-12-01

    The Siyom Valley section in eastern Arunachal Pradesh exposes an inverted metamorphic succession (Nandini & Thakur, 2011), metapelitic assemblages increasing in grade northwards from chlorite, through biotite, garnet-staurolite and kyanite-bearing schist to kyanite-sillimanite migmatite. Grade changes are mostly controlled by shallowly north, and northwest-dipping fault structures. Two textural stages of garnet growth can be identified in the ilmenite-bearing amphibolite facies rocks, staurolite having formed late in, or after, deformation responsible for the main penetrative foliation (S2). Kyanite and rutile inclusions in garnet indicate that their growth in migmatite preceded that of matrix sillimanite, ilmenite and cordierite, though unrecrystallized kyanite is also common in the feldspathic matrix. Preliminary data indicate the pronounced tectonic thinning of metasedimentary protoliths during exhumation, and the probability of a pronounced step in grade in the middle part of the river section. Similarities with sections in the Sikkim (Dasgupta et al., 2004) and western Arunachal Pradesh (Goswami et al., 2009) Himalaya reflect the lateral continuity of the south-vergent thrusts that controlled the exhumation of the high-grade rocks, with debate concerning the location and significance of the Main Central Thrust zone begging protolith and metamorphic age data. Dasgupta, S.,Ganguly, J. & Neogi, S., 2004. Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P-T gradient and implications. Journal of Metamorphic Geology, 22, 395-412. Goswami, S., Bhowmik, S.K. & Dasgupta, S., 2009. Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India. Journal of Asian Earth Sciences, 36, 390-406. Nandini, P. & Thakur, S.S., 2011. Metamorphic evolution of the Lesser Himalayan Crystalline Sequence, Siyom Valley, NE Himalaya, India. Journal of Asian Earth Sciences, 40, 1089-1100

  12. Permo-Triassic and Paleoproterozoic metamorphism related to continental collision in Yangpyeong, South Korea

    Science.gov (United States)

    Oh, Chang Whan; Imayama, Takeshi; Lee, Seung Yeol; Yi, Sang-Bong; Yi, Keewook; Lee, Byung Choon

    2015-02-01

    Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet-biotite gneiss and sillimanite-garnet-biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888-1871 Ma) at P-T conditions of 760-820 °C and 8-10 kbar and 710-750 °C and 5-7 kbar, respectively. These rocks were overprinted by low-P/T type metamorphism (590-650 °C, 3-4 kbar) during the Triassic (ca. 237 Ma). In contrast, a cordierite-rich migmatite near the post-collisional Triassic igneous complex in the eastern part of the area was strongly metamorphosed during the Triassic (ca. 235 Ma) at 750-790 °C and 7-8 kbar. The similar Triassic ages in the western and eastern areas suggest that low-P/T type metamorphism occurred as a second stage of regional metamorphism, which is characterized by the production of cordierite with an irregularly shaped garnet. The metamorphic grade of the Triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions, and the Paleoproterozoic metamorphism is well preserved in the western part with low grade Triassic metamorphism. The new discovery of the Triassic metamorphic event in the Yangpyeong area, in addition to the previously reported Triassic post-collision igneous event, supports the idea that the continental collision belt between the North and South China blocks extends from the Hongseong area into the Odesan area through the Yangpyeong area in South Korea.

  13. P-T-t-D paths of the North Himalayan metamorphic rocks: Implications for the Himalayan orogeny

    Science.gov (United States)

    Ding, Huixia; Zhang, Zeming; Hu, Kaiming; Dong, Xin; Xiang, Hua; Mu, Hongchen

    2016-06-01

    Metamorphic P-T-t-D paths have been determined in the Yardoi gneiss dome of the eastern Himalaya that place new constraints on the genesis of the North Himalayan Gneiss Dome and tectonic evolution of the Himalayan orogen. Our study shows that the schists from the Yardoi dome underwent at least three stages of metamorphism and deformation. The first stage of prograde metamorphism is represented by the core of porphyroblastic garnet, showing distinct growth compositional zoning, and hosting mineral inclusions of biotite, muscovite, quartz and ilmenite; the curved inclusion trails define the first stage of deformation foliation (S1). The second stage of peak metamorphism is represented by the rim of porphyroblastic garnet and the aligned matrix minerals of biotite, muscovite, plagioclase, quartz, kyanite and staurolite, which define the main deformation foliation (S2). The third stage of retrograde metamorphism is characterized by occurrence of biotite, plagioclase, muscovite and sillimanite within the distributed shear bands, which define the late deformation foliation (S3). Phase equilibrium modeling reveals that Yardoi schists witnessed a peak metamorphism of upper amphibolite-facies under P-T conditions of 7-8 kbar and 615-665 °C, and records a clockwise P-T-t-D path with a prograde process of both temperature and pressure increase and subsequent retrogression of isothermal decompression. The zircon U-Pb dating indicates that the prograde process occurred during the Middle Eocene of 48-36 Ma and the retrograde metamorphism probably lasted to ca. 16 Ma. The present result shows that the formation of the Yardoi schists was related to the subduction and exhumation of Indian crust, the initial collision of Indian and Asian continents occurred at ca. 50 Ma, the northeastern margin of the Indian continent was shallowly underthrusted beneath the Asian continent, and distinct tectonic discontinuity is present in the Greater Himalayan Sequence.

  14. Tectonic evolution of the Brooks Range ophiolite, Alaska

    Energy Technology Data Exchange (ETDEWEB)

    Harris, R.A. (West Virginia Univ., Morgantown, WV (United States). Dept. of Geology)

    1993-04-01

    Detailed studies of the composition, internal structure, and age of the Brooks Range ophiolite (BRO) and its metamorphic sole reveal new constraints for its tectonic evolution. The BRO consists of six separate thrust masses of consanguineous composition, internal organization, structure and age. Subophiolite metamorphic rocks are locally preserved along its structural base, which is well exposed in several places. The metamorphic sole is locally transitional with mafic volcanic sequences, chert, tuffs, and minor clastic sedimentary material of the Copter Peak Complex, which is correlative with the Angayucham terrane. This terrane is much older than, and chemically distinct from the BRO. The internal structure of the BRO is characterized by NE-SW trending igneous layers that expose the transition zone from crust to mantle. Residual mantle material consists of tectonized peridotite in abrupt contact with dunite pods up to 4 km thick. Ductile and brittle structures of the BRO preserve various phases of its dynamic evolution from a magma body to a fragmented thrust sheet. The earliest deformational effects are recorded by ductile lattice and shape fabrics in dunites and the layered series of the BRO. Magmatic flow planes generally parallel the petrologic moho, and dip 40[degree]--70[degree] to the NW and SE. Flow lineations consistently plunge ESE-ENE from 39[degree]--54[degree]. Igneous laminations and compositional layers represent patterns of magmatic flow in, and plastic deformation of, a cumulate sequence -- not the deposition pattern of cumulate layers. In the upper layered series, amphiboles with a shape-preferred orientation yield Ar/Ar plateau ages of 163--169 Ma. These ages overlap with plateau ages of the same kind from amphibolite of the metamorphic sole. This concordance in age indicates that cooling of the BRO coincided with its tectonic emplacement.

  15. Discrimination of iron ore deposits of granulite terrain of Southern Peninsular India using ASTER data

    Science.gov (United States)

    Rajendran, Sankaran; Thirunavukkarasu, A.; Balamurugan, G.; Shankar, K.

    2011-04-01

    This work describes a new image processing technique for discriminating iron ores (magnetite quartzite deposits) and associated lithology in high-grade granulite region of Salem, Southern Peninsular India using visible, near-infrared and short wave infrared reflectance data of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Image spectra show that the magnetite quartzite and associated lithology of garnetiferrous pyroxene granulite, hornblende biotite gneiss, amphibolite, dunite, and pegmatite have absorption features around spectral bands 1, 3, 5, and 7. ASTER band ratios ((1 + 3)/2, (3 + 5)/4, (5 + 7)/6) in RGB are constructed by summing the bands representing the shoulders of absorption features as a numerator, and the band located nearest the absorption feature as a denominator to map iron ores and band ratios ((2 + 4)/3, (5 + 7)/6, (7 + 9)/8) in RGB for associated lithology. The results show that ASTER band ratios ((1 + 3)/2, (3 + 5)/4, (5 + 7)/6) in a Red-Green-Blue (RGB) color combination identifies the iron ores much better than previously published ASTER band ratios analysis. A Principal Component Analysis (PCA) is applied to reduce redundant information in highly correlated bands. PCA (3, 2, and 1 for iron ores and 5, 4, 2 for granulite rock) in RGB enabled the discrimination between the iron ores and garnetiferrous pyroxene granulite rock. Thus, this image processing technique is very much suitable for discriminating the different types of rocks of granulite region. As outcome of the present work, the geology map of Salem region is provided based on the interpretation of ASTER image results and field verification work. It is recommended that the proposed methods have great potential for mapping of iron ores and associated lithology of granulite region with similar rock units of granulite regions of Southern Peninsular India. This work also demonstrates the ability of ASTER's to provide information on iron ores, which is valuable

  16. Geochemistry and Petrogenesis of Neoarchean Metamorphic Mafic Rocks in the Wutai Complex

    Institute of Scientific and Technical Information of China (English)

    L(U) Yongjun; LIU Shuwen; Guochun ZHAO; LI Qiugen; Jian ZHANG; LIU Chaohui; K. H. PARK; Y. S. SONG

    2006-01-01

    Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns (Lan/Ybn=0.86-1.3), the lowest total REEs (29-52 ppm), and weak negative to positive Eu anomalies (Eun/Eun*=0.84-1.02), nearly flat primitive mantle normalized patterns and strong negative Zr(Hf) anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs (34-116 ppm), LREE-enriched (Lan/Ybn=1.76-4.34) chondrite normalized REE patterns with weak Eu anomalies (Eun/Eun*=0.76-1.16), and negative Nb, Ta, Zr(Hf), Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slabderived fluids. Rocks in Group #3 are characterized by the highest total REEs (61-192 ppm), the strongest LREEs enrichment (Lan/Ybn=7.12-16) with slightly negative Eu anomalies (Eun/Eun*=0.81-0.95) in the chondrite normalized diagram. In the primitive mantle normalized diagram,these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies.They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts.The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.

  17. Oppositely dipping thrusts and transpressional imbricate zone in the Central Eastern Desert of Egypt

    Science.gov (United States)

    Abd El-Wahed, Mohamed A.

    2014-12-01

    This paper documents the 40-60 km wide ENE-WSW trending Mubarak-Barramiya shear belt (MBSB) in the Central Eastern Desert of Egypt by examining its structural styles, kinematics and geometry. Our study revealed the existence of prevalent dextral and minor sinistral conjugate shear zones. The MBSB is metamorphic belt (greenschist-amphibolite) characterized by at least three post-collisional (740-540 Ma) ductile Neoproterozoic deformation events (D1, D2 and D3) followed by a brittle neotectonic deformation (D4). D1 event produced early top-to-the-northwest thrust displacements due to NW-SE shortening. D2 produced discrete zones of NNW-trending upright folds and culminated in initiation of major NW-trending sinistral shear zones of the Najd Fault System (NFS, at c. 640-540 Ma ago) as well as steeply dipping S2 foliation, and shallowly plunging L2 lineation. NW-to NNW-trending F2 folds are open to steep and vary in plunge from horizontal to vertical. D2 deformational fabrics are strongly overprinted by D3 penetrative structures. D3 is characterized by a penetrative S3 foliation, steeply SE- to NW-plunging and shallowly NE-plunging stretching lineations (L3), asymmetric and sheath folds (F3) consistent with dextral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2-D3 represent a non-coaxial progressive event formed in a dextral NE- over NW-sinistral shear zone during a partitioned transpression in response to E-W-directed compression during oblique convergence between East and West Gondwana developed due to closure of the Mozambique Ocean and amalgamation of the Arabian-Nubian Shield in Cryogenian-early Ediacaran time.

  18. Chromium isotope signature during continental crust subduction recorded in metamorphic rocks

    Science.gov (United States)

    Shen, Ji; Liu, Jia; Qin, Liping; Wang, Shui-Jiong; Li, Shuguang; Xia, Jiuxing; Ke, Shan; Yang, Jingsui

    2015-11-01

    The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE) like, with δ53CrNIST979 of greenschists, amphibolites, and eclogites ranging from -0.06‰ to -0.17‰, -0.05‰ to -0.27‰, and -0.01‰ to -0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ53Cr (Δ53CrCpx-Gt) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, respectively, indicating that significant inter-mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite-free peridotites (-0.21‰ to -0.11‰) < chromite-bearing peridotite (-0.07‰) < chromite (-0.06‰). These findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite.

  19. Taconian retrograde eclogite from northwest Connecticut, USA, and its petrotectonic implications

    Science.gov (United States)

    Chu, Xu; Ague, Jay J.; Axler, Jennifer A.; Tian, Meng

    2016-01-01

    Mafic lenses hosted by felsic paragneiss in a Taconic thrust slice (Canaan Mountain Formation) from northwest Connecticut (New England, USA) contain relict mineral assemblages and decompression textures indicative of high pressure (HP) precursors. Symplectic intergrowths consisting mostly of diopside + plagioclase or biotite + plagioclase are pseudomorphous after omphacite and phengite, respectively. Pseudosection analysis and thermobarometry demonstrate that the inferred peak assemblage of garnet + clinopyroxene + phengite formed at > 14 kbar and ~ 710 °C in the eclogite facies. Bulk-rock geochemistry and field relations indicate that the protoliths of the mafic gneisses were likely rifting-related mafic intrusions. Zircon U-Pb dating by ion probe yields a 456 ± 4.6 Ma (2σ) metamorphic age for the mafic gneiss. The zircons from the felsic host rocks have an identical 456 ± 11 Ma metamorphic rim age and Grenvillian detrital cores. The HP metamorphism and the coeval arc magmatism reflect the collision between the Laurentian passive margin and a Taconic arc complex over an east-dipping subduction zone that was active until ~ 456 Ma in Connecticut (southern New England). The P-T path is characterized by post-peak-T compression, suggesting that the eclogite-facies metamorphism was associated with the deformation of the collision zone after the initial continent-arc collision. After the culmination of collision the subduction polarity switched; metamorphic ages decrease southward along the orogen suggesting that this reversal occurred ~ 10 Myr later in New England than in Newfoundland. Nearly all mafic rocks in the study area crop out as fairly ordinary-looking amphibolites, so it is reasonable to speculate that HP metamorphism was more extensive than currently recognized in New England but has been obscured by thermal overprinting and retrogression.

  20. REE, Sm-Nd and U-Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): Evidence for crustal contamination

    International Nuclear Information System (INIS)

    A geochemical and geochronological study of the Alpine External Crystalline Massifs (AECM) of Aiguilles Rouges, Belledonne and Argentera was undertaken in order to constrain the geodynamic evolution of this segment of the Variscan foldbelt. Another aim of the study is to characterize the behaviour of isotopic markers, in particular the U-Pb zircon system, under high-grade metamorphic conditions. The whole-rock geochemistry of eclogites and amphibolites was investigated using major and trace element (including the REE) analytical techniques; isotopic studies were performed by application of the Sm-Nd whole-rock and U-Pb zircon methods. In terms of regional geological history, the early development of metamorphic and magmatic activity in the AECM is typical of the extensional tectonic regime observed throughout the Variscan foldbelt during the Cambro-Ordovician (i.e. basic magmatism dated at 475-450 Ma). The composition of the metabasic rocks is closely similar to tholeiites emplaced into thinned continental crust which are generally associated with the initial stages of oceanic rifting. The source regions for these metabasics are characterized by initial εNd values between +6 and +8, suggesting depleted mantle sources influenced by a weak crustal component and/or the existence of a metasomatised lithosphere. The multi-stage eclogite-facies metamorphism is dated at 425-395 Ma (i.e. Silurian). An application of the U-Pb method, associated with the artificial abrasion of zircon grains, has led to the recognition of a weak crustal contamination in the metabasic protoliths. This is implied by the Archaean and Lower Proterozoic upper intercepts on Concordia - devoid of geological significance - which reflect the presence of a pre-existing basement to the AECM. (orig./WL)

  1. Geochemistry, Petrology and Zircon U-Pb Dating for Bibi Maryam Granitoid, NE of Nehbandan, East of Iran

    Directory of Open Access Journals (Sweden)

    Mohammad Hosie Zarrinkoub

    2011-04-01

    Full Text Available Bibi Maryam granitoid body with about 5 Km2 has intruded into ophiolite mélange complex in sistan suture zone, in east of Iran. Thermal effects of this body on the host rocks show that this activity occurred after emplacement of ophiolite mélange. This intrusive body with general northwest – southeast trend is mainly tonalite-quartz diorite but there is a minor unit as dykes of granodiorite in the northwest side. Main texture in tonalite - quartz diorite is granular and in granodiorite part, granular, myrmekitic and graphic are popular. Quartz, plagioclase (oligoclase-andesin and biotite are the main minerals and the minors are amphibol, apatite, zircon and opaque in the main body. Quartz, sodium plagioclase and alkali feldspar are the main minerals and the minors are garnet, apatite and opaque minerals in granodiorite unit. Bibi Maryam granitoid is magmatic arc type and has plotted in calc-alkaline, metaluminous to peraluminous field. High ratio of LREE / HREE, Sr/Y (average is 38.7, and SiO2 amount(average is 69.48 with absent of Eu negative anomaly show these rocks are very similar to high SiO2 adakites. High ratio of Sr/Y can be interpreted of high depth melting in plagioclase unstability and garnet stability field. The source of this magma can be an eclogite or garnet amphibolites which derived of subducted metamorphosed oceanic plate in east of Iran. Dating on zircon grains in the main body (tonalite-quartzdiorite with zircon uranium – lead shows this magmatic event has occurred at 71.5 ± 0.6 Ma (maestrichtian. Based on this dating, the emplacement of host ophiolite mélange should occurred before maestrichtian.

  2. Geochemistry of PGE in mafic rocks of east Khasi Hills, Shillong Plateau, NE India

    Indian Academy of Sciences (India)

    Sampa Hazra; Jyotisankar Ray; C Manikyamba; Abhishek Saha; S S Sawant

    2015-03-01

    The mafic rocks of east Khasi Hills of the Meghalaya Plateau, northeastern India, occur as an intrusive body which cut across the weakly metamorphosed Shillong Group of rocks. Other than Shillong Group of rocks, high grade Archaean gneissic rocks and younger porphyritic granites are also observed in the study area. The studied mafic rocks of east Khasi Hills cover an area of about 4 km2 and represent structurally controlled intrusion and varying grades of deformation. Structurally, these mafic rocks can be divided into massive type of mafic rocks, which are more or less deformation free and foliated type of mafic rocks that experienced deformation. Petrographically, this massive type can be classified as leuco-hornblende-gabbro whereas foliated type can be designated as amphibolite. On the basis of major oxide geochemistry, the investigated mafic rocks can be discriminated into high titanium (HT) (TiO2 > 2 wt%) and low titanium (LT) types (TiO2 < 2 wt%). Use of several geochemical variation diagrams, consideration of chondrite-normalized and mantle-normalized REE and PGE plots suggest role of magmatic differentiation (with almost no role of plagioclase fractionation) in a subduction controlled tectonic environment. The PGE trends of the studied rocks suggest relative enrichment of palladium group of PGE (PPGE) compared to iridium group PGE (IPGE). Critical consideration of Sm vs. La, Cu vs. La, Pd vs. La and Cu/Pd vs. La/Sm plots strongly favours generation of the parent magma at a columnar melting regime with batch melting of cylindrical column of the parent mantle to the tune of ∼25%. The characteristic PGE behaviours of the presently investigated mafic rocks of east Khasi Hills can be typically corroborated as `orogenic' (discordant) type. These rocks have an enriched mantle affinity with a co-magmatic lineage and they have been generated by slab-dehydration, wedge-melting and assimilation fractional crystallization process at a continental margin arc setting.

  3. Mafic rocks from Erinpura gneiss terrane in the Sirohi region: Possible ocean-floor remnants in the foreland of the Delhi Fold Belt, NW India

    Indian Academy of Sciences (India)

    M K Pandit; H De Wall; H Daxberger; J Just; M Bestmann; K K Sharma

    2011-08-01

    A small isolated mafic body occurs to the south of Sirohi near village Daba within the Neoproterozoic Erinpura Granite in the southern sector of the Proterozoic Delhi Fold Belt in NW India. This mafic body occurs close to a 100 m wide NE–SW trending shear zone (Daba Shear Zone) which overprints the felsic rock fabrics. Further south, a small mafic body near village Kui was also sampled which forms the southern limit of the Phulad Ophiolite Suite which is a 300 km long major NE–SW trending lineament, described as Western Margin Fault. Some of the lithological components of the Daba mafic body show locally preserved magmatic fabric but completely transformed mineralogies under lower amphibolites facies metamorphic conditions where two-stage deformation has been inferred. Magnetic fabric analysis underlines a general correspondence of structural elements in both felsic and mafic lithologies. Binary correlations of Zr with other high field strength elements underline fractionation as the main process in the evolution of Daba and Kui rocks. Geochemical characteristics indicate subalkaline tholeiitic basalt affinity for these mafic rocks. The trace element characteristics, such as enriched LIL elements, high Th, absence of negative Nb anomalies and depletion in compatible elements in Daba samples suggest an enriched mantle source and lower degree of melting. The trace and rare earth element characteristics for Kui (Th anomaly, Nb–Ta trough and less spiked patterns, flat REE trends) indicate derivation from a refractory mantle source affected by fluids derived from subduction. Distinct differences in trace and REE characteristics between Daba and Kui can be interpreted in terms of different stages of ophiolite development.

  4. Fluid-induced dissolution breakdown of monazite from Tso Morari complex, NW Himalayas: evidence for immobility of trace elements

    Science.gov (United States)

    Upadhyay, Dewashish; Pruseth, Kamal Lochan

    2012-08-01

    Primary igneous monazite from the Polokongka La granite of the Tso Morari complex in the western Himalayas has been partially replaced by a three-layered corona of metamorphic fluor-apatite, allanite + U- and Th-bearing phases (huttonite + brabantite), and epidote. The alteration is related to high-pressure amphibolite-facies (10-11 kbar and 587-695 °C) fluid-induced retrogression of the ultra-high-pressure granite during exhumation after India-Asia collision. The corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid-mediated coupled dissolution-reprecipitation process. Mass balance calculations using the volume proportions and compositions of coronal minerals show that the REE, U, Th, Pb, Ba and P were conserved and not transported outside the alteration corona. The formation of fluor-apatite, allanite, huttonite and coffinite from monazite and the immobility of REE, U and Th require an influx of alkali- and F-bearing, Ca-rich fluid having high Ca/Na into the corona. We are aware of only two other occurrences of such alteration textures, and these have several similarities in terms of geodynamic setting and P-T histories of the host rocks. We suggest that there may be a common mechanism of exhumation style, and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high-P metamorphism in continental collision zones, which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks.

  5. Element redistribution and mobility during upper crustal metamorphism of metasedimentary rocks: an example from the eastern Mount Lofty Ranges, South Australia

    Science.gov (United States)

    Hammerli, Johannes; Spandler, Carl; Oliver, Nicholas H. S.

    2016-04-01

    We present a detailed study on element mobility during prograde metamorphism of metasedimentary rocks of the eastern Mt. Lofty Ranges, South Australia. Mineral and bulk rock compositions were monitored across a regional metamorphic gradient from ≈350-400 °C to migmatite grade (≈650-700 °C) at ≈0.3-0.5 GPa, where pervasive up-temperature fluid flow during metamorphism has been proposed previously. Major and most trace elements (including rare earth elements) are isochemical during metamorphism as they are effectively redistributed into newly formed major and/or accessory minerals. Monazite or allanite and xenotime control the whole rock concentration of rare earth elements (REEs), whereas apatite and titanite are minor REE hosts. The only non-volatile elements that are demonstrably mobilized by metamorphic fluids are Zn, Pb, Ag, Cs, Sb, Bi and As, whose concentrations decreased with increasing metamorphic grade. Depletion of Zn, Sb and Pb was progressive with increasing temperature in staurolite-absent psammopelites, with losses of ≈80 % of the original Zn and >80 % of the protolithic Sb and ≈50 % of the original Pb from the rocks from high-grade metamorphic zones. Pronounced depletion of As and Cs occurs at the greenschist/amphibolite facies boundary and the transition to migmatite grade, respectively, while Ag and Bi contents decrease between 500 and 550 °C where >50 % of the original Ag and Bi is lost. While for most elements, unmetamorphosed sedimentary sequences can be considered chemical equivalents of metasedimentary rocks occupying deeper crust levels, in some cases, such as the extensive flow of Cl-rich fluid documented here, metals such as Zn, Pb and Ag may be stripped and may serve as a metal source for orebody formation. The decrease of As, Bi and Sb contents during prograde metamorphism might be a more universal feature that is linked with sulphide phase transitions.

  6. Structural, petrological and geochronological analysis of the lithotypes from the Pien region (Parana State, Brazil) and adjacences

    International Nuclear Information System (INIS)

    The Pien area presents the major geotectonic domains separated by the Pien Shear Zone (PSZ). The northern one is the Rio Pien Granite-Mylonitic Suite composed by calc-alkaline granitoids of Neoproterozoic age. The southern domain is represented by the Amphibolite-Granulite where high grade metamorphism took place at the end of paleoproterozoic time. Considering the identified lithotypes, their geochemical affinity (particularly the Ti, Cr, Ni and REE content) and the geological context observed in the area, a geotectonical model of active continental margin related to subduction SSZ (Supra-Subduction Zone) is proposed. K-Ar on plagioclase from gabbronorites gave Neoproterozoic ages although Sm-Nd whole rock isochron yielded Paleoproterozoic ages. Based in geochemical data, it is proposed that the biotite gneiss and biotite-amphibole-gneiss which occur near the PSZ have a shoshonitic to high-K calc-alkaline features which are characteristic of active continental margins. K-Ar on biotite extracted from these rocks, gave Neoproterozoic ages. The available radiometric data for the Rio Pien mylonitic granitoids show that between 650-595 Ma the generation, deformation and cooling below the isotherm of 250 deg C occurred. On the other hand, the geochronological data for the Agudos do Sul Massif are in the 590-570 Ma interval showing its younger generation. The Sr87 / Sr86 initial ratios for both granitoids suggest more involvement of the continental crust in the origins of Agudos do Sul granitic Massif. The analyses of the entire set of the available data for the Pien area allows the suggestion of a geotectonic scenery related to the evolution of an active continental margin during the collages associated to the Brasiliano Cycle

  7. Pressure-temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

    Science.gov (United States)

    Cruciani, Gabriele; Franceschelli, Marcello; Massonne, Hans-Joachim; Carosi, Rodolfo; Montomoli, Chiara

    2013-08-01

    Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S1 and S2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7 mol% and an increase of pyrope component from 4 to 6 mol% from core to rim. The grossular content firstly increases from the inner (Grs~ 21) to the outer core (Grs~ 27) and then decreases towards the outermost rim (Grs~ 15). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6 pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P-T pseudosections were calculated in the system Na2O-K2O-CaO-FeO-MnO-MgO-Al2O3-TiO2-SiO2-H2O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8 GPa and temperatures of 460-500 °C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540-570 °C and 0.7-1.0 GPa. These results suggest an HP-metamorphic imprint during the D1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D2 folding and shearing occurred at decreasing P-T conditions during the exhumation of the metamorphic complex.

  8. Cabacal belt, southern Amazon craton in Mato Grosso-Brazil: Implications for the tectonic evolution (more complications)

    International Nuclear Information System (INIS)

    The Amazon Craton covers an area of about 250000Km2 on the northern part of the South America. The Precambrim time history of the craton has been divided in geochronological provinces (Cordani et al. 1979; Teixeira et al. 1989; Tassinari et al. 1996; Tassinari and Macambira, 1999; and Santos et al. 2000). The Cabacal Belt covers an area of approximately 350 square kilometers in the southern region of the Amazon Craton, Mato Grosso-Brazil. It is a narrow NNW trending belt of greenchist to amphibolite grade volcano-sedimentary rocks intruded by plutonic rocks of tonalitic to granitic composition and mafic sills/dikes (Pinho et al., 1997). To the East it is bounded by Mesoproterozoic sediments of the Aguapei Group. To the West it is separated from the Araputanga Belt by a metamorphic complex, chiefly made of orthogneisses. The Cabacal Belt is one of the three NNW trending belts from the Alto Jauru Greenstone Belt. As a whole, the supracrustal rocks of the Cabacal Belt consist of three units: a basal unit comprising mafic lavas and breccias; a middle unit of felsic to intermediate metavolcanics rocks with interlayered tuffs and sediments; and a top unit of dacite-rhyodacit lavas, tuffs, and sediments (clastic and chemical). Monteiro et al. (1986) named these units as Mata Preta (MPF), Manuel Leme (MLF) and Rancho Grande (RGF) formations respectively. Leite et al. (1986) used the name Quatro Meninas Complex for the basic unit We analyzed zircons from seven samples in the Cabacal Belt, including samples from the volcano-sedimentary sequence, from intrusive bodies, and from the gneiss that separates the Cabacal Belt from the Araputanga Belt (au)

  9. Archaean tonalitic-trondhjemitic and granitic plutonism in the Gaviao block, Sao Francisco craton, Bahia, Brazil: geochemical and geochronological characteristics

    International Nuclear Information System (INIS)

    The granitic-gneissic terranes of the Gaviao Block in the central-southern Sao Francisco Craton are a Key area to the understanding of the evolution of the South American Platform. The Archaean granitic-gneissic rocks are intimately associated with the Umburanas and Contendas Mirante greenstone belts. The metamorphic grades vary from greenschist to amphibolite facies. These rocks were intensely deformed and intruded by Paleoproterozoic granites. The main evolution of the Gaviao Block is market by the formation of granitoid nuclei during various episodes of TTG plutonism between 3.2 and 3.4 Ga. The TDM Sm-Nd model ages for these granitoids range from 3.2 and 3.6 Ga, indicating involvement of sialic crust in their genesis, in agreement with the εNd(t) values between -4.0 and -1.3. Between 3.1 and 2.5 Ga, the Gaviao Block was affected by volcanic and sedimentary activity associated with the formation of the Umburanas and Contendas Mirante greenstone belts, intruded by granites about 2.75 and 2.5 Ga ago. During the paleoproterozoic, the Gaviao Block was regionally deformed and metamorphosed, and intruded by granites between 2.1 and 1.9 Ga. During the mesoproterozoic, tectonic and metamorphic activity occurred between 1.2 and 1.0 Ga as suggested by resetting of Rb-Sr and K-Ar systematics. Finally, regional tectono-thermal overprints in the areas occurred during the Neoproterozoic. These episodes are accompanied by the intrusion of mafic dikes at about 0.9 Ga K-Ar ages given by biotites fall within the range 0.5-0.7 Ga and mark the end of the cratonization stage. (author)

  10. Microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep, recovery, and grain boundary sliding

    Science.gov (United States)

    Massey, M.A.; Prior, D.J.; Moecher, D.P.

    2011-01-01

    Optical microscopy, electron probe microanalysis, and electron backscatter diffraction methods have been used to examine a broad range of garnet microstructures within a high strain zone that marks the western margin of a major transpression zone in the southern New England Appalachians. Garnet accommodated variable states of finite strain, expressed as low strain porphyroclasts (Type 1), high strain polycrystalline aggregates (Type 2), and transitional morphologies (Type 3) that range between these end members. Type 1 behaved as rigid porphyroclasts and is characterized by four concentric Ca growth zones. Type 2 help define foliation and lineation, are characterized by three Ca zones, and possess a consistent bulk crystallographic preferred orientation of (100) symmetrical to the tectonic fabric. Type 3 show variable degrees of porphyroclast associated with aggregate, where porphyroclasts display complex compositional zoning that corresponds to lattice distortion, low-angle boundaries, and subgrains, and aggregate CPO mimics porphyroclast orientation. All aggregates accommodated a significant proportion of greenschist facies deformation through grain boundary sliding, grain rotation and impingement, and pressure solution, which lead to a cohesive behavior and overall strain hardening of the aggregates. The characteristic CPO could not have been developed in this manner, and was the result of an older phase of partitioned amphibolite facies dislocation creep, recovery including chemical segregation, and recrystallization of porphyroclasts. This study demonstrates the significance of strain accommodation within garnet and its affect on composition under a range of PT conditions, and emphasizes the importance of utilizing EBSD methods with studies that rely upon a sound understanding of garnet. ?? 2010 Elsevier Ltd.

  11. Field characteristics, petrography, and geochronology of the Hohonu Batholith and the adjacent Granite Hill Complex, North Westland, New Zealand