Sample records for amphibolitised mafic dykes

  1. Ultradepleted adcumulates from a late-stage mafic dyke of the Bayantsagaan layered intrusion, Mongolia (United States)

    Chistyakova, Sofya; Latypov, Rais


    It is commonly expected that late-stage dykes in mafic layered intrusions must be strongly enriched in all incompatible components that are concentrated in the evolved residual melt. Our recent study of one late-stage dyke from the Bayantsagaan layered intrusion (Mongolia) has revealed, however, that this is not always the case. This 11 cm thick dyke is composed of quite fresh, fully crystalline troctolite with no signs of chilling against host leucotroctolite. Texturally, both dyke and leucotroctolite are composed of medium-grained plagioclase-olivine-magnetite cumulate. The dyke is, however, finer-grained and more adcumulate than leucotroctolite as evident from a much less amount of intercumulus material mainly represented by interstitial amphibole (5 vol.% versus 10 vol.%). The dyke shows three distinctive geochemical features. Firstly, it is compositionally more evolved than leucotroctolite as indicated by notably lower real and normative An-content of plagioclase, whole-rock Mg-number and Cr and higher whole-rock TiO2. Secondly, despite of being more evolved, the dyke is notably depleted in all incompatible components (e.g. K2O, Y, REE). Their concentrations in the dyke are several times lower than in host leucotroctolite (e.g. 0.015 ppm versus 0.112 ppm for Dy). Thirdly, the dyke reveals internal reverse zonation with an inward increase in compatible MgO, TiO2 and normative An-content and a decrease in all incompatible components (e.g. REE). Such a distribution of incompatible components is indicative of an inward decrease in the amount of trapped melt, in other words, the dyke becomes inwards progressively more adcumulate. In layered intrusions, adcumulates are commonly attributed to the almost complete removal of interstitial liquid enriched in incompatible elements from cumulate pile by some primary cumulus (e.g. in situ growth) or post-cumulus processes (e.g. compaction, compositional convection, thermal migration). These processes are, however, not

  2. ArcGIS studies and field relationships of Paleoproterozoic mafic dyke swarms from the south of Devarakonda area, Eastern Dharwar Craton, southern India: Implications for their relative ages

    Indian Academy of Sciences (India)

    Amiya K Samal; Rajesh K Srivastava; Lokesh K Sinha


    Google Earth Image and cross-cutting field relationships of distinct Paleoproterozoic mafic dykes from south of Devarakonda area in the Eastern Dharwar Craton has been studied to establish relative emplacement ages. The Devarakonda, covering an area of ∼700 km2, shows spectacular cross-cutting field relationships between different generations of mafic dykes, and is therefore selected for the present study. Although some recent radiometric age data are available for distinct Paleoproterozoic mafic dykes from the Eastern Dharwar Craton, there is no analogous age data available for the study area. Therefore, relative age relationships of distinct mafic dykes have been established for the study area using cross-cutting field relationships and GIS techniques, which shows slightly different picture than other parts of the Eastern Dharwar Craton. It is suggested that NE–SW trending mafic dykes are youngest in age (probably belong to ∼1.89 Ga dyke swarm), whereas NNW–SSE trending mafic dykes have oldest emplacement age. Further, the NNW–SSE mafic dykes are older to the other two identified mafic dyke swarms, i.e., WNW–ESE (∼2.18 Ga) and N–S trending (∼2.21 Ga) mafic dyke swarms, as dykes of these two swarms cross-cut a NNW–SSE dyke. It provides an evidence for existence of a new set of mafic dykes that is older to the ∼2.21 Ga and probably younger to the ∼2.37 Ga swarm. Present study also supports existence of two mafic dyke swarms having similar trend (ENE–WSW to NE–SW) but emplaced in two different ages (one is ∼2.37 Ga and other ∼1.89 Ga).

  3. The discovery of the Neoarchean mafic dyke swarm in Hengshan and reinterpretation of the previous "Wutai greenstone belt"

    Institute of Scientific and Technical Information of China (English)


    The structural mapping and section study indicate that the "greenstone belts" in the southern to central parts of Hengshan were intensively sheared and transposed mafic dyke swarm,which originally intruded into the Neoarchean grey gneiss and high-pressure granulite terrain(HPGT).The HPGT is characterized by flat-dipping structures,to the south it became steep and was cut by the Dianmen mafic dyke swarm.After high-pressure granulite-facies metamorphic event,the mafic dyke swarm occurred,and was associated with the extensional setting and reworked by the late strike-slip shearing.The zircon age dating proves that the Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma,followed by late tectonothermal reworking.The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton(NCC),providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other cratonic blocks of the world.

  4. The discovery of the Neoarchean mafic dyke swarm in Hengshan and reinterpretation of the previous “Wutai greenstone belt”

    Institute of Scientific and Technical Information of China (English)

    李江海; 张志强; 黄雄南


    The structural mapping and section study indicate thai the "greenstone belts" in the southern to central parts of Hengshan were intensively sheared and transposed mafic dyke swarm, which originally intruded into the Neoarchean grey gneiss and nigh-pressure granulite terrain (HPGT). The HPGT is characterized by flat-dipping structures, to the south it became steep and was cut by the Dianmen mafic dyke swarm. After high-pressure; granulite-facies metamorphic event, the mafic dyke swarm occurred, and was associated with the extensional setting and reworked by the late strike-slip shearing. The zircon age dating proves that the Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma, followed by late tectonothermal reworking. The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton (NCC), providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other e

  5. Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway) (United States)

    Degli Alessandrini, G.; Menegon, L.; Malaspina, N.; Dijkstra, A. H.; Anderson, M. W.


    A dry mafic dyke from a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway) experienced syn-kinematic melt-rock interaction. Viscous shearing occurred at T ≈ 800 °C, P ≈ 0.75-0.95 GPa and was coeval with infiltration of felsic melt from adjacent migmatitic metapelites. The dyke has a mylonitic microstructure where porphyroclasts of orthopyroxene, clinopyroxene and plagioclase are wrapped by a fine-grained (4-7 μm) polyphase mixture of clinopyroxene + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar ± apatite. Microstructural observations and electron backscatter diffraction analysis indicate that the porphyroclasts deformed by a combination of dislocation glide and fracturing, with only a limited record of dislocation creep, recovery and dynamic recrystallization. We identified diffusion creep as the dominant deformation mechanism in the mixture based on the small grain size, phase mixing and weak crystallographic preferred orientation of all phases (interpreted as the result of oriented grain growth during viscous flow). The polyphase mixture did not form by dynamic recrystallization or by mechanical fragmentation of the porphyroclasts, but rather by melt-rock interaction. Thermodynamic models indicate that the syn-kinematic mineral assemblage results from the chemical interaction between a pristine mafic dyke and ca. 10 vol.% of felsic melt infiltrating from the adjacent partially molten metapelites. Extrapolation of laboratory-derived flow laws to natural conditions indicates that the formation of interconnected layers of fine-grained reaction products deforming by diffusion creep induces a dramatic weakening in the mafic granulites, with strain rates increasing up to 2-3 orders of magnitude. The reaction weakening effect is more efficient than the weakening associated with melt-assisted diffusion creep in the presence of up to 10 vol.% of infiltrated melt without formation of fine-grained reaction products

  6. Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark:

    DEFF Research Database (Denmark)

    Holm, Paul Martin; Pedersen, Lise E.; Højsteeen, Birte


    ), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively.The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated.The Kelseaa type magmas...

  7. Structure and tectonic setting of the SE Sardinia mafic dyke swarm. Insights for the stress state during magma emplacement in the upper crust (United States)

    Martinez-Poza, Ana Isabel; Druguet, Elena


    The SE Sardinia mafic dyke swarm intruded in Permian times into the Late Variscan granitoids of the Sàrrabus massif. In order to characterize the structure of the dyke swarm and to assess the stress state and tectonic regime at the time of dyking, we have performed a multi-methodological study which encompasses paleostress analysis using the orientation distribution of dykes, estimation of crustal extension from dyke thickness/spacing ratios and true dilation direction from the matching geometry of offsets. The mean orientation of the dyke swarm is ∼N150°, with secondary main trends orientated ∼N77° and ∼N10°. The fracture network (mostly joints) exhibits multiple orientations grouped in four or more sets, being the orientation and density patterns in granitoid and dykes significantly different. Some of the pre-existing joints were exploited by the dykes, while others were reactivated as faults during and after dyke intrusion. The paleostress analysis yielded σ1 sub-vertical, indicating that an extensional tectonic regime predominated in the area during the emplacement of the dyke swarm. σ3 was sub-horizontal, sub-parallel to the mean NE-SW dilation direction. The inferred relative stress magnitudes are characterized by a σ1 much larger than the other two principal stresses. In this context, intrusion of the dyke swarm occurred under low magma pressure conditions and preferentially into a pre-existing joint nerwork during a regional extensional event. The SE Sardinian dyke swarm has been compared with other contemporary mafic dykes of the western Mediterranean region, by attempting to restore the effects of the Neogene and eo-Alpine rotational events. The reconstruction gives a complex crustal-scale pattern in terms of dyke orientation, a fact that could be partly attributed to differential rotations associated with localized strike-slip movements that might have operated coevally with regional extension or transtension during Permian times.

  8. Ordovician and Triassic mafic dykes in the Wudang terrane: Evidence for opening and closure of the South Qinling ocean basin, central China (United States)

    Nie, Hu; Wan, Xin; Zhang, He; He, Jian-Feng; Hou, Zhen-Hui; Siebel, Wolfgang; Chen, Fukun


    We report zircon ages and geochemical composition for mafic dykes that intruded Neoproterozoic volcanic-sedimentary sequences in the southern part of Wudang area, South Qinling. The results indicate that the dykes were emplaced during the Early Paleozoic (c. 460 Ma) and Early Mesozoic (c. 220 Ma). The dykes share similar major element composition, but have distinctive trace element pattern and Sr-Nd-Pb isotope distribution. Early Paleozoic mafic dykes are characterized by enrichment in LREEs, LILEs and HFSEs and EM II-type isotopic features. These geochemical features suggest derivation from an OIB-type mantle source that had undergone metasomatism during earlier subduction events. The Early Mesozoic mafic dykes can be subdivided into two distinct geochemical groups. Dykes of Group 1 are depleted in LREEs, LILEs and HFSEs and show depleted isotope compositions, indicating an origin by partial melting of asthenospheric mantle material. Dykes of Group 2 have high Rb-, Ba-, and K-contents and EM I-type isotopic features, suggesting input of lower crustal material to the magma source during Mesozoic subduction. We propose that the Early Paleozoic dykes are related to the opening of an oceanic basin separating South Qinling from the Yangtze Block, while the Early Mesozoic dykes were derived from partial melting of up-welling asthenosphere during the final amalgamation of these two blocks in the Early Mesozoic. A slab break-off model could explain not only the petrogenesis of the Mesozoic mafic dykes, but also the distinct geological features between the Dabie-Sulu and South Qinling orogens. We propose that slab break-off occurred at great depth in the Dabie-Sulu orogen and hence rare magmatism occurred. Whereas in South Qinling the break-off occurred at a shallow depth, the asthenospheric mantle material could rise further up into the overlying mantle where it experienced decompression and melting. As a consequence, crustal sections were heated up to produce extensive

  9. Mafic dykes at the southwestern margin of Eastern Ghats belt: Evidence of rifting and collision

    Indian Academy of Sciences (India)

    S Bhattacharya; A K Chaudhary; W Teixeira


    The southwestern margin of the Eastern Ghats Belt characteristically exposes ma fic dykes intruding massif-type charnockites. Dykes of olivine basalt of alkaline composition have characteristic trace element signatures comparable with Ocean Island Basalt (OIB). Most importantly strong positive Nb anomaly and low values of Zr/Nb ratio are consistent with OIB source of the mafic dykes. K –Ar isotopic data indicate two cooling ages at 740 and 530 Ma. The Pan-African thermal event could be related to reactivation of major shear zones and represented by leuco-granite vein along minor shear bands. And 740 Ma cooling age may indicate the low grade metamorphic imprints, noted in some of the dykes. Although no intrusion age could be determined from the present dataset, it could be constrained by some age data of the host charnockite gneiss and Alkaline rocks of the adjacent Prakasam Province. Assuming an intrusion age of ∼1 .3 Ga, Sr –Nd isotopic composition of the dykes indicate that they preserved time-integrated LREE enrichment. In view of the chemical signatures of OIB source, the ma fic dykes could as well be related to continental rifting, around 1.3 Ga, which may have been initiated by intra-plate volcanism.

  10. Petrogenesis and tectonic implications of the Neoproterozoic Datian mafic-ultramafic dykes in the Panzhihua area, western Yangtze Block, SW China (United States)

    Yang, Yi-Jin; Zhu, Wei-Guang; Bai, Zhong-Jie; Zhong, Hong; Ye, Xian-Tao; Fan, Hong-Peng


    Mafic-ultramafic dykes are important geological markers that can punctuate the onset of crustal extension during the breakup of a continent and provide valuable information on the mantle source. This study reports secondary ion mass spectroscopy zircon and baddeleyite U-Pb ages, elemental and Nd isotopic data for the Datian mafic-ultramafic dykes in the Panzhihua area, western Yangtze Block, SW China. Two kinds of rocks are confirmed: the picritic rock and the dolerite. Based on petrographic and geochemical features, the dolerite dykes are further subdivided into two groups: Group I mafic and Group II mafic dykes, which emplaced at 760 Ma (zircon U-Pb) and 800 Ma (zircon and baddeleyite U-Pb), respectively. All samples from the picritic rocks and the Group I mafic rocks show the features of high-Ti and alkaline basaltic magma in composition as well as "humped" trace element patterns, which are similar to those of typical alkaline basalts associated with continental rifts except for the slightly negative Nb-Ta anomalies. The Group II mafic rocks display the features of low-Ti and tholeiitic magma, moderately enriched in LILE and LREE, and characterized by distinctively negative Nb-Ta anomalies. The primary magmas of the picritic rocks and the Group I mafic rocks were generated from ca 25 % partial melting of an OIB-like, Nd isotopically depleted but incompatible elements relatively enriched mantle source within a garnet stable field. The Group II mafic rocks crystallized from crustal contaminated mafic magmas that were derived from a spinel-bearing sub-continental lithospheric mantle source, because of low ratios of La/Yb, Ti/Y and Sm/Yb. Geochemical features suggest that these groups of mafic-ultramafic dykes were formed in a continental rift setting, but derived from different mantle sources. In combination with other Neoproterozoic igneous rocks in the western margin of Yangtze Block, it is suggested that the Datian mafic-ultramafic dykes in the Panzhihua area

  11. Dyke-sill relationships in Karoo dolerites as indicators of propagation and emplacement processes of mafic magmas in the shallow crust (United States)

    Coetzee, A.; Kisters, A. F. M.


    This paper describes the spatial and temporal relationships between Karoo-age (ca. 180 Ma) dolerite dykes and a regional-scale saucer-sill complex from the Secunda (coal mine) Complex in the northeastern parts of the Karoo Basin of South Africa. Unlike parallel dyke swarms of regional extensional settings, mafic dykes commonly show curved geometries and highly variable orientations, short strike extents and complex cross-cutting and intersecting relationships. Importantly, the dyke networks originate from the upper contacts of the first-order dolerite sill-saucer structure and are not the feeders of the saucer complex. Cross-cutting relationships indicate the largely contemporaneous formation of dykes and the inner sill and inclined sheets of the underlying saucer. Systematic dykes form a distinct boxwork-type pattern of two high-angle, interconnected dyke sets. The formation and orientation of this dyke set is interpreted to be related to the stretching of roof strata above elongated magma lobes that facilitated the propagation of the inner sill, similar to the ;cracked lid; model described for large saucer complexes in Antarctica. Dyke patterns generally reflect the saucer emplacement process and the associated deformation of wall rocks rather than far-field regional stresses.

  12. Fluid Inclusion Study of Quartz Xenocrysts in Mafic Dykes from Kawant Area, Chhota Udaipur District, Gujarat, India

    Directory of Open Access Journals (Sweden)

    Randive Kirtikumar


    Full Text Available Unusual mafic dykes occur in the proximity of the Ambadongar Carbonatite Complex, Lower Narmada Valley, Gujarat, India. The dykes contain dense population of quartz xenocrysts within the basaltic matrix metasomatised by carbonate-rich fluids. Plagioclase feldspars, relict pyroxenes, chlorite, barite, rutile, magnetite, Fe-Ti oxides and glass were identified in the basaltic matrix. Quartz xenocrysts occur in various shapes and sizes and form an intricate growth pattern with carbonates. The xenocrysts are fractured and contain several types of primary and secondary, single phase and two-phase fluid inclusions. The two-phase inclusions are dominated by aqueous liquid, whereas the monophase inclusions are composed of carbonic gas and the aqueous inclusions homogenize to liquid between 226°C and 361°C. Majority of the inclusions are secondary in origin and are therefore unrelated to the crystallization of quartz. Moreover, the inclusions have mixed carbonic-aqueous compositions that inhibit their direct correlation with the crustal or mantle fluids. The composition of dilute CO2-rich fluids observed in the quartz xenocrysts appear similar to those exsolved during the final stages of evolution of the Amba Dongar carbonatites. However, the carbonates are devoid of fluid inclusions and therefore their genetic relation with the quartz xenocrysts cannot be established.

  13. Geochronologic and geochemical constraints of the petrogenesis of Permian mafic dykes in the Wuding area, SW China: Implications for Fe-Ti enrichment in mafic rocks in the ELIP (United States)

    Fan, Hong-Peng; Zhu, Wei-Guang; Bai, Zhong-Jie; Yang, Yi-Jin


    Mafic dykes are widely distributed and well preserved in the Wuding area, SW China. The dykes are composed of fine-grained diabases, which are further subdivided into two groups (Group I and Group II). The rocks from Group I have relatively higher MgO (4.95-6.16 wt%) and lower TiO2 (2.26-2.78 wt%) contents than those from the Group II rocks (MgO = 4.04-4.58 wt%; TiO2 = 4.09-4.55 wt%). Two baddeleyite ages of 264 ± 3 Ma and 256 ± 5 Ma from the Wuding dykes suggest that the ages of these dykes are similar to the plutonic bodies and the associated volcanic rocks of the Emeishan large igneous province (ELIP). The compositions and isotopic signatures of the dykes are also similar to those of the ELIP volcanic rocks, indicating derivation from the same magmatic event at 260 Ma. The primary melts of the Wuding dykes were originated by partial melting of a long-term depleted OIB-like mantle source. The Group I dykes were mainly formed by wallrock assimilation and fractional crystallization processes. The magmas parental to the Group II rocks were much more evolved than the magma parental to the Group I rocks and probably produced by wallrock assimilation and fractional crystallization of the parent magma of the Group I rocks. The increasing enrichment of Fe and Ti from the Group I to the Group II rocks were attributed to a fractional crystallization process which may be an epitome of enrichment of Fe and Ti in the Fe-Ti rich mafic intrusions of the ELIP.

  14. Petrological and geochemical constraints on the origin of mafic dykes intruding the composite Kaçkar Pluton from the eastern Blacksea magmatic arc, NE Turkey (United States)

    Aydin, Faruk; Oguz, Simge; Baser, Rasim; Uysal, Ibrahim; Sen, Cüneyt; Karsli, Orhan; Kandemir, Raif


    Geological, petrographical and geochemical data of mafic dykes intruding the composite Kaçkar Pluton from the eastern Blacksea magmatic arc (EBMA), NE Turkey, provide new insights into the nature of the metasomatizing agents in subcontinental lithospheric mantle beneath the region during the late Mesozoic-early Tertiary. Mafic dykes from the Çaykara and Hayrat (Trabzon), and also Ikizdere (Rize) areas from the northern margin of the EBMA consist of basalts, dolerites, lamprophyres (basic member) and lesser basaltic andesites and trachyandesites (evolved member). All dykes have no deformation and metamorphism. The outcrops of these dykes vary, with thickness from 0.2 to 10 m. and visible length from 3 to 20 m. In general, the mafic dykes dip steeply and cut directly across the Kaçkar Pluton, and show NW- and NE-trending, roughly parallel to the orientations of the EBMA main faults. Most of the dyke samples display subaphyric to porphyritic texture with phenocrysts of plagioclase (up to 10%), clinopyroxene (5-20%), amphibole (5-15%), and some contain variable amount of biotite (5-20%), lesser quartz (1-2%), and minor euhedral zircon, apatite and Fe-Ti oxides. The basic members of the mafic dykes have SiO2 of 44.1-51.9%, MgO of 4.5-12.1%, and TiO2 >mostly 0.8% (up to 2.3%) with K2O+Na2O of 1.3-6.6% with mostly subalkaline character. They are relatively high in mg-number (0.45-0.73) and transition metals (V=171-376 ppm, Co=22-45 ppm, Ni=3-148 ppm, and Sc=21-49 ppm). The evolved members of the dykes exhibit relatively higher SiO2 (57.1-60.2%) and K2O+Na2O (5.6-9.0%), and lower MgO (2.2-5.9%) and TiO2 (0.5-0.8%) contents than those from the basic dykes. Also, these samples have slightly low mg-number (0.41-0.65) and transition metals (V=99-172 ppm, Co=9-22 ppm, Ni=1-43 ppm, and Sc=9-20 ppm). In the Harker diagrams, all samples of the mafic dykes form a continuous array, and exhibit similar geochemical characteristics. In general, SiO2 inversely correlates with MgO, Fe

  15. New constraints on the geochronology and Sm-Nd isotopic characteristics of Bas-Drâa mafic dykes, Anti-Atlas of Morocco (United States)

    El Bahat, Abdelhakim; Ikenne, Moha; Cousens, Brian; Söderlund, Ulf; Ernst, Richard; Klausen, Martin B.; Youbi, Nasrrddine


    The Precambrian inliers of the Anti-Atlas belt across the southern part of Morocco contain numerous mafic dyke and sill swarms, which are thought to represent the erosional remnants of Proterozoic Large Igneous Provinces (LIPs). As part of an extended research campaign to date and characterize these LIPs, two amphiboles from one dyke within the Bas-Drâa inlier (western Anti-Atlas) yielded consistent 40Ar-39Ar ages of 611 Ma ± 1.3; i.e., evidence of magmatic activity during the Ediacaran (635-542 Ma). Furthermore, Nd isotope ratios and model ages identifies four groups of dykes within the Bas-Drâa inlier, all of which were derived from a previously enriched mantle reservoir with very low 143Nd/144Nd ratios (0.51065-0.51170) and corresponding εNd(t) values (-16.44 to -2.45). Such low Nd-ratios arguably reflect either a contribution of crustal components, such as the Paleoproterozoïc schisto-granitic basement, or metasomatic modification of the sub-continental lithospheric mantle.

  16. Sr-Nd-Pb isotopes of the Early Paleozoic mafic-ultramafic dykes and basalts from South Qinling belt and their implications for mantle composition

    Institute of Scientific and Technical Information of China (English)


    Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are characterized by eNd( t ) = +3.28―+5.02, (87Sr/86Sr)i=0.70341―0.70555, (206Pb/204Pb)i = 17.256―18.993, (207Pb/204Pb)i= 15.505―15.642, (208Pb/204Pb)i=37.125―38.968, △8/4=21.18―774.43, △7/4=8.11―18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMII and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.

  17. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: Origin and evolution of the Kangâmiut mafic dyke swarm, West Greenland

    Directory of Open Access Journals (Sweden)

    Lesher, Charkes E.


    Full Text Available The Kangâmiut dyke swarm in West Greenland intruded Archaean terrains at 2.04 Ga, and its northern portion was subsequently metamorphosed to granulite facies during the Nagssugtoqidian orogeny(c. 1.8 Ga. Mineral and whole-rock major and trace element compositions show that the parental magmas for the dyke swarm differentiated by the fractionation of olivine, clinopyroxene, plagioclase and late stage Fe-Ti oxides. Petrographical observations and the enrichment of K2O during differentiation argue that hornblende was not an important fractionating phase. Field observations suggest emplacement at crustal levels above the brittle–ductile transition, and clinopyroxene geothermobarometry constrains dyke emplacement depths to less than 10 km. Granulite facies metamorphism of the Kangâmiut dykes and their host rocks in the northern portion of the swarm requires subsequent burial to c. 30 km, related to roughly 20 km of crustal thickening between the time of dyke emplacement and peak metamorphism during the Nagssugtoqidian orogeny. Kangâmiut dykes are characterised by low Ba/La ratios (12 ±5, and high Nb/La ratios (0.8 ±0.2, compared to subduction relatedbasalts (Ba/La c. 25; Nb/La c. 0.35. These geochemical characteristics argue that the Kangâmiut dykes are not related to subduction processes. Forward modelling of rare-earth element data requires that primitive magmas for the Kangâmiut dykes originated from a moderately depleted mantle source with a mantle potential temperature of c. 1420°C. The inferred potential temperature is consistent with potential temperature estimates for ambient mantle at 2.0 Ga derived from secular cooling models and continental freeboard constraints. The geochemistry and petrology of the Kangâmiut dykes support a model that relates the dyke activity to passive rifting of the proposed Kenorland supercontinent rather than to mantle plume activity or subduction.

  18. Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India

    Indian Academy of Sciences (India)

    Rajesh K Srivastava; Anup K Sinha


    Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic dykes had undergone extreme differentiation (∼50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼10–15% melting of the mantle source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived

  19. The Ediacaran volcanic rocks and associated mafic dykes of the Ouarzazate Group (Anti-Atlas, Morocco): Clinopyroxene composition, whole-rock geochemistry and Sr-Nd isotopes constraints from the Ouzellarh-Siroua salient (Tifnoute valley) (United States)

    Belkacim, Said; Gasquet, Dominique; Liégeois, Jean-Paul; Arai, Shoji; Gahlan, Hisham A.; Ahmed, Hassan; Ishida, Yoshito; Ikenne, Moha


    Belonging to the huge Ouarzazate volcanic Group that covered the whole Anti-Atlas during the late Ediacaran (580-545 Ma), the Tifnoute valley volcanic formations are mainly pyroclastic and show a large composition, from trachybasalt to rhyolite and are crosscut by dolerite dykes. The Tifnoute valley volcanic rocks are located within a rigid salient of the Anti-Atlas that gives them special extreme characteristics. Due to the heavy greenschist alteration that affects this volcanic group, we focused the more immobile elements, but as REE can also be affected, we used the composition of unaltered clinopyroxene crystals to determine the nature of these volcanic rocks. The clinopyroxene is an augite diopside in the basalt, an augite in the andesite and an augite-salite in the dolerite. Petrography of the Tifnoute mafic volcanic rocks and clinopyroxene compositions indicate the presence of two magmatic series: (i) older high-K calc-alkaline (alkali-calcic) andesite and basalt characterized by the early crystallization of Fe-Ti oxides and of the late fractionation of plagioclase, the modal proportion of the latter increasing from the basalt to the andesite and (ii) younger alkalic dolerite dykes. With clinopyroxene trace element compositions obtained using laser ablation ICP-MS, we calculated the composition of the melts in equilibrium with the pyroxenes. The volcanic rocks of the Tifnoute Valley have positive εNd570 (+1.7 to +5.0), low Sri (volcanic rocks emplaced in a Pan-African transtensive post-collisional environment that evolved towards the major rifting event that will give rise to the Rheic ocean, in a similar way to what occurred just after the Variscan orogeny during the Triassic period that evolved to the Tethys ocean opening.

  20. Tectonic breccia of the Cabo Frio area, State of Rio de Janeiro, Brazil, intruded by Early Cretaceous mafic dyke: evidence of the Pan-African brittle tectonism? Brecha tectônica da área de Cabo Frio - RJ, intrudida por dique máfico do Eocretáceo: evidência do tectonismo rúptil do Pan-Africano?

    Directory of Open Access Journals (Sweden)

    Akihisa Motoki


    Full Text Available This paper presents the field descriptions and microscopic observations of a tectonic breccia in the basement gneiss of the Cabo Frio and Arraial do Cabo areas, State of Rio de Janeiro, Brazil, and its intrusive contact with the Early Cretaceous mafic dyke. At the sea cliff close to the Ilha do Japonês, there is an excellent contact outcrop between them. The tectonic breccia zone is 10 to 20m wide and has N30ºE direction. The breccia clasts are angular and characterized by auto-brecciation texture, and composed of breccia with similar aspect of the host tectonic breccia. The matrix is firmly consolidated by hydrothermalism and following silicification. The mafic dyke is 7 to 10m wide and of N45ºE direction. Along the contact, the dyke chilled margin featured by fine-grained basalt and prismatic joints can be observed. At the Conchas Beach and Arraial do Cabo city, there are four outcrops demonstrating the mafic dyke intrusion into the consolidated tectonic breccias. These outcrops prove that the tectonic breccias are older than the Early Cretaceous tholeiitic dykes. The fault breccias could have been formed during the brittle-phase tectonism of the last stage of the Pan-African Orogeny by hydrothermalism without magmatic activities, namely tectonic hydrothermalism. The existence of the clasts constituent of the breccia that are composed of breccia suggests that the fault movement and following hydrothermalism occurred repeatedly.Esse trabalho apresenta as descrições de campo e observações petrográficas de uma brecha tectônica no embasamento gnáissico das áreas de Cabo Frio e Arraial do Cabo, RJ, e seu contato intrusivo com um dique máfico do Eocretáceo. Na proximidade da ilha do Japonês, ocorre um excelente afloramento de contato entre esses litotipos. A zona da brecha tectônica tem 10 a 20 m de largura e tem direção de N30ºE. Os clastos da brecha são angulosos e caracterizados pela textura de auto-brechação, sendo similares

  1. The paleomagnetism and geological significance of Meso- proterozoic dyke swarms in the central North China Craton

    Institute of Scientific and Technical Information of China (English)


    The Mesoproterozoic mafic dyke swarms are extensively distributedin the central North China Craton(NCC) including North Shanxi, Wutai and Lüliang areas, which are not deformed and metamorphic but high magnetic, so the dyke swarms become the mark to compare the high meta-morphic rock areas in magnetism. Based on the analysis of paleomagnetism of mafic dyke swarms in North Shanxi, Wutai and Lüliang areas, NCC inclined southward about 18° so that North Shanxi lifted up and rotated 10° left to Wutai area. The dyke swarms in Lüliang developed later than in North Shanxi and Wutai area. The NNW-trending and WNW-trending dyke swarms developed in Lüliang while the North China Plate moved northward consistently so that the paleomagnetism of dyke swarms in Lüliang is greatly different from North Shanxi and Wutai area.

  2. The late Paleoproterozoic extension event:aulacogens and dyke Swarms in the North China craton

    Institute of Scientific and Technical Information of China (English)


    The extension structures and tectonic implication in the North China Craton (NCC) are discussed in this paper based on the mafic dyke swarms and geochronology, combining with the geochronology of aulacogens. The late Paleoproterozoic time is the important turning point in the Precambrian evolution of the NCC. The extension system (e. g. aulacogens and dyke swarms) is widespread in the NCC, which marks the carbonization of the NCC with the rigid characteristic similar to the modern plate. The paleostress field modeling suggests that the dyke swarms and aulacogens are arogenic extension marking the start of the supercontinent, not synorogenic and postorogenic extension. The mafic dyke swarms in the NCC mainly ranged from 1.83 to 1.77 Ga. The extension of the NCC is very limited brittle extension, the average extension ratio is only 0.35 % given by mafic dyke swarms extension calculation, so most of extension in the NCC is contributed by the aulacogens. The mafic dyke swarms are related with the aulacogens in the origin.

  3. U-Pb baddeleyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events (United States)

    Heaman, L. M.; Tarney, J.


    PRECISE and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events, calibration of apparent polar-wander paths and deciphering mechanisms of dyke emplacement. Precambrian dyke swarms, such as the Scourie dyke swarm in north-west Scotland, are invaluable time-markers, so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains. Precise dating is often difficult, however, because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems. Recent advances in U-Pb geochronology1-3 combined with the discovery that some mafic dykes contain trace amounts of uranium-bearing minerals such as baddeleyite (ZrO2) and/or zircon have made it possible to obtain U-Pb ages for these rocks with a precision typically on the order of 1-2 Myr4,5. Some of the first detailed U-Pb studies of Precambrian mafic dyke swarms5,6 have shown that large volumes of mafic magma, 100,000 km3, were emplaced into the continental crust in surprisingly short periods of time (<2 Myr). Here we report the results from a U-Pb study of three members of the Scourie dyke swarm, and find at least two periods of dyke emplacement at 2,418 and 1,992 Myr BP. We speculate on a potential global correlation of early Proterozoic mafic magmatism and hence on the origin of dyke swarms.

  4. The paleomagnetism and geological significance of Meso- proterozoic dyke swarms in the central North China Craton

    Institute of Scientific and Technical Information of China (English)

    HOU; Guiting


    [1]Ma Xinghua, Zhang Zhengkun, Paleomagnetism and its application to plate tectonics, in The Basic Problems of Plate Tectonics (in Chinese), Beijing: Seismology Press, 1985, 119-142.[2]Zhang Wenzhi, Li Pu, The paleomagnetic characteristics of Sub-Sinian Erathem in Jixian of China, Bulletin of the Chinese Academy of Geological Science, Branch Bulletin of Geological Institute of Tianjin, 1980, 1(1): 111[3]Halls, H. C., Paleomagnetism, structure and longitudinal correlation of middle Precambrian dykes from northwestern Ontario and Minnesota, Can. J. Earth Sci., 1985, 23: 142.[4]Qian Xianglin, Chen Yaping, Late Precambrian mafic swarms of the north China Craton, in Mafic Dyke Swarms, Geological Association of Canada (eds. Halls, H. C., Fahrig, W. F.), 1987, Special Paper, 34: 385.[5]Chen Yaping, Qian Xianglin, The paleomagnetism study of Late Precambrian mafic dyke swarms in Lüliang inlier of Shanxi, Acta Scientiarum Naturalium Universitatis Pekinensis (in Chinese), 1987, 2: 87.[6]Ernst, R. E., Head, J. W., Parfitt, E. et al, Giant radiating dyke swarms on Earth and Venus, Earth-Science Reviews, 1995, 4: 229[7]Radhakrishns, T., Joseph. M., Proterozoic palaeomagnetism of the mafic dyke swarms is the high-grade region of southern India, Precambrian Research, 1996, 76: 31.[8]Li Jianghai, Qian Xianglin, Zhai Mingguo, The tectonic classification of granulite facies of North China and its Early Precambrian tectonic evolution, Scientia Geologica Sinica (in Chinese), 1997, 32: 254.[9]Hou Guiting, Zhang Chen, Qian Xianglin, The formation mechanism and tectonic stress field of Mesoproterozoic mafic dyke swarms in the North China Craton, Geological Review (in Chinese), 1998, 44(3): 309.[10]Li Jianghai, He Wenyuan, Qian Xianglin, Genetic mechanism and tectonic setting of Proterozoic mafic dyke swarm: its implication for paleoplate reconstruction, Geological Journal of China University (in Chinese), 1997, 3(3): 272.[11]Hou Guiting, Mu

  5. Tectonic significance of dykes in the Sarnu-Dandali alkaline complex, Rajasthan, northwestern Deccan Traps

    Institute of Scientific and Technical Information of China (English)

    Anjali Vijayan; Hetu Sheth; Kamal Kant Sharma


    Whether swarms of preferentially oriented dykes are controlled by regional stress fields, or passively exploit basement structural fabric, is a much debated question, with support for either scenario in individual case studies. The Sarnu-Dandali alkaline complex, near the northwestern limit of the Deccan Traps continental flood basalt province, contains mafic to felsic alkaline volcano-plutonic rocks and carbonatites. The complex is situated near the northern end of the 600 km long, NNWeSSE-trending Barmer-Cambay rift. Mafic enclave swarms in the syenites suggest synplutonic mafic dykes injected into a largely liquid felsic magma chamber. Later coherent dykes in the complex, of all compositions and sizes, dominantly strike NNWeSSE, parallel to the Barmer-Cambay rift. The rift formed during two distinct episodes of extension, NWeSE in the early Cretaceous and NEeSW in the late Cretaceous. Control of the southern Indian Dharwar structural fabric on the rift trend, as speculated previously, is untenable, whereas the regional Precambrian basement trends (Aravalli and Malani) run NEeSW and NNEeSSW. We therefore suggest that the small-scale Sarnu-Dandali dykes and the much larger-scale Barmer-Cambay rift were not controlled by basement structure, but related to contemporaneous, late Cretaceous regional ENEeWSW extension, for which there is varied independent evidence.

  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


    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. Dyke-Davidoff-Masson syndrome

    Directory of Open Access Journals (Sweden)

    Manas R Behera


    Full Text Available A 14-month-old male child presented with recurrent generalized seizures, spastic hemiplegia, microcephaly and had developmental delay in motor and speech domains. CT of the brain revealed characteristic features diagnostic of infantile type of cerebral hemiatrophy or Dyke-Davidoff-Masson syndrome.

  8. Dyke-Davidoff-Masson Syndrome


    Zawar, Ifrah; Khan, Ashfa A.; Sultan, Tipu; Rathore, Ahsan W.


    The Dyke-Davidoff-Masson Syndrome (DDMS) results from an insult to the growing brain in utero or early infancy, which lead to loss of neurons compromising the growth of the brain. Clinical presentation includes seizures, hemiparesis, facial asymmetry, and learning disability. Radiological findings include cerebral atrophy on one side. Here, we present a case with status epilepticus who had underlying DDMS. It is a rare syndrome and uncommon cause for status epilepticus. Infections of CNS, hyp...

  9. Dyke swarms and their role in the genesis of world-class gold deposits: Insights from the Jiaodong peninsula, China (United States)

    Li, Lin; Li, Sheng-Rong; Santosh, M.; Li, Qing; Gu, Yue; Lü, Wen-Jie; Zhang, Hua-Feng; Shen, Jun-Feng; Zhao, Guo-Chun


    Jiaodong peninsula, located at the southeastern margin of the North China Craton, is well known for its rich endowment of super-large gold deposits. This region is also characterized by hundreds of intermediate - mafic dykes that cluster into several dyke swarms. In this study, we present zircon U-Pb ages which show that the timing of dyke emplacement as ca. 120 Ma, which followed the crystallization of the (quartz-) diorite porphyry at ca. 130 Ma. These ages coincide with the peak ages reported for magmatism and metallogeny in the central North China Craton. The various ages (2450-2570 and 154 Ma) from the inherited zircon crystals in these rocks suggest that substantial lower to middle crustal basement rocks and Jurassic granitoids were involved during dyke emplacement. The dyke swarms, varying between alkaline and subalkaline, are compositionally low-Si low-Ti lamprophyre and low-Si high-Ti dolerite porphyry in the western of the Jiaodong peninsula, whereas those in the eastern part are composed of both high-Si low-Ti and low-Si high-Ti lamprophyres. These features imply multiple sources for the dykes from convective asthenospheric mantle to ancient enriched lithospheric mantle with magma generation at different depths involving mixture of slab-derived hydrous fluids. The source magmas of the dykes were hydrous and enriched in volatiles as well as ore components, and their evolution occurred under high oxygen fugacity conditions. The relatively rapid emplacement and cooling of the dyke systems enabled the migration of fluids into ore-controlling faults or fractures. We envisage that the dyke swarms played an important role in generating the world-class gold mineralization of Jiaodong.

  10. Magma deformation and emplacement in rhyolitic dykes (United States)

    McGowan, Ellen; Tuffen, Hugh; James, Mike; Wynn, Peter


    Silicic eruption mechanisms are determined by the rheological and degassing behaviour of highly-viscous magma ascending within shallow dykes and conduits. However, we have little knowledge of how magmatic behaviour shifts during eruptions as dykes and conduits evolve. To address this we have analysed the micro- to macro-scale textures in shallow, dissected rhyolitic dykes at the Tertiary Húsafell central volcano in west Iceland. Dyke intrusion at ~3 Ma was associated with the emplacement of subaerial rhyolitic pyroclastic deposits following caldera formation[1]. The dykes are dissected to ~500 m depth, 2-3 m wide, and crop out in two stream valleys with 5-30 m-long exposures. Dykes intrude diverse country rock types, including a welded ignimbrite, basaltic lavas, and glacial conglomerate. Each of the six studied dykes is broadly similar, exhibiting obsidian margins and microcrystalline cores. Dykes within pre-fractured lava are surrounded by external tuffisite vein networks, which are absent from dykes within conglomerate, whereas dykes failed to penetrate the ignimbrite. Obsidian at dyke margins comprises layers of discrete colour. These display dramatic thickness variations and collapsed bubble structures, and are locally separated by zones of welded, brecciated and flow-banded obsidian. We use textural associations to present a detailed model of dyke emplacement and evolution. Dykes initially propagated with the passage of fragmented, gas-charged magma and generation of external tuffisite veins, whose distribution was strongly influenced by pre-existing fractures in the country rock. External tuffisites retained permeability throughout dyke emplacement due to their high lithic content. The geochemically homogenous dykes then evolved via incremental magma emplacement, with shear deformation localised along emplacement boundary layers. Shear zones migrated between different boundary layers, and bubble deformation promoted magma mobility. Brittle

  11. The antecryst compositional influence on Cretaceous alkaline lamprophyre dykes, SE Brazil

    Directory of Open Access Journals (Sweden)

    Saulo Gobbo Menezes

    Full Text Available The question of whether the antecryst assemblage affects the bulk composition of lamprophyre dykes, and could be responsible for the compositional zonation between their centers and borders is addressed through a detailed study involving four monchiquite and camptonite dykes (basanites and tephrites representative of the Arco de Ponta Grossa and Serra do Mar alkaline provinces. In them, antecrysts are interpreted as early-crystallized minerals that are no longer in equilibrium with their host-liquid, albeit still linked to the same magmatic system. They represent recycled crystals of earlier stages of the magmatic system at depth. The antecryst microtextures, such as zoned clinopyroxene megacrysts (augite cores and titanaugite rims with partly corroded cores, olivine crystals with corroded rims surrounded by biotite coronas, chrome-spinel inclusions in clinopyroxene and olivine megacryst cores, and titanomagnetite crystals surrounded by biotite coronas, suggest chemical re-equilibrium with the matrix. The greatest antecryst cargo in these dykes is found in their centers. After subtracting the antecryst volume from the center analyses of each body, the calculated compositions are very similar to the border analyses. The mafic antecryst cargo of each occurrence proportionally leads to enrichment of MgO, FeO, TiO2, CaO, compatible trace elements (Cr, Ni and Co, and depletion of SiO2, K2O, Na2O, Al2O3 and incompatible trace elements (Ba, Sr and REE. The whole-rock geochemical analyses of each dyke represent the combination of accumulated crystals and melt. The compositional zonation of the studied dykes is associated with the antecryst cargo rather than different magmatic pulses.

  12. Physical volcanology of the mafic segment of the subaqueous New Senator caldera, Abitibi greenstone belt, Quebec, Canada

    Energy Technology Data Exchange (ETDEWEB)

    Moore, Lyndsay N; Mueller, Wulf U [Universite du Quebec a Chicoutimi, 555 boul. du l' Universite, Chicoutimi, Quebec, G7H2B1 (Canada)], E-mail:


    Archean calderas provide valuable insight into internal geometries of subaqueous calderas. The New Senator caldera, Abitibi greenstone belt, Canada, is an Archean example of a subaqueous nested caldera with a basal stratigraphy dominated by gabbro-diorite dykes and sills, ponded magmas and basalt and andesite lava flows. The aim of our study is to focus on the use of physical volcanology to differentiate between the various mafic units found at the base of the New Senator caldera. Differentiation between these various mafic units is important from an exploration point of view because in modern subaqueous summit calders (e.g. Axial Seamount) margins of ponded magmas are often sites of VMS formation.

  13. New U-Pb ages from dykes cross-cutting the Demirci metamorphics, NW Turkey: Implications for multiple orogenic events (United States)

    Sen, Fatih; Koral, Hayrettin; Peytcheva, Irena


    A high-grade metamorphic sequence in the Sünnice Mountains, Bolu, NW Turkey, is represented by migmatitic quartzo-feldspathic gneisses in amphibolite facies called the Demirci metamorphics/paragneisses, and a sequence of low-grade meta-volcanics containing meta-andesites with minor meta-rhyolites and meta-sedimentary rocks called the Yellice meta-volcanics. They are intruded by the Dirgine granite with an age of Upper Ediacaran (576-565 Ma) and are considered a part of the İstanbul-Zonguldak Tectonic Unit. The Demirci paragneisses are also intruded by a number of dykes in various directions, traditionally considered without radiometric dating to have been emplaced in a single magmatic phase in the Eocene related to post-collision regime of the Anatolide-Tauride platform. Mafic-intermediate-felsic dykes cross-cutting the Demirci paragneisses have been investigated in maps of 1/1000 scale, and their U-Pb zircon age, major-trace element and kinematic data have been obtained. The mafics dykes cross-cutting the Yellice meta-volcanics, equivalents of those in the Demirci paragneisses, occur in N400-500E orientations and have calc-alkaline basalt compositions with a subduction signature. The intermediate dykes occur in N650W, N800W orientations and have calc-alkaline basaltic andesite to andesitic compositions with a subduction signature. Some felsics occur in N150W and N800E orientations and have calc-alkaline dacitic compositions with a collisional tectonic setting. Other calc-alkaline granitic dykes occur in N750E orientation and calc-alkaline granitic compositions with a subduction signature. U-Pb LA-ICP-MS dating of zircons yield ages from 485.7±3.6 Ma (i.e. Cambro-Ordovician) for N800E trending dacite dykes; 443.0±5.4 Ma (i.e. Ordovician-Silurian) for N150W trending dacite dykes; 301.0±1.6 Ma (i.e. Upper Pennsylvanian-Carboniferous) for N650W trending basaltic andesite dykes; 268.2±2.4 Ma (i.e. Guadalupian-Permian) for N40-500E trending basalt dykes; 262.9±3

  14. Determining dyke-propagation paths at Santorini volcano, Greece (United States)

    Drymoni, Kyriaki; Browning, John; Lecoeur, Nora; Gudmundsson, Agust


    The volcanic Island of Santorini constitutes a complex of collapse calderas which has experienced a range of explosive and effusive volcanic eruptions and is still active. Numerous stratigraphic horizons which constitute the upper part of the volcano have widely different mechanical properties, resulting in local stresses that may act as dyke-traps, preventing the dykes from reaching the surface to erupt. Several caldera collapses (<3.6 ka) have exposed part of the stratigraphy and a dyke swarm (composed of at least 63 dykes, many arrested and some feeders) within a section of the northern caldera wall, allowing detailed examination. This ongoing study will (1) document the petrological and structural characteristics of feeder and non-feeder (arrested) dykes and estimate their frequency; (2) determine the physiochemical and mechanical conditions that control dyke arrest/dyke penetration at contacts between layers; (3) explore the fluid and mechanical conditions of the associated magma chamber(s) that must be satisfied for chamber rupture and dyke injection to occur; (4) make numerical and probabilistic models as to the likely dyke paths in heterogeneous and anisotropic crustal segments/volcanoes (such as Santorini), including the likelihood of injected dykes reaching the surface during an unrest period in a volcano of a given type; (5) compare the data collected from Santorini with existing data on dykes worldwide, particularly those on dykes in Tenerife and Iceland. The principal aim of the study is to provide models that, during an unrest period in Santorini and other similar volcanoes, allow us to forecast (a) the condition for magma-chamber rupture and dyke injection, and (b) the likely path of the resulting dyke. The latter includes assessment of the likelihood as to dyke arrest versus dyke propagation to the surface, the latter resulting in an eruption. For dyke-fed eruptions, the study will also provide methods for forecasting the likely volumetric flow

  15. Mineralogical and chemical character- istics of newer Dolerite Dyke around Keonjhar, Orissa: Implication for hydrothermal activity in subduction zone setting

    Indian Academy of Sciences (India)

    Piyali Sengupta; Arijit Ray; Sayantani Pramanik


    The newer dolerite dykes around Keonjhar within the Singbhum Granite occur in NE–SW, NW–SE and NNE–SSW trends. The mafic dykes of the present study exhibit several mineralogical changes like clouding of plagioclase feldspars, bastitisation of orthopyroxene, and development of fibrous amphibole (tremolite–actinolite) from clinopyroxene, which are all considered products of hydrothermal alterations. This alteration involves addition and subtraction of certain elements. Graphical analyses with alteration index and elemental abundances show that elements like Rb, Ba, Th, La and K have been added during the alteration process, whereas elements like Sc, Cr, Co, Ni, Si, Al, Fe, Mg and Ca have been removed. It is observed that in spite of such chemical alteration, correlation between major and trace elements, characteristic of petrogenetic process, is still preserved. This might reflect systematic alteration (addition or subtraction) of elements without disturbing the original element to element correlation. It has also been established by earlier workers that the evolution of newer dolerite had occurred in an arc-back arc setting which may also be true for newer dolerites of the present study. This is evident from plots of pyroxene composition and whole rock composition of newer dolerite samples in different tectonic discrimination diagrams using immobile elements. The newer dolerite dykes of the Keonjhar area may thus be considered to represent an example of hydrothermal activity on mafic rocks in an arc setting.

  16. Vegetation dynamics and erosion resistance of sea dyke grassland

    NARCIS (Netherlands)

    Sprangers, H.


    In the Netherlands, in addition to the width and height of the dyke body itself, renewed measures for reconstruction and maintenance of dykes have stressed the importance to the safeguarding of the dyke, of the grass cover's protection of the clay-layer against erosion.From an inventory of different

  17. Dyke belt in North Western margin of Siberian platform (United States)

    Ryabov, Victor


    The Early Triassic dyke swarm belt is strengthening at about 500 km (50-60 km width) along the northern margin of Siberian platform. Its locations is limited from the north by rift structure of the Yenisei-Khatanga trough, and from the south by Kystyktah-Ayan-Ambardah uprise. From west to east dyke belt is crossing Norilsk, Kamensky and Maimecha-Kotuiskaya province. In this direction the composition of dykes changing from basic rthrough alkali-basic to alkali-ultramafic varieties. The sickness of dykes varies from 0,5-10m up to 90 m and length - from 5-15 m to 140 km. The orientation of the bulk of dykes coincides with the direction of the major structural and tectonic lineaments of the region. Dykes are often refer to the fault planes. The composition of dykes may vary along the stretches. The density of hundreds of basaltic dykes and small intrusions in the dyke swarms is not permanent and sometimes essentially uneven. They form a compact dyke swarms of dykes, stocks and sub layering bodies veins. In dyke package on Huor-Uyallah river here are 20 subparallel dykes of different chemical composition at a distance of 2 km Lack of the evidence of the dykes crossing allows to assume their formation during the same magmatic cycle. Dykes cross cut through the entire incision of basaltic plateau. They did not created the flow effusions and refer to the final stage trap magmatism. Ar/Ar age of the youngest dykes in the province Kamenska 238-247 Ma. the age of lamproite dyke in Norilsk province is 235 Ma. Dykes are represented by dolerites trachydolerites, syenites, minettes, lamprophyres (camptonite, spessartite, vosgesite), avgitites, melanephelinite, alnoites, limburgites alkaline picrites, meimechites. Their content vary widely: SiO2 - 35,7-62,6; TiO2- 0,4-7,5; Al2O3- 4,4-17,5; Fe2O3- 4,6-20,6; MnO- 0,08-0,44; MgO- 0,8-31,5; CaO- 0,7-15,4; Na2O- 0,01-6,5; K2O 0,8-5,3 wt.%; P2O5 0,1-1,2 wt.%. The h Bolsheavamskaya volcanic basin in Kamensky province is most abundant in

  18. Dyke Davidoff Masson Syndrome: A case report

    Directory of Open Access Journals (Sweden)

    More Sumeet S, Jadhav Aravinash L, Garkal Shailendra M, Tewari Suresh C


    Full Text Available Dyke-Davidoff-Masson syndrome (DDMS is characterized by seizures, facial asymmetry, contralateral hemiplegia and mental retardation. The characteristic radiologic features are cerebral hemiatrophy with homolateral hypertrophy of the skull and sinuses. We report a case of DDMS in an 18years old girl who presented with a history of generalized tonic – clonic seizures, hemiparesis and hemiatrophy of right side.

  19. The Origin and Emplacement Patterns of Paleoproterozoic (2.5-1.8 Ga) Mafic/Ultramafic Magmatism (United States)

    Heaman, L. M.


    The Paleoproterozoic (~2.5-1.8 Ga) is a stunning period in Earth history where numerous giant dyke swarms were emplaced and can be found transecting Archean cratons worldwide, the onset of this magmatism follows closely the ~2.6 Ga stabilization of most Archean cratons. Based on conservative estimates, the volumes of basaltic magma produced in the Paleoproterozoic rival more contemporary LIPs (>1M km3). Often, only the root zones of Paleoproterozoic LIPs are preserved (mafic/ultramafic dyke swarms and layered mafic intrusions) so the complete erosion of the volcanic succession in many examples, the repeated emplacement of dyke swarms into zones of crustal weakness over protracted periods of time (e.g. >400 m.y.), and the fact that some Proterozoic LIPs are metamorphosed adds to the difficulty in deciphering their origin. Equally challenging is determining whether this period of voluminous outpouring of mafic magma could be the cause of several unique features of Earth evolution at this time; for example, triggering extreme climate conditions, initiating the abrupt increase in the oxygen content of the atmosphere, promoting the rapid production of banded iron formations, causing isobaric high-grade metamorphism and granite production in the lower crust, and contributing to substantial downward continental root growth through underplating of basaltic magma. Based on a compilation of more than 200 precise and accurate U-Pb dates for Paleoproterozoic mafic/ultramafic dykes, sills and layered mafic intrusions, at least three synchronous global events (~2.50, 2.23, and 1.88 Ga) and two active mantle-derived magmatic cycles can be identified in the period 2.5-1.8 Ga (2520-2370 and 2230-1820 Ma), with a distinct hiatus between 2370 and 2230 Ma. The beginning of each magmatic cycle is represented by the widespread emplacement of dyke swarms on several cratons within a relatively short time interval of ~20 m.y. In the Superior Province, Canada the onset of 2.52-2.50 Ga mafic

  20. Three Paleoproterozoic A-type granite intrusions and associated dykes from Kainuu, East Finland

    Directory of Open Access Journals (Sweden)

    Hannu Huhma


    Full Text Available Mafic and felsic intrusive rocks aged 2.5–2.4 Ga have been observed over a large area in eastern and northern Finland, as well as in adjacent northwestern Russia. We describe three granite intrusions and associated dykes from Kainuu, Finland, that belong to this bimodal magmatic event. All these three granites show clear A2-type chemical affinities with high Y/Nb, HREE, Fe/Mg, Ga and Nb. Two of the intrusions, Rasinkylä and Pussisvaara, were dated at 2425±3 and 2427±3 Ma, respectively, using thermal ionisation mass spectrometry utilizing the chemical abrasion method (CA-TIMS. CA-TIMS ages are supported by single-grain age determinations obtained by using Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICPMS. New data on the previously described Rasimäki granite from southern Kainuu is presented, including an age of 2389±5 Ma obtained with LA-MC-ICPMS. The variable magnetite content of the granites is proposed to reflect the differences in the oxidation state of the source, which in our interpretation is the local Archean lower crust. Partial melting and the emplacement of the granites occurred in an extensional environment. Heat for the partial melting was provided by mafic magmas under and intraplating the extended crust.

  1. Engineering geotextile tubes to be used in dyke construction

    Institute of Scientific and Technical Information of China (English)

    ZHU Yuan-sheng


    Geotextile tube technology has been increasingly used in dykes. In this work reinforcement theory and circle method were employed to examine the allowable tensile limit of the geotextile tube and the stability factor of the slip surface of the dyke. The formulas to calculate the layer-to-layer spacing and size of geotextile tubes applied to double prism dykes were deduced. The application of these formulas was illustrated by several examples. The calculation results indicate that unequal spacing arrangement is more economical than equal spacing and the layer number of required geotextile tubes decreases with the increase of allowable tensile strength of the geotextile.

  2. Petrological, geochemical and isotopic investigations on a carbonate-dyke and enclosed pyroxenite xenoliths from Val Mastallone (Ivrea-Verbano Zone): evidence of a cumulate carbonatite in the lower crust? (United States)

    Galli, Andrea; Grassi, Daniele; Burg, Jean-Pierre; Schwab, Leo; Rickli, Jörg; Gianola, Omar


    The Ivrea-Verbano Zone (Italy/Switzerland) represents one of the best exposed mantle-crust sections worldwide. Its geological evolution has been governed by the Permian underplating of mantle-derived basic magmas („Mafic Complex") into the high-grade basement of the Southern Alps. In the Ivrea-Verbano Zone, marbles occur as concordant bodies or partly discordant carbonate-dykes. Generally, these dykes are constituted of calcite, diopside, scapolite, contain enclave of the host rocks and display sharp contacts to the host lithologies without evidences of alteration zones. In Val Mastallone, an up to 40 m thick carbonate-dyke with different characteristics occurs within mafic granulites. This dyke is composed of calcite, clinopyroxene and subordinate allanite and zircon. No scapolite is observed. The contacts to the host granulites are characterized by alteration zones composed of actinolite, chlorite, clinozoisite, plagioclase and calcite. The carbonate-dyke bears enclave of phlogopite-amphibole-apatite-rutile-ilmenite ± garnet or spinel clinopyroxenites. These rock type is not outcropping elsewhere in the proximity of the dyke, suggesting a significant transport. Host mafic granulite enclave are found exclusively at the margin of the dyke. Calcite dykelets rich in zircon, baddeleyite and other Ba, U, Th, REE-rich phases cut across the enclave. The carbonate-dyke shows an enrichment of LREE over HREE ((La/Yb)N = 14), with a Σ REE = 338 and Y/Ho = 27. On the chondrite-normalized REE abundances diagram, no Eu anomaly is observed. Mantle-normalized pattern shows strong negative anomalies at Cs, Rb, K, Pb, P, Zr, Hf, Ti and positive Ba, Th, Sr, Nd anomalies, similarly to the "world average carbonatites". Measured absolute trace element concentrations are lower than average carbonatites but significantly higher than typical limestones and similar to cumulate carbonatites found elsewhere in the world (e.g. India, China, Brazil). Grt-bearing clinopyroxenite enclave

  3. Link between extension, dyking and subsidence as the reconstruction tool of intraplate rifting mechanism (backstripping data, modelling and geochronology) (United States)

    Polyansky, Oleg P.; Reverdatto, Vladimir V.; Babichev, Alexey V.


    predicted over a range of γ = 0.01-0.12 (average value 0.06, i.e., ~6%). This value seems acceptable if we consider cumulative sills and dykes thicknesses in the outcrops and borehole sections in the Upper Devonian-Lower Carboniferous sediments. For better rifting age resolution we sampled dolerites from the western, eastern and central dyke swarms, which were then analyzed by stepwise 40Ar/39Ar heating. The determined ages of the monomineral plagioclase and pyroxene fractions from the dyke samples (345-378 Ma) coincide well with fast subsidence and sedimentation phase in the Devonian defined by backstripping data. Dating of dyke intrusions shows following succession of rifting episodes: extension by dyking starts near the central axes (364-378 Ma), extending to the periphery areas on the opposite sides of the rift through (345-362 Ma). The established sequence of dyke intrusions is alternative to the oceanic crust spreading, at which the age of intrusions is increased from the spreading center in rift-perpendicular direction. Results of thermo-mechanical modelling of formation of the Vilyui rift basin are presented. The goal of 2D finite elements modelling is to demonstrate that the deep troughs can form in the continental crust under its limited extension. The 2D numerical simulation shows the possibility of considerable localized subsidence near the rift axis and explains why mafic dike swarms are concentrated on the basin periphery. Thus, the mechanism of the Vilyui basin formation by means of magmatic rifting is corroborated [Polyansky at al., 2013]. RFBR grant no. 14-05-00188.

  4. Evolution of the Antarctic Peninsula lithosphere: Evidence from Mesozoic mafic rocks (United States)

    Riley, T. R.; Curtis, M. L.; Flowerdew, M. J.; Whitehouse, M. J.


    New geochronology from a thick (> 800 m) basaltic succession along the eastern margin of the Antarctic Peninsula confirm a Middle Jurassic age (178 ± 1 Ma). This marginally postdates the adjacent Ferrar large igneous province of the Transantarctic Mountains and predates the extensive silicic volcanism of the Mapple Formation (~ 170 Ma) of the Antarctic Peninsula. The geochemistry of other rare, but broadly contemporaneous, basaltic successions of the Antarctic Peninsula, along with Cretaceous-age mafic dykes, are used to interpret the influences of lithospheric and asthenospheric mantle sources during the Mesozoic. Two significant high magmatic addition rate events occurred along the Antarctic Peninsula continental margin at 170 and 110 Ma and can be correlated to events along the South American Cordillera. These 'flare-up' events are characterised by extensive silicic (mostly ignimbrite) volcanism of the Chon Aike Province (V2 event: 170 Ma) and significant granitoid batholith emplacement of the Lassiter Coast intrusive suite (110 Ma). The 170 Ma event is exposed across large parts of the northern Antarctic Peninsula, whilst the 110 Ma event is more widespread across the southern Antarctic Peninsula. The basaltic volcanism described here precedes the 'flare-up' event at 170 Ma and has geochemical characteristics that indicate a thickened lithosphere prevailed. A major dyke swarm that followed the 170 Ma event indicates that extensive lithospheric thinning had occurred, which allowed the ascent of depleted mafic melts. The thinning was the direct result of widespread lower crustal/upper lithospheric melting associated with the silicic volcanism. In the southern Antarctic Peninsula, the lithosphere remained over thickened until the emplacement of the major batholiths of the Lassiter Coast intrusive suite at 110 Ma and was then immediately followed by the emplacement of more asthenosphere-like melts indicating extensive lithospheric thinning.

  5. Petrology, geochemistry of hornblende gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting

    Indian Academy of Sciences (India)

    Aditi Mandal; Arijit Ray; Mayukhee Debnath; Sankar Prasad Paul


    Paharpur gabbroic intrusive is an arcuate body running east–west paralleling the foliation of Chhotanagpur Granite Gneiss which acts as country rock. The main gabbroic body is intruded by a number of dolerite dykes running north–south. It is composed of clinopyroxene (Wo48En40Fs12–Wo51En40Fs09, mg no. 72–82), plagioclase (An52–An90), hornblende (magnesian hornblende to ferro-tschermackite), orthopyroxene (En76–En79) and ilmenite. Hornblende occurs as large poikilitic grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65–73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fractionation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogical and chemical compositions of intrusive rocks also have some similarity with mafic rocks of ophiolite complex of subduction zone.

  6. Peridotites and mafic igneous rocks at the foot of the Galicia Margin: an oceanic or continental lithosphere? A discussion

    Energy Technology Data Exchange (ETDEWEB)

    Korprobst, J.; Chazot, G.


    An ultramafic/mafic complex is exposed on the sea floor at the foot of the Galicia Margin (Spain and Portugal). It comprises various types of peridotites and pyroxenites, as well as amphibole-diorites, gabbros, dolerites and basalts. For chronological and structural reasons (gabbros were emplaced within peridotites before the continental break-up) this unit cannot be assigned to the Atlantic oceanic crust. The compilation of all available petrological and geochemical data suggests that peridotites are derived from the sub-continental lithospheric mantle, deeply transformed during Cretaceous rifting. Thus, websterite dykes extracted from the depleted MORB mantle reservoir (DMM), were emplaced early within the lithospheric harzburgites; subsequent boudinage and tectonic dispersion of these dykes in the peridotites, during deformation stages at the beginning of rifting, resulted in the formation of fertile but isotopically depleted lherzolites. Sterile but isotopically enriched websterites, would represent melting residues in the peridotites, after significant partial melting and melt extraction related to the thermal erosion of the lithosphere. The latter melts are probably the source of brown amphibole metasomatic crystallization in some peridotites, as well as of the emplacement of amphibole-diorite dykes. Melts directly extracted from the asthenosphere were emplaced as gabbro within the sub-continental mantle. Mixing these DMM melts together with the enriched melts extracted from the lithosphere, provided the intermediate isotopic melt-compositions - in between the DMM and Oceanic Islands Basalts reservoir - observed for the dolerites and basalts, none of which are characterized by a genuine N-MORB signature. An enriched lithospheric mantle, present prior to rifting of the Galicia margin, is in good agreement with data from the Messejana dyke (Portugal) and more generally, with those of all continental tholeiites of the Central Atlantic Magmatic Province (CAMP

  7. Local stresses, dyke arrest and surface deformation in volcanic edificesand rift zones

    Directory of Open Access Journals (Sweden)

    L. S. Brenner


    Full Text Available Field studies indicate that nearly all eruptions in volcanic edifices and rift zones are supplied with magma through fractures (dykes that are opened by magmatic overpressure. While (inferred dyke injections are frequent during unrest periods, volcanic eruptions are, in comparison, infrequent, suggesting that most dykes become arrested at certain depths in the crust, in agreement with field studies. The frequency of dyke arrest can be partly explained by the numerical models presented here which indicate that volcanic edifices and rift zones consisting of rocks of contrasting mechanical properties, such as soft pyroclastic layers and stiff lava flows, commonly develop local stress fields that encourage dyke arrest. During unrest, surface deformation studies are routinely used to infer the geometries of arrested dykes, and some models (using homogeneous, isotropic half-spaces infer large grabens to be induced by such dykes. Our results, however, show that the dyke-tip tensile stresses are normally much greater than the induced surface stresses, making it difficult to explain how a dyke can induce surface stresses in excess of the tensile (or shear strength while the same strength is not exceeded at the (arrested dyke tip. Also, arrested dyke tips in eroded or active rift zones are normally not associated with dyke-induced grabens or normal faults, and some dykes arrested within a few metres of the surface do not generate faults or grabens. The numerical models show that abrupt changes in Young's moduli(stiffnesses, layers with relatively high dyke-normal compressive stresses (stress barriers, and weak horizontal contacts may make the dyke-induced surface tensile stresses too small for significant fault or graben formation to occur in rift zones or volcanic edifices. Also, these small surface stresses may have no simple relation to the dyke geometry or the depth to its tip. Thus, for a layered crust with weak contacts, straightforward

  8. Geochronological Significance of the Post-Orogenic Mafic-Ultramafic Rocks in the Hongqiling Area of Jilin Province, Northeast China

    Institute of Scientific and Technical Information of China (English)

    Zhang Guangliang; Wu Fuyuan


    Mafic-ultramafic complexes are widespread in Hongqiling in central Jinlin Province, NE China. The Hongqiling complex comprises pyroxenite, olivine websterite, lherzolite, gabbro and leucogabbro. Based on the latest geochronological results, these intrusions yield a zircon U-Pb age of about 216 Ma, implying that they emplaced in the late Triassic period and that the mafic-ultramafic complexes are post-orogenic in origin. These ages are coeval with the emplacement of A-type granites in the area, but slightly younger than syn-orogenic granitic magmatism. The composition of the parent magma during the equilibration of the accumulated olivine crystallizing is basaltic with high MgO, and it comes from depleted or juvenile lithospheric mantle, according to the results of trace elements and Sr-Nd isotopes. Amount of crustal material contaminated the magmatic source, implying that the mafic-ultramafic rocks originate from the mixing product of crust- and mantle-derived magma. Therefore, the magmatic source was contaminated by an amount of crustal material, and the subsequent crystal fractionation resulted in the Cu-Ni mineralization. Dynamic analyses indicate that, after the collision of the North China Craton and Jiamusi Massif, the lithospheric delamination during post-orogeny resulted in the upwelling of asthenosphere and the intruding of the mass and underplating of the mantle-derived magma, which led to the formation of the primary mafic-ultramafic magma due to the mantle-derived magma and partial melting of the lower crust. This result suggests that the mafic-ultramafic complexes belong to a typically important magmatic suite that evolved during post-orogenic processes, and they were exposed as maficultramafic dyke swarms that existed in the post-orogenic extension environment. It also implies that the orogenic process finished ultimately in this regional geological setting. The widespread occurrence of mafic-ultramafic complexes in the Xing'an-Mongolia Orogenic Belt

  9. Dyke Foundation Classification%关于堤基的分段分类

    Institute of Scientific and Technical Information of China (English)

    ZHANG; Jia-fa


    Main factors, which should be considered in the classification of dyke foundation, are discussed in this paper. Engineering conditions should be taken into accoun t when the levee safety is appraised based on engineering geologic appraisement and classificatio n. On the basis of safety appraisement, dyke foundation may be classified with regard of suitable r einforcement measures. Examples are presented to illustrate the instructive significance of dyke foundation cla ssification to dyke reinforcement design.

  10. Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr-Nd-O isotope systematics of ultramafic dykes

    Indian Academy of Sciences (India)

    Abhijit Roy; A Sarkar; S Jeyakumar; S K Aggrawal; M Ebihara; H Satoh


    Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; ( HREE)N ∼ 2-3 times chondrite, (Gd/Yb)N∼1). The Nd(t) values vary from +1.23 to −3.27 whereas 18O values vary from +3.16‰ to +5.29‰ (average +3.97‰ ± 0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (±silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable Nd, low Sri(0.702) and low 18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.

  11. Temporal geoelectric behaviour of dyke aquifers in northern Deccan Volcanic Province, India

    Indian Academy of Sciences (India)

    Gautam Gupta; Vinit C Erram; Suyash Kumar


    Vertical electrical resistivity soundings (VES) were carried out over four major dykes of Nandurbar district in the northern Deccan Volcanic Province (DVP) of Maharashtra to investigate the subsurface geological conditions, with an aim of identifying zones with groundwater resource potential. Dykes can act as pathways or barrier to the groundwater flow depending upon the intensity of fracturing in the dyke rock. Whether the dykes act as water conduits or as barriers depends on their structure, location and orientation with respect to the groundwater flow. The Nandurbar district is known for occurrence of dykes and dyke swarms. A total of 33 dykes were demarcated in the study region and four major dykes (D4, D5, D6, and D7) from these were chosen for detailed VES studies. Data were acquired over these four dykes during pre-monsoon and post-monsoon periods to observe the seasonal variation in groundwater movement. These studies revealed changes in field characters, their attitudes, thickness and structure of the dykes. Longitudinal geoelectrical sections along these dykes demonstrated carrier as well as barrier stretches which identified potential aquifers up to depths of 25–30m below which hard and compact rock exists. These studies also indicated that dykes with sufficient width, length and favourable hydrogeological structure form potential aquifers for the occurrence and movement of groundwater in the study area.

  12. Mafic Plinian eruptions: Is fast ascent required? (United States)

    Szramek, Lindsay Ann


    It has been hypothesized that for a Plinian eruption of mafic magma to occur, that magma must ascend rapidly from the chamber to cause it to fragment into a jet containing juvenile and nonjuvenile tephra. To determine how fast mafic Plinian magmas need to travel to the level of fragmentation, a number of decompression experiments were carried out on two hydrous mafic magmas, and the results are compared to the products of two well-documented mafic Plinian eruptions: the basaltic andesite Fontana eruption of Masaya (Nicaragua) and the hawaiite 122 B.C. eruption of Etna (Italy). Comparison of natural and experimental textures shows that the Fontana eruption can be replicated in the lab at decompression rates between 0.1 MPa s-1 and 0.2 MPa s-1. This decompression rate is faster than any previously determined experimentally rate for more silicic eruptions. The hawaiite was unable to be reproduced in the lab. The natural groundmass is highly crystalline, which would have raised the viscosity of the initial melt by 1-2 orders of magnitude, which may not be enough to cause fragmentation.

  13. Wave overtopping erosion tests at Groningen sea dyke

    NARCIS (Netherlands)

    Akkerman, G.J.; van Gerven, K.A.J.; Schaap, H.A.; van der Meer, J.W.


    In the present report field erosion tests of the inner slope of a sea dyke in the province of Groningen (near Delfzijl) are described for the situation of severe wave overtopping. Three types of tests have been performed: tests at the present grass cover, tests at a reinforced grass cover and tests

  14. Age and origin of fluorapatite-rich dyke from Baranec Mt. (Tatra Mts., Western Carpathians: a key to understanding of the post-orogenic processes and element mobility

    Directory of Open Access Journals (Sweden)

    Gawęda Aleksandra


    Full Text Available On the southeastern slope of the Baranec Mount in the Western Tatra Mountains (Slovakia an apatite-rich pegmatite-like segregation was found in the subvertical fault zone cutting metapelitic rocks. Two zones: felsic (F and mafic (M were found, differing in mineral assemblages and consequently in chemistry. Fluorapatite crystals yield a LA-ICP-MS U-Pb age of 328.6 ± 2.4 Ma. A temperature decrease from 634 °C to 454 °C at a pressure around 500 to 400 MPa with oxygen fugacity increasing during crystallization are the possible conditions for formation of the pegmatite-like segregation, while secondary alterations took place in the temperature range of 340 – 320 °C. The Sr-Nd isotope composition of both apatite and whole rock point toward a crustal origin of the dike in question, suggesting partial melting of (P, F, H2O-rich metasedimentary rocks during prolonged decompression of the Tatra Massif. The original partial melt (felsic component was mixed with an external (F, H2O-rich fluid, carrying Fe and Mg fluxed from more mafic metapelites and crystallizing as biotite and epidote in the mafic component of the dyke.

  15. Dyke emplacement and propagation: a new laboratory approach (United States)

    Bakker, R.; Benson, P.; Vinciguerra, S.


    It is well known that magma ascends trough the crust by the process of dyking. During dyke movement crustal rocks, specifically the volcano's basement rocks, fracture due to the stress imposed by the ascending magma, thus providing conduits for magma transport. Dykes are frequently seen in the field and have been reproduced via numerical and analogue studies. However, a number of assumptions regarding rock mechanical behavior frequently has to be made as such data are very hard to directly measure at the pressure/temperature conditions of interest: high temperatures at relatively shallow depths. Such data are key to simulating the magma intrusion dynamics through the lithologies that underlie the volcanic edifice. To bridge this gap, we have performed a suite of rock deformation experiments in both compressive and tensile regimes, using a Paterson-type triaxial apparatus on representative lithologies present in the basement of Mt. Etna. In the compressive regime, we find that sedimentary rocks present at a depth of aproximately 2 km show a B/D transition at around 300 degC and significant weakening with temperatures exceeding 400 degC. Volcanic rocks (basalt) shows a significant change in deformation behavior only at temperatures exceeding 800 degC. Such a large contrast in mechanical properties could be favorable for dyke deflection or dyke arrest. However, in the tensile regime, it remains a significant technical challenge to precisely reproduce the conditions of dyking in the lab. As a starting point, we are now testing an analogue material to replace the magma to avoid such high temperatures, relying of maintaining similar temperature/viscosity ratios between magma/country rock in the laboratory and the field. We chose PMMA (a.k.a. plexiglass) for this task as it displays a large range in viscosities (log(visc)range = 10 - 1) with temperatures between 100 and 300 degC, making it an excellent analogue material. In addition PMMA solidifies after the sample cools

  16. Geochemistry of eastern North American CAMP diabase dykes (United States)

    Callegaro, S.; Marzoli, A.; Bertrand, H.; Reisberg, L. C.; Chiaradia, M.; Bellieni, G.


    The Piedmont area of the Appalachians and the coastal plains of eastern North America (ENA) were intruded between 202 and 195 Ma[1,2] by swarms of diabase (dolerite) dykes and a few sills of the Central Atlantic magmatic province (CAMP). Different trends are observed for the dykes, from NW (more frequent in the South), to N-S and NE, thus a radial pattern and a coeval emplacement driven by the bulging effect of a mantle plume were initially inferred[3]. Conversely, based on field data, an age progression can be defined from NW- to N- and NE-oriented dykes, supporting a diachronous northward rift-to-drift transition during Pangaea break-up. New geochemical data on 74 ENA dykes suggest a dominant lithospheric mantle source for these magmatic suite. A deep enriched mantle source is further discarded by the crystallization temperatures (ca.1350°C) calculated[4] for high-Fo (up to Fo89) olivines, which do not reflect very hot (i.e. mantle-plume) potential mantle temperatures. Incompatible trace element contents are fairly homogeneous and generally low, e.g. (La/Yb)Ch (0.54-2.39), typical of melts derived from a quite depleted shallow (spinel) mantle-source. However, isotopic compositions of ENA dykes display a considerable spread in initial isotopic signatures, do not show correlation with incompatible trace elements, and are independent of the orientation and age of the dykes, i.e. 87Sr/86Sr200Ma (0.7043-0.7088), ɛNd200Ma (-6.67-+2.42) and 206Pb/204Pb200Ma (17.46-18.61). Pb isotopic compositions plot above the NHRL, at positive Δ7/4 (10-17) and Δ8/4 (19-73), calling for an enriched ancient component in the mantle source. Unradiogenic 187Os/188Os200Ma ratios (0.127-0.144) argue for negligible amounts of crustal contamination and, coupled with the large range of Sr-Nd-Pb isotopic compositions, suggest generation from a strongly heterogeneous mantle source, probably metasomatized lithosphere. Lithospheric mantle underlying the Appalachian orogen may have undergone

  17. New paleomagnetic data from 1.80-1.75 Ga mafic intrusions of Fennoscandia and Sarmatia: implications for the late Paleoproterozoic paleogeography of Baltica and Laurentia (United States)

    Pisarevsky, Sergei; Lubnina, Natalia; Sokolov, Svetoslav; Bogdanova, Svetlana


    A series of recently dated 1.78-1.75 Ga lamprophyre and shoshonite intrusions are exposed north of Ladoga Lake in southern Karelia (Fennoscandia). We carried out a paleomagnetic study of these intrusions and an additional study of the coeval gabbro-dolerite Ropruchey sill near the Onega Lake. All studied rocks carry a stable primary remanence supported by positive contact tests. We also studied 14 mafic dykes and 1 mafic sill from Ukrainian shield (Sarmatia). Most of these intrusions have been dated or re-dated recently by U-Pb (baddeleyite) method at 1.80-1.75 Ga. Ukrainian dykes also carry a consistent stable bipolar remanence. Two positive contact tests suggest that this remanence is primary. A comparison of new and previously published paleomagnetic data shows a significant difference between Fennoscandian and Sarmatian 1.80-1.75 Ga paleopoles. This implies that the final assemble of Baltica by docking of Volgo-Sarmatia and Fennoscandia occurred after 1.75 Ga. Consequently these two parts of Baltica should be considered as independent blocks in pre-1.75 Ga paleogeographic reconstructions. Using late Paleoproterozoic paleomagnetic data from Laurentia together with geological constraints we have built a new kinematic paleogeographic model for Laurentia and Baltica in the Statherian.

  18. Effect of host-rock rheology on dyke shape, thickness and magma overpressure (United States)

    Vachon, Rémi; Hieronymus, Christoph F.


    The size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport, and because large numbers of dykes often comprise a major proportion of the volcanic edifice and of the underlying crust. Standard elastic models predict dyke geometry to be elliptic in cross-section for constant overpressure and uniform host-rock properties, whereas observations show that dyke thickness is typically more nearly constant with a sharp taper at the ends. Moreover, the predicted overpressures required to inflate dykes in a purely elastic medium are often significantly higher (>150 MPa and up to 2 GPa) than those estimated by other means (about 1-50 MPa). In this study, we use 2-D finite element models to test whether other host-rock rheologies lead to more realistic dyke shapes and overpressures. We examine three different rheologies, each of which is affected by the presence of the dyke itself: (1) elasticity with reduced moduli in regions of low pressure or tension; (2) elastoplasticity with plastic failure in the high-stress regions surrounding the dyke tips; (3) viscoelasticity with a viscosity decrease due to heating by the dyke. We use rheological parameters obtained from laboratory experiments whenever possible, and assume static conditions for the final dyke shape. We find that all three rheologies tend to make the dyke more rectangular relative to the elliptical dykes of the linearly elastic models. The change in shape is due to enhanced deformation in the high-stress zone surrounding the dyke tip. We also find that the overpressure required to inflate an initially thin dyke to a given thickness is reduced for all three rheologies. The greatest decrease in overpressure by a factor of about 0.1 is observed for the elastoplastic model, and for the viscoelastic model if the dyke intrudes into moderately pre-heated host-rock. We discuss our results with respect to dyke observations from Rum Island (Scotland


    Directory of Open Access Journals (Sweden)

    Biswadev Basu


    Full Text Available ABSTRACT : Dyke – Davidoff – Masson Syndrome (DDMS, is a rare clinical condition characterized by clinical triad of seizures, contralateral spastic hemiplegia or hemiparesis, with or without mental retardation. Diagnosis requires presence of cerebral hemiatrophy with homolateral hypertrophy of the skull and sinuses on brain imaging. Here we report a case of DDMS in a 16 year old girl who presented with seizures and hemiparesis.MRI of her brain showed hemiatrophy involving the left cerebral hemisphere with enlargement of ipsilateral sinuses and ventricles

  20. Dyke Monitorin by the Means of Persistent Scattering Interferometry at the Coast of Northern Germany (United States)

    Seidel, M.; Marzahn, P.; Ludwig, R.


    40 percent of the world's population are presently living in coastal areas or along the main rivers. Taking into account that the vulnerability of these areas is increasing due to sea-level rise and coastal hazards such as storm surges or extreme rainfall events accompanied with floods, the importance of safety structures such as dykes is increasing as well. Hence, a spatial distributed dyke monitoring should be part of a sustainable adaptation strategy. Due to increasing amount of SAR-data from various satellites with high spatial and temporal coverage, the means of SAR-interferometry could be an essential tool to ensure this kind of required monitoring. Given this prospect, Persistent Scattering Interferometry (PSI) will be a very suitable monitoring technique for dyke structures to identify dyke movement with the accuracy of few millimetres. This procedure focuses on pixels that show a stable scattering behaviour in a sequence of multiple SAR-scenes. In opposition to ground-measurements, the spatial coverage of this technique provides comparable results for different parts of the dyke; furthermore weak segments with particular high movements could be identified in advance. This could prevent future dyke crevasses and help to reduce risks in high-populated areas. This paper attempts to describe the potential of the PSI technique for a spatial distributed dyke monitoring at the coast in northern Germany. 21 ERS-2 scenes and 16 Envisat ASAR scenes were analysed. Those Scenes cover an area of a sea shore dyke including a flood regulation barrage and results point out the potential for this technique to monitor dyke structures. Even though the observed dyke doesn't show any significant deformation rates, the two datasets show the same signal for the whole dyke.

  1. Mafic and related complexes in Galicia: an excursion guide

    NARCIS (Netherlands)

    Arps, C.E.S.; Calsteren, van P.W.C.; Hilgen, J.D.; Kuijper, R.P.; Tex, den E.


    In Galicia occur several polymetamorphic complexes that contain mafic and ultramafic rocks. Mafic volcanics and gabbros are situated in or near the complexes. An episode of metamorphism and granitization encompassed the generation of the volcanics and gabbros. Gravity surveys revealed the existence

  2. Fault-related carbonate breccia dykes in the La Chilca area, Eastern Precordillera, San Juan, Argentina (United States)

    Castro de Machuca, Brígida; Perucca, Laura P.


    Carbonate fault breccia dykes in the Cerro La Chilca area, Eastern Precordillera, west-central Argentina, provide clues on the probable mechanism of both fault movement and dyke injection. Breccia dykes intrude Upper Carboniferous sedimentary rocks and Triassic La Flecha Trachyte Formation. The timing of breccia dyke emplacement is constrained by cross cutting relationships with the uppermost Triassic unit and conformable contacts with the Early Miocene sedimentary rocks. This study supports a tectonic-hydrothermal origin for these breccia dykes; fragmentation and subsequent hydraulic injection of fluidized breccia are the more important processes in the breccia dyke development. Brecciation can be triggered by seismic activity which acts as a catalyst. The escape of fluidized material can be attributed to hydrostatic pressure and the direction of movement of the material establishes the direction of least pressure. Previous studies have shown that cross-strike structures have had an important role in the evolution of this Andean segment since at least Triassic times. These structures represent pre-existing crustal fabrics that could have controlled the emplacement of the dykes. The dykes, which are composed mostly of carbonate fault breccia, were injected upward along WNW fractures.

  3. Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland. (United States)

    Sigmundsson, Freysteinn; Hooper, Andrew; Hreinsdóttir, Sigrún; Vogfjörd, Kristín S; Ófeigsson, Benedikt G; Heimisson, Elías Rafn; Dumont, Stéphanie; Parks, Michelle; Spaans, Karsten; Gudmundsson, Gunnar B; Drouin, Vincent; Árnadóttir, Thóra; Jónsdóttir, Kristín; Gudmundsson, Magnús T; Högnadóttir, Thórdís; Fridriksdóttir, Hildur María; Hensch, Martin; Einarsson, Páll; Magnússon, Eyjólfur; Samsonov, Sergey; Brandsdóttir, Bryndís; White, Robert S; Ágústsdóttir, Thorbjörg; Greenfield, Tim; Green, Robert G; Hjartardóttir, Ásta Rut; Pedersen, Rikke; Bennett, Richard A; Geirsson, Halldór; La Femina, Peter C; Björnsson, Helgi; Pálsson, Finnur; Sturkell, Erik; Bean, Christopher J; Möllhoff, Martin; Braiden, Aoife K; Eibl, Eva P S


    Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

  4. The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do Pontal Orogen: Onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil (United States)

    Salgado, Silas Santos; Ferreira Filho, Cesar Fonseca; Caxito, Fabrício de Andrade; Uhlein, Alexandre; Dantas, Elton Luiz; Stevenson, Ross


    The Brejo Seco mafic-ultramafic Complex (BSC) occurs at the extreme northwest of the Riacho do Pontal Orogen Internal Zone, in the northern margin of the São Francisco Craton in Northeast Brazil. The stratigraphy of this medium size (3.5 km wide and 9 km long) layered intrusion consists of four main zones, from bottom to top: Lower Mafic Zone (LMZ; mainly troctolite), Ultramafic Zone (UZ; mainly dunite and minor troctolite); Transitional Mafic Zone (TMZ; mainly troctolite) and an Upper Mafic Zone (UMZ; gabbro and minor anorthosite, troctolite, and ilmenite magnetitite). Ni-Cu-PGE mineralization occurs at the contact of the UZ with the TMZ, consisting of an up to 50 m thick stratabound zone of disseminated magmatic sulfides. An Mg-tholeiitic affinity to the parental magma is indicated by the geochemical fractionation pattern, by the magmatic crystallization sequence and by the elevated Fo content in olivine. A Smsbnd Nd isochron yielded an age of 903 ± 20 Ma, interpreted as the age of crystallization, with initial εNd = 0.8. Evidence of interaction of the BSC parental magma with sialic crust is given by the Rare Earth and trace element patterns, and by slightly negative and overall low values of εNd(900 Ma) in between -0.2 and +3.3. Contrary to early interpretations that it might constitute an ophiolite complex, based mainly on the geochemistry of the host rocks (Morro Branco metavolcanosedimentary complex), here we interpret the BSC as a typical layered mafic-ultramafic intrusion in continental crust, related to an extensional regime. The BSC is chrono-correlated to mafic dyke swarms, anorogenic granites and thick bimodal volcanics of similar age and tectonic setting in the São Francisco Craton and surrounding areas. Intrusion of the BSC was followed by continued lithospheric thinning, which led to the development of the Paulistana Complex continental rift volcanics around 888 Ma and ultimately to plate separation and the generation of new oceanic crust (Monte

  5. Looking for Asian butch-dykes: exploring filmic representations of East Asian butch-dykes in Donna Lee's Enter the Mullet. (United States)

    Lin, Hui-Ling


    Asian butch-dykes have been overlooked in analyses of Chinese cinema, studies that often concentrate on "feminized" transgender roles. This article examines cinematic representations of Asian butch-dykes through film analysis of Enter the Mullet (2004), a five-minute short, and in-depth interviews with the filmmaker, Donna Lee, a Chinese-Canadian in Vancouver. Lee's film is inspired by Enter the Dragon (1973), starring Bruce Lee, the most recognized icon of Asian masculinity. Combining with the mullet hairstyle, which is often associated with White working-class, the filmmaker introduces viewers to the hybrid masculinity of Asian butch-dykes. The article argues that Asian female masculinity can be a strategic means of destabilizing the hegemony of White-male-middle-class masculinity.

  6. Mangrove forest against dyke-break-induced tsunami on rapidly subsiding coasts (United States)

    Takagi, Hiroshi; Mikami, Takahito; Fujii, Daisuke; Esteban, Miguel; Kurobe, Shota


    Thin coastal dykes typically found in developing countries may suddenly collapse due to rapid land subsidence, material ageing, sea-level rise, high wave attack, earthquakes, landslides, or a collision with vessels. Such a failure could trigger dam-break tsunami-type flooding, or "dyke-break-induced tsunami", a possibility which has so far been overlooked in the field of coastal disaster science and management. To analyse the potential consequences of one such flooding event caused by a dyke failure, a hydrodynamic model was constructed based on the authors' field surveys of a vulnerable coastal location in Jakarta, Indonesia. In a 2 m land subsidence scenario - which is expected to take place in the study area after only about 10-20 years - the model results show that the floodwaters rapidly rise to a height of nearly 3 m, resembling the flooding pattern of earthquake-induced tsunamis. The depth-velocity product criterion suggests that many of the narrow pedestrian paths behind the dyke could experience strong flows, which are far greater than the safe limits that would allow pedestrian evacuation. A couple of alternative scenarios were also considered to investigate how such flood impacts could be mitigated by creating a mangrove belt in front of the dyke as an additional safety measure. The dyke-break-induced tsunamis, which in many areas are far more likely than regular earthquake tsunamis, cannot be overlooked and thus should be considered in disaster management and urban planning along the coasts of many developing countries.

  7. Sedimentary dykes in the Oskarshamn-Vaestervik area. A study of the mechanism of formation

    Energy Technology Data Exchange (ETDEWEB)

    Roeshoff, Kennert [BBK AB, Solna (Sweden); Cosgrove, John [Imperial College, London (United Kingdom). Dept. of Earth Sciences and Engineering


    This study of the sedimentary dykes from the Oskarshamn-Vaestervik area, near Aespoe and surrounding region, is aimed at understanding the mechanism of their formation. In particular it is important to establish whether or not they formed by the injection of high pressure fluidized sediments and if so what the likely effect of any future over pressured sediments will be on the stability of the fracture network in the basement rocks at Aespoe. This report is made up of a review of the literature on sedimentary dykes, a discussion of the various mechanical models for hydraulic fracturing and a description of the field and laboratory study carried out on the sedimentary dykes. The literature review indicates a remarkable consensus on the mode of formation of these structures based on their fabric (particularly layering generated in part by variation in clast size) and the composition of the infilling material. Two modes of origin have been recognised. These are the passive infilling of dykes where the dyke material has entered an open fracture under the influence of gravity, and active, i.e. forceful injection of a fluidized sediment under high pressure into a pre-existing fracture or into a fracture generated by the high pressure fluid. The discussion of the theory of fluid induced fracturing leads to the recognition of three systems which are the two end members and an intermediate form of a complete spectrum of materials ranging from unconsolidated and incohesive sediments, through cemented but porous rocks to crystalline rocks with no intrinsic porosity and whose only porosity relates to that imparted by the fracture network that the rock contains. The theory best suited to analyses this latter system is one based on fracture mechanics and is known as the theory of external hydraulic fracturing. From the point of view of the sedimentary dykes in the study area around the Aespoe Hard Rock Laboratory, where the dykes occur in the fractured granitic basement, this is

  8. Experimental and numerical investigation of the effect of basaltic dykes on transient saltwater intrusion (United States)

    Abdoulhalik, Antoifi; Ahmed, Ashraf; Hamill, Gerrard


    Heterogeneity in porous media is one of the major factors that control mixing zone and saltwater wedge dynamics. Basaltic dykes are natural geological structures that can significantly affect groundwater flow and solute transport in coastal aquifers. Few recent studies have investigated the effects of heterogeneity on the freshwater-saltwater mixing zone. However, most of these studies have focused on steady-state conditions. Furthermore, as per our knowledge no one has so far completed experiments to study the impact of basaltic dykes on transient saltwater intrusion. In this study, we have performed experiments in a laboratory-scale aquifer model to study saltwater intrusion process under different heterogeneous settings; a dyke was set at different locations and several values of thickness and permeability were tested. Using a variable-head groundwater system, a head difference was imposed across the porous media and images of the transient saltwater-wedge were recorded at thirty second intervals. The experimental data sets were simulated using the MODFLOW-family variable density flow code SEAWAT. The results show that under intruding-wedge conditions, the width of the mixing zone is almost two orders of magnitude larger in the presence of the dyke than it is in the homogeneous case, while it is more than ten times larger under receding-wedge conditions. A slight increase of the toe length was observed (around nine percent) in the presence of the dyke under intruding-wedge and receding-wedge conditions. When the permeability of the dyke is further reduced, it acts as an impervious barrier that almost prevents the progression of the saltwater wedge. Thus, saltwater is accumulated along the side boundary, and then forced to drain along the bottom boundary. The results highlight the significant effects of macroscopic variations in aquifer properties (such as volcanic dykes) in the temporal evolution of mixing zone dynamics (widening and narrowing) and the

  9. Thermal models of dyke intrusion during development of continent-ocean transition (United States)

    Daniels, K. A.; Bastow, I. D.; Keir, D.; Sparks, R. S. J.; Menand, T.


    A consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching: dyke intrusion also plays an important role. Magma intrusion can accommodate extension at lower yield stresses than are required to extend thick, strong, unmodified continental lithosphere mechanically, thereby aiding the breakup process. Dyke intrusion is also expected to heat and thereby weaken the plate, but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood. To address this issue, a numerical solution to the heat-flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting. The thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in Ethiopia. Finite difference models demonstrate that magmatic extension rate exerts a first-order control on the crustal thermal structure. Once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly (less than 1 Ma). In the Main Ethiopian Rift (MER), the majority of present-day extension is focused on ∼20 km-wide Quaternary-Recent axial magmatic segments that are mostly seismogenic to mid-crustal depths and show P-wave seismic velocities characteristic of heavily intruded continental crust. When reviewed in light of our models, these observations require that no more than half of the MER's extension since ∼2 Ma has been achieved by dyke intrusion. Magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm/yr extension rate. In the older, faster extending (16 mm/yr) Red Sea rift (RSR) in Afar, dyke intrusion is expected to have had a more dramatic impact on crustal rheology. Accordingly, effective elastic plate

  10. U-Pb age of the Anuri dyke: paleoproterozoic potassic alkaline magmatism and implications for the Transamazonic orogeny in the Sao Francisco Craton;Idade U-Pb do dique sienitico Anuri, sul da Bahia: magmatismo alcalino-potassico paleoproterozoico e sua implicacao para a orogenia transamazonica no Craton do Sao Francisco

    Energy Technology Data Exchange (ETDEWEB)

    Conceicao, Herbet; Rios, Debora Correia, E-mail: herbet@ufba.b, E-mail: debora@cpgg.ufba.b [Universidade Federal da Bahia (LPAPM/IGEO/UFBA), Salvador, BA (Brazil). Centro de Pesquisa em Geofisica e Geologia. Lab. de Petrologia Aplicada a Pesquisa Mineral; Rosa, Maria de Lourdes da Silva, E-mail: lourdes@cpgg.ufba.b [Universidade Federal da Bahia (UFBA), Salvador, BA (Brazil); Oberli, Felix, E-mail: oberli@erdw.ethz.c [Institute Isotopengeologie/Mineral, Zuerich (Switzerland)


    The Anuri Syenitic Dyke (72 km{sup 2}) is the south representative of a 1000 km N-S alignment of syenitic dykes which occur at the East Bahia. As the others bodies, Anuri is essentially composed of hypersolvus ultrapotassic syenites, which show the records of its differentiation by the presence of mafic-ultramafic apatite-rich cumulates. Its crystallization age of 2095- 4 Ma (U-Pb{sub zircao}) is quite similar with results obtained for the other syenitic dykes, which suggest that their intrusions occur almost at the same time, sincronically, after the Transamazonic Orogeny climax. The geochemical data reveal that Anuri syenites are Ba, Sr, P, LREE enriched and show negative anomalies of Ti and Nb, suggesting a mantelic source subduction related. The initial Sr ratio and {epsilon}{sub Nd} values confirms the presence of this anomalous mantle. These data suggest that at the end of Transamazonic Orogeny, at the Sao Francisco Craton, there were particular conditions which allow the generation of alkaline magmas through the melting of an EMI reservoir. (author)

  11. Relamination of mafic subducting crust throughout Earth's history (United States)

    Maunder, Ben; van Hunen, Jeroen; Magni, Valentina; Bouilhol, Pierre


    Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as ;relamination;) is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting. Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80-120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for

  12. Experiment and Simulation of Turbulent Flow in Local Scour around a Spur Dyke

    Institute of Scientific and Technical Information of China (English)

    Hao ZHANG; Hajime NAKAGAWA; Kenji KAWAIKE; Yasuyuki BABA


    The turbulent flow in the local scour hole around a single non-submerged spur dyke is investigated with both experimental and numerical methods.The experiments are conducted under clear-water scour regime with an impermeable spur dyke.The scour geometry and flow velocities are measured in details with a high-resolution laser displacement meter,electro-magnetic velocimetries and PIV (Particle image velocimetry).A 3D non-linear κ-ε model is developed to simulate the complex local flow field around the scour area.The numerical model is formulated using FVM (Finite volume method) on a collocated unstructured mesh,capable of resolving complex geometries and boundaries.It is found that the simulation results are reasonably consistent with those of the experimental measurements.Based on the study results,the nature of the flow structrue around a spur dyke with local scour hole is analyzed.

  13. 堤防加固设计中的若干技术问题%Problems about Design of Dyke Reinforcement

    Institute of Scientific and Technical Information of China (English)

    ZHANG; Jia-fa


    The dyke experiences very high flood level more and more frequently. The timely issued " The Standard of Dyke Design" has played an important role in the design, measure fea sibility study and examination of dyke reinforcement. Safety appraisal is required before dyke rein forcement design. But, the conducted item is safety demonstration in the process of dyke reinforcement design at present. This paper suggests that at least 8 aspects should be considered in safety demo nstration. The effective water head is considered as the most important factor for dyke safety and reinfo rcement design. Seepage control measure design is an important part of dyke reinforcement design . Its correct train of thought is discussed in this paper. Some other factors for dyke reinforcement d esign are also pointed out.

  14. Controls on sill and dyke-sill hybrid geometry and propagation in the crust: The role of fracture toughness (United States)

    Kavanagh, J. L.; Rogers, B. D.; Boutelier, D.; Cruden, A. R.


    Analogue experiments using gelatine were carried out to investigate the role of the mechanical properties of rock layers and their bonded interfaces on the formation and propagation of magma-filled fractures in the crust. Water was injected at controlled flux through the base of a clear-Perspex tank into superposed and variably bonded layers of solidified gelatine. Experimental dykes and sills were formed, as well as dyke-sill hybrid structures where the ascending dyke crosses the interface between layers but also intrudes it to form a sill. Stress evolution in the gelatine was visualised using polarised light as the intrusions grew, and its evolving strain was measured using digital image correlation (DIC). During the formation of dyke-sill hybrids there are notable decreases in stress and strain near the dyke as sills form, which is attributed to a pressure decrease within the intrusive network. Additional fluid is extracted from the open dykes to help grow the sills, causing the dyke protrusion in the overlying layer to be almost completely drained. Scaling laws and the geometry of the propagating sill suggest sill growth into the interface was toughness-dominated rather than viscosity-dominated. We define KIc* as the fracture toughness of the interface between layers relative to the lower gelatine layer KIcInt / KIcG. Our results show that KIc* influences the type of intrusion formed (dyke, sill or hybrid), and the magnitude of KIcInt impacted the growth rate of the sills. KIcInt was determined during setup of the experiment by controlling the temperature of the upper layer Tm when it was poured into place, with Tm < 24 °C resulting in an interface with relatively low fracture toughness that is favourable for sill or dyke-sill hybrid formation. The experiments help to explain the dominance of dykes and sills in the rock record, compared to intermediate hybrid structures.

  15. Dyke-path formation in relation to the eruptions of Eyjafjallajökull 2010 and Bardarbunga-Holuhraun 2014 (United States)

    Gudmundsson, Agust


    Dykes are extension fractures and form when the magmatic overpressure is high enough to rupture (break) the host rock. Their formation is entirely analogous to that of many joints and human-made hydraulic fractures, such as are used to increase permeability in reservoirs. When generating their paths, dykes use existing weaknesses (e.g., cooling joints) in the host rock. The maximum depth of large tension fractures below the surface of a rift zone, however, is mostly less than a few hundred metres. If the fractures extend to greater depths, they must change into closed normal faults which are generally not used as magma paths. There are thus no large tension fractures or wide-open faults at great depths ready to be filled with magma to form a dyke. While magma flow in dykes, as in other fluid-driven fractures, is at any point in various directions dyke segmentation may indicate the overall large-scale flow direction. Thus, dykes composed of large-spaced disconnected segments in lateral sections are primarily formed in vertical magma flow at segmentation depth whereas those composed of large-spaced disconnected segments in vertical sections are primarily formed in lateral magma flow. The far-field displacement and stress fields of segmented dykes are similar to those generated by single, continuous dykes of similar dimensions, particularly when the distances between the nearby tips of the segments become small in comparison with segment lengths. Most dykes become arrested and never supply magma to eruptions. Feeder-dykes normally reach the surface only along parts of their lengths (strike-dimensions). The volumetric flow or effusion rate of magma through a feeder-dyke or volcanic fissure depends on the aperture (opening) of the dyke or fissure in the 3rd power. All these theoretical and observational results are here applied to the dyke emplacements associated with the eruptions of Eyjafjallajökull 2010 and Bardarbunga-Holuhraun 2014. The results make it possible to

  16. Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco) (United States)

    Neres, Marta; Silva, Pedro F.; Ikene, Moha; Martins, Sofia; Hafid, Ahmid; Mata, João; Almeida, Francisco; Youbi, Nasrrddine; Boumehdi, Ahmed


    Paleomagnetic data able to constrain the paleoposition of the West African Craton (WAC) during Paleo-Mesoproterozoic are absent, mainly due to gaps on the sedimentary record and intense remagnetizations. Dykes that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) have recently been subjected to geochronological studies, which revealed ages between Paleoproterozoic and early Neoproterozoic. Therefore, these dykes represent a window of opportunity for paleomagnetic studies aiming to infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. We determined the paleomagnetic directions and evaluated their meaning by rock magnetic and mineral analyses, complemented by petrographic observations. Results revealed that these rocks record the presence of a complex history of remagnetization events, mostly assigned to several Phanerozoic thermal/chemical events. In particular, we found components assigned to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases (magnetite and hematite). The primary (magmatic) thermo-remanent magnetization of the dykes was obliterated during these events through multiple thermal and chemical remagnetizations. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma. The authors wish to acknowledge FCT (Portugal) - CNRST (Morocco) bilateral agreement for its major contribution without which this work wouldn't be possible. Publication supported by project FCT UID/GEO/50019/2013 - Instituto Dom Luiz.

  17. Mineral reactions and strain localization in a sheared mafic granulite infiltrated by melt (Seiland Igneous Province, Norway) (United States)

    degli Alessandrini, Giulia; Menegon, Luca; Malaspina, Nadia; Dijkstra, Arjan; Anderson, Mark


    This study investigates the deformation mechanisms of a metagabbroic dyke experiencing syn-kinematic melt-rock interaction in a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway). Solid state shearing occurred at T ≈750-820 °C, P ≈0.75-0.95 GPa and was coeval with melt infiltration from dehydration melting of adjacent metasediments, as evident from thin leucosome veinlets within the dykes. The mylonite consists of cpx [Ca0.47,Mg0.35,Fe0.18]SiO3 + opx [Ca0.1,Mg0.5,Fe0.4]SiO3 + pl (An77Ab22Or1) porphyroclasts with localized grt and ilm coronas, embedded in a fine grained matrix of cpx + opx + pl + qtz + ilm ± kfs. Porphyroclasts range in size (diameter) between 25 to 650 μm, whereas the fine grain matrix is consistently below 10 μm (average 4-7 μm). Porphyroclasts show varying degrees of elongation, with the opx reaching aspect ratios of 1:16 and the cpx reaching rare maxima of 1:7. Cpx and pl porphyroclasts are micro-fractured and micro-boudinaged with fine-grained material infill. Texturally, opx porphyroclasts display a marked crystallographic preferred orientation (CPO) and activity of the {100} and minor {100} slip systems, whereas cpx and pl porphyroclasts are randomly oriented. All porphyroclasts have strong internal misorientations (undulatory and sweeping extinction) and lack recovery features (subgrains). The fine-grained polyphase matrix wrapping the porphyroclasts displays weak to absent CPO, with the exception of opx that shows a {100} poles-to-planes maxima perpendicular to the foliation. Based on the microstructure, we argue that a large part of the matrix is the product of metamorphic reactions in the presence of melt. To test this hypothesis, the interaction between the studied mafic dyke and an adjacent felsic leucosome was modelled using PerpleX for P-T conditions ranging between 7-9 kbar and 700-1000°C. Results show that the syn-kinematic mineral assemblage (opx + cpx + pl + qtz + ilm ± kfs) can be

  18. An unusual occurrence of mafic accretionary lapilli in deep-marine volcaniclastics on 'Eua, Tonga: Palaeoenvironment and process (United States)

    Cunningham, J. K.; Beard, A. D.


    Reports of occurrences of accretionary lapilli on Earth, whether in historic time or in the geological record, are restricted to subaerial environments or to shallow marine environments when faunal evidence exists to determine palaeodepths. The proximity of the deep ocean to subduction zones/island arcs (where moist explosive volcanism conducive to ash aggregate formation is common) makes this surprising. In this paper, accretionary lapilli are reported within Middle Miocene mafic glass-rich volcaniclastics on 'Eua, the island closest to the Tonga Trench, a persistent high in the frontal arc basin. The glass in the accretionary lapilli has been subjected to advanced palagonitisation, but concentric layers marked by micro-aggregates containing shard-shaped particles survive to determine one group of occurrences as layered accretionary lapilli. The palaeoenvironment, as established by pelagic microfauna, is clearly deep marine, not less than 1600 m. The host rocks, typically gravel/sand in grain size, contain sedimentary structures (normal grading to inverse and normal-to-inverse grading, lack of grading, large-scale cross-bedding, slump bedding and sedimentary dykes) suggesting that the full spectrum of sediment gravity flow types, including less ordered debris flows, has been active. In an island arc environment, a range of sediment gravity flow types can be initiated, some by pyroclastic flows entering the sea. However, the thin beds of accretionary lapilli do not exhibit features of sediment gravity flow deposits or those of submarine pyroclastic flows. Possible transport processes must account for the matrix between the ash aggregates, which is either coarse-grained or absent. Modelling of particle descent times to 1600 m through a sea water column provides one explanation for the features displayed.

  19. Study on the carbonate ocelli-bearing lamprophyre dykes in the Ailaoshan gold deposit zone, Yunnan Province

    Institute of Scientific and Technical Information of China (English)

    HUANG; Zhilong(黄智龙); LIU; Congqiang(刘丛强); XIAO; Huayun(肖化云); HAN; Runsheng(韩润生); XU; Cheng(许成); LI; Wenbo(李文博); ZHONG; Kunming(钟昆明)


    Three carbonate ocelli-bearing lamprophyre dykes have been found in the Laowangzhai and Beiya gold orefields in the northern sector of the Ailaoshan gold deposit zone, Yunnan Province. Ocelli in the lamprophyre dykes are carbonates composed mainly of dolomite and calcite. Their trace elements, REE and C isotopic compositions are characteristic of carbonatite and the main mineral assemblages, major elements, trace elements and REE in the matrix are similar to those in the carbonate ocelli-barren lamprophyre dykes in the orefields, which are calc-alkaline lamprophyres that derived from the fertile mantle. The results indicate that the carbonate ocelli-bearing lamprophyre dykes in this area were produced at the time when the Himalayan lamprophyre magma evolved to a relatively late stage of silicate-carbonate liquid immiscibility. In the process of magmatic evolution there took place magmatic degassing with CO2 and H2O as the dominant released gases.

  20. Radial patterns of bitumen dykes around Quaternary volcanoes, provinces of northern Neuquén and southernmost Mendoza, Argentina (United States)

    Cobbold, Peter R.; Ruffet, Gilles; Leith, Leslie; Loseth, Helge; Rodrigues, Nuno; Leanza, Hector A.; Zanella, Alain


    Where the Neuquén Basin of Argentina abuts the Andes, hundreds of veins of solid hydrocarbon (bitumen) are visible at the surface. Many of these veins became mines, especially in the last century. By consensus, the bitumen has resulted from maturation of organic-rich shales, especially the Vaca Muerta Fm of Late Jurassic age, but also the Agrio Fm of Early Cretaceous age. To account for their maturation, recent authors have invoked regional subsidence, whereas early geologists invoked magmatic activity. During 12 field seasons (since 1998), we have tracked down the bitumen localities, mapped the veins and host rocks, sampled them, studied their compositions, and dated some of them. In the provinces of northern Neuquén and southernmost Mendoza, the bitumen veins are mostly sub-vertical dykes. They tend to be straight and continuous, crosscutting regional structures and strata of all ages, from Jurassic to Palaeocene. Most of the localities lie within 70 km of Tromen volcano, although four are along the Rio Colorado fault zone and another two are at the base of Auca Mahuida volcano. On both volcanic edifices, lavas are of late Pliocene to Pleistocene age. Although regionally many of the bitumen dykes tend to track the current direction of maximum horizontal tectonic stress (ENE), others do not. However, most of the dykes radiate outward from the volcanoes, especially Tromen. Thicknesses of dykes tend to be greatest close to Tromen and where the host rocks are the most resistant to fracturing. Many of the dykes occur in the exhumed hanging walls of deep thrusts, especially at the foot of Tromen. Here the bitumen is in places of high grade (impsonite), whereas further out it tends to be of medium grade (grahamite). A few bitumen dykes contain fragments of Vaca Muerta shale, so that we infer forceful expulsion of source rock. At Curacó Mine, some shale fragments contain bedding-parallel veins of fibrous calcite (beef) and these contain some bitumen, which is

  1. Use of gravity drainage and quasi-homogenous dykes for containment of oil sands tailings

    Energy Technology Data Exchange (ETDEWEB)

    Nichols, L.; Czajewski, K. [Terracon Geotechnique Ltd., Calgary, AB (Canada)


    Solutions to the disposal of byproducts from oil sands processing into tailings ponds in Alberta were examined. Gravity drainage of composite tailings (CT) was used as a mechanism to facilitate the consolidation of CT in critical areas of the containment ponds through the use of internal sand layers within the body of the deposit. Critical areas were defined as the areas around the perimeters of the containment ponds. The aim of the solution was to provide long-term stability to the disposal area through the creation of stiff deposits around the perimeter and through the facilitation of reclamation efforts on the surface. It was concluded that use of the quasi-homogenous dykes is the only commercially proven method of reducing the fine tailings inventory of the oil sands industry. It was concluded that the use of gravity drainage and quasi-homogenous dykes for oil sands containment structures will assist the industry in reducing its environmental footprint.

  2. Variscan potassic dyke magmatism of durbachitic affinity at the southern end of the Bohemian Massif (Lower Austria) (United States)

    Zeitlhofer, Helga; Grasemann, Bernhard; Petrakakis, Konstantin


    Dykes in the Strudengau area (SW Moldanubian Zone, Austria) can be mineralogically divided into lamprophyres (spessartites and kersantites) and felsic dykes (granite porphyries, granitic dykes and pegmatoid dykes). Geochemical analyses of 11 lamprophyres and 7 felsic dykes show evidence of fractional crystallization. The lamprophyres are characterized by metaluminous compositions, intermediate SiO2 contents and high amounts of MgO and K2O; these rocks have high Ba (800-3000 ppm) and Sr (250-1000 ppm) contents as well as an enrichment of large-ion lithophile elements over high field strength elements, typical for enriched mantle sources with variable modifications due to fractionation and crustal contamination. This geochemical signature has been reported from durbachites (biotite- and K feldspar-rich mela-syenites particularly characteristic of the Variscan orogen in Central Europe). For most major elements, calculated fractionation trends from crystallization experiments of durbachites give an excellent match with the data from the Strudengau dykes. This suggests that the lamprophyres and felsic dykes were both products of fractional crystallization and subsequent magma mixing of durbachitic and leucogranitic melts. Rb-Sr geochronological data on biotite from five undeformed kersantites and a locally deformed granite porphyry gave cooling ages of c. 334-318 Ma, indicating synchronous intrusion of the dykes with the nearby outcropping Weinsberger granite (part of the South Bohemian Batholith, c. 330-310 Ma). Oriented matrix biotite separated from the locally deformed granite porphyry gave an Rb-Sr age of c. 318 Ma, interpreted as a deformation age during extensional tectonics. We propose a large-scale extensional regime at c. 320 Ma in the Strudengau area, accompanied by plutonism of fractionated magmas of syncollisional mantle-derived sources, mixed with crustal components. This geodynamic setting is comparable to other areas in the Variscan belt documenting an

  3. Geochemical Characteristics of a Carbonatite Dyke Rich in Rare Earths from Bayan Obo, China

    Institute of Scientific and Technical Information of China (English)


    The whole-rock geochemistry of a rare earths rich carbonatite dykes that locates at Dulahala and lies 3 km north-east to the East Ore body of the giant Bayan Obo RE-Nb-Fe deposit was analysed. The dyke cuts cross H1 coarse quartz sandstone and H2 fine quartzite of the Proterozoic Bayan Obo group. RE content in the dyke varies greatly up to 20%(mass fraction), which comprises rich RE ores. Light RE in carbonatites are extremely enriched and strongly fractionated relative to heavy RE, but no Eu anomaly. The carbonatite may be produced by mechanisms as follows: the carbonatite magma is directly formed by very low degree(F<1%) partial melting of enriched lithospheric mantle, leaving residual minerals characterized by abundant garnet;then the magma arises into a chamber within the crust where they will undergo fractional crystallization, which makes RE further concentrated in carbonatite. The RE patterns and spider diagrams of the carbonatite are identical to those fine-grained dolomite marble that is the ore-host rock for the Bayan Obo deposit. However, the carbonatite is calcic, which is different from the fine-grained dolomite marble in major element geochemistry. The difference is suggested to be resulted from that the carbonatite dyke is not affected by a large scale dolomitization, while the fine-grained dolomite marble might be the product of dolomitized carbonatite intrusive body that might set up a hydrothermal system in the region, which transported Mg from the Bayan Obo sediments, especially form the shales to the carbonatite intrusion.

  4. Understanding and using technological affordances: a commentary on Conole and Dyke

    Directory of Open Access Journals (Sweden)

    Tom Boyle


    Full Text Available The paper by Conole and Dyke sets the context by pointing to a number of problems that inhibit the widespread, effective use of Information and Communication Technologies (ICT to support learning. They argue that this situation highlights the need to explore a theoretical basis for the use of ICT to support learning. The central argument of the paper is that the notion of affordances can make a significant contribution to this endeavour. The paper aims to articulate the potential impact of these affordances primarily through the development of a taxonomy. It draws on social constructivist theory to help understand and articulate the impact of these affordances. The concept of affordances is potentially both rich and provocative. Conole and Dyke provide a refreshing and diverse look at the theoretical basis for the use of ICT to support learning. We have structured our commentary around six questions that are provided by Conole and Dyke in their Discussion section. Whilst examining these questions we highlight various issues raised by the paper, which we believe, need further consideration and clarification.

  5. Monitoring dyke injection and strain field evolution using shear-wave splitting. (United States)

    Kendall, J.-M.; Verdon, J. P.; Keir, D.; Baird, A.


    Magma storage and dyke injection in the shallow crust is a fundamental process in rifting and volcanic environments. The dyking will tend to align with directions of maximum compressive stress, and the associated aligned fracturing and melt migration provides a very effective means of generating seismic anisotropy. Observations of shear-wave splitting provide one of the most unambiguous indicators of such anisotropy. As such, shear-wave splitting can be used to monitor the evolving strain field in volcanic and rifting environments. Here we apply lessons learned from monitoring fracture propagation during the hydraulic stimulation of tight-gas reservoirs. In a number of experiments we observe spatial and temporal variations in shear-wave splitting magnitude and orientation. We invert shear-wave observations for fracture properties, including the tangential and normal compliance, the ratio of which is a good indicator of fluid flow and permeability. Frequency dependent affects can be also used to indicate the length scales of the causative cracks or fractures. We apply these insights to microseismic data recently acquired across the volcanically active Afar triple junction in Ethiopia. The pattern of S-wave splitting in Afar is best explained by anisotropy from deformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increased density of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres. The results help in our understanding of the role of melt in strain accommodation in rifting and volcanic environments.

  6. Petrology of the prehistoric lavas and dyke of the Barren Island, Andaman Sea, Indian Ocean

    Indian Academy of Sciences (India)

    M A Alam; D Chandrasekharam; O Vaselli; B Capaccioni; P Manetti; P B Santo


    Although Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions during historic times, the eruptions that led to the formation of this volcanic island occurred mainly during prehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appears to be characterized by a constructive phase with the piling up of lava flows and scoria deposits and Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possible caldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmatic activity, whose deposits presently mantle inner and outer caldera walls, a new phase of intracalderic Vulcanian activities took place. A prominent dyke in the SE inner side of the caldera wall was recognized. Petrographically the lava flows and dyke are similar but they differ in their chemical composition (viz., SiO2, MgO, Ni, Cr) significantly. Similarity in major, minor and trace element composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondrite normalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of the Barren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that Barren Island must have evolved from a source similar to that of Sunda Arc lavas during the Quaternary Period.

  7. Dyking at EPR 16°N hypermagmatic ridge segment: Insights from near-seafloor magnetics (United States)

    Szitkar, Florent; Dyment, Jérôme; Le Saout, Morgane; Honsho, Chie; Gente, Pascal


    High-resolution, near-seafloor magnetic data have been acquired over the 16°N hypermagmatic segment of the East-Pacific Rise using an Autonomous Underwater Vehicle. This survey proves to be ideal to test the relative efficiency of various inversion methods applied to data acquired at a more or less constant altitude above the seafloor. Unlike other methods, a recently published Bayesian inversion preserves the short wavelengths and allows for the resolution of a high-resolution reduced-to-the-pole magnetic anomaly. This anomaly unveils the presence of several laterally adjacent dykes associated with individually separated Axial Summit Troughs. The observation of such anomalies, and therefore of shallow dykes, confirms the hypermagmatic character of the segment in a location where complex magma chambers have been imaged in seismic reflection studies. Variable intensity of the magnetic anomalies reflects the depth of the dyke swarms and, ultimately, the timing and style of eruptive events, helping to constrain the spreading axis evolution.

  8. Cloning, expression, and characterization of β-mannanase from Bacillus subtilis MAFIC-S11 in Pichia pastoris. (United States)

    Lv, Junnan; Chen, Yiqun; Pei, Honglei; Yang, Wenhan; Li, Zhimin; Dong, Bing; Cao, Yunhe


    The β-mannanase gene (1,029 nucleotide) from Bacillus subtilis MAFIC-S11, encoding a polypeptide of 342 amino acids, was cloned and expressed in Pichia pastoris. To increase its expression, the β-mannanase gene was optimized for codon usage (mannS) and fused downstream to a sequence-encoding modified α-factor signal peptide. The expression level was improved by 2-fold. This recombinant enzyme (mannS) showed its highest activity of 24,600 U/mL after 144-h fermentation. The optimal temperature and pH of mannS were 50 °C and 6.0, respectively, and its specific activity was 3,706 U/mg. The kinetic parameters V max and K m were determined as 20,000 U/mg and 8 mg/mL, respectively, representing the highest ever expression level of β-mannanase reported in P. pastoris. In addition, the enzyme exhibited much higher binding activity to chitin, chitosan, Avicel, and mannan. The superior catalytic properties of mannS suggested great potential as an effective additive in animal feed industry.

  9. The campsite dykes: A window into the early post-solidification history of the Skaergaard Intrusion, East Greenland (United States)

    Holness, Marian B.; Richardson, Chris; Andersen, Jens C. Ø.


    The Skaergaard Intrusion of East Greenland is cut by several generations of dykes, the earliest of which is thought to have intruded shortly after solidification of the Skaergaard. Two ~ 6 m wide doleritic dykes from the earliest generation are exposed in the campsite area near Homestead Bay of the Skaergaard Peninsula. One of the dykes (the Campsite Dyke) locally contains abundant xenoliths of troctolitic cumulate. The other (the Plagioclase-phyric Dyke) contains abundant large plagioclase phenocrysts. Cross-cutting relationships between the two dykes are not exposed. The median clinopyroxene-plagioclase-plagioclase dihedral angle, Θcpp, in the Campsite Dyke is 88-89.5°, whereas that of the Plagioclase-phyric Dyke is 79°. Using an empirical relationship between Θcpp and the duration of crystallisation derived from dolerite sills, the observed Θcpp suggests that the Campsite Dyke is the older of the two, intruding the Skaergaard when it had cooled to 920-970 °C. The Plagioclase-phyric Dyke intruded later, once the Skaergaard had cooled below 670 °C. The troctolitic xenoliths divide into two separate groups. Type A xenoliths have microstructures similar to those of the Skaergaard Layered Series although mineral compositions are generally more primitive than those of the exposed cumulates - this type of xenolith is likely to have been derived from either deeper levels in the Skaergaard Intrusion or from a closely-related underlying magma chamber. One Type A xenolith has mineral compositions and Θcpp consistent with an origin in LZb of the Layered Series - this xenolith contains partially inverted pigeonite, suggesting that inversion of low-Ca pyroxene in the lower part of the Layered Series took place after the intrusion had completely solidified. Type B xenoliths are characterized by plagioclase containing large and abundant melt inclusions. Comparison with the microstructures of glassy crystalline nodules from Iceland points to a multi-stage cooling history

  10. Textural and chemical consequences of interaction between hydrous mafic and felsic magmas: an experimental study (United States)

    Pistone, Mattia; Blundy, Jonathan D.; Brooker, Richard A.


    Mantle-derived, hydrous mafic magmas are often invoked as a mechanism to transfer heat, mass and volatiles to felsic plutons in the Earth's crust. Field observations suggest that mafic, water-rich magmas often intrude viscous felsic crystal-rich mushes. This scenario can advect water from the crystallising mafic magma to the felsic magma, leading to an increase in melt fraction in the felsic mush and subsequent mobilisation, at the same time as the mafic magma becomes quenched through a combination of cooling and water loss. To investigate such a scenario, we conducted experiments on a water-undersaturated (4 wt% H2O in the interstitial melt) dacitic crystal mush (50-80 vol% quartz crystals) subject to volatile supply from a water-saturated (≥6 wt% H2O) andesite magma at 950 °C and 4 kbar. Our experimental run products show unidirectional solidification textures (i.e. comb layering) as crystals nucleate at the mafic-felsic interface and grow into the mafic end-member. This process is driven by isothermal and isobaric undercooling resulting from a change in liquidus temperature as water migrates from the mafic to the felsic magma. We refer to this process as "chemical quenching" and suggest that some textures associated with natural mafic-felsic interactions are not simply cooling-driven in origin, but can be caused by exsolution of volatiles adjacent to an interface, whether a water-undersaturated felsic magma (as in our experiments) or a fracture.

  11. Water-driven undercooling during the interaction of mafic and felsic magmas (United States)

    Pistone, M.; Blundy, J. D.; Brooker, R. A.; Hinton, R.


    Mantle-derived mafic magmas are often invoked as a mechanism to transfer heat, mass and volatiles to felsic plutons that reside in the Earth's crust. This process has been suggested as a means of sustaining shallow magmatic bodies and triggering volcanic eruptions. Various field observations suggest that mafic water-rich magmas might intrude a viscous felsic crystal-rich mush. This scenario might be expected to produce water advection from the crystallizing mafic magma to the felsic magma, leading to an increase of melt fraction in the felsic mush and subsequent mobilization whilst the mafic magma is simultaneously quenched. To investigate certain features of this scenario we conducted 24-hour experiments to establish the petrological evolution of a water-saturated (4 wt.% H2O in the interstitial melt) dacitic crystal mush (50-80 vol.% quartz crystals) subject to a volatile supply released from a water-saturated (≥ 6 wt.% H2O) andesitic magma at 950 °C and 500 MPa (15 km depth). Run products were characterised by SEM, EPMA, SIMS and Raman. Our results show unidirectional solidification textures (comb layering) as crystal nucleate at the mafic-felsic interface and grow into the mafic end-member. This is a direct effect of isothermal undercooling that results from a change in liquidus temperatures of the interacting magmas with changing water content. This is the first study exploring felsic-mafic magma interaction under "natural conditions" and shows that textures associated with mafic-felsic interactions found in the field may not be simply cooling-driven in origin. These experiments allow us to explore some essential concepts for understanding the origin of mafic enclaves, how volatiles contribute to crystal mush remobilisation within the Earth's crust and can trigger explosive volcanic eruptions during recharge of mafic inputs into felsic reservoirs, and how degassing processes might be traced by textural features that indicate the direction of volatile

  12. Fault and dyke detectability in high resolution seismic surveys for coal: a view from numerical modelling* (United States)

    Zhou, Binzhong 13Hatherly, Peter


    Modern underground coal mining requires certainty about geological faults, dykes and other structural features. Faults with throws of even just a few metres can create safety issues and lead to costly delays in mine production. In this paper, we use numerical modelling in an ideal, noise-free environment with homogeneous layering to investigate the detectability of small faults by seismic reflection surveying. If the layering is horizontal, faults with throws of 1/8 of the wavelength should be detectable in a 2D survey. In a coal mining setting where the seismic velocity of the overburden ranges from 3000 m/s to 4000 m/s and the dominant seismic frequency is ~100 Hz, this corresponds to a fault with a throw of 4-5 m. However, if the layers are dipping or folded, the faults may be more difficult to detect, especially when their throws oppose the trend of the background structure. In the case of 3D seismic surveying we suggest that faults with throws as small as 1/16 of wavelength (2-2.5 m) can be detectable because of the benefits offered by computer-aided horizon identification and the improved spatial coherence in 3D seismic surveys. With dykes, we find that Berkhout's definition of the Fresnel zone is more consistent with actual experience. At a depth of 500 m, which is typically encountered in coal mining, and a 100 Hz dominant seismic frequency, dykes less than 8 m in width are undetectable, even after migration.

  13. Oxygen isotope composition of mafic magmas at Vesuvius (United States)

    Dallai, L.; Cioni, R.; Boschi, C.; D'Oriano, C.


    The oxygen isotope composition of olivine and clinopyroxene from four plinian (AD 79 Pompeii, 3960 BP Avellino), subplinian (AD 472 Pollena) and violent strombolian (Middle Age activity) eruptions were measured to constrain the nature and evolution of the primary magmas of the last 4000 years of Mt. Vesuvius activity. A large set of mm-sized crystals was accurately separated from selected juvenile material of the four eruptions. Crystals were analyzed for their major and trace element compositions (EPMA, Laser Ablation ICP-MS), and for 18O/16O ratios. As oxygen isotope composition of uncontaminated mantle rocks on world-wide scale is well constrained (δ18Oolivine = 5.2 ± 0.3; δ18Ocpx = 5.6 ± 0.3 ‰), the measured values can be conveniently used to monitor the effects of assimilation/contamination of crustal rocks in the evolution of the primary magmas. Instead, typically uncontaminated mantle values are hardly recovered in Italian Quaternary magmas, mostly due to the widespread occurrence of crustal contamination of the primary magmas during their ascent to the surface (e.g. Alban Hills, Ernici Mts., and Aeolian Islands). Low δ18O values have been measured in olivine from Pompeii eruption (δ18Oolivine = 5.54 ± 0.03‰), whereas higher O-compositions are recorded in mafic minerals from pumices or scoria of the other three eruptions. Measured olivine and clinopyroxene share quite homogeneous chemical compositions (Olivine Fo 85-90 ; Diopside En 45-48, respectively), and represent phases crystallized in near primary mafic magmas, as also constrained by their trace element compositions. Data on melt inclusions hosted in crystals of these compositions have been largely collected in the past demonstrating that they crystallized from mafic melt, basaltic to tephritic in composition. Published data on volatile content of these melt inclusions reveal the coexistence of dissolved water and carbon dioxide, and a minimum trapping pressure around 200-300 MPa, suggesting

  14. Lithological Discrimination of the Mafic-Ultramafic Complex, Huitongshan, Beishan, China:Using ASTER Data

    Institute of Scientific and Technical Information of China (English)

    Lei Liu; Jun Zhou; Dong Jiang; Dafang Zhuang; Lamin R Mansaray


    The Beishan area has more than seventy mafic-ultramafic complexes sparsely distributed in the area and is of a big potential in mineral resources related to mafic-ultramafic intrusions. Many mafic-ultramafic intrusions which are mostly in small sizes have been omitted by previous works. This research takes Huitongshan as the study area, which is a major district for mafic-ultramafic occur-rences in Beishan. Advanced spaceborne thermal emission and reflection radiometer (ASTER) data have been processed and interpreted for mapping the mafic-ultramafic complex. ASTER data were processed by different techniques that were selected based on image reflectance and laboratory emis-sivity spectra. The visible near-infrared (VNIR) and short wave infrared (SWIR) data were trans-formed using band ratios and minimum noise fraction (MNF), while the thermal infrared (TIR) data were processed using mafic index (MI) and principal components analysis (PCA). ASTER band ratios (6/8, 5/4, 2/1) in RGB image and MNF (1, 2, 4) in RGB image were powerful in distinguishing the subtle differences between the various rock units. PCA applied to all five bands of ASTER TIR imagery high-lighted marked differences among the mafic rock units and was more effective than the MI in differen-tiating mafic-ultramafic rocks. Our results were consistent with information derived from local geolog-ical maps. Based on the remote sensing results and field inspection, eleven gabbroic intrusions and a pyroxenite occurrence were recognized for the first time. A new geologic map of the Huitongshan area was created by integrating the results of remote sensing, previous geological maps and field inspection. It is concluded that the workflow of ASTER image processing, interpretation and ground inspection has great potential for mafic-ultramafic rocks identifying and relevant mineral targeting in the sparsely vegetated arid region of northwestern China.

  15. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia (United States)

    Pallister, J.S.; McCausland, W.A.; Jonsson, Sigurjon; Lu, Zhiming; Zahran, H.M.; El, Hadidy S.; Aburukbah, A.; Stewart, I.C.F.; Lundgren, P.R.; White, R.A.; Moufti, M.R.H.


    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees.

  16. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia (United States)

    Pallister, John S.; McCausland, Wendy A.; Jónsson, Sigurjón; Lu, Zhong; Zahran, Hani M.; Hadidy, Salah El; Aburukbah, Abdallah; Stewart, Ian C. F.; Lundgren, Paul R.; White, Randal A.; Moufti, Mohammed R. H.


    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8km long with 91cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees.

  17. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    KAUST Repository

    Pallister, John S.


    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees. © 2010 Macmillan Publishers Limited. All rights reserved.

  18. Palaeointensity and palaeodirection determinations of Paleoproterozoic dykes in the Kaapvaal Craton (South Africa) (United States)

    Shcherbakova, V. V.; Lubnina, N. V.; Shcherbakov, V. P.; Zhidkov, G. V.


    A combined palaeodirectional and palaeointensity study of a representative collection from the Bushveld Igneous Complex from 27 dolerite dykes from the 2.9, 2.7, and 1.8 Ga age swarms radiating SE, E and NE, respectively [Olsson et al., 2010] was carried out. Conventional progressive thermal or AF demagnetization was applied to all specimens. The palaeomagnetic directions have been calculated after thermal demagnetization. The ChRMs were isolated over the temperature interval 440-590 C and their intensities amount to 95% of total NRMs. Paleopole calculate from the primary high-temperature component, separated in the 2.9 Ga SE-dykes, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78 Ga volcanics. The paleopole calculated for the 2.7 Ga age E-trending dykes of the eastern region does not correspond to any of the previously obtained Archean-Paleoproterozoic paleopoles for the Kaapvaal Craton. The paleopole calculated for some NE-trending dykes of the Black Ridge swarm in the NE region is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Palaeointensity determinations were carried out on rocks from ten dykes of different ages using Thellier-Coe method with the "check-points" procedure on specimens of 1 cm in edge length cut from either drilled cores or hand samples. Rock magnetic measurements were made on sister specimens. Curie temperatures and the thermal stability of magnetic minerals were estimated from thermomagnetic heating-cooling cycles to incrementally higher temperatures Ti with a Curie balance in an external magnetic field H = 0.45 T. To assess the magnetic hardness and mineralogy of samples, measurements of magnetic susceptibility and hysteresis loop parameters were performed. The domain structure (DS) was estimated also from the thermomagnetic criterion by evaluating the tails of pTRMs. Wilson's method of palaeointensity determination based on comparison of thermodemagnetization curves

  19. The influence of volatiles on the interaction of mafic and felsic magmas (United States)

    Pistone, M.; Jarvis, P.; Blundy, J. D.


    Mantle-derived mafic magmas provide heat, mass and volatiles to felsic plutons emplaced in the Earth's crust. Inputs of mafic magma lead to a wide range of physical and chemical interactions and have been frequently invoked as a means of sustaining shallow magmatic bodies and even triggering volcanic eruptions, like the case of the 2010 Eyjafjallajökull volcanic eruption where a mafic batch induced remobilization and explosion of a felsic reservoir. There is the general need to understand how hydrous mafic magmas interact with compositionally evolved plutons and how volatiles contribute to physical and chemical changes operating in mafic-felsic magma interaction. Thus, we experimentally constrained the effect of volatiles (mainly H2O) on the interaction of water-saturated mafic magmas and water-saturated felsic crystal mushes in the shallow crust (solid crystals (50 to 80 vol.% quartz crystals) and water-saturated (6 wt.% H2O) andesitic glass (initially crystal-free mafic end-member), for maximum 24 hour duration of experiment. The two specimens were encapsulated in welded Au capsules, buffered with NNO +1 to +2. Microstructural and chemical results of the run products display interesting features such as: i) reduction of crystal size in the mafic end-member towards to the interface of the two samples ('chemical quenching'); ii) mafic percolation into the felsic crystal mush; iii) formation of a dacitic melt-enriched hornblende-bearing 'hybrid front' at the interface of the two samples; iv) efflux of water into the felsic mush (5 to 8 wt.% in the residual melt) and generation of hornblende coronas around quartz crystals close to the interface of the two samples; v) lack of alkali (particularly, K) 'uphill diffusion' in the mafic end-member. The experimental and analytical results are against the existing state-of-the-art that does not consider the influence of volatiles in a realistic scenario where two magmas display different initial composition, water content

  20. High level triggers for explosive mafic volcanism: Albano Maar, Italy (United States)

    Cross, J. K.; Tomlinson, E. L.; Giordano, G.; Smith, V. C.; De Benedetti, A. A.; Roberge, J.; Manning, C. J.; Wulf, S.; Menzies, M. A.


    Colli Albani is a quiescent caldera complex located within the Roman Magmatic Province (RMP), Italy. The recent Via dei Laghi phreatomagmatic eruptions led to the formation of nested maars. Albano Maar is the largest and has erupted seven times between ca 69-33 ka. The highly explosive nature of the Albano Maar eruptions is at odds with the predominant relatively mafic (SiO2 = 48-52 wt.%) foiditic (K2O = 9 wt.%) composition of the magma. The deposits have been previously interpreted as phreatomagmatic, however they contain large amounts (up to 30%vol) of deep seated xenoliths, skarns and all pre-volcanic subsurface units. All of the xenoliths have been excavated from depths of up to 6 km, rather than being limited to the depth at which magma and water interaction is likely to have occurred, suggesting an alternative trigger for eruption. High precision geochemical glass and mineral data of fresh juvenile (magmatic) clasts from the small volume explosive deposits indicate that the magmas have evolved along one of two evolutionary paths towards foidite or phonolite. The foiditic melts record ca. 50% mixing between the most primitive magma and Ca-rich melt, late stage prior to eruption. A major result of our study is finding that the generation of Ca-rich melts via assimilation of limestone, may provide storage for significant amounts of CO2 that can be released during a mixing event with silicate magma. Differences in melt evolution are inferred as having been controlled by variations in storage conditions: residence time and magma volume.

  1. Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes (United States)

    Jakobsen, Jakob K.; Tegner, Christian; Brooks, C. Kent; Kent, Adam J. R.; Lesher, Charles E.; Nielsen, Troels F. D.; Wiedenbeck, Michael


    Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47.4-49.0 wt.% SiO2, 13.4-14.9 wt.% Al2O3 and 10.7-14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas, overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism.

  2. Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes

    DEFF Research Database (Denmark)

    Jakobsen, J.K.; Tegner, Christian; Brooks, Kent;


    Abstract Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47...

  3. Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps) (United States)

    Langone, Antonio; José Alberto, Padrón-Navarta; Zanetti, Alberto; Mazzucchelli, Maurizio; Tiepolo, Massimo; Giovanardi, Tommaso; Bonazzi, Mattia


    A detailed structural, geochemical and geochronological survey was performed on zircon grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke consists mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding meta-diorites show evidence of ductile deformation occurred under amphibolite-facies conditions. Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by fine grained plagioclase-dominated matrix and pressure shadows, typically filled by oxides. Fractures and cracks within zircon are common and can be associated with grain displacement or they can be filled by secondary minerals (oxides and chlorite). Cathodoluminescence (CL) images show that zircon grains have internal features typical of magmatic growth, but with local disturbances. However EBSD maps on two selected zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca. 10o. The majority of the domains of the mosaic texture are related to parting and fractures, but some domains show no clear relation with brittle features. Rotation angles related to the mosaic texture are not crystallographically controlled. In addition, one of the analysed zircons shows clear evidence of plastic deformation at one of its corners due to indentation. Plastic deformation results in gradual misorientations of up to 12o, which are crystallographically controlled. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections and designed to cover the entire exposed surface of selected grains. Such investigations revealed a strong

  4. Dyke-Davidoff-Masson Syndrome. An unusual cause of status epilepticus. (United States)

    Zawar, Ifrah; Khan, Ashfa A; Sultan, Tipu; Rathore, Ahsan W


    The Dyke-Davidoff-Masson Syndrome (DDMS) results from an insult to the growing brain in utero or early infancy, which lead to loss of neurons compromising the growth of the brain. Clinical presentation includes seizures, hemiparesis, facial asymmetry, and learning disability. Radiological findings include cerebral atrophy on one side. Here, we present a case with status epilepticus who had underlying DDMS. It is a rare syndrome and uncommon cause for status epilepticus. Infections of CNS, hypoxic ischemic encephalopathy, intracranial bleed, trauma, congenital vascular malformations are the common causes of this syndrome. Diagnosis is established after clinical history, examination, and MRI. Intractable seizures can be controlled with appropriate anticonvulsants. Subsequently, these children may require physiotherapy, speech therapy, and occupational therapy in addition to the anticonvulsant medication. Outcome is better if the seizures are controlled.

  5. Dyke propagation and tensile fracturing at high temperature and pressure, insights from experimental rock mechanics. (United States)

    Bakker, Richard; Benson, Philip; Vinciguerra, Sergio


    It is well known that magma ascends trough the crust by the process of dyking. To enable dyke emplacement, basement rocks typically fail in a mode 1 fracture, which acts as conduits for magma transport. An overpressure of the ascending magma will further open/widen the fracture and permit the fracture to propagate. In order to further understand the emplacement and arrest of dykes in the subsurface, analogue and numerical studies have been conducted. However, a number of assumptions regarding rock mechanical behaviour frequently has to be made as such data are very hard to directly measure at the pressure/temperature conditions of interest: high temperatures at relatively shallow depths. Such data are key to simulating the magma intrusion dynamics through the lithologies that underlie the volcanic edifice. Here we present a new laboratory setup, which allows us to investigate the tensile fracturing properties under both temperature and confining pressure, and the emplacement of molten material within the newly formed fracture. We have modified a traditional tri-axial test assembly setup to be able to use a Paterson type High Pressure, High Temperature deformation apparatus. Sample setup consists of cylindrical rock samples with a 22 mm diameter and a 8 mm bore at their centre, filled with a material chosen as such that it's in a liquid state at the experimental temperature and solid at room temperature to enable post-experiment analysis. The top and lower parts of the rock sample are fitted with plugs, sealing in the melt. The assembly is then placed between ceramic pistons to ensure there are no thermal gradients across the sample. The assembly is jacketed to ensure the confining medium (Ar) cannot enter the assembly. A piston is driven into the sample such that the inner conduit materials pressure is slowly increased. At some point a sufficient pressure difference between the inner and outer surfaces causes the sample to deform and fail in the tensile regime

  6. Adult Presentation of Dyke-Davidoff-Masson Syndrome: A Case Report

    Directory of Open Access Journals (Sweden)

    Ujjawal Roy


    Full Text Available Dyke-Davidoff-Masson syndrome (DDMS is a rare disease which is clinically characterized by hemiparesis, seizures, facial asymmetry, and mental retardation. The classical radiological findings are cerebral hemiatrophy, calvarial thickening, and hyperpneumatization of the frontal sinuses. This disease is a rare entity, and it mainly presents in childhood. Adult presentation of DDMS is unusual and has been rarely reported in the medical literature. Key Messages: DDMS is a rare disease of childhood. However, it should be kept in mind as a diagnostic possibility in an adult who presents with a long duration of progressive hemiparesis with seizures and mental retardation. Cerebral hemiatrophy, calvarial thickening, and hyperpneumatization of the frontal sinuses are diagnostic for this illness on brain imaging.

  7. Shoshonitic and ultrapotassic post-collisional dykes from northern Karakorum (Sinkiang, China) (United States)

    Pognante, Ugo


    High-K calc-alkaline, shoshonitic and ultrapotassic post-collisional dykes of Neogene age have been found in the remote and little known region of northern Karakorum located around the Shaksgam valley, north of the K2-Gasherbrum range (China). The dykes derive from more or less comparable basic magma(s) and display rather unusual petrographic and geochemical characters with respect to the other K-rich rocks. The geochemical data are consistent with derivation of the basic magma(s) from small degrees of partial melting of garnet-lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the Indian plate beneath Karakorum. The spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene, phlogopite, plagioclase, garnet±amphibole followed by the precipitation of abundant alkali feldspar, amphibole±apatite±quartz. Additionally, assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz ( ± plagioclase) xenocrysts and by the occasional presence of granitic xenoliths. An apparent connection exists between magmatism and tectonism in the complex Karakorum Fault Zone (KFZ). It is suggested that, during the Neogene, the strike-slip KFZ and some adjacent post-metamorphic faults transiently behaved as extensional fault systems down to deep levels, triggering ascent and emplacement of the K-rich magma. The subsequent (re)activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper Baltoro Glacier and K2-Gasherbrum range, relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the Shaksgam valley area.

  8. 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


    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.

  9. Sapphirine-bearing high pressure mafic granulite and its implications in the south Altyn Tagh

    Institute of Scientific and Technical Information of China (English)

    ZHANG Jianxin; MENG Fancong


    Sapphirine-bearing high pressure mafic granulites are recognized in the Bashiwake area of the south Altyn Tagh. Mafic granulites and associated peridotites occur as lenses within kyanite-bearing high pressure felsic granulites. Sapphirine occurs as intergrowths with corundum, spinel and Ca-rich plagioclase in symplectite in between garnet and clinopyroxene. Reaction textures and mineral compositions indicate that sapphirine-bearing symplectites formed by reactions consuming kyanite, garnet and clinopyroxene. Therefore, both petrographic observations and the mineral compositions suggest that these quartz-free mafic granulites had an eclogite origin with an assemblage of garnet + clinopyroxene + kyanite, which is estimated to be stable at T = 1000℃ and P≥2.1 Gpa. The sapphirine-bearing symplectites are resulted from isothermal decompression or decompression with slight cooling during exhumation of the host rock.

  10. Orogenic potassic mafic magmatism, a product of alkaline-peraluminous mixing ? Variscan 'calc-alkaline' rocks from the Central Iberian and Ossa Morena Zones, Central Spain. (United States)

    Scarrow, Jane H.; Cambeses, Aitor; Bea, Fernando; Montero, Pilar; Molina, José F.; Moreno, Juan Antonio


    Orogenic magmatic rocks provide information about mantle and crust melt-generation and -interaction processes. In this context, minor potassic mafic stocks which are formed of enriched mantle and crustal components and are common as late-orogenic intrusions in granitic plutons give insight into the timing of new crust formation and crustal recycling. Potassic mafic stocks are prevalent, albeit low volume, constituents of granite batholiths all through the European Variscan (350-280 Ma). In the Central Iberia Zone, Spanish Central System, crustal-melt, S-type, granitoid plutons are intruded by minor concomitant ultramafic-intermediate appinitic-vaugneritic stocks. Notwithstanding their whole-rock calc-alkaline composition, the stocks apparently did not have a subduction-related origin. Recent studies have attributed their genesis to mixing of alkaline mantle and peraluminous crustal melts. Their primary alkaline character, as indicated by amphibole and biotite mineral chemistry data, points, rather, towards an extension-related genesis. In the Ossa Morena Zone, south of the Central Iberian Zone, the igneous rocks also have a whole-rock calc-alkaline composition which has been considered to be the result of northward subduction of the South Portuguese Zone. Nevertheless, identification of a 'sill' of significant volume of mafic magma in the middle crust, the ´IBERSEIS reflective body', in a seismic profile across the Ossa Morena and South Portuguese Zones has cast doubt upon the calc-alkaline magmatism-subduction model; leading, instead, to the magmatism being attributed to intra-orogenic extension related to a mantle plume active from 340 Ma to 330 Ma. The aim here, then, is to reinvestigate the petrogenesis and age of the calc-alkaline rocks of the Ossa Morena Zone to determine their tectonomagmatic context be it subduction-, plume- or extension-related, and establish what they may reveal about mantle-crust interactions. Focussing, initially, on the Valencia del

  11. Magma Emplacement and Mafic-Felsic Magma Hybridisation: Structural, Microstructural and Geochemical Evidences From the Pan-African Negash Pluton, Northern Ethiopia (United States)



    The Negash pluton (50 sq. km) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics and metasediments. This almost circular massif consists of monzogranites, granodiorites, monzodiorites, monzogabbros, and hybrid quartz monzodiorites. The rocks are enriched in LIL-elements, depleted in HFS-elements, have fractionated REE patterns, low 87Sr/86Sri (0.702344 - 0.703553) and 143Nd/144Ndi (0.512031 - 0.512133) ratios, positive ɛ Nd values (3.46 to 5.40), and Pan-African model Nd ages (0.83 to 1.08 Ga). These data, along with single zircon U-Pb dating, show that the pluton was emplaced at 608 Ma from primitive source (underplated material or juvenile island arc crust) with contamination by the country rocks. The pluton shows widespread mafic-felsic magma interactions of two types: (i) homogeneous and heterogeneous hybrid monzodiorites at the northwestern part; and (ii) mingled interfaces at the diorite-granodiorite contact zones in the Eastern and Southeastern parts. Detailed structural (using the method of the anisotropy of magnetic susceptibility) and microstructural studies have been applied to understand how these interactions occurred with respect to pluton construction. The monzodiorites constituting the northwestern part of the pluton, which are composed of complexly zoned plagioclases or unzoned plagioclase laths, euhedral hornblende with biotite cores and acicular apatites, are characterized by abundant net veining, synplutonic dikes, microgranitoid enclaves, and juxtaposed series of discrete mafic-felsic pulses of hybrid nature with vertical syn-emplacement structures. The mingled interfaces between the diorites and granodiorites, on the other hand, are characterized by lobate contacts with interfingering of diorites into granodiorites at the decametric scale, abundant inclined to horizontal granitic pipes, breccia dykes and veins, which are strongly enriched in megacrysts of K-feldspars, and numerous swarms of

  12. A Geochemical Study of an REE-rich Carbonatite Dyke at Bayan Obo, Inner Mongolia, Northern China

    Institute of Scientific and Technical Information of China (English)


    An REE-rich carbonatite dyke was found in Dulahala, close to the Bayan Obo superlarge REE-Nb-Fe mineral deposit in Inner Mongolia, northern China. The REE content in the dyke varies greatly, from 1% up to 20% (wt), which might constitute rich REE ores. Light REEs in the carbonatite are enriched and highly fractionated relative to heavy REEs and there is no Eu anomaly. The REE and trace element distribution patterns of the carbonatite are identical to those of fine-grained dolomite marble which is the host rock of the Bayan Obo REE-Nb-Fe superlarge mineral deposit. This indicates a petrogenetic linkage between the REE-rich carbonatite and the mineralizations in this region.

  13. Magnetic resonance imaging depiction of acquired Dyke-Davidoff-Masson syndrome with crossed cerebro-cerebellar diaschisis: Report of two cases

    Directory of Open Access Journals (Sweden)

    Ranjana Gupta


    Full Text Available Acquired Dyke-Davidoff-Masson syndrome, also known as hemispheric atrophy, is characterized by loss of volume of one cerebral hemisphere from an insult in early life. Crossed cerebellar diaschisis refers to dysfunction/atrophy of cerebellar hemisphere which is secondary to contralateral supratentorial insult. We describe magnetic resonance imaging findings in two cases of acquired Dyke-Davidoff-Masson syndrome with crossed cerebro-cerebellar diaschisis.

  14. Magnetic resonance imaging depiction of acquired Dyke-Davidoff-Masson syndrome with crossed cerebro-cerebellar diaschisis: Report of two cases. (United States)

    Gupta, Ranjana; Joshi, Sandeep; Mittal, Amit; Luthra, Ishita; Mittal, Puneet; Verma, Vibha


    Acquired Dyke-Davidoff-Masson syndrome, also known as hemispheric atrophy, is characterized by loss of volume of one cerebral hemisphere from an insult in early life. Crossed cerebellar diaschisis refers to dysfunction/atrophy of cerebellar hemisphere which is secondary to contralateral supratentorial insult. We describe magnetic resonance imaging findings in two cases of acquired Dyke-Davidoff-Masson syndrome with crossed cerebro-cerebellar diaschisis.

  15. Geochemistry of Metamafic Dykes from the Quanji Massif:Petrogenesis and Further Evidence for Oceanic Subduction, Late Paleoproterozoic, NW China

    Institute of Scientific and Technical Information of China (English)

    Hassan Abdelslam Mustafa; Qinyan Wang; Nengsong Chen; Fanxi Liao; Min Sun; Meshaal Abdelgadir Salih


    A suite of ~1.84–1.92 Ga metamafic dykes within the paragneiss suite (khondalite) of the Quanji massif in NW China, has been chosen in this study for further understanding the tectonic evo-lution and possible links to the global Columbia supercontinent. Occurrence and field relations sug-gest that they were formed coevally with a previous studied ~1.83–1.85 Ga metamafic dyke swarms. Whole-rock major and trace elemental geochemistry suggests precursor magma of the amphibolites being generated from a volcanic arc-related tectonic setting rather than a back-arc environment where the metamafic dyke swarms were emplaced. The metamafic dykes show enrichment of LREE and strongly negative anomalies for Ta-Nb, Zr-Hf and Ti, have high SiO2 (49.3 wt.%–52.5 wt.%) but low MgO (6.40 wt.%–7.76 wt.%) contents and Mg# (Mg#=[100×(MgO/40.3)]/[MgO/40.3+FeO/71.8]) values (45.7–52.1), suggesting evolved precursor magma. The high values of La/Ta (22.2–42.8) and La/Nb (1.71–2.47), mildly negativeεNd(t) values (-2.51–0.15), with depleted mantle model ages (TDM) of 2.45–2.84 Ga, suggest that their precursor magmas were possibly derived from a subduction-related fluid metasomatized Archean sub-continental lithospheric mantle. This study provides further evi-dence for oceanic plate subduction prevailing before or around ~1.85 Ga, which confirms a prolonged subduction-accretion-collision history in the NW China which is possibly linked to the assembly of the Columbia supercontinent.

  16. Quasi 2D hydrodynamic modelling of the flooded hinterland due to dyke breaching on the Elbe River

    Directory of Open Access Journals (Sweden)

    S. Huang


    Full Text Available In flood modeling, many 1D and 2D combination and 2D models are used to simulate diversion of water from rivers through dyke breaches into the hinterland for extreme flood events. However, these models are too demanding in data requirements and computational resources which is an important consideration when uncertainty analysis using Monte Carlo techniques is used to complement the modeling exercise. The goal of this paper is to show the development of a quasi-2D modeling approach, which still calculates the dynamic wave in 1D but the discretisation of the computational units are in 2D, allowing a better spatial representation of the flow in the hinterland due to dyke breaching without a large additional expenditure on data pre-processing and computational time. A 2D representation of the flow and velocity fields is required to model sediment and micro-pollutant transport. The model DYNHYD (1D hydrodynamics from the WASP5 modeling package was used as a basis for the simulations. The model was extended to incorporate the quasi-2D approach and a Monte-Carlo Analysis was used to conduct a flood sensitivity analysis to determine the sensitivity of parameters and boundary conditions to the resulting water flow. An extreme flood event on the Elbe River, Germany, with a possible dyke breach area was used as a test case. The results show a good similarity with those obtained from another 1D/2D modeling study.

  17. Dyke-Davidoff-Masson syndrome: case report of fetal unilateral ventriculomegaly and hypoplastic left middle cerebral artery. (United States)

    Piro, Ettore; Piccione, Maria; Marrone, Gianluca; Giuffrè, Mario; Corsello, Giovanni


    Prenatal ultrasonographic detection of unilateral cerebral ventriculomegaly arises suspicion of pathological condition related to cerebrospinal fluid flow obstruction or cerebral parenchimal pathology. Dyke-Davidoff-Masson syndrome is a rare condition characterized by cerebral hemiatrophy, calvarial thickening, skull and facial asymmetry, contralateral hemiparesis, cognitive impairment and seizures. Congenital and acquired types are recognized and have been described, mainly in late childhood, adolescence and adult ages. We describe a female infant with prenatal diagnosis of unilateral left ventriculomegaly in which early brain MRI and contrast enhanced-MRI angiography, showed cerebral left hemiatrophy associated with reduced caliber of the left middle cerebral artery revealing the characteristic findings of the Dyke-Davidoff-Masson syndrome. Prenatal imaging, cerebral vascular anomaly responsible for the cerebral hemiatrophy and the early clinical evolution have never been described before in such a young child and complete the acquired clinical descriptions in older children. Differential diagnosis, genetic investigations, neurophysiologic assessments, short term clinical and developmental follow up are described. Dyke-Davidoff-Masson syndrome must be ruled out in differential diagnosis of fetal unilateral ventriculomegaly. Early clinical assessment, differential diagnosis and cerebral imaging including cerebral MRI angiography allow the clinicians to diagnose also in early infancy this rare condition.

  18. Retraction notice to "Ca. 2.5 billion year old mafic dykes in western Shandong Province: Implications for hybridization between subducted continental crust and the North China Craton, China" (United States)

    Liu, Shen; Feng, Caixia; Zhai, Mingguo; Hu, Ruizhong; Gao, Shan; Lai, Shaocong; Zou, Haibo; Yan, Jun


    This article has been retracted: please see Elsevier Policy on Article Withdrawal. This article has been retracted at the request of the Editors-in-Chief. The article duplicates significant parts of a paper that had already appeared in, and was withdrawn from Precambrain Research (") One of the conditions of submission of a paper for publication is that authors declare explicitly that the paper is not under consideration for publication elsewhere. Re-use of any data should be appropriately cited. As such this article represents a severe abuse of the scientific publishing system. The scientific community takes a very strong view on this matter and apologies are offered to readers of the journal that this was not detected during the submission process.

  19. Acute myeloblastic leukemia-associated Marfan syndrome and Davidoff-Dyke-Masson syndrome: a case report

    Directory of Open Access Journals (Sweden)

    Ahmet Faik Öner


    Full Text Available We present herein a 23-year-old man with acute myeloblastic leukemia (AML associated with Davidoff-Dyke-Masson syndrome (DDMS and Marfan syndrome (MS. The diagnosis of DDMS was based on findings including left facial asymmetry, left hemiparesis, mental retardation, right cerebral hemiatrophy, dilatation of the ipsilateral lateral ventricle and calvarial thickening. The diagnosis of MS was based on clinical findings including tall stature, myopia, retinitis pigmentosa, blue scleras, scoliosis, pectus excavatum, arachnodactyly and low ratio of upper/lower body segment. The patient developed hepatosplenomegaly, gingival hypertrophy and pancytopenia. Peripheral blood film and bone marrow examination showed that most of nucleated cells were blasts; immunophenotype of those cells showed CD11+, CD13+, CD14+, CD33+ and HLA-DR+. These findings confirmed the diagnosis of AML (FAB-M5. After induction chemotherapy, remission was obtained. To the best of our knowledge, our case is the third report of AML in MS syndrome, while AML associated with DDMS and MS has not been previously reported in the literature.

  20. Nb/Ta variations of mafic volcanics on the Archean-Proterozoic boundary: Implications for the Nb/Ta imbalance

    Institute of Scientific and Technical Information of China (English)

    LIU Yongsheng; GAO Shan; WANG Xuance; HU Shenghong; WANG Jianqi


    The HFSE and REE of the Precambrian mafic volcanics from the North China craton demonstrate obvious A(Archean)-P(Proterozoic) boundary. The Neoarchean mafic vol-canics show weak correlation between HFSE and TiO2. Their superchondritic Nb/Ta ratio (18.8(1.2) could be attributed to partial melting of mantle peridotite in the presence of garnet. Compared with Neoarchean mafic volcanics, the Paleoproterozoic ones have higher HFSE contents and lower Nb/Ta ratio (15.6(2.9). The significantly elevated HFSE and REE contents of Paleoproterozoic mafic volcanics imply metasomatic enrichment of mantle source, in which Ti-rich silicates could be present as suggested by significant positive correlations between TiO2 and HFSE. The global database of Precambrian mafic volcanics shows a similar A-P boundary. 23 Archean mafic volcanic suites yield an average Nb/Ta ratio of 17.8(1.9 higher than or close to the PM value; Proterozoic mafic volcanics from 28 suites yield an average Nb/Ta ratio of 14.7(4.1 deficit could be mainly formed in post-Archean time. Archean mafic volcanics could be one of the geochemical reservoirs complementing the low Nb/Ta of the post-Archean continental crust and DM.

  1. Can we see the distal dyke communicate with the caldera? Examples of temporal correlation analysis using seismicity from the Bárðarbunga volcano (United States)

    Jónsdóttir, Kristín; Jónasson, Kristján; Tumi Guðmundsson, Magnús; Hensch, Martin; Hooper, Andrew; Holohan, Eoghan; Sigmundsson, Freysteinn; Halldórsson, Sæmundur Ari; Vogfjörð, Kristín; Roberts, Matthew; Barsotti, Sara; Ófeigsson, Benedikt; Hjörleifsdóttir, Vala; Magnússon, Eyjólfur; Pálsson, Finnur; Parks, Michelle; Dumont, Stephanie; Einarsson, Páll; Guðmundsson, Gunnar


    The Bárðarbunga volcano is composed of a large oval caldera (7x11 km) and fissures extending tens of kilometers away from the caldera along the rift zone, which marks the divergent plate boundary across Iceland. On August 16th, 2014 an intense seismic swarm started below the Bárðarbunga caldera and in the two weeks that followed a dyke migrated some 47 km laterally in the uppermost 6-10 km of the crust along the rift. The dyke propagation terminated in lava fields just north of Vatnajökull glacier, where a major (1.5 km3) six months long eruption took place. Intense earthquake activity in the caldera started in the period August 21-24 with over 70 M5 earthquakes accompanying slow caldera collapse, as verified by various geodetic measurements. The subsidence is likely due to magma withdrawal from a reservoir at depth beneath the caldera. During a five months period, October-February, the seismic activity was separated by over 30 km in two clusters; one along the caldera rims (due to piecewise caldera subsidence) and the other at the far end of the dyke (as a result of small shear movements). Here we present statistical analysis comparing the temporal behaviour of seismicity recorded in the two clusters. By comparing the earthquake rate in the dyke in temporal bins before and after caldera subsidence earthquakes to the rate away from these bins (background rate), we show posing a statistical p-value test, that the number of dyke earthquakes was significantly higher (p earthquake (>M4.6) in the caldera. Increased dyke seismicity was also observed 0-3 hours following a large caldera earthquake. Elevated seismicity in the dyke before a large caldera earthquake may occur when a constriction in the dyke was reduced, followed by pressure drop in the chamber. Assuming that the large caldera earthquakes occurred when chamber pressure was lowest, the subsiding caldera piston may have caused temporary higher pressure in the dyke and thereby increased the likelihood of an

  2. Geochronology and Geochemistry of Mafic Dikes from Hainan Island and Tectonic Implications

    Institute of Scientific and Technical Information of China (English)

    CAO Jianjin; HU Ruizhong; LIU Shen; XIE Guiqing


    In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K-Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole-rock K-Ar dating yielded a magmatic duration of 61-98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements (LREE), and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial ~(87)Sr/~(86)Sr ratios and εSr(t) of the mafic dikes are 0.70634-0.71193 and +27.7 to +112.2, respectively. In the ~(87)Sr/~(86)Sr versus εNd(t) diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle (source region) of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophile elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basaits mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages: (1) the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; (2) the thinning and extension stage during 61-98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and (3) the large-scale thinning and extension stage after 61 Ma. The large-scale asthenospheric upwelling results in the strong

  3. Mineral chemistry of monazite-(Nd, xenotime-(Y, apatite, fluorite and zircon hosting in lamprophyre dyke in Abu Rusheid area, South Eastern Desert, Egypt

    Directory of Open Access Journals (Sweden)

    Mohamed A. Ali


    Full Text Available The studied mineralized lamprophyre dyke in Abu Rusheid area is trending NNW-SSE, and occurs withinAbu Rusheid mineralized shear zone, measuring 0.2 - 1.0 m in width and 0.5 - 1.0 km in length. It was emplacedparallel with the Abu Rusheid shear zone. The dyke is mainly composed of plagioclases, amphiboles, mica (musco-vite and biotite, relics of pyroxenes with K-feldspars and quartz derived from surrounding country rocks asphenocrysts embedded in fine-grained groundmass. The lamprophyre dyke hosts REE-minerals monazite-(Nd,xenotime-(Y, and REE-bearing minerals apatite, fluorite, zircon-(Hf, rutile with inclusions of xenotime and ironoxides. The emplacement of lamprophyre dyke caused heating in the mineralized shear zone of Abu Rusheid area.The lamprophyre dyke was subsequently affected by hydrothermal alterations (e.g. chlorite-carbonate, muscovitization,fluoritization.The REE were remobilized from the mineralized shear zones by hydrothermal solutionsand re-precipitatedas REE-minerals xenotime-(Y and monazite-(Nd around flourapatite, fluorite, zircon andrutile. The solid solutions between monazite-(Nd and xenotime-(Y were formed as a product precipitation fromhydrothermal solutions. Also, the apatite mineral in the lamprophyre dyke was subjected to the heating duringthe emplacement, which lead to its alteration and breakdown with concominant precipitation of xenotime-(Yand monazite-(Nd. The chemistry of monazite-(Nd and xenotime-(Y obtained by scanning electron microscopy(SEM, and electron probe microanalysis (EPMA, showed that these minerals are enriched in U and Th. Themonazite-(Nd associated with fluorapatite in the studied dyke is poor in Th (0.02 ≤ Th ≤ 0.81 wt%, but usuallyrich in U (0.92 ≤ U ≤ 2.91 wt%, which indicates that monazite formed as a result of flourapatite metasomatism.

  4. Flood risk control of dams and dykes in middle reach of Huaihe River

    Directory of Open Access Journals (Sweden)

    Zhen-kun MA


    Full Text Available Three stochastic mathematical models for calculation of the reservoir flood regulation process, river course flood release, and flood risk rate under flood control were established based on the theory of stochastic differential equations and features of flood control systems in the middle reach of the Huaihe River from Xixian to the Bengbu floodgate, comprehensively considering uncertain factors of hydrology, hydraulics, and engineering control. They were used to calculate the flood risk rate with flood regulation of five key reservoirs, including the Meishan, Xianghongdian, Nianyushan, Mozitan, and Foziling reservoirs in the middle reach of the Huaihe River under different flood frequencies, the flood risk rate with river course flood release under design and check floods for the trunk of the Huaihe River in conjunction with relevant flood storage areas, and the flood risk rate with operation of the Linhuaigang Project under design and check floods. The calculated results show that (1 the five reservoirs can withstand design floods, but the Xianghongdian and Foziling reservoirs will suffer overtopping accidents under check floods; (2 considering the service of flood storage areas under the design flood conditions of the Huaihe River, the mean flood risk rate with flood regulation of dykes and dams from Xixian to the Bengbu floodgate is about 0.2, and the trunk of the Huaihe River can generally withstand design floods; and (3 under a check flood with the flood return period of 1 000 years, the risk rate of overtopping accidents of the Linhuaigang Project is not larger than 0.15, indicating that it has a high flood regulation capacity. Through regulation and application of the flood control system of the Linhuigang Project, the Huaihe River Basin can withstand large floods, and the safety of the protected area can be ensured.

  5. Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province, USA (United States)

    Wolff, John A.; Ellwood, Brooks B.; Sachs, Scott D.


    Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.

  6. Petrological and geochemical studies of ultramafic–mafic rocks from the North Puruliya Shear Zone (eastern India)

    Indian Academy of Sciences (India)

    Aditi Mandal; Arijit Ray


    Ultramafic and mafic rocks occur within a linear belt, trending nearly E–W along North Puruliya Shear Zone of the Chhotanagpur Gneissic Complex (CGC). These rocks are classified as gabbro, norite, gabbronorite, dolerite, diorite, olivine-websterite and lherzolite. Mafic rocks (Group 1) often occur in association with ultramafic variants (Group 2) and sometimes in isolation. A genetic link has been established between these mafic and ultramafic rocks using disposition of ultramafic and mafic rocks in the outcrop, systematic variation in modal mineralogy, co-linearity of plots in biaxial chemical variation diagram. Chemical composition of biotite and clinopyroxene reveal calc-alkaline nature and arc signature in these mafic–ultramafic rocks and whole rock geochemical characters indicate similarity with arc magma in subduction zone setting. The high values of Mg no. (47–81) and Al2O3 (5.5–17.9) of mafic rocks indicate primitive, aluminous nature of the parental melt and presence of amphibole and biotite indicate its hydrous nature. The parent mafic melt evolved through fractionation of olivine, spinel, clinopyroxene and plagioclase. The crystal cumulates gave rise to the ultramafic rocks and the associated mafic rocks formed from residual melt. Crustal contamination played an important role in magmatic evolution as evident from variation in abundance of Rb in different lithomembers. Mafic–ultramafic rocks of the present study have been compared with intra-cratonic layered complexes, mafic–ultramafic rocks of high grade terrain, Alaskan type ultramafic–mafic complex and ophiolites. It is observed that the ultramafic–mafic rocks of present study have similarity with Alaskan type complex.

  7. Spatial greenstone-gneiss relationships: Evidence from mafic-ultramafic xenolith distribution patterns (United States)

    Glikson, A. Y.


    The distribution patterns of mafic-ultramafic xenoliths within Archaean orthogneiss terrain furnish an essential key for the elucidation of granite-greenstone relations. Most greenstone belts constitute mega-xenoliths rather than primary basin structures. Transition along strike and across strike between stratigraphically low greenstone sequences and xenolith chains demonstrate their contemporaneity. These terrains represent least deformed cratonic islands within an otherwise penetratively foliated deformed gneiss-greenstone crust. Whereas early greenstone sequences are invariably intruded by tonalitic/trondhjemitic/granodioritic gneisses, stratigraphically higher successions may locally overlap older gneiss terrains and their entrained xenoliths unconformably. The contiguity of xenolith patterns suggests their derivation as relics of regional mafic-ultramafic volcanic crustal units and places limits on horizontal movements between individual crustal blocks.

  8. Sentinel-1 TOPS interferometry for geophysical applications: Dyke intrusion imaged during 2014 Pico do Fogo eruption (United States)

    Gonzalez, Pablo J.; Marinkovic, Petar; Samsonov, Sergey; Hooper, Andrew; Larsen, Yngvar; Wright, Tim


    apply Sentinel-1 TOPS-InSAR to illuminate the deformation accompanying a recent volcanic eruption at Pico do Fogo volcano, Fogo (Cape Verde). The detected deformation is consistent with a shallow near-vertical dyke intrusion, which fed the fissure eruption. The modelling of the deformation interferograms do not seem affected largely by the variable LoS vector, in this case. The presented results prove the potential of Terrain observation by progressive scans (TOPS) interferometry for geophysical applications, in particular using ESA Sentinel-1 mission.

  9. A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

    Institute of Scientific and Technical Information of China (English)

    Y.A. Cook; I.V. Sanislav; J. Hammerli; T.G. Blenkinsop; P.H.G.M. Dirks


    Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.%MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf ¼ 32e35, Ti/Zr ¼ 107e147), producing flat REE and HFSE profiles [(La/Yb)pm ¼ 0.9 e1.3], with abundances of 3e10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm ¼ 0.6e0.8] and Th [(Th/La)pm ¼ 0.6e0.9]. Initial isotope compositions (3Nd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping w50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between w2830 and w2820 Ma; are compo-sitionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.

  10. The age and constitution of Cerro Campanario, a mafic stratovolcano in the Andes of central Chile (United States)

    Hildreth, W.; Singer, B.; Godoy, E.; Munizaga, F.


    Cerro Campanario, a towering landmark on the continental divide near Paso Pehuenche, is a glacially eroded remnant of a mafic stratovolcano that is much younger than previously supposed. Consisting of fairly uniform basaltic andesite, rich in olivine and plagioclase, the 10-15 km3 edifice grew rapidly near the end of the middle Pleistocene, about 150-160 ka, as indicated by 40Ar/39Ar and unspiked K-Ar analyses of its lavas.

  11. Physical abrasion of mafic minerals and basalt grains: application to Martian aeolian deposits (United States)

    Cornwall, Carin; Bandfield, Joshua L.; Titus, Timothy N.; Schreiber, B. C.; Montgomery, D.R.


    Sediment maturity, or the mineralogical and physical characterization of sediment deposits, has been used to locate sediment source, transport medium and distance, weathering processes, and paleoenvironments on Earth. Mature terrestrial sands are dominated by quartz, which is abundant in source lithologies on Earth and is physically and chemically stable under a wide range of conditions. Immature sands, such as those rich in feldspars or mafic minerals, are composed of grains that are easily physically weathered and highly susceptible to chemical weathering. On Mars, which is predominately mafic in composition, terrestrial standards of sediment maturity are not applicable. In addition, the martian climate today is cold, dry and sediments are likely to be heavily influenced by physical weathering rather than chemical weathering. Due to these large differences in weathering processes and composition, martian sediments require an alternate maturity index. Abrason tests have been conducted on a variety of mafic materials and results suggest that mature martian sediments may be composed of well sorted, well rounded, spherical basalt grains. In addition, any volcanic glass present is likely to persist in a mechanical weathering environment while chemically altered products are likely to be winnowed away. A modified sediment maturity index is proposed that can be used in future studies to constrain sediment source, paleoclimate, mechanisms for sediment production, and surface evolution. This maturity index may also provide details about erosional and sediment transport systems and preservation processes of layered deposits.

  12. A 100 Ma bimodal composite dyke complex in the Jiamusi Block, NE China: An indication for lithospheric extension driven by Paleo-Pacific roll-back (United States)

    Sun, Ming-Dao; Chen, Han-Lin; Zhang, Feng-Qi; Wilde, Simon A.; Dong, Chuan-Wan; Yang, Shu-Feng


    A 125 m-wide bimodal composite dyke complex, consisting of rhyolite and dolerite dykes, was emplaced into Cretaceous volcanic strata of the Songmuhe Formation in the Jiamusi Block of NE China. The dolerite dykes are sub-vertical, strike north-south, and intruded into both the country rocks and rhyolite dykes soon after the latter solidified. SHRIMP zircon U-Pb dating shows that the rhyolite dykes were emplaced at 100 ± 2 Ma and the dolerite dykes were also most likely emplaced at 100 ± 2 Ma. The rhyolite is characterized by enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and depletion in high-field strength elements (HFSE). It shows a significant negative Eu anomaly, and has ɛNd(t) values ranging from 0.49 to 1.66 and two groups of initial 87Sr/86Sr ratios at 0.7045 and 0.7061. The rhyolite displays the compositional signature of Peraluminous Ferroan Granitoid, indicating it was derived by either differentiation of basalt and/or low pressure partial melting of crust. The dolerite is also characterized by enrichment in LILE and LREE, and depletion in HFSE. It has a weak negative Eu anomaly and has ɛNd(t) = - 1.22 to + 3.26, and (87Sr/86Sr)i = 0.7057-0.7074. The dolerite originated from partial melting of lithospheric mantle which was affected by subducted oceanic crust, and experienced different amounts of crustal contamination. Such bimodal dyke complexes are an important indicator of crustal extension under the influence of mantle processes. Thus the dyke complex in the Jiamusi Block indicates mid-Cretaceous intra-plate extension in NE China related to the subduction of the paleo-Pacific plate along the eastern Eurasian continental margin. When compared with Mesozoic bimodal magmatism further to the west, our new data support a temporal eastward migration of magmatism over a distance > 1000 km from ~ 160 Ma to ~ 100 Ma. This was most likely associated with roll-back of the paleo-Pacific Plate and consequent upwelling of

  13. Polarization of mafic and felsic rocks In the Skaergaard Layered Series (Invited) (United States)

    McBirney, A. R.; Johnston, A.; Webster, J. D.


    When it became apparent that plagioclase could never sink in a magma as iron rich and dense as that of the Skaergaard Intrusion, we were faced with the problem of explaining the formation of anorthositic layers, foundered blocks, and schlieren that consist almost entirely of plagioclase but were obviously stable on the floor of the intrusion. Sonnenthal (Jour. Pet., 1998, 39: 633-661) found that gabbroic blocks that fell from the roof series were originally more gabbroic and were altered metasomatically to anorthosites after they reached the floor were buried it the advancing front of crystallization. Their mafic components were expelled into the surrounding gabbro and replaced by plagioclase. Similar processes appear to have transformed parts of the Layered and Marginal Border Series into strongly polarized anorthosites and olivine pyroxenites. The studies of Filiberto and Treiman (Chem. Geol. 2009, 263: 50-68) of the effect of chlorine on the liquidus of basalts, together with our on-going experimental investigation of its effect on the system plagioclase-pyroxene, has opened new possibilities that may enable us to explain these enigmatic rocks. When Webster and his co-workers (Geoch Cosmoch. Acta, 2009, 73: 559-581) showed that, unlike water, chlorine is much more soluble in mafic silicate melts than felsic ones, these contrasting properties suggested that the effect of chlorine on the cotectic relations of plagioclase and pyroxene might be the opposite of that of water: it could depress the melting temperature of pyroxene just as water depressed that of plagioclase. Experiments currently underway at the American Museum of Natural History and University of Oregon are showing that this is indeed the case. Consider what might happen if a crystallizing gabbro were infiltrated by a chlorine-rich aqueous fluid of the kind that Larsen and Brooks (Jour. Pet., 1997, 35: 1651-79) have shown permeated the rocks while they were still at a high temperature. Flux melting

  14. Decoupling of whole-rock Nd-Hf and zircon Hf-O isotopic compositions of a 284 Ma mafic-ultramafic intrusion in the Beishan Terrane, NW China (United States)

    Su, Ben-Xun; Qin, Ke-Zhang; Lu, Ying-Huai; Sun, He; Sakyi, Patrick Asamoah


    Abundant Permian mafic-ultramafic intrusions in the Beishan Terrane, NW China, are parts of the Tarim large igneous province. Among these intrusions, Luodong intrusion is composed of dunite, wehrlite and gabbro. These rocks have whole-rock Sr-Nd-Hf and zircon Hf-O isotope compositions that display significant decoupling. The decoupling of these generally well-correlated systems demonstrates contrasting evolving trends. Systematic compositional and mineralogical controls on decoupling have been investigated. Wehrlites and gabbros show MORB-like trace element patterns with negligible crustal contamination. They have high initial 143Nd/144Nd [ɛNd(t) = +6.6 to +11.2] and 176Hf/177Hf [ɛHf(t) = +12.2 to +16.9] and low initial 87Sr/86Sr [(87Sr/86Sr)i = 0.702949-0.704098] ratios and plot within the MORB field, indicating that their parental magmas were derived from a depleted mantle source. The enrichment features are present in the zircon crystals separated from the gabbro. These homogeneously unzoned zircon crystals have high U concentrations, low Th/U ratios and a U-Pb age of 284.0 ± 2.3 Ma. They are therefore interpreted as having formed rapidly in a highly fractionated/evolved magma. Their ɛHf(t) and δ18O values range from -9.3 to -6.7 and +10.25 ‰ to +11.42 ‰, respectively. The decoupling is linked with crustal contamination by ancient crust (probably Proterozoic schist in the Beishan Terrane) that occurred during zircon crystallization. However, the contamination signature in the whole-rock composition was soon overprinted by magma mixing process. The inference is evidenced by zoning textures preserved in plagioclase, clinopyroxene, spinel and particularly olivine, and the presence of coeval diabase dykes cutting through the intrusion. The isotope decoupling observed in this mantle-plume-related mafic-ultramafic intrusion supports the idea that Nd-Hf decoupling and Lu/Hf and Sm/Nd parent/daughter variations exist only on a small hand-size scale in a

  15. Study on Carbonate Limestone Dykes (Neptunian Dykes) of Upper Devonian in Nanbiancun, Guilin, Guangxi%广西桂林南边村上泥盆统沉积灰岩脉研究

    Institute of Scientific and Technical Information of China (English)

    彭阳; 陆刚; 胡贵昂


    、亮晶胶结物、铁泥质等,与孔隙水混合形成粘稠的液化颗粒灰岩流,灌入这些张裂隙.二者裂隙成因均与台缘同沉积断层的构造—地震活动密切相关.即:同沉积断层发生时,在南边村剖面层45中产生刚性张裂隙,而其上覆地层的沉积物还处在弱固结的塑性状态,发生混合、颗粒化形成塑性液化颗粒流灌入下伏裂隙中.因此,南边村剖面层45中的沉积灰岩脉是软硬岩层对同沉积断层活动诱发地震的不同响应.而南边村剖面晚泥盆世所处的台缘靠近斜坡的古地理位置是其沉积灰岩脉的内部结构构造与众不同的原因.%Carbonate neptunian dykes occurred commonly in Late Palaeozoic carbonate plateform margins in Guilin and Northwestern Youjiang basin, Guangxi Autonomous Region. In the Nanbiancun International Devonian- Carboniferous (bed 55 ~ 56) Boundary (DCB) Parastratotype Section of Guilin of Guangxi, carbonate neptunian dykes are identified in Bed 45 (a liquefied grainstone vein) and Bed 50 (some fine crystal fiber calcite veins, as signal of tension structure background) and is considered far different from those in the Miaotou section of Guilin and Youjiang Basin. In the Miaotou section of Guilin and the Youjiang Basin, the infillings of neptunian dyke are afterwards normal sea water sediments, while in the Nanbiancun section of Guilin are simultaneous overlain liquefied soft-sediment-flow, shown as skeletals and non-skeletal grains ( with dark Fe-bearing clays, fine calcites and fine skeletals) mixture. The non-skeletal grains deformed and impinged each other with very few cements among them That means they are not normal sea water deposits. The skeletal grains are mainly brachiopod and crinoid stems crowded floating in non-skeletal grains. A single Brachiopods shell are folded as "U" shape and then cut into several continuous parts in situ. That means a slowly soft press surrounding and wholly flowing. The upper part of the

  16. Dyke leakage localization and hydraulic permeability estimation through self-potential and hydro-acoustic measurements: Self-potential 'abacus' diagram for hydraulic permeability estimation and uncertainty computation (United States)

    Bolève, A.; Vandemeulebrouck, J.; Grangeon, J.


    In the present study, we propose the combination of two geophysical techniques, which we have applied to a dyke located in southeastern France that has a visible downstream flood area: the self-potential (SP) and hydro-acoustic methods. These methods are sensitive to two different types of signals: electric signals and water-soil pressure disturbances, respectively. The advantages of the SP technique lie in the high rate of data acquisition, which allows assessment of long dykes, and direct diagnosis in terms of leakage area delimitation and quantification. Coupled with punctual hydro-acoustic cartography, a leakage position can be precisely located, therefore allowing specific remediation decisions with regard to the results of the geophysical investigation. Here, the precise localization of leakage from an earth dyke has been identified using SP and hydro-acoustic signals, with the permeability of the preferential fluid flow area estimated by forward SP modeling. Moreover, we propose a general 'abacus' diagram for the estimation of hydraulic permeability of dyke leakage according to the magnitude of over water SP anomalies and the associated uncertainty.

  17. Integration of geotechnical and geophysical techniques for the characterization of a small earth-filled canal dyke and the localization of water leakage (United States)

    Bièvre, Grégory; Lacroix, Pascal; Oxarango, Laurent; Goutaland, David; Monnot, Guy; Fargier, Yannick


    This paper investigates the combined use of extensive geotechnical, hydrogeological and geophysical techniques to assess a small earth dyke with a permanent hydraulic head, namely a canal embankment. The experimental site was chosen because of known issues regarding internal erosion and piping phenomena. Two leakages were visually located following the emptying of the canal prior to remediation works. The results showed a good agreement between the geophysical imaging techniques (Electrical Resistivity Tomography, P- and SH-waves Tomography) and the geotechnical data to detect the depth to the bedrock and its lateral variations. It appeared that surface waves might not be fully adapted for dyke investigation because of the particular geometry of the studied dyke, non-respectful of the 1D assumption, and which induced depth and velocity discrepancies retrieved from Rayleigh and Love waves inversion. The use of these classical prospecting techniques however did not allow to directly locate the two leakages within the studied earth dyke. The analysis of ambient vibration time series with a modified beam-forming algorithm allowed to localize the most energetic water flow prior to remediation works. It was not possible to detect the leakage after remediation works, suggesting that they efficiently contributed to significantly reduce the water flow. The second leakage was not detected probably because of a non-turbulent water flow, generating few energetic vibrations.

  18. Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides

    DEFF Research Database (Denmark)

    Svenningsen, Olaf


    that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally...

  19. 华北克拉通孔兹岩带中段大青山-乌拉山变质杂岩立甲子基性麻粒岩年代学及地球化学研究%Geochronological and geochemical study of the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex, the central Khondalite Belt in the North China Craton

    Institute of Scientific and Technical Information of China (English)

    刘平华; 刘福来; 蔡佳; 刘建辉; 施建荣; 王舫


    The Iijiazi mafic granulites are predominantly composed of Amp Cpx-Opx granulite, Grt-bearing Amp Cpx-Opx granulite and Bt Amp Cpx-Opx granulite, and have been found as irregular lenses and deformed dykes within the Al-rich gneisses and granitic gneisses in the Daqingshan-Wulashan metamorphic complex. The apparent207 Pb/206 Pb ages recorded by the metamorphic zircon domains with the middle-to low-pressure granulite-facies mineral inclusions of Cpx + Amp + Pl, which are consistent to the matrix-type metamorphic assemblages of the mafic granulites, range from 1933 ±39Ma to 1834 ±40Ma with a weighted mean age of 1892 ±7Ma, which are interpreted as the timing of the middle,-to low-pressure granulite-facies metamorphism of the mafic granulites. Mobile elements such as the large ion lithophile elements (K, Na, Sr, Rb) of the mafic granulites changed in significant levels, high field strength elements (such as Nb, Zr, Ti) and rare earth elements basically unchanged and keep stable during the metamorphic process. The Lijiazi mafic granulites from the Daqingshan complex belong to tholeiitic-basaltic rock series, with the concentrations of SiO2 at 45. 58% ~51. 40%, and Mg# value mainly ranging from 41 to 54. The composition of the rare earth elements of the mafic granulites is characterized by the flat, (La/Yb )CN =1. 30 ~ 1.51) with no Eu anomalies (Eu/Eu * = 0. 93 ~ 1. 04) on the chondrite-normalized patterns. All samples of the mafic granulites share obvious negative anomaly in Nb, Zr and Ti of the Phanerozoic island arc basalts. According to the geochemical and geochronological data of the Lijiazi mafic granulites, it is suggested that the protolith ages of the Lijiazi mafic granulites is 2450 ~ 1930Ma, the protoliths of the mafic granulites are undergone middle-to low-pressure granulite-facies metamorphism at ~ 1900Ma, the composition of the major and trace elements of the mafic granulites is characterized by the island arc tholeiitic basaltic rocks, and

  20. Ultrapotassic dykes in the Moldanubian Zone and their significance for understanding of the post-collisional mantle dynamics during Variscan orogeny in the Bohemian Massif (United States)

    Kubínová, Šárka; Faryad, Shah Wali; Verner, Kryštof; Schmitz, Mark; Holub, František


    We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE-SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs. The vaugnerite and syenite porphyry have high K2O (6 to 7 wt.%) and mg-number (0.66-0.74), but low CaO and Na2O contents. The vaugnerite is markedly rich in P2O5 (> 2 wt.%) and comparing to syenite porphyry has higher amount of mantle-incompatible elements (e.g. Rb, Cs, Ba, Pb, Th, U), V and Cr. ID-TIMS analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338.59 ± 0.68 and 337.87 ± 0.21 Ma, respectively. Mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent. Formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post-collisional mantle dynamics initiated by slab break-off and asthenosphere upwelling into the Moldanubian accretionary complex during the Variscan Orogeny.

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

    Python, Marie; Abily, Bénédicte; France, Lydéric


    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

  2. Palaeomagnetism of Precambrian dyke swarms in the North China Shield: The ˜1.8 Ga LIP event and crustal consolidation in late Palaeoproterozoic times (United States)

    Piper, John D. A.; Jiasheng, Zhang; Huang, Baochung; Roberts, Andrew P.


    The North China Shield (NCS) is cut by a laterally-extensive dyke swarm emplaced at 1.78-1.76 Ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by ˜1.85 Ga. Palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with NE to N declinations and rare opposite polarity. Dykes with NE shallow magnetic declination (A1, D/ I = 36/-1°) recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations (A2, D/ I = 8/2°) in lower grade metamorphic terranes to the south. Contact tests indicate a primary cooling-related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable. Petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at ˜1.85 Ga. A temporal migration A2 → A1 is implied by the higher crustal level and earlier acquisition of the former, and the deeper source and later acquisition of the latter. A third population of dyke magnetisations (A3, D/ I = 18/43°) is distributed towards steeper inclinations and close to the Mesozoic-Recent palaeofield. These are either partial or complete overprints of A1-A2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra, or are much younger dykes of Mesozoic-Tertiary age. A minority fourth (later Precambrian but presently undated) dual polarity population has a magnetisation (11 dykes, D/ I = 108/7°) with contact tests indicating a primary cooling-related origin. The ˜1.78-1.76 Ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major Large Igneous Province (LIP)-related events. The new definition of

  3. P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

    Institute of Scientific and Technical Information of China (English)

    HU Daogong; WU Zhenhan; JIANG Wan; YE Peisheng


    Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (Mi) is preserved only in the granulites and represented by plagioclase+homblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of gamet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of horublende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of homblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of gamet+clinopyroxene+plagioclase+homblende were estimated at 769-905°C and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800°C and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708°C and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.

  4. Lithologic mapping of mafic intrusions in East Greenland using Landsat Thematic Mapper data (United States)

    Naslund, H. Richard; Birnie, R. W.; Parr, J. T.


    The East Greenland Tertiary Igneous Province contains a variety of intrusive and extrusive rock types. The Skaergaard complex is the most well known of the intrusive centers. Landsat thematic mapping (TM) was used in conjunction with field spectrometer data to map these mafic intrusions. These intrusions are of interest as possible precious metal ore deposits. They are spectrally distinct from the surrounding Precambrian gneisses. However, subpixel contamination by snow, oxide surface coatings, lichen cover and severe topography limit the discrimination of lithologic units within the gabbro. Imagery of the Skaergaard and surrounding vicinity, and image processing and enhancement techniques are presented. Student theses and other publications resulting from this work are also listed.

  5. Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes


    J. K. Jakobsen; C. Tegner; C. K. Brooks; A. J. R. Kent; C.-E. Lesher; T.F.D. Nielsen; Michael Wiedenbeck


    Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7–14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lo...

  6. Impacts of dyke development in flood prone areas in the Vietnamese Mekong Delta to downstream flood hazard (United States)

    Khanh Triet Nguyen, Van; Dung Nguyen, Viet; Fujii, Hideto; Kummu, Matti; Merz, Bruno; Apel, Heiko


    The Vietnamese Mekong Delta (VMD) plays an important role in food security and socio-economic development of the country. Being a low-lying coastal region, the VMD is particularly susceptible to both riverine and tidal floods, which provide, on (the) one hand, the basis for the rich agricultural production and the livelihood of the people, but on the other hand pose a considerable hazard depending on the severity of the floods. But despite of potentially hazardous flood, the area remain active as a rice granary due to its nutrient-rich soils and sediment input, and dense waterways, canals and the long standing experience of the population living with floods. In response to both farmers' requests and governmental plans, the construction of flood protection infrastructure in the delta progressed rapidly in the last twenty years, notably at areas prone to deep flooding, i.e. the Plain of Reeds (PoR) and Long Xuyen Quadrangle (LXQ). Triple rice cropping becomes possible in farmlands enclosed by "full-dykes", i.e. dykes strong and high enough to prevent flooding of the flood plains for most of the floods. In these protected flood plains rice can be grown even during the peak flood period (September to November). However, little is known about the possibly (and already alleged) negative impacts of this fully flood protection measure to downstream areas. This study aims at quantifying how the flood regime in the lower part of the VMD (e.g. Can Tho, My Thuan, …) has been changed in the last 2 recent "big flood" events of 2000 and 2011 due to the construction of the full-dyke system in the upper part. First, an evaluation of 35 years of daily water level data was performed in order to detect trends at key gauging stations: Kratie: upper boundary of the Delta, Tan Chau and Chau Doc: areas with full-dyke construction, Can Tho and My Thuan: downstream. Results from the Mann-Kendall (MK) test show a decreasing trend of the annual maximum water level at 3 stations Kratie, Tan

  7. Petrogenesis of Mesoproterozoic lamproite dykes from the Garledinne (Banganapalle) cluster, south-western Cuddapah Basin, southern India (United States)

    Rao, N. V. Chalapathi; Atiullah; Kumar, Alok; Sahoo, Samarendra; Nanda, Purnendu; Chahong, Ngazimpi; Lehmann, B.; Rao, K. V. S.


    We report mineral chemistry and whole-rock major and trace-element geochemistry for a recent find of Mesoproterozoic (~1.4 Ga) lamproites from the Garledinne (Banganapalle) cluster, south-western part of the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The Garledinne lamproites occur as WNW-ESE-trending dykes that have undergone varying degree of pervasive silicification and carbonate alteration. Nevertheless, their overall texture and relict mineralogy remain intact and provide important insights into the nature of their magmas. The lamproite dykes have porphyritic to weakly porphyritic textures comprising pseudomorphed olivine macrocrysts and microphenocrysts, titanian phlogopite microphenocrysts, spinel having a compositional range from chromite to rarely magnesiochromite, Sr-rich apatite and niobian rutile. The Garledinne and other Cuddapah Basin lamproites (Chelima and Zangamarajupalle) collectively lack sanidine, clinopyroxene, potassic richterite, and titanite and are thus mineralogically distinct from the nearby Mesoproterozoic lamproites (Krishna and Ramadugu) in the Eastern Dharwar Craton, southern India. The strong correlation between various major and trace elements coupled with high abundances of incompatible and compatible trace elements imply that alteration and crustal contamination have had a limited effect on the whole-rock geochemistry (apart from K2O and CaO) of the Garledinne lamproites and that olivine fractionation played an important role in their evolution. The Garledinne lamproites represent small-degree partial melts derived from a refractory (previously melt extracted) peridotitic mantle source that was subsequently metasomatised (enriched) by carbonate-rich fluids/melts within the garnet stability field. The involvement of multiple reservoirs (sub-continental lithospheric mantle and asthenosphere) has been inferred in their genesis. The emplacement of the Garledinne lamproites is linked to extensional events, across the various

  8. Layered amphibolite sequence in NE Sardinia, Italy: remnant of a pre-Variscan mafic silicic layered intrusion? (United States)

    Franceschelli, Marcello; Puxeddu, Mariano; Cruciani, Gabriele; Dini, Andrea; Loi, Marilisa


    A banded amphibolite sequence of alternating ultramafic, mafic (amphibolite) and silicic layers, tectonically enclosed within Variscan migmatites, outcrops at Monte Plebi (NE Sardinia) and shows similarities with leptyno-amphibolite complexes. The ultramafic layers consist of amphibole (75-98%), garnet (0-20%), opaque minerals (1-5%) and biotite (0-3%). The mafic rocks are made up of amphibole (65-80%), plagioclase (15-30%), quartz (0-15%), opaque minerals (2-3%) and biotite (0-2%). The silicic layers consist of plagioclase (60-75%), amphibole (15-30%) and quartz (10-15%). Alteration, metasomatic, metamorphic and hydrothermal processes did not significantly modify the original protolith chemistry, as proved by a lack of K2O-enrichment, Rb-enrichment, CaO-depletion, MgO-depletion and by no shift in the rare earth element (REE) patterns. Field, geochemical and isotopic data suggest that ultramafic, mafic and silicic layers represent repeated sequences of cumulates, basic and acidic rocks similar to macrorhythmic units of mafic silicic layered intrusions. The ultramafic layers recall the evolved cumulates of Skaergaard and Pleasant Bay mafic silicic layered intrusions. Mafic layers resemble Thingmuli tholeiites and chilled Pleasant Bay mafic rocks. Silicic layers with Na2O: 4-6 wt%, SiO2: 67-71 wt% were likely oligoclase-rich adcumulates common in many mafic silicic layered intrusions. Some amphibolite showing a strong Ti-, P-depletion and REE-depletion are interpreted as early cumulates nearly devoid of ilmenite and phosphates. All Monte Plebi rocks have extremely low Nb, Ta, Zr, Hf content and high LILE/HFSE ratios, a feature inherited from the original mantle sources. The mafic and ultramafic layers show slight and strong LREE enrichment respectively. Most mafic layer samples plot in the field of continental tholeiites in the TiO2-K2O-P2O5 diagram and are completely different from N-MORB, E-MORB and T-MORB as regards REE patterns and Nd, Sr isotope ratios but show

  9. Oxygen isotopes of some trondhjemites, siliceous gneisses, and associated mafic rocks (United States)

    Barker, F.; Friedman, I.; Hunter, D.R.; Gleason, J.D.


    Analyses of oxygen isotopes in whole-rock samples of 58 Precambrian and Phanerozoic trondhjemites and siliceous gneisses and of 28 cogenetic mafic to intermediate rocks from North America, Fennoscandia, and southern Africa give the following results: 1. (1) 47 trondhjemites, tonalites, and mostly Archean acidic gneisses that apparently are not isotopically disturbed show an overage ?? 15O of +7.3??? and a range of 5.2-8.9???; 11 other samples are slightly to moderately disturbed and show higher values; and 2. (2) the mafic rocks show a wide range of ??-values, from about 0-9??? but the undisturbed ones give an average ?? 18O of 5.2???. The ?? 18O values of the trondhjemitic intrusives and siliceous gneisses of similar composition are lower than those of most granitic rocks and support models for derivation of these rocks from basaltic parents. This approach, however, cannot be used to determine if individual bodies formed by differentiation or by partial melting. ?? 1976.

  10. Petrology and geochemistray of Imam-Zadeh Hashem mafic and ultramafic bodies, southern Guilan province

    Directory of Open Access Journals (Sweden)

    Mojgan Salavati


    Full Text Available Mafic and ultramafic plutonic igneous bodies, with small and big outcrops, between Shemshak rock units (Jurassic are observed in east of Imam Zadeh Hashem, in southern Guilan province. Ultramafic cumulates consist of clinopyroxenite, and plagiofer clinopyroxenite, olivine clinopyroxenite, and mafic rocks, based on mineralogy consist of gabbros, olivine gabbros, biotite gabbros and amphibole gabbros. According to geochemical data, studied rocks have tholeiitic nature and in the tectonic setting diagrams, display arc characteristic. The chondrite normalized REE patterns show low enrichment in LREEs relative to HREEs. The negative Nb and Ti anomalies in primitive mantle and MORB-normalized multi-element diagrams of the rocks are characteristic of island arc magmas. Also, enrichment in LILE and depletion of HFSE may indicate a subduction-related tectonic setting. According to geological and geochemical evidence, Imam-Zadeh Hashem ultramafic and sub-alkaline gabbro rocks can be as a part of the Southern Caspian Sea Ophiolite sequence (SCO that formed in a suprasubduction tectonic system.

  11. Quantitative EPMA Compositional Mapping of NWA 2995: Characterization, and Petrologic Interpretation of Mafic Clasts (United States)

    Carpenter, P. K.; Hahn, T. M.; Korotev, R. L.; Ziegler, R. A.; Jolliff, B. L.


    We present the first fully quantitative compositional maps of lunar meteorite NWA 2995 using electron microprobe stage mapping, and compare selected clast mineralogy and chemistry. NWA 2995 is a feldspathic fragmental breccia containing numerous highland fine grained lithologies, including anorthosite, norite, olivine basalt, subophitic basalt, gabbro, KREEP-like basalt, granulitic and glassy impact melts, coarse-grained mineral fragments, Fe-Ni metal, and glassy matrix [1]. Chips of NWA 2995, representing these diverse materials, were analyzed by INAA and fused-bead electron-probe microanalysis (EPMA); comparison of analytical data suggests grouping of lunar meteorites NWA 2995, 2996, 3190, 4503, 5151, and 5152. The mean composition of NWA 2995 corresponds to a 2:1 mixture of feldspathic and mare material, with approximately 5% KREEP component [2]. Clast mineral chemistry and petrologic interpretation of paired stone NWA 2996 has been reported by Mercer et al. [3], and Gross et al. [4]. This study combines advances in quantitative EPMA compositional mapping and data analysis, as applied to selected mafic clasts in a polished section of NWA 2995, to investigate the origin of mafic lithic components and to demonstrate a procedural framework for petrologic analysis.

  12. K-Ar Geochronology of Mesozoic Mafic Dikes in Shandong Province, Eastern China:Implications for Crustal Extension

    Institute of Scientific and Technical Information of China (English)

    LIU Shen; HU Ruizhong; ZHAO Junhong; FENG Caixia


    Based on K-Ar isotope analyses, Mesozoic mafic (and alkali ultramafic) dikes from western and eastern Shandong Province, China, are dated at 88.2±1.70 Ma to 169.5±3.7 Ma with the majority of ages ranging from 90 Ma to 140 Ma. The emplacement of the dikes suggests a major Yanshanian (Cretaceous) crustal extension in Shandong province.Together with other available age data, this study suggests four periods of crustal extension at about 80 Ma, 100 Ma, 120 Ma and 140 Ma, respectively. Besides the effect of collapse of the Yanshanian orogenic belt on the emplacement of the mafic dikes in Shandong in the Cretaceous, the mantle plume and the extensive left-lateral advection and extension of the Tanlu fault also have controlled the crustal extension and the emplacement of the mafic dikes in eastern Shandong and western Shandong, respectively.

  13. Platinum-group element concentrations in pyrite from the Main Sulfide Zone of the Great Dyke of Zimbabwe (United States)

    Piña, R.; Gervilla, F.; Barnes, S.-J.; Oberthür, T.; Lunar, R.


    The Main Sulfide Zone (MSZ) of the Great Dyke of Zimbabwe hosts the world's second largest resource of platinum-group elements (PGE) after the Bushveld Complex in South Africa. The sulfide assemblage of the MSZ comprises pyrrhotite, pentlandite, chalcopyrite, and minor pyrite. Recently, several studies have observed in a number of Ni-Cu-PGE ore deposits that pyrite may host significant amounts of PGE, particularly Pt and Rh. In this study, we have determined PGE and other trace element contents in pyrite from the Hartley, Ngezi, Unki, and Mimosa mines of the Great Dyke by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Based on the textures and PGE contents, two types of pyrite can be differentiated. Py1 occurs as individual euhedral or subhedral grains or clusters of crystals mostly within chalcopyrite and pentlandite, in some cases in the form of symplectitic intergrowths, and is PGE rich (up to 99 ppm Pt and 61 ppm Rh; 1.7 to 47.1 ppm Ru, 0.1 to 7.8 ppm Os, and 1.2 to 20.2 ppm Ir). Py2 occurs as small individual euhedral or subhedral crystals within pyrrhotite, pentlandite, and less frequently within chalcopyrite and silicates and has low PGE contents (<0.11 ppm Pt, <0.34 ppm Rh, <2.5 ppm Ru, <0.37 ppm Ir, and <0.40 ppm Os). Py1 contains higher Os, Ir, Ru, Rh, and Pt contents than the associated pyrrhotite, pentlandite, and chalcopyrite, whereas Py2 has similar PGE contents as coexisting pyrrhotite and pentlandite. Based on the textural relationships, two different origins are attributed for each pyrite type. Py1 intergrowth with pentlandite and chalcopyrite is inferred to have formed by late, low temperature (<300 °C) decomposition of residual Ni-rich monosulfide solid solution, whereas Py2 is suggested to have formed by replacement of pyrrhotite and pentlandite caused by late magmatic/hydrothermal fluids.

  14. Contribution of the FUTUREVOLC project to the study of segmented lateral dyke growth in the 2014 rifting event at Bárðarbunga volcanic system, Iceland (United States)

    Sigmundsson, Freysteinn; Hooper, Andrew; Hreinsdóttir, Sigrún; Vogfjörd, Kristín S.; Ófeigsson, Benedikt; Rafn Heimisson, Elías; Dumont, Stéphanie; Parks, Michelle; Spaans, Karsten; Guðmundsson, Gunnar B.; Drouin, Vincent; Árnadóttir, Thóra; Jónsdóttir, Kristín; Gudmundsson, Magnús T.; Samsonov, Sergey; Brandsdóttir, Bryndís; White, Robert S.; Ágústsdóttir, Thorbjörg; Björnsson, Helgi; Bean, Christopher J.


    The FUTUREVOLC project (a 26-partner project funded by FP7 Environment Programme of the European Commission, addressing topic "Long-term monitoring experiment in geologically active regions of Europe prone to natural hazards: the Supersite concept) set aims to (i) establish an innovative volcano monitoring system and strategy, (ii) develop new methods for near real-time integration of multi-parametric datasets, (iii) apply a seamless transdisciplinary approach to further scientific understanding of magmatic processes, and (iv) to improve delivery, quality and timeliness of transdisciplinary information from monitoring scientists to civil protection. The project duration is 1 October 2012 - 31 March 2016. Unrest and volcanic activity since August 2014 at one of the focus areas of the project in Iceland, at the Bárðarbunga volcanic system, near the middle of the project duration, has offered unique opportunities for this project. On 16 August 2014 an intense seismic swarm started in Bárðarbunga, the beginning of a major volcano-tectonic rifting event forming over 45 km long dyke extending from the caldera to Holuhraun lava field outside the northern margin of Vatnajökull. A large basaltic, effusive fissure eruption began in Holuhraun on 31 August which had by January formed a lava field with a volume in excess of one cubic kilometre. We document how the FUTUREVOLC project has contributed to the study and response to the subsurface dyke formation, through increased seismic and geodetic coverage and joint interpreation of the data. The dyke intrusion in the Bárðarbunga volcanic system, grew laterally for over 45 km at a variable rate, with an influence of topography on the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred over 14 days, was revealed by propagating seismicity, ground

  15. Re-interpretation of zircon date in a carbonatite dyke at the Bayan Obo giant REE-Fe-Nb deposit,China

    Institute of Scientific and Technical Information of China (English)


    @@ Recent re-valuation of the dating of the carbonatite dykes associated with the REE-Fe-Nb giant deposit at Bayan Obo has revealed an anomaly with one of the dates. Inspection of the post-1990 literature for the carbonatite dykes and H8 dolomite rocks and ores of Bayan Obo provides age data in the range 1223 +65 Ma to 1656 Ma, as Table 1 shows. Since Sm and Nd are relatively immobile elements, most of the Sm-Nd dates are considered to be little changed by the subsequent mineralization processes. That the dates occur in two clusters, 1656 ~ 1500 Ma and 1312 ~ 1223 Ma might refer to two related periods of carbonatitic magmatic activity, but that is not relevant at this moment.

  16. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. (United States)

    Tang, Ming; Chen, Kang; Rudnick, Roberta L


    The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago.

  17. The fate of mafic and ultramafic intrusions in the continental crust (United States)

    Roman, Alberto; Jaupart, Claude


    Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

  18. Neogene to Recent Mafic Volcanism in Death Valley Reveals Architecture of Deep Mojavia Lithosphere (United States)

    Rämö, O. T.; Calzia, J.


    At 10-0 m.y., the evolution of the Death Valley region, SE California, was characterized by repeated extrusion of mafic and intermediate lavas. The volume of these lavas appears to have diminished with time, from the relatively extensive (now faulted) Sheephead, Death Valley, Shoshone, and Funeral basalts to small monogenic volcanic centers (Split Cinder Cone, Ubehebe). A common denominator of these lavas is their transitional alkaline character (in general, trachybasalts to trachydacites with Na > K), relatively low MgO (1.5 to 6 wt.%), varying Fe2O3(tot) (4 to 12 wt.%) and Ni (10 to 80 ppm). They are all strongly enriched in the LREE: Chondrite-normalized La/Yb varies between 10 and 28, with the earlier, more voluminous lavas being less enriched (10 to 18) than the youngest volcanics (27 to 28). The initial Nd isotope composition of the lavas is outstandingly varying and does not correlate with fractionation stage. Their epsilon-Nd values vary from -10 to -2 and none of them thus registers a major asthenospheric component. Overall, these transitional mafic magmas probably represent relatively low-degree melts from the subcontinental lithosphere underneath the cratonic Mojavia crust. The 147Sm/144Nd ratios of the lavas show a restricted range (0.0930 to 0.1140) and, together with the measured 143Nd/144Nd ratios of the lavas, define Meso- to Neoproterozoic depleted mantle model ages between 800 and 1200 Ma. These have now spatial control across the Death Valley region and are clearly lower than those measured for the exposed craton. This suggest that the Mojavia cratonic mantle lithosphere may be a random amalgamation of ultramafic domains that vary in the degree of metasomatism and rejuvenation.

  19. The role of dyking and fault control in the rapid onset of eruption at Chaitén Volcano, Chile (United States)

    Wicks, C.; De La, Llera; Lara, L.E.; Lowenstern, J.


    Rhyolite is the most viscous of liquid magmas, so it was surprising that on 2 May 2008 at Chaitén Volcano, located in Chile’s southern Andean volcanic zone, rhyolitic magma migrated from more than 5 km depth in less than 4 hours and erupted explosively with only two days of detected precursory seismic activity. The last major rhyolite eruption before that at Chaitén was the largest volcanic eruption in the twentieth century, at Novarupta volcano, Alaska, in 1912. Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives. Here we present surface deformation data related to the Chaitén eruption based on radar interferometry observations from the Japan Aerospace Exploration Agency (JAXA) DAICHI (ALOS) satellite. The data on this explosive rhyolite eruption indicate that the rapid ascent of rhyolite occurred through dyking and that melt segregation and magma storage were controlled by existing faults.

  20. The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile. (United States)

    Wicks, Charles; de la Llera, Juan Carlos; Lara, Luis E; Lowenstern, Jacob


    Rhyolite is the most viscous of liquid magmas, so it was surprising that on 2 May 2008 at Chaitén Volcano, located in Chile's southern Andean volcanic zone, rhyolitic magma migrated from more than 5 km depth in less than 4 hours (ref. 1) and erupted explosively with only two days of detected precursory seismic activity. The last major rhyolite eruption before that at Chaitén was the largest volcanic eruption in the twentieth century, at Novarupta volcano, Alaska, in 1912. Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives. Here we present surface deformation data related to the Chaitén eruption based on radar interferometry observations from the Japan Aerospace Exploration Agency (JAXA) DAICHI (ALOS) satellite. The data on this explosive rhyolite eruption indicate that the rapid ascent of rhyolite occurred through dyking and that melt segregation and magma storage were controlled by existing faults.

  1. Geological and Geochemical Characteristics of Early Cretaceous Mafic Dikes in Northern Jiangxi Province, SE China and Their Geodynamic Implications

    Institute of Scientific and Technical Information of China (English)

    XIE Guiqing; HU Ruizhong; MAO Jingwen; LI Ruiling; CAO Jinjian; JIANG Guohao; QI Liang


    The development of Early Cretaceous mafic dikes in northern and southern Jiangxi allows an understanding of the geodynamic setting and characteristics of the mantle in southeast China in the Cretaceous. Geological and geochemical characteristics for the mafic dikes from the Wushan copper deposit and No. 640 uranium deposit are given in order to constrain the nature of source mantle, genesis and tectonic implications. According to the mineral composition,the mafic dikes in northern Jiangxi can be divided into spessartite and olive odinite types, which belong to slightly potassium-rich calc-alkaline lamprophyre characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), large depletion in high strength field elements (HSFE) and with negative Nb, Ta and Ti anomalies, as well as 87Sr/86Sr ratios varying from 0.7055 to 0.7095 and 143Nd/144Nd ratios varying from 0.5119 to 0.5122.All features indicate that the magma responsible for the mafic dikes was derived mainly from metasomatic lithosphere mantle related to dehydration and/or upper crust melting during subduction. Differences in geochemical characteristics between the mafic dikes in northern Jiangxi and the Dajishan area, southern Jiangxi were also studied and they are attributed to differences in regional lithosphefic mantle components and/or magma emplacement depth. Combining geological and geochemical characteristics with regional geological history, we argue that southeast China was dominated by an extensional tectonic setting in the Early Cretaceous, and the nature of the mantle source area was related to enrichment induced by asthenosphere upwelling and infiltration of upper crust-derived fluids responding to Pacific Plate subduction.

  2. P-T history and geochemical characteristics of mafic granulites and charnockites from west of Periya, North Kerala, southern India (United States)

    Prakash, D.; Chandra Singh, P.; Arima, M.; Singh, Triveni


    The study region forms the northern part of Kerala (south India) and constitutes part of granulite-facies rocks of the exhumed Proterozoic south Indian Granulite Terrain (SGT). The SGT consists of a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockite, mafic granulite, gneiss, schist, anorthosite, granite and minor meta-sedimentary rocks. Garnet-bearing mafic granulites occur as small enclaves within charnockites. We report for the first time P-T constraints on the prograde path preceding peak metamorphism in the northernmost part of Kerala. An increase of the Mg, Fe and decrease of Ca and Mn contents from the core towards the rim of garnet in the mafic granulites suggest prograde garnet growth. The prograde path was followed by peak metamorphism at a temperature of c. 800 °C and a pressure of c. 7.5 kbar as computed by isopleths of XMg garnet, XCa garnet and XAn plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene-plagioclase, biotite-quartz and hornblende-plagioclase symplectites characterize the subsequent stages of metamorphism. The PT path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust. Diffusion modeling of Fe-Mg exchange between garnet and hornblende suggests a near-peak cooling rate of 10-70 °C/myr. Such cooling rates are too high to be accounted for by normal isostatic uplift and erosion and suggest that the terrain was tectonically exhumed. Charnockites are richer in SiO2 and lower in MgO and CaO when compared to mafic granulites. Their REE patterns are relatively flat and show prominent negative europium anomalies. The mafic granulites are metamorphosed tholeiitic basalts as revealed by major- and trace-element geochemistry.

  3. Petrogenesis of the Alaskan-type mafic-ultramafic complex in the Makkah quadrangle, western Arabian Shield, Saudi Arabia (United States)

    Habtoor, Abdelmonem; Ahmed, Ahmed Hassan; Harbi, Hesham


    The Makkah quadrangle is a part of the Jeddah terrane in the Precambrian basement, Western Arabian Shield of Saudi Arabia. Gabal Taftafan mafic-ultramafic complex lies within the central part of the Makkah quadrangle. The Taftafan mafic-ultramafic complex is a well-differentiated rock association which comprises of dunite core, hornblende- and plagioclase-bearing peridotites, troctolite, clinopyroxenite and marginal gabbro, in a distinctive zonal structure. The bulk-rock geochemistry of the Taftafan mafic-ultramafic rocks is characterized by a tholeiitic/sub-alkaline affinity with high Mg in the ultramafic core (0.84) and is systematically decreased towards the marginal gabbro (0.60). The patterns of trace elements show enrichment in the fluid-mobile elements (Sr, Ba) and a pronounced negative Nb anomaly which reflect a hydrous parental magma generated in a subduction tectonic setting. The mafic-ultramafic rocks of the Taftafan complex have low total rare earth elements (REE) displaying sub-parallel patterns leading to the assumption that these rocks are comagmatic and are formed by fractional crystallization from a common magma type. The platinum-group elements (PGE) content of all rock types in the Taftafan complex is very low, with ∑ PPGE > ∑ IPGE; displaying slightly positive slopes of the PGE distribution patterns. The chemistry of ferromagnesian minerals is characterized by a high forsterite (Fo) olivine with wide range (Fo91-67), from ultramafic core to the marginal gabbro, Ca-rich diopsidic clinopyroxene, and calcic hornblende. Orthopyroxene is almost absent from all rock types, or very rare when present. Hornblende and Ca-plagioclase possess the longest crystallization history since they are present in almost all rock types of the complex. Spinels in the dunite and hornblende-bearing peridotite core show homogeneous composition with intermediate Cr# (0.53-0.67). Plagioclase-bearing peridotite and troctolite have two exsolved types of spinel; Al

  4. Emplacement age for the mafic-ultramafic plutons in the northern Dabie Mts. (Hubei): Zircon U-Pb, Sm-Nd and 40Ar/39Ar dating

    Institute of Scientific and Technical Information of China (English)


    The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9(0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende 40Ar/39Ar ages of 116.1(1.1 Ma and 106.6(0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500(C. The hornblende 40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15-25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.

  5. Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications (United States)

    Ishwar-Kumar, C.; Sajeev, K.; Windley, B. F.; Kusky, T. M.; Feng, P.; Ratheesh-Kumar, R. T.; Huang, Y.; Zhang, Y.; Jiang, X.; Razakamanana, T.; Yagi, K.; Itaya, T.


    The occurrence of high-pressure mafic-ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet-clinopyroxene-quartz coronas that formed after the breakdown of orthopyroxene-plagioclase-ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicate a pressure-temperature (P-T) evolution from an isothermal (780 °C) pressure up to c. 24 kbar to decompression and cooling. Such a P-T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U-Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K-Ar dating of muscovite and biotite from biotite-kyanite-sillimanite gneiss and garnet-biotite gneiss yields age of 486 ± 9 Ma and 459 ± 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal

  6. Evaluation of the Lithospheric Contribution to Southern Rio Grande Rift Mafic Melts (United States)

    Konter, J. G.; Crocker, L.; Anaya, L. M.; Rooney, T. O.


    As continental rifting proceeds, the accommodation of lithospheric thinning by mechanical extension and magmatic intrusion represents an important but poorly constrained tectonic process. Insight into role of the magmatic component may come from the composition of volcanic products, which can record magma-lithosphere interactions. The volcanic activity in continental rift environments is frequently characterized by bimodal associations of mafic and silicic volcanism with heterogenous lithospheric contributions. We present a new integrated data set from several mafic volcanic fields in the Rio Grande Rift, consisting of major and trace element compositions, as well as isotopes. This data set provides insight into asthenospheric melting processes and interactions with the overlying lithosphere. The melting processes and the related extensional volcanism is the result of foundering of the Farallon slab. Large volume silicic eruptions such as those in the Sierra Madre Occidental originate from a large contribution of lithospheric melting, with a subordinate asthenospheric contribution. In contrast, Late Tertiary and Quaternary basaltic volcanic fields in the Rio Grande Rift were likely sourced in the asthenosphere and did not reside in the lithosphere for substantial periods. As a result the region is the ideal natural laboratory to investigate the interaction of asthenospheric melts with the lithosphere. In particular the wide array of volcanic fields contain multiple xenolith localities, such as Kilbourne Hole, providing direct samples of lithosphere and crust. Although previous studies have focused on correlations between amount of extension related to Farallon slab foundering, volcanic compositions, and their mantle sources, we present data that suggest that some compositional signatures may pre-date current tectonic processes. Radiogenic isotope data from several volcanic fields in New Mexico show a converging pattern in Pb isotope compositions, focusing on the

  7. Cl-rich hydrous mafic mineral assemblages in the Highiș massif, Apuseni Mountains, Romania (United States)

    Bonin, Bernard; Tatu, Mihai


    The Guadalupian (Mid-Permian) Highiș massif (Apuseni Mountains, Romania) displays a bimodal igneous suite of mafic (gabbro, diorite) and A-type felsic (alkali feldspar granite, albite granite, and hybrid granodiorite) rocks. Amphibole is widespread throughout the suite, and yields markedly high chlorine contents. Three groups are identified: Cl-rich potassic hastingsite (2.60-3.40 wt% Cl) within A-type felsic rocks and diorite, mildly Cl-rich pargasite to hornblende (0.80-1.90 wt% Cl) within gabbro, and low F-Cl hornblende within gabbro and hybrid granodiorite. Coexisting biotite is either Cl-rich within diorite, or F-Cl-poor to F-rich within A-type felsic rocks. Chlorine and fluorine are distributed in both mafic phases, according to the F-Fe and Cl-Mg avoidance rules. The low-Ti contents suggest subsolidus compositions. Cl-rich amphibole within diorite and A-type felsic rocks yields a restricted temperature range - from 575 °C down to 400 °C, whereas mildly Cl-rich amphibole within gabbro displays the highest range - from 675 to 360 °C. Temperatures recorded by Cl-rich biotite within diorite range from 590 to 410 °C. Biotite within A-type felsic rocks yields higher temperatures than amphibole: the highest values- from 640 to 540 °C - are recorded in low-F-Cl varieties, whereas the lowest values- from 535 to 500 °C - are displayed by F-rich varieties. All data point to halogen-rich hydrothermal fluids at upper greenschist facies conditions percolating through fractures and shear zones and pervasively permeating the whole Highiș massif, with F precipitating as interstitial fluorite and Cl incorporating into amphibole, during one, or possibly several, hydrothermal episodes that would have occurred during a ~ 150 My-long period of time extending from the Guadalupian (Mid-Permian) to the Albian (Mid-Cretaceous).

  8. Geochronology and geochemistry of a suite of mafic rocks in Chencai area, South China: Implications for petrogenesis and tectonic setting (United States)

    Zhao, Lei; Zhai, Mingguo; Zhou, Xiwen; Santosh, M.; Ma, Xudong


    Paleozoic crust-mantle interaction and geodynamic setting of the South China Block have remained largely elusive due to the scarcity of mafic rock suites. Here we report data from a suite of mafic rocks including norite, mafic granulite, garnet-bearing amphibolite, biotite-bearing amphibolite, and amphibolite from the Chencai region in the northeastern Cathaysia Block of South China, that mostly record amphibolite to granulite facies metamorphism. U-Pb dating of zircons from the mafic granulite and biotite-bearing amphibolite show broadly coeval magmatism, and amphibolite- to granulite-facies metamorphism during 435-438 Ma. The garnet-bearing amphibolite also records metamorphism at 436 Ma and the magmatic age of its protolith is constrained to be between 436 Ma and 496 Ma. The norite has an emplacement age of 422 Ma. Geochemically, the amphibolite and granulite have high Al2O3 contents and belong to the high-alumina group. The garnet-bearing amphibolite belongs to the intermediate-alumina group, whereas the norite and biotite-bearing amphibolite belong to the low-alumina group. A cumulate origin is proposed for the norite with orthopyroxene as the main cumulus phase. Based on their similar trace element and Lu-Hf isotopic signatures, it is inferred that the protoliths of amphibolite and granulite belonging to high-alumina group were likely generated from the same source. Furthermore, their LREE enrichment relative to HREE, positive Eu anomaly, and negative Nb, Ta and Zr anomalies suggest derivation from the same enriched mantle source, perhaps in a continental arc environment. Zircons from the garnet-bearing amphibolite belonging to intermediate-alumina group have high positive ɛHf (t) values with a peak of + 14 and trace element signatures similar to N-MORB, suggesting a depleted asthenospheric mantle source. The rock suite from Chencai suggests formation in an active continental margin, with the garnet-bearing amphibolite representing relics of the Paleozoic

  9. Melanite garnet-bearing nepheline syenite minor intrusion in Mawpyut ultramafic–mafic complex, Jaintia Hills, Meghalaya

    Indian Academy of Sciences (India)

    Monoj Maitra; J S David; S Bhaduri


    Mawpyut igneous suite in Jaintia Hills of Meghalaya plateau comprises differentiated suite of ultramafic–mafic rocks. The complex differs from other ultramafic–alkaline–carbonatite igneous emplacements of Shillong plateau and Mikir Hills like Jesra, Sung, Samchampi complexes, by the absence of alkaline–carbonatite rocks as major litho-units. Melanite garnet-bearing nepheline syenite, occurs as late phase minor intrusion in Mawpyut igneous complex, posseses alkaline character and shows inubiquitous relation with the host ultramafic–mafic rocks. On the other hand, this alkaline intrusive bodies of the Mawpyut igneous complex shows chemico-mineralogical resemblance with garnet-bearing nepheline syenite, ijolite litho-members of Jesra, Sung, Samchampi complexes of the region. It is interpreted that melanite garnet-bearing nepheline syenite intrusion in Mawpyut is contemporaneous with Jesra, Sung, Samchampi ultramafic–alkaline–carbonatite complexes and the host rocks of Mawpyut complex is an earlier magmatic activity possibly from a comparatively least enriched source.

  10. Petrogenesis of permian sulfide-bearing mafic-ultramafic intrusions insoutheast Chinese Altay and east Tianshan, NW China


    Gao, Jianfeng; 高剑峰


    The Central Asia Orogenic Belt is one of the largest accretionary orogenic belts in the world. In this belt, many sulfide‐bearing mafic‐ultramafic intrusions occur along faults, including the Kalatongke complex in southeast Chinese Altay and the Huangshandong intrusion in east Tianshan. The Kalatongke complex is a composite body including ~308Ma dioritic intrusion and 287Ma sulfide‐bearing mafic intrusion. The dioritic intrusion consists of biotite‐hornblende gabbro, diorite and quartz d...

  11. Integrated Approach (Geophysics and Remote Sensing) to identify Water-bearing Dyke Swarms and Fractured Basement in the Sinai Peninsula, Egypt (United States)

    Mohamed, L.; Sultan, M.; Ahmed, M. E.; Sauck, W.; Abouelmagd, A. A.; Chouinard, K.


    An integrated approach utilizing Very Low Frequency (VLF) and magnetic field surveying and temporal remote sensing data including: (1) Advanced Space Borne Thermal Emission and Reflection (ASTER) data, (2) European Remote Sensing (ERS-1 and ERS-2) radar imagery, and (3) Tropical Rainfall Measuring Mission (TRMM) was used to delineate water-bearing sub-vertical shear zones within the basement complex of the Sinai Peninsula. The following steps were undertaken: (1) the shear zones and dyke swarms within the basement complex were delineated using false color ASTER band and band ratio images; (2) the spatial and temporal precipitation events over the basement complex were then identified from TRMM data, and (3) finally, observations extracted from temporal radar and thermal ASTER bands were used to identify the water-bearing shear zones and dyke swarms. A fracture or dyke was deemed to be water bearing if: (1) it witnessed a large increase in its reflectivity and emissivity compared to its surroundings following a precipitation event, and maintained such differences for periods ranging from days to months. Field observations and VLF investigations were then applied to test the validity of our satellite-based methodologies for locating targeted aquifer types and for refining the satellite-based selections. The VLF detects conductive water-saturated subvertical breccia zones in bedrock. Thirty two VLF transects were collected in September of 2011 and July of 2012 along with 10 magnetic profiles at the same VLF locations. Both VLF and magnetic transects were acquired along a traverse perpendicular to the dike orientations with station separations ranging from 10 to 25 m. The VLF receiver (T-VLF) measures the distortion of the normally horizontal electromagnetic flux lines by local electrical conductors. At each VLF station, and for each frequency used, the following were measured: the tilt of the electromagnetic field, from the horizontal (given in percentage), the

  12. MR and CT imaging in the Dyke-Davidoff-Masson syndrome: report of three cases and contribution to pathogenesis and differential diagnosis

    Directory of Open Access Journals (Sweden)



    Full Text Available Cerebral hemiatrophy or Dyke-Davidoff-Masson syndrome is a condition characterized by seizures, facial asymmetry, contralateral hemiplegia or hemiparesis, and mental retardation. These findings are due to cerebral injury that may occur early in life or in utero. The radiological features are unilateral loss of cerebral volume and associated compensatory bone alterations in the calvarium, like thickening, hyperpneumatization of the paranasal sinuses and mastoid cells and elevation of the petrous ridge. The authors describe three cases. Classical findings of the syndrome are present in variable degrees according to the extent of the brain injury. Pathogenesis is commented.

  13. Analysis of the deconstruction of Dyke Marsh, George Washington Memorial Parkway, Virginia-Progression, geologic and manmade causes, and effective restoration scenarios (United States)

    Litwin, Ronald J.; Smoot, Joseph P.; Pavich, Milan J.; Markewich, Helaine W.; Oberg, Erik; Helwig, Ben; Steury, Brent; Santucci, Vincent L.; Durika, Nancy J.; Rybicki, Nancy B.; Engelhardt, Katharina M.; Sanders, Geoffrey; Verardo, Stacey; Elmore, Andrew J.; Gilmer, Joseph


    This report is a synthesis of the latest findings from an ongoing study of Dyke Marsh, an eroding freshwater tidal wetland that is scheduled for federal restoration. Its purpose is to provide an accurate and up-to-date temporal and geological framework for the marsh, of which most is new information (plus a compilation of historical and recent information), that is directly relevant to the restoration effort and also is relevant to short-term and long-term land management decisions regarding this natural resource.

  14. Transfer of volatiles and metals from mafic to felsic magmas in composite magma chambers: An experimental study (United States)

    Guo, Haihao; Audétat, Andreas


    In order to determine the behavior of metals and volatiles during intrusion of mafic magma into the base of silicic, upper crustal magma chambers, fluid-rock partition coefficients (Dfluid/rock) of Li, B, Na, S, Cl, K, Mn, Fe, Rb, Sr, Ba, Ce, Cu, Zn, Ag, Cd, Mo, As, Se, Sb, Te, W, Tl, Pb and Bi were determined experimentally at 2 kbar and 850 °C close to the solidus of mafic magma. In a first step, volatile-bearing mafic glasses were prepared by melting a natural basaltic trachyandesite in the presence of volatile-bearing fluids at 1200 °C/10 kbar in piston cylinder presses. The hydrous glasses were then equilibrated in subsequent experiments at 850 °C/2 kbar in cold-seal pressure vessels, which caused 80-90% of the melt to crystallize. After 0.5-2.0 days of equilibration, the exsolved fluid was trapped by means of in-situ fracturing in the form of synthetic fluid inclusions in quartz. Both the mafic rock residue and the fluid inclusions were subsequently analyzed by laser-ablation ICP-MS for major and trace elements. Reverse experiments were conducted by equilibrating metal-bearing aqueous solutions with rock powder and then trapping the fluid. In two additional experiments, information on relative element mobilities were obtained by reacting fluids that exsolved from crystallizing mafic magma with overlying silicic melts. The combined results suggest that under the studied conditions S, Cl, Cu, Se, Br, Cd and Te are most volatile (Dfluid/rock >10), followed by Li, B, Zn, As, Ag, Sb, Cs, W, Tl, Pb and Bi (Dfluid/rock = 1-10). Less volatile are Na, Mg, K, Ca, Mn, Fe, Rb, Sr, Mo and Rb (Dfluid/rock 0.1-1), and the least fluid-mobile elements are Al, Si, Ti, Zr, Ba and Ce (Dfluid/rock <0.1). This trend is broadly consistent with relative element volatilities determined on natural high-temperature fumarole gases, although some differences exist. Based on the volatility data and measured mineral-melt and sulfide-melt partition coefficients, volatile fluxing in

  15. Enigmatic Late-Stage Textures In Mafic Cumulates: Skaergaard Intrusion, East Greenland. (United States)

    Stripp, G.; Holness, M.; Veksler, I.


    The complexities of slow solidification in deep-seated silicate liquid bodies are poorly understood. Late-stage melt migration, due to compaction of the crystal mush, drives re-crystallisation and metasomatism which can have significant effects on chamber-wide chemical evolution. In this contribution we present novel textural observations from mafic cumulates of the Skaergaard Layered Intrusion which may shed light on liquid movement during the last stages of solidification of the crystal mush. Previously undescribed mafic symplectites are widespread in the Skaergaard Layered Series, and comprise vermicular intergrowths of plagioclase and olivine, which may be replaced by clinopyroxene in the outer parts of the symplectite. The symplectites grow outwards from Fe-Ti oxide grains, consuming adjacent cumulus plagioclase. In the Middle Zone of the Layered Series (where symplectites are best developed) symplectite plagioclase adjacent to the Fe-Ti oxide grains contains 1.2 wt% FeOtot which decreases to 0.6 wt% FeOtot at the symplectite margin, compared to a ~ 0.35 wt% average for adjacent cumulus plagioclase. Symplectite plagioclase is up to 40 mol% more An-rich than the adjacent cumulus grains. Olivine compositions range from Fo45 to Fo32 along the growth direction of the symplectite, compared to ~ Fo44 for cumulus olivine at this level in the intrusion. Biotite commonly replaces olivine. Texturally- and compositionally-related patches of intergrown clinopyroxene and An-rich plagioclase occur locally on plagioclase triple junctions and plagioclase grain boundaries. Symplectites are present, but rare, in the lower parts of the Skaergaard Layered Series; increase significantly in volumetric importance in Lower Zone b; are very common in Middle Zone and disappear in UZ. The symplectites resemble those formed by hydrous partial melting of oceanic gabbros (Koepke et al., 2005) but important differences include the presence of clinopyroxene and Fe-rich plagioclase, and the

  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


    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. Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau? (United States)

    Phillips, Bethan A.; Kerr, Andrew C.; Mullen, Emily K.; Weis, Dominique


    The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene-Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.

  18. Evolution of the East African rift: Drip magmatism, lithospheric thinning and mafic volcanism (United States)

    Furman, Tanya; Nelson, Wendy R.; Elkins-Tanton, Linda T.


    The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.

  19. Determination of rare-earths and other trace elements in neo proterozoic-neo paleozoic dykes from Ceara state, Brazil, by neutron activation analysis

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    Anjos, Rafael Martins dos; Figueiredo, Ana M.G., E-mail: rafael.anjos@usp.b, E-mail: anamaria@ipen.b [Instituto de Pesquisas Energeticas e Nucleares (IPEN/CNEN-SP), Sao Paulo, SP (Brazil). Centro do Reator Nuclear de Pesquisas. Lab. de Analise por Ativacao com Neutrons; Cardoso, Gustavo Luan; Marques, Leila S., E-mail: leila@iag.usp.b [Universidade de Sao Paulo (IAG/USP), SP (Brazil). Inst. de Astronomia, Geofisica e Ciencias Atmosfericas


    Trace elements such as rare earths, U, Th, Ta, Ba and Hf can be very useful in petrogenetic studies of igneous and metamorphic rocks, giving information about the origin and evolution of magmas. Instrumental Neutron Activation Analysis (INAA) is an accurate and precise for trace element analysis in geological samples, and provides the information required for this kind of studies. In this study, rare earths and incompatible trace elements were determined by INAA in the geological reference materials GS-N and BE-N, to quality control, and for the investigation of acid dykes of neo proterozoic-neo paleozoic ages, which outcrop in the Medio Coreau and Ceara Central domains from the Borborema Province (Ceara State). The powdered samples (particle sizes less than 100 mesh), crushed by using a mechanical agate mortar grinder, were irradiated at the IEA-R1 nuclear reactor at IPEN-CNEN/SP, and the induced activity was measured by high resolution gamma-ray spectrometry. The accuracy and precision of the method were evaluated and preliminary results of dyke samples are presented. (author)

  20. Variations in the chemical composition of lamprophyllite-group minerals and the crystal structure of fluorine-rich barytolamprophyllite from new peralkaline dyke (United States)

    Akimenko, M. I.; Aksenov, S. M.; Sorokhtina, N. V.; Kogarko, L. N.; Kononkova, N. N.; Rastsvetaeva, R. K.; Rozenberg, K. A.


    The variations in the chemical composition of lamprophyllite-group minerals from a peralkaline dyke of the Mokhnatye Roga area (Kandalaksha region, Kola Peninsula), which are crystallized during the entire period of dyke formation and form several generations, have been investigated. The early generations differ in a steadily high fluorine content, while the later ones exhibit reduced amount of fluorine, impurity elements, and sodium, with a simultaneous increase in the potassium content. The crystal structure of fluorine- rich barytolamprophyllite (potentially a new representative of the lamprophyllite group, differing by the predominance of fluorine in the anion X site) has been analyzed by single crystal X-ray diffraction. This mineral is found to have a monoclinic unit cell with the following parameters: a = 19.5219(8) Å, b = 7.0915(2) Å, c = 5.3925(2) Å, β = 96.628(3)°, and sp. gr. C2/ m. The structure is refined to R = 5.73% in the anisotropic approximation of the atomic displacement parameters using 3668 I > 2σ( I). The idealized formula ( Z = 2) is (Ba,Sr)2[Na(Na,Fe)2(Ti,Mg)F2][Ti2(Si2O7)2O2].

  1. Temporal magma dynamics during solidification of the mafic-silicic complex of Isle au Haut, Maine (United States)

    Patwardhan, Kaustubh

    The Isle au Haut Igneous Complex provides a unique opportunity to examine in detail the in situ physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas. The complex contains a 600 m thick sequence of 11 alternating layers of gabbro and diorite (typically 15--40 m thick). Purely on the basis of density contrasts (2.65 g/cm 3 gabbro vs. 2.55 g/cm3 diorite), the entire system should have undergone wholesale instability and mixing; it is instead arrested in a grossly unstable state of interaction while molten. Chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near liquidus gabbroic magma invaded partly crystalline, cooler diorite. Mineral assemblages, chemical analyses, and phase equilibria calculations indicate initial temperatures during emplacement of ˜1180°C (gabbro) and ˜1000°C (diorite). Conductive thermal models yield solidification timescales of 15--60 years for individual gabbro layers and about a thousand years for the entire complex. There is ample evidence for two phases of small-scale interfacial Rayleigh-Taylor type instabilities of dioritic melt into the gabbros. Phase I occurred immediately upon gabbro emplacement whereas evenly spaced, slender more silicic pipes represent a much later stage (Phase II). Pipe geometry and spacing, estimated viscosities of the gabbroic magma and silicic melt, and the sudden increase in silica near the upper contact of the diorite, all indicate a thin (˜18--53 cm) buoyant layer at the upper contact of the diorite as the source of the pipes. Compaction of the diorite produced this layer over a period of about ten years. Simultaneous solidification along the lower contact of the overlying gabbro, thickening inwards, increased viscosity enough to arrest pipe ascent after a few meters. Crystal size distribution (CSD) analyses of the gabbro layers yield crystal growth rates (G0 = 2--4 x 10-10 cm/s) and nucleation rates

  2. Experimental and textural constraints on mafic enclave formation in volcanic rocks (United States)

    Coombs, M.L.; Eichelberger, J.C.; Rutherford, M.J.


    We have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at Southwest Trident volcano, Alaska. Andesite enclaves are present throughout the four dacite lava flows produced by the eruption, and resemble mafic enclaves commonly found in other volcanic rocks. Our experiments replicate the pressure-temperature-time path taken by enclave-forming andesite magma as it is engulfed in dacite during magma mixing. Pressure and temperature information for the andesite and dacite are from [Coombs et al., Contrib. Mineral. Petrol. 140 (2000) 99-118]. The andesite was annealed at 1000??C, and then cooled to 890??C at rates of 110??C h1, 10??C h1 and 2??C h1. Once cooled to 890??C, andesite was held at this lower temperature from times ranging from 1 to 40 h. The andesite that was cooled at the slower rates of 2??C h1 and 10??C h1 most resembles enclave groundmass texturally and compositionally. Based on simple thermal calculations, these rates are more consistent with cooling of the andesite groundmass below an andesite-dacite interface than with cooling of enclave-sized spheres. If enclaves do crystallize as spheres, post-crystallization disaggregation must occur. Calculations using the MELTS algorithm [Ghiorso and Sack, Contrib. Mineral. Petrol. 119 (1995) 197-212] show that for incoming andesite to become less dense than the dacite to become less dense ???34 volume % of its groundmass must crystallize to undergo~18 volume % vesiculation; these values are similar to those determined for Southwest Trident enclaves. Thus such crystallization may lead to 'flotation' of enclaves and be a viable mechanism for enclave formation and dispersal. The residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments. Decompression experiments on Southwest Trident dacite suggest an average ascent rate for the eruption of ???2

  3. Experimental effects of pressure and fluorine on apatite saturation in mafic magmas, with reference to layered intrusions and massif anorthosites (United States)

    Tollari, N.; Baker, D. R.; Barnes, S.-J.


    Apatite is a cumulate phase in the upper parts of some mafic layered intrusions and anorthositic complexes. We investigated the effect of pressure and fluorine on apatite saturation in mafic magmas to better understand under which conditions this mineral crystallizes. Apatite saturation gives information about the formation of silicate rocks, and is of interest in explaining the formation of apatite-oxide-rich rocks (e.g. nelsonites comprising approximately, one-third apatite and two-third Fe-Ti oxide). Two models of formation are proposed for this rock type: crystal fractionation followed by accumulation of apatite and Fe-Ti oxides and liquid immiscibility. New experiments carried out with mafic compositions at 500 MPa confirm that the most important variables on phosphate saturation are SiO2 and CaO. Fluorine addition leads to apatite saturation at lower SiO2 and higher CaO concentrations. Comparison of our results with those of previous experimental studies on liquid-liquid immiscibility at upper-to-mid-crustal conditions allows us to investigate the relative importance of apatite saturation versus liquid-liquid immiscibility in the petrogenesis of nelsonites and similar rocks. The liquid line of descent of three natural examples studied (the Sept-Îles intrusive suite, the anorthositic Complex of the Lac-St-Jean and the Skaergaard layered intrusion) do not cross the liquid-liquid immiscibility field before they reach apatite saturation. Thus, the apatite-oxide-rich rock associated with these three intrusive suites are best explained by crystal fractionation followed by accumulation of apatite and Fe-Ti oxides.

  4. Metallogenic Series Related to Permian Mafic Complex in North Xinjiang: Post-collisional Stage or Mantle Plume Result?

    Institute of Scientific and Technical Information of China (English)

    WANG Yuwang; WANG Jingbin; WANG Lijuan; LONG Lingli


    There are four deposit types related to a Permian mafic complex in northern Xinjiang, i.e.,copper-nickel sulfide deposit, vanadic titanomagnetite deposit, magnetite (-cobalt) deposit and Cu-Ni-VTiFe composite deposit. The deposits are distributed spanning tectonic units with dose andconsecutive metallogenic ages. A transitional deposit type can occur among the end-member deposits.Trace elements of host rocks show that they can derive from similar source area. Hence, theyconstitute a particular metallogenic series related to a mafic-ultramafic complex that is also a symbolseries of the post-collisional stage of the Central Asia Metallogenic Province (CAMP). Themetallogenic ages of the series are between 260 Ma and 300 Ma throughout the Permian. Unlikemineralization from a mantle plume, the metallogenic period of this series spans at least 40 Ma.Compared with related deposits of the Emeishan mantle plume, the North Xinjiang series has asimilar ore-forming element assemblage but has preferably developed Cu-Ni sulfide deposits ratherthan vanadic titanomagnetite deposits. In concomitance with this series, North Xinjiang area hasdeveloped a set of syntectonic Au-Cu-Mo metailogenic series related to a felsic volcanic-intrusivecomplex, which might indicate that there is no direct relationship with mantle plume activity. Fromearly to late, i.e., the sequence of copper-nickel sulfide to magnetite (-cobalt) to vanadictitanomagnetite deposit, the host rock series evolves from mafic-ultramafic and tholeiite series tomafic and alkalic series, the ∑REE content tends to increase with increasing of REE fractionation, andsome of the trace elements (particularly LIL) also show an increasing tendency. The aboveevolutionary regularity possibly reflects a course where the magma source deepens and thermalinterface moves down, energy gradually exhausts, and neo-continentai crust forming in the post-collision stage tends to stabilize.

  5. Mafic Volcanism Along the Sinaloa Coast, Mexico, and its Relation to the Opening of the Gulf of California (United States)

    Orozco Esquivel, T.; Ferrari, L.; Lopez Martinez, M.


    We report on new localities with mafic volcanism along the Sinaloa coast, which record changes in the magma generation processes along the eastern margin of the Gulf of California. South of Culiacán, Sinaloa, isolated outcrops of basaltic lavas built a ca. 60 km long belt aligned to the SE. The similarity in the mineralogy and composition of the lavas suggest that these outcrops could have been part of a single flow. Lavas contain abundant plagioclase (up to 3 mm), and olivine (up to 1.5 mm) phenocrysts, and scarce clinopyroxene, in a relatively coarse matrix. In multiement diagrams, the lavas show the negative Nb and Ta, and positive Pb and Sr anomalies characteristic of subduction related rocks. The age determination of these rocks is in process, nevertheless, rocks with similar compositions are known from ~11 Ma mafic dikes that outcrop in southern Sinaloa. The Pericos volcanic field, located about 25 km to the NW of Culiacán is composed by lava flows, shield volcanoes, and cinder cones of basaltic composition that cover an area of aprox. 20 x 32 km, and have a well preserved morphology suggestive of a Pliocene-Quaternary age. Lavas are porphyritic and contain olivine, plagioclase and clinopyroxene in a microcrystalline matrix. Some lava flows contain abundant megacrysts of green clinopyroxene (up to 8 cm), olivine (up to 1 cm), and/or plagioclase (up to 1 cm), or aggregates of olivine and clinopyroxene. Trace element abundances are remarkably uniform among all analyzed samples and are characteristic of intraplate magmas. Rocks with very similar composition, mineralogy, and also containing megacrysts, have been reported in the Pliocene Punta Piaxtla and Mesa Cacaxtla, located 200 km to the SSE at the Sinaloa coast. Those similarities indicate that mafic intraplate volcanism related to the opening of the Gulf of California is more broadly represented in the area than previously considered.

  6. Rb-Sr and Nd-Sr isotope geochemistry and petrogenesis of the Misho Mountains mafic dikes (NW Iran

    Directory of Open Access Journals (Sweden)

    Maryam Ahankoub


    Full Text Available Introduction There are some theories about the Paleotethys event during the Paleozoic that have been proposed by geologists (Metcalfe, 2006. Some scientist offered some pieces of evidence about the northern margin of Gondwana (Zhu et al., 2010. The Paleotethys Ocean and Hercynian orogenic report first in Iran, have been Offered from the Morrow and Misho Mountain (Eftekharnejad, 1981. Misho Mountains is located between the north and south Misho faults and cause the formation of a positive flower structure (Moayyed and Hossainzade, 2011. There is theory about Misho southern fault as the best candidate of the Paleotethys suture zone (Moayyed and Hossainzade, 2011. Geochemistry and Sr –Nd isotopic data of the A2 granitic and Synite rocks of the East Misho, indicate that the magmatism post collision has occurred in the active continental margin by extensional zones of the following the closure of the Paleotethys (Ahankoub, 2012. Granite and syenite rocks have been cut by mafic dikes. Mafic dikes are often formed in extensional tectonic settings related to mantle plume or continental break –up (Zhu et al., 2009. In this paper, we use the geochemistry and Nd-Sr isotope data to determined petrogenesis, tectono-magmatic setting and age of Misho mafic dikes. Materials and methods After petrography study of 30 thin sections of mafic dike rocks, 8 samples were selected for whole-rock chemical analyses using ICP-MS and ICP-AES instruments by ACME Company in Vancouver, Canada. We prepared 6 sample For Sm-Nd and Rb-Sr analysis. Sr and Nd isotope ratios were measured with a thermal ionization mass spectrometer, VG Sector 54–30 at the Nagoya University. The isotope abundances of Rb, Sr, Nd, and Sm were measured by the ID method with a Finnigan MAT Thermoquad THQ thermal ionization quadrupole mass spectrometer at the Nagoya University. NBS987 and JNdi-1 were measured as natural Sr and Nd isotope ratio standards (Tanaka et al., 2000. Averages and 2σ errors

  7. Mapping the 3-D extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data (United States)

    Finn, Carol A.; Bedrosian, Paul A.; Cole, Janine; Khoza, Tshepo David; Webb, Susan J.


    Geophysical models image the 3D geometry of the mafic portion of the Bushveld Complex north of the Thabazimbi-Murchison Lineament (TML), critical for understanding the origin of the world's largest layered mafic intrusion and platinum group element deposits. The combination of the gravity and magnetic data with recent seismic, MT, borehole and rock property measurements powerfully constrains the models. The intrusion north of the TML is generally shallowly buried (generally modeled area of ∼160 km × ∼125 km. The modeled thicknesses are not well constrained but vary from ∼12,000 m, averaging ∼4000 m. A feeder, suggested by a large modeled thickness (>10,000 m) and funnel shape, for Lower Zone magmas could have originated near the intersection of NS and NE trending TML faults under Mokopane. The TML has been thought to be the feeder zone for the entire Bushveld Complex but the identification of local feeders and/or dikes in the TML in the models is complicated by uncertainties on the syn- and post-Bushveld deformation history. However, modeled moderately thick high density material near the intersection of faults within the central and western TML may represent feeders for parts of the Bushveld Complex if deformation was minimal. The correspondence of flat, high resistivity and density regions reflect the sill-like geometry of the Bushveld Complex without evidence for feeders north of Mokopane. Magnetotelluric models indicate that the Transvaal sedimentary basin underlies much of the Bushveld Complex north of the TML, further than previously thought and important because the degree of reaction and assimilation of the Transvaal rocks with the mafic magmas resulted in a variety of mineralization zones.

  8. Variable sediment flux in generation of Permian subduction-related mafic intrusions from the Yanbian region, NE China (United States)

    Guo, Feng; Li, Hongxia; Fan, Weiming; Li, Jingyan; Zhao, Liang; Huang, Miwei


    This paper presents petrology, mineralogy, zircon U-Pb ages, and whole-rock major, trace element and Sr-Nd-Hf isotopic compositions of four Permian (273-253 Ma) subduction-related mafic intrusions (including the Qinggoushan and Qianshan gabbros, and the Wangqing and Shuguang diorites) from the Yanbian region, NE China, with aims to understand the role of subducted sediment flux in generation of arc mafic cumulates. These intrusions have mineral assemblages crystallized in water-saturated parental magmas and show variable degrees of crystal accumulation as observed in mafic cumulates in subduction zones. Mass-balance consideration indicates that their parental magmas were calc-alkaline with arc-type trace element features (enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depletions in Nb-Ta). They also have Sr-Nd-Hf isotopic compositions, i.e., 87Sr/86Sr(i) = 0.7029-0.7047, εNd(t) = + 0.9 ~ + 6.8, εHf(t) = + 5.6 ~ + 14.6, similar to modern arc basalts. The parental magmas were likely derived from a mantle wedge variably metasomatized by sediment melt and fluid from the subducting paleo-Asian Oceanic slab. Combined trace elemental and isotopic modeling results suggest that the parental magma of Qinggoushan gabbro was formed through 5-20% melting of the mantle wedge with 1% and 1.5% additions of sediment fluid and sediment melt, respectively; 5-10% melting of the mantle wedge through inputs of 1% sediment fluid and 2% sediment melt produced the Qianshan gabbro; 10-20% melting of the mantle wedge with additions of 1% sediment fluid and 3% sediment melt formed the Wangqing diorite; whereas 5-20% melting of the mantle wedge through an input of 1.5% sediment melt produced the Shuguang diorite. The Hf-Nd isotopic array of the Yanbian Permian mafic intrusions reflected the existence of an Indian Ocean-type mantle, which was isotopically distinct from the Pacific-type mantle during early Paleozoic in the Central Asian Orogenic

  9. Geochemical characteristics and tectonic setting of the Tuerkubantao mafic-ultramafic intrusion in West Junggar, Xinjiang, China

    Directory of Open Access Journals (Sweden)

    Yufeng Deng


    Full Text Available Mineral chemistry, whole-rock major oxide, and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion, in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt. The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite, olivine pyroxenite, gabbro, and diorite. The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks. The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB. In addition, the Tuerkubantao intrusion displays relatively low Th/U and Nb/U (1.13–2.98 and 2.53–7.02, respectively and high La/Nb and Ba/Nb (1.15–4.19 and 37.7–79.82, respectively. These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting. The trace element patterns of peridotites, gabbros, and diorite in the Tuerkubantao intrusion have sub-parallel trends, suggesting that the different rock types are related to each other by differentiation of the same primary magma. The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite. However, the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic setting, internal lithological zoning, and geochemistry. The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions. In combination with the Devonian magmatism and porphyry mineralization, we propose that subduction of the oceanic slab has

  10. Bimodal magmatism during the Diego Hernández Formation, Tenerife, Canary Islands: genesis and eruption-triggering of phonolitic magmas during ongoing mafic volcanism (United States)

    Olin, P. H.; Wolff, J. A.; Edgar, C. J.; Cas, R.; Martí, J.


    The Diego Hernández Formation (DHF) represents the explosive eruption of nearly 70 cubic km of phonolite over approximately 200 k.y. from the Las Cañadas caldera on Tenerife. Four chemostratigraphic units are distinguished on the basis of trace element contents: DHF bs (represented by the 370 ka Fortaleza and 347 ka Roque Members), DHF I (319 ka Aldea, 309 ka Fasnia, and 268 ka Poris Members), DHF II (Arafo and 223 ka Caleta Members), and DHF III (Cruz Sequence and the 196 ka Abrigo Member); all named units involve plinian and/or ignimbrite components that devastated a significant fraction of the island [1]. These chemostratigraphic units demarcate two dominant compositional trends distinct in incompatible element contents, and in Nb/Ta and REE ratios. DHF bs and DHF III plot along a high-Nb trend, and DHF I and DHF II plot along a low-Nb trend, a feature consistent with divergent fractionation histories involving titanite. Mafic magma was an important component of the DHF magmatic system and flanking mafic volcanism was ongoing during DHF time. Major phonolitic eruptions are conformably bounded by basanitic lavas and scoria deposits. Mafic magmatic components are identifiable in many of the phonolitic pyroclastic deposits as mafic, mingled and banded pumices, or as quenched mafic enclaves. Mafic components in the Abrigo, Caleta, and Poris Members are nearly geochemically identical to the underlying scoria or lava, suggesting that flanking mafic volcanism may in some cases be associated with subcaldera intrusive events that remobilize phonolitic magma to trigger major explosive eruptions. We envisage that the DHF represents a time when the intrusion of mantle-derived mafic magma in the lower crust supplied heat sufficient for the generation of intermediate tephriphonolite and phonotephrite magmas via melting of gabbroic/basaltic crust. Some of these intermediate magmas evolved to phonolite by crystal fractionation, a scenario consistent with DHF III

  11. Global absorption center map of the mafic minerals on the Moon as viewed by CE-1 IIM data

    Institute of Scientific and Technical Information of China (English)


    Determining the global distribution of minerals on the Moon has been an important goal of lunar science. Hyperspectral remote sensing is an important approach to acquiring minerals on the Moon on the global scale. The wavelength of the absorption band center is the key parameter for identifying minerals with reflectance spectra as well as remote sensing data. The global absorption center map of the mafic minerals of the Moon was produced for the first time with the Chang’E-1 IIM data. This map shows the global distribution of mafic minerals such as orthopyroxenes, clinopyroxenes, and olivine and even plagioclase feldspar of the Moon. The validation for some representative areas indicates that the global map is reliable and even more detailed than the results derived from Clementine-data. Moreover, our method is insensitive to the topography and viewing and illumination geometries. The global absorption band center map not only contributes to the lunar science research, but also has other implications to be further studied. Moreover, the preprocessing methods such as calibration and correction introduced in this study can be useful in other research with IIM data.

  12. The Cerro Bitiche Andesitic Field: petrological diversity and implications for magmatic evolution of mafic volcanic centers from the northern Puna (United States)

    Maro, Guadalupe; Caffe, Pablo J.


    The Cerro Bitiche Andesitic Field (CBAF) is one of the two largest mafic volcanic fields in northern Puna (22-24° S) and is spatially and temporally associated with ignimbrites erupted from some central Andean Altiplano-Puna Volcanic Complex calderas. The CBAF comprises seven scoria cones and widespread high-K calcalkaline lava flows that cover an area of 200 km2. Although all erupted rocks have a relatively narrow chemical range (56-62 % SiO2, 3-6 % MgO), there is a broad diversity of mineral compositions and textures. The least evolved lavas (˜58-61 % SiO2) are high-Mg andesites with scarce (andesites (˜62 wt% SiO2), on the other hand, are porphyritic rocks with plagioclase + orthopyroxene + biotite and ubiquitous phenocryst disequilibrium textures. These magmas were likely stored in crustal reservoirs, where they experienced convection caused by mafic magma underplating, magma mixing, and/or assimilation. Trace element and mineral compositions of CBAF lavas provide evidence for complex evolution of distinct magma batches.

  13. Why do mafic arc magmas contain ˜4wt% water on average? (United States)

    Plank, Terry; Kelley, Katherine A.; Zimmer, Mindy M.; Hauri, Erik H.; Wallace, Paul J.


    The last 15 yr have seen an explosion of data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions trapped in volcanic phenocrysts, aliquots of magma that have presumably escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations in the least degassed melt inclusions from each volcano. Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6 wt% H2O. The average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with a global average of 3.9±0.4 wt% H2O. Significant variations occur from volcano to volcano within each arc, but the means are indistinguishable within one s.d. The narrow range and common average value for H2O are in stark contrast to the concentrations of most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of arc melt inclusions. One possibility is that melt inclusion H2O values reflect vapor saturation at the last storage depth in the crust prior to eruption. In this scenario, magmas rise from the mantle with variable H2O contents (>4 wt%), become vapor-saturated and start degassing, and continue to degas up until the depth at which they stall. If the stalling depths are ∼6 km, which is common for storage depths beneath volcanoes, magmas would be saturated at ∼4 wt% H2O, and melt inclusions, most of which become closed during further ascent, would thus record ≤4 wt% H2O. Another possibility is that the mantle melting process modulates water content in the melt such that magmas rise out of the mantle with ∼4 wt% H2O. A strong relationship between the water content of the source, H2O(o) and the degree of melting (F) maintains nearly constant water contents in the melt for a restricted range in mantle temperature. Magmas with

  14. A Study of Sr, Nd and O Isotopes of the K-rich Melanocratic Dykes in the Late Mesozoic Gold Field in the Jiaodong Peninsula

    Institute of Scientific and Technical Information of China (English)


    Geologic, petrographic and petrochemical studies of the lateMesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr)i-0.70895-0.71140) and low (143Nd/144Nd)i ratios (varying from 0.51135 to 0.51231); and its δ18OsMow. whole rock values vary from +5.8‰ to +10.6‰ with a mean of +7.1‰. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced me tasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enriched mantle in its mature stage in the back-arc spreading environment. The evolution of magmas is associated with two trends, i.e., fractional crystallization and mixing with or intensive contamination by palaeo-crust materials or metamorphic rocks. The former process is evident in the gold field system of quartz-vein type, whereas the latter is dominated in the gold field system of the altered-rock type. This conclusion is very important for more detailed study of petrogenesis and mineralization through the crust-mantle interaction (exchange) in the Mesozoic in this region.

  15. 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


    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.

  16. Eruption dynamics of the 7.7 ka Driftwood pumice-fall suggest mafic injection is a common eruption mechanism for Makushin Volcano, Alaska (United States)

    Lerner, A.; Crowley, P.; Hazlett, R. W.; Nicolaysen, K. E.


    Makushin Volcano on Unalaska Island, AK is potentially the most threatening volcano in the Aleutian chain, being close to the largest Aleutian towns of Dutch Harbor and Unalaska. This study reports the eruption chronology and triggering mechanism for the most recent highly explosive event, the 7.7 ka Driftwood Pumice-fall event. The Driftwood Pumice reaches thicknesses of over 2 m, and isopach contours estimate a total deposit volume of 0.3-0.9 km3, covering an area of at least 8100 km2. These reconstructions show an eruption on the scale of the 1980 Mt. St. Helens eruption, with a VEI of 4-5. In the field, the deposit was divided into four stratigraphic horizons from bottom to top, and tephra within these layers becomes systematically more mafic upward through the section, ranging from a basal low-SiO2 dacite (64 wt.% SiO2) to an upper medium-SiO2 andesite (61.5 wt.% SiO2). High-Ca plagioclase (An75-83) and high-Mg olivine (Mg69-75) grains within the pumice are in great disequilibrium with the dacitic glass (64-69 wt.% SiO2), suggesting their origin in a more mafic magma. Geochemical trends, disequilibrium mineral populations, and mineral zonation patterns within these plagioclase and olivine xenocrysts show evidence of magma mixing between a bulk siliceous magma chamber and a mafic injection. The amount of the mafic component increases upward within the deposit, ranging from 0-25% throughout the section. The mafic injection is calculated to have been ~110-200 °C hotter than the siliceous magma chamber. The thermal pulse provided by the injection likely initiated convection and volatile exsolution within the siliceous magma body, ultimately causing the Driftwood Pumice eruption. Diffusion rates based on the thickness of lower-Mg rim zonations (<10 µm thick rims of Mg64) in the olivine xenocrysts show a lag-time of ~1 year between the basaltic injection and the resulting eruption. Similar delays between mafic injections and eruptions are seen in numerous other

  17. Petrogenesis of the Neoproterozoic Ngondo Plutonic complex (Cameroon, west central Africa): a case of late-collisional ferro-potassic magmatism (United States)

    Tagne-Kamga, Gabriel


    The Ngondo complex is a late-collisional pluton, which was emplaced around 600 Ma along a N030° E strike-slip shear zone in the southwestern part of the Neoproterozoic Fold Belt of Cameroon. It comprises three successively emplaced plutonic groups of rocks: (i) mafic to felsic intrusive rocks (MFR), (ii) fine-grained granites (FGG) and (iii) coarse-grained granites (CGG). Late aplitic and pegmatite dykes were emplaced along brittle fractures in these plutons. The complex is metaluminous to weakly peraluminous, high-K, calc-alkaline to " trans-alkaline" ferro-potassic, with mineralogical and geochemical characteristic of I-type granites. The plutonic rocks are characterised by high Ba, Sr, Rb and ∑REE concentrations and low Ni and Cr contents in the mafic members. They also display chondrite-normalised REE patterns characterised by variable LREE enrichment, moderate to minor HREE fractionation with moderate to pronounced negative Eu anomalies (Eu/Eu∗: 0.6-0.9 (MFR), 0.2-0.5 (FGG), 0.3-0.7 (CGG)). Trace element distribution patterns for the three plutonic groups are similar with a distinctive depletion in Nb, Sr and Ti relative to other trace elements and a greater enrichment in LILE compared to HFSE. These plutonic groups present distinct evolutionary trends precluding their origin from differentiation of a single parental magma. The geochemical and isotopic data indicate that they derived from partial melting of heterogeneous (meta)-igneous mafic lower crustal materials, having possibly the composition of amphibolitised high-K calc-alkaline basaltic andesites and andesitic metagreywackes. Petrographic evidences such as the presence of quartz-ocelli, xenocrysts of feldspar, fragments of country rocks (migmatitic gneisses) strongly indicate that crustal contamination may have played an important role in the genesis of the plutonic rocks. This contamination process is further supported by the variation of major and trace elements together with Sr-Nd isotopic data

  18. Mafic and ultramafic inclusions along the San Andreas Fault System: their geophysical character and effect on earthquake behavior, California, USA (United States)

    Ponce, D. A.; Langenheim, V. E.; Jachens, R. C.; Hildenbrand, T. G.


    Mafic and ultramafic rocks along the San Andreas Fault System (SAFS) influence earthquake processes where their geologic setting often provides information on the tectonic evolution of these large-scale strike-slip faults. In the northern part of the SAFS, along the Hayward Fault (HF), inversion of gravity and magnetic data indicate that seismicity avoids the interior of a large gabbro body and mechanical models may be able to explain how this massive mafic block influences the distribution of stress. Aftershocks of the M6.7 1989 Loma Prieta earthquake are also spatially related to the distribution of a gabbro body, clustering along the SAF and terminating at the NW end of the gabbro body where it abuts the fault surface. Based on geophysical modeling and a three-dimensional view of the subsurface geology and seismicity, aftershocks do not occur in the interior of the buried gabbro body. In the southern part of the SAFS, aftershocks and ruptures of the M7.1 1999 Hector Mine and M7.3 1992 Landers earthquakes avoid the interior of a Jurassic diorite that extends to depths of approximately 15 km and was probably an important influence on the rupture geometry of the these earthquakes. Seismicity prior to the Landers earthquake also tend to avoid the diorite, suggesting that it affects strain distribution. The San Jacinto Fault (SJF), a discontinuity within the Peninsular Ranges batholith (PRB), separates mafic, dense, and magnetic rocks of the western PRB from more felsic, less dense, and weakly magnetic rocks of the eastern PRB. The geophysical gradients do not cross the SJF zone, but instead bend to the northwest and coincide with the fault zone. Because emplacement of the PRB presumably welded across this older crustal boundary, the SJF zone probably developed along the favorably oriented margin of the dense, stronger western PRB. Two historical M6.7 earthquakes may have nucleated along the PRB discontinuity suggesting that the PRB may continue to affect how strain

  19. Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: Occurrence of Archean oceanic plateau (United States)

    El Atrassi, Fatima; Debaille, Vinciane; Mattielli, Nadine; Berger, Julien


    While Archean terrains are mainly composed of a TTG (Tonalite-trondhjemite-granodiorite) suite, more mafic lithologies such as amphibolites are also a typical component of those ancient terrains. Although mafic rocks represent only ~10% of the Archean cratons, they may provide key evidence of the role and nature of basaltic magmatism in the formation of the Archean crust as well as the evolution of the Archean mantle. This study focuses on the Archean crust from the West African craton in Mauritania (Amsaga area). The Amsaga Archean crust mainly consists of TTG and thrust-imbricated slices of mafic volcanic rocks, which have been affected by polymetamorphic events from the amphibolite to granulite facies. We report the results of a combined petrologic, Sm-Nd isotopic, major element and rare earth element (REE) study of the Archean amphibolites in the West African craton. This study was conducted in order to characterize these rocks, to constrain the time of their formation and to evaluate their tectonic setting and their possible mantle source. Our petrological observations show that these amphibolites have fine to medium granoblastic and nematoblastic textures. They are dominated by amphibolite-facies mineral assemblages (mainly amphibole and plagioclase), but garnet and clinopyroxene occur in a few samples. These amphibolites have tholeiitic basalt composition. On a primitive mantle-normalized diagram, they display fairly flat patterns without negative anomalies for either Eu or Nb-Ta. We have shown using Sm-Nd whole rock isotopic data that these amphibolites formed at 3.3 ±0.075 Ga. They have positive ɛNdi values (+5.2 ± 1.6). These samples show isotopically juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source and that they have a

  20. 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


    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

  1. Amphibole-rich intrusive mafic and ultramafic rocks in arc settings: implications for the H2O budget (United States)

    Tiepolo, M.; Langone, A.; Morishita, T.; Esna-Ashari, A.; Tribuzio, R.


    Although amphibole is rarely a phenocryst of arc lavas, many intermediate and silicic magmas in arc settings are considered residual after cryptic amphibole crystallization at mid-low crustal levels (e.g., Davidson et al., 2007). Amphibole-rich mafic and ultramafic intrusive rocks (hornblendites, amphibole-gabbros to amphibole-diorites) are reported worldwide in orogenic settings. These amphibole-rich plutonics could be the "hidden" amphibole reservoir invoked in the arc crust. They usually possess chemical and textural heterogeneities recording the magmatic processes occurring in the mid to low crust (e.g., Tiepolo et al., 2011). Being amphibole-rich, these intrusive rocks are an important source of information on the possible role played by amphibole in arc magma petrogenesis. In particular, for the capability of amphibole to incorporate H2O and elements with a marked affinity for the fluid phase, these rocks are also useful to track the origin and evolution of subduction related fluids. We present here geochemical and geochronologic data on amphibole-rich ultramafic intrusive rocks from different localities worldwide: i) Alpine Orogen (Adamello Batholith and Bregell intrusions); ii) Ross Orogen (Husky Ridge intrusion - Antarctica); iv) Japan Arc (Shikanoshima Island intrusion); v) Sanandaj-Sirjan Zone, Central Iran (Aligoordaz granitoid complex). The coupling of textural information, micro-chemical data and "in situ" zircon geochronology has allowed us to show that these ultramafic intrusive rocks share striking petrologic and geochemical similarities. They are thus the expression of a common magmatic activity that is independent from the age and from the local geological setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks are retained a common feature of collisional-systems worldwide. Amphibole is thus expected to play a major role in the differentiation of arc magmas and in particular in the H2O

  2. The timing of compositionally-zoned magma reservoirs and mafic 'priming' weeks before the 1912 Novarupta-Katmai rhyolite eruption (United States)

    Singer, Brad S.; Costa, Fidel; Herrin, Jason S.; Hildreth, Wes; Fierstein, Judy


    The June, 6, 1912 eruption of more than 13 km3 of dense rock equivalent (DRE) magma at Novarupta vent, Alaska was the largest of the 20th century. It ejected >7 km3 of rhyolite, ∼1.3 km3 of andesite and ∼4.6 km3 of dacite. Early ideas about the origin of pyroclastic flows and magmatic differentiation (e.g., compositional zonation of reservoirs) were shaped by this eruption. Despite being well studied, the timing of events that led to the chemically and mineralogically zoned magma reservoir remain poorly known. Here we provide new insights using the textures and chemical compositions of plagioclase and orthopyroxene crystals and by reevaluating previous U-Th isotope data. Compositional zoning of the magma reservoir likely developed a few thousand years before the eruption by several additions of mafic magma below an extant silicic reservoir. Melt compositions calculated from Sr contents in plagioclase fill the compositional gap between 68 and 76% SiO2 in whole pumice clasts, consistent with uninterrupted crystal growth from a continuum of liquids. Thus, our findings support a general model in which large volumes of crystal-poor rhyolite are related to intermediate magmas through gradual separation of melt from crystal-rich mush. The rhyolite is incubated by, but not mixed with, episodic recharge pulses of mafic magma that interact thermochemically with the mush and intermediate magmas. Hot, Mg-, Ca-, and Al-rich mafic magma intruded into, and mixed with, deeper parts of the reservoir (andesite and dacite) multiple times. Modeling the relaxation of the Fe-Mg concentrations in orthopyroxene and Mg in plagioclase rims indicates that the final recharge event occurred just weeks prior to the eruption. Rapid addition of mass, volatiles, and heat from the recharge magma, perhaps aided by partial melting of cumulate mush below the andesite and dacite, pressurized the reservoir and likely propelled a ∼10 km lateral dike that allowed the overlying rhyolite to reach the

  3. Comprehensive Pb-Sr-Nd-Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials (United States)

    Fourny, Anaïs.; Weis, Dominique; Scoates, James S.


    Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a comprehensive Pb-Sr-Nd-Hf isotopic and trace element concentration data set, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM-P), five basalts (BHVO-2, BIR-1a, JB-3, BE-N, GSR-3), a diabase (W-2), a dolerite (DNC-1), a norite (NIM-N), and an anorthosite (AN-G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC-ICP-MS and TIMS, and multielement analyses by HR-ICP-MS are reported with 4-12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR-1a). For Pb isotopic reproducibility, several of the basalts (JB-3, BHVO-2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (<50 ppm), however, obtaining good reproducibility for Pb and Hf isotopic ratios is more challenging for NIM-P, NIM-N, and AN-G due to a variety of factors, including postcrystallization Pb mobility and the presence of accessory zircon. Collectively, these results form a comprehensive new database that can be used by the geochemical community for evaluating the radiogenic isotope and trace element compositions of volcanic and plutonic mafic-ultramafic rocks.

  4. The origin of spheroidal patterns of weathering in the Pados-Tundra mafic-ultramafic complex, Kola Peninsula, Russia

    Directory of Open Access Journals (Sweden)

    A.Y. Barkov


    Full Text Available We document a new and unusual occurrence of patterns of protruding spheroidal weathering developed in a dunitic rock of the Pados-Tundra mafic-ultramafic complex of Early Proterozoic age, Kola Peninsula, Russia. It provides an example similar to that reported recently from a mineralized harzburgite in the Monchepluton layered complex in the same region. These patterns are genetically different from common results of “normal spheroidal weathering” sensu stricto. The spheroidally weathered dunite at Pados-Tundra consists of a high-Fo olivine, Ol (Fo 87. 5, which is, in fact, not altered. Accessory grains of aluminous chromite are present. Relief spheroids (1.5 to 4 cm in diameter; up to ~5 vol. % are distributed sparsely and heterogeneously. They are hosted by the olivine matrix and composed of talc, Tlc, and tremolite, Tr, (Mg# = 95-96 formed presumably at the expense of orthopyroxene, Opx, (i.e., pre-existing oikocrysts during a deuteric (autometasomatic alteration. In contrast, oikocrystic Opx (En 86.0 is quite fresh in related spheroids at Monchepluton, in which only minor deuteric alteration (Tlc + Tr are observed. We infer that (1 the ball-shaped morphology of the weathered surface is a reflection of the presence of oikocrysts of Opx, which crystallized after Ol at the magmatic stage; they were entirely replaced by the deuterically induced Tlc + Tr at Pados-Tundra. (2 Differential rates of weathering are implied for rock-forming minerals in these ultramafic rocks, with a higher resistance of Opx vs. Fo-rich Ol, and Tlc + Tr vs. Fo-rich Ol. (3 The ball-like shape of the large spheroids, produced by magmatic processes, may likely represent an additional factor of their higher stability to weathering in the superficial environment. Similar patterns can be expected in other mafic-ultramafic complexes, especially in layered intrusions.

  5. Geochemistry and zircon ages of mafic dikes in the South Qinling, central China: evidence for late Neoproterozoic continental rifting in the northern Yangtze block (United States)

    Zhu, Xiyan; Chen, Fukun; Liu, Bingxiang; Zhang, He; Zhai, Mingguo


    Neoproterozoic volcanic-sedimentary sequences of the southern Qinling belt, central China, were intruded by voluminous mafic dikes. secondary ion mass spectrometry zircon U-Pb dating indicates that these dikes were emplaced at 650.8 ± 5.2 Ma, coeval with mafic rocks occurring at the northern margin of the Yangtze block. The dikes are characterized by enrichment of large ion lithophile elements, high Ti contents (up to 3.73 wt%) and Nb/Ta ratios between 14.5 and 19.6, suggesting a mantle source of oceanic island basalt affinity. Initial 87Sr/86Sr ratios show positive correlation with SiO2 contents and negative correlation with Zr/Nb ratios, implying that these rocks were affected by crustal contamination during the magma ascend and emplacement process. The dikes have initial ɛ Nd values of +0.2 to +3.3, low 206Pb/204Pb ratios of 16.96-17.45, and moderate 87Sr/86Sr ratios of 0.7043-0.7076, likely pointing to the involvement of an enriched mantle source. The mafic dikes and coeval mafic volcanic equivalents in the South Qinling and the northern Yangtze are hypothesized to be related with the prolonged breakup of the supercontinent Rodinia, suggesting that continental rifting lasted until ca. 650 Ma.

  6. Petrogenesis and tectonic implications of Triassic mafic complexes with MORB/OIB affinities from the western Garzê-Litang ophiolitic mélange, central Tibetan Plateau (United States)

    Liu, Bin; Ma, Chang-Qian; Guo, Yu-Heng; Xiong, Fu-Hao; Guo, Pan; Zhang, Xin


    Although numerous Paleo-Tethyan ophiolites with mid-oceanic ridge basalts (MORB) and/or oceanic-island basalt (OIB) affinities have been reported in the central Tibetan Plateau (CTP), the origin and tectonic nature of these ophiolites are not well understood. The petrogenesis, mantle sources and geodynamic setting of the mafic rocks from these ophiolites are unclear, which is the main reason for this uncertainty. In this paper, we present new geochronological, mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone (GLS), a typical Paleo-Tethyan suture crossing the CTP. Zircon LA-ICP-MS U-Pb ages of 234 ± 3 Ma and 236 ± 2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E-MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB-like components. The ratios of Zn/Fet, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel-bearing peridotite or spinel ± minor garnet-bearing peridotite. We propose that back-arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB-type asthenosphere mantle and partial melting of sub-continental lithosphere were metasomatized by plume

  7. Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

    Indian Academy of Sciences (India)

    Ashima Saikia; Bibhuti Gogoi; Mansoor Ahmad; Talat Ahmad


    The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, crosscutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

  8. Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust

    Directory of Open Access Journals (Sweden)

    S. Bhattacharya


    Full Text Available Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB. They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / YbN ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km. The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ε-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.

  9. Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex (United States)

    Saikia, Ashima; Gogoi, Bibhuti; Ahmad, Mansoor; Ahmad, Talat


    The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, cross-cutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

  10. Petrology of continental tholeiitic magmas forming a 350-km-long Mesozoic dyke swarm in NE Brazil: Constraints of geochemical and isotopic data (United States)

    Ngonge, Emmanuel Donald; de Hollanda, Maria Helena Bezerra Maia; Archanjo, Carlos José; de Oliveira, Diógenes Custódio; Vasconcelos, Paulo Marcosde Paula; Muñoz, Patrício Rodrigo Montecinos


    The Ceará Mirim dyke swarm (northeastern Brazil) is composed of Cretaceous tholeiites with plagioclase, clinopyroxene (± olivine), Fe-Ti oxides and pigeonite in their groundmass. These tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2 ≥ 1.5 wt.%; Ti/Y > 360), and low-Ti tholeiites (TiO2 ≤ 1.5 wt%; Ti/Y ≤ 360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The low-Ti tholeiites exhibit some contamination with crustal (felsic) materials during ascent. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic 87Sr/86Sr (~ 0.7035-0.7039) combined with (in part) radiogenic 143Nd/144Nd and 206Pb/204Pb (> 19.1) ratios, which together reveal a likely contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show variable Sr-Nd ratios with relatively consistent 206Pb/204Pb ratios clustering towards an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Thus, we propose that FOZO and EMI components coexisted (including minor mixing with E-MORB magmas) and contributed in varying extents to the generation of the Ceará-Mirim dyke swarm primary melts, which segregated at 75 to 60 km in depth around the garnet-spinel facies transition zone. The mechanism that promoted melting was most likely non-plume related. We suggest that plate-boundary forces linked to the opening of the Atlantic Ocean promoted passive rifting and that the resulting asthenospheric

  11. 沙尘在防沙堤附近沉积特性的数值模拟%Numerical Simulation on Sedimentation Characteristics of Sand Dust Around Sand Preventing Dyke

    Institute of Scientific and Technical Information of China (English)

    王志强; 何艺峰; 黄晟敏; 李国申; 高鸿恩; 富宝锋


    为研究沙尘在防沙堤附近的沉积特点,基于FLUENT软件,采用标准K-ε湍流模型和DPM模型对沙尘在防沙堤附近的运动轨迹进行数值模拟.结果表明,沙尘的运动轨迹受自身粒径、风速及防沙堤迎风面坡度的综合影响,沙粒粒径越小,风速越大,防沙堤迎风面坡度越缓,沙粒越过防沙堤的能力越强;除风速较低时部分小粒径颗粒沉积在防沙堤后端外,其余情况下沙尘沉积主要发生在防沙堤前端.%To study the sedimentation characteristics of sand dust around sand preventing dyke, the particle tracks were simulated by the standard A' -e model and DPM model based on FLUENT software. The results show that particles moving tracks are affected by particles size, wind velocity and sand preventing dyke l/h ratio of windward side synthetically. The sand dust quantity passing sand preventing dyke increases with wind velocity and l/h, and decreases but the case is just converse with particle size. Customarily most sand dust accumulate in front of the sand preventing dyke, only a little fine particles accumulate in the leeward at lower wind speed.

  12. Investigation and Evaluation of Ecological Method in Sea Shore Dyke in Dapeng Peninsula of Shenzhen%深圳市大鹏半岛海堤生物安全防护现状调查与评价

    Institute of Scientific and Technical Information of China (English)



    The location of sea shore dyke, type of coastline and vegetation, species of plants and its distribution were investigated. The importance of storm tide prevention, the importance of biodiversity conservation and the importance of existing biological forest shelter belts were evaluated The results showed that there are 15 locations in protecting sea shore dyke within the study area rating biological protection very importance, 5 for importance,4 for general. The importance of ecological method in protecting sea shore dyke also has been evaluated.%对深圳市大鹏半岛海堤分布、海岸类型、植被类型、植物种类及分布等进行详细调查.对防风暴潮重要性、生物多样性保护重要性和现有生物防护林带保护重要性3个方面进行评价.结果表明,研究区内有15处海堤生物防护重要性等级为极重要,5处为重要,4处为一般.对海堤生物安全防护的重要性进行综合评价.

  13. Using Different Grazing Practices for Increasing Plant Biodiversity in the Dykes and Embankments Along the Rhône River (Southern France) (United States)

    Moinardeau, Cannelle; Mesléard, François; Dutoit, Thierry


    Extensive grazing by domestic herbivores is a widespread management practice used since the 80s in many European agro-ecosystems such as semi-natural grasslands to maintain open habitats and to enhance biodiversity. Such grazing systems have principally been tested in cultural ecosystems of high nature value threatened by grazing abandonment. However, there have been few case studies of grazing management in very anthropized ecosystems, such as the new ecosystems created by urban or industrial conversions. In Southern France, the Rhône channeling for navigation and electricity production generated in the 1950s the construction of thousands of hectares of dams and dykes which were colonized naturally by diverse plant communities. Yet shrub encroachment and the consequent recourse to mechanical cutting to facilitate control and maintenance, raise the question of how best to maintain and manage these new habitats. Consequently, since 1999, different low-intensity grazing management systems using rustic breeds of cattle, horses and goats have been tested on a protected reserve of 1454 ha located in the lower part of the Rhône river. Extensive grazing, more than cutting or no management, positively modified vegetation heterogeneity (beta-diversity), the target open grassland species, but not plant species richness (alpha-diversity). However, the current monitoring shows that these benefits of grazing will be confirmed only if low-intensity grazing systems are sustained and if new adaptations can be also made, such as the use of mixed stocking and the establishment of multiyear contracts with breeders.

  14. Note: Modified π-type Butterworth-Van Dyke model for dual-mode Lamb-wave resonator with precise two-port Y-parameter characterizations. (United States)

    Wang, Yong; Hong, Yan; Goh, Wang Ling; Mu, Xiaojing


    Dual-mode Lamb-wave resonator has become a powerful component for clock reference and sensing applications, enabling efficient compensations of temperature effects, concurrent measurements of multiple environmental parameters, etc. An equivalent circuit model for the dual-mode Lamb-wave resonator is indispensable as it provides a means as well as being an effective tool for evaluating device characteristics and to aid the designing of circuitry for the resonators. This could be the first time ever that an efficient equivalent-circuit model, i.e., modified π-type Butterworth-Van Dyke model for dual-mode Lamb-wave resonators is reported. Evaluated by experiments, this model attains noteworthy agreements on both the magnitudes and phases of Y11 and Y21 of the measurement results. Compared to literature, the proposed model is capable of modeling the dual resonances efficiently. Moreover, this work also proves more accurate when viewing the Y-parameters across a wide frequency range. The gained features of this model are most beneficial for the analysis of the dual-mode Lamb-wave resonator and also for the designing of circuits.

  15. Note: Modified π-type Butterworth-Van Dyke model for dual-mode Lamb-wave resonator with precise two-port Y-parameter characterizations (United States)

    Wang, Yong; Hong, Yan; Goh, Wang Ling; Mu, Xiaojing


    Dual-mode Lamb-wave resonator has become a powerful component for clock reference and sensing applications, enabling efficient compensations of temperature effects, concurrent measurements of multiple environmental parameters, etc. An equivalent circuit model for the dual-mode Lamb-wave resonator is indispensable as it provides a means as well as being an effective tool for evaluating device characteristics and to aid the designing of circuitry for the resonators. This could be the first time ever that an efficient equivalent-circuit model, i.e., modified π-type Butterworth-Van Dyke model for dual-mode Lamb-wave resonators is reported. Evaluated by experiments, this model attains noteworthy agreements on both the magnitudes and phases of Y11 and Y21 of the measurement results. Compared to literature, the proposed model is capable of modeling the dual resonances efficiently. Moreover, this work also proves more accurate when viewing the Y-parameters across a wide frequency range. The gained features of this model are most beneficial for the analysis of the dual-mode Lamb-wave resonator and also for the designing of circuits.

  16. Nevus vascularis mixtus (cutaneous vascular twin nevi) associated with intracranial vascular malformation of the Dyke-Davidoff-Masson type in two patients. (United States)

    Ruggieri, Martino; Milone, Pietro; Pavone, Piero; Falsaperla, Raffaele; Polizzi, Agata; Caltabiano, Rosario; Fichera, Marco; Gabriele, Anna Lia; Distefano, Angela; De Pasquale, Rocco; Salpietro, Vincenzo; Micali, Giuseppe; Pavone, Lorenzo


    The term twin spotting refers to phenotypes characterized by the spatial and temporal co-occurrence of two (or more) different nevi arranged in variable cutaneous patterns, and can be associated with extra-cutaneous anomalies. Several examples of twin spotting have been described in humans including nevus vascularis mixtus, cutis tricolor, lesions of overgrowth, and deficient growth in Proteus and Elattoproteus syndromes, epidermolytic hyperkeratosis of Brocq, and the so-called phacomatoses pigmentovascularis and pigmentokeratotica. We report on a 28-year-old man and a 15-year-old girl, who presented with a previously unrecognized association of paired cutaneous vascular nevi of the telangiectaticus and anemicus types (naevus vascularis mixtus) distributed in a mosaic pattern on the face (in both patients) and over the entire body (in the man) and a complex brain malformation (in both patients) consisting of cerebral hemiatrophy, hypoplasia of the cerebral vessels and homolateral hypertrophy of the skull and sinuses (known as Dyke-Davidoff-Masson malformation). Both patients had facial asymmetry and the young man had facial dysmorphism, seizures with EEG anomalies, hemiplegia, insulin-dependent diabetes mellitus (IDDM), autoimmune thyroiditis, a large hepatic cavernous vascular malformation, and left Legg-Calvé-Perthes disease (LCPD) [LCPD-like presentation]. Array-CGH analysis and mutation analysis of the RASA1 gene were normal in both patients.

  17. Petrogenesis and tectonic setting of the Bondla mafic-ultramafic complex, western India: Inferences from chromian spinel chemistry (United States)

    Ishwar-Kumar, C.; Rajesh, V. J.; Windley, B. F.; Razakamanana, T.; Itaya, T.; Babu, E. V. S. S. K.; Sajeev, K.


    Crustal-scale shear/suture zones hold prime importance because they are one of the critical parameters used for paleogeographic configurations of supercontinental assemblies. The Kumta suture, located on the western margin of peninsular India, has been interpreted as the eastern extension of the Betsimisaraka suture zone of Madagascar. This suture separates the Karwar block (ca. 3200 Ma tonalite-trondhjemite-granodiorite (TTG) and amphibolite) in the west from a quartzite-dominated shelf that overlies ca. 2571 Ma quartzo-feldspathic gneisses of the Dharwar block in the east. The NW/SE-trending Bondla ultramafic-mafic complex, situated in the arc just west of the Kumta suture, comprises gabbro, troctolite, wehrlite, dunite, peridotite, pyroxenite, chromitite and chromian spinel-bearing serpentinite. In this paper, we study the chemistry of Cr-spinels in chromitites and serpentinites to help understand their paleo-tectonic environments. The Cr-spinel in Bondla chromitites and serpentinites shows variations in Cr# [Cr/(Cr + Al)] ranging from 0.54 to 0.58 and 0.56 to 0.64 respectively; also, the Mg# [Mg/(Mg + Fe)] varies from 0.56 to 0.67 and 0.41 to 0.63 respectively. The Cr-spinels in serpentinites have strong chemical zoning with distinctive ferrian chromite rims (Mg# 0.41-0.63), whereas the Cr-spinels in chromitites are generally homogeneous with only occasional weak zoning. The spinel-core crystallization temperature in the serpentinite is estimated to be above 600 °C (the spinel stability field was calculated for equilibrium with Fo90 olivine), which suggests the core composition is chemically unaltered. The Cr-spinels in all studied samples have low-Al2O3 (15-23 wt%) and moderate to high-Cr# (0.54-0.69), suggesting derivation from a supra-subduction zone arc setting. The chemistry of clinopyroxene in serpentinite indicates a wide range of crystallization temperatures from 969 °C to 1241 °C at 1.0 GPa. The calculated parental magma composition was similar to

  18. Crystal Zoning Constrains on the Processes and Time Scales Involved in Monogenetic Mafic Volcanism (Tenerife, Canary Islands) (United States)

    Albert, H.; Costa Rodriguez, F.; Marti, J.


    Most of the historical eruptive activity in Tenerife has been relatively mafic and mildly-explosive monogenetic eruptions, and thus it seems that this activity is the most likely in the near future. Here we investigate the processes and time scales that lead to such eruptions with the aim to better interpret and plan for any possible unrest in the island. We focus on three historical eruptions: Siete Fuentes (December 31 1704-January 1705), Fasnia (January 5-January 13 1705) and Arafo (February 2-February 26 1705) issued from a 10 km long basaltic fissure eruption oriented N45E and covering an area of 10.4 km2. The erupted volume increases by 5-fold from the first to the last eruption. All magmas are tephritic, although the bulk-rock becomes more mafic with time due to accumulation of olivine with Cr-spinel inclusions, and clinopyroxene rather than to the appearance of a truly more primitive melt. Olivine core compositions of the three eruptions range between Fo79 and Fo87. Frequency histograms show three main populations: at Fo79-80, Fo80-82 and Fo84-87 displaying normal and reverse zoning. Thermodynamic calculations show that only cores with Fo80-82 are in equilibrium with the whole rock. Clinopyroxene phenocrysts can have large pools of matrix glass and show rims of different composition. Only the rims, with Mg#84-86, are in equilibrium with the whole-rock. Considering olivine cores and clinopyroxene rims in equilibrium we obtained a temperature range of 1150-1165°C, and MELTS calculations suggest pressures of 1 to 5 kbar. The variety of olivine core populations reflects mixing and mingling between three different magmas, and their proportions have changed with time from Siete Fuentes to Arafo. Most crystals have complex zoning profiles that record two events: (1) one of magma mixing/mingling at depth, (2) another of magma transport and ascent to the surface. Magma mixing at depth ranges from about 3 months to two years and is similar for the three eruptions

  19. The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences (United States)

    Giovanardi, Tommaso; Girardi, Vicente A. V.; Correia, Ciro T.; Sinigoi, Silvano; Tassinari, Colombo C. G.; Mazzucchelli, Maurizio


    The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite-facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in ɛNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.

  20. Characteristics and origin of mafic and ultramafic xenoliths in trachyandesite lavas from Heikongshan volcano,Tengchong,Yunnan Province,China

    Institute of Scientific and Technical Information of China (English)


    The trachyandesite lavas from Heikongshan volcano of the Tengchong volcanic cluster, Yunnan Province contain relatively abundant mafic and ultramafic small xenoliths comprising mainly gabbro, pyroxenite and rare lherzolites, all less than 2.5 cm in size. Gabbro xenoliths are characterized by "open" texture represented by relatively abundant vesicles and ground-mass fillings, while pyroxenite xenoliths are characterized by equigranular and poikilitic texture. Their mineral compositions (clinopyroxene ± orthopyroxene ± plagioclase) are similar to those of phenocrysts in lavas, and their equilibration temperature (1000-1125°C) is consistent with the crystallization temperature of the phenocrysts (998-1108°C). Thus, the textures and compositions of these xenoliths are different from those of lower crustal and mantle xenoliths, and were derived from the magma chamber. Among them, the gabbroic xenoliths come from the crystal-bubble-liquid zone at the top of the magma reservoir, while the pyroxenite xenoliths come from the cumulates in the bottom of magma reservoir. The studied single lherzolite xenolith with porphyroclastic texture has similar mineral compositions to the mantle xenoliths from eastern China, and is considered to be of upper mantle origin. It was brought into magma reservoir by replenishment magma, and might have experienced a cooling event before eruption. The formation of xenoliths in trachyandesite flows largely reflects the ascent, storage, evolution and eruption processes of magma.

  1. PGE distribution in the Chromite bearing mafic-ultramafic Kondapalli Layered Complex, Krishna district, Andhra Pradesh, India

    Directory of Open Access Journals (Sweden)

    Meshram Tushar M.


    Full Text Available The Kondapalli Layered Complex (KLC is a dismembered mafic-ultramafic layered intrusion, mainly composed of gabbroic and anorthositic rocks with subordinate ultramafics and chromitite. Chromitite occurs as lenses, pods, bands and disseminations. Platinum group of minerals (PGMs occur as inclusions within chromite and silicates. The study indicates an inhomogeneous distribution of PGMs and distinct dominance of IPGEs over the PPGEs. The average ΣPGE content of chromite of KLC varies from 64 ppb to 576 ppb with Pt ranging from 5 to 495 ppb, Pd 5 to 191 ppb, Ir 3 to 106 ppb, Ru 3 to 376 ppb and Rh 3 to 135 ppb. The PGMs identified in the KLC indicate primary deposition of the IPGE, preceding chromite, indicating its orthomagmatic nature. Most of the PGM grains are usually below 10 μm. The identified PGMs are Laurite (RuS2, irarsite (Ir, As, S, iridosmine (Os, Ir, undetermined Os-Ir sulphide and Ru-Os-Ir-Zn alloys. Chromite also contains inclusions of pentlandite, millerite, chalcopyrite and pyrite. Study indicating that the KLC have orthomagmatic origin for PGE which are dominated by IPGE group and formed under surpa-subduction zone peridotite setting.

  2. Hydration vs. oxidation: Modelling implications for Fe–Ti oxide crystallisation in mafic intrusions, with specific reference to the Panzhihua intrusion, SW China

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    Geoffrey H. Howarth


    The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable. H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f(O2 Mt–Uv crystallises earlier than plagioclase and clinopyroxene. Furthermore, the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe–Ti oxide ore layers.

  3. Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

    Institute of Scientific and Technical Information of China (English)

    HOU; Zhenhui; LI; Shuguang; CHEN; Nengsong; LI; Qiuli; LIU


    The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet + clinopyroxene + hypersthene) yields an age of 136(±)18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3―10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716―780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136±18 Ma) defined by Sm-Nd isochron and K-Ar age of 123―127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

  4. Magmatic structure and geochemistry of the Luanga Mafic-Ultramafic Complex: Further constraints for the PGE-mineralized magmatism in Carajás, Brazil (United States)

    Mansur, Eduardo Teixeira; Ferreira Filho, Cesar Fonseca


    The Luanga Complex is part of the Serra Leste Magmatic Suite, a cluster of PGE-mineralized mafic-ultramafic intrusions located in the northeastern portion of the Carajás Mineral Province. The Luanga Complex is a medium-sized layered intrusion consisting of three main zones: i. the lower Ultramafic Zone comprising ultramafic adcumulates (peridotite), ii. the Transition Zone comprising interlayered ultramafic and mafic cumulates (harzburgite, orthopyroxenite and norite) and iii. the upper Mafic Zone comprising a monotonous sequence of mafic cumulates (norite) with minor orthopyroxenite layers. Several PGE-mineralized zones occur in the Transition Zone but the bulk of the PGE resources are hosted within a 10-50 meter thick interval of disseminated sulfides at the contact of the Ultramafic and Transition Zones. The compositional range of cumulus olivine (Fo78.9-86.4) is comparable to those reported for layered intrusions originated from moderate primitive parental magmas. Mantle normalized alteration-resistant trace element patterns of noritic rocks are fractionated, as indicated by relative enrichment in LREE and Th, with negative Nb and Ta anomalies, suggesting assimilation of older continental crust. Ni contents in olivine in the Luanga Complex (up to 7500 ppm) stand among the highest values reported in layered intrusions globally. The highest Ni contents in olivine in the Luanga Complex occur in distinctively PGE enriched (Pt + Pd > 1 ppm) intervals of the Transition Zone, in both sulfide-poor and sulfide bearing (1-3 vol.%) rocks. The origin of the PGE- and Ni-rich parental magma of the Luanga Complex is discussed considering the upgrading of magmas through dissolution of previously formed Ni-rich sulfide melts. Our results suggest that high Ni contents in olivine and/or orthopyroxene provide an additional exploration tool for Ni-PGE deposits, particularly useful for target selection in large magmatic provinces.

  5. Caribbean affinities of mafic crust from northern Colombia: preliminary geochemical results from basaltic rocks of the Sinu-San Jacinto belt (United States)

    Bustamante, C.; Cardona, A.; Valencia, V.; Weber, M.; Guzman, G.; Montes, C.; Ibañez, M.; Lara, M.; Toro, M.


    The petrotectonic characterization of accreted mafic remnants within the northern Andes and the Caribbean yield major insights on the growth and evolution of oceanic plates, as well as in the identification of the role of terrane accretion within the northern Andes orogeny. Within the northern termination of the Andes, in northern Colombia, several exposures of mafic and ultramafic rocks have been identified. However, extensive sedimentary cover and difficulties in field access have left the petogenetic analysis and tectonic implications of this rocks scarcely studied. Preliminary geochemical constrains from volcanic rocks obtained in outcrops and as clasts from a Paleocene-Eocene conglomerate indicate that the mafic rocks are mainly andesitic in composition, with well defined enrichment in Th and Ce and depletion in Nb and flat to weakly enriched LREE. These features suggest a relatively immature intra-oceanic volcanic arc setting for the formation of these rocks. Hornblende-dioritic dikes in peridotites also attest to the role of water in the magmatic evolution, and the affinity to a subduction related setting. The tectonic implications of this arc remnants and the relation between these rocks and other oceanic domains in the northern Andes suggest that the compositional and tectonic setting on the different accreted margins of the Caribbean plate are heterogeneous.

  6. Late Mesozoic crust-mantle interaction and lower crust components in South China: A geochemical study of mafic granulite xenoliths from Cenozoic basalts

    Institute of Scientific and Technical Information of China (English)

    YU; Jinhai; (于津海); XU; Xisheng; (徐夕生); ZHOU; Xinmin; (周新民)


    Mafic granulite xenoliths collected from Cenozoic basalts in SE China can be classified as magmatic granulite and cumulate granulite. Magmatic granulites are characterized by highly concentrated Al2O3, K2O, P2O5, Ba, Sr, Pb and REE, and low contents of Nb, Zr, Hf and Th, and have an incompatible element abundance pattern similar to that of continental arc basalts. Cumulate granulites aredepleted in K2O, P2O5, Rb, Cs and Ba. These granulite xenoliths were the products of crystallization and recrystallization of the basaltic magma underplating into crust-mantle boundary in Late Mesozoic. Sr and Nd isotopic compositions and variation trend of these mafic rocks are the result of crust-mantle mixing and controlled by assimilation and fractional crystallization process (AFC). However,trace element and major element variations were mainly controlled by fractionalcrystallization. The granulites are similar in geochemistry to surface Late Mesozoic gabbro and basalt in the study area, suggesting a close petrogenetic link between them. Late Mesozoic basaltic magma activities are the most important cause for the formation of extensive contemporaneous granite and rhyolite in the study area. This study and previous data indicate that the lower crust beneath South China is composed of a variety of Paleo- to Meso-proterozoic metamorphic rocks and Late Mesozoic mafic granulites.

  7. Precursors to dyke-fed eruptions at basaltic volcanoes: insights from patterns of volcano-tectonic seismicity at Kilauea volcano, Hawaii (United States)

    Bell, Andrew F.; Kilburn, Christopher R. J.


    To investigate the physical controls on volcano-tectonic (VT) precursors to eruptions and intrusions at basaltic volcanoes, we have analyzed the spatial and temporal patterns of VT earthquakes associated with 34 eruptions and 23 dyke intrusions that occurred between 1960 and 1983 at Kilauea, in Hawaii. Eighteen of the 57 magmatic events were preceded by an acceleration of the mean rate of VT earthquakes located close to the main shallow magma reservoir. Using a maximum-likelihood technique and the Bayesian Information Criterion for model preference, we demonstrate that an exponential acceleration is preferred over a power-law acceleration for all sequences. These sequences evolve over time-scales of weeks to months and are consistent with theoretical models for the approach to volcanic eruptions based on the growth of a population of fractures in response to an excess magma pressure. Among the remaining 40 magmatic events, we found a significant correlation between swarms of VT earthquakes located in the mobile south-flank of Kilauea and eruptions and intrusions. The behaviour of these swarms suggests that at least some of the magmatic events are triggered by transient episodes of elevated rates of aseismic flank movement, which could explain why many eruptions and intrusions are not preceded by longer-term precursory signals. In none of the 57 cases could a precursory sequence be used to distinguish between the approach to an eruption or an intrusion, so that, even when a precursory sequence is recognized, there remains an empirical chance of about 40% (24 intrusions from 57 magmatic events) of issuing a false alarm for an imminent eruption.

  8. Cambrian intermediate-mafic magmatism along the Laurentian margin: Evidence for flood basalt volcanism from well cuttings in the Southern Oklahoma Aulacogen (U.S.A.) (United States)

    Brueseke, Matthew E.; Hobbs, Jasper M.; Bulen, Casey L.; Mertzman, Stanley A.; Puckett, Robert E.; Walker, J. Douglas; Feldman, Josh


    The Southern Oklahoma Aulocogen (SOA) stretches from southern Oklahoma through the Texas panhandle and into Colorado and New Mexico, and contains mafic through silicic magmatism related to the opening of the Iapetus Ocean during the early Cambrian. Cambrian magmatic products are best exposed in the Wichita Mountains (Oklahoma), where they have been extensively studied. However, their ultimate derivation is still somewhat contentious and centers on two very different models: SOA magmatism has been suggested to occur via [1] continental rifting (with or without mantle plume emplacement) or [2] transform-fault related magmatism (e.g., leaky strike-slip faults). Within the SOA, the subsurface in and adjacent to the Arbuckle Mountains in southern Oklahoma contains thick sequences of mafic to intermediate lavas, intrusive bodies, and phreatomagmatic deposits interlayered with thick, extensive rhyolite lavas, thin localized tuffs, and lesser silicic intrusive bodies. These materials were first described in the Arbuckle Mountains region by a 1982 drill test (Hamilton Brothers Turner Falls well) and the best available age constraints from SOA Arbuckle Mountains eruptive products are ~ 535 to 540 Ma. Well cuttings of the mafic through intermediate units were collected from that well and six others and samples from all but the Turner Falls and Morton wells are the focus of this study. Samples analyzed from the wells are dominantly subalkaline, tholeiitic, and range from basalt to andesite. Their overall bulk major and trace element chemistry, normative mineralogy, and Srsbnd Nd isotope ratios are similar to magmas erupted/emplaced in flood basalt provinces. When compared with intrusive mafic rocks that crop out in the Wichita Mountains, the SOA well cuttings are geochemically most similar to the Roosevelt Gabbros. New geochemical and isotope data presented in this study, when coupled with recent geophysical work in the SOA and the coeval relationship with rhyolites, indicates

  9. Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes (United States)

    Moore, Nicole E.; Debari, Susan M.


    Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3-50.3 wt% SiO2, Mg# 64-65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7-52.6 wt% SiO2, Mg# 58-62) and Sulphur Creek (51.2-54.6 wt% SiO2, Mg# 56-57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8-54.0 wt% SiO2, Mg# 68-70) and the HMA of Glacier Creek (58.3-58.7 wt% SiO2, Mg# 63-64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2-52.6 wt% SiO2, Mg# 55-58), is an Mg-poor differentiate of the Type 3 endmember. The calc

  10. Petrological evidence for non-linear increase of magmatic intrusion rates before eruption at open vent mafic volcanoe (United States)

    Ruth, D. C. S.; Costa Rodriguez, F.


    The most active volcanoes on earth erupt in a yearly to decadal time scales, typically erupt mafic magmas and are open-vent systems with prominent degassing plumes (e.g. Mayon, Arenal, Llaima, Etna). Here we investigate the plumbing systems, dynamics, and processes that drive eruptions at these systems. These are key questions for improving hazard evaluation, and better understanding the unrest associated with these types of volcanoes. The petrology and geochemistry from six historical eruptions (1947-2006) of Mayon volcano (Philippines) shows that all lavas are basaltic andesite with phenocrysts of plagioclase + orthopyroxene (Opx) + clinopyroxene. Opx crystals show a variety of compositions and zoning patterns (reverse, normal or complex) with Mg# (= 100 *Mg/[Mg+Fe]) varying from 67 to 81. The simplest interpretation is that the low Mg# parts of the crystals resided on an upper crustal and crystal rich reservoir that was intruded by more primitive magmas from which the high Mg# parts of the crystals grew. Modelling Mg-Fe diffusion in Opx shows that times since magma injection and eruption range from a few days up to 3.5 years in all of the investigated eruptions. The longest diffusion times are shorter than the repose times between the eruptions, which implies that crystal recycling between eruptive events is negligible. This is a surprising result that shows that for each eruption a different part of the evolved crystal-rich plumbing system is activated. This can be due to random intrusion location or an irreversibility of the plumbing system that prevents multiple eruptions from the same crystal-rich part. Moreover, we find that the number of intrusions markedly increases before each eruption in a non-linear manner. Such an increased rate of intrusions with time might reflect non-linear rheological properties of the crystal-rich system, of the enclosing rocks, or the non-linear evolution of crystal-melt reaction-dissolution fronts during magma intrusions.

  11. Design of the dam and south dyke at the Toulnustouc hydroelectric development project; Conception du barrage et de la digue sud de l'amenagement hydroelectrique de la Toulnustouc

    Energy Technology Data Exchange (ETDEWEB)

    Beausejour, N.; Bouzaiene, H. [RSW Inc., Montreal, PQ (Canada); Hammamji, Y.; Bigras, A.; Bergeron, A. [Hydro-Quebec Equipment, Montreal, PQ (Canada)


    The Toulnustouc hydroelectric dam is located approximately 120 km north of Baie-Comeau, Quebec and has been in operation since 2005. New development involved expanding the existing Lac-Sainte-Anne reservoir by 22 km{sup 2} through the construction of a new 77 m high and 535 m long dam and a 46 m high and 400 m long south dyke. This paper described the geology of the area and presented details of the main dam, instrumentation and stability analysis. The main concrete face rockfill dam (CFRD) lies on a foundation of bedrock. This paper presented the predictions of the upstream face displacements and the internal movements that develop in the rockfill embankment during the construction phase and the reservoir filling. The materials for the embankment dam were assumed to be non-linear, stress dependent and stress history dependent. The prediction displacements of the upstream face were compared to the displacement measured by monitors installed on the concrete face during the filling of the reservoir. The south dyke was constructed in an effort to control and reduce water infiltration under the foundation and to raise the crest of the reservoir. The principal criteria used for the design of the south dyke and the CFRD was presented. These included design wave and freeboard; ice loading on the upstream slab; geometry of the fill; material types with layer thickness and compaction criteria; stress and deformation of the fill; stability of the slope in both static and seismic conditions; extruded curb as stabilization method of the upstream slope, concrete slab, plinth, parapet, perimeter joint and waterstops; foundation treatment; and, instrumentation. During reservoir filling, the measured deformations and seepage were found to be lower than expected. 7 refs., 1 tab., 3 figs.

  12. Controle da antracnose e qualidade de mangas (Mangifera indica L. cv. van dyke, após tratamento hidrotérmico e químico Anthracnose control and mango quality (Mangifera indica L. cv. van dyke after hydrothermal and chemical treatment

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    Mário Sérgio Carvalho Dias


    Full Text Available A antracnose causa severos danos à casca da manga. Por isso, há a necessidade de se fazer o tratamento fitossanitário pós-colheita dos frutos, utilizando a imersão em calda de fungicidas, a qual, associada ao tratamento hidrotérmico, tem resultado em melhores benefícios. Com o presente trabalho objetivou-se estudar o controle da antracnose e qualidade pós-colheita de mangas (Mangifera indica L., cv. Van Dyke, após tratamento hidrotérmico associado ao químico mediante as análises física, físico-químicas, químicas e fitopatológica. As mangas, provenientes de pomar comercial do município de Janaúba - MG, da safra 2000, foram colhidas em estádio de maturação comercial. Após seleção de acordo com a uniformidade de cor, tamanho e ausência de injúrias mecânicas e fisiológicas, as mesmas foram transportadas em caixas plásticas para a EPAMIG - CTNM - Nova Porteirinha - MG, Laboratório de Pós-colheita de frutos, onde as amostras foram submetidas ao tratamento hidrotérmico (55°C por 5minutos e banho frio em soluções contendo os fungicidas Thiabendazole (Tecto 400 mL/100 L, Prochloraz (Sportak 110 mL/100 L e Imazalil (Magnate 200 mL/100 L. Após secagem ao ar, os frutos foram acondicionados em bandejas plásticas e armazenados à temperatura ambiente (25±2°C e UR 70%, por um período de 12 dias e avaliados quanto às características intrínsecas de qualidade a cada 4 dias. O experimento foi conduzido em delineamento inteiramente casualizado, com 8 tratamentos, 4 repetições e unidade experimental composta de 4 frutos. As variações de pH, sólidos solúveis totais, acidez total titulável e açúcares solúveis totais não comprometeram as características organolépticas de mangas cv. Van Dyke armazenadas sob temperatura ambiente (25±2°C e UR 70% até os 8 dias de armazenamento. A associação do tratamento hidrotérmico com o químico foi eficiente no controle da antracnose dos frutos por até 12 dias de

  13. Petrographic, geochemical and isotopic evidence of crustal assimilation processes in the Ponte Nova alkaline mafic-ultramafic massif, SE Brazil (United States)

    Azzone, Rogério Guitarrari; Montecinos Munoz, Patricio; Enrich, Gaston Eduardo Rojas; Alves, Adriana; Ruberti, Excelso; Gomes, Celsode Barros


    Crustal assimilation plus crystal fractionation processes of different basanite magma batches control the evolution of the Ponte Nova cretaceous alkaline mafic-ultramafic massif in SE Brazil. This massif is composed of several intrusions, the main ones with a cumulate character. Disequilibrium features in the early-crystallized phases (e.g., corrosion and sieve textures in cores of clinopyroxene crystals, spongy-cellular-textured plagioclase crystals, gulf corrosion texture in olivine crystals) and classical hybridization textures (e.g., blade biotite and acicular apatite crystals) provide strong evidence of open-system behavior. All samples are olivine- and nepheline-normative rocks with basic-ultrabasic and potassic characters and variable incompatible element enrichments. The wide ranges of whole-rock 87Sr/86Sri and 143Nd/144Ndi ratios (0.70432-0.70641 and 0.512216-0.512555, respectively) are indicative of crustal contribution from the Precambrian basement host rocks. Plagioclase and apatite 87Sr/86Sr ratios (0.70422-0.70927) obtained for the most primitive samples of each intrusion indicate disequilibrium conditions from early- to principal-crystallization stages. Isotope mixing-model curves between the least contaminated alkaline basic magma and heterogeneous local crustal components indicate that each intrusion of the massif is differentiated from the others by varied degrees of crustal contribution. The primary mechanisms of crustal contribution to the Ponte Nova massif involve the assimilation of host rock xenoliths during the development of the chamber environment and the assimilation of partial melts from the surrounding host rocks. Thermodynamic models using the melts algorithm indicate that parental alkaline basic magmas can be strongly affected by contamination processes subsequently to their initial stages of crystallization when there is sufficient energy to assimilate partial melts of crustal host rocks. The assimilation processes are considered to

  14. Volatile contents of mafic-to-intermediate magmas at San Cristóbal volcano in Nicaragua (United States)

    Robidoux, P.; Aiuppa, A.; Rotolo, S. G.; Rizzo, A. L.; Hauri, E. H.; Frezzotti, M. L.


    San Cristóbal volcano in northwest Nicaragua is one of the most active basaltic-andesitic stratovolcanoes of the Central American Volcanic Arc (CAVA). Here we provide novel constraints on the volcano's magmatic plumbing system, by presenting the first direct measurements of major volatile contents in mafic-to-intermediate glass inclusions from Holocene and historic-present volcanic activity. Olivine-hosted (forsterite [Fo] 1500 μg/g) found in Nicaragua at Cerro Negro, Nejapa, and Granada. Models of H2O and CO2 solubilities constrain the degassing pathway of magmas up to 425 MPa ( 16 km depth), which includes a deep CO2 degassing step (only partially preserved in the MI record), followed by coupled degassing of H2O and S plus crystal fractionation at magma volatile saturation pressures from ∼ 195 to < 10 MPa. The variation in volatile contents from San Cristóbal MI is interpreted to reflect (1) Holocene eruptive cycles characterized by the rapid emplacement of basaltic magma batches, saturated in volatiles, at depths of 3.8-7.4 km, and (2) the ascent of more-differentiated and cogenetic volatile-poor basaltic andesites during historic-present eruptions, having longer residence times in the shallowest (< 3.4 km) and hence coolest regions of the magmatic plumbing system. We also report the first measurements of the compositions of noble-gas isotopes (He, Ne, and Ar) in fluid inclusions in olivine and pyroxene crystals. While the measured 40Ar/36Ar ratios (300-304) and 4He/20Ne ratios (9-373) indicate some degree of air contamination, the 3He/4He ratios (7.01-7.20 Ra) support a common mantle source for Holocene basalts and historic-present basaltic andesites. The magmatic source is interpreted as generated by a primitive MORB-like mantle, that is influenced to variable extents by distinct slab fluid components for basalts (Ba/La 76 and U/Th 0.8) and basaltic andesites (Ba/La 86 and U/Th 1.0) in addition to effects of magma differentiation. These values for the

  15. Contamination in mafic mineral-rich calc-alkaline granites: a geochemical and Sr-Nd isotope study of the Neoproterozoic Piedade Granite, SE Brazil

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    Leite Renato J.


    Full Text Available The Piedade Granite (~600 Ma was emplaced shortly after the main phase of granite magmatism in the Agudos Grandes batholith, Apiaí-Guaxupé Terrane, SE Brazil. Its main units are: mafic mineral-rich porphyritic granites forming the border (peraluminous muscovite-biotite granodiorite-monzogranite MBmg unit and core (metaluminous titanite-bearing biotite monzogranite BmgT unit and felsic pink inequigranular granite (Bmg unit between them. Bmg has high LaN/YbN (up to 100, Th/U (>10 and low Rb, Nb and Ta, and can be a crustal melt derived from deep-seated sources with residual garnet and biotite. The core BmgT unit derived from oxidized magmas with high Mg# (~45, Ba and Sr, fractionated REE patterns (LaN/YbN= 45, 87Sr/86Sr(t~ 0.710, epsilonNd(t ~ -12 to -14, interpreted as being high-K calc-alkaline magmas contaminated with metasedimentary rocks that had upper-crust signature (high U, Cs, Ta. The mafic-rich peraluminous granites show a more evolved isotope signature (87Sr/86Sr(t = 0.713-0.714; epsilonNd(t= -14 to -16, similar to Bmg, and Mg# and incompatible trace-element concentrations intermediate between Bmg and BmgT. A model is presented in whichMBmgis envisaged as the product of contamination between a mafic mineral-rich magma consanguineous with BmgT and pure crustal melts akin to Bmg.

  16. Early Jurassic subduction of the Paleo-Pacific Ocean in NE China: Petrologic and geochemical evidence from the Tumen mafic intrusive complex (United States)

    Guo, Feng; Li, Hongxia; Fan, Weiming; Li, Jingyan; Zhao, Liang; Huang, Miwei; Xu, Wenliang


    Subduction of the Paleo-Pacific Oceanic Plate is widely considered to have caused extensive Mesozoic magmatism, lithospheric deformation and mineralization in East Asia. However, it is still unclear when this subduction began. Here we report an Early Jurassic (~ 187 Ma) mafic intrusive complex (including olivine norite, gabbro, and diorite) from the Tumen area in NE China. The olivine norite contains a mineral assemblage of olivine, pyroxene, Ca-plagioclase, and hornblende that crystallized in a water-saturated parental magma. The rocks in the complex show variable degrees of plagioclase and ferromagnesian mineral accumulation as reflected by positive Sr and Eu anomalies in primitive mantle-normalized incompatible element patterns. Mass-balance calculations indicate that the parental magma was calc-alkaline with arc-type trace element features (i.e., large ion incompatible and light rare earth element enrichment and Nb-Ta depletion). It also had Sr-Nd-Hf isotopic compositions (87Sr/86Sr(i) = 0.7042 to 0.7044, εNd(t) = + 2.5 to + 3.5 and εHf(t) = + 8.4 to + 10.5) similar to those of modern arc basalts. The parental magma was likely derived from 5 to 20% melting of a mantle wedge metasomatized by an addition of 3-4% hydrous sediment melt from the subducting Paleo-Pacific Oceanic slab. The Tumen mafic intrusive complex, together with other contemporaneous mafic intrusions, I-type granitoids, and felsic lavas, constitutes an Early Jurassic N-S-trending arc magmatic belt that was formed by westward subduction of the Paleo-Pacific Ocean.

  17. Compositional variability in mafic arc magmas over short spatial and temporal scales: Evidence for the signature of mantle reactive melt channels (United States)

    Rawson, Harriet; Keller, Tobias; Fontijn, Karen; Pyle, David M.; Mather, Tamsin A.; Smith, Victoria C.; Naranjo, José A.


    Understanding arc magma genesis is critical to deciphering the construction of continental crust, understanding the relationship between plutonic and volcanic rocks, and for assessing volcanic hazards. Arc magma genesis is complex. Interpreting the underlying causes of major and trace element diversity in erupted magmas is challenging and often non-unique. To navigate this complexity mafic magma diversity is investigated using sample suites that span short temporal and spatial scales. These constraints allow us to evaluate models of arc magma genesis and their geochemical implications based on physical arguments and recent model results. Young volcanic deposits (≲18 kyr) are analysed from the Southern Volcanic Zone (SVZ), Chile, in particular suites of scoria cones on the flanks of arc stratovolcanoes that have erupted relatively primitive magmas of diverse compositions. Our study is centred on the high-resolution post-glacial tephrochronological record for Mocho-Choshuenco volcano where tight age constraints and a high density of scoria cones provide a spatially well-resolved mafic magma dataset. Two compositional trends emerge from the data. Firstly, magmas from cones on the flanks of the main edifice become more mafic with distance from the central vent. This is attributed to fractional crystallisation processes within the crust, with distal cones sampling less differentiated magmas. Secondly, there is a set of cones with distinct major and trace element compositions that are more primitive but enriched in incompatible elements relative to the central system and other 'normal SVZ' magmas. This distinct signature - termed the 'Kangechi' signature - is observed at three further clusters of cones within the SVZ. This is attributed to greater preservation of the enriched melt signature arising from reactive melt transport within the mantle wedge. Our model has important implications for arc magma genesis in general, and in particular for the spatial and temporal

  18. A high-pyrite semianthracite of Late Permian age in the Songzao Coalfield, southwestern China: Mineralogical and geochemical relations with underlying mafic tuffs

    Energy Technology Data Exchange (ETDEWEB)

    Dai, Shifeng; Wang, Xibo; Chen, Wenmei [State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, (China); Li, Dahua [Research Center of State Key Laboratory of Coal Resources and Safe Mining, Chongqing 400042, (China); Chou, Chen-Lin [Illinois State Geological Survey (Emeritus), 615 East Peabody Drive, Champaign, IL 61820, (United States); Zhou, Yiping [Yunnan Institute of Coal Geology Prospection, Kunming 650218, (China); Zhu, Changsheng; Li, Hang [Research Center of State Key Laboratory of Coal Resources and Safe Mining, Chongqing 400042, (China); Zhu, Xingwei; Xing, Yunwei; Zhang, Weiguo; Zou, Jianhua [State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, (China)


    The No. 12 Coal (Late Permian) in the Songzao Coalfield, Chongqing, southwestern China, is characteristically high in pyrite and some trace elements. It is uniquely deposited directly above mafic tuff beds. Samples of coal and tuffs have been studied for their mineralogy and geochemistry using inductively coupled plasma-mass spectrometry, X-ray fluorescence, plasma low-temperature ashing plus powder X-ray diffraction, and scanning electron microscopy equipped with energy-dispersive X-ray analysis. The results show that the minerals of the No. 12 Coal are mainly composed of pyrite, clay minerals (kaolinite, chamosite, and illite), ankerite, calcite, and trace amounts of quartz and boehmite. Kaolinite and boehmite were mainly derived from sediment source region of mafic tuffs. Chamosite was formed by the reaction of kaolinite with Fe-Mg-rich fluids during early diagenesis. The high pyrite (S{sub p,d} 8.83%) in the coal was related to marine transgression over peat deposits and abundant Fe derived from the underlying mafic tuff bed. Ankerite and calcite were precipitated from epigenetic fluids. Chemical compositions of incompatible elements indicate that the tuffs were derived from enriched mantle and the source magmas had an alkali-basalt character. Compared to other coals from the Songzao Coalfield and common Chinese coals, the No. 12 Coal has a lower SiO{sub 2}/Al{sub 2}O{sub 3} (1.13) but a higher Al{sub 2}O{sub 3}/Na{sub 2}O (80.1) value and is significantly enriched in trace elements including Sc (13.5 {mu}g/g), V (121 {mu}g/g), Cr (33.6 {mu}g/g), Co (27.2 {mu}g/g), Ni (83.5 {mu}g/g), Cu (48.5 {mu}g/g), Ga (17.3 {mu}g/g), Y (68.3 {mu}g/g), Zr (444 {mu}g/g), Nb (23.8 {mu}g/g), and REE (392 {mu}g/g on average). Above mineralogical compositions, as well as similar ratios of selected elements (e.g., SiO{sub 2}/Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}/Na{sub 2}O) and similar distribution patterns of incompatible elements (e.g., the mantle-normalized diagram for

  19. A high-pyrite semianthracite of Late Permian age in the Songzao Coalfield, southwestern China: Mineralogical and geochemical relations with underlying mafic tuffs (United States)

    Dai, S.; Wang, X.; Chen, W.; Li, D.; Chou, C.-L.; Zhou, Y.; Zhu, Chen; Li, H.; Zhu, Xudong; Xing, Y.; Zhang, W.; Zou, J.


    The No. 12 Coal (Late Permian) in the Songzao Coalfield, Chongqing, southwestern China, is characteristically high in pyrite and some trace elements. It is uniquely deposited directly above mafic tuff beds. Samples of coal and tuffs have been studied for their mineralogy and geochemistry using inductively coupled plasma-mass spectrometry, X-ray fluorescence, plasma low-temperature ashing plus powder X-ray diffraction, and scanning electron microscopy equipped with energy-dispersive X-ray analysis.The results show that the minerals of the No. 12 Coal are mainly composed of pyrite, clay minerals (kaolinite, chamosite, and illite), ankerite, calcite, and trace amounts of quartz and boehmite. Kaolinite and boehmite were mainly derived from sediment source region of mafic tuffs. Chamosite was formed by the reaction of kaolinite with Fe-Mg-rich fluids during early diagenesis. The high pyrite (Sp,d=8.83%) in the coal was related to marine transgression over peat deposits and abundant Fe derived from the underlying mafic tuff bed. Ankerite and calcite were precipitated from epigenetic fluids.Chemical compositions of incompatible elements indicate that the tuffs were derived from enriched mantle and the source magmas had an alkali-basalt character. Compared to other coals from the Songzao Coalfield and common Chinese coals, the No. 12 Coal has a lower SiO2/Al2O3 (1.13) but a higher Al2O3/Na2O (80.1) value and is significantly enriched in trace elements including Sc (13.5??g/g), V (121??g/g), Cr (33.6??g/g), Co (27.2??g/g), Ni (83.5??g/g), Cu (48.5??g/g), Ga (17.3??g/g), Y (68.3??g/g), Zr (444??g/g), Nb (23.8??g/g), and REE (392??g/g on average). Above mineralogical compositions, as well as similar ratios of selected elements (e.g., SiO2/Al2O3 and Al2O3/Na2O) and similar distribution patterns of incompatible elements (e.g., the mantle-normalized diagram for incompatible elements and chondrite-normalized diagram for rare earth elements) of coal and tuff, indicated that

  20. New Elemental and Isotopic Data From Mafic Lavas on the Puna Plateau and Re-Examining the Geochemical Signature of Convective Lithospheric Removal in the Central Andes (United States)

    Murray, K. E.; Ducea, M. N.; Reiners, P. W.


    Foundering or delamination of the lower lithosphere into the convecting mantle is required by mass balance in convergent orogens such as the central Andes. In the central Andean volcanic zone (CVZ), late Miocene to Recent mafic lavas erupted on the Puna plateau are small volume fissure flows and cinder cones classically cited as evidence of convective lithospheric removal, in concert with a suite of observations including high surface elevation (>4000m) and anomalously thin lithosphere relative to other parts of the CVZ. Mafic lavas provide the best available geochemical window into the recent history of the upper mantle in this and other regions. However, an increasing number of elemental and isotopic data suggest that these melts are less distinct from the neighboring arc magmatism than originally predicted. This observation weakens the hypothesis that there is a distinct geochemical fingerprint for so-called delamination magmatism, while advancing our understanding of the size of delaminating bodies and the timescales over which they detach from the lithosphere and interact with the mantle wedge. In this contribution, we present elemental and radiogenic isotopic data from 20 newly sampled mafic lavas from the Puna plateau (24.5°S to 27°S). Preliminary major element analyses show that the Puna lavas are high-K to shoshonitic in composition, in broad agreement with other mafic lavas sampled though out the region. Several sampled flows contain xenotliths of granitoid composition, which likely represent the crustal end member that contributed to the more evolved lavas. Along with major, trace and rare earth element analyses, we will present 87Sr/86Sr and 143Nd/144Nd data to further characterize source regions of these melts. In sum, these data will allow us to (1) expand the spatial coverage of this dataset in the central Andes, (2) contribute to the effort to parse contributions from the subcontinental lithosphere, asthenosphere, subduction-related fluids, and

  1. Key technologies for design of sea dykes of Tlaozini 1st stage project in Jiangsu Province%江苏条子泥Ⅰ期匡围海堤设计关键技术

    Institute of Scientific and Technical Information of China (English)

    徐亮; 吴彩娥; 陈茹


    根据江苏东台条子泥边滩海域的特性,结合施工的具体条件,从工程实际情况出发,因地制宜,对条子泥Ⅰ期匡围工程的海堤设计关键技术进行探讨.结果表明,海堤防护采取的防护措施合理,能满足工程质量、安全的要求,同时取得了良好的经济效益.%There' re a series of radial sand ridges in the near-shore shallow sea area of Jianggang Town in Dongtai City of Jiangsu Province. Tiaozini sand ridge is located in the center of the radial sandbar area and adjacent to the beach, whose northern land boundary is Iiangduo River estuary and the southern one is Xinchuan Port. According to the characteristics of Tiaozini sea area and combined with the specific construction conditions, the key technologies for the design of sea dykes of Tiaozini 1* stage project are discussed based on the actual situations and local conditions. The results indicate that the measures for protecting the sea dykes are rational, and they can meet the requirements of project quality and safety. Accordingly, good economic benefits are achieved.

  2. Parasitic analysis and π-type Butterworth-Van Dyke model for complementary-metal-oxide-semiconductor Lamb wave resonator with accurate two-port Y-parameter characterizations. (United States)

    Wang, Yong; Goh, Wang Ling; Chai, Kevin T-C; Mu, Xiaojing; Hong, Yan; Kropelnicki, Piotr; Je, Minkyu


    The parasitic effects from electromechanical resonance, coupling, and substrate losses were collected to derive a new two-port equivalent-circuit model for Lamb wave resonators, especially for those fabricated on silicon technology. The proposed model is a hybrid π-type Butterworth-Van Dyke (PiBVD) model that accounts for the above mentioned parasitic effects which are commonly observed in Lamb-wave resonators. It is a combination of interdigital capacitor of both plate capacitance and fringe capacitance, interdigital resistance, Ohmic losses in substrate, and the acoustic motional behavior of typical Modified Butterworth-Van Dyke (MBVD) model. In the case studies presented in this paper using two-port Y-parameters, the PiBVD model fitted significantly better than the typical MBVD model, strengthening the capability on characterizing both magnitude and phase of either Y11 or Y21. The accurate modelling on two-port Y-parameters makes the PiBVD model beneficial in the characterization of Lamb-wave resonators, providing accurate simulation to Lamb-wave resonators and oscillators.

  3. Duration of a large Mafic intrusion and heat transfer in the lower crust: A SHRIMP U-Pb zircon Study in the Ivrea-Verbano Zone (Western Alps, Italy) (United States)

    Peressini, G.; Quick, J.E.; Sinigoi, S.; Hofmann, A.W.; Fanning, M.


    The Ivrea-Verbano Zone in the western Italian Alps contains one of the world's classic examples of ponding of mantle-derived, mafic magma in the deep crust. Within it, a voluminous, composite mafic pluton, the Mafic Complex, intruded lower-crustal, high-grade paragneiss of the Kinzigite Formation during Permian-Carboniferous time, and is now exposed in cross-section as a result of Alpine uplift. The age of the intrusion is still debated because the results of geochronological studies in the last three decades on different rock types and with various dating techniques range from 250 to about 300 Ma. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age determinations on 12 samples from several locations within the Mafic Complex were performed to better constrain the age of the igneous event. The results indicate a long history of magma emplacement and cooling, which reconciles the spread in previously published ages. The main intrusive phase took place at 288 ?? 4 Ma, causing a perturbation of the deep-crustal geotherm, which relaxed to the Sm-Nd closure temperature in garnet-free mafic rocks after about 15-20 Myr of sub-solidus cooling at c. 270 Ma. These results suggest that large, deep crustal plutons, such as those identified geophysically at depths of 10-20 km within extended continental crust (e.g. Yellowstone, Rio Grande Rift, Basin and Range) may have formed rapidly but induced a prolonged thermal perturbation. In addition, the data indicate that a significant thermal event affected the country rock of the Mafic Complex at about 310 Ma. The occurrence of an upper amphibolite- to granulite-facies thermal event in the Kinzigite Formation prior to the main intrusive phase of the Mafic Complex has been postulated by several workers, and is corroborated by other geochronological investigations. However, it remains uncertain whether this event (1) was part of a prolonged perturbation of the deep-crustal geotherm, which started long before the onset of

  4. Geochemistry and spatial distribution of late-Paleozoic mafic volcanic rocks in the surrounding areas of the Gonghe Basin: Implications for Majixueshan triple-junction and east Paleotethyan archipelagic ocean

    Institute of Scientific and Technical Information of China (English)


    The late-Paleozoic mafic volcanic rocks occurring in the surrounding areas of the Gonghe basin are distributed in the A'nyêmaqên ophiolite zone, Zongwulong tectonic zone and Kuhai-Saishitang volcanic zone. The mafic volcanics in the A'nyêmaqên zone formed an ancient ridge-centered hotspot around the Majixueshan OIB, the Kuhai-Saishitang mafic rocks consist of E-MORB and continental rift basalts and the Zongwulong volcanic rocks are enriched N-MORB. The regionally low Nb/U and Ce/Pb ratios reflect the influence of the OIB material on the mafic magma source. From geochemistry, spatial distribution and tectonic relationship of the mafic rocks, an ancient triple-junction centered at the Majixueshan can be inferred. The existence of the Kuhai-Saishitang aulacogen may have provided a tectonic channel for the Majixueshan OIB materials metasomatizing the magma source for the Zongwulong rocks. The formation of the triple-junction and the rifting of the Zongwulong zone have separated the orogens and massifs in the region.

  5. Triggers on sulfide saturation in Fe-Ti oxide-bearing, mafic-ultramafic layered intrusions in the Tarim large igneous province, NW China (United States)

    Cao, Jun; Wang, Christina Yan; Xu, Yi-Gang; Xing, Chang-Ming; Ren, Ming-Hao


    Three Fe-Ti oxide-bearing layered intrusions (Mazaertag, Wajilitag, and Piqiang) in the Tarim large igneous province (NW China) have been investigated for understanding the relationship of sulfide saturation, Platinum-group element (PGE) enrichment, and Fe-Ti oxide accumulation in layered intrusions. These mafic-ultramafic layered intrusions have low PGE concentrations (convecting mantle, without appreciable input of lithospheric mantle. The Mazaertag and Wajilitag intrusions have near-chondritic γOs(t) values (+13 to +60) against restricted ɛ Nd(t) values (-0.4 to +2.8), indicating insignificant crustal contamination. Rocks of the Piqiang intrusion have relatively low ɛ Nd(t) values of -3.1 to +1.0, consistent with ˜15 to 25 % assimilation of the upper crust. The rocks of the Mazaertag and Wajilitag intrusions have positive correlation of PGE and S, pointing to the control of PGE by sulfide. Poor correlation of PGE and S for the Piqiang intrusion is attributed to the involvement of multiple sulfide-stage liquids with different PGE compositions or sulfide-oxide reequilibration on cooling. These three layered intrusions have little potential of reef-type PGE mineralization. Four criteria are summarized in this study to help discriminate between PGE-mineralized and PGE-unmineralized mafic-ultramafic intrusions.

  6. U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains--Crust-mantle interaction and LOMU component

    Institute of Scientific and Technical Information of China (English)

    黄方; 李曙光; 周红英; 李惠民


    The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are characterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first observed that some samples with lower 206Pb/204Pb and higher 207Pb/204Pb ratios show typical characters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional mafic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.

  7. Petrology of the Motaghairat mafic-ultramafic complex, Eastern Desert, Egypt: A high-Mg post-collisional extension-related layered intrusion (United States)

    Abdel Halim, Ali H.; Helmy, Hassan M.; Abd El-Rahman, Yasser M.; Shibata, Tomoyuki; El Mahallawi, Mahmoud M.; Yoshikawa, Masako; Arai, Shoji


    The geodynamic settings of the Precambrian mafic-ultramafic complexes in the Eastern Desert of Egypt have important bearing on understanding the geotectonic evolution of the Arabian Nubian Shield. We present a detailed petrological study on a layered mafic-ultramafic intrusion that is located at the contact between the Precambrian continental crust and the Miocene Red Sea oceanic crust. The Motaghairat layered intrusion consists of basal lherzolite, orthopyroxenite, troctolite, olivine gabbro and anorthosite on the top. Variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high-Mg tholeiitic parent melt through fractional crystallization processes. The parental magma was derived from a metasomatised mantle source. The primitive mantle-normalized patterns of the calculated melts exhibit enrichment in U relative to Th and Ba relative LREE which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt. Geological and petrological evidences suggest that the layered Motaghairat intrusion was emplaced during post-orogenic extension following subduction break-off and lithospheric delamination after the collision between the amalgamated island arc terranes and the Saharan Metacraton. The heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction-break-off.

  8. Integrated Experimental and Modeling Studies of Mineral Carbonation as a Mechanism for Permanent Carbon Sequestration in Mafic/Ultramafic Rocks

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Zhengrong [Yale Univ., New Haven, CT (United States); Qiu, Lin [Yale Univ., New Haven, CT (United States); Zhang, Shuang [Yale Univ., New Haven, CT (United States); Bolton, Edward [Yale Univ., New Haven, CT (United States); Bercovici, David [Yale Univ., New Haven, CT (United States); Ague, Jay [Yale Univ., New Haven, CT (United States); Karato, Shun-Ichiro [Yale Univ., New Haven, CT (United States); Oristaglio, Michael [Yale Univ., New Haven, CT (United States); Zhu, Wen-Iu [Univ. of Maryland, College Park, MD (United States); Lisabeth, Harry [Univ. of Maryland, College Park, MD (United States); Johnson, Kevin [Univ. of Hawaii, Honolulu, HI (United States)


    A program of laboratory experiments, modeling and fieldwork was carried out at Yale University, University of Maryland, and University of Hawai‘i, under a DOE Award (DE-FE0004375) to study mineral carbonation as a practical method of geologic carbon sequestration. Mineral carbonation, also called carbon mineralization, is the conversion of (fluid) carbon dioxide into (solid) carbonate minerals in rocks, by way of naturally occurring chemical reactions. Mafic and ultramafic rocks, such as volcanic basalt, are natural candidates for carbonation, because the magnesium and iron silicate minerals in these rocks react with brines of dissolved carbon dioxide to form carbonate minerals. By trapping carbon dioxide (CO2) underground as a constituent of solid rock, carbonation of natural basalt formations would be a secure method of sequestering CO2 captured at power plants in efforts to mitigate climate change. Geochemical laboratory experiments at Yale, carried out in a batch reactor at 200°C and 150 bar (15 MPa), studied carbonation of the olivine mineral forsterite (Mg2SiO4) reacting with CO2 brines in the form of sodium bicarbonate (NaHCO3) solutions. The main carbonation product in these reactions is the carbonate mineral magnesite (MgCO3). A series of 32 runs varied the reaction time, the reactive surface area of olivine grains and powders, the concentration of the reacting fluid, and the starting ratio of fluid to olivine mass. These experiments were the first to study the rate of olivine carbonation under passive conditions approaching equilibrium. The results show that, in a simple batch reaction, olivine carbonation is fastest during the first 24 hours and then slows significantly and even reverses. A natural measure of the extent of carbonation is a quantity called the carbonation fraction, which compares the amount of carbon removed from solution, during a run, to the maximum amount

  9. Emplacement and Solidification of Inter-Layerd Silicic and Mafic Magmas, Isle au Haut Igneous Complex, Maine (United States)

    Patwardhan, K.; Marsh, B. D.


    The Isle au Haut Igneous Complex, Maine, presents an intriguing association of basaltic and granitic rocks in the form of a composite layered sequence of alternating gabbroic and dioritic units. The 413 Ma complex may have formed by periodic replenishment or invasion of mafic magma into an evolving, more silicic magma chamber (Chapman & Rhodes, 1992; JVGR). The interaction of a denser magma overlying a less dense magma promotes Rayleigh-Taylor instabilities (RTI), resulting in overturning initiated through pipes and the eventual formation of a stable configuration of diorite over gabbro (Chapman & Rhodes, 1992; JVGR). Field exposures on the southern parts of the island covering a 160 m section through the sequence shows four gabbroic-dioritic pairs (i.e., gabbro over diorite) exhibiting strong interaction between the two in the forms of diapiric pipes of diorite intruding gabbro and gabbro pillows caught in diorite. It is clear that the two magmas were simultaneously molten, and this presents an opportunity to observe the physics and chemistry of interaction between gabbroic magma and partly molten diorite. Emplacement of denser, sill like gabbroic magma over solidifying, less dense dioritic magma may also have given rise to structures resembling load casts and flame structures common to sedimentary rocks. That the diorite was significantly solidified (e.g., ~55% crystals) at the time of intrusion of the gabbroic magma is reflected in field and textural relationships suggesting that overturning due to gravitational instability was initiated but did not go to completion. That is, the diorite was strong enough to act as a rigid medium for gabbro emplacement and subsequent support without wholesale mixing, but at the same time was weak enough to yield low density melt, perhaps through compaction, to undergo RTI at the interfaces and form extensive ensembles of diapiric pipes in the overlying gabbro. We examine the physics and chemistry of this process beginning with

  10. High-resolution seismic imaging of the Kevitsa mafic-ultramafic Cu-Ni-PGE hosted intrusion, northern Finland (United States)

    Malehmir, Alireza; Koivisto, Emilia; Wjins, Chris; Tryggvason, Ari; Juhlin, Christopher


    Kevitsa, in northern Finland, is a large nickel/copper ore body hosted by a massive mafic-ultramafic intrusion with measured and indicated resources of 240 million tons (cutoff 0.1%) grading 0.30% Ni and 0.41% Cu. Mining started in 2012 with an open pit that will extend down to about 550-600 m depth. The expected mine life is more than 20 years. Numerous boreholes are available in the area, but the majority of them are shallow and do not provide a comprehensive understanding of the dimensions of the intrusion. However, a number of boreholes do penetrate the basal contact of the intrusion. Most of these are also shallow and concentrated at the edge of the intrusion. A better knowledge of the geometry of the intrusion would provide a framework for near-mine and deep exploration in the area, but also a better understanding of the geology. Exact mapping of the basal contact of the intrusion would also provide an exploration target for the contact-type mineralization that is often more massive and richer in Ni-Cu than the disseminated mineralization away from the contact. With the objective of better characterizing the intrusion, a series of 2D profiles were acquired followed by a 3D reflection survey that covered an area of about 3 km by 3 km. Even though the geology is complex and the seismic P-wave velocity ranges between 5 to 8 km/s, conventional processing results show gently- to steeply-dipping reflections from depths of approximately 2 km to as shallow as 100 m. Many of these reflections are interpreted to originate from either fault systems or internal magmatic layering within the Kevitsa main intrusion. Correlations between the 3D surface seismic data and VSP data, based upon time shifts or phase changes along the reflections, support the interpretation that numerous faults are imaged in the volume. Some of these faults cross the planned open-pit mine at depths of about 300-500 m, and it is, therefore, critical to map them for mine planning. The seismic 3D

  11. Integrated Experimental and Modeling Studies of Mineral Carbonation as a Mechanism for Permanent Carbon Sequestration in Mafic/Ultramafic Rocks

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Zhengrong [Yale Univ., New Haven, CT (United States); Qiu, Lin [Yale Univ., New Haven, CT (United States); Zhang, Shuang [Yale Univ., New Haven, CT (United States); Bolton, Edward [Yale Univ., New Haven, CT (United States); Bercovici, David [Yale Univ., New Haven, CT (United States); Ague, Jay [Yale Univ., New Haven, CT (United States); Karato, Shun-Ichiro [Yale Univ., New Haven, CT (United States); Oristaglio, Michael [Yale Univ., New Haven, CT (United States); Zhu, Wen-Iu [Univ. of Maryland, College Park, MD (United States); Lisabeth, Harry [Univ. of Maryland, College Park, MD (United States); Johnson, Kevin [Univ. of Hawaii, Honolulu, HI (United States)


    A program of laboratory experiments, modeling and fieldwork was carried out at Yale University, University of Maryland, and University of Hawai‘i, under a DOE Award (DE-FE0004375) to study mineral carbonation as a practical method of geologic carbon sequestration. Mineral carbonation, also called carbon mineralization, is the conversion of (fluid) carbon dioxide into (solid) carbonate minerals in rocks, by way of naturally occurring chemical reactions. Mafic and ultramafic rocks, such as volcanic basalt, are natural candidates for carbonation, because the magnesium and iron silicate minerals in these rocks react with brines of dissolved carbon dioxide to form carbonate minerals. By trapping carbon dioxide (CO2) underground as a constituent of solid rock, carbonation of natural basalt formations would be a secure method of sequestering CO2 captured at power plants in efforts to mitigate climate change. Geochemical laboratory experiments at Yale, carried out in a batch reactor at 200°C and 150 bar (15 MPa), studied carbonation of the olivine mineral forsterite (Mg2SiO4) reacting with CO2 brines in the form of sodium bicarbonate (NaHCO3) solutions. The main carbonation product in these reactions is the carbonate mineral magnesite (MgCO3). A series of 32 runs varied the reaction time, the reactive surface area of olivine grains and powders, the concentration of the reacting fluid, and the starting ratio of fluid to olivine mass. These experiments were the first to study the rate of olivine carbonation under passive conditions approaching equilibrium. The results show that, in a simple batch reaction, olivine carbonation is fastest during the first 24 hours and then slows significantly and even reverses. A natural measure of the extent of carbonation is a quantity called the carbonation fraction, which compares the amount of carbon removed from solution, during a run, to the maximum amount

  12. Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate mafic lavas (United States)

    Zhang, L.-Y.; Prelević, D.; Li, N.; Mertz-Kraus, R.; Buhre, S.


    Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O > 4 wt.%; K2O/Na2O > 1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 - thin, Group 2 - middle, and Group 3 - thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a near north-south direction along a broad zone where the lithosphere thickness increases gradually. We present a detailed petrographical and geochemical study on olivine macrocrysts in combination with new geochemical data on their host lavas, including major and trace element abundances as well as Sr, Nd, and Pb isotopic signatures. Our ultimate aim is to quantitatively and qualitatively determine the role of lithospheric mantle thickness (named as "lid effect") and composition in the variation of mafic lavas and olivine composition. When corrected to Mg# = 0.72, a number of major elements in the lavas correlate with increasing lithospheric thickness (L): Si72 and Al72 decrease, whereas Mg72, Fe72, Ti72 and P72 increase. Sm/Yb ratios in the lavas increase, implying that lithospheric thickness exerts an important control. Group 3 mafic lavas are ultrapotassic (showing lamproite affinity) with K2O/Na2O > 4: their La/Sm and Pb isotope ratios deviate from the above correlations, indicating that the lavas from the thickest part of the basin exhibit the highest extent of metasomatic enrichment of the mantle source. Several parameters (e.g. [Ni], Ni/Mg, Ni/(Mg/Fe), Mn/Fe and Ca/Fe) in melt-related olivine from Group 1 and Group 2 lavas are controlled by variable lithosphere thickness. Olivine

  13. Late Holocene hydrous mafic magmatism at the Paint Pot Crater and Callahan flows, Medicine Lake Volcano, N. California and the influence of H2O in the generation of silicic magmas (United States)

    Kinzler, R.J.; Donnelly-Nolan, J. M.; Grove, T.L.


    This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years B.P.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ~3000 14C years B.P.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes.

  14. Occurrence model for magmatic sulfide-rich nickel-copper-(platinum-group element) deposits related to mafic and ultramafic dike-sill complexes: Chapter I in Mineral deposit models for resource assessment (United States)

    Schulz, Klaus J.; Woodruff, Laurel G.; Nicholson, Suzanne W.; Seal, Robert R., II; Piatak, Nadine M.; Chandler, Val W.; Mars, John L.


    Magmatic sulfide deposits containing nickel (Ni) and copper (Cu), with or without (±) platinum-group elements (PGE), account for approximately 60 percent of the world’s nickel production. Most of the remainder of the Ni production is derived from lateritic deposits, which form by weathering of ultramafic rocks in humid tropical conditions. Magmatic Ni-Cu±PGE sulfide deposits are spatially and genetically related to bodies of mafic and/or ultramafic rocks. The sulfide deposits form when the mantle-derived mafic and/or ultramafic magmas become sulfide-saturated and segregate immiscible sulfide liquid, commonly following interaction with continental crustal rocks.

  15. Dating sub-20 micron zircons in granulite-facies mafic dikes from SW Montana: a new approach using automated mineralogy and SIMS U-Pb geochronology (United States)

    Ault, A. K.; Mahan, K. H.; Flowers, R. M.; Chamberlain, K.; Appleby, S. K.; Schmitt, A. K.


    Geochronological data is fundamental to all tectonic studies, but a major limitation for many lithologies is a paucity of sizeable zircons suitable for conventional U-Pb techniques. In particular, mafic dike swarms provide important time markers for tectonometamorphic activity in Precambrian terranes, but commonly yield little or no zircon or baddeleyite sufficient for TIMS or standard ion-probe analysis of crystal separates. We apply a new approach involving in-situ automated mineralogy and high spatial resolution Secondary Ion Mass Spectrometry (SIMS) geochronology to a mafic dike swarm exposed in the Northern Madison Range of SW Montana. The dikes cross-cut early fabrics but are also variably deformed and metamorphosed to P-T conditions as high as 1.2 GPa and 850 C. The swarm emplacement age is inferred to be ca. 2.1 Ga based on similarities to dated dikes in the adjacent Tobacco Root Mountains. Resolving the timing of dike emplacement and high-grade metamorphism in the study area is important for understanding the extent of post-Archean modification to the northwest margin of the Wyoming craton. Identification and textural characterization of zircons were facilitated by in-situ automated mineralogical analysis, in contrast to a standard elemental X-ray mapping approach. Our technique uses an SEM-based platform coupling calibrated BSE data with X-ray data collected by multiple energy dispersive spectrometers to rapidly identify target accessory phases at high spatial resolution. Whole thin section search maps were generated in ~30 minutes at 4 µm pixel resolution. Our dike thin sections commonly contained >300 zircons in a variety of textural settings, with 80% having a short dimension 95% for 206Pb. SIMS data for 22 zircons from a granulite-facies mafic dike thin section define a chord with upper and lower intercepts of 1753.1 ± 9.5 Ma and 63.2 ± 7.9 Ma, respectively (2 sigma error, MSWD = 1.6). A positive correlation between U concentration and degree of

  16. Generation of the Early Cenozoic adakitic volcanism by partial melting of mafic lower crust, Eastern Turkey: Implications for crustal thickening to delamination (United States)

    Karsli, Orhan; Dokuz, Abdurrahman; Uysal, İbrahim; Aydin, Faruk; Kandemir, Raif; Wijbrans, Jan


    Early Cenozoic (48-50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO 2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474-0.70640) and low ɛNd (50 Ma) values (- 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr-Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the

  17. Petrogenesis of carbonic dykes within southern Tibetan Plateau, and climatic effects%藏南碳酸岩脉成因及其气候效应

    Institute of Scientific and Technical Information of China (English)



    released back to atmosphere through hot springs. Most carbon had, however, transferred into deep interior of Tibetan Plateau along with subducted Indian crust. The biotite within the subducted slab underwent dehydration to form granitic magmas beneath south Tibetan Plateau. The metasomatic reactions between the granitic magmas and the subducted carbon-rich Siwalik formation took place to release high-temperature CO2-rich fluids beneath Tibetan Plateau, regarded as crustal-derived carbonic magmas in this study. The magmas/fluids intruded into south Tibetan upper crust to form carbonic dykes. The huge atmospheric CO2 has, therefore, been transformed into carbonic magmas within thickened crust of southern Tibetan Plateau during the collision between India and Asia. The carbon emitted by hot springs as well as volcanoes within Tibetan Plateau was originated from atmosphere. It is recycling carbon. The carbon emissions from Tibetan Plateau are slightly less than those sinked by Tibetan Plateau. Otherwise,the carbonic dykes,formed by consuming huge atmospheric CO2,never occurred within Tibetan Plateau. This clearly suggests that Himalayan Mountains and south Tibetan Plateau are a huge reservoir for atmospheric CO2, leading to global cooling during Cenozoic times. Moreover,the changing of atmospheric CO2 was mainly derived by Earth's tectonic activities, and not by mankind. Global changing is only a natural phenomenon, without any relationships carbon emissions of human.%始新世末期以来,全球大气CO2浓度持续下降,但长期以来不清楚为何这一时期全球大气CO2浓度下降,巨量的大气CO2赋存于何处.深入研究该问题有助于准确理解未来大气CO2浓度变化的趋势,特别是有助于进一步评估人类自身碳排放的后果.这一时期,小印度陆块持续与大亚洲陆块汇聚,导致了以喜马拉雅为代表的山脉群和青藏高原的形成.很早就有学者从地球表层碳循环的角度提出了“青藏高原的隆升

  18. Chemical and isotopic relationship of mafic and felsic magmas in a sub-volcanic reservoir: The Guadalupe Igneous Complex (GIC), Sierra Nevada, California (United States)

    Ratschbacher, B. C.; Paterson, S. R.; Putirka, K. D.


    It is commonly believed that the interaction of mafic and felsic melts in the form of mixing/mingling as well as their genetic link in the form of fractionation play an important role in the formation of continental crust. The combination of whole rock major element content and isotopic signature, as presented in this study, is a powerful tool to identify the origin and genetic relation of mafic and felsic melts in magmatic arc settings where new material is added to the crust. The GIC is part of the Jurassic Sierran magmatic arc exposed in the Western Metamorphic Belt and contains two main units consisting of mafic (up to 9 wt. % MgO and 49 to 56 wt. % SiO2) and felsic (around 75 wt. % SiO2) rocks, which locally mingled and mixed to different proportions at a shallow emplacement level. In the lower parts of the GIC, fine-grained gabbros gradually evolve into the overlying diorite to meladiorite unit. A mingling zone separates these mafic rocks from granites, granophyres and overlying rhyolites in the upper part of the complex. Major element whole rock analyses show that the GIC is bimodal with gabbros and granitoids acting as endmembers in SiO2, MgO and CaO contents. For Al2O3, Na2O and other element oxides, the different units strongly overlap in compositions. Recent work using single grain zircon U-Pb dating found ages for both the gabbros and the felsic part of the complex of 151 Ma within uncertainty (Saleeby et al., 1989; Ernst et al., 2009, and unpublished data from this study). These ages are in agreement with Rb-Sr data from each unit, which fall on a 152×7 Ma isochron and therefore imply closed-system evolution. Major oxide data show that assimilation of the exposed surrounding host rocks is unlikely and cannot serve as an assimilant to reproduce the observed felsic compositions from the gabbroic rocks. Sri, Nd and Pb systematics show that all units except for capping granophyres and rhyolites plot close together implying a shared parental melt, which is

  19. Timing of the Wudangshan, Yaolinghe volcanic sequences and mafic sills in South Qinling: U-Pb zircon geochronology and tectonic implication

    Institute of Scientific and Technical Information of China (English)

    LING WenLi; REN BangFang; DUAN RuiChun; LIU XiaoMing; MAO XinWu; PENG LianHong; LIU ZaoXue; CHENG JianPing; YANG HongMei


    The Wudangshan, Yaolinghe volcanic-sedimentary sequences and doleritic-gabbroic sills comprise the largest exposed Precambrian basement in South Qinling. Zircons separated from 5 volcanic-pyroclas-tic samples of the Wudangshan Group, 2 volcanic samples of the Yaolinghe Group and one sample for the mafic sills were used for U-Pb dating by laser ablation-inductively coupled plasma mass spec-trometry (LA-ICPMS). The results reveal that the Wudangshan volcanic sequence was formed at (755±3) Ma (a weighted mean from the 5 samples, MSWD=0.47), whereas the Yaolinghe volcanic suite and the mafic sill were crystallized at (685:L±5) (2 samples, MSWD=0.36) and (679±J:3) Ma (MSWD=1.6), respectively, which are equal to each other within analysis errors. These ages are markedly younger than those previously documented for the rocks. The newly obtained ages for the Wudangshan and Yaolinghe Groups are identical to those of the bottom Liantuo and slightly older than those of the Nantuo Forms-tions, respectively, lower strata of the Nanhua (middle to late Neoproterozoic) stratotype section in eastern Three Gorges, Yangtze craton. A range of inherited magmatic zircons was recognized with ages of 830 to 780 Ma, which are typical of Neoprotzrozoic magmatisms recorded along the margins and interior of the Yangtze craton. Thus, there is Neoproterozoic basement comprising 830-780 Ma igne-ous suites in South Qinling; the inherited zircons were detrital sediments derived from the northern margin of the Yangtze craton. Accordingly, it is suggested that the South Qinling is a segment of the Yangtze craton before the Qinling Orogeny.

  20. Mesoproterozoic island arc magmatism along the south-eastern margin of the Indian Plate: Evidence from geochemistry and zircon U-Pb ages of mafic plutonic complexes (United States)

    Subramanyam, K. S. V.; Santosh, M.; Yang, Qiong-Yan; Zhang, Ze-ming; Balaram, V.; Reddy, U. V. B.


    The Prakasam Igneous Province within the Nellore Schist Belt (NSB) preserves important imprints of mafic magmatism along the south-eastern margin of the Indian plate. Here we report petrology, geochemistry and zircon U-Pb age data from three gabbro plutons namely Purimetla, Kanigiri and P C Palle which intruded into the high grade rocks of the region. LA-ICP-MS U-Pb data on zircons from the three plutons reveal prominent late Mesoproterozoic ages of 1334 ± 15 Ma, 1338 ± 27 Ma and 1251.2 ± 9.4 Ma. The cumulative 207Pb/206Pb mean age of 1315 ± 11 Ma is interpreted to represent the timing of mafic magmatism in the Prakasam Igneous Province. These rocks show adcumulus to mesocumulus and poikilitic textures indicating fractional crystallization of plagioclase and clinopyroxenes in the Purimetla pluton whereas the Kanigiri and P C Palle intrusions possess hornblende and biotite suggesting the role of water during partial melting. The rocks show LREE enrichment (∑LREE/∑HREE = 2.2-15.0), marked Eu-anomalies (Eu/Eu∗ = 0.8-2.2) and fractionated patterns (LaN/YbN = 3-79). Primitive mantle normalised trace element spider diagrams indicate subduction modified arc signatures with LILE enrichment and depletion of Nb, Ti and Zr relative to Th and La. Tectonic discrimination diagrams show arc magmatic affinities for the three gabbro plutons consistent with subduction zone setting. We propose a tectonic model involving intra oceanic island arc accretion during late Mesoproterozoic along the eastern margin of the Indian continent.

  1. The deep crustal structure of the mafic-ultramafic Seiland Igneous Province of Norway from 3-D gravity modelling and geological implications (United States)

    Pastore, Zeudia; Fichler, Christine; McEnroe, Suzanne A.


    The Seiland Igneous Province (SIP) is the largest complex of mafic and ultramafic intrusions in northern Fennoscandia intruded at ca. 580-560 Ma. The depth extent and the deep structure of the SIP are mainly unknown apart from three profiles modelled by gravity and refraction seismic data. Utilizing 3-D gravity modelling, a complex model of the deep subsurface structure of the SIP has been developed. The structure is presented in a multiprofile model ranging from the surface to the Moho. The mafic/ultramafic rocks of the SIP are modelled with densities of 3100 and 3300 kg m-3, the surrounding rocks by densities of 2700 and 2900 kg m-3 for upper and lower crust, respectively. This density model explains the pronounced positive Bouguer gravity anomaly of up to 100 mGal above background. Its minimum volume is estimated from the subsurface model to 17 000 km3 and as such we revise downwards the earlier estimations of 25 000 km3. The new subsurface model suggests that most of the SIP has a thickness between 2 and 4 km. An area with roots in an annular pattern is found and two deep-reaching roots have been identified located below the islands of Seiland and Sørøy. The depth of these roots is estimated to approximatively 9 km. The SIP is presently interpreted to be in the Caledonian Kalak Nappe Complex and the roots depth constrains its minimum thickness which is larger than earlier estimated. Furthermore, the rather undisturbed shape of the annular root pattern indicates that the SIP has not been subjected to strong tectonic reworking during the Caledonian orogeny.

  2. 新疆北天山黄山东含铜镍矿镁铁-超镁铁岩体的岩石因:主量元素、微量元素和Sr-Nd同位素证据%Petrogenesis of the Huangshandong Ni-Cu Sulfide-Bearing Mafic-Ultramafic Intrusion, Northern Tianshan, Xinjiang: Evidence from Major and Trace Elements and Sr-Nd Isotope

    Institute of Scientific and Technical Information of China (English)

    邓宇峰; 宋谢炎; 颉炜; 程松林; 李军


    本文对新疆黄山-镜儿泉铜镍成矿带中黄山东岩体进行了主量元素、微量元素和Sr-Nd同位素的研究,探讨了岩体成因和区域构造演化.黄山东岩体有三次岩浆侵入,第一次侵入形成了橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体,其中橄榄辉长岩位于岩体中部,角闪辉长岩位于岩体上部和下部,闪长岩沿岩体边缘分布;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为二辉橄榄岩,位于岩体下部,为主要的赋矿岩相.黄山东岩体样品除橄榄辉长岩和角闪辉长岩TiO2含量之外主量元素投影点都处于橄榄石、单斜辉石、斜方辉石和斜长石矿物化学成分之间,指示它们主要由橄榄石、单斜辉石、斜方辉石和斜长石组成.与塔里木地区地幔柱成因二叠纪镁铁-超镁铁岩体相比,黄山东岩体亏损Nb、Ta、Ti等高场强元素,La、Ba、Th、U等不相容元素含量较低,ENd (t)较高(+7.32‰~+8.29‰),并且具有较低的Th/U比值(1.13~2.98)和Nb/U比值(2.53~7.02),较高的La/Nb比值(1.15~4.19)和Ba/Nb比值(37.7~79.8).这些特征与俯冲成因的火山岩和侵入岩相似,暗示其原始岩浆主要来源于受俯冲流体交代地幔的部分熔融,软流圈地幔的上涌可能为部分熔融提供了热源.%The major and trace elements and Sr-Nd isotopes of the Huangshandong mafic-ultramafic intrusion in the Huangshan-Jing'erquan magmatic Cu-Ni mineralization belt are discussed to constrain its formation and regional tectonic evolution. Three magmatic pulses of the Huangshandong intrusion have been identified. The first pulse formed olivine gabbro, hornblende gabbro and diorite, which constitute the main body of the complex, with olivine gabbro in the central section, hornblende gabbro in upper and lower parts, and olivine gabbro along the margin of the complex. The second pulse formed gabbronorite dykes occurring as

  3. Geochemistry of the Peramora Mélange and Pulo do Lobo schist: geochemical investigation and tectonic interpretation of mafic mélange in the Pangean suture zone, Southern Iberia (United States)

    Dahn, Dustin R. L.; Braid, James A.; Murphy, J. Brendan; Quesada, Cecilio; Dupuis, Nicole; McFarlane, Christopher R. M.


    The Peramora Mélange is part of an accretionary complex between the South Portuguese Zone (a fragment of Laurussia) and the Ossa Morena Zone (para-autochthonous Gondwana) and is an expression of the Pangean suture zone in southwestern Iberia. The suture zone is characterized by fault-bounded units of metasedimentary rocks, mélanges, and mafic complexes. Detailed geologic mapping of the Peramora Mélange reveals a complex pattern of imbricated schists and mafic block-in-matrix mélanges. Geochemical signatures of the Pulo do Lobo schist (PDL) are consistent with derivation from both mafic and continental sources. The mafic block-in-matrix mélange displays normal mid-ocean ridge basalt (NMORB) geochemical signature, juvenile Sm-Nd isotopic compositions, and a range of zircon ages similar to those observed in the PDL, suggesting a sedimentary component. Taken together, these data suggest a complex tectonic history characterized by erosion of a NMORB source, mélange formation, and imbrication during underplating occurring during the final stages of continent-continent collision.

  4. Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians (United States)

    Coish, Raymond; Kim, Jonathan; Twelker, Evan; Zolkos, Scott P.; Walsh, Gregory J.


    The Moretown Formation, exposed as a north-trending unit that extends from northern Vermont to Connecticut, is located along a critical Appalachian litho-tectonic zone between the paleomargin of Laurentia and accreted oceanic terranes. Remnants of magmatic activity, in part preserved as metamorphosed mafic rocks in the Moretown Formation and the overlying Cram Hill Formation, are a key to further understanding the tectonic history of the northern Appalachians. Field relationships suggest that the metamorphosed mafic rocks might have formed during and after Taconian deformation, which occurred at ca. 470 to 460 Ma. Geochemistry indicates that the sampled metamorphosed mafic rocks were mostly basalts or basaltic andesites. The rocks have moderate TiO2 contents (1–2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. Their chemistry is similar to compositions of basalts from western Pacific extensional basins near volcanic arcs. The metamorphosed mafic rocks of this study are similar in chemistry to both the pre-Silurian Mount Norris Intrusive Suite of northern Vermont, and also to some of Late Silurian rocks within the Lake Memphremagog Intrusive Suite, particularly the Comerford Intrusive Complex of Vermont and New Hampshire. Both suites may be represented among the samples of this study. The geochemistry of all samples indicates that parental magmas were generated in supra-subduction extensional environments during lithospheric delamination.

  5. Density and velocity relationships for digital sonic and density logs from coastal Washington and laboratory measurements of Olympic Peninsula mafic rocks and greywackes (United States)

    Brocher, Thomas M.; Christensen, Nikolas I.


    for each well log. We also present two-way traveltimes for 15 of the wells calculated from the sonic velocities. Average velocities and densities for the wells having both logs can be reasonably well related using a modified Gardner’s rule, with p=1825v1/4, where p is the density (in kg/m3) and v is the sonic velocity (in km/s). In contrast, a similar analysis of published well logs from Puget Lowland is best matched by a Gardner’s rule of p=1730v1/4, close to the p=1740v1/4 proposed by Gardner et al. (1974). Finally, we present laboratory measurements of compressional-wave velocity, shear-wave velocity, and density for 11 greywackes and 29 mafic rocks from the Olympic Peninsula and Puget Lowland. These units have significance for earthquake-hazard investigations in Puget Lowland as they dip eastward beneath the Lowland, forming the “bedrock” beneath much of the lowland. Average Vp/Vs ratios for the mafic rocks, mainly Crescent Formation volcanics, lie between 1.81 and 1.86. Average Vp/Vs ratios for the greywackes from the accretionary core complex in the Olympic Peninsula show greater scatter but lie between 1.77 and 1.88. Both the Olympic Peninsula mafic rocks and greywackes have lower shear-wave velocities than would be expected for a Poisson solid (Vp/Vs=1.732). Although the P-wave velocities and densities in the greywackes can be related by a Gardner’s rule of p=1720v1/4, close to the p=1740v1/4 proposed by Gardner et al. (1974), the velocities and densities of the mafic rocks are best related by a Gardner’s rule of p=1840v1/4. Thus, the density/velocity relations are similar for the Puget Lowland well logs and greywackes from the Olympic Peninsula. Density/velocity relations are similar for the Washington coastal well logs and mafic rocks from the Olympic Peninsula, but differ from those of the Puget Lowland well logs and greywackes from the Olympic Peninsula.

  6. Petrogenesis and geodynamic setting of Early Cretaceous mafic-ultramafic intrusions, South China: A case study from the Gan-Hang tectonic belt (United States)

    Qi, Youqiang; Hu, Ruizhong; Liu, Shen; Coulson, Ian M.; Qi, Huawen; Tian, Jianji; Zhu, Jingjing


    A study using whole-rock major-trace elements and Sr-Nd isotopes as well as zircon U-Pb dating has been carried out on Early Cretaceous mafic-ultramafic intrusions from the Gan-Hang tectonic belt (GHTB), South China, to understand the origin of mantle sources and the sequential evolution of the underlying Late Mesozoic lithospheric mantle of this area. The study focused on two intrusions, one at Quzhou and the other at Longyou (see Fig. 1). They are primarily composed of mafic-ultramafic rocks with wide range of chemical compositions. The Quzhou mafic rocks have relatively narrow ranges of SiO2 (48.94-51.79 wt%), MgO (6.07-7.21 wt%), Fe2O3 (10.48-11.56 wt%), CaO (8.20-8.81 wt%), and Mg# (51.7-56.5) with relatively low K2O (0.56-0.67 wt%) and Na2O (3.09-3.42 wt%). By contrast, the ultramafic rocks from Longyou have distinct lower SiO2 (41.50-45.11 wt%) and higher MgO (9.05-9.90 wt%), Fe2O3 (12.14-12.62 wt%), CaO (8.64-10.67 wt%), and Mg# (59.5-61.1) with relatively higher K2O (1.32-1.75 wt%) and Na2O (4.53-5.08 wt%). They are characterized by Ocean Island Basalts (OIB)-type trace element distribution patterns, with a significant enrichment of light rare earth elements (LREE), large ion lithophile elements (LILE, i.e., Rb, Ba, K, and Sr) and high field strength elements (HFSE, i.e., Nb, Ta), and slight depletion of Th, U, Ti, and Y. The intrusions exhibit relatively depleted Sr-Nd isotope compositions, with (87Sr/86Sr)i range of 0.7035 to 0.7055 (143Nd/144Nd)i of 0.51264 to 0.51281 and εNd(t) values of + 3.0 to + 6.6. Zircon U-Pb dating of Longyou and Quzhou intrusions yields consistent magma emplacement ages of 129.0 ± 3.9 to 126.2 ± 2.4 Ma, respectively. The dating results are consistent with the peak of extension in Early Cretacerous throughout the Gan-Hang tectonic belt. Their magmas were principally derived from near-solidus partial melting of pyroxenites with different content of silica, and the pyroxenites were resulted from a juvenile SCLM peridotite

  7. Draped aeromagnetic survey in Transantarctic Mountains over the area of the Butcher Ridge igneous complex showing extent of underlying mafic intrusion (United States)

    Behrendt, John C.; Damaske, D.; Finn, C.A.; Kyle, P.; Wilson, T.J.


    A draped aeromagnetic survey over the area surrounding the Butcher Ridge igneous complex (BRIC), Transantarctic Mountains, was acquired in 1997-1998 as part of a larger Transantarctic Mountains Aerogeophysical Research Activity survey. The BRIC is a sill-like hypoabyssal intrusion ranging in composition from tholeiitic basalt to rhyolite. An 40Ar/39 Ar age of 174 Ma and the chemical character of the basaltic rocks show the BRIC to be part of the widespread Jurassic Ferrar suite of continental tholeiitic rocks, that extends for 3500 km across Antarctica. The aeromagnetic survey shows a horseshoe-shaped pattern of anomalies reaching amplitudes as great as 1900 nT generally associated with the bedrock topography where it is exposed. It is apparent that the high-amplitude anomaly pattern is more extensive than the 10-km-long exposed outcrop, first crossed by a single 1990 aeromagnetic profile. The highest-amplitude anomalies appear south of the profile acquired in 1990 and extend out of the survey area. The new aeromagnetic data allow determination of the extent of the interpreted Butcher mafic(?) intrusion beneath exposures of Beacon sedimentary rock and ice in the area covered, as well as beneath the small BRIC exposure. The magnetic anomalies show a minimum area of 3000 km2, a much greater extent than previously inferred. Magnetic models indicate a minimum thickness of ???1-2 km for a horizontal intrusion. However, nonunique models with magnetic layers decreasing in apparent susceptibility with depth are consistent with of a 4- to 8-km-thick layered intrusion. These magnetic models indicate progressively deeper erosion of the interpreted mafic-layered body from the south to north. The erosion has removed more magnetic upper layers that mask the magnetic effects of the lower less magnetic layers. The probable minimum volume of the intrusion in the area of the survey is ???6000 km3. An alternate, but less likely, interpretation of a series of dikes can also fit the

  8. Middle Triassic magma mixing in an active continental margin: Evidence from mafic enclaves and host granites from the Dewulu pluton in West Qinling, central China (United States)

    Huang, X.; Mo, X.; Yu, X.


    The Qinling-Dabie-Sulu orogen was formed through the collision of the North and South China blocks, but the precise timing of the closure of the Paleo-Tethys ocean between the two blocks remains debated. Large volumes of Triassic granites associated with mafic microgranular enclaves (MMEs) were emplaced in the Qinling terrane. This paper presents field observations, petrography, geochronology and geochemistry of the MMEs and their host granites from the Dewulu pluton in West Qinling. The host rocks comprise granodiorite and granodioritic porphyry, and the The MMEs range in composition from gabbroic diorite to diorite. Zircon LA-ICP-MS U-Pb ages suggest that the granites and MMEs were coeval at ca. 245 Ma. The granites are relatively enriched in LILE and depleted in HFSE, and have evolved Sr-Nd-Pb and zircon Hf isotopic compositions [initial 87Sr/86Sr = 0.7070-0.7076, ɛNd(t) = -7.5 to -6.8, ɛHf(t) = -8.2 to -4.2], indicative of an origin from the amphibolitic lower crust. The near-primitive gabbro-dioritic MMEs bear a remarkable geochemical resemblance to the high-magnesium andesite (HMA), such as moderate SiO2 (~55 wt.%), low FeOT/MgO (~0.75), high Cr (268-308 ppm) and MgO (8.58-8.77 wt.%) with Mg# of ~70. Additionally, they exhibit lower initial 87Sr/86Sr, higher ɛNd(t) and ɛHf(t), and more radiogenic Pb isotopes than the dioritic MMEs which share similar isotopic compositions with the granites. These features, together with the presence of the specific minerals in the MMEs (e.g., felsic xenocrysts and acicular apatite), point to mixing process between the lower crust-derived magmas and the melts produced by the reaction of the subducting sediment-derived components and the overlying mantle. Taking into account the regional occurrence of synchronous plutonic-volcanic complexes (250-234 Ma) ranging from basaltic to granitic variants, we suggest that the Dewulu pluton formed in an active continental margin in response to the local extension triggered by the

  9. Zircon geochronology and geochemistry of mafic xenoliths from Liaoning kimberlites:Track the early evolution of the lower crust, North China Craton

    Institute of Scientific and Technical Information of China (English)

    ZHENG Jianping; YU Chunmei; LU Fengxiang; LI Huimin


    Mafic xenoliths from Paleozoic Fuxian kimberlites (Liaoning Province) mainly include garnet granulite, with minor pyroxene amphibolite, meta-gabbro and pyroxene syenite. The SiO2 contents of the xenoliths are from 47.3 wt% to 49.9 wt%. The granulites are mainly coarse-medium grained and reasonably with well-developed granoblastic (mostly polygonal)texture, contain the assemblage of garnet + plagioclase + pyroxene + K-feldspar ± phlogopite.The mineral assemblage of the amphibolites is plagioclase + pyroxene + amphibole ± K-feldspar,with the equilibrium temperature and pressure conditions of 744-821 ℃ and 0.76-0.88 GPa.The granulites are regarded as derivation from the lower crust depth (more than 29 km), below the amphibolites. Garnet granulites compositionally correspond to sub-alkalic basalt, and have wide ranges of Ni abundance (133-840 ppm), and Nb/Y (0.12-1.85), Nb/U (3.51-53.86) and Ta/U (0.38-2.48). The amphibolite and the syenite correspond to alkalic basalt. The Fuxian mafic xenoliths are regarded as the metamorphic product of the underplated magmas (including fractional crystallization or not) experienced the contamination with the pre-existed crustal components, and partly effected by kimberlitic magmas. The concordant ages of zircons for the gabbro (2610-2580 Ma) and the near-concordant upper intercept ages of zircons for the garnet granulite and pyroxene amphibolite (2578-2538 Ma) indicate that they are currently known as the oldest deep-seated xenoliths from the lower crust of the North China Craton. These ages recorded the formation of the united Eastern Block of the North China Craton, That is, Neoarchean (2.6-2.5 Ga) is an important continental crustal growth period of the North China Craton.The lower intercept age of the garnet granulite (1853 Ma) recorded an important tectonic thermal event in Paleoproterozoic. This event was probably related to collision of the East and West block,and resulted in the final assembly of the North China Craton

  10. Zircon geochronology and geochemistry of mafic xenoliths from Liaoning kimberlites: Track the early evolution of the lower crust, North China Craton

    Institute of Scientific and Technical Information of China (English)

    ZHENG; Jianping; YU; Chunmei; LU; Fengxiang; LI; Huimin


    Mafic xenoliths from Paleozoic Fuxian kimberlites (Liaoning Province) mainly include garnet granulite, with minor pyroxene amphibolite, meta-gabbro and pyroxene syenite. The SiO2 contents of the xenoliths are from 47.3 wt% to 49.9 wt%. The granulites are mainly coarse-medium grained and reasonably with well-developed granoblastic (mostly polygonal)texture, contain the assemblage of garnet + plagioclase + pyroxene + K-feldspar ± phlogopite.The mineral assemblage of the amphibolites is plagioclase + pyroxene + amphibole ± K-feldspar,with the equilibrium temperature and pressure conditions of 744-821 ℃ and 0.76-0.88 GPa.The granulites are regarded as derivation from the lower crust depth (more than 29 km), below the amphibolites. Garnet granulites compositionally correspond to sub-alkalic basalt, and have wide ranges of Ni abundance (133-840 ppm), and Nb/Y (0.12-1.85), Nb/U (3.51-53.86) and Ta/U (0.38-2.48). The amphibolite and the syenite correspond to alkalic basalt. The Fuxian mafic xenoliths are regarded as the metamorphic product of the underplated magmas (including fractional crystallization or not) experienced the contamination with the pre-existed crustal components, and partly effected by kimberlitic magmas. The concordant ages of zircons for the gabbro (2610-2580 Ma) and the near-concordant upper intercept ages of zircons for the garnet granulite and pyroxene amphibolite (2578-2538 Ma) indicate that they are currently known as the oldest deep-seated xenoliths from the lower crust of the North China Craton. These ages recorded the formation of the united Eastern Block of the North China Craton, That is, Neoarchean (2.6-2.5 Ga) is an important continental crustal growth period of the North China Craton.The lower intercept age of the garnet granulite (1853 Ma) recorded an important tectonic thermal event in Paleoproterozoic. This event was probably related to collision of the East and West block,and resulted in the final assembly of the North China Craton

  11. Petrogenesis and ore genesis of the Permian Huangshanxi sulfide ore-bearing mafic-ultramafic intrusion in the Central Asian Orogenic Belt, western China (United States)

    Mao, Ya-Jing; Qin, Ke-Zhang; Li, Chusi; Xue, Sheng-Chao; Ripley, Edward M.


    The Permian Huangshanxi mafic-ultramafic intrusion hosts one of the two largest magmatic sulfide deposits in the Eastern Tianshan which is situated in the southern margin of the Central Asian Orogenic Belt. In this paper we use mineral compositions and whole-rock geochemical data to decipher the genetic relationship between magma evolution and sulfide mineralization. The Huangshanxi intrusion consists of three separate intrusive units. Important sulfide mineralization occurs in the base of the last intrusive unit, an elongated, layered ultramafic body composed of lherzolite at the bottom, olivine websterite in the middle and websterite at the top. Based on olivine-liquid equilibria and mass balance, the MgO and FeO contents in the parental magma for a lherzolite sample are estimated to be 8.71 and 8.36 wt.%, respectively. The Huangshanxi mafic-ultramafic intrusive rocks and the estimated "trapped liquids" for several olivine-orthopyroxene cumulate rocks all show light rare earth element enrichments relative to heavy rare earth elements and significant Nb depletions relative to Th and La, which are similar to the characteristics of coeval basalts in the region. The arc-like geochemical features are attributed to pre-Permian mantle metasomatism by slab-derived fluids. Partial melting of the previously-modified mantle is thought to have resulted from heating by upwelling asthenosphere associated with post-subduction lithosphere delamination or by mantle plume activity. The relationship between the Fo and Ni contents of olivine crystals from the Huangshanxi sulfide-poor ultramafic rocks (source mantle. Stratigraphic reversals in olivine Fo contents and bulk sulfide PGE tenors suggest that multiple magma replenishments occurred during the development of the Huangshanxi magmatic Ni-Cu sulfide deposit. The sulfide ore formation can be divided into two stages: a conduit stage during which immiscible sulfide droplets and olivine crystals were brought up by ascending magma

  12. The Late Pliocene mafic lavas from the Camusú Aike volcanic field (˜50°S, Argentina): Evidence for geochemical variability in slab window magmatism (United States)

    D'Orazio, M.; Innocenti, F.; Manetti, P.; Haller, M. J.; Di Vincenzo, G.; Tonarini, S.


    The Camusú Aike volcanic field (CAVF), part of the discontinuous N-S-trending belt of Cenozoic mafic lava formations that occur in a backarc position along extra-Andean Patagonia, is located in southern Patagonia (˜50°S, Santa Cruz province), approximately 70 km east of the extensive Meseta de las Vizcachas and just south of the upper Río Santa Cruz valley. The CAVF volcanics cover a surface of ˜200 km 2 and occur mainly as lava flows and scoria cones. They are subdivided into two groups: Group I volcanics are high-TiO 2, low-Mg# olivine-hypersthene-normative basalts and trachybasalts that erupted at about 2.9 Ma; Group II lavas are much less abundant, more primitive basaltic andesites that erupted at about 2.5 Ma. Both groups show a within-plate geochemical signature, though it is more marked in Group I lavas. The main geochemical characteristics, age, and location of CAVF volcanics are consistent with the slab window opening model proposed by different authors for the genesis of the Miocene-Recent mafic magmatism of Patagonia south of 46.5°S. The whole-rock geochemical and Sr-Nd isotope features of Group I lavas ( 87Sr/ 86Sr=0.7035-0.7037; 143Nd/ 144Nd=0.51288-0.51291) indicate a genetic link between these lavas and the primitive basalts in southernmost Patagonia (Pali Aike volcanic field and Estancia Glencross area), which have been interpreted as melting products of an isotopically depleted asthenosphere. The relatively evolved compositions of the erupted Group I magmas are modeled by a polybaric crystal fractionation process without significant involvement of crustal contamination. The more primitive Group II lavas are strongly depleted in incompatible elements, have slightly higher (LREE+Ba+Th+U)/HFSE ratios, and have more enriched Sr-Nd isotope compositions ( 87Sr/ 86Sr≈0.7039; 143Nd/ 144Nd≈0.51277) that are more akin to the Patagonian basalts farther to the north. The most likely explanation for the geochemical features of Group II lavas is the

  13. Primera edad U-Pb en circón usando LA-ICP-MS de un dique traquiandesítico emplazado en el granito tipo-A Los Árboles, Sierras Pampeanas Orientales First age U-Pb zircon LA-ICP-MS from trachyandesitic dyke emplaced in the A-type Los Árboles pluton, Eastern Sierras Pampeanas

    Directory of Open Access Journals (Sweden)

    Juan A Dahlquist


    Full Text Available La edad de cristalización de diques subvolcánicos emplazados en plutones graníticos carboníferos, Sierras Pampeanas Orientales, permanece incierta. La primera edad U-Pb en circón usando LA-ICP-MS obtenida en un dique que intruye al plutón Los Árboles (sierra de Fiambalá, revela una edad de cristalización de 311 ± 3 Ma.The crystallization age of subvolcanic dykes emplaced in Carboniferous granitic plutons of the Sierras Pampeanas Orientales remains uncertain. The first age precise U-Pb zircon LA-ICP-MS obtained from a dyke emplaced in the Los Árboles pluton (Sierra de Fiambalá yield a crystallization age of 311 ± 3 Ma.

  14. 白云鄂博矿床碳酸岩墙/脉和赋矿白云岩中流体包裹体研究%Fluid inclusion study of carbonatite dykes/veins and ore-hosted dolostone at the Bayan Obo ore deposit

    Institute of Scientific and Technical Information of China (English)

    秦朝建; 裘愉卓; 周国富; 王中刚; 张台荣; 肖国望


    Fluid inclusion study has been carried out for the carbonatite dykes/veins and the ore-hosted dolostone of the Bayan Obo superlarge REE-Fe-Nb deposit in order to provide the evidence and constraint for their origin. Based on the detailed geological observation and mineralogical investigation, the heating and cooling stage and laser Raman spectroscopy were mainly used for the laboratory work of this study. Following results have been obtained: (1) The discovery of melt and melt-fluid inclusions from carbonatite dykes/veins in the Dulahara and Jianshan Mountains, combined with the fine-grained (aplitic) texture of rocks, as well as the types and features of fluid inclusions such as CO2 enrichment, higher homogenization temperature, provides a direct evidence for the magmatic origin of these dykes/veins. (2) The carbonatite dykes/veins distributed in two regions, nearby the axis of the Kuangou anticline and in the east to Bayan Obo town, mainly show coarse-grained texture. No melt inclusion was found, and the fluid inclusions possess features of less CO2, lower homogenization temperature and higher salinity. They are tentatively identified as veins formed by some carbonate-rich hydrothermal solution. (3) Bedding carbonate layers/lens within the hanging wall and foot wall of ore-hosted dolostone, previously recognized as of magmatic origin, mainly composed by dolomite. The existence of single phase pure aqueous inclusions with very low homogenization temperature indicates their sedimentary origin. (4) The ore-hosted dolostone possesses apparent bedding and laminated structures. No melt inclusion was found, but, both single phase pure aqueous inclusions and CO2bearing multi-phase fluid inclusions coexisted in dolostones. In the direction towards ore-bodies, the homogenization temperature and CO2 contents of fluid inclusion show an increasing tendency. It indicates the sedimentary origin of dolostone superimposed by late fluid metasomatism.

  15. Structural, Petrographic and Geochemical Characteristics of Mafic Dikes Intrusive in Metasedimentary Rocks of the Crixás Greenstone Belt, Goiás

    Directory of Open Access Journals (Sweden)

    Hardy Jost


    Full Text Available Mafic dikes and stocks are a common feature in the Archean terrain of Goiás, Central Brazil, where they occur asswarms of 2.3 - 2.5 Ga within the granite-gneiss complexes (2.8 e 2.7 Ga, as well as intrusions related to the komatiite andbasalt flows of the greenstone belts lower stratigraphic units, but were unknown within the upper metasedimentary units.Detailed study of core sections from several drill-sites in the area of the Crixás greenstone belt gold deposits showed thatdike intrusion occurred after the main Paleoproterozoic deformation and metamorphism of the metasedimentary units,and literature data indicate that the magmatic zircons from the dikes yielded an age of 2,170 ± 17 Ma. Petrographic andgeochemical data show that they have the composition of epicratonic high-Ti diabases, which are similar to the largecontinental flood basalts of the Paraná Basin. The age of the intrusions may be correlated with the short time-interval of theRhyacian (2.20 to 2.17 Ga, during which successive localized episodes of mantle plume volcanism occurred on the Earth.

  16. U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goiás, Brazil (United States)

    Giovanardi, T.; Girardi, V. A. V.; Correia, C. T.; Sinigoi, S.; Tassinari, C. C. G.; Mazzucchelli, M.


    The Cana Brava Complex is the northernmost and least well known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises theNiquelândia and Barro Alto complexes. Available geochronological data from a range of techniques (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complexwhich suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age ofNiquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the embedding Palmeirópolis Sequence.

  17. Plume-proximal mid-ocean ridge origin of Zhongba mafic rocks in the western Yarlung Zangbo Suture Zone, Southern Tibet (United States)

    He, Juan; Li, Yalin; Wang, Chengshan; Dilek, Yildirim; Wei, Yushuai; Chen, Xi; Hou, Yunling; Zhou, Aorigele


    The >2000 km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of the Mesozoic Neotethyan oceanic lithosphere, and is divided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. Zircon U-Pb dating of a doleritic rock from the northern branch has revealed a concordant age of 160.5 ± 1.3 Ma. All of the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display consistent REE and trace element patterns similar to those of modern OIB-type rocks. The geochemical and Sr-Nd-Pb isotopic signatures of these OIB-type rocks from the western segment are identical with those of OIB-type and alkaline rocks from other ophiolite massifs along the central and eastern segments of the YZSZ, suggesting a common mantle plume source for their melt evolution. The enriched Sr-Nd-Pb isotopic character of the gabbroic dike rocks from the southern branch points to a mantle plume source, contaminated by subducted oceanic crust or pelagic sediments. We infer that the mafic rock associations exposed along the YZSZ represent the remnants of a Neotethyan oceanic lithosphere, which was developed as part of a plume-proximal seafloor-spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean.

  18. Determination of trace elements in Mesozoic dykes of the Serra do Mar by neutron activation; Determinacao de elementos tracos em diques mesozoicos da Serra do Mar por meio de ativacao com neutrons

    Energy Technology Data Exchange (ETDEWEB)

    Vicentini, Caio M.; Marques, Leila S., E-mail:, E-mail: [Universidade de Sao Paulo (IAG/USP), Sao Paulo, SP (Brazil). Instituto de Astronomia, Geofisica e Ciencias Atmosfericas; Figueiredo, Ana Maria G., E-mail: [Instituto de Pesquisas Energeticas e Nucleares (IPEN/CNEN-SP), Sao Paulo, SP (Brazil)


    The analysis of trace elements such as rare earths, Th, U, Ta, Hf, Ba, Rb and Ba, is a very important tool for petrogenetic studies. In order to study these processes in dykes of Enxame Serra do Mar (Coast of Sao Paulo and Rio de Janeiro), belonging to the Parana Magmatic Province (PMP), one of the most significant provinces of continental basalts in the world, were perform analyzes by neutron activation in these dikes. The technique, employed in Centro de Reator de Pesquisa of the Instituto de Pesquisas Energeticas e Nucleares, provided concentrations of trace elements with accuracy levels of 10% and 9%, which are suitable for petrogenetic studies. Due to the low concentrations of the elements analyzed, the experimental routine sample preparation processes covered very careful to avoid contamination. The samples investigated can be divided into four groups: basic rocks (SiO{sub 2} <55%) with Ti / Y> 500; basic rocks with Ti / Y <500; intermediate rocks (55% 63%). Dikes of intermediate and acid composition only occur at the Sao Sebastiao Island and adjacent coastal region. The concentrations of major and minor elements, as well as the abundance patterns of rare earths and other incompatible elements of these more differentiated rocks, show significant similarities with the type of the volcanic Chapeco, suggesting similar genesis, in other worlds, including also processes of crustal contamination.

  19. U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of mafic intrusive rocks in the Motuo, SE Tibet constrain on their petrogenesis and tectonic implication (United States)

    Pan, Fa-Bin; Zhang, Hong-Fei; Xu, Wang-Chun; Guo, Liang; Luo, Bi-Ji; Wang, Shuai


    Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA-ICP-MS U-Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50 Ma. These mafic intrusive rocks are characterized by variable SiO2 (44.60-57.60 wt.%), high Al2O3 (17.19-20.86 wt.%), and low MgO (1.85-5.38 wt.%) with Mg# of 31-55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr-Nd-Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7064 to 0.7086, εNd(t) = - 3.41 to + 1.22, and zircon εHf(t) = - 3.8 to + 6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr-Nd-Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr-Nd isotopic composition of the Motuo Late Cretaceous-Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas.

  20. Tectonic transition from Late Carboniferous subduction to Early Permian post-collisional extension in the Eastern Tianshan, NW China: Insights from geochronology and geochemistry of mafic-intermediate intrusions (United States)

    Zhang, Xiaoran; Zhao, Guochun; Eizenhöfer, Paul R.; Sun, Min; Han, Yigui; Hou, Wenzhu; Liu, Dongxing; Wang, Bo; Liu, Qian; Xu, Bing; Zhu, Chloe Yanlin


    The closure of the Junggar Ocean between the Central Tianshan and Junggar terranes is essential in understanding the final assembly of the southern Central Asian Orogenic Belt. This study presents new whole-rock geochemical, Sr-Nd and zircon U-Pb-Hf isotopic data for mafic-intermediate intrusions from the Central Tianshan block to provide robust constraints on the final closure of the ocean. LA-ICP-MS U-Pb dating on magmatic-type zircons yields weighted mean 206Pb/238U ages of ca. 310 Ma and ca. 290 Ma, which are interpreted as the crystallization ages of the intrusions. Petrographic and geochemical analyses of the Late Carboniferous mafic-intermediate rocks, characterized by typical subduction-related signatures, low Sm/Yb (plate. In contrast, the Early Permian mafic magmatism exhibits typical within-plate basalt affinities, such as high TiO2 (2.7-3.2 wt.%) contents, elevated Ti/V (86.0-115.1) and Zr/Y (4.9-9.3) ratios, OIB-like trace element patterns and high εNd(t) (+ 1.1 to + 4.5) and zircon εHf(t) (+ 3.0 to + 9.8) values. In association with previous investigations, we suggest that their protoliths were most probably derived from the partial melting of an asthenospheric mantle source in the garnet stability field, plausibly induced by asthenosphere upwelling during the slab break-off of the Junggar oceanic plate, which agrees well with the linear distributions of Permian mafic-ultramafic rocks in the Eastern Tianshan. Collectively, our data pinpoint a tectonic transition from oceanic subduction to post-collisional extension during Late Carboniferous to Early Permian time, probably triggered by the closure of the Junggar Ocean and subsequent arc-continent collision between the Central Tianshan and Junggar terranes that gave rise to the final assembly of the Eastern Tianshan.

  1. A back-arc setting for mafic rocks of the Honeysuckle Beds, southeastern N.S.W.: the use of trace and rare earth element abundances determined by INAA

    Energy Technology Data Exchange (ETDEWEB)

    Dadd, K.A. [University of Technology, Sydney, NSW (Australia)


    Major, trace and rare earth elements abundance in mafic rocks of the Honeysuckle Beds was determined by x-ray fluorescence and neutron activation analysis . A comparison with typical mid-ocean ridge basalt compositions reveals an enrichment in light rare earths elements (Ba, Rb, and Th) and depletion in Nb, Ta and Ti, consistent with modifications of the source by subduction-related fluids. 9 refs., 6 figs.

  2. Subduction of fore-arc crust beneath an intra-oceanic arc: The high-P Cuaba mafic gneisess and amphibolites of the Rio San Juan Complex, Dominican Republic (United States)

    Escuder-Viruete, Javier; Castillo-Carrión, Mercedes


    The Rio San Juan metamorphic complex (RSJC) exposes a segment of a high-P accretionary prism, built during Late Cretaceous subduction below the intra-oceanic Caribbean island-arc. In this paper we present new detailed maps, tectonostratigraphy, large-scale structure, mineral chemistry, in situ trace element composition of clinopyroxene (Cpx), and bulk rock geochemical data for representative garnet-free peridotites and mafic metaigneous rocks of the Cuaba and Helechal tectonometamorphic units of the southern RSJC. The Cuaba subcomplex is composed of upper foliated amphibolites and lower garnet amphibolites, retrograded (coronitic) eclogites, and heterogeneous metagabbros metamorphosed to upper amphibolite and eclogite-facies conditions. The lenticular bodies of associated peridotites are Cpx-poor harzburgites. The underlying Helechal subcomplex is composed of Cpx-poor harzburgites, Cpx-rich harzbugites, lherzolites and rare dunites. The presented data allow us to argue that the Cuaba subcomplex: (a) represents tectonically deformed and metamorphosed crust of the Caribbean island-arc, (b) contains fragments of its supra-subduction zone mantle, and (c) includes different geochemical groups of mafic protoliths generated by varying melting degrees of diverse mantle sources. These geochemical groups include mid-Ti tholeiites (N-MORB), normal IAT and calc-alkaline rocks, low-Ti IAT, metacumulates of boninitic affinity, and HREE-depleted IAT, that collectively record a multi-stage magmatic evolution for the Caribbean island-arc, prior to the Late Cretaceous high-P metamorphism. Further, these mafic protoliths present comparable geochemical features to mafic igneous rocks of the Puerca Gorda Schists, Cacheal and Puerto Plata complexes, all of them related to the Caribbean island-arc. These relations suggest that the southern RSJC complex represents part of the subducted fore-arc of the Caribbean island-arc, which experienced initial subduction, underplating below the arc

  3. Time scales of the dynamics of mafic-silicic magma interaction during solidification of the Isle au Haut gabbro-diorite layered complex, Maine. (United States)

    Patwardhan, K.; Marsh, B. D.


    The opportunity to examine in detail the in situ physical and chemical interaction of juxtapositioned mafic and silicic magma is vital to understanding the general dynamics of interaction of basaltic and silicic magmas. The Isle au Haut Igneous Complex (413 Ma) is one of many along the coast of Maine exhibiting a clear silicic-mafic magmatic association in the form of a layered gabbro-diorite sequence emplaced within granitic country rock (Chapman and Rhodes 1992, Wiebe 1994, 2001). Although, purely on the basis of density contrasts, the entire system should have undergone wholesale instability and mixing, that it has not and is instead arrested in a grossly unstable state with many indications of interaction while molten, makes this an especially valuable sequence. The entire sequence (~600 m thick) has five pairs of gabbro-diorite layers with individual units typically 20-40 m (gabbro) and 15-35 m (diorite) thick. Based on the mineral assemblage, chemical analyses and phase equilibria, the initial temperatures at the time of emplacement of the two magmas were ~1180 oC (gabbro) and ~1020 oC (diorite). A conductive thermal model for the solidification of a typical gabbroic layer (~20 m) indicates a timescale on the order of a few years, whereas the whole complex solidified in about a thousand years. Essentially crystal-free gabbro invaded the partially crystallized and lower temperature diorite, forming a series of large horizontal 'fingers' or layers. Evidence of this process is in the form of distinct chilled margins of gabbro against diorite and that the gabbro has not fallen into the diorite in spite of its higher density. Yet the diorite has spawned fields of evenly spaced finger-like siliceous plumes (~6 cm radius) penetrating the gabbro to high levels at every interface, reaching to within ~2 m of the upper gabbro contact. The flattened plumes at this horizon, compositionally similar to the underlying undifferentiated diorite, represent an early stage of

  4. The behaviour of the extended HFSE group (Nb, Ta, Zr, Hf, W, Mo) during the petrogenesis of mafic K-rich lavas: The Eastern Mediterranean case (United States)

    Kirchenbaur, M.; Münker, C.


    In arc lavas, elements of the extended high field strength element group (HFSE; Nb, Ta, Zr, Hf, W, and Mo) are valuable tracers to unravel magma source processes. These elements can also help to identify residual mineral assemblages in subducting slabs and in the mantle. Most high-precision studies on HFSE behaviour to date only focused on intra-oceanic arc suites and data for mafic lavas of the K-rich series (medium-K, high-K and shoshonitic) are scarce. Arguably, K-rich series are the most incompatible element-rich end-members of subduction zone magmatism, and they often record sediment recycling into the mantle. Understanding HFSE fractionation in K-rich lavas can therefore provide important insight into the global HFSE budget. Here we present a comprehensive extended HFSE dataset obtained by isotope dilution on well-characterised K-rich lavas from the Eastern Mediterranean, also including subducting sediment samples drilled during DSDP Leg 13 and ODP Leg 160 South and West of Crete. The volcanic samples include mafic calc-alkaline lavas from the active Aegean Island arc (Santorini) and post-collisional Tertiary lavas from SE Bulgaria. The Santorini lavas record a hydrous sediment melt-mediated source overprint of a depleted mantle source by components from the subducting African plate. The Bulgarian lavas tap lithospheric mantle sources that were overprinted by fluid- and melt-like subduction components during Eocene subduction of the African Plate. The sediments in this study comprise silts/sands, marl oozes, limestones and clay-rich debris flows and approximate the bulk sediment subducted beneath the Hellenic arc. The marked enrichment of all HFSE in the lavas is controlled by the composition of the subducted sediments as shown by low 176Lu/177Hf (0.008630-0.02433) and Zr/Nb (11.3-29.4), combined with variable εHf (-3 to +11) and elevated W contents (up to 2.45 ppm) in the lavas. Nevertheless, the lavas display unfractionated ratios of Nb/Ta and Zr/Hf of 12

  5. Trace-element fingerprints of chromite, magnetite and sulfides from the 3.1 Ga ultramafic-mafic rocks of the Nuggihalli greenstone belt, Western Dharwar craton (India) (United States)

    Mukherjee, Ria; Mondal, Sisir K.; González-Jiménez, José M.; Griffin, William L.; Pearson, Norman J.; O'Reilly, Suzanne Y.


    The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic-mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite-trondhjemite-granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)-peridotite-pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length ≈30-500 m; width ≈2-15 m) are hosted by the serpentinite-peridotite unit. Unaltered chromites from massive chromitites (>80 % modal chromite) of the Byrapur and Bhaktarhalli chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78-86 and moderate Mg# (100 Mg/(Mg + Fe2+)) of 45-55. In situ trace-element analysis (LA-ICPMS) of unaltered chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe3+# versus Ti/Fe3+# diagram, the Byrapur chromites plot in the field of suprasubduction zone (SSZ) chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the chromites, where the calculated parental melt of the Byrapur chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli chromite was derived from Archean high-Mg basalt. The major-element chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe-Ti-V-rich magnetite bands in layered intrusions, and magnetites from

  6. High-K andesite petrogenesis and crustal evolution: Evidence from mafic and ultramafic xenoliths, Egmont Volcano (Mt. Taranaki) and comparisons with Ruapehu Volcano, North Island, New Zealand (United States)

    Price, Richard C.; Smith, Ian E. M.; Stewart, Robert B.; Gamble, John A.; Gruender, Kerstin; Maas, Roland


    This study uses the geochemistry and petrology of xenoliths to constrain the evolutionary pathways of host magmas at two adjacent andesitic volcanoes in New Zealand's North Island. Egmont (Mt. Taranaki) is located on the west coast of the North Island and Ruapehu lies 140 km to the east at the southern end of the Taupo Volcanic Zone, the principal locus of subduction-related magmatism in New Zealand. Xenoliths are common in the eruptives of both volcanoes but the xenoliths suites are petrographically and geochemically different. Ruapehu xenoliths are predominantly pyroxene-plagioclase granulites derived from Mesozoic meta-greywacke basement and the underlying oceanic crust. The xenolith population of Egmont Volcano is more complex. It includes sedimentary, metamorphic and plutonic rocks from the underlying basement but is dominated by coarse grained, mafic and ultramafic igneous rocks. Gabbroic xenoliths (Group 1) are composed of plagioclase, clinopyroxene and amphibole whereas ultramafic xenoliths are dominated by amphibole (Group 2) or pyroxene (Group 3) or, in very rare cases, olivine (Group 4). In Group 1 xenoliths plagioclase and clinopyroxene and in some cases amphibole show cumulate textures. Amphibole also occurs as intercumulate poikilitic crystals or as blebs or laminae replacing pyroxene. Some Group 2 xenoliths have cumulate textures but near monomineralic amphibole xenoliths are coarse grained with bladed or comb textures. Pyroxene in Group 3 xenoliths has a polygonal granoblastic texture that is commonly overprinted by veining and amphibole replacement. Group 1 and most Group 2 xenoliths have major, trace element and Sr, Nd and Pb isotope compositions indicating affinity with the host volcanic rocks. Geochemical variation can be modelled by assimilation fractional crystallisation (AFC) and fractional crystallisation (FC) of basaltic parents assuming an assimilant with the composition of average crystalline basement and Group 1 xenoliths have

  7. Upper Paleozoic mafic and intermediate volcanic rocks of the Mount Pleasant caldera associated with the Sn-W deposit in southwestern New Brunswick (Canada): Petrogenesis and metallogenic implications (United States)

    Dostal, Jaroslav; Jutras, Pierre


    Upper Paleozoic ( 365 Ma) mafic and intermediate volcanic rocks of the Piskahegan Group constitute a subordinate part of the Mount Pleasant caldera, which is associated with a significant polymetallic deposit (tungsten-molybdenum-bismuth zones 33 Mt ore with 0.21% W, 0.1% Mo and 0.08% Bi and tin-indium zones 4.8 Mt with 0.82% Sn and 129 g/t In) in southwestern New Brunswick (Canada). The epicontinental caldera complex formed during the opening of the late Paleozoic Maritimes Basin in the northern Appalachians. The mafic and intermediate rocks make up two compositionally distinct associations. The first association includes evolved rift-related continental tholeiitic basalts, and the second association comprises calc-alkaline andesites, although both associations were emplaced penecontemporaneously. The basalts have low Mg# 0.34-0.40, smooth chondrite-normalized REE patterns with (La/Yb)n 5-6, primitive mantle-normalized trace element patterns without noticeable negative Nb-Ta anomalies, and their ɛNd(T) ranges from + 2.5 to + 2.2. The basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of 70-90 km. The calc-alkaline andesites of the second association have chondrite-normalized REE patterns that are more fractionated, with (La/Yb)n 7-8.5, but without significant negative Eu anomalies. Compared to the basaltic rocks, they have lower ɛNd(T) values, ranging from + 0.5 to + 1.9, and their mantle-normalized trace element plots show negative Nb-Ta anomalies. The ɛNd(T) values display negative correlations with indicators of crustal contamination, such as Th/La, Th/Nb and SiO2. The andesitic rocks are interpreted to have formed by assimilation-fractional crystallization processes, which resulted in the contamination of a precursor basaltic magma with crustal material. The parent basaltic magma for both suites underwent a different evolution. The tholeiitic basalts experienced shallow-seated fractional

  8. Impingement of Deep Mantle-Derived Upwelling Beneath Northern, Subducted Extension of the East Pacific Rise and Palinspastically Restored Cenozoic Mafic Magmatism in Western North America (United States)

    Rowley, D. B.; Moucha, R.; Forte, A. M.; Mitrovica, J. X.; Simmons, N. A.; Grand, S. P.


    Reconstruction of the retrodicted whole mantle flow, based on presently imaged distribution of variations in seismic velocity and its correlation to density (Simmons et al. 2009), over the past 30 Ma, in the North American fixed frame of reference, reveals that the northern, now subducted, extension of the East Pacific Rise is coincident with mantle buoyancy arising from near the core-mantle boundary and extending to the base of the lithosphere (Moucha et al. 2009 GRL, in press). Divergence of the reconstructed flow near the surface is independent of the surface plate(s) and results in predicted geological manifestations distinct from those predicted by traditional plate driven models of flow. Most particularly the retrodicted flow-related dynamic topography results in progressive west to east sweep of surface uplift, that is now centered on the Colorado Plateau (Moucha et al. 2008, 2009 GRL, in press). In addition, and the primary focus of the current study is the relationship between this retrodicted mantle-wide flow and the history of magmatism within the western U.S. and adjacent Mexico. There is a close spatial correlation between the impingement of upwelling with palinspastic restored western North America (McQuarrie and Wernicke, 2005) and onset and distribution of magmatism, particularly of mafic compositions as revealed in the Navdat ( database. Although often attributed to effects of opening of a slab window (Snyder and Dickinson, 1979, McQuarrie and Oskins, 2008) associated with continued plate-driven separation, this model predicts active mantle flow induced upwelling and divergence resulting in mantle melting that sweeps across east-northeast across southern Basin and Range to the Rio Grande Rift with time and as seen in the distribution of magmatism in this region.

  9. High-Mg adakitic rocks and their complementary cumulates formed by crystal fractionation of hydrous mafic magmas in a continental crustal magma chamber (United States)

    Ma, Qiang; Xu, Yi-Gang; Zheng, Jian-Ping; Sun, Min; Griffin, William L.; Wei, Ying; Ma, Liang; Yu, Xiaolu


    Understanding how adakitic magmas form is important for understanding the formation of the continental crust. Generating such high-Sr/Y rocks by crystal fractionation of basalts/basaltic andesites in magma chambers has been proposed in a wide range of tectonic settings. However, the complementary cumulates predicted by this scenario have rarely been observed. The late Triassic (~ 227 Ma) Ningcheng complex from the North China Craton is composed of a websterite - (Ol -/Hbl-) pyroxenite - gabbro unit and a quartz-diorite unit. They are interpreted as the products (cumulates and derivative melts, respectively) of fractionation from hydrous mafic magmas at mid- to lower-crustal pressures (4.9 ~ 8.3 kbar). The quartz diorites are high-Mg intermediate rocks with moderate SiO2 (57.0 ~ 62.9 wt%), high Mg# (> 49) and adakitic trace element signatures, such as high Sr (≥ 636 ppm) and light rare earth elements (REEs), low Y (≤ 17 ppm) and heavy REEs (Yb ≤ 1.8 ppm), lack of obvious Eu anomalies, and high Sr/Y (≥ 31) and La/Yb (≥ 24)). These adakitic signatures reflect differentiation of hydrous mantle-derived magmas in the deep crust, leaving behind a plagioclase-free residual solid assemblage in the early stages, which is represented by the coeval websterite-pyroxenite complex. This study therefore not only demonstrates that hydrous crystal fractionation is an important mechanism to form adakitic rocks, but also presents an example of a preserved fractionating system, i.e. high-Sr/Y rocks and their complementary cumulates. A geochemical comparison is made between representative adakitic rocks formed by fractionation of hydrous magmas and Archean TTGs. It is suggested that crystal fractionation is an efficient process for making Phanerozoic high Sr/Y rocks but was not responsible for the formation of Archean granitoids.

  10. Mineralogical and numerical approaches to establish the pre-eruptive conditions of the mafic Licán Ignimbrite, Villarrica Volcano (Chilean Southern Andes) (United States)

    Lohmar, Silke; Parada, Miguel; Gutiérrez, Francisco; Robin, Claude; Gerbe, Marie Christine


    The Licán Ignimbrite represents a large explosive basaltic andesite event in the eruptive history of the Villarrica Volcano, an active stratovolcano of the Southern Andean Volcanic Zone. A mineralogical study of this ignimbrite indicates important disequilibrium features. Resoption textures were observed around Ca-rich (An74-92; Group 1) and more sodic (An35-44; Group 2) plagioclase phenocryst cores. Rims on these phenocrysts as well as microlites grew with an intermediate composition (An47-63; Group 3). Low- and high-Mg clinopyroxene and orthopyroxene phenocrysts were also identified; most of them showing reverse Mg zoning. Based on these mineralogical data and thermodynamic modeling, an increase of about 200 °C in temperature (from ~ 900-1000° to ~ 1100-1150 °C), a decompression of ~ 400 bar (equivalent to ~ 1.5 km) and a decrease in H2O content in the melt of ~ 1.6 wt.% are identified during crystallization. Addition of hot magma from below would have caused heating of the reservoir with the consecutive ascent of magma pulses, H2O exsolution and convective mixing (compositional and/or thermal mixing). A numerical model using the Finite Element Method was developed to assess the response of the Licán Ignimbrite reservoir upon conductive heating by 200 °C by mafic underplating; this model indicates a rapid (tens of years) temperature homogenization of this system, with ascending magma plumes with velocities as high as 6 m/h. Because the Licán Ignimbrite erupted at the end of the Pleistocene glaciation, glacial unloading could have also contributed to depressurization of the reservoir.

  11. 唱凯堤决口应急封堵抢险施工技术与组织管理%Construction techniques and organizational management of emergency risk-elimination for closuring breaching of Changkai Dyke

    Institute of Scientific and Technical Information of China (English)



    本文详细介绍了江西抚州市唱凯堤决口的概况、应急封堵抢险的设计方案和施工方案.针对应急抢险工程工期紧、施工条件差、社会关注度高等特点,采取了加大设备、人力等资源投入等强化施工措施,提出了抢险工程的设备及人力等资源投入应为平时施工的2~3倍,且必须一次到位的观点.介绍了提高现场组织指挥人员级别、明确任务目标、加强抢险过程中的统计分析、加强关键环节组织协调等组织管理措施.对类似工程施工和应急抢险工作有一定的参考作用.%Both the design scheme and construction scheme of the emergency risk-elimination for closuring the breaching of Changkai Dyke in Fuzhou of Jiangxi Province are described herein in detail During the emergency construction, the intensified construction measures such as increasing the inputs of equipment and labor resource, reinforcing the statistical analysis; strengthening the organization and coordination of the key-links during the construction, etc. Are adopted in accordance with the characteristics of limited construction period, poor construction condition and high concern from the society. Therefore, the emergency risk-elimination of this time has some references for the similar emergency construction concerned.

  12. Emplacement age for the mafic-ultramafic plutons in the northern Dabie Mts. (Hubei): Zircon U-Pb, Sm-Nd and 40Ar/39Ar dating

    Institute of Scientific and Technical Information of China (English)

    WANG; Jianghai


    [1]Zhang, Q., Ma, B. L., Liu, R. X. et al., A remnant of continental lithospheric mantle above a subduction zone: Geochemical constraints on ultramafic rocks from Raobazhai area in Anhui Province, Sci. in China, Ser. B, 1995, 38(12): 1522- 1529.[2]Zhai, M. G., Cong, B. L., Petro-tectonics of the Sulu-Dabie Shan metamorphic belt, Central and East China, Sci. in China, Ser. D, 1996, 39(3): 319-328.[3]Li, S. G., Hart, S. R., Zheng, S. G. et al., Timing of collision between the North and South China Blocks-- the Sm-Nd isotopic age evidence, Sci. in China, Ser. B, 1989, 32(11): 1393-1400.[4]Ni, Y. H., Li, S. G., Sm-Nd ages of the syn-collisional mafic-ultramafic intrusions in the Dabie Mountains, Chinese Sci. Bull., 1998, 43(2): 160-163.[5]Chen, D. G., Wu, Y. B., Xia, Q. K. et al., Sm-Nd ages for the Jiaoziyan gabbro pluton and the characteristics of Nd isotopes, Acta Geosci. Sinica, 1997, 18(Suppl.): 9-11.[6]Jahn, B. M., Wu, F. Y., Lo, C. H. et al., Crust-mantle interaction induced by deep subduction of the continental crust: geochemical and Sr-Nd isotopic evidence from post-collisional mafic-ultramafic intrusions of the northern Dabie complex, central China, Chem. Geol., 1999, 157: 119-146.[7]Li, S. G., Jagoutz, E., Zhang, Z. Q. et al., Structure of high-metamorphic belt in the Dabie mountains and its tectonic implications, Chinese Sci. Bull., 1995, 40(Suppl.): 138-140.[8]Zhang, R. Y., Liou, J. G., Tsai, C. H., Petrogenesis of a high-temperature metamorphic terrain: a new interpretation for the north Dabie Shan, central China, J. Metamorphic Geol., 1996, 14: 319-333.[9]Wang, J. H., The Early Evolution of the Dabie Complex, with Special Reference to the Genetic Mechanism of Migmatites, Wuhan: China University of Geosciences Press, 1991, 1-140.[10]Suo, S. T., Zhong, Z. Q., You, Z. D., Extensional deformation of post ultrahigh-pressure metamorphism and exhumation process of ultrahigh-pressure metamorphic rocks in the Dabie massif

  13. Intensive hydration of the wedge mantle at the Kuril arc - NE Japan arc junction: implications from mafic lavas from Usu Volcano, northern Japan (United States)

    Kuritani, T.; Tanaka, M.; Yokoyama, T.; Nakagawa, M.; Matsumoto, A.


    The southwestern part of Hokkaido, northern Japan, is located at the junction of the NE Japan arc and the Kuril arc. The subducting Pacific plate under this region shows a hinge-like shape due to the dip change of the subducting plate along the trench. Because of the interest in this unique tectonic setting, this arc-arc junction has been the focus of extensive geophysical studies (e.g. Kita et al., 2010, Morishige and van Keken, 2014; Wada et al., 2015). This region is also known as an area in which magmatism has been intense; there are many active volcanoes such as Usu, Tarumae, and Komagatake, and large calderas including Toya, Shikotsu, and Kuttara. In this region, the temporal and spatial evolution of the volcanism and the chemical compositions of the volcanic rocks are well characterized (e.g. Nakagawa, 1992). However, the generation conditions of magmas have not been estimated for these volcanoes, probably because of the scarcity of basaltic products. Therefore, a possible link between the tectonic setting and the intense magmatism is still unclear. In this study, we carried out a petrological and geochemical study on mafic lavas (49.6-51.3 wt.% SiO2) from Usu Volcano, and estimated the conditions under which the magmas were generated. By application of a plagioclase-melt hygrometer to the plagioclase and the host magma, the water content of ~6.5 wt.% was obtained for the basaltic magma. Using this information, as well as the olivine maximum fractionation model (Tatsumi et al., 1983), the composition of the primary magma is estimated to be 47.9 wt.% SiO2, 15.1 wt.% MgO, and 4.1 wt.% H2O. Analyses using the multi-component thermodynamics suggest that the primary magma was generated in the source mantle with 0.9 wt.% H2O at 1310ºC and at 1.6 GPa. The water content of 0.9 wt.% of the source mantle is significantly higher than the estimates for the source mantle in the main NE Japan arc (hinge-like shape, and many fractures might have been developed in the slab

  14. Late Eocene-Early Oligocene two-mica granites in NW Turkey (the Uludağ Massif): Water-fluxed melting products of a mafic metagreywacke (United States)

    Topuz, Gültekin; Okay, Aral I.


    hornblende, (iii) occurrence in a ductile strike-slip zone, and (iv) relatively low magma temperatures can be accounted for by water-fluxed melting of a metagreywacke source with substantial mafic component at middle to lower crustal depths.

  15. A LA-ICP-MS study of minerals in the Rocche Rosse magmatic enclaves: Evidence of a mafic input triggering the latest silicic eruption of Lipari Island (Aeolian Arc, Italy) (United States)

    Davì, Marcella; De Rosa, Rosanna; Barca, Donatella


    The volcanic products of Lipari Island (Aeolian Arc, Italy) younger than 10 ka are mostly aphyric rhyolitic pumices and obsidians emitted during unusual strombolian-type eruptions, which ended with the emplacement of lava flows. The last volcanic activity on the island dates back to 1230 ± 40 AD, with the extrusion of Rocche Rosse (RR) obsidian lava flow. Recently, mafic enclaves of latitic to trachytic composition have been identified and an evolution process between these enclaves and the rhyolitic magma has been documented in detail [Davì, M., 2007. The Rocche Rosse rhyolitic lava flow (Lipari, Aeolian Islands): magmatological and volcanological aspects. Plinius, supplement to the European Journal of Mineralogy 33, 1-8]. In this work textural and trace-element investigation of mineral phases of the RR enclaves, such as crystals of clinopyroxene, olivine, plagioclase, alkali-feldspar and biotite, was carried out to delineate the most recent feeding system of the island, since such a reconstruction could be significant in terms of hazard forecasting. The results indicate that most of the mineral phases are reversely or oscillatory zoned with respect to both major and trace elements, suggesting an early crystallization under low fO 2 conditions from melts of intermediate composition, followed by a later growth from a more mafic (presumably shoshonitic-basaltic) magma than that from which their cores crystallized. Crystals of magnesium-rich pyroxene and forsteritic-rich olivine are indicative of the presence of this shoshonitic basaltic magma. Based on microanalytical data, it is suggested here that the feeding system of recent Lipari volcanic activity was characterized by a shoshonitic-basaltic magma originating from a deep reservoir, which may have evolved and stopped in the crust, generating zoned magma chambers at different depths, in which latitic and rhyolitic magmas reside. The sudden arrival of a new input of mafic melt may have interacted with these

  16. Geochemistry and age relationships of metamorphosed mafic sills from Sierra de Enmedio and Sierra de Carrascoy (Eastern Betic zone, Southeastern Spain

    Directory of Open Access Journals (Sweden)

    Torres-Roldán, R. L.


    Full Text Available The presence of fairly abundant shallow-intrusive mafic bodies is a common feature of the almagride units, a recently defined group of tectonic elements of the Eastern Betic Zone whose paleogeographic and tectonic interpretation is particuIarly controversial. In this paper we focuse on the bulk geochemistry (including K/Ar data and possible age relationships of tbese rocks, and discuss them in view of their significance regarding possible tectonic framework of emplacement and subsequent crustal evolution. The analyses (49-55% SiO2 point to a derivation from tholeiitic magmas that interacted with continental crust, as indicated by enrichment in the less compatible elements, such as Ba, Rb, Th and K, whose amounts are similar to those of well known continental tholeiitic provinces. Thus, the magmatic event is inferred to have been generated in a tectonic environment comparable to that of continental rifts. Emplacement may have taken place in Upper Triassic to Middle Jurassic times, as bracketed by their intrusion within Triassic beds and one whole-rock KIAr date of 178±4 Ma from a less altered sample. Much younger analytical K/Ar ages, scattering between 41±5 and 57±2 Ma, are obtained from common samples, however, reflecting an Eocene or younger metamorphic event that caused partial recrystallisation under low-grade greenchist, to actinolite-pumpelIyite facies conditions. As compared to other mafic complexes in the Betics, the observed chemical evidence for crustal contamination makes the almagride metabasites more similar to those in the external zona (e.g. the so-calIed «opbites» than in tbe Nevado-Filábride Ensemble, thus being consistent with proposals that correlate these units with the Subbetic in the Murcia area.La frecuente presencia de cuerpos de metabasitas es una característica común de las unidades almágrides, un grupo de unidades tectónicas de la Zona Bética Oriental, de reciente definición, cuya interpretaci

  17. Tephra architecture, pyroclast texture and magma rheology of mafic, ash-dominated eruptions: the Violent Strombolian phase of the Pleistocene Croscat (NE Spain) eruption. (United States)

    Cimarelli, C.; Di Traglia, F.; Vona, A.,; Taddeucci, J.


    A broad range of low- to mid-intensity explosive activity is dominated by the emission of ash-sized pyroclasts. Among this activity, Violent Strombolian phases characterize the climax of many mafic explosive eruptions. Such phases last months to years, and produce ash-charged plumes several kilometers in height, posing severe threats to inhabited areas. To tackle the dominant processes leading to ash formation during Violent Strombolian eruptions, we investigated the magma rheology and the field and textural features of products from the 11 ka Croscat basaltic complex scoria cone in the Quaternary Garrotxa Volcanic Field (GVF). Field, grain-size, chemical (XRF, FE-SEM and electron microprobe) and textural analyses of the Croscat pyroclastic succession outlined the following eruption evolution: activity at Croscat began with fissural, Hawaiian-type fountaining that rapidly shifted towards Strombolian style from a central vent. Later, a Violent Strombolian explosion included several stages, with different emitted volumes and deposit features indicative of differences within the same eruptive style: at first, quasi-sustained fire-fountaining with ash jet and plume produced a massive, reverse to normal graded, scoria deposit; later, a long lasting series of ash-explosions produced a laminated scoria deposit. The eruption ended with a lava flow breaching the western-side of the volcano. Scoria clasts from the Croscat succession ubiquitously show micrometer- to centimeter-sized, microlite-rich domains (MRD) intermingled with volumetrically dominant, microlite-poor domains (MPD). MRD magmas resided longer in a relatively cooler, degassed zone lining the conduit walls, while MPD ones travelled faster along the central, hotter streamline, the two interminging along the interface between the two velocity zones. The preservation of two distinct domains in the short time-scale of the eruption was favoured by their rheological contrast related to the different microlite

  18. A Late Carboniferous-Early Permian slab window in the West Junggar of NW China: Geochronological and geochemical evidence from mafic to intermediate dikes (United States)

    Yin, Jiyuan; Long, Xiaoping; Yuan, Chao; Sun, Min; Zhao, Guochun; Geng, Hongyan


    The Late Carboniferous to Early Permian is a critical period for the formation of particular high-temperature magmatic associations in West Junggar, NW China and the geodynamic processes of ridge subduction and related slab windows in the Central Asian Orogenic Belt. Mafic to intermediate dikes, exposed in the central and southeastern West Junggar extensively, mainly consist of Nb-enriched and magnesian suites. The Nb-enriched dikes (305 Ma) are characterized by high Nb contents (5.63-9.08 ppm) and large variations of major element contents, exhibiting high-K to medium-K calc-alkaline characteristics with high Na2O/K2O (1.33-3.85) and low Sr/Y (9-32) ratios. The dikes have moderate Mg# (44.6-52.9), LREE-rich and sub-horizontal HREE patterns ((La/Yb)N = 3.05-7.18; (Gd/Yb)N = 1.43-1.78) with weak Eu anomalies (Eu*/Eu = 0.81-1.10) and positive εNd (t) values (+ 5.2 to + 8.0), and likely originate from partial melting of mantle peridotite that was metasomatized by subducted sediment-derived melts and slab fluids. The medium-K magnesian dikes (305 Ma) are characterized by high MgO (3.05-7.28 wt.%), Cr (22.3-311 ppm) and Ni (32.6-199 ppm) contents and all plot in the field of high-Mg andesite. Some have very high Mg numbers (Mg# > 60), suggesting a primitive melt composition. These samples display positive Ba, Sr, K and negative Nb-Ta-Ti anomalies as well as strongly-depleted HREE patterns with weak positive Eu anomalies. They have high Sr/Y (27-126) and relatively high εNd (t) values (+ 6.0 to + 8.3), which suggest a similar petrogenesis as the Nb-enriched dikes, but were metasomatized by adakitic melts. The high-K magnesian dikes (284 Ma) are high-K calc-alkaline and have lower MgO (3.63-5.57 wt.%) compared with the medium-K suite. They have high Sr (422-819 ppm), Sr/Y (26-62) ratios and εNd (t) values (+ 7.0 to + 7.1) with relatively flat REE patterns ((La/Yb)N = 2.4-5.5), probably originating from partial melting of K-enriched mantle wedge peridotite metasomatized

  19. Isotopic and trace element geochemistry of alkalic-mafic-ultramafic-carbonatitic complexes and flood basalts in NE India: Origin in a heterogeneous Kerguelen plume (United States)

    Ghatak, Arundhuti; Basu, Asish R.


    The Archean East Indian cratonic margin was affected by the Kerguelen plume (KP) ˜117 Ma, causing flood-basalt eruptions of the Rajmahal-Bengal-Sylhet Traps (RBST). The RBST cover ˜one million km2 in and around the Bengal Basin as alkalic-ultrabasic intrusives in the west and Sikkim in the north, and Sylhet basalts and alkalic-carbonatitic-ultramafic complexes in the Shillong plateau - Mikir hills farther east of the Rajmahal-Bengal Traps. We provide new Nd-Sr-Pb-isotopic and trace element data on 21 unreported discrete lava flows of the Rajmahal Traps, 56 alkalic-carbonatitic-mafic-ultramafic rocks from four alkalic complexes, and three dikes from the Gondwana Bokaro coalfields, all belonging to the RBST. The data allow geochemical correlation of the RBST with some contemporaneous Kerguelen Plateau basalts and KP-related volcanics in the southern Indian Ocean. Specifically, the new data show similarity with previous data of Rajmahal group I-II basalts, Sylhet Traps, Bunbury basalts, and lavas from the southern Kerguelen Plateau, indicating a relatively primitive KP source, estimated as: ɛNd(I) = +2, 87Sr/86Sr(I) = 0.7046, with a nearly flat time-integrated rare earth element (REE) pattern. We model the origin of the uncontaminated RBST basalts by ˜18% batch melting with a 2× chondritic KP source in the spinel-peridotite stability depths of 60-70 km in the mantle. The new geochemical data similar to the Rajmahal group II basalts indicate a light REE enriched average source at ɛNd(I) = -5, 87Sr/86Sr(I) = 0.7069. Our geochemical modeling indicates these lavas assimilated granulites of the Eastern Ghats, reducing the thickness of the continental Indian lithosphere. Lack of an asthenospheric MORB component in the RBST province is indicated by various trace element ratios as well as the Nd-Sr isotopic ratios. Three alkalic complexes, Sung, Samchampi, and Barpung in NE India, and one in Sikkim to the north are of two groups: carbonatites, pyroxenites, lamproites

  20. Melt inclusions in the olivine from the Nantianwan intrusion: Implications for the parental magma of Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions of the ∼260 Ma Emeishan large igneous province (SW China) (United States)

    Zhang, Le; Ren, Zhong-Yuan; Wang, Christina Yan


    Olivine-hosted melt inclusions provide an archive of the parental magma and early magma history that is unavailable from bulk-rock analyses of cumulates. For those olivine-bearing mafic-ultramafic intrusions, a combined in situ analysis of major elements and Pb isotopic compositions for the melt inclusions and host olivine crystals may provide an effective way to understand the nature of the parental magma of the intrusions. In this study, we take the Nantianwan intrusion in the Emeishan large igneous province (SW China) as an example to analyze the melt inclusions and the host olivine. The Nantianwan intrusion is mainly composed of gabbronorite, with minor olivine gabbro. The olivine crystals in the olivine gabbro have Fo contents varying from 81.1 to 89.2 and Ni from 0.05 to 0.30 wt.%. The melt inclusion hosted in the most Mg-rich olivine has 50.9 wt.% SiO2, 1.0 wt.% TiO2, 15.1 wt.% MgO and 2.9 wt.% Na2O + K2O, indicating that the parental magma of the intrusion was of high-Mg basaltic composition. The melt inclusions overall have 208Pb/206Pb ratios of 2.0567-2.1032 and 207Pb/206Pb of 0.8287-0.8481, similar to the Pb isotopic compositions of the Emeishan flood basalts and consistent with insignificant crustal contamination. Given that the Nantianwan intrusion contains the most Mg-rich olivine among the Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions in the Emeishan LIP, we infer that the composition of the melt inclusion in the most Mg-rich olivine from the Nantianwan intrusion may represent the least evolved parental magma of the Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions in the Emeishan LIP. This can be further used to constrain the magma process related to Ni-Cu-(PGE) sulfide mineralization.

  1. Diagnosis and Differential Diagnosis of the Dyke-Davidoff-Masson Syndrome by CT and MRI: Report of 3 Cases and Literature Review%戴克-大卫杜夫-梅森综合征CT、MRI诊断及鉴别诊断(附3例报告并文献复习)

    Institute of Scientific and Technical Information of China (English)

    熊伟; 蒋秋燕; 欧波; 龙从杰


    Objective To discuss the clinical and imaging findings of Dyke-Davidoff-Masson syndrome(DDMS).Methods The clinical and imaging features of DDMS in 3 patients were analyzed retrospectively with literature review.Results 3 patients suffered from cerebral hemiatrophy with ipsilateral ventricular dilatation and displacement of the midline structures, 2 cases with encephalomalacia and porencephaly,one case with thalamus hypoplasia and lentiform nucleus hypoplasia were found.Unilateral calvarial thickening was seen in 3 cases.There were hyperpneumatization of paranasal sinuses in 2 cases,hypoplasia of the cranial fossa in 2 cases,hyperpneumatization of mastoid cells and elevation of the petrous ridge in one case.Conclusion DDMS displays relatively the typical imaging features,and it can be diagnosed correctly according to imaging findings in combination with clinical characters.%目的 探讨戴克-大卫杜夫-梅森综合征(Dyke-Davidoff-Masson syndrome,DDMS)的临床及影像学表现.方法 回顾性分析3例DDMS患者的临床及影像学资料并作文献复习.结果 3例均出现单侧大脑萎缩,同侧侧脑室扩大,中线结构向患侧偏移,1例出现脑软化及脑穿通畸形,1例出现丘脑及豆状核萎缩.3例出现患侧颅骨增厚,2例出现鼻窦过度发育,2例颅窝缩小,1例乳突窦增大及岩骨嵴抬升.结论 DDMS具有比较典型的影像学表现,结合临床表现影像学可以正确诊断.

  2. Experience with fermentation of grass and grass silage from extensively used grassland. Feasibility study on monofermentation of grass silage from contaminated sites - biomass from grasslands of the Elbe dyke foreland; Betriebserfahrungen mit der Vergaerung von Gras und Grassilagen von extensiv genutztem Gruenland. Machbarkeitsuntersuchung zur Monovergaerung von Grassilagen schadstoffkontaminierter Standorte am Beispiel der eingesetzten Biomasse von Gruenlandflaechen aus dem Deichvorland der Elbe

    Energy Technology Data Exchange (ETDEWEB)

    Heuer, Hans-Juergen [Landwirtschaftskammer Niedersachsen, Bezirksstelle Uelzen (Germany)


    This feasibility study was concluded in 2011; monofermentation of grass silage from contaminated sites of the Elbe dyke foreland was investigated. It was found that a biogas operated in monofermentation of grass silage from extensively used grasslands is technically and economically feasible in batch operation. It was also shown that the fermentation residue can be used as agricultural fertilizer, provided that it is first turned in a compost turning unit and then worked into the soil. In view of the high pollutant concentration of the soils in the Elbe dyke foreland, the results of the project give the agricultural businesses in the Elbe valley grasslands an alternative use of the land. [German] Die Machbarkeitsuntersuchung zur Monovergaerung von Grassilagen schadstoffkontaminierter Standorte am Beispiel der eingesetzten Biomasse von Gruenlandflaechen aus dem Deichvorland der Elbe wurde Anfang Mai 2011 abgeschlossen. Im Ergebnis wurde herausgearbeitet, dass sich eine Biogaslage im Monovergaerungsverfahren mit ausschliesslich nur Grassilagen von extensiv gefuehrten Gruenlandflaechen technisch und wirtschaftlich in einer Batch-Anlage betreiben laesst. Auch wurde der Nachweis gefuehrt, dass die Biomasse aus dem Deichvorland der Elbe als Gaerrest auf das Gruenland im Deichvorland unter Beruecksichtigung von naturschutzfachlichen Vorgaben und nach den Vorgaben der guten fachlichen Praxis beim Duengen zurueckgefuehrt werden kann. Vor einer Aufbringung auf dem Gruenland sollte der feste Gaerrest jedoch mit einem Kompost-Umsetzer aufbereitet werden. Nach dem Ausstreuen auf dem Gruenland ist der Gaerrest mit einer Wiesenschleppe einzureiben. Vor dem Hintergrund der hohen Schadstoffbelastung der Boeden im Deichvorland der Elbe, tragen die im Projekt erarbeiteten Ergebnisse dazu bei, den landwirtschaftlichen Betriebsleitern in der Elbtalaue eine alternative Nutzung zur bisherigen Lebensmittelproduktion unterbreiten zu koennen.

  3. Petrology and geochemistry of mafic magmatic rocks from the Sarve-Abad ophiolites (Kurdistan region, Iran): Evidence for interaction between MORB-type asthenosphere and OIB-type components in the southern Neo-Tethys Ocean (United States)

    Saccani, Emilio; Allahyari, Khalil; Rahimzadeh, Bahman


    The Sarve-Abad (Sawlava) ophiolites crop out in the Main Zagros Thrust Zone and represent remnants of the Mesozoic southern Neo-Tethys Ocean that was located between the Arabian shield and Sanandaj-Sirjan continental block. They consist of several incomplete ophiolitic sequences including gabbroic bodies, a dyke complex, and pillow lava sequences. These rocks generally range from sub-alkaline to transitional character. Mineral chemistry and whole-rock geochemistry indicate that they have compositions akin to enriched-type mid-ocean ridge basalts (E-MORB) and plume-type MORB (P-MORB). Nonetheless, the different depletion degrees in heavy rare earth elements (HREE), which can be observed in both E-MORB like and P-MORB like rocks enable two main basic chemical types of rocks to be distinguished as Type-I and Type-II. Type-I rocks are strongly depleted in HREE (YbN 9.0). Petrogenetic modeling shows that Type-I rocks originated from 7 to 16% polybaric partial melting of a MORB-type mantle source, which was significantly enriched by plume-type components. These rocks resulted from the mixing of variable fractions of melts generated in garnet-facies and the spinel-facies mantle. In contrast, Type-II rocks originated from 5 to 8% partial melting in the spinel-facies of a MORB-type source, which was moderately enriched by plume-type components. A possible tectono-magmatic model for the generation of the southern Neo-Tethys oceanic crust implies that the continental rift and subsequent oceanic spreading were associated with uprising of MORB-type asthenospheric mantle featuring plume-type component influences decreasing from deep to shallow mantle levels. These deep plume-type components were most likely inherited from Carboniferous mantle plume activity that was associated with the opening of Paleo-Tethys in the same area.

  4. Rb-Sr isotopic composition of granites in the Western Krušné hory/Erzgebirge pluton, Central Europe: record of variations in source lithologies, mafic magma input and postmagmatic hydrothermal events (United States)

    Dolejš, David; Bendl, Jiří; Štemprok, Miroslav


    The late Variscan (327-318 Ma) Western Krušné hory/Erzgebirge pluton (Czech Republic and Germany) represents a multiply emplaced intrusive sequence ranging from low-F biotite monzogranites (with rare minor bodies of gabbrodiorites and granodiorites) to high-F topaz-zinnwaldite alkali-feldspar granites. This granite suite is characterized by progressively increasing concentrations of incompatible elements (Li, Rb, F), monotonous decrease in mafic components and compatible elements (FeOtot, MgO, TiO2, CaO, Sr) with increasing silica. Consequently, this leads to extreme variations in the Rb/Sr ratios (0.52 to 59), which impose highly variable 87Rb/86Sr and 87Sr/86Sr signatures. The low-F biotite monzogranites represent isotopically heterogeneous mixture with (87Sr/86Sr)323 = 0.707-0.709 between partial melts from the Saxothuringian metasediments and mantle-derived mafic precursors. The medium-F two-mica microgranites show variable (87Sr/86Sr)323 = 0.708-0.714, indicating involvement of multiple precursors and more mature crustal protoliths. The evolved high-F topaz-zinnwaldite alkali-feldspar granites were derived from a precursor with (87Sr/86Sr)320 = 0.707-0.708 at 324-317 Ma by differentiation, which produced the extreme Rb/Sr enrichment and variations. The Li/Rb ratios remain nearly constant (~0.5), thus insensitive to the degree of geochemical differentiation. In comparison to terrestrial variations, the high Li/Rb values indicate derivation of granitic magmas from predominantly sedimentary precursors, in accord with 7Li-6Li and 143Nd-144Nd isotope composition reported previously. The Rb-Sr element variations in each granite unit are sligthly different and indicate ascent and emplacement of separate magma batches, which do not form a single liquid line of descent. We consider the enrichment of granites in incompatible elements (Li, Rb, F) and compatible depletion of ferromagnesian components, CaO and Sr as a combined effect of multiple precursors, changes in

  5. The Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion, northeast of Beijing (North China): Implications for the mantle source of the giant Late Mesozoic magmatic event in the North China Craton (United States)

    Liu, Ping-Ping; Zhou, Mei-Fu; Yan, Dan-Ping; Zhao, Guo-Chun; Su, Shang-Guo; Wang, Xiao-Lin


    The Early Cretaceous Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion in the Yanshan Belt northeast of Beijing is coeval with the giant Late Mesozoic igneous province in the eastern part of the North China Craton (NCC). This magmatic event was associated with lithospheric thinning and thus the igneous rocks have been used to characterize the nature of the Mesozoic mantle beneath the NCC. The Shangzhuang mafic pluton intruded a large granodioritic complex and crystallized at ~ 850-872°C at a depth of 13-14 km. It is composed, from the base upward, of troctolite, Fe-Ti oxide-bearing gabbronorite and gabbro. Rocks from this intrusion have low initial 87Sr/86Sr ratios (0.7053-0.7058), negative initial εNd values (- 9.4 to - 10.7), highly differentiated LREE and nearly flat HREE patterns indicative of an EMI-like mantle source unaffected by upper crustal contamination. The occurrence of Fe-Ti oxide ore layers, magnetite-ilmenite exsolution lamellae in hornblende and high TiO2 contents of the silicate rocks indicate that they formed from Fe- and Ti-rich ferrobasaltic magmas, which may have been generated by addition of magmas from a deeper mantle source. The presence of orthopyroxene, high-Mg ilmenite (up to 8.5 wt %), hornblende, biotite and high oxygen fugacities calculated from coexisting titanomagnetite-ilmenite pairs can be explained by derivation from an enriched EMI-type mantle modified by fluids from a subducted slab and mixed with asthenospheric or deeper-mantle materials in an extensional setting. Exposure of the complex occurred during large-scale uplift (at least 13 km) and exhumation of the Yanshan orogenic belt in the Early Cretaceous. Chemical metasomatism triggered by water and enhanced by heat from a deep magma source may have played an important role in removing the ancient cratonic root, generating partial melting of the lithospheric mantle and producing coeval magmatic activity in the Mesozoic eastern NCC.

  6. Sm-Nd data for mafic-ultramafic intrusions in the Svecofennian (1.88 Ga Kotalahti Nickel Belt, Finland – implications for crustal contamination at the Archaean/Proterozoic boundary

    Directory of Open Access Journals (Sweden)

    Hannu V. Makkonen


    Full Text Available Sm-Nd data were determined for eight mafic-ultramafic intrusions from the Svecofennian (1.88 Ga Kotalahti Nickel Belt, Finland. The intrusions represent both mineralized and barren types and are located at varying distances from the Archaean/Proterozoic boundary.The samples for the 23 Sm-Nd isotope analyses were taken mostly from the ultramafic differentiates. Results show a range in initial εNd values at 1880 Ma from -2.4 to +2.0. No relationship can be found between the degree of Ni mineralization and initial εNd values, whilea correlation with the geological domain and country rocks is evident. The Majasaari and Törmälä intrusions, which have positive εNd values, were emplaced within the Svecofennian domain in proximity to 1.92 Ga tonalitic gneisses, which have previously yielded initialεNd values of ca. +3. In contrast, the Luusniemi intrusion, which has an εNd value of -2.4 is situated close to exposed Archaean crust. Excluding two analyses from the Rytky intrusion, all data from the Koirus N, Koirus S, Kotalahti, Rytky and Kylmälahti intrusions, withinerror limits, fall in the range -0.7 ± 0.3. The results support the concept of contamination by Archaean material in proximity to the currently exposed craton margin. The composition of the proposed parental magma for the intrusions is close to EMORB, with initialεNd values near +4.

  7. A non-plume model for the Permian protracted (266-286 Ma) basaltic magmatism in the Beishan-Tianshan region, Xinjiang, Western China (United States)

    Xue, Sheng-Chao; Li, Chusi; Qin, Ke-Zhang; Tang, Dong-Mei


    The convenient mantle plume model for the Permian protracted mafic-ultramafic intrusions and mafic dykes (266-286 Ma) in the Beishan-Tianshan region, northern Xinjiang, western China can be rejected, because their temporal-spatial distribution does not show a hotspot track predicted by such model. New zircon U-Pb ages reveal that two small mafic dyke clusters (Podong, 280.5 ± 2 Ma; Luodong, 266.2 ± 3.2 Ma) that are separated by only ~ 20 km in the Pobei area, the southernmost part of the Beishan-Tianshan region, have a large age difference of ~ 18 Ma. The older mafic dykes are characterized by nearly flat mantle-normalized rare-earth-element patterns, pronounced negative Nb-Ta anomalies and positive εNd(t) values from 5.5 to 7.5, similar to the majority of the Permian mafic-ultramafic intrusions in the region. The younger mafic dykes are characterized by significant light rare-earth-element enrichments as well as pronounced negative Nb-Ta anomalies, plus lower εNd(t) (- 1.2 to 2.6) values and higher initial 87Sr/86Sr ratios than the older mafic dykes. The observed compositional variations can be explained by source mantle heterogeneity plus different degrees of crustal contamination. Overall, the Permian mafic-ultramafic rocks in the Beishan-Tianshan region are geochemically consistent with the products of basaltic magmatism induced by lithospheric delamination and asthenosphere upwelling in a convergent tectonic zone.

  8. 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:, e-mail:; Girardi, Vicente Antonio Vitorio [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica], e-mail:; 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:


    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)

  9. Geochemical and isotopic studies of the Lady of the Lake Intrusion and associated tobacco root Batholith: Constraints on the genetic relation between Cretaceous mafic and silicic magmatism in Southwestern Montana (United States)

    Sarkar, Arindam; Brophy, James G.; Ripley, Edward M.; Li, Chusi; Kamo, Sandra L.


    Small volumes of alkalic mafic intrusions are spatially associated with Cretaceous to Early Tertiary granodioritic to granitic intrusions in the batholithic province of southwestern Montana. The mafic rocks generally occur near the contacts of the Boulder, Pioneer, and Tobacco Root Batholiths with country rocks, but their genetic relation with the batholiths is uncertain. The Lady of the Lake Intrusion is a small layered body composed of melagabbro and gabbro that occurs along the south-central margin of the Tobacco Root Batholith near its contact with Archean country rocks. A diorite unit, spatially distinct from the granodiorite/quartz monzonite of the Batholith intrudes the gabbroic rocks of the Lady of the Lake Intrusion. Zircon crystals from the melagabbro and diorite units give U-Pb ages that are very similar to that of the Tobacco Root Batholith at 74.88 ± 0.17 Ma and 76.24 ± 0.08 Ma, respectively. Mineral chemistry, whole rock major and trace element compositions, and oxygen and sulfur isotope ratios have been utilized to evaluate the genetic relation between the Lady of the Lake Intrusion, the diorite, and the Tobacco Root Batholith. No significant variation in the composition of clinopyroxene is observed in different rock units of the Lady of the Lake Intrusion. Minor olivine with Fo 64 in the melagabbro unit is interpreted to represent early crystallization in the base of the intrusion. Whole rock major and trace element compositions, as well as results from modeling using the MELTS program, are consistent with the premise that the diorite was produced by fractional crystallization of the same magma that was parental to the gabbros of the Lady of the Lake Intrusion. Both whole rock chemistry and oxygen isotopes support the interpretation that the parental magma was an uncontaminated mantle-derived basaltic magma. In contrast, trace element and oxygen isotopes indicate that the quartz monzonitic and granodioritic rocks of the Tobacco Root Batholith and

  10. Natural factors and mining activity bearings on the water quality of the Choapa basin, North Central Chile: insights on the role of mafic volcanic rocks in the buffering of the acid drainage process. (United States)

    Parra, Amparo; Oyarzún, Jorge; Maturana, Hugo; Kretschmer, Nicole; Meza, Francisco; Oyarzún, Ricardo


    This contribution analyzes water chemical data for the Choapa basin, North Central Chile, for the period 1980-2004. The parameters considered are As, Cu Fe, pH, EC, SO₄⁻², Cl⁻¹, and HCO[Formula: see text], from samples taken in nine monitoring stations throughout the basin. Results show rather moderate contents of As, Cu, and Fe, with the exception of the Cuncumén River and the Aucó creek, explained by the influence of the huge porphyry copper deposit of Los Pelambres and by the presence of mining operations, respectively. When compared against results obtained in previous researches at the neighboring Elqui river basin, which host the El Indio Au-Cu-As district, a much reduced grade of pollution is recognized for the Choapa basin. Considering the effect of acid rock drainage (ARD)-related Cu contents on the fine fraction of the sediments of both river basins, the differences recorded are even more striking. Although the Los Pelambres porphyry copper deposit, on the headwaters of the Choapa river basin, is between one and two orders of magnitude bigger than El Indio, stream water and sediments of the former exhibit significantly lower copper contents than those of the latter. A main factor which may explain these results is the smaller degree of H( + )-metasomatism on the host rocks of the Los Pelambres deposit, where mafic andesitic volcanic rocks presenting propylitic hydrothermal alteration are dominant. This fact contrast with the highly altered host rocks of El Indio district, where most of them have lost their potential to neutralize ARD.

  11. U-Pb zircon geochronology and phase equilibria modelling of a mafic eclogite from the Sumdo complex of south-east Tibet: Insights into prograde zircon growth and the assembly of the Tibetan plateau (United States)

    Weller, O. M.; St-Onge, M. R.; Rayner, N.; Waters, D. J.; Searle, M. P.; Palin, R. M.


    The Sumdo complex is a Permian-Triassic eclogitic metamorphic belt in south-east Tibet, which marks the location of a suture zone that separates the northern and southern Lhasa terranes. An integrated geochronological and petrological study of a mafic eclogite from the complex has constrained its tectonometamorphic history and provides a case study of zircon growth in eclogite as a product of prograde dissolution-precipitation. In situ U-Pb geochronology indicates that the eclogite contains a single population of zircon with a crystallisation age of 273.6 ± 2.8 Ma. The morphology and chemistry of the zircon grains are consistent with growth by dissolution-precipitation of protolith magmatic zircon. The presence of zircon grains as inclusions in the cores of peak phases indicates that zircon dissolution-precipitation occurred during prograde metamorphism, and calculated pressure and temperature conditions over which mineral inclusions in zircon are stable suggest that the zircon most likely precipitated at 15.5-16.5 kbar and 500-560 °C. Subsequent peak metamorphism is calculated to have reached pressure-temperature conditions of 27 ± 1 kbar and 670 ± 50 °C. Previous studies, which have documented a range of peak metamorphic conditions from high- to ultrahigh-pressure at c. 266-230 Ma, indicate that the Sumdo complex is a composite belt that experienced protracted eclogite exhumation. The results of this study are consistent with this interpretation, and extend the age range of high-pressure metamorphism in the complex to over 40 Myr. Analysis of published pressure-temperature-time data indicates two systematic behaviours within this spread. First, peak metamorphic temperatures declined over time. Second, eclogite exhumation occurred in two discrete intervals: soon after formation, and during the demise of the subduction zone. The latter behaviour serves as a reminder that eclogite exhumation is the exception rather than the rule.

  12. Design and construction of a rockfill spur dyke on the Rupert River (PK 85) under the Eastmain1-A/LaSarcelle/Rupert diversion project; Conception et construction d'un epi en enrochement sur la riviere Rupert (PK 85) dans le cadre du projet Eastmain-1-A/LaSarcell/derivation Rupert

    Energy Technology Data Exchange (ETDEWEB)

    Pelletier, Pierre; Fecteau Sebastien, Thibodeau; Patrick [GENIVAR, Quebec, (Canada); Cote, Pierre; Grenon, Alain [Societe d' Energie de la Baie James, Montreal, (Canada)


    The Eastmain-1-A/Sarcelle plants and Rupert diversion project involved the partial diversion of the Rupert River in the direction of the La Grande complex. This diversion point on the Rupert River is located at the 314th kilometre, where a dam with a regulation works was constructed. A network of nine hydraulic works was designed and constructed to maintain the water levels in the slow flow section of the river to limit the environmental impact. A rockfill spur dyke was constructed at the 85th kilometre of the Rupert River, for control of the water levels over a distance of 10.8 kilometres. This paper presented the challenges facing the construction operation of this hydraulic work on the Rupert River. The design of this structure was first simulated by 2-D hydrodynamic models. A scale model built at the 1:40 scale was used to assess the stability of the temporary piers created for shelter during the construction process.

  13. 基于MlKE11水动力模型的大洋河丹东有堤段行洪能力分析%Analysis on Flood Discharge Capacity of Dayang River Dandong Section with Dyke Based on MIKE11 Hydrodynamic Model

    Institute of Scientific and Technical Information of China (English)



    河道现状行洪能力是河道所在地区防洪安全的核心,也是防汛主管管部门汛期指导防汛的重要依据。为分析大洋河丹东有堤段的行洪能力,采用水动力模型MIKE11计算、模拟河道及河网的一维洪水演进过程,准确判断其行洪能力,为保护沿岸居民防洪安全提供理论基础。%Flood discharge capacity of watercourse status is the core of flood control safety in the watercourse area, and is also as an important basis for flood prevention authorities directing the work of flood prevention during flood season. In order to analyze the flood discharge capacity of Dayang River Dandong section with dyke, we used hydrodynamic model MIKE11 to calculate and imitate one-di-mensional flood routing process of watercourse and river network, and estimated the flood discharge capacity correctly, providing the theoretical basis for preventing coastwise residents flood control safety.

  14. Low grade metamorphism of mafic rocks (United States)

    Schiffman, Peter


    Through most of this past century, metamorphic petrologists in the United States have paid their greatest attention to high grade rocks, especially those which constitute the core zones of exhumed, mountain belts. The pioneering studies of the 50's through the 80's, those which applied the principles of thermodynamics to metamorphic rocks, focused almost exclusively on high temperature systems, for which equilibrium processes could be demonstrated. By the 1980's, metamorphic petrologists had developed the methodologies for deciphering the thermal and baric histories of mountain belts through the study of high grade rocks. Of course, low grade metamorphic rocks - here defined as those which form at pressures and temperatures up to and including the greenschist facies - had been well known and described as well, initially through the efforts of Alpine and Circum-Pacific geologists who recognized that they constituted an integral and contiguous portion of mountain belts, and that they underlay large portions of accreted terranes, many of oceanic origins. But until the mid 80's, much of the effort in studying low grade rocks - for a comprehensive review of the literature to that point see Frey (1987) - had been concentrated on mudstones, volcanoclastic rocks, and associated lithologies common to continental mountain belts and arcs. In the mid 80's, results of the Deep Sea Drilling Project (DSDP) rather dramatically mitigated a shift in the study of low grade metamorphic rocks.

  15. Petrochemical composition characteristics of mafic microgranular enclaves in granites in SE China and their significance.%华南花岗岩暗色微粒包体的岩石化学组成特征及其意义

    Institute of Scientific and Technical Information of China (English)

    彭卓伦; Rodney Grapes; 庄文明; 张献河


    Inhomogeneous enclave is the most popular type in the marie microgranular enclave (MME) of gran ites in SE China, which is generally with a dark or more mafic tim. The lithologic units in the rim of MME range from monzonite to quartz monzonite and to monzodiorite, while those in the core from quartz monzodior ite to tonalite. A MME generally has decreasing Si and Ca contents and Na/K ratios, and increasing K, Al,Ti, P and Fe+Mg contents from its core to rim, indicating that the MMEs should have been mainly derived from restites of the partial melting rock. Further research revealed that the composition of restite in granites varies in the direction of increasing biotite as the melting degree of the rock increases and the biotite-rich en claves should be the final product of rock melting process.%对华南诸多花岗岩体暗色微粒包体的野外调查表明, 非均一包体是包体中最普遍的类型,常具有暗色基性边.岩石化学分析结果表明,包体边缘的岩性为二长岩-石英二长岩-二长闪长岩,中心则为石英二长闪长岩-英云闪长岩.从包体中心到边缘,具有Si、Ca含量和Na/K比值降低,K、Al、Ti、P和Fe+Mg含量升高的变化趋势,揭示包体应来自岩石部分熔融的残余熔渣.进一步研究还表明,花岗岩中残余熔渣(暗色包体)的组分随着岩石熔融程度升高而向黑云母增多的方向演化,暗示富云包体应是岩石熔融演化至末阶段的产物.

  16. 赣抚大堤防洪保护区洪水风险图编制洪水分析方案设置研究%Study on setting flood analysis scheme for flood risk mapping of flood protection area of Ganfu Grand Dyke

    Institute of Scientific and Technical Information of China (English)

    张秀平; 丁志雄


    编制洪水风险图是落实防汛工作从"控制洪水"向"洪水管理"转变和开展洪水风险管理的重要基础支撑,也是国内防洪减灾工作中重要的非工程类措施之一.洪水风险图编制是一个复杂的过程,涉及的基础资料较多,所需考虑的要素复杂,而洪水分析方案的设置是洪水风险图编制中至关重要的部分,关系到洪水风险图编制结果的实用性.本文以赣抚大堤防洪保护区为例,从洪水来源、洪水分析频率、洪水组合方式、可能溃口位置、溃口宽度、溃决时机等方面,并权衡实用性和易用性对设计方案优化组合,简述赣抚大堤洪水风险图编制过程中洪水分析方案的设置规则,为江西省及其他地区全面开展洪水风险图编制提供参考.%Flood risk mapping is an important foundation for the implementation of flood control work from flood control to flood management and provides an important foundation support for flood risk management.It is one of the improtant non-engineering measures for flood control and disaster relief in China.Flood risk mapping is a complicated process ,involving much basic informa-tion and the complex factors need to be considered.Flood analysis schemes are one of a vital part of the preparation of flood risk mapping,which is related to the practicality of flood risk mapping results.In this paper,we took flood protectition area of Ganfu Grand Dyke as an example,setting the rules for flood analysis schemes in the process of flood risk mapping of Ganfu Grand Dyke from these aspects ssuch as flood sources,flood analysis frequency,flood combinations,may breach position,breach wideth and out-burst opportunity and weighing the practicality and usability in order to optimize the analysis scheme are stated.It provides a refer-ence for this province and other areas to carry out a comprehensive flood risk mapping.

  17. 诸广花岗岩体南部油洞断裂带辉绿岩脉的Ar-Ar年龄及其地球化学特征%The Ar-Ar Age and Geochemical Characteristics of Diabase Dykes of the Youdong Fault Zone in South of Zhuguang Granite Pluton

    Institute of Scientific and Technical Information of China (English)

    曹豪杰; 黄国龙; 许丽丽; 黄乐真; 王小冬; 吴建勇; 王春双


    The Ar-Ar age of diabase dykes in the Youdong fault zone is 110.6±2.0 Ma,indicating that diabase dykes were emplaced in late Yanshaninan.Diabase is rich in SiO2 (mean value 50.65%),Al2O3 (mean value 15.10%),CaO(mean value 10.48%),MgO(mean value 7.80%) and poor in TiO2 (mean value 1.13%),FeOt(mean value 9.90%),Na2O(mean value 2.28%),K2O (mean value 0.90%),and P2O5 (mean value 0.11%.In rare earth elements,it is rich in large-ion lithophile Rb,Th,Ba,K,and poor in Nb,Ta,Zr,Hf,P and Ti.Total rare earth is low,and bear the right-wing-type distribution patten eith obviously Eu deplation(δEu mean value 0.92).Its characteristic is similar with that of active continental margin basalt,and this phenomenon reflects that primitive magma came from relative enrichment of the lithospheric mantle,didn't come from original mantle.%油洞断裂带中辉绿岩脉的全岩Ar-Ar年龄为110.6±2.0 Ma,属于燕山晚期岩浆活动产物.辉绿岩具有较高的SiO2(平均为50.65%)、Al2O3(平均为15.10%)、CaO(平均为10.48%)、MgO(平均为7.80%)含量和较低的TiO2(平均为1.13%)、FeOt(平均为9.90%)、Na2 O(平均为2.28%)、K2O(平均为0.90%)、P2 O5(平均为0.11%)含量.在微量元素方面,大离子元素Rb、Th、Ba、K的富集明显,而Nb、Ta、Zr、Hf、P、Ti显示亏损.稀土元素总量低,轻稀土相对富集,分布模式略呈右倾型,Eu亏损弱(δEu平均为0.92).辉绿岩的上述元素地球化学特征非常相似于活动大陆边缘钙碱性玄武岩,反映原始岩浆来源于受太平洋板块俯冲产生的流体对上覆地幔楔交代改造形成的富集地幔源区.

  18. 新疆北山地区罗东镁铁质-超镁铁质层状岩体岩石成因%Petrogenesis of Luodong mafic-ultramafic layered intrusion, Beishan region, Xinjiang

    Institute of Scientific and Technical Information of China (English)

    凌锦兰; 夏明哲; 郭娜欣; 汪帮耀; 夏昭德; 姜常义


    罗东镁铁质-超镁铁质岩体位于塔里木板块东北部的新疆北山地区,岩体平面形态为眼球状,出露面积约2.1 km2.由纯橄岩、单辉橄榄岩、斜长二辉橄榄岩、橄榄二辉岩、方辉辉石岩、橄长岩、橄榄辉长岩、辉长岩、苏长辉长岩和淡色辉长岩组成,堆晶结构和堆晶韵律发育,属于层状岩体.岩浆演化过程中主要分离结晶/堆晶相是橄榄石和单斜辉石,此外还有斜长石、铬尖晶石和斜方辉石.岩石地球化学特征表明岩体与围岩之间的同化混染较弱.原生岩浆属于苦橄质岩浆(MgO=14.7%),并且具有高的岩浆结晶温度(1412℃).OIB型Nd、Sr同位素组成和高温苦橄质原生岩浆暗示,罗东岩体的形成可能与地幔柱活动有关,是地幔柱轴部部分熔融的产物.由此表明,塔里木板块东北部早二叠世幔源岩浆岩也应该隶属于塔里木大火成岩省.%Lens shaped Luodong mafic-ultramafic intrusion lies in Beishan region of Xinjiang in Northeastern Tarim Plate and covers an area of 2.1 km2. It consists of dunite, wehrlite, plagio-lherzolite, olivine websterite, peridotite pyroxenite, troctolite, olivine gabbro, gabbro, noritegabbro and leucogabbro. The Luodong intrusion is a layered intrusion with obvious cumulus textures and rhythmics. The mainly fractional crystallized or cumulated phases are olivine and clinopyroxene, together with minor plagioclase, orthopyroxene and Cr-spinel during magma evolution. The geochemical feature indicates weak crustal contamination. The primary magma is picritic with MgO contents up to 14.7% and high crystallization temperature of 1412 ℃. OIB-like Nd, Sr isotopic compositions and high temperature picritic primary magma indicate that Luodong intrusion may have an affinity with mantle plume and be formed by partial melting of axis plume. The Early Permain mantle-derived magmatic rocks in Northeastern Tarim Plate are thus a part of Tarim Large Igneous Province.

  19. Characteristics and Magmatism of MaficUltramafic Layered Intrusions%镁铁质-超镁铁质层状岩体基本特征及岩浆作用

    Institute of Scientific and Technical Information of China (English)

    夏昭德; 姜常义; 夏明哲; 凌锦兰; 卢荣辉


    Mafic- ultramafic layered intrusions can occur in various tectonic settings and have typical layered structure and prosodic structure. The magmatisms for layered rocks are mainly gravity differentiation, double-diffusive convection, compaction, assimilation contamination, and magma mixing. Gravitational differentiation well explains the vertical changes of rock types and variations of mineral composition in layered intrusions. Double-diffusive convection and magma mixing play an important role in the formation process of rhythmic beddings. Compaction plays a key role in the late stage of magma evolution for the formation of adcumulatic texture. Assimilation contamination can change the composition of magma,resulting in changes in the composition of magma differentiation. The types of crystal structures depend on the different activities of inter-cumulus liquid and penetration rate of crystal mush, and adcumulatic texture formation requires high permeability. When the inter-cumulus liquid is not active, it is favorable for the formation of ortho-cumulatic texture. Rock association and textures, mineral compositions and change rules are good records for magma differentiation process, magma supplying, and other information, which can be used to discuss the petro-genesis of layered intrusions: temperature and pressure conditions of magma, primitive magma and magmatic evolution.%镁铁质-超镁铁质层状岩体可以产于多种构造环境,岩体具有典型的层状构造和韵律结构.层状岩体形成时主要的岩浆作用有重力分异作用、双扩散对流作用、压实作用、同化混染和岩浆混合作用等,其中重力分异作用很好地解释了层状岩体垂向上岩石类型的变化和矿物组分的变化,双扩散对流作用和岩浆混合作用在韵律层理形成过程中起了很重要的作用,而压实作用对岩浆演化晚期阶段补堆晶结构的形成作用明显,同化混染作用可改变岩浆的成分,导致岩浆分

  20. Speleothem and biofilm formation in a granite/dolerite cave, Northern Sweden

    DEFF Research Database (Denmark)

    Sallstedt, T.; Ivarsson, M.; Lundberg, J.


    Tjuv-Antes grotta (Tjuv-Ante's Cave) located in northern Sweden is a round-abraded sea cave ('tunnel cave'), about 30 m in length, formed by rock-water abrasion in a dolerite dyke in granite gneiss. Abundant speleothems are restricted to the inner, mafic parts of the cave and absent on granite...

  1. Metasomatismo en ortoanfibolitas de la Faja máfica-ultramáfica del río de las tunas, Mendoza Metasomatism in orthoamphibolites from the Río de Las Tunas mafic-ultramafic belt, Mendoza

    Directory of Open Access Journals (Sweden)

    M. Florencia Gargiulo


    áticos y la secuencia de reemplazo observada en ellos, sugieren que tales asociaciones se generaron a causa de un proceso metasomático, acontecido en condiciones de bajo grado a una presión menor a 4 kbar, durante los estadios finales del ciclo de metamorfismo regional evidenciado en el área.In this contribution, petro-mineralogical and chemical evidences of metasomatism in orthoamphibolites from the Río de Las Tunas mafic-ultramafic belt in Frontal Cordillera of Mendoza province are provided. These orthoamphibolites integrate the metamorphic basement of the Frontal Cordillera and they belong to the Guarguaráz Metamorphic Complex. These rocks show mineral associations and textural relationships indicating disequilibrium conditions between some of the mineral phases. Hornblendic amphibole crystals show a reaction border with development of clinopyroxene (ferroan dioside; meanwhile the calcicplagioclase is mostly replaced by a fine-grained matrix integrated by the mineral association of garnet-epidotes.l -prehnite-pumpellyite- albite. The amphibole and clinopyroxene crystals are also partially replaced by pumpellyite and/or chlorite, meanwhile the matrix is slightly replaced by muscovite and calcite. Garnet compositions belong to the grossular-andradite series and their average composition is: Alm05Adr53Prp01Sps04Grs36Uv01. Some crystals with Cr2O3 content between 11.69-13.17 wt.% and with a subtle chemical zonation (uvarovitic core: Alm03Adr13Prp00Sps02Grs35Uv47 with more grossularic border: Alm03Adr12 Prp00Sps01Grs44Uv40 occur in relationship with magnetite crystals with up to 20% of the chromite component. Pressure estimations based on the compositional zoning of amphibole crystals gave values between 2.5-4.5 kbar. The observed mineral associations together with the textural and chemical characteristics of these rocks show that this kind of orthoamphibolites is completely different from the classical amphibolites of igneous protoliths outcropping in the study area

  2. SHRIMP zircon U-Pb and Re-Os dating of No. 10 intrusive body and associated ores in Pobei mafic-ultramafic belt of Xinjiang and its significance%新疆坡北基性-超基性岩带10号岩体SHRIMPU-Pb和矿石Re-Os同位素定年及其意义

    Institute of Scientific and Technical Information of China (English)

    李华芹; 梅玉萍; 屈文俊; 蔡红; 杜国民


    Located in Beishan rift in the northeastern part of Tarim plate, the Pobei No. 10 mafic-ultramafic body in-truded into Early Carboniferous Hongliuyuan Formation. It is large in size and well differentiated, thus serving as one of the most important intrusive bodies in search for copper-nickel sulfides. According to its emplacement strata, it was previously thought that this intrusive body was formed in Late Carboniferous-Early Permian. The uthors performed dating by using such techniques as zircon U-Pb, disseminated ores Re-Os and Sr-Nd isotopic tracer with the purpose of systematic determination of the magmatie and mineralization ages, on such a basis, discussed the metallogenic significance of systematic differences between the ages dated by different isotopic sys-tems. Radiometric dating of the gabbro from the Pobei No. 10 mafic-ultramafie body yielded rather scattered zir-con age information, but obtained a main body zircon SHRIMP U-Pb age of (289± 13)Ma(95 % confidence, MSWD=4.9, n = 8), which is interpreted as the formation age of the host intrusive. The disseminated ores give an apparent Re-Os isochron age of (413 ± 20) Ma with an initial 187Os/188Os ratio of (0.226 ± 0.032). However, it is found that the apparent Re-Os isochron ages of disseminated ores are older than both the forma-tion age of the host intrusions and the ages of Pobei No. 10 mafic-ultramafic emplacement strata, which indicates that the apparent Re-Os isochron ages of disseminated ores are not reliable. It is suggested that the Pobei No. 10 mafic-ultramafic body and the deposit were formed at about 280 Ma ago. Because of heterogeneity in initial ~(187)Os/~(188)Os ratios of disseminated ores caused by crust contamination, the apparent dates older than the true age were obtained. Cu-Ni mineraliztion shows a close time-space relationship with the mafic-ultramafic intrusive body, implying a product of magmatic liquation. It is therefore concluded that the intrusion of Pobei No. 10 mafic

  3. Condiciones físicas de formación de gabros ymigmatitas derivadas de rocas máficas en el centro de la Sierra de ValleFértil, San Juan Physical conditions for the formation ofgabbros and migmatites derived from mafic rocks in the center of Sierra deValle Fértil, San Juan

    Directory of Open Access Journals (Sweden)

    AlinaM. Tibaldi


    resultadosexperimentales se demuestra que la transición desde rocas ígneas máficas amigmatitas máficas ocurrió por enfriamiento isobárico, y que el emplazamientocontinuo de magmas debió ser la fuente de calor que mantuvo la secuencia enfacies de granulitas sin dejar que el enfriamiento fuera rápido. Se interpretala geología del centro de la sierra de Valle Fértil como un ejemplo del núcleoplutónico-metamórfico del arco magmático Famatiniano donde el gradientemetamórfico anormalmente alto refleja que un volumen importante de magmasmáficos alcanzaban, y dominaban, en paleo-profundidades de entre 16 y 20kilómetros.A sequence of plutonicmafic rocks inter-stratified with both mafic- and metasedimentary-derivedmigmatites is found along the San Juan valley in the center of the Sierra deValle Fértil. This natural example shows the transition from igneous tometamorphic petrologic processes which occurred during the crystallization ofmafic magmas and the subsequent partial melting of crystallized gabbroic rocks.This work studies the mineralogical changes associate to this petrologictransition. Thermobarometric estimates based on amphibole-plagioclase indicatethat the mafic magmas crystallized at around 1100ºC and 5 ± 0.5 kbar. Theconditions under which gabbroic rocks were partially melted are estimated usingtwo pyroxenes thermometry and amphibole-plagioclase thermobarometry. Similarphysical conditions in the range between 740 and 840ºC and 5 to 6.5 kbar arerecovery from mineral assemblages in the mesosomes and leucosomes of maficmigmatites. The main mineral compositional changes that accompanied the partialprocess of the gabbroic rocks are: 1 depletion of aluminium content and Mg#ratio in pyroxenes; 2 depletion of anorthite mole fraction of theplagioclases; and 3 depletion of the Mg# ratio in amphiboles. These mineralcompositional variations are consistent with those found by experimentallymelting mafic protoliths. Experimental results showed that the temperature forpromoting

  4. Caracterización de rocas ultramáficas, máficas y metasedimentarias del cordón del Peñasco, Precordillera Occidental, Mendoza Characterization of ultramafic, mafic and metasedimentary rocks of the Cordón del Peñasco, western Precordillera, Mendoza

    Directory of Open Access Journals (Sweden)

    Florencia L Boedo


    Full Text Available En el cordón del Peñasco, ubicado en el sector norte de la provincia de Mendoza, dentro del ámbito de la Precordillera Occidental, aforan metasedimentitas en contacto tectónico con una asociación máfica-ultramáfica. Dentro de las metasedimentitas en facies esquistos verdes se han reconocido dos asociaciones: metalimolitas carbonáticas y metaareniscas y metapelitas. Las rocas máficas-ultramáficas están representadas por serpentinitas derivadas de harzburgitas y dunitas, gabros bandeados metamorfizados en facies granulita; diabasas constituyendo diques o filones capa, metabasaltos amigdaloides y metahialoclastitas con bajo grado de metamorfismo. Se realizaron estimaciones termométricas en una granulita máfica (gabro bandeado utilizando el par granate-clinopiroxeno, obteniéndose temperaturas de 884°C. Las mismas permiten estimar que estos cuerpos experimentaron metamorfismo de alta temperatura, para posteriormente retrogradar a facies esquistos verdes, sobre la base de aspectos mineralógicos y texturales. Con respecto a las unidades metasedimentarias, el paleoambiente inferido para la unidad de metaareniscas y metapelitas corresponde a pie de talud continental con predominio de sedimentación gravitacional sobre la base de estructuras sedimentarias reconocidas. Mientras que la unidad de metalimolitas carbonáticas pertenecerían a un ambiente marino profundo. Estas unidades han sido asignadas al Eopaleozoico a partir de criterios estratigráficos y deformadas y metamorfizadas hacia fines del Devónico como consecuencia de la orogenia Chánica.In Cordón del Peñasco area, located in northern Mendoza province, western Precordillera, metasedimentites are in tectonic contact with a mafic-ultramafic association. The former have been under greenschist facies metamorphism and two associations have been recognized: carbonate metasiltstones and metasandstones and metapelites. The mafic-ultramafic association is represented by serpentinized

  5. Petrología y geoquímica de las rocas máficas-ultramáficas del área río Las Águilas - arroyo de Los Manantiales, Provincia de San Luis Petrology and geochemistry of mafic-ultramafic rocks in the Las Águilas River-of the Manantiales stream area, San Luis

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    G. Ferracutti


    Full Text Available Dada la carencia de información de campo detallada, se efectuó el mapeo y muestreo de los cuerpos máficos-ultramáficos aflorantes entre el río de Las Águilas y el arroyo de Los Manantiales en la provincia de San Luis. La escasa información de campo, petrográfica y geoquímica de las unidades de éste área dificultaba la vinculación entre las rocas aflorantes y aquellas reconocidas en los testigos de perforación. Los cuerpos ígneos de interés se hallan en contacto con el basamento metamórfico a través de fallas de alto ángulo de buzamiento y/o contactos intrusivos. En Las Higueras y Virorco, se identificaron afloramientos de rocas máficas-ultramáficas con textura magmática primaria de tipo estratificada no-uniforme de regularidad intermitente. El estudio de estos cuerpos generó un desafío particular debido a los procesos deformación, metamorfismo y alteración que los han afectado. Estos factores no solo modificaron la mineralogía de las rocas sino que ademas generaron bloques basculados reconocidos por la presencia de texturas de estratificación magmaticas cuya posición espacial no se corresponde con la magmática originaria. Los patrones de los elementos de tierras raras normalizados a condrita permiten identificar un proceso de diferenciación, con mayor enriquecimiento en los contenidos de tierras raras en las rocas más evolucionadas. En base a los contenidos de tierras raras de las rocas ultramáficas, se infiere que la cristalización magmática estuvo controlada por la presencia de olivino y ortopiroxeno.Geological mapping was completed in the area between Las Águilas River and the Manantiales stream in San Luis province due to the absence of a detailed geologic map of the mafic-ultramafic bodies which occur in this region. Also the lack of petrographic and geochemical information constituted a draw back when attempting to relate surface and subsurface rocks. In this contribution a geological map of the area and

  6. Constraints on the formation of geochemically variable plagiogranite intrusions in the Troodos Ophiolite, Cyprus (United States)

    Freund, Sarah; Haase, Karsten M.; Keith, Manuel; Beier, Christoph; Garbe-Schönberg, Dieter


    The geochemistry and petrology of tonalitic to trondhjemitic samples ( n = 85) from eight different plagiogranite intrusions at the gabbro/sheeted dyke transition of the Troodos Ophiolite were studied in order to determine their petrogenetic relationship to the mafic plutonic section and the lava pile. The plagiogranitic rocks have higher SiO2 contents than the majority of the glasses of the Troodos lava pile, but lie on a continuation of the chemical trends defined by the extrusive rocks, indicating that the shallow intrusions generally represent crystallised magmas. We define three different groups of plagiogranites in the Troodos Ophiolite based on different incompatible element contents and ratios. The first and most common plagiogranite group has geochemical similarities to the tholeiitic lavas forming the lavas and sheeted dyke complex in the Troodos crust, implying that these magmas formed at a spreading axis. The second plagiogranite group occurs in one intrusion that is chemically related to late-stage and off-axis boninitic lavas and dykes. One intrusion next to the Arakapas fault zone consists of incompatible element-enriched plagiogranites which are unrelated to any known mafic crustal rocks. The similarities of incompatible element ratios between plagiogranites, lavas and mafic plutonic rocks, the continuous chemical trends defined by plagiogranites and mafic rocks, as well as incompatible element modelling results, all suggest that shallow fractional crystallisation is the dominant process responsible for formation of the felsic magmas.

  7. Geología y petrología del cuerpo Máfico-Ultramáfico Las Juntas, Sierra de Valle Fértil, Provincia de San Juan Geology and petrology of mafic-ultramafic body from Las Juntas, Valle Fértil, San Juan

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    Baliani Ignacio


    Full Text Available En el centro de la sierra de Valle Fértil aflora un cuerpo lenticular orientado en forma aproximada N-S que se encuentra encajado en un basamento ígneo de naturaleza máfica-ultramáfica. La morfología y posición actual de dicho cuerpo se debe a su emplazamiento tectónico, y la fábrica magmàtica interna presenta rumbo similar al del cuerpo y buzante al este. Las rocas que conforman el cuerpo de Las Juntas exponen una variedad litológica con las siguientes composiciones: norita olivínica, gabro anfibólico, dunita, gabronorita anfibólica y peridotita anfibólica piroxénica. Se caracterizan por presentar un encapado ígneo a escala de afloramiento y textura cumular a escala de muestra de mano, mientras que la microtextura predominante en estas rocas es de tipo coronítica. La relación cogenética entre las rocas máficas y ultramáficas de Las Juntas es determinada por la composición de silicatos de Mg y Fe presentes en ellas. El coeficiente de intercambio Fe-Mg entre olivino y clinopiroxeno con el líquido muestra un magma con una relación de #Mg (Mg/Mg + Fe-2 alrededor de 0,6; siendo inferior a la esperada para magmas generados en el manto peridotítico. Esto demuestra que el magma formador de estas rocas se habría originado en el manto litosférico, posteriormente ascendido y emplazado en niveles inferiores de la corteza (20 - 25 km. Este proceso petrológico es característico de magmatismo asociado a subducción, y relacionado al arco magmático Famatiniano.An elongate body of mafic and ultramafic plutonic rock crops out in central Sierra de Valle Fértil. The igneous body was tectonically emplaced and its internal magmatic fabrics strike roughly north-south and dip to the east. Rocks in the body are different from their hosting plutonic rocks in showing a layered igneous structure associated with pristine cumulate textural features at all observational scales. The layered body is composed of gabbronorite cumulates, ultramafic

  8. Significado de fábricas magnéticas en rocas volcánicas cuaternarias de la Isla Decepción (Shetland del Sur, Antártida)


    Gil Imaz, Andrés; Gil Peña, Inmaculada; Galindo Zaldívar, Jesús; Rey Salgado, Jorge; Maestro González, Adolfo; Pueyo Anchuela, Óscar; Soto Marín, Ruth; Oliva Urcia, Belén; López Martínez, Jerónimo


    This work deals with the magnetic fabric (AMS technique) of different lithotypes related to quaternary volcanism of Deception Island (South Shetland Islands). The studied rocks include lava flows from basaltic to basaltic-andesitic composition, mafic dykes and several types of pyroclastic deposits. By combining the analysis of parametric aspects of the magnetic ellipsoids and their directional pattern, four types of magnetic fabrics have been recognized. Type I fabrics are related...

  9. Grain to outcrop-scale frozen moments of dynamic magma mixing in the syenite magma chamber, Yelagiri Alkaline Complex, South India

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    M.L. Renjith


    Full Text Available Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Complex, South India. Yelagiri syenite is a reversely zoned massif with shoshonitic (Na2O + K2O=5–10 wt.%, Na2O/K2O = 0.5–2, TiO2 <0.7 wt.% and metaluminous character. Systematic modal variation of plagioclase (An11–16 Ab82–88, K-feldspar (Or27–95 Ab5–61, diopside (En34–40Fs11–18Wo46–49, biotite, and Ca-amphibole (edenite build up three syenite facies within it and imply the role of in-situ fractional crystallization (FC. Evidences such as (1 disequilibrium micro-textures in feldspars, (2 microgranular mafic enclaves (MME and (3 synplutonic dykes signify mixing of shoshonitic mafic magma (MgO = 4–5 wt.%, SiO2 = 54–59 wt.%, K2O/Na2O = 0.4–0.9 with syenite. Molecular-scale mixing of mafic magma resulted disequilibrium growth of feldspars in syenite. Physical entity of mafic magma preserved as MME due to high thermal-rheological contrast with syenite magma show various hybridization through chemical exchange, mechanical dilution enhanced by chaotic advection and phenocryst migration. In synplutonic dykes, disaggregation and mixing of mafic magma was confined within the conduit of injection. Major-oxides mass balance test quantified that approximately 0.6 portions of mafic magma had interacted with most evolved syenite magma and generated most hybridized MME and dyke samples. It is unique that all the rock types (syenite, MME and synplutonic dykes share similar shoshonitic and metaluminous character; mineral chemistry, REE content, coherent geochemical variation in Harker diagram suggest that mixing of magma between similar composition. Outcrop-scale features of crystal accumulation and flow fabrics also significant along with MME and synplutonic dykes in syenite suggesting that Yelagiri syenite magma chamber had evolved

  10. Eruptive history of Mammoth Mountain and its mafic periphery, California (United States)

    Hildreth, Wes; Fierstein, Judy


    This report and accompanying geologic map portray the eruptive history of Mammoth Mountain and a surrounding array of contemporaneous volcanic units that erupted in its near periphery. The moderately alkaline Mammoth eruptive suite, basaltic to rhyodacitic, represents a discrete new magmatic system, less than 250,000 years old, that followed decline of the subalkaline rhyolitic system active beneath adjacent Long Valley Caldera since 2.2 Ma (Hildreth, 2004). The scattered vent array of the Mammoth system, 10 by 20 km wide, is unrelated to the rangefront fault zone, and its broad nonlinear footprint ignores both Long Valley Caldera and the younger Mono-Inyo rangefront vent alignment.

  11. Vesta Evolution from Surface Mineralogy: Mafic and Ultramafic Mineral Distribution (United States)

    DeSanctis, M. C.; Ammannito, E.; Palomba, E.; Longobardo, A.; Mittlefehldt, D. W.; McSween, H. Y; Marchi, S.; Capria, M. T.; Capaccioni, F.; Frigeri, A.; Pieters, C. M.; Ruesch, O.; Tosi, F.; Zambon, F.; Hiesinger, H.; Magni, G.; McFadden, L. A.; Raymond, C. A.; Russell, C. T.; Sunshine, J. M.


    Vesta is the only intact, differentiated, rocky protoplanet and it is the parent body of HED meterorites. Howardite, eucrite and diogenite (HED) meteorites represent regolith, basaltic-crust, lower-crust and possibly ultramafic-mantle samples of asteroid Vesta. Only a few of these meteorites, the orthopyroxene-rich diogenites, contain olivine, a mineral that is a major component of the mantles of differentiated bodies, including Vesta. The HED parent body experienced complex igneous processes that are not yet fully understood and olivine and diogenite distribution is a key measurement to understand Vesta evolution. Here we report on the distribution of olivine and its constraints on vestan evolution models.

  12. Magnetic fabrics in characterization of magma emplacement and tectonic evolution of the Moyar Shear Zone, South India

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    P. Pratheesh


    Full Text Available The Moyar Shear Zone (MSZ of the South Indian granulite terrain hosts a prominent syenite pluton (∼560 Ma and associated NW-SE to NE-SW trending mafic dyke swarm (∼65 Ma and 95 Ma. Preliminary magnetic fabric studies in the mafic dykes, using Anisotropy of Magnetic Susceptibly (AMS studies at low-field, indicate successive emplacement and variable magma flow direction. Magnetic lineation and foliation in these dykes are identical to the mesoscopic fabrics in MSZ mylonites, indicating shear zone guided emplacement. Spatial distribution of magnetic lineation in the dykes suggests a common conduit from which the source magma has been migrated. The magnetic foliation trajectories have a sigmoidal shape to the north of the pluton and curve into the MSZ suggesting dextral sense of shear. Identical fabric conditions for magnetic fabrics in the syenite pluton and measured field fabrics in mylonite indicate syntectonic emplacement along the Proterozoic crustal scale dextral shear zone with repeated reactivation history.

  13. Complete preservation of ophiolite suite from south Andaman, India: A mineralchemical perspective

    Indian Academy of Sciences (India)

    Abhishek Saha; Avik Dhang; Jyotisankar Ray; Suvankar Chakraborty; David Moecher


    Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11° 39′ N: 92° 45′E) to Chiriyatapu (11° 30′ 24′′N: 92° 42′ 30′′E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic–mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic–mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be `Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7–An35.5) whereas groundmass plagioclase are relatively sodic (An33–An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite.

  14. Geochemistry and Petrogenesis of Gouli Mafic Enclaves and Their Host Rocks in Eastern Kunlun%东昆仑沟里地区暗色包体及其寄主岩石地球化学特征及成因

    Institute of Scientific and Technical Information of China (English)

    陈广俊; 孙丰月; 李碧乐; 王冠; 钱烨; 许庆林; 刘国才


    The authors report petrographical and whole-rock geochemical data of the host diorite and mafic microgranular enclaves(MMEs)within the Asiha diorite pluton at Gouli area in eastern Qinghai East Kunlun,and ascertain their petrogenesis and tectonic setting.The Asiha diorite pluton contains abundant MMEs which are mainly amphibole gabbro.The MMEs display igneous-texture and some of them are characterized by plastic flow deformation.The MMEs have plagioclase,amphibole and quartz xenocrysts and acicular apatite, which belongs to petrographical characteristics of magma mixing. Authors systematically studied petrological and geochemical characteristics of the host rocks and the MMEs,and conclude that both are meta-aluminous,high-K calc-alkaline series-shoshonitic series rocks. The selected major oxides of the MMEs and the host rocks show a linear correlation in Harker diagrams. In addition, Al2 O3/K2 O CaO/K2 O and SiO2/CaO K2 O/CaO identical oxide ratio denominator covariant diagram of the MMEs and their host rocks also show a linear correlation,which indicate their composition change is related to the magma mixing and mingling.Trace elements data show that they are both enriched in large ion lithophile elements (Rb,K)and depleted in high field strength elements (Nb,Ta,P and Ti).The MMEs are characterized by low SiO2 contents (SiO2=50.70% 53.88%), high MgO,Fe2O3 and CaO contents,as well as a higher Mg# value (Mg#=0.52 0.59),which indicates that the primary magma was derived from the partial melting of the mantle wedge.The Rb/Sr ratio of the host rocks varies from 0.22 to 0.27,which is close to but less than average values of the earth crust.The Nb/Ta ratio of the host rocks varies from 14.5 to 15.2,which is between the average values of the crust and mantle,suggesting that the magma of the host rocks is mainly from crust and experienced magma mixing.Combined with regional tectonic evolution and discrimination of tectonic setting,we suggest that the Asiha diorite pluton

  15. Characteristics Research on Chromite from the Poyi Cu-Ni Sulfide-bearing Mafic-ultramafic Intrusions in the Beishan Block,Xinjiang%新疆北山坡-含铜镍镁铁-超镁铁质岩体铬铁矿特征研究

    Institute of Scientific and Technical Information of China (English)

    柴凤梅; 夏芳; 陈斌; 卢鸿飞


    The Poyi maiic-ultramafic intrusion is one of the most important Cu-Ni bearing complexes in Beishan area. This intrusion is composed of peridotite, pyroxenite and gabbro. Chromites occurs in the peridotite as an enhedral, subhedral and anhedral accessory mineral. The characteristics of chromites suggested that the primary magma of the Poyi intrusions must have been mantle-derived S-undersaturated mafic magma derived from asthenosphere mantle source even related to mantle plume and have experienced two stages evolution to form this intrusion. Some chromites enclosed within olivine may be crystallized from the primary magma in the deep magma chamber, and others enclosed within pyroxene and being interstitial crystals may be formed from the olivine- and sulfide-laden crystal mush in the high-level magma chamber. The chromites formed in the early stage of magma evolution are very important for the Cu-Ni sulphide deposit.%坡-镁铁-超镁铁质岩体为新疆北山地区一重要的含铜镍硫化物的侵入体.该岩体主要由橄榄岩、辉石岩和辉长岩组成.铬铁矿主要以副矿物形式现于橄榄岩中.它们以自形-半自形以及它形存在于橄榄石颗粒间或其内,部分为辉石包裹,偶见其包裹橄榄石.结合电子探针研究,认为坡一岩体母岩浆是来源于软流圈地幔或者与地幔柱有关的基性岩浆,原生岩浆经过了两个阶段的演化形成了目前的坡一岩体.包裹于橄榄石中的铬铁矿为岩浆早期深部岩浆房中结晶的产物,其他形式存在的铬铁矿为富含橄榄石和铬铁矿“晶粥”的演化岩浆结晶的结果.铬铁矿的结晶对坡一铜镍矿床的形成具有重要的意义.

  16. 东天山白石泉镁铁-超镁铁杂岩体的Nd-Sr-Os同位素成分及其对岩浆演化的意义%Nd-Sr-Os isotopic data of the Baishiquan mafic-ultramafic complex from East Tianshan, and implications for petrogenesis

    Institute of Scientific and Technical Information of China (English)

    陈斌; 贺敬博; 陈长健; 木合塔尔·扎日


    本文报道东天山有铜镍硫化物矿化的白石泉镁铁-超镁铁杂岩体(分布在中天山地块北部)的微量元素和Nd-Sr-Os同位素成分,以探讨其地幔源区性质和壳幔相互作用过程.白石泉杂岩体的地球化学特征是富集大离子亲石元素和轻稀土元素,但亏损高场强元素.全岩Sr和Nd同位素初始比值变化较大,分别是(87Sr/86Sr)i =0.7032~0.7066和εNd(t)=5.6~-0.9,两者呈反相关关系.全岩Os含量在80×10-12~29×10-12之间,富含放射性成因Os(187 Os/188 Os=0.295~ 1.18).据此,得出结论:(1)白石泉杂岩体的母岩浆在上升侵位过程中,受到地壳物质的强烈混染,导致各岩石类型的Nd-Sr同位素成分变化很大和Os同位素成分富集放射性成因Os的特征,这与前人认为的白石泉岩体侵位过程中没有地壳混染作用的模式明显不同;(2)白石泉杂岩体的母岩浆来自被古生代俯冲带熔/流体交代过的年轻岩石圈地幔,这要求我们重新认识中天山前寒武地块的性质和规模.%We report trace elements and Nd-Sr-Os isotopic data for the Baishiquan mafic-ultramafic complex that occurs in the East Tianshan Mountains (in the northern part of the Mid-Tianshan Precambrian block) and is accompanied by Cu-Ni deposits, to understand its mantle source characteristics and processes of mantle-crustal interaction. Geochemically, the complex is characterized by enrichment of LILEs and depletion of HFSEs. The whole-rock Sr-Nd isotopic compositions of the Baishiquan complex vary significantly, with (87Sr/ 86Sr)i =0. 7032 ~0. 7066 and εNd(t) =5. 6 ~ -0. 9, and the Nd-Sr isotopic ratios are negatively correlated. The complex shows very radiogenic Os isotopic compositions, with (187Os/188Os)i =0.295~1. 18 and Os=80 ×l0-12 ~29 ×l0-12. Based on these data, we conclude that (1) the parent magma of the Baishiquan complex experienced significant crustal contamination during magma emplacement, as is revealed by the negatively

  17. 新疆东天山镁铁-超镁铁岩体中橄榄石成分特征及其成因意义:以黄山东和图拉尔根为例%Compositions of olivine from the mafic-ultramafic complexs in eastern Tianshan, Xinjiang and implications to petrogenesus: Examples from Huangshandong and Tulargen complexes

    Institute of Scientific and Technical Information of China (English)

    刘艳荣; 吕新彪; 梅微; 惠卫东


    A dozen of mafic-ultramafic complexes related to copper-nickel sulphide mineralization were found in Eastern Tianshan, Xinjiang. This paper takes Huangshandong and Tulargen complexes for examples, and carries out mineral chemistry tests of fresh olivine from different lithologies. Electron microprobe analyses of olivine show that the Fo of olivine in hornblende-gabbro facies and ultramafic rock facies range from 67 to 73 and from 81 to 84, respectively; Ni contents range from 308 to 1762 ug/g in Huangshandong complex. Olivine crystals in the Tulargen complex have Fo numbers varying from 82 to 85, with Ni from 857 to 1818 μg/g. Modeling of olivine fractional crystallization and sulfide segregation show that the parental magma for Huangshandong complex has experienced 19% olivine fractional crystallization and extensive sulfide segregation (~0.3%). Re-equilibration of the early formed olivine crystals with trapped liquid can account for the variations of olivine compositions. The Tulargen complex has experienced 9% olivine fractional crystallization and extensive sulfide segregation (~0.5%).%新疆东天山发育有十多个与铜镍硫化物成矿有关的镁铁-超镁铁杂岩体,对选取于黄山东和图拉尔根岩体岩体内不同岩性中的新鲜橄榄石进行了矿物化学成分测试.黄山东岩体角闪辉长岩相的橄榄角闪辉长岩中橄榄石Fo为67~73,超镁铁岩相的角闪(斜长)橄榄岩、角闪二辉橄榄岩和斜长单辉橄榄岩中橄榄石Fo为81~84,Ni含量为308~1762 μg/g;图拉尔根岩体中橄榄石Fo为82~85,Ni含量变化范围为857~1818μg/g.利用最高Fo含量的橄榄石成分计算得到黄山东岩体母岩浆MgO为10.3%,图拉尔根岩体母岩浆MgO为11.47%.橄榄石分离结晶和硫化物熔离模拟结果显示,黄山东岩体发生了19%的橄榄石分离结晶,由橄榄石分离结晶导致硫化物熔离的程度约为0.3%,晶间硫化物熔浆的物质交换作用对黄山东橄榄石

  18. Mineralogía y termo-barometría de los Complejos máficos Sol de Mayo y Suya Taco, Norte de las Sierras de Comechingones, Córdoba Mineralogy and thermo-barometry of the Sol de Mayo and Suya Taco mafic complexes from northern Sierra de Comechingones, Córdoba

    Directory of Open Access Journals (Sweden)

    Alina M. Tibaldi


    Full Text Available Este trabajo presenta un estudio detallado de la petrografía, mineralogía química y termo-barometría de dos complejos máficos que afloran al norte de la sierra de Comechingones. Se aplicaron un conjunto de termómetros y barómetros empíricos con el propósito de determinar los rangos de P y T absolutas de cristalización de estas rocas. Las temperaturas de equilibrio en pares de piroxenos (cpx-opx se estimaron usando tres termómetros independientes. Dos calibraciones termométricas (Wells 1977, Kretz 1982 dan resultados consistentes y razonables indicando que los piroxenos registran condiciones de cristalización magmática (920-1150ºC, mientras que la tercera calibración (Lindsley 1983 estima temperaturas muy inferiores. Esta discrepancia puede estar relacionada con el hecho de que la última calibración no sería aplicable a rocas que cristalizaron a partir de magmas ricos en hierro. Por su parte, la termometría Amph-Pl sugiere temperaturas de equilibrio comprendidas entre 750-850ºC, para composiciones típicas de núcleos, mientras que para composiciones representativas de los bordes de grano las temperaturas estimadas son significativamente inferiores (This work presents a detailed study including petrography, mineral chemistry and thermo-barometry of igneous rock from two mafic complexes that outcrop in the north portion of the sierra de Comechingones. The range of absolute P and T under which these rocks crystallized is retrieved using a set of empiric barometers and thermometers. The estimations of equilibrium temperature determined in pairs of pyroxenes (cpx-opx were performed through three independent thermometers. Two of the thermometric formulations (Wells 1977, Kretz 1982 yield consistent and reasonable results indicating that these phases record magmatic crystallizations conditions (920-1150ºC, whereas the third calibration by Lindsley (1983 gives lower temperatures. This discrepancy is thought to reflect the fact that

  19. Evolución metamórfica de los gabros coroníticos de El Arenal, faja máfica-ultramáfica El Destino-Las Águilas, sierra de San Luis Metamorphic evolution of the coronitic gabbrons of the El Arenal, El Destino-Las Aguilas, mafic-ultramafic belt, Sierra de San Luis

    Directory of Open Access Journals (Sweden)

    N. Brogioni


    Full Text Available Las meta-melagabronoritas coroníticas se localizan en uno de los cuerpos de El Arenal, pertenecientes a la faja máfica-ultramáfica El Destino-Las Aguilas del flanco oriental de la sierra de San Luis. Son rocas de grano fino y representan fraccionados poco evolucionados, con alto contenido de Cr-Ni y [Mg/Mg+FeT++] comprendidas entre 0,72 y 0,75, derivados de un magma toleítico olivínico de bajo Ti, cuyo emplazamiento precedió a la deformación penetrativa y metamorfismo famatinianos. Las microestructuras en coronas alrededor de olivino magnesiano y anortita registran tres estadios metamórficos de crecimiento mineral bajo condiciones de facies granulita, anfibolita y esquistos verdes que tuvieron lugar durante el ascenso y enfriamiento lento del magma favorecido por las condiciones P-T elevadas, ca. facies anfibolita, propias del nivel cortical profundo de locación. Luego de una ligera alteración deutérica de los componentes primarios, que ahora se conservan como relictos, se produjeron las transformaciones mineralógicas y microestructurales metamórficas más significativas. La desestabilización del par olivino-plagioclasa durante el estadio granulítico dio lugar a la formación de coronas de Opx y de simplectitas de Cpx + Spl. La deshidratación de las rocas de caja adyacentes, causada por el desarrollo de reacciones metamórficas, permitió durante el estadio anfibolítico la incorporación de fluidos acuosos indispensables para la cristalización de hornblenda, tanto en coronas de Amp + Spl simplectíticos como reemplazando al Opx relicto. La mineralogía de menor temperatura es relacionable con la exhumación de las rocas y representaría la última etapa de la trayectoria evolutiva antihoraria experimentada por las mismas durante el Paleozoico inferior.The coronitic meta-melagabbronorites are part of the El Arenal bodies which belong to the El Destino-Las Aguilas mafic to ultramafic belt, outcroping on the eastern flank of the

  20. 西南天山东德沟镁铁-超镁铁岩中尖晶石的矿物学特征及其地质意义%Mineralogy of spinel from mafic-ultramafic rocks in Dongdegou, southwestern Tianshan and its geological significance

    Institute of Scientific and Technical Information of China (English)

    孔凡梅; 李旭平; 李守军; 吴苏


    中国西南天山东德沟镁铁-超镁铁岩出露于天山伊犁-中天山板块和塔里木板块之间的碰撞造山带中,岩石类型主要为(方辉)橄榄岩,且普遍经历了含水流体(熔体)的渗透交代作用,含有形态和成分变化复杂的尖晶石,表现为从红褐色到黄褐色、再到深绿色和浅绿色,尖晶石的Cr2O3和FeO的含量依次降低,Al2O3和MgO的含量逐渐增高.研究认为红褐色和黄褐色的铬尖晶是原生矿物,而绿色的镁铝尖晶石为次生矿物.原生铬尖晶石的Cr#为0.41~0.87,Mg#为0.21~0.61,且成分变化大,显示其可能具多期熔融的复杂成因;次生绿色的尖晶石形态不一、成分相对有较大变化,表明其至少形成于两个期次的变质作用或流体交代过程.原生铬尖晶石具有SSZ环境橄榄岩的特征,指示其寄主岩石可能是俯冲上板片地幔楔岛弧环境的橄榄岩残片.%The mafic-ultramafic rocks of Dongdougou are located in the orogenic belt between the Tarim plate and the Yili-central Tianshan plate, southwestern Tianshan. Harzburgite is the main type of peridotite, generally metasomatized by hydrous melt or fluid. It includes different types of spinels, being red-brown, brown, dark green, and light green in color. Cr2O3 and FeO decrease gradually whereas AI2O3 and MgO increase with the metasomatism. Studies indicate that red-brown and brown picotiles constitute the primary type, while green magnesium spinels possess the second place. The compositions of the primary picotiles are variable with Cr and Mgtt in the range of 0.41~0.87 and 0.21-0.61 respectively, which may indicate polyphase origin of melting. Secondary spinels have different shapes, and their compositions vary tremendously, exhibiting two stages of metasomatism or metamorphism. The primary picotiles have characteristics of peridotite in a subducting upper plate mantle wedge zone, which implies the host rocks might have been the fragments of peridotite from a SSZ

  1. Geochemical characteristics of platinum group elements and Re-Os isotopes of the Hongshishan Cu-Ni sulfide-bearing mafic-ultramafic intrusion in Beishan block of Xinjiang and their metallogenic significance%新疆北山红石山含铜镍镁铁-超镁铁质岩体PGE和Re-Os同位素地球化学特征及成矿意义

    Institute of Scientific and Technical Information of China (English)

    夏芳; 柴凤梅; 陈斌; 卢鸿飞; 严玉圃


    The Pobei mafic-ultramafic belt is an impotant Ni-Cu sulfide metallogenic prospective province in Beis-han area, eastern Xinjiang. About 20 intrusions (e.g. , Bijiashan, Hongshishan, Cihai, Poyi, Poshi and Lu-odong) were recognized along the fault in this area and almost all the intrusions are associated with Ni-Cu sulf ide deposits. The Hongshishan mafic-ultramafic complexs are located in the eastern part of the Pobei mafic-ultramafic belt and the southern part of the Baidiwa-Yunihe fault in this area. They intruded into quartz schist of the Lower Carboniferous Hongliuhe Formation. They are mainly composed of dunite, peridotite, troctolite, pyrox-enite, olivine gabbro, gabbro and diorite. These rocks were emplaced in two stages. The first stage consists ofperidotite and olivine gabbro, and the second stage comprises gabbro and diorite. The dunite, pyroxene peri-dotite and troctolite are the host rocks for Ni-Cu sulfide ores. The platinum-group elements and Re-Os isotopic compositions of ore and rocks from the Hongshishan Ni-Cu sulfide-bearing mafic-ultramafic intrusions were analyzed, and the result shows that the total PGE content is quite low, ranging from 0.54×l0~9to 15.84×1CT9 and increasing with the increase of sulfide content. The PGE primitive mantle-normalized patterns display a positive slope and low fractionation between IPGE and PPGE. PGE, Cu and Ni are depleted relative to the primitive mantle with low and a narrow Pd/Ir range (3.19 - 10.5), indicating a weak influence of later hydrothermal alteration. Furthermore, the Ir displays positive correlations with Pt, Pd, Os, Ru and Rh, indicating a weak contribution of the fractionation and the partial melt to PGE depletion. The Cu/Pd ratios(73 × 103 ~ 1 670 × 103)and Ti/Pd ratios (87 × 103~2 857 × 103)of these rocks suggest that sulfur in the primary magmas had reached saturation, and immiscible sulfides droplets were segregated from silicate magmas before their emplacement The relationship between Ni

  2. Petrogenesis and geodynamics of plagiogranites from Central Turkey (Ekecikdağ/Aksaray): new geochemical and isotopic data for generation in an arc basin system within the northern branch of Neotethys (United States)

    Köksal, Serhat; Toksoy-Köksal, Fatma; Göncüoglu, M. Cemal


    In the Late Cretaceous, throughout the closure of the Neotethys Ocean, ophiolitic rocks from the İzmir-Ankara-Erzincan ocean branch were overthrusted the northern margin of the Tauride-Anatolide Platform. The ophiolitic rocks in the Ekecikdaǧ (Aksaray/Central Turkey) region typify the oceanic crust of the İzmir-Ankara-Erzincan branch of Neotethys. The gabbros in the area are cut by copious plagiogranite dykes, and both rock units are intruded by mafic dykes. The plagiogranites are leucocratic, fine- to medium-grained calc-alkaline rocks characterized mainly by plagioclase and quartz, with minor amounts of biotite, hornblende and clinopyroxene, and accessory phases of zircon, titanite, apatite and opaque minerals. They are tonalite and trondhjemite in composition with high SiO2 (69.9-75.9 wt%) and exceptionally low K2O (<0.5 wt%) contents. The plagiogranites in common with gabbros and mafic dykes show high large-ion lithophile elements/high-field strength element ratios with depletion in Nb, Ti and light rare-earth elements with respect to N-MORB. The plagiogranites together with gabbros and mafic dykes show low initial 87Sr/86Sr ratios (0.70419-0.70647), high ƐNd(T) (6.0-7.5) values with 206Pb/204Pb (18.199-18.581), 207Pb/204Pb (15.571-15.639) and 208Pb/204Pb (38.292-38.605) ratios indicating a depleted mantle source modified with a subduction component. They show similar isotopic characteristics to the other supra-subduction zone (SSZ) ophiolites in the Eastern Mediterranean to East Anatolian-Lesser Caucasus and Iran regions. It is suggested that the Ekecikdaǧ plagiogranite was generated in a short time interval from a depleted mantle source in a SSZ/fore-arc basin setting, and its nature was further modified by a subduction component during intra-oceanic subduction.

  3. Late Caledonian microdiorites and felsic porphyrites from Northern Scotland: implications for the petrogenesis of high Ba-Sr granites (United States)

    Fowler, Mike; Jacklin, Holly


    A suite of sharply cross-cutting microdiorite - felsic porphyrite dykes, emplaced into the southern part of the Northern Highlands Terrane, is thought to be coeval with the local Caledonian high Ba-Sr granites. On occasion they can be seen to pillow into, and mix with the granites (e.g. Strontian) in the manner of synplutonic dykes. In the least-deformed examples small-scale mixing and mingling textures are preserved between basic and acid variants, and the felsic porphyrites (rarely) have mafic marginal facies. Microdiorites also grade into rocks of the appinite suite. Thus, the compositional range of the suite is considerable, linking mafic magmas to more evolved compositions via many intermediate stages. These therefore offer a window into the processes of Caledonian magma evolution. A selection of some 50 dykes has ben collected and analysed for major and trace elements, mostly from the environs of Strontian, but also as far north as Loch Quoich and west to Arisaig. They show a continuous chemical range from 47% to 74% SiO2, 18% to high-Mg andesite of sanukite affinity. Petrogenetically-informative trace elements bear the hallmarks of a subduction-related source, with general enrichment in LILEs and relative depletion in HFSEs (in particular Nb-Ta). The chemistry of the felsic porphyrites is closely comparable with the local Strontian and Cluanie granites, and cumulus-enriched mafic microdiorites are chemically similar to local discrete appinites. Such data can therefore be used to test alternative petrogenetic hypotheses: that the high Ba-Sr granites evolved by crystal fractionation (± crustal contamination) from mantle-derived appinitic parents, or that they are crustal melts associated with genetically unrelated but contemporaneous mafic magmas.

  4. Palaeointensity determinations on rocks from Palaeoproterozoic dykes from the Kaapvaal Craton (South Africa) (United States)

    Shcherbakova, V. V.; Shcherbakov, V. P.; Zhidkov, G. V.; Lubnina, N. V.


    Palaeointensity study of the Proterozoic-Archean volcanic rocks from the Kaapvaal Craton South Africa) are reported. Palaeomagnetic study of this collection was performed earlier by Olsson et al. Electron microscope observations, thermomagnetic and hysteresis measurements indicate the presence of single-domain and pseudo-single-domain (SD-PSD) magnetite grains as the main magnetic mineral. The samples demonstrated a very good stability to heating, the electron micrograph observations revealed magnetite-ilmenite exsolution structure. Palaeointensity determinations were obtained by Coe-modified Thellier procedure. A total 58 samples from 14 sites were studied but only seven samples from one site NL28 of the Early Proterozoic age of 1.9 Ga passed palaeointensity selection criteria. Reliable palaeointensity determinations were obtained by both Thellier and Wilson methods on 18 cubes (subsamples) from site NL28 yielding rather low mean virtual dipole moment (VDM) = (2.82 ± 0.12) ×1022 Am2 which is in agreement with the suggestion of existence of the `Proterozoic dipole low period'.

  5. Are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows? (United States)

    Jessop, D. E.; Gilchrist, J.; Jellinek, A. M.; Roche, O.


    Turbulent volcanic jets are produced by highly-energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground-hugging pyroclastic flows. Ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate, disrupt global air travel and cause respiratory health problems. Pyroclastic flows, by contrast, are potentially catastrophic to populations and infrastructure close to the volcano. Key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large (entraining) eddies forming the jet edge changes the density of the air-ash mixture: low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes. Recent experiments on particle-laden (multi-phase) volcanic jets from flared and straight-sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region. This newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles, an effect that has generally been overlooked in previous studies. Building on this work, here we use an extensive series of experiments on multi-phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera-forming (CCF) eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes. Our results, understood through scaling analyses and a one-dimensional theoretical model, show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies. However, because the size of the entraining eddies scales approximately with vent width, the extent of entrainment is reduced as the vent width becomes small in comparison to its length. Consequently, our work shows that for specified mass eruption rates, the high length-to-width ratio vents typical of CCF events are more likely to produce pyroclastic flows. We suggest that the enigmatic trend in the geological record for the largest CCF eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece-wise continuous caldera ring fractures.

  6. Homosexuals, dykes and queers: political positions in the lesbian and gay movement

    Directory of Open Access Journals (Sweden)

    Coll-Planas, Gerard


    Full Text Available (English In this article I aim to explore conflicts among the variety of political projects in the lesbian and gay movement.I base the article on Alberto Mira’s (2004 proposed models for the expression of homosexuality: decadentist, homophile and camp. I discuss the main theoretical and political debates that arise from these models. Firstly, I ask whether life on the margins of mainstream social rules allows gays and lesbians to live a freer existence or merely means internalised homophobia (using Jean Genet’s and Michel Foucault’s approaches as examples. Secondly, I question whether the demand for “normality” entails the creation of new exclusions (following Judith Butler’s approach to normality: Finally, I consider whether camp culture reproduces and/or subverts gender rules (introducing a discussion between Sheila Jeffreys and Judith Butler’s positions. On the basis of this exploration of Mira's three cultural models, I develop a typology of political positions ('normalization of homosexuality', 'transformation' and 'queer', which I apply to the case study of the Catalan lesbian and gay movement.

  7. Homosexuals, dykes and queers: political positions in the lesbian and gay movement

    Directory of Open Access Journals (Sweden)

    Gerard Coll-Planas


    Full Text Available In this article I aim to explore conflicts among the variety of political projects in the lesbian and gay movement.I base the article on Alberto Mira’s (2004 proposed models for the expression of homosexuality: decadentist, homophile and camp. I discuss the main theoretical and political debates that arise from these models. Firstly, I ask whether life on the margins of mainstream social rules allows gays and lesbians to live a freer existence or merely means internalised homophobia (using Jean Genet’s and Michel Foucault’s approaches as examples. Secondly, I question whether the demand for “normality” entails the creation of new exclusions (following Judith Butler’s approach to normality: Finally, I consider whether camp culture reproduces and/or subverts gender rules (introducing a discussion between Sheila Jeffreys and Judith Butler’s positions. On the basis of this exploration of Mira's three cultural models, I develop a typology of political positions ('normalization of homosexuality', 'transformation' and 'queer', which I apply to the case study of the Catalan lesbian and gay movement.

  8. Utility of natural generalised inverse technique in the interpretation of dyke structures

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, M.M.M.; Murty, T.V.R.; Rao, P.R.; Lakshminarayana, S.; Subrahmanyam, A.S.; Murthy, K.S.R.

    been performed to build Generalised Inverse Operator (GIO). This GIO is operated on the observed anomaly (with reference to the calculated) to yield improved model parameters. Data and model resolution matrices are computed to check the correctness...

  9. The flood, the channels, and the dykes : managing legal information a globalized and digital world


    Breuker, Joost; Casanovas, Pompeu; Klein, Michel C. A.; Francesconi, Enrico


    Information search and retrieval are part of daily routines of the legal profession. Lawyers, judges, prosecutors, and legal clerks usually access a number of electronic resources to browse, search, select, or update legal contents. Legal databases have currently become large digital libraries where the tasks related to information-seeking may sometimes be cumbersome. Adding semantics to support information search may provide significant results in terms of efficiency, efficacy, and user sati...

  10. SHRIMP dating of the Bangong Lake SSZ-type ophiolite:Constraints on the closure time of ocean in the Bangong Lake-Nujiang River, northwestern Tibet

    Institute of Scientific and Technical Information of China (English)

    SHI RenDeng


    The Bangong Lake ophiolite is located in the westernmost part of the Bangong Lake-Nujiang River and massive lavas and mafic dykes with SSZ-type ophiolitic geochemical affinity formed at the end of a Wilson circle. The SHRIMP U-Pb ages of the co-magmatic zircon domains from one gabbroic dyke (Sample 01Y-155) range from 162.5±8.6 Ma to 177.1±1.4 Ma with an average of 167.0+1.4 Ma (n = 12,MSWD = 1.2), suggesting that the subduction of the Bangong Lake Neo-Tethyan Ocean started before the Middle Jurassic. It is inferred that the tectonic transform from spreading to subduction of the Neo-Tethyan Ocean began before the Middle Jurassic in the Bangong Lake area.

  11. Contact metamorphism, partial melting and fluid flow in the granitic footwall of the South Kawishiwi Intrusion, Duluth Complex, USA (United States)

    Benko, Z.; Mogessie, A.; Molnar, F.; Severson, M.; Hauck, S.; Lechler, P.; Arehart, G.


    The footwall of the South Kawishiwi Intrusion (SKI) a part of the Mesoproterozoic (1.1 Ga) Duluth Complex consists of Archean granite-gneiss, diorite, granodiorite (Giant Range Batholith), thin condensed sequences of Paleoproterozoic shale (Virginia Fm.), as well as banded iron formation (Biwabik Iron Fm). Detailed (re)logging and petrographic analysis of granitic footwall rocks in the NM-57 drillhole from the Dunka Pit area has been performed to understand metamorphic processes, partial melting, deformation and geochemical characteristics of de-volatilization or influx of fluids. In the studied drillhole the footwall consists of foliated metagranite that is intersected by mafic (dioritic) dykes of older age than the SKI. In the proximal contact zones, in the mafic dykes, the orthopyroxene+clinopyroxene+plagioclase+quartz+Fe-Ti-oxide+hornblende±biotite porphyroblasts embedded in a plagioclase+K-feldspar+orthopyroxene+apatite matrix indicate pyroxene-hornfels facies conditions. Migmatitization is revealed by the euhedral crystal faces of plagioclase and pyroxene against anhedral quartz crystals in the in-situ leucosome and by the presence of abundant in-source plagioclase±biotite leucosome veinlets. Amphibole in the melanosome of mafic dykes was formed with breakdown of biotite and implies addition of H2O to the system during partial melting. Towards the deeper zones, the partially melted metatexite-granite can be characterized by K-feldspar+plagioclase+quartz+ortho/clinopyroxene+biotite+Fe-Ti-oxide+apatite mineral assemblage. The felsic veins with either pegmatitic or aplititic textures display sharp contact both to the granite and the mafic veins. They are characterized by K-feldspar+quartz±plagioclase±muscovite mineral assemblage. Sporadic occurrence of muscovite suggest local fluid saturated conditions. Emplacement of gabbroic rocks of the SKI generated intense shear in some zones of the granitic footwall resulting in formation of biotite-rich mylonites with

  12. Assimilation of carbonate country rock by the parent magma of the Panzhihua Fe-Ti-V deposit (SW China: Evidence from stable isotopes

    Directory of Open Access Journals (Sweden)

    Clément Ganino


    Full Text Available The Panzhihua intrusion in southwest China is part of the Emeishan Large Igneous Province and host of a large Fe-Ti-V ore deposit. During emplacement of the main intrusion, multiple generations of mafic dykes invaded carbonate wall rocks, producing a large contact aureole. We measured the oxygen-isotope composition of the intrusions, their constituent minerals, and samples of the country rock. Magnetite and plagioclase from Panzhihua intrusion have δ18O values that are consistent with magmatic equilibrium, and formed from magmas with δ18O values that were 1–2‰ higher than expected in a mantle-derived magma. The unmetamorphosed country rock has high δ18O values, ranging from 13.2‰ (sandstone to 24.6–28.6‰ (dolomite. The skarns and marbles from the aureole have lower δ18O and δ13C values than their protolith suggesting interaction with fluids that were in exchange equilibrium with the adjacent mafic magmas and especially the numerous mafic dykes that intruded the aureole. This would explain the alteration of δ18O of the dykes which have significantly higher values than expected for a mantle-derived magma. Depending on the exact δ18O values assumed for the magma and contaminant, the amount of assimilation required to produce the elevated δ18O value of the Panzhihua intrusion was between 8 and 13.7 wt.%, assuming simple mixing. The exact mechanism of contamination is unclear but may involve a combination of assimilation of bulk country rock, mixing with a melt of the country rock and exchange with CO2-rich fluid derived from decarbonation of the marls and dolomites. These mechanisms, particularly the latter, were probably involved in the formation of the Fe-Ti-V ores.

  13. Emplacement of a Thick Mafic Extrusive Body: A CSD Solution to a Map-Generated Question (United States)

    Pollock, M.; Sloan, J. A.; Siler, D. L.; Karson, J.


    A petrological study of crystallization in thick extrusive bodies is motivated by the geologic mapping of glacially excavated volcanic crust exposed along the Vatnsdalsfjall mountain range in northwest Iceland. Vatnsdalsfjall reveals a regional flexure zone, in which ~7 Ma lava flows dip 5-10° westward toward the extinct Skagi-Hunafloi rift axis. The regional flexure is disrupted by a local bowl-shaped depression, where lava flows steepen to ~50° as they dip below a ~250 m thick, ~3 km long igneous unit known as the Hjallin lens (HL). Its lenticular shape and well-developed columnar jointing most likely led Annels (1968) to initially map the HL as a shallow-level intrusion. However, more recent field investigations by McClanahan and Wobus (2004) reveal that the HL is an extrusive body, as evidenced by a distinct basal contact that consists of a flow-banded rhyolitic ash flow tuff underlain by a sedimentary layer containing organic material. Based on preliminary petrologic and geochemical analyses, McClanahan and Wobus (2004) describe the HL as a fine-grained, aphyric basalt that is nearly uniform in mineralogy, texture, and composition. Further field work by Siler (2011) shows that the HL overlies basaltic lavas that thicken toward the center of the bowl-shaped depression, indicating that the lava flows were emplaced during subsidence. The mapped relationship of the HL to the underlying lava flows suggests that the lens is a syn- (or post-) subsidence extrusion emplaced into an existing paleotopographic low. When considered with the field relationships, the homogeneous and fine-grained character of the lens naturally presents a petrological question: Was the anomalously thick extrusive body emplaced in a single eruptive event or is the HL the product of multiple flows that erupted in rapid succession? To understand the formation the HL, we measured the crystal size distribution (CSD) of plagioclase in backscattered electron (BSE) images of 20 samples from three separate vertical transects. CSDs preserve the cooling history of igneous rocks and can be related to crystallization parameters, including cooling rate (e.g., Cashman, 1993; Resmini, 2007; Pupier et al., 2008). Variations in the lens CSDs are nonsystematic, revealing that the extrusive unit is not texturally uniform. Although the base of the lens generally contains fine plagioclase microlites (~3 μm long) set in a glassy matrix while plagioclase in the interior occurs as well-formed (~100 μm) laths, the irregular variations in CSDs suggest that cooling rate is not simply related to distance from the base. Furthermore, cooling rates interpreted from CSDs are at least 2 orders of magnitude larger than cooling rates expected by conductive cooling alone. Thus, CSDs are inconsistent with the formation of the HL as a simple, single body. CSDs will be combined with geochemical data to evaluate possible modes of emplacement for thick extrusive bodies (e.g., multiple flows; lava lake with catalyzed cooling). Understanding the mode of emplacement will yield insights into the relationship of the lens to the underlying lava flows and the timing of volcanic activity relative to subsidence.

  14. Somma-Vesuvius Plinian Eruptions fed by mafic magma: insights from bubbles in melt inclusions (United States)

    Esposito, R.; Redi, D.; Cannatelli, C.; Danyushevsky, L. V.; Lima, A.; Bodnar, R. J.; De Vivo, B.


    Mt. Somma-Vesuvius Plinian eruptions were first described by Pliny the younger in 79 AD during the infamous eruption that destroyed Pompeii. Today, such eruptions are still a concern to the nearly 3 million people living in the Naples metropolitan area. Understanding the source for Mt. Somma-Vesuvius magma and the coexisting volatile phase is vital to better constrain the long-term eruptive behavior of this volcano. In the present study, ~ 50 olivine phenocrysts were selected from lavas and pumices produced during mild effusive events referred to as inter-Plinian eruptions, and from highly explosive Plinian eruptions that occurred at Mt. Somma-Vesuvius between 33000 ka and 1631 AD. Selected olivine phenocrysts containing MI were examined petrographically and analyzed for Fo content. Fo varies from 69 to 73 mole% for inter-Plinian olivine crystals and from 84 to 90 mole% with one zoned olivine containing 76-81 mole% Fo, for Plinian olivine crystals. Investigated MI vary from slightly crystallized to highly crystallized. Selected crystallized MI were reheated using the Vernadsky stage, and quenched to a homogeneous glass (Group 1) or glass plus a vapor bubble (Group 2). On one hand, MI of Group 1 are hosted in olivine ranging from Fo72 to Fo76 and were all erupted from the Pompeii eruption (white pumice deposit). On the other hand, MI of Group 2 are trapped in olivine ranging from Fo69 to Fo81 and from Fo84 to Fo90, and the hosts are representative of both Plinian and inter-Plinian events. The only eruption where Group-1 and Group-2 MI coexist is the Pompeii eruption. Group 2 MIs were further analyzed by Raman to test for the presence of volatiles (CO2 or H2O) in the vapor bubbles. CO2 was detected in all MI analyzed. CO2 density was determined using the distance between the two Fermi-diad peaks, and ranges between 0.14 and 0.55 g/cm3. Six MI also showed evidence for H2O in the vapor bubble. In addition, carbonates were detected at the glass-vapor interface of five MI. This study shows that the CO2-rich fluid phase, which might exsolve from Plinian melts contain a significant amount of H2O. In addition, the first melting temperature of the fluids in the vapor bubble (~ -58ºC) suggests that volatile components, other than CO2, are included in the vapor bubbles.

  15. Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

    Indian Academy of Sciences (India)

    Shantanu Keshav; Gudmundur H Gudfinnsson


    Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites, nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth’s history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace element geochemistry, it has been suggested that these lavas represent low-degree melts that have undergone little crystal fractionation en route to the surface. Many of these lavas also carry highpressure mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that these magmas cooled to variable extents during their passage. We note subtle, yet important, differences in terms of CaO, Al2O3, CaO/Al2O3, and CaO/MgO. High-pressure experimental melting studies in CMAS-CO3 (3–8GPa) and natural lherzolitic systems (3GPa) demonstrate that at an isobar increasing leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet (≥ 85km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with anhydrous ones, also have higher CaO content in the coexisting melt at a given and . Comparison of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally determined high-pressure melts indicates that melting of anhydrous mantle lherzolite or garnet pyroxenite is not able to explain many of the major element systematics of the lavas. However, high-pressure partial melts of carbonated lherzolite have the right major element trends. Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced partial melting. On continents, lavas from South Africa and certain localities in Germany are the products of a very low degree of partial melting, and those from Texas and certain other localities in Germany are products of a slightly more advanced degree of partial melting of a carbonated lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas does not necessarily mean that the erupted lavas represent direct mantle melts.

  16. Structural control on basaltic dike and sill emplacement, Paiute Ridge mafic intrusion complex, southern Nevada

    Energy Technology Data Exchange (ETDEWEB)

    Carter Krogh, K.E.; Valentine, G.A.


    Late Miocene basaltic sills and dikes in the Paiute Ridge area of southern nevada show evidence that their emplacement was structurally controlled. Basaltic dikes in this area formed by dilating pre-existing vertical to steeply E-dipping normal faults. Magma propagation along these faults must have required less energy than the creation of a self-propagated fracture at dike tips and the magma pressure must have been greater than the compressive stress perpendicular to the fault surface. N- to NE-trending en echelon dikes formed locally and are not obviously attached to the three main dikes in the area. The en echelon segments are probably pieces of deeper dikes, which are segmented perhaps as a result of a documented rotation of the regional stresses. Alternatively, changes in orientation of principal stresses in the vicinity of each en echelon dike could have resulted from local loads associated with paleotopographic highs or nearby structures. Sills locally branched off some dikes within 300 m of the paleosurface. These subhorizontal bodies occur consistently in the hanging wall block of the dike-injected faults, and intrude Tertiary tuffs near the Paleozoic-Tertiary contact. The authors suggest that the change in stresses near the earth`s surface, the material strength of the tuff and paleozoic rocks, and the Paleozoic bedding dip direction probably controlled the location of sill formation and direction of sill propagation. The two largest sills deflected the overlying tuffs to form lopoliths, indicating that the magma pressure exceeded vertical stresses at that location and that the shallow level and large size of the sills allowed interaction with the free (earth`s) surface. 32 refs., 4 figs., 1 tab.

  17. Constraining the Thickness of the Crystal Mush in Layered Mafic Intrusions (United States)

    Holness, M. B.; Tegner, C.; Nielsen, T. F.


    When basaltic magma stalls in the crust, cooling leads to growth of a marginal mushy layer. The thickness of this crystal mush can be constrained using the step-changes in textural maturity (quantified by the median augite(cpx)-plag-plag dihedral angle, Θcpp) caused by the change in fractional latent heat accompanying the arrival of a new liquidus phase. At the instant of its saturation in the bulk magma, the top of the mush is marked by the first appearance of the new primocryst phase. At this moment, the high porosity upper zone of the mush comprises poorly consolidated material, with no cpx-plag-plag junctions: at deeper levels porosity decreases by primocryst overgrowth, growth of interstitial augite, and compaction. In the context of dihedral angle populations, the mush zone can be divided into 3: the upper zone, where melt is adjacent to all plag-plag junctions; the middle zone where augite fills some of the pore corners; and the lower zone where augite fills all pore corners. For our purposes, the base of this lower zone corresponds to the point at which diffusive change of grain boundary orientations has effectively ceased. The change in fractional latent heat accompanying the addition to the liquidus assemblage will be fully recorded within the upper zone since all cpx-plag-plag junctions are created after the change and its consequent decrease in the contribution of sensible heat to the total enthalpy loss: Θcpp will be high. The middle zone will record a mixture of the new and old thermal regime: junctions which were melt-filled at the moment of arrival of the new phase will have less opportunity to increase the cpx-plag-plag angle, while those which were already filled by augite will have higher angles. Θcpp will therefore range from the new higher value at the top of middle zone, to some lower value at the base. Cumulates in the lower zone had no melt-filled junctions: Θcpp will increase from a low value corresponding to the previous value of fractional latent heat recorded at the base of the lower zone to meet the lowest angles recorded at the base of the middle zone. The width and shape of the step-change in Θcpp thus tells us about mush thickness and structure. The arrival of cumulus apatite in the Skaergaard Layered Series coincides with the associated step-change in Θcpp in the centre of the intrusion, constraining mush thickness to be of the order of a few metres. However, the first appearance of cumulus apatite in floor cumulates adjacent to the intrusion walls is offset by 150m from the associated step-change in Θcpp, consistent with a mush some 150m thick. These differences are associated with significant variations in trapped liquid content calculated from bulk rock P2O5 concentration: the cumulates forming in the centre of the intrusion floor contain ~ 5 vol.% trapped liquid, while those near the walls contain ~15 vol.% trapped liquid. Thick mushes in the Skaergaard chamber thus are more orthocumulate in nature than the thin mush. These differences probably reflect variable contributions of eroded material from the vertical chamber walls, compounded by less efficient compaction in the more rapidly cooled marginal regions.

  18. Extreme iron enrichment and liquid immiscibility in mafic intrusions: Experimental evidence revisited (United States)

    Veksler, Ilya V.


    This paper examines phase equilibria and mass balance constraints on Fe enrichment in tholeiitic liquids in plutonic environments. The peak of Fe enrichment is thought to roughly coincide with magma saturation in Fe-Ti oxides; and olivine starts to react with the liquid at about the same time. This crucial stage of crystallization is examined in detail using a compilation of chemical analyses of 64 experimental charges that comprise liquids (quenched glasses) equilibrated with the liquidus assemblage of olivine, plagioclase, high-Ca pyroxene, and low-Ca pyroxene. Some samples also contain Fe-Ti oxides. It is shown that the 4-phase liquidus assemblage does not constrain a narrow range of liquid compositions. The concentrations of SiO 2 in the selection of experimental glasses vary broadly from 42 to 66 wt.%. Silica content shows strong negative correlations with FeO and CaO/Al 2O 3, and strong positive correlation with alkalis. Extreme Fe enrichment above 22 wt.% FeO is observed only in alkali-free or alkali-poor liquids. Broad compositional variations for the multiply-saturated liquids are attributed to strong non-ideality and complex concentration-activity relationships in ferrobasaltic melts. Liquid immiscibility characteristic of Fe-rich silicate liquids is the ultimate consequence of non-ideality. Petrogenetic implications of phase equilibria and mass balance constraints are discussed for a classical example of the Skaergaard intrusion in East Greenland, where the trend of extreme Fe enrichment has been in contention. It is proposed that seemingly conflicting results of experiments on Skaergaard natural cumulate rocks and model melt compositions can be reconciled if it is assumed that silicate liquid immiscibility in Skaergaard started not at the very end of crystallization but earlier, soon after the start of ilmenite and magnetite crystallization.

  19. Geochronology and geochemistry of the Heihe mafic pillow lavas in the Qinling Mountains, China

    Institute of Scientific and Technical Information of China (English)

    ZHANG; Zongqing


    [1]Lee, C. Y., A preliminary study of plate tectonics of China, Bull. Chinese Acad. Geol. Sci. (in Chinese), 1980, 2(1): 11-19.[2]Lee, C. Y., Tectonic evolution of Asia, Bull. Chinese Acad. Geol. Sci. (in Chinese), 1984, 10: 3-11.[3]Ren Jishun, Jiang Chunfa, Zhang Zhengkun et al., The Geotectonic Evolution of China (in Chinese), Beijing: Science Press, 1980.[4]Ren Jishun, Chen Tingyu, Niu Baogui et al., Tectonic Evolution of the Continental Hithospheres and Metallogeny in Eastern China and Adjacent Areas (in Chinese), Beijing: Science Press, 1990.[5]Zhang Qiusheng, Zhu Yongzheng, The Paleozoic ophiolite in the east Qinling, J. Changchun College of Geology (in Chi-nese), 1984, 3: 1-13.[6]Xiao Siyun, Zhang Weiji, Song Ziji, The Metamorphic Strata of the Northern Qinling in China (in Chinese), Xi'an: Xi'an Jiaotong University Press, 1988.[7]Xu Zhiqin, Lu Yilun, Tang Yaoqing et al., Formation of the Composite Eastern Qinling Chains (in Chinese), Beijing: China Environmental Science Press, 1988.[8]Zhang Guowei, Formation and evolution of the Qinling orogenic belt (in Chinese), Xi'an: Northwest University Press, 1988.[9]Zhang Guowei, Meng Qingren, Yu Zaiping, Orogenesis and dynamics of the Qinling orogen, Science in China, Ser. D, 1996, 39(3):225-234.[10]Wu Liren, Xu Guizhong, Geological Evolution in the East Qinling-Dabieshan Collision Orogen (in Chinese), Beijing: Science Press, 1988.[11]Sun Yong, Yu Zaiping, A discussion of an ancient ocean and Caledonian in the east Qinling, in A Selection of Papers Pre-sented at the Conference on the Qinling Orogenic Belt (in Chinese), Xi'an: Northwest University Press, 1991, 167-173.[12]Zhang Zongqing, Liu Dunyi,Fu Guomin et al., Geochronology of Metamorphic Strata in the North Qinling Area (in Chi-nese), Beijing: Geological Publishing House, 1994.[13]Cui Zhilin, Sun Yong, Wang Xueren, Discovery of radiolarian fossils within the Danfeng ophiolite in the Qinling and its significance, Chinese Science Bull., 1995, 40(19):1686-1688.[14]Pei Xianzhi, Composition and Tectinic Evolution of the Shangdan Structural Zone in the East Qinling, China (in Chinese), Xi'an: Xi'an Maps Publishing House, 1997.[15]Jiang Changyi, Su Shengrui, Zhao Taiping et al., Intruded Rock Belt in the North Qinling Area and Jinning Movement (in Chinese), Beijing: Geological Publishing House, 1988.[16]Zhang Zongqing, Zhang Guowei, Fu Guomin et al., Geochronology of metamorpic strata in the Qinling Mountains and its tectonic implications, Science in China, Ser. D, 1996, 39(3): 283-292.[17]Zhang Qi, Zhang Zongqing, Sun Yong et al., Trace element and isotopic geochemistry of metebasalts from Danfeng Group in Shangxian-Danfeng area, Shaanxi Province, Acta Petrologica Sinica (in Chinese), 1995, 11(1): 43-54.[18]Zhang Benren, Luo Tingchuan, Ouyang Jianping et al., Geochemical Study of the Lithosphere, Tectonism and Metallo-genesis in the Qingling-Dabashan Region (in Chinese), Wuhan: China University of Geosciences Press, 1994.[19]Li, S., Hart, S. R., Zheng, S. et al., Timing of collision between the North and South China blocks: The Sm-Nd isotopic age evidence, Science in China, Ser. B, 1989, 32(3): 312-319.[20]Li, S., Chen, Y. Z., Zhang Guowei, A l Ga B. P. Alpine peridotite body emplaced into the Qingling group: Evidence for the existence of the late proterozoic plate tectonics in the North Qinling area, Geol. Review (in Chinese), 1991, 37(3): 235-242.[21]Zhang Guowei, Meng Qingren, Lai Shaocong, Tectonics and structure of Qinling orogenic belt, Science in China, Ser. B, 995, 38(10): 1379.[22]Liu Guohui, Zhang Shouguang, You Zhendong et al., The Major Metamorphic Rock Groups in the Qinling Orogenic Belt and Their Metamorphic Evolution (in Chinese), Beijing: Geological Publishing House, 1993.[23]You Zhendong, Suo Shutian, Han Yujing et al., The major characteristcs of the metamorphic complex of the Qinling oro-genic belt and their implication of the construction of the eastern Qinling continental crust, in A Selection of Papers Pre-sented at the Conference on the Qinling Orogenic Belt (in Chinese), Xi'an: Northwest University Press, 1991, 1-14.[24]Wang Tao, Li Wuping, Wang Xiaoxia, Zircon age of the Niujiaoshan granitoid gneisses in the Qinling complex of the Qinling orogenic belt, Regional Geology of China (in Chinese), 1998, 17(3): 262-265.[25]Lu Xinxiang, Dong You, Wei Xiangdong, Age of the Tuwushan A-type granite and its tectonic implications, Chinese Science Bulletin, 1999, 44(9): 975-978.

  20. The mafic, ultramafic and metamorphic xenoliths in triassic evaporite complexes, North West Algeria

    Energy Technology Data Exchange (ETDEWEB)

    Midoun, M.; Seddiki, A.


    In northwestern Algeria, triassic evaporate bodies contain varied xenoliths extracted from the lower crust and upper mantle and are interpreted as evidence of crustal thinning at the beginning of the Triassic period. Similar materials are known to occur in the internal areas of the western Mediterranean chains, which allow us to propose the existence of a wide area of crustal thinning during the Triassic along the future Tethyan axis. (Author)

  1. Bedout basement rise, offshore northwestern Australia: evidence of an unshocked mafic volcanic hyaloclastite volcanic breccia (United States)

    Glikson, A.


    Core samples from Bedout-1 (3035.8-3044.95 m.), Bedout basement rise, offshore northwestern Australia, were examined by optical microscopy, SEM, EDS and WDS spectrometry. At this stratigraphic depth level Becker et al. (2004) interpret cryptocrystalline alteration zones around and within plagioclase in terms of shock-induced transformation of feldspar into diaplectic maskelynite glass _u postulating a ~200 km-large impact structure and thereby an impact connection of the Permian-Triassic boundary mass extinction. However, the breccia is dominated by fragments of microlitic basalt and ophitic-textured dolerite with well preserved igneous textures, showing no evidence of shock metamorphism. Euhedral pseudomorphs of chlorite and amphibole, probably after pyroxene, protrude into or are enveloped by euhedral albite-twinned calcic plagioclase (andesine to bytownite). Minor phases include euhedral ilmenite needles and subhedral magnetite grains. Plagioclase is altered by cryptocrystalline albite and microcrystalline albite-chlorite matrix along crystal boundaries, along twin lamella and within internal oscillatory crystal zones, consistent with burial metamorphosed hydrovolcanic basalts and spilites (e.g. Amstutz, 1974). The volcanic fragments are set within, and injected by, microcrystalline intergranular mesostasis of mixed mineral fragments and volcanic meta-glass. Becker et al. (2004) refer to the breccia in part as product of Mg-rich sediments (e.g. dolomites). However, apart from the pristine igneous textures of the breccia, the transition element levels (chlorite in dolerite fragment "C Ni 97-160 ppm; Co 75-152 ppm; Cu 69-204 ppm; mesostasis "C Ni 29-45 ppm; Co 18-52 ppm; Cu 26-110 ppm) are consistent with Fe-rich basalts but exceed common abundances in carbonates and marls (BVTP, 1981; Wedepohl, 1978). No shock metamorphic features, such as planar deformation features (PDF), are observed in the feldspar or in any other phases. No criteria for discriminating maskelynite and volcanic meta-glass are indicated by Becker et al. (2004). As PDF formation (10-35 GPa) necessarily precedes diaplectic transformation into maskelynite (35-45 GPa) (French, 1998), a presence of maskelynite is inconsistent with the absence of PDF in the plagioclase _u a phase prone to the development of shock effects (e.g. Mory, 2000). Little evidence exists for the hydrothermal activity which typically follows impact events. However, thanks to a partial development of a rim syncline-like structure in Triassic sediments around the Bedout rise, further testing of the origin of this remarkable structure is warranted. Amstutz, G.C., 1974, Spilites and Spilitic Rocks, Springer-Verlag, Berlin; Becker, L. et al., 2004, Science Express, 13.5.04; BVSP - Basaltic Volcanism Study Project, 1981, Pergamon; French, B.M., 1998, Traces of Catastrophe, Lunar and Planetary Contributions 954; Mory et al., 2000, Earth and Planetary Science, 177, 119-128; Wedepohl, K.H., Handbook of Geochemistry, Springer-Verlag, 1978.

  2. Magmatic Cu-Ni sulfide mineralization of the Huangshannan mafic-untramafic intrusion, Eastern Tianshan, China (United States)

    Zhao, Yun; Xue, Chunji; Zhao, Xiaobo; Yang, YongQiang; Ke, Junjun


    The Huangshannan Ni-Cu (-PGE) sulfide deposit, a new discovery from geological prospecting in Eastern Tianshan, is in a belt of magmatic Ni-Cu (-PGE) sulfide deposits along the southern margin of the Central Asian Orogenic Belt. The host intrusion of the Huangshannan deposit is composed of a layered ultramafic sequence and a massive gabbro-diorite unit. The major sulfide orebodies occur mainly within websterite and lherzolite in the layered ultramafic sequence. In-situ zircon U-Pb dating analyses yielded a crystallization age of 282.5 ± 1.4 Ma, similar to the ages of the Permian Tarim mantle plume. Samples from the Huangshannan intrusion are characterized by nearly flat rare earth elements patterns, negative Zr, Ti and Nb anomalies, arc-like Th/Yb and Nb/Yb ratios, and significantly lower rare earth element and immobile trace element contents than the Tarim basalts. These characteristics suggest that the Huangshannan intrusion was not generated from the Tarim mantle plume. The primary magma for the Huangshannan intrusion and its associated sulfide mineralization were formed from different pulses of picritic magma with different degrees of crustal contamination. The first pulse underwent an initial removal of 0.016% sulfide in the deep magma chamber. The evolved magma reached sulfide saturation again in the shallow magma chamber and formed sulfide ores in lherzolite. The second pulse of magma reached a level of 0.022% sulfide segregation at staging chamber before ascending up to the shallow magma chamber. In the shallow conduit system, this sulfide-unsaturated magma mixed with the first pulse of magma and with contamination from the country rocks, leading to the formation of sulfide ores in websterite. The third magma pulse from the deep chamber formed the unmineralized massive gabbro-diorite unit of the Huangshannan intrusion.

  3. Non-traditional stable isotope behaviors in immiscible silica-melts in a mafic magma chamber. (United States)

    Zhu, Dan; Bao, Huiming; Liu, Yun


    Non-traditional stable isotopes have increasingly been applied to studies of igneous processes including planetary differentiation. Equilibrium isotope fractionation of these elements in silicates is expected to be negligible at magmatic temperatures (δ(57)Fe difference often less than 0.2 per mil). However, an increasing number of data has revealed a puzzling observation, e.g., the δ(57)Fe for silicic magmas ranges from 0‰ up to 0.6‰, with the most positive δ(57)Fe almost exclusively found in A-type granitoids. Several interpretations have been proposed by different research groups, but these have so far failed to explain some aspects of the observations. Here we propose a dynamic, diffusion-induced isotope fractionation model that assumes Si-melts are growing and ascending immiscibly in a Fe-rich bulk magma chamber. Our model offers predictions on the behavior of non-traditional stable isotope such as Fe, Mg, Si, and Li that are consistent with observations from many A-type granitoids, especially those associated with layered intrusions. Diffusion-induced isotope fractionation may be more commonly preserved in magmatic rocks than was originally predicted.

  4. Application of Ground Penetrating Radar Surveys and GPS Surveys for Monitoring the Condition of Levees and Dykes

    Directory of Open Access Journals (Sweden)

    Tanajewski Dariusz


    Full Text Available This paper analyses the possibility of using integrated GPS (Global Positioning System surveys and ground penetrating radar surveys to precisely locate damages to levees, particularly due to the activity of small fossorial mammals. The technology of intercommunication between ground penetrating radar (GPR and an RTK (Real-Time Kinematic survey unit, and the method of data combination, are presented. The errors which may appear during the survey work are also characterized. The procedure for processing the data so that the final results have a spatial character and are ready to be implemented in digital maps and geographic information systems (GIS is also described.

  5. ESI-VI10 Jost Van Dyke, U.K. Virgin Islands 2000 (Environmental Sensitivity Index Map) (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Environmental Sensitivity Index (ESI) maps are an integral component in oil-spill contingency planning and assessment. They serve as a source of information in the...

  6. 护砂保堤固我"长城"%Protect the Sand Protect the Dyke Make the "Great Wall" Durable

    Institute of Scientific and Technical Information of China (English)



    @@ 84km长的盐场海堤宛如一条巨龙横卧在黄海海岸线上,它不仅为金桥盐业有限公司挡潮抵浪,同时也保护着赣榆、灌云、新浦、连云两县两区2 000km2、150万人口的生命财产安全.

  7. Application of Ground Penetrating Radar Surveys and GPS Surveys for Monitoring the Condition of Levees and Dykes (United States)

    Tanajewski, Dariusz; Bakuła, Mieczysław


    This paper analyses the possibility of using integrated GPS (Global Positioning System) surveys and ground penetrating radar surveys to precisely locate damages to levees, particularly due to the activity of small fossorial mammals. The technology of intercommunication between ground penetrating radar (GPR) and an RTK (Real-Time Kinematic) survey unit, and the method of data combination, are presented. The errors which may appear during the survey work are also characterized. The procedure for processing the data so that the final results have a spatial character and are ready to be implemented in digital maps and geographic information systems (GIS) is also described.

  8. Dykes of standards supporting polders of data : The practices used in the Netherlands for making archaeological data available and accessible

    NARCIS (Netherlands)

    Gilissen, V.


    In the Netherlands, the archiving and publication of archaeological research data has led to the establishment of EDNA, the e-Depot for Dutch Archaeology, accommodated at DANS (Data Archiving and Networked Services) EDNA is a collaboration between DANS and

  9. Application of generalized inverse for analysis of magnetic anomalies due to a dyke model - Some numerical experiments

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, M.M.M.; Murty, T.V.R.; SuryaPrakash, S.; Chandramouli, P.; Murthy, K.S.R.

    the determination of the parameters of the model is to be carried out comes up in observation blocks, some of which are over-determined while some are under-determined leading to non-uniqueness of the solution. The generalized inverse technique solves such a...),  is diagonal matrix of non-zero singular values (l i 2 = λ i ’) of M arranged in the decreasing order. Once U,  and V are obtained by solving above Eigenvalue problem (Eqs.12 and 13) the generalised inverse solution (Jackson 18 ) is given...

  10. The Fazenda Gavião granodiorite and associated potassic plutons as evidence for Palaeoproterozoic arc-continent collision in the Rio Itapicuru greenstone belt, Brazil (United States)

    Costa, Felipe G.; Oliveira, Elson P.; McNaughton, Neal J.


    Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, São Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gavião granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with Archaean migmatite-gneiss basement. The pluton is mostly composed of hornblende granodiorite, occasionally crosscut by syn-plutonic mafic dykes. The FGG is metaluminous, medium- to high-K calc-alkaline with relatively constant silica abundances (SiO2 ˜ 63-66 wt%), high Sr (900-800 ppm) and high Ba (1000-1500 ppm). The associated mafic dykes are ultrapotassic, with high abundances of Ba, Sr, MgO, Ni, Cr, and light rare earth elements, suggesting derivation from partial melts of an enriched mantle source. The FGG originated probably by fractional crystallization from a primitive K-rich mafic magma that interacted with crustal melts. Its zircon U-Pb SHRIMP age of 2106 ± 6 Ma indicates that the FGG is younger than the early (2163-2127 Ma) tonalite-trondhjemite-granodiorite (TTG) and calc-alkaline arc plutons of the greenstone belt, and is closely related in time and space with potassic to ultrapotassic plutons (ca. 2110-2105 Ma). The negative ɛNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive ɛNd(t) of the older arc plutons, indicating a major change of isotope signatures in granites of the Rio Itapicuru greenstone belt with time. This isotope shift may be related to magma contamination with older continental material and/or derivation of the parental potassic magma from enriched lithospheric mantle sources. We suggest that the K-rich plutons were emplaced during or shortly after Palaeoproterozoic arc-continent collision.

  11. Sr-Nd-Hf isotopes of the intrusive rocks in the Cretaceous Xigaze ophiolite, southern Tibet: Constraints on its formation setting (United States)

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


    The Cretaceous Xigaze ophiolite is best exposed at the central part of the Yarlung-Zangbo Suture Zone, Tibet Plateau. It consists of a thick section of mantle peridotites, but a relatively thin mafic sequence. This study presents geochronological and geochemical data for intrusive dykes (both mafic and felsic) and basalts to revisit the formation setting of the Xigaze ophiolite. The rodingites are characterized by high CaO and low Na2O contents relative to mafic dykes and show big variations in trace element compositions. Both gabbros and diabases have similar geochemical compositions, with MgO contents of 6.42-11.48 wt% and Mg# of 0.56-0.71. They display REE patterns similar to N-MORB and are variably enriched in large ion lithophile elements. Basalts have fractionated compositions and display LREE-depleted patterns very similar to N-MORB. They do not show obvious enrichment in LILE and depletion in high-field-strength elements, but a negative Nb anomaly is present. The studied plagiogranites have compositions of trondhjemite to tonalite, with high Na2O and low K2O contents. They have low TiO2 contents less than 1 wt%, consistent with melts formed by anatexis of gabbros rather than by differentiation of basalts. Zircons from seven samples, including three rodingites, three plagiogranites, and one gabbro, have been dated and yielded U-Pb ages of 124.6 ~ 130.5 Ma, indicating the Xigaze ophiolite was formed during the Early Cretaceous. They have mantle-like δ18O values of + 4.92 ~ + 5.26‰ and very positive εHf(t) values of + 16 ~ + 13.3. Ages of the rodingites and less altered gabbros indicate that serpentinization was occurred at ~ 125 Ma. Occurrence of both gabbroic and diabase dykes within the serpentinites suggests that the mantle lithosphere of the Xigaze ophiolite was rapidly exhumed. Both mafic and felsic dykes have slightly more radiogenic 87Sr/86Sr ratios relative to MORB, but depleted Hf-Nd isotpe compositions. They have a limited range of ε

  12. The giant Kalgoorlie Gold Field revisited

    Institute of Scientific and Technical Information of China (English)

    Noreen Mary Vielreicher; David Ian Groves; Neal Jesse McNaughton


    The Neoarchaean Kalgoorlie Gold Field contains the giant Golden Mile and world-class Mt Charlotte deposits, which have been the subject of much research for over 100 years. The Golden Mile deposit is a complex array of ductile to brittle vein and breccia lodes that are predominantly hosted in the highly-fractionated Golden Mile Dolerite sill. The Fimiston lodes comprise an array of narrow lodes that evolved broadly syn- to late-formation of the regional D2 NW-trending foliation. The lodes are charac-terized by pyrite veinlets and disseminations, quartz veinlets and breccias, and banded quartz-carbonate veins with sericite, carbonate, and pyrite-dominated alteration. Bonanza Green-Leader, or Oroya-style, lodes, with grades in excess of 1000 g/t Au, are similar to the Fimiston-style lodes, but are character-ized by abundant visible gold, native tellurium and more abundant telluride minerals within roscoelite-bearing alteration zones. The arguably structurally younger Mt Charlotte-style lodes are characterized by a pipe-shaped, coarse-grained quartz, carbonate and scheelite vein-stockwork with distinct vertically-zoned, carbonate-sericite-albite-pyrite ? pyrrhotite dominant alteration assemblages around veins within Unit 8 of the Golden Mile dolerite and porphyry dykes. The network of steep-and gently-dipping extension and shear fracture-fill veins are associated with NE-trending fault sets that cross cut the regional NW-trend. The deposit area is intruded by swarms of porphyry dykes, including syn-volcanic mafic dykes, early and volumetrically most significant c. 2.67 Ga feldspar-phyric porphyry dykes, as well as later c. 2.66‒2.65 Ga calc-alkaline hornblende-phyric dykes associated with younger c. 2.65‒2.64 Ga lamprophyre dykes. All post-volcanic dykes have similar orientations to the Fimiston lodes. The feldspar dykes are clearly overprinted by all styles of mineralization, although the relationship between hornblende-phyric and lamprophyre dykes and gold

  13. The Bero Volganic Group: New Lithological, Stratigraphic, and Geochemical Data of this Extension of the Parana-Etendeka Igneous Province into SW Angola with Implications (United States)

    Marsh, J.; Swart, R.


    The Bero Volcanic Group, an extension of the Etendeka-Paraná Igneous Province into SW Angola, forms the eroded basement to the on-shore Namibe Basin, an Early Cretaceous-Cenozoic terrestrial and marine sedimentary sequence. The igneous suite outcrops between latitudes 14.68o and 15.25o S and comprises quartz latite rheoignimbrites/lavas, tholeiitic basaltic lavas, pyroclastic/volcaniclastic deposits, minor aeolian sandstones, and mafic tholeiitic dykes and gabbroic sheets. Quartz latite lithologies dominate. In the Rio Bero area in the S quartz latites are underlain by several thin flows of basalt interbedded with, and underlain by, thin discontinuous lenses of aeolian sandstone. This sequence is consistent with the general stratigraphic sequence in the northern Etendeka of Namibia. To the N basalts and aeolian sandstones are absent and the quartz latites lie directly on Precambrian basement rocks in places. To date, data for a quartz latite correlated with a Chapecó rhyolites of the Paraná are available from only one locality in Angola. This study's wider sampling and major and trace element and radiogenic isotope analysis reveals the following: (1) all mafic rocks are high-Ti, the lavas being equivalent to the Khumiba/Urubici type; (2) mafic dykes cutting the quartz latites having affinities to the Paranapanema-Ribeira mafic lavas; (3) five quartz latite geochemical types are present, three of which are known from Etendeka/Paraná (Sarusas/Guarapuava, Khoraseb/Ourinhos and Ventura) and their stratigraphic relationships in Angola are consistent with those in the Etendeka and Paraná; (4) their Angolan occurrence significantly extends the area covered by, and potential eruptive volumes of, these silicic types; (5) two other quartz latite types are unknown in the Etendeka and Paraná and are probably products of low-volume, local eruptions. The Chinguau type is geochemically similar to the low-Ti quartz lalites of the southern Etendeka but has lower Epsilon Nd

  14. The geology, structure and mineralisation of the Oyu Tolgoi porphyry copper-gold-molybdenum deposits, Mongolia: A review

    Directory of Open Access Journals (Sweden)

    T.M. (Mike Porter


    Mineralisation is characterised by varying, telescoped stages of intrusion and alteration. Early A-type quartz veined dykes were followed by Cu-Au mineralisation associated with potassic alteration, mainly K-feldspar in quartz-monzodiorite and biotite-magnetite in basaltic hosts. Downward reflux of cooled, late-magmatic hydrothermal fluid resulted in intense quartz-sericite retrograde alteration in the upper parts of the main syn-mineral intrusions, and an equivalent chlorite-muscovite/illite-hematite assemblage in basaltic host rocks. Uplift, facilitated by syn-mineral longitudinal faulting, brought sections of the porphyry deposit to shallower depths, to be overprinted and upgraded by late stage, shallower, advanced argillic alteration and high sulphidation mineralisation. Key controls on the location, size and grade of the deposit cluster include (i a long-lived, narrow faulted corridor; (ii multiple pulses of overlapping intrusion within the same structure; and (iii enclosing reactive, mafic dominated wall rocks, focussing ore.

  15. Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

    Directory of Open Access Journals (Sweden)

    Brian G.J. Upton


    Full Text Available The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of <4 km and are essentially undisturbed by later events. Magmatism in the southern zone began with the emplacement of two giant, ≤800 m wide dykes and involved intrusion of transitional olivine basaltic, high Al/Ca magmas crystallising to troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c. 1163 Ma Younger giant dyke complex (YGDC mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokkenis considered to be coeval and presumably comagmatic with the YGDC. At the unconformitybetween the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustalassimilation.A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik. Three post-extensional complexes (Tugtutôq,Ilímaussaq and late Igdlerfigssalik along the former rift mark the end of magmatism at c. 1140 Ma. The latter two complexes have

  16. Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits (United States)

    Patten, Clifford G. C.; Pitcairn, Iain K.; Teagle, Damon A. H.; Harris, Michelle


    Volcanogenic massive sulphide (VMS) deposits are commonly enriched in Cu, Zn and Pb and can also be variably enriched in Au, As, Sb, Se and Te. The behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known. Ocean Drilling Program (ODP) Hole 1256D penetrates a complete in situ section of the upper oceanic crust, providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration. A representative suite of samples was analysed for Au, As, Sb, Se and Te using low detection limit methods, and a mass balance of metal mobility has been carried out through comparison with a fresh Mid-Oceanic Ridge Basalt (MORB) glass database. The mass balance shows that Au, As, Se, Sb, S, Cu, Zn and Pb are depleted in the sheeted dyke and plutonic complexes by -46 ± 12, -27 ± 5, -2.5 ± 0.5, -27 ± 6, -8.4 ± 0.7, -9.6 ± 1.6, -7.9 ± 0.5 and -44 ± 6 %, respectively. Arsenic and Sb are enriched in the volcanic section due to seawater-derived fluid circulation. Calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as VMS mineralisation. The quantity of Au mobilised and the ratio of Au to base metals are similar to those of mafic VMS, and ten times enrichment of Au would be needed to form a Au-rich VMS. The Cu-rich affinity of mafic VMS deposits could be explained by base metal fractionation both in the upper sheeted dykes and during VMS deposit formation.

  17. The Voisey's Bay Ni-Cu-Co Sulfide Deposit,Labrador,Canada:Emplacement of Silicate and Sulfide-Laden Magmas into Spaces Created within a Structural Corridor%The Voisey's Bay Ni-Cu-Co Sulfide Deposit,Labrador, Canada: Emplacement of Silicate and Sulfide-Laden Magmas into Spaces Created within a Structural Corridor

    Institute of Scientific and Technical Information of China (English)

    Peter C.Lightfoot; Dawn Evans-Lamswood; Robert Wheeler


    Abstract:The Voisey's Bay Ni-Cu-Co sulfide deposit is hosted in a 1.34 Ga mafic intrusion that is part of the Nain Plutonic Suite in Labrador,Canada.The Ni-Cu-Co sulfide mineralization is associated with magmatic breccias that are typically contained in weakly mineralized olivine gabbros,troctolites and ferrogabbros,but also occur as veins in adjacent paragneiss.The mineralization is associated with a dyke-like body which is termed the feeder dyke.This dyke connects the shallow differentiated Eastern Deeps chamber in the east to a deeper intrusion in the west termed the Western Deeps Intrusion.Where the conduit is connected to the Eastern Deeps Intrusion,the Eastern Deeps Deposit is developed at the entry line of the dyke along the steep north wall of the Eastern Deeps Intrusion.The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized Variable-Textured Troctolite (VTT) that reaches a maximum thickness above the ENE-WSW axis of the Eastern Deeps Deposit.At depth to the west,the conduit is adjacent to the south side of the Western Deeps Intrusion,where the dyke and intrusion contain disseminated magmatic sulfide mineralization.The Reid Brook Zone plunges to the east within the dyke,and both the dyke and adjacent paragneiss are mineralized.The Ovoid Deposit comprises a bowl-shaped body of massive sulfide where the dyke widens near to the present-day surface.It is not clear whether this deposit was developed as a widened-zone within the conduit or at the entry point into a chamber that is now lost to erosion.The massive sulfides and breccia sulfides of the Eastern Deeps are petrologically and chemically different when compared to the disseminated sulfides in the VTT; there is a marked break in Ni tenor (Ni content in 100% sulfide,abbreviated to [Ni] 100) and Ni/Co of sulfide between the two.The boundary of the sulfide types is often marked by strong sub-horizontal alignment of heavily digested and metamorphosed paragneiss fragments

  18. Subduction-related oceanic crust in the Khantaishir ophiolite (western Mongolia). (United States)

    Gianola, O.; Schmidt, M. W.; Jagoutz, O. E.


    Most of the oceanic crust is generated at mid oceanic ridges and only a minor portion results from magmatism related to subduction zones (i.e. back-arc basins or in incipient arcs). However it has been observed that several ophiolites preserve an oceanic crust displaying a subduction zone signature. Such a signature is also found in the Khantaishir ophiolite located in western Mongolia. This ~570 m.y. old ophiolite is ~480 km2 in size and displays a complete sequence, tectonically slightly dismembered during the emplacement process. The ophiolite exposes ~130 km2 of highly refractory harzburgitic mantle with local dunite channels and lenses. Towards its top the mantle is replaced by sub-horizontal km-wide discrete zones of pyroxenites situated either in the mantle or forming a crust-mantle transition zone overlain by gabbros. The crust is then composed of various gabbros and minor gabbronorite (both in part replaced by pyroxenites and/or cut by intermediate dykes), by a dyke/sill-complex and by pillow lavas. The entire ophiolite is re-equilibrated at lower greenschist facies conditions. Major and trace elements of the crustal rocks of the Khantaishir ophiolite show trends similar to those observed for the Izu-Bonin-Mariana subduction system. Mafic dykes/sills and pillow lavas of the Khantaishir ophiolite have overall basaltic-andesite compositions, resembling high-Mg andesites with an average SiO2 of 57 wt%. Their low TiO2 (high-Mg andesites and boninites from modern island arcs. This evidence suggests that the Kantaishir crust might represent the submarine initial stage of an incipient arc, probably when the preexisting oceanic crust is spread and incipient island arc crust is formed.

  19. Platinum-group elements in the Eastern Deccan volcanic province and a comparison with platinum metals of the western Deccan

    Indian Academy of Sciences (India)

    James Crocket; Dalim Paul; Trisha Lala


    This study is the first detailed investigation of the platinum-group elements (PGE) at the eastern margin of the Deccan volcanic province of India. One of the PGE, osmium, is not included largely because of analytical problems. The study is focused on mafic volcanics and dykes from four areas including Amarkantak, Umaria, Shahdol and Chirimiri. The first two localities represent two lava piles of about 170 and 400 m thickness respectively. In Umaria, 16 flows have been demarcated based on petrography and field studies. The Shahdol samples are basal lava formations overlying Gondwana sediments (Carboniferous) and the Chirimiri samples are dykes. In this study, the western Deccan province is defined as the Western Ghats plus Kutch. On average, the PGE are ∼20% higher in Amarkantak than Umaria and the flows are ∼13% higher in PGE than the dykes. A Zr vs. Pd scattergram found a strong positive correlation for these two elements except for one Umaria sample which indicated severe Pd loss. A comparison of west and east parts of the Deccan volcanic province using primitive mantle normalization showed that higher values prevailed in the western province suite in the Ni-Ir-Ru-Pt region. In contrast, eastern province values dominated in the Pd-Au-Cu region at the ‘Cu’ end of the profiles. A strong dominance of Pd in the eastern Deccan was also of interest. A number of factors, for example, percentage partial melting of the source rock and the temperature and pressure of partial melting strongly influence the character of these profiles. The observed PGE profile characteristics probably result in part from a long distance of subsurface transport of Deccan magma from the western to eastern regions.

  20. Timing of late Palaeoproterozoic metamorphism in the northern Belomorian Belt, White Sea region: conclusions from U-Pb isotopic data and P-T evidence

    Directory of Open Access Journals (Sweden)

    Torbjörn Skiöld


    Full Text Available Brilliant-looking zircon crystals from a garnet-amphibolitic, metamorphosed mafic dyke at Lyagkomina in the Lake Kovdozero area, northern Belomorian Belt, most probably mark the 1875±4 Ma age of peak late Palaeoproterozoic regional metamorphism which followed upon dyke intrusion associated with deformation of the Lapland-Kola orogenic tectonic collage. U-Pb ages of titanites (ca. 1860 Ma and rutiles (ca. 1750 Ma help reconstruct the retrograde path and the cooling rates which decreased with time. The new titanite and rutile ages agree well with similar age data previously obtained on a regional scale from the central part of the belt and suggest absence of variation in mineral ages as a function of lithological differences. P-T analysis of the metamorphic parageneses indicates initial sharp decompression after peak metamorphism and jump uplift with unroofing of ca. 9-10 km of the crust in the Belomorian Belt previously thickened by late Palaeoproterozoic orogenic thrusting and stacking.

  1. Crystallization and emplacement of the Lac St-Jean anorthosite massif (Quebec, Canada) (United States)

    Woussen, G.; Dimroth, E.; Corriveau, L.; Archer, P.


    The Lac St-Jean anorthosite massif underlies an area of over 20,000 km2 and has been emplaced into migmatitic gneisses of the central granulite terrain of the Grenville Province of the Canadian shield. Field data and petrography in an area straddling the anorthosite-gneiss contact, close to Chicoutimi (Quebec) permits an outline of its tecto-magmatic evolution. Depositional magmatic textures in the massif reveals that it crystallized from a magma in a relatively calm tectonic environment. The absence of fusion in pelitic gneisses at the contact proves that the crystallization did not take place at the level presently exposed. The parallelism of subvertical foliation in the enveloping gneisses and the anorthosite indicates that both were deformed together. It is suggested that the deformation results from a diapiric ascent of the anorthosite massif after its consolidation at depth. The depth of consolidation of the anorthosite is estimated at ˜ 25 30 km from subsolidus reaction between plagioclase and olivine. The diapiric ascent is further substantiated by the fact that three sets of mafic dykes of different ages, intrusive into the anorthosite, have a mineralogy which indicates successively decreasing P, T conditions of emplacement from granulite fades to amphibolite facies. An evolution of the basement gneisses and the anorthosite is proposed as a working hypothesis; it relies on the fact that metabasite dyke swarms in the basement gneisses represent a period of major crustal extension and could be used as a stratigraphic subdivision of the Grenville Province.

  2. Cenozoic diatreme field in Chubut (Argentina) as evidence of phreatomagmatic volcanism accompanied with extensive Patagonian plateau basalt volcanism?

    Institute of Scientific and Technical Information of China (English)

    Károly Németh; Ulrike Martin; Miguel J. Haller; Viviana L Alric


    @@ In Patagonia, Argentina, at the northern border of the Patagonian Cenozoic mafic plateau lava fields, newly discovered diatremes stand about 100 m above the surrounding plain. These diatremes document phreatomagmatic episodes associated with the formation of the volcanic fields. The identified pyroclastic and intrusive rocks are exposed lower diatremes of former phreatomagmatic volcanoes and their feeding dyke systems.These remotely located erosional remnants cut through Paleozoic granitoids and Jurassic/Cretaceous alternating siliciclastic continental successions that are relatively easily eroded. Plateau lava fields are generally located a few hundreds of metres above the highest level of the present tops of the preserved diatremes suggesting a complex erosional history and potential interrelation-ships between the newly identified diatremes and the surrounding lava fields. Uprising magma from theunderlying feeder dyke into the diatreme root zone intruded the clastic debris in the diatremes, inflated them and mingled with the debris to form subterranean peperite. The significance of identifying diatremes in Patagonia are twofold: 1) in the syn-eruptive paleoenvironment, water was available in various "soft-sediments", commonly porous, media aquifer sources, and 2) the identified abundant diatremes that form diatreme fields are good source candidates for the extensive lava fields with phreatomagmatism facilitating magma rise with effective opening of fissures before major lava effusions.

  3. Sphene-centered ocellar texture as a petrological tool to unveil the mechanism facilitating magma mixing (United States)

    Gogoi, Bibhuti; Saikia, Ashima; Ahmad, Mansoor


    The sphene-centered ocellar texture is a unique magma mixing feature characterized by leucocratic ocelli of sphene enclosed in a biotite/hornblende-rich matrix (Hibbard, 1991). The ocelli usually consist of plagioclase, K-feldspar and quartz with sphene crystals at its centre. Although geochemical and isotopic data provide concrete evidence for the interaction between two compositionally distinct magmas, the exact processes by which mixing takes place is yet uncertain. So, textural analysis can be used to decipher the behaviour of two disparate magmas during mixing. Presented work is being carried out on the sphene ocelli, occurring in hybrid rocks of the Nimchak Granite Pluton (NGP), to understand its formation while two compositionally different magmas come in contact and try to equilibrate. The NGP is ca. 1 km2in extent which has been extensively intruded by number of mafic dykes exhibiting well preserved magma mixing and mingling structures and textures in the Bathani Volcano-Sedimentary Sequence (BVSS) located on the northern fringe of the Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) of eastern Indian Shield. From petrographic and mineral chemical studies we infer that when basaltic magma intruded the crystallizing granite magma chamber, initially the two compositionally different magmas existed as separate entities. The first interaction that took place between the two phases is diffusion of heat from the relatively hotter mafic magma to the colder felsic one followed by diffusion of elemental components like K and incompatible elements from the felsic to the mafic domain. Once thermal equilibrium was attained between the mafic and felsic melts, the rheological contrasts between the two phases were greatly reduced. This allowed the felsic magma to back-vein into the mafic magma. The influx of back-veined felsic melt into the mafic system disrupted the equilibrium conditions in the mafic domain wherein minerals like amphibole, plagioclase and biotite



    KURT, Hüseyin; Kerim KOÇAK; ASAN, Kürşad; KARAKAŞ, Mustafa


    Biotite granitoid contain mainly quartz, biotite, plagioclase, K-feldspar, muscovite as main phases with minor amphiboles, and apatite, zircon, allanite and chlorite and serisite as accessory components. In contrast, enclaves are composed mainly of plagioclase, amphibole, augite, biotite, with accessory sphene, zircon and calcite and epidote. The granitoids including enclaves with metaluminous composition, display chemical and mineralogical characteristics of S-type granitoids, such as peralu...

  5. A model of sulphur solubility for hydrous mafic melts: application to the determination of magmatic fluid compositions of Italian volcanoes

    Directory of Open Access Journals (Sweden)

    M. Pichavant


    Full Text Available We present an empirical model of sulphur solubility that allows us to calculate f S2 if P, T, fO2 and the melt composition, including H2O and S, are known. The model is calibrated against three main experimental data bases consisting in both dry and hydrous silicate melts. Its prime goal is to calculate the f S2 of hydrous basalts that currently lack experimental constraints of their sulphur solubility behaviour. Application of the model to Stromboli, Vesuvius, Vulcano and Etna eruptive products shows that the primitive magmas found at these volcanoes record f S2 in the range 0.1-1 bar. In contrast, at all volcanoes the magmatic evolution is marked by dramatic variations in f S2 that spreads over up to 9 orders of magnitude. The f S2 can either increase during differentiation or decrease during decompression to shallow reservoirs, and seems to be related to closed versus open conduit conditions, respectively. The calculated f S2 shows that the Italian magmas are undersaturated in a FeS melt, except during closed conduit conditions, in which case differentiation may eventually reach conditions of sulphide melt saturation. The knowledge of f S2, fO2 and fH2O allows us to calculate the fluid phase composition coexisting with magmas at depth in the C-O-H-S system. Calculated fluids show a wide range in composition, with CO2 mole fractions of up to 0.97. Except at shallow levels, the fluid phase is generally dominated by CO2 and H2O species, the mole fractions of SO2 and H2S rarely exceeding 0.05 each. The comparison between calculated fluid compositions and volcanic gases shows that such an approach should provide constraints on both the depth and mode of degassing, as well as on the amount of free fluid in magma reservoirs. Under the assumption of a single step separation of the gas phase in a closed-system condition, the application to Stromboli and Etna suggests that the main reservoirs feeding the eruptions and persistent volcanic plumes at these volcanoes might contain as much as 5 wt% of a free fluid phase. Consideration of the magma budget needed to balance the amounts of volatiles emitted in the light of these results shows that the amount of nonerupted magma could be overestimated by as much as one order of magnitude.

  6. Low to Extremely Low Water Abundances Measured in Nominally Anhydrous Minerals in Mafic to Granitic Apollo Rock Clasts (United States)

    Simon, J. I.; Christoffersen, R.; Wang, J.; Alexander, C. M. O'D.; Mills, R. D.; Hauri, E. H.


    Lunar sample-based volatile studies have focused on assessing the inventory and distribution of water in the Moon. Some have focused on the relatively young mare basalts and pyroclastic glasses, which result from partial melting of the relatively young lunar mantle. Less certain is the water inventory for the oldest materials available, which have the greater potential to record the earliest history of volatiles in the Moon (and thus provide evidence for the "wet" vs. "dry" accretion hypotheses of the Earth-Moon system. Studies of volatiles in ancient lunar rocks have largely focused on apatite. One recent FTIR (Fourier Transform Infrared Radiometer) study of plagioclase reported a relatively "wet" (approximately 320 parts per million) magma for primordial ferroan anorthosites (FANs). Another, a NanoSIMS study of alkali feldspar, reported a "wet" (approximately 1 weight percentage) felsic magma, but due to the differentiation processes required for silicic magmatism in the lunar crust, predicted an essentially "dry" (less than 100 parts per million) bulk Moon. Thus, despite evidence that appears to complicate the early "dry" Moon paradigm, there is no apparent unanimity among the measurements, even those on apatite. This disparity is clearly seen by the order of magnitude different water estimates for lunar "alkali-rich suite rocks" (Fig. 1). Some of the apparent differences may be explained by recent improvements in the apatite-based water estimates that better account for relative compatibilities of OH-, Cl, and F. In the present work, we seek to expand our understanding of the volatile abundances in early formed lunar magmas, their source reservoirs, and to address the potential role that felsic magmas play on the lunar hydrogen budget over time by employing NanoSIMS analysis of nominally anhydrous minerals.

  7. Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury (United States)

    Namur, Olivier; Charlier, Bernard; Holtz, Francois; Cartier, Camille; McCammon, Catherine


    Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200-1750 °C), low- to high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S2- and forms complexes with Fe2+, Mg2+ and Ca2+. The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from 10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity (fO2), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW- 5.4 ± 0.4. We also calculate that the mantle of Mercury contains 7-11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.

  8. Petrogenesis and timing of mafic magmatism, South Taimyr, Arctic Siberia: A northerly continuation of the Siberian Traps? (United States)

    Reichow, M. K.; Saunders, A. D.; Scott, R. A.; Millar, I. L.; Barfod, D.; Pringle, M. S.; Rogers, N. W.; Hammond, S.


    The Siberian large igneous province (LIP) forms the world's most extensive continental exposure of basalt and has several sub-provinces surrounding it, which may be genetically related. The Taimyr peninsula of north Siberia is one of these sub-provinces and is frequently assumed to be the northerly continuation of the basalts exposed at Noril'sk, the best-studied area of the Siberian LIP. However, the correlation is uncertain. We present new major and trace element data from 35 samples of extrusive and intrusive rocks from Taimyr, with Sr and Nd isotope data from a subset of ten. The Taimyr rocks fall into two groups with low (~ 7 wt.%) and elevated (~ 9 wt.%) MgO concentrations. The high-MgO rocks display a restricted range of initial 87Sr/86Sr (0.705 to 0.706) and 143Nd/144Nd (0.5122 to 0.5124) ratios, and share bulk silicate earth normalised rare earth element patterns strikingly similar to data observed in the ore-related Noril'sk intrusions. The remaining low-MgO group samples have a broader range with higher Sr and lower Nd isotope values and higher incompatible trace element ratios (e.g., Th/Ta > 5.3 and La/Smn > 1.7) similar to the crustally-contaminated Nadezhdinsky and Morongovsky suite basalts of the Noril'sk region. The major and trace element data for both groups are consistent with a process of fractional crystallisation coupled with small degrees of assimilation of incompatible-element-enriched lower crust involving different contaminants. Trace element model calculations indicate a process of magma formation at large degrees of partial melting and at pressures of less than 3 GPa, probably within the garnet-spinel transition zone or the spinel stability field of the asthenospheric mantle. We obtained an argon plateau age of ~ 252 (252.7 ± 1.5) Ma and a ~ 239 Ma total fusion age from a Taimyr lava and intrusive sample, respectively, confirming that volcanism is only partly contemporaneous with the activity of the Siberian LIP. Although this is in agreement with previous interpretations, we argue that the age difference between both events is only ~ 13 Ma and probably less (~ 5 Ma) although further investigation of the relationship is required. Our data allow correlation with distinct Noril'sk members and most importantly to the ore-bearing (Ni-Cu) intrusions implying that whole rock chemistry could have value as a prospecting tool in Taimyr.

  9. The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement (United States)

    Petri, B.; Mohn, G.; Štípská, P.; Schulmann, K.; Manatschal, G.


    Contact aureoles rimming plutonic rocks are the locus of metamorphism and deformations witnessing magma emplacement mechanisms in the crust. In this study, structural and petrological observations are combined to mineral equilibria modelling to unravel the polyphase tectono-metamorphic history of the Permian Sondalo gabbro and its host rock, the Campo unit (Eastern Alps). The Campo unit consists of Grt-St-Ms-Bt-Pl-Qtz ± Sil ± And ± Crd mica schists attesting of a Carboniferous prograde P- T path, reaching 6 kbar/600 °C and subsequently 5.6 kbar/650 °C. This metamorphism is coeval with the formation of a sub-vertical NE-SW trending foliation (S1) and its overprint by a sub-vertical NW-SE trending foliation (S2). The heat brought by the Permian intrusives subsequently caused heating of the Campo unit at around 3-4 kbar/540 °C reflected by regional static crystallization of cordierite and andalusite porphyroblasts. During the intrusion of the Sondalo gabbro, thermal peak conditions are recorded by Grt-Sil-Spl-Crd-Ilm granulitic xenoliths at ~5.5 kbar/930 °C, subsequently exhumed at ~4 kbar during the development of a new foliation (S3). This foliation is localized around the pluton and moderately dips away from the centre of the pluton. In the migmatitic contact aureole, Grt-Sil-Bt-Pl-Qtz-Ilm and Grt-Sil-Crd-Spl-Bt-Kfs-Ilm residual rocks bear the new foliation (S3) and document a decompression from 6 kbar/750 °C to 5 kbar/725 °C and from 5.2 kbar/800 °C to reach 4.8 kbar/770 °C, respectively. The P- T- d paths recorded by the host rock and the xenoliths point to a two-step magma emplacement. First the Sondalo gabbro intruded the Campo unit causing heating of the host rock without deformation at 3-4 kbar. Second, the ductile flow along the pluton margins developed a new foliation (S3) during exhumation of the pluton and its immediate contact aureole from 6 to 4 kbar. Altogether, it indicates a progressive increase in mechanical coupling between the pluton and the host rock during magma emplacement.

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

    NARCIS (Netherlands)

    Arps, C.E.S.


    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-blastbeari

  11. The Formation of Carbonate Minerals and the Mobility of Heavy Metals during Water-CO2-Mafic Rock Interactions

    DEFF Research Database (Denmark)

    Olsson, Jonas

    are consistent with values of controlled laboratory experiments from the literature for Ba, Cd, Co, Cu, Mg, Mn, Na, Ni, Sr and Zn. The calcium carbonates also scavenge other elements, including rare earth elements (REE) and the toxic metals As and Pb. This and the next study can be considered natural analogues...... minerals, the reaction products are known to be manifold and reflect the complex composition of the basaltic material. Formation of secondary products, such aluminium and iron (hydr)oxides, are considered undesirable, because (1) they consume the cations that could be used to sequester CO2, thus compete...... with the process of carbonation, and (2) they can form a passivating layer, which inhibit dissolution of the basaltic material and slow down the carbonation process. The purpose of this thesis was to identify formation products, relevant to CarbFix, and assess their ability to immobilize toxic metals released from...

  12. The preliminary unit price analysis of the vertical seepage control for the dyke%垂直防渗单价初析

    Institute of Scientific and Technical Information of China (English)

    杨国顺; 刘伟; 党爱琴



  13. Was the Devonian geomagnetic field dipolar or multipolar? Palaeointensity studies of Devonian igneous rocks from the Minusa Basin (Siberia) and the Kola Peninsula dykes, Russia (United States)

    Shcherbakova, V. V.; Biggin, A. J.; Veselovskiy, R. V.; Shatsillo, A. V.; Hawkins, L.; Shcherbakov, V. P.; Zhidkov, G. V.


    Defining variations in the behaviour of the geomagnetic field through geological time is critical to understanding the dynamics of Earth's core and its response to mantle convection and planetary evolution. Furthermore, the question of whether the axial dipole dominance of the recent palaeomagnetic field persists through the whole of Earth's history is fundamental to determining the reliability of palaeogeographic reconstructions and the efficacy of the magnetosphere in shielding Earth from solar wind radiation. Previous palaeomagnetic directional studies have suggested that the palaeofield had a complex configuration in the Devonian period (419-359 Ma). Here we present new palaeointensity determinations from rocks aged between 408 and 375 Ma from the Minusa Basin (southern Siberia), and the Kola Peninsula to investigate the strength of the field during this enigmatic period. Palaeointensity experiments were performed using the thermal Thellier, microwave Thellier, and Wilson methods on 165 specimens from 25 sites. Six out of eight successful sites from the Minusa Basin and all four successful sites from the Kola Peninsula produced extremely low palaeointensities (uniformitarian view of the palaeomagnetic field: field intensities of nearly an order of magnitude lower than Neogene values (except during relatively rare geomagnetic excursions and reversals) together with the widespread appearance of strange directions found in the Devonian suggest that the Earth's field during this time may have had a dominantly multipolar geometry. A persistent, low intensity multipolar magnetic field and associated diminished magnetosphere would increase the impact of solar particles on the Earth's magnetosphere, ionosphere and atmosphere with potential major implications for Earth's climate and biosphere.

  14. Combined Paleomagnetic And Petromagnetic Study Of Oligocene Dyke Swarms From Tecalitlan Area: Implications For Relative Motion Of Jalisco And Michoacan Blocks (United States)

    Aguilar Reyes, B.; Rosas Elguera, J.; Gogichaishvili, A.; Alva Valdivia, L.


    Results of detailed paleomagnetic studies from Tecalitlan area are reported. Sexteen sites (about 150 oriented samples) were collected including one Ar-Ar dated ignimbrite. Rock-magnetic experiments permitted identification of the magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature, in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Judging from the ratios of hysteresis parameters, all samples appear to fall in the pseudo- single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Declination is significantly different from that expected which may suggests a counterclockwise tectonic rotation of 19°.

  15. Platinum-group elements: so many excellent properties (United States)

    Zientek, Michael L.; Loferski, Patricia J.


    The platinum-group elements (PGE) include platinum, palladium, rhodium, ruthenium, iridium, and osmium. These metals have similar physical and chemical properties and occur together in nature. The properties of PGE, such as high melting points, corrosion resistance, and catalytic qualities, make them indispensable to many industrial applications. PGE are strategic and critical materials for many nations because they are essential for important industrial applications but are mined in a limited number of places and have no adequate substitutes. Exploration and mining companies have found approximately 104,000 metric tons of PGE (with minor gold) in mineral deposits around the world that could be developed. For PGE, almost all known production and resources are associated with three geologic features: the Bushveld Complex, a layered mafic-to-ultramafic intrusion in South Africa; the Great Dyke, a layered mafic-to-ultramafic intrusion in Zimbabwe; and sill-like intrusions associated with flood basalts in the Noril’sk-Talnakh area, Russia. To help predict where PGE supplies might be located, USGS scientists study how and where PGE resources are concentrated in the Earth's crust and use that knowledge to assess the likelihood that undiscovered PGE deposits may exist. Techniques used for assessing mineral resources were developed by the USGS to support the stewardship of Federal lands and evaluate mineral resource availability in a global context. The USGS also compiles statistics and information on the worldwide supply, demand, and flow of PGE. These data are all used to inform U.S. national policymakers.

  16. Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica:Neoarchean magmatism and Neoproterozoic high-grade metamorphism

    Institute of Scientific and Technical Information of China (English)

    Toshiaki Tsunogae; Daniel J. Dunkley; Kenji Horie; Takahiro Endo; Tomoharu Miyamoto; Mutsumi Kato


    We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss), mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750e850 ?C, approximately 150 ?C lower than those estimated for met-asedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ? 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc mag-matism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay) by subduction/collision events during the as-sembly of Gondwana supercontinent, and subsequently underwent w850 ?C granulite-facies meta-morphosed during Neoproterozoic to Cambrian final collisional event.

  17. Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

    Directory of Open Access Journals (Sweden)

    Toshiaki Tsunogae


    Full Text Available We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss, mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750–850 °C, approximately 150 °C lower than those estimated for metasedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ± 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc magmatism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay by subduction/collision events during the assembly of Gondwana supercontinent, and subsequently underwent ∼850 °C granulite-facies metamorphosed during Neoproterozoic to Cambrian final collisional event.

  18. An eastern Mediterranean analogue for the Late Palaeozoic evolution of the Pangaean suture zone in SW Iberia (United States)

    Brendan Murphy, J.; Braid, James A.; Quesada, Cecilio; Dahn, Dustin; Gladney, Evan; Dupuis, Nicolle


    It has long been recognized that the Late Paleozoic evolution of SW Iberia preserves a record of collision and suturing between Laurussia (South Portuguese Zone) and Gondwana (Ossa Morena Zone), which is one of the key events in the development of the Variscan orogen and the amalgamation of Pangea. The suture zone (Pulo do Lobo Zone) is classically considered to be an accretionary complex and is characterized by an assemblage of greenschist facies, polydeformed and imbricated meta-sedimentary rocks, mélanges, and mafic complexes. However recent work has shown some of the metasedimentary rocks and mélange were likely derived from neither the upper or lower plates. Mafic complexes in the mélange have NMORB compositions, highly depleted Sm-Nd isotopic signatures and geochronological data imply that their protoliths probably formed between ca. 350 and 340 Ma. Geochronological data also imply that components of the mafic mélange contain a paucity of ancient continental detritus. The Pulo do Lobo Zone was also intruded by ca. 360-310 Ma composite plutons and related dykes ranging from gabbro to granite in composition. The oldest phases of these intrusions are syn- to late-tectonic with respect to the deformation. Taken together these recent observations suggest that much of the tectonic evolution of the Pulo do Lobo suture zone post-dates the onset of collisional tectonics elsewhere in the Variscan orogen, and is broadly analogous to the complex Cenozoic tectonic evolution of the eastern Mediterranean oceanic tracts relative to the ongoing collision between the African, Eurasian and Arabian plates.

  19. Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia (United States)

    José Fernández, Francisco; Llana-Fúnez, Sergio; Valverde-Vaquero, Pablo; Marcos, Alberto; Castiñeiras, Pedro


    High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480-488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP-HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr-1, and the exhumation path involved a cooling of ˜ 20 °C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P-T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz

  20. New paleomagnetic data from Bornholm granitoids testing whether the East-European Craton rotated during the 1.50-1.45 Ga Danopolonian orogeny (United States)

    Lubnina, N.; Bogdanova, S.; Cecys, A.


    According to the palaeogeographic reconstructions, the East-European Craton (EEC) was part of the Palaeo- to Mesoproterozoic supercontinent Nuna / Columbia (Hoffman, 1997; Rogers and Santosh, 2002). Particularly important was the period between 1.5 and 1.3 Ga, when incipient break-up of this supercontinent occurred (Condie, 2002) but the EEC ("Baltica") still remained in close connection with other continental blocks. During the entire Mesoproterozoic, however, the EEC featured different geodynamic regimes in its presently western and eastern parts. In the west, these were convergent, while rifting prevailed in the east (Bogdanova et al., 2008). Previously, paleomagnetic studies of the Mesoproterozoic Ladoga Lake mafic rocks in NE Russia and the Dalarna mafic dykes in Sweden have disclosed a regular trend from the older Dalarna dykes to the younger dolerites of Lake Ladoga, suggesting an anticlockwise rotation of about 20 degrees. That rotation could either have affected the entire EEC as a result of the Danopolonian orogeny at ca. 1.50-1.45 Ga or have been associated with local block-displacement events in the Pasha-Ladoga graben (Lubnina et al., 2005, 2007). In the present study, we have tested these alternative possibilities by carrying out new paleomagnetic studies of Mesoproterozoic granitoids from the Danish island of Bornholm in the South Baltic Sea, which is a key area of the Danopolonian orogeny. On SW Bornholm, the 1.46 Ga Ronne granodiorites, which are cut by NNW trending thin dolerite dykes have been sampled in the Klippelokke quarry. Remanence measurements were performed using a 2G cryogenic magnetometer at the Palaeomagnetic Laboratory of the Department of Geology, Lund University, Sweden. Conventional progressive thermal or alternating field (AF) demagnetizations were applied to all specimens. During the stepwise thermal and AF demagnetization experiments, two components of NRM were isolated in the majority of the granitoid specimens. The low

  1. Petrological and geochemical characteristics of Paleoproterozoic ultramafic lamprophyres and carbonatites from the Chitrangi region, Mahakoshal supracrustal belt, central India

    Indian Academy of Sciences (India)

    Rajesh K Srivastava


    A number of ENE–WSW trending Paleoproterozoic dykes and plugs of mafic, ultramafic, alkaline and carbonatite rocks intrude Mahakoshal supracrustal belt (MSB), which is a part of the Central Indian Tectonic Zone (CITZ). Best exposures of these intrusions are found in the eastern parts of the MSB, particularly in and around Chitrangi area. Many of these intrusions have greenschist facies mineral composition and show sharp contact with supracrustal rocks. However, igneous textures, such as porphyritic/glomeroporphyritic, are still preserved in the form of partly pseudomorphed olivines, phlogopites and pyroxenes. Striking feature observed in some ultramafic samples is the presence of melanite garnet and rounded or elliptical carbonate ocelli. The petrographic characteristics suggest occurrence of carbonate-rich ultramafic lamprophyres; close to aillikite composition. Coarse-grained carbonatites show hypidiomorphic texture and mostly composed of calcite with appreciable amount of silicate minerals like clinopyroxene, phlogopite and olivine (often pseudomorphed by calcite, amphibole and chlorite). It is difficult to establish any direct genetic relationship between carbonatite and ultramafic lamprophyre samples on the basis of their chemistry; they were likely derived from distinct parental melts. High Mg#(up to ∼78), and high Ni and Cr contents (up to ∼1700 and ∼1100, respectively) and low HREE concentration in few ultramafic lamprophyre samples apparently suggest their derivation from a near-primary mantle-derived melts originated at great depths. Geochemistry and presence of carbonate ocellae in ultramafic lamprophyre samples suggest genesis of these silicate rocks and associated carbonatites through liquid immiscibility, however possibility of their derivation through vein-plus-wall-rock melting model cannot be ignored. A multi-stage veined mantle melting model is suitable in the latter case. It is suggested that early stages of rifting in the Mahakoshal

  2. Geology of the Eoarchean, > 3.95 Ga, Nulliak supracrustal rocks in the Saglek Block, northern Labrador, Canada: The oldest geological evidence for plate tectonics (United States)

    Komiya, Tsuyoshi; Yamamoto, Shinji; Aoki, Shogo; Sawaki, Yusuke; Ishikawa, Akira; Tashiro, Takayuki; Koshida, Keiko; Shimojo, Masanori; Aoki, Kazumasa; Collerson, Kenneth D.


    The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U-Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, > 3.95 Ga, and forms the Iqaluk-Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk-Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the > 3.95 Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

  3. Origin of the Rubian carbonate-hosted magnesite deposit, Galicia, NW Spain: mineralogical, REE, fluid inclusion and isotope evidence (United States)

    Kilias, Stephanos P.; Pozo, Manuel; Bustillo, Manuel; Stamatakis, Michael G.; Calvo, José P.


    The Rubian magnesite deposit (West Asturian—Leonese Zone, Iberian Variscan belt) is hosted by a 100-m-thick folded and metamorphosed Lower Cambrian carbonate/siliciclastic metasedimentary sequence—the Cándana Limestone Formation. It comprises upper (20-m thickness) and lower (17-m thickness) lens-shaped ore bodies separated by 55 m of slates and micaceous schists. The main (lower) magnesite ore body comprises a package of magnesite beds with dolomite-rich intercalations, sandwiched between slates and micaceous schists. In the upper ore body, the magnesite beds are thinner (centimetre scale mainly) and occur between slate beds. Mafic dolerite dykes intrude the mineralisation. The mineralisation passes eastwards into sequence of bedded dolostone (Buxan) and laminated to banded calcitic marble (Mao). These show significant Variscan extensional shearing or fold-related deformation, whereas neither Rubian dolomite nor magnesite show evidence of tectonic disturbance. This suggests that the dolomitisation and magnesite formation postdate the main Variscan deformation. In addition, the morphology of magnesite crystals and primary fluid inclusions indicate that magnesite is a neoformed hydrothermal mineral. Magnesite contains irregularly distributed dolomite inclusions (modified basinal brines that have reacted and equilibrated with intercalated siliciclastic rocks. Magnesite formation is genetically linked to regional hydrothermal dolomitisation associated with lithospheric delamination, late-Variscan high heat flow and extensional tectonics in the NW Iberian Belt. A comparison with genetic models for the Puebla de Lillo talc deposits suggests that the formation of hydrothermal replacive magnesite at Rubian resulted from a metasomatic column with magnesite forming at higher fluid/rock ratios than dolomite. In this study, magnesite generation took place via the local reaction of hydrothermal dolostone with the same hydrothermal fluids in very high permeability zones at

  4. Structural and metamorphic analysis of the Sucuru-Paraba region: implications for evolution of the Alto Moxotó Terrane, Borborema Province

    Directory of Open Access Journals (Sweden)

    Lauro Cézar Montefalco de Lira Santos


    Full Text Available Inserted on the Alto Moxot Terrane of the Borborema Province, the Sucuru region (Paraba state, Brazil includes two dominantly pre-Ediacaran tectono-stratigraphic domains, bounded by an expressive thrust shear zone (Carmo SZ. The first domain has a metaplutonic nature, being formed by granodioritic to migmatitic orthogneisses (Floresta Complex cut by several intrusive suites. These suites comprise a unit of mafic-ultramafic nature (Malhada Vermelha, a second one mainly of granitic-granodioritic composition (Pedra d'gua and a last suite whose composition ranges from syenitic to syenogranitic (Serra da Barra. On the second domain predominate migmatitic paragneisses from of the Sertnia Complex. All these units were cut by Cambrian-Ediacaran felsic dykes and A-type granites. Three tectonic events were recognized. The first Dn episode represents a thrusting with tectonic transport to NW-NNW, being important the Sucuru and Carmo shear zones. The Dn+1 episode is a transcurrent event of Ediacaran age, being important in regional context the Coxixola and Congo shear zones. The final progressive late Ediacaran-Cambrian Dn+2 episode, culminated with the emplacement of the Sucuru dyke swarm and the Prata and Serra da Engabelada A-type granites. Petrographic evidences show that the Dn episode reached its metamoprhic peak on the granulitic or eclogitic facies, whose paragenesis was after re-equilibrated to the amphibolite during the Dn+1 event. The Dn+1 episode developed mylonitic corridors on the amphibolite facies with associated migmatization, while the Dn+2, event reached a greenschist facies restrict to influence of the late and post-tectonic intrusives. The occurrence of garnet amphibolites with symplectite texture along the metamorphic path Mn-Mn+1 suggests that the thrust event represented a high pressure metamorphic episode, indicating a probable Paleoproterozoic suture.

  5. Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications (United States)

    Ranganai, Rubeni T.; Whaler, Kathryn A.; Ebinger, Cynthia J.


    Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures

  6. The geology, structure and mineralisation of the Oyu Tolgoi porphyry copper-gold-molybdenum deposits, Mongolia:A review

    Institute of Scientific and Technical Information of China (English)

    T.M. Mike Porter


    The Oyu Tolgoi cluster of seven porphyry Cu-Au-Mo deposits in southern Mongolia, define a narrow, linear, 12 km long, almost continuously mineralised trend, which contains in excess of 42 Mt of Cu and 1850 t of Au, and is among the largest high grade porphyry Cu-Au deposits in the world. These deposits lie within the Gurvansayhan island-arc terrane, a fault bounded segment of the broader Silurian to Carboniferous Kazakh-Mongol arc, located towards the southern margin of the Central Asian Orogenic Belt, a collage of magmatic arcs that were periodically active from the late Neoproterozoic to Permo-Triassic, extending from the Urals Mountains to the Pacific Ocean. Mineralisation at Oyu Tolgoi is associated with multiple, overlapping, intrusions of late Devonian (w372 to 370 Ma) quartz-monzodiorite intruding Devonian (or older) juvenile, probably intra-oceanic arc-related, basaltic lavas and lesser volcaniclastic rocks, unconformably overlain by late Devonian (w370 Ma) basaltic to dacitic pyroclastic and volcano sedimentary rocks. These quartz-monzodiorite intrusions range from early-mineral porphyritic dykes, to larger, linear, syn-, late- and post-mineral dykes and stocks. Ore was deposited within syn-mineral quartz-monzodiorites, but is dominantly hosted by augite basalts and to a lesser degree by overlying dacitic pyroclastic rocks. Following ore deposition, an allochthonous plate of older Devonian (or pre-Devonian) rocks was overthrust and a post-ore biotite granodiorite intruded at w365 Ma. Mineralisation is characterised by varying, telescoped stages of intrusion and alteration. Early A-type quartz veined dykes were followed by Cu-Au mineralisation associated with potassic alteration, mainly K-feldspar in quartz-monzodiorite and biotite-magnetite in basaltic hosts. Downward reflux of cooled, late-magmatic hydrothermal fluid resulted in intense quartz-sericite retrograde alteration in the upper parts of the main syn-mineral intrusions, and an equivalent chlorite

  7. Petrology, geochemistry and tectonic settings of the mafic dikes and sills associated with the evolution of the Proterozoic Cuddapah Basin of south India

    Indian Academy of Sciences (India)

    Nilanjan Chatterjee; Somdev Bhattacharji


    In this article we summarize the petrological, geochemical and tectonic processes involved in the evolution of the Proterozoic intracratonic Cuddapah basin. We use new and available ages of Cuddapah igneous rocks, together with field, stratigraphic, geophysical and other criteria, to arrive at a plausible model for the timing of these processes during basin evolution. We present petrological and geochronological evidence of dike emplacement along preferred lineament directions around the basin in response to stresses, which may have been responsible for the evolution of the basin itself. Basaltic dike intrusion started on the south Indian shield around 2400 Ma and continued throughout the Cuddapah basin evolution and sedimentation. A deep mantle perturbation, currently manifested by a lopolithic cupola-like intrusion under the southwestern part of the basin, may have occurred at the onset of basin evolution and played an important role in its development. Paleomagnetic, gravity and geochronological evidence indicates that it was a constant thermal source responsible for dike and sill emplacement between 1500 and 1200 Ma both inside and outside the basin. Lineament reactivation in the NW-SE and NE-SW directions, in response to the mantle perturbation, intensified between 1400 and 1200 Ma, leading to the emplacement of several cross cutting dikes. Fe-Mg partition coefficients of olivine and augite and Ca-Na partition coefficient of plagioclase, calculated from the composition of these minerals and bulk composition of their host rocks, indicate that the dikes outside the Cuddapah basin are cumulates. The contemporary dikes may be related by fractional crystallization as indicated by a positive correlation between their plagioclase Ca#(atomic Ca/[Ca+Na]) and augite Mg#(atomic Mg/[Mg+Fe]). A few NW-SE and NE-SW cross cutting dikes of the period between 1400 and 1200 Ma, preserve petrographic evidence of episodic magmatic intrusive activity along preferred directions. Petrological reasoning indicates that a magmatic liquid reacted with a set of cross cutting dikes, intruding into one that was already solidified and altering the composition of the magma that produced the other dike. The Cuddapah basin tholeiites may be related by fractional crystallization at 5 kb and 1019-1154°C, which occurred in the lopolithic cupola near the southwestern margin of the basin. Xenolith bearing picrites, which occur near the periphery of the cupola, originated by the accumulation of xenoliths in the tholeiites. This is indicated by the composition of the olivine in the xenoliths (Fo78.7−81.9), which are closely similar to calculated olivine compositions (Fo77.8−78.3) in equilibrium with the tholeiites under the same P-T conditions. It is inferred that fractionation in the cupola resulted in crystals settling on its walls. Hence, the xenolith-bearing sills occur at the periphery of the lopolithic body.

  8. Integrated Paleomagnetism and U-Pb Geochronology of Mafic Dikes of the Eastern Anabar Shield Region, Siberia: Implications for Mesoproterozoic Paleolatitude of Siberia and Comparison with Laurentia. (United States)

    Ernst; Buchan; Hamilton; Okrugin; Tomshin


    This article reports the first joint paleomagnetic and U-Pb geochronologic study of Precambrian diabase dikes in the Anabar Shield and adjacent Riphean cover of Siberia. It was undertaken to allow comparison with similar published studies in Laurentia and to test Proterozoic reconstructions of Siberia and Laurentia. An east-trending Kuonamka dike yielded a provisional U-Pb baddeleyite emplacement age of 1503+/-5 Ma and a virtual geomagnetic pole at 16 degrees S, 221 degrees E (dm=17&j0;, dp=10&j0;). A paleomagnetic pole at 6 degrees N, 234 degrees E (dm=28&j0;, dp=14&j0;) was obtained from five Kuonamka dikes. An east-southeast-trending Chieress dike yielded a U-Pb baddeleyite emplacement age of 1384+/-2 Ma and a virtual geomagnetic pole at 4 degrees N, 258 degrees E (dm=9&j0;, dp=5&j0;). Kuonamka and Chieress poles are interpreted to be primary but do not average out secular variation. Assuming that the Siberian Plate has remained intact since the Mesoproterozoic, except for mid-Paleozoic opening of the Viljuy Rift, then the above results indicate that the Siberian Plate was in low latitudes at ca. 1503 and 1384 Ma, broadly similar to low latitudes determined for Laurentia from well-dated paleopoles at 1460-1420, 1320-1290, and 1267 Ma. This would allow Laurentia and Siberia to have been attached in the Mesoproterozoic, as suggested in several recent studies based on geological criteria. However, because paleomagnetic results from the Anabar Shield region do not average out secular variation and the ages of poles from Siberia and Laurentia are not well matched, it is not yet possible to distinguish between these reconstructions or to rule out other configurations that also maintain the two cratons at low paleolatitudes.

  9. 镁铁-超镁铁层状侵入体研究现状%Current Study on the Mafic-Ultramafic Layered Intrusions

    Institute of Scientific and Technical Information of China (English)

    胡素芳; 周新华


    General characteristics and genetic mechanism of themafic-ultramafic layered intrusions are presented,and the petrological and geochemical characteristics of Skaergaard, Bushveld, Stillwater layered intrusions and layered intrusions in Panxi which are famous in the world are discussed in this paper.%本文主要探讨了镁铁-超镁铁层状侵入体的一般特征及成因机制,并对世界上著名的斯开利戛德、布什维尔德、斯蒂尔沃特层状侵入体和我国攀西层状侵入体的岩石学和地球化学特征进行了总结和对比。

  10. Applications of Landsat Thematic Mapper and ground-based spectrometer data to a study of the Skaergaard and other mafic intrusions of East Greenland (United States)

    Birnie, Richard W.; Naslund, H. Richard; Nichols, Jennifer D.; Turner, Patricia A.; Parr, J. Thomas


    Landsat TM data have been used in conjunction with field spectrometer data to map the lithologic units associated with a series of gabbroic intrusions in the East Greenland Tertiary Igneous Province. The general lack of vegetation combined with the difficulty of access to these intrusions make them ideal candidates for lithologic mapping using remote sensing techniques. In addition, these bodies are of interest as possible precious metal ore deposits. The intrusions are spectrally distinct from the surrounding Precambrian gneisses; however, subpixel contamination by snow, oxide surface coatings, and lichen cover and severe topography limit the discrimination of lithologic units within the gabbro. The spectral nature of the surface contaminants was evaluated with a Barringer Hand Held Ratioing Radiometer (HHRR). These HHRR data indicate that bare rock exposures have distinct TM signatures for each lithologic unit but that even small amounts of subpixel contamination are enough to mask these differences because of the large differences between the TM signatures of the rocks and the contaminants.

  11. Petrological insights into the 1976-2000 eruption episode of White Island, New Zealand: an eruption fuelled by repeated mafic recharge (United States)

    Kilgour, Geoff; Moune, Severine; Della Pasqua, Fernando; Christenson, Bruce


    White Island is a partially submerged volcano located ~ 50 km to the NE of North Island, New Zealand. It is New Zealand's most active volcano and the island is a popular tourist destination. Surprisingly, little is known about the magmatic processes that led to historical eruptions given the potential vulnerability of tourists and its high level of historical activity. In addition, the volcano has been monitored with an array of methods since 1967; from geodetic surveying techniques to geochemical analyses of fluids and gases. Access to an excellent sample record has allowed us to examine a temporal record of magmatic processes, through the careful analysis of scoria clasts. We have then been able to compare apparent timescales of magmatic events to monitored signals. In this work we examine an extended eruptive episode (1976-2000) in order to determine the causes of signals produced throughout this sequence. Previous petrological and geophysical analysis of the 1976-2000 eruptive episode suggested that magma resided at a very shallow depth (i.e., ~ 500 m) for extended periods. This was primarily based on the focussed deformation signal and the low H2O ( 1 km depth prior to eruption. Significant magma degassing into the hydrothermal system was likely responsible for the deformation signal and is unrelated to very shallow magma residence. Taken together, these data provide an important link between petrology and geophysical monitoring at White Island.

  12. Petrography and Metamorphism of the Metasedimentary Country-Rocks of the Jacurici Valley Chromitite-Hosting Mafic-Ultramafic Complexes, Bahia, Northeastern Brazil

    Directory of Open Access Journals (Sweden)

    Eliane A. Del Lama


    Full Text Available This paper deal with on the metasedimentary country-rocks of the chromite-bearing ultramafic rocks that occur in the “Jacurici River Valley Chromium District” northeastern Bahia, Brazil. This region presents a complex geologic-petrologic framework of rocks that were intensely deformed, metamorphosed and transformed by metasomatic processes, making it difficult to interpret their volutionary/metamorphic record. Although the metasedimentary country rocks have also been affected by such processes, it is possible to distinguish evidence of a previous high-grade metamorphism that affected them. Thermobarometric data for the observed mineralogical associations indicate P-T conditions around 750-800ºC and 7-8 kb for the metamorphic peak, based mainly on the presence of olivine in marbles and the cordierite-garnet-sillimanitespinel association in aluminous gneisses.

  13. Rb-Sr and Sm-Nd geochronology of the Cana Brava layered mafic-ultramafic intrusion, Brazil, and considerations regarding its tectonic evolution

    Energy Technology Data Exchange (ETDEWEB)

    Correia, Ciro Teixeira; Girardi, Vicente A.V.; Tassinari, Colombo C.G. [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias; Jost, Hardy [Brasilia Univ., DF (Brazil). Dept. de Geociencias


    The Cana Brava complex is an anorogenic stratiform complex, whose layers dip from 30{sup 0} to 50{sup 0} NW. The massif is made up of five units composed of layers containing several associations of cumulus phases and variable amounts of inter-cumulus minerals. Transition between units are characterized by abrupt changes in the composition of these phases. From base to top, the sequence consists of amphibolites (PICB1), overlain by serpentinites (PICB2), metawebsterites (PICB3), and metagabbros (PICB4 and PICB5). This sequence was originally formed by microgabbros, peridotites, websterites, and gabbros. Isotopic data indicate that the parental magma of the Cana Brava complex evolved as follows: mantle derivation at about 2.5 Ga and subsequent retention at subcrustal levels; intrusion within the Palmeiropolis volcano-sedimentary sequence and igneous crystallization at about 2.0 Ga; (c) Metamorphism and ductile-ruptile deformation under compression at about 1.3 Ga; and (d) Further metamorphic reequilibration during the Brasiliano Cycle at about 0.77 Ga. (Author) 27 refs., 7 figs., 2 tabs.

  14. Late-Pleistocene to precolumbian behind-the-arc mafic volcanism in the eastern Mexican Volcanic Belt; implications for future hazards (United States)

    Siebert, Lee; Carrasco-Núñez, Gerardo


    An area of widespread alkaline-to-subalkaline volcanism lies at the northern end of the Cofre de Perote-Citlaltépetl (Pico de Orizaba) volcanic chain in the eastern Mexican Volcanic Belt (MVB). Two principal areas were active. About a dozen latest-Pleistocene to precolumbian vents form the 11-km-wide, E-W-trending Cofre de Perote vent cluster (CPVC) at 2300-2800 m elevation on the flank of the largely Pleistocene Cofre de Perote shield volcano and produced an extensive lava field that covers >100 km 2. More widely dispersed vents form the Naolinco volcanic field (NVF) in the Sierra de Chiconquiaco north of the city of Jalapa (Xalapa). Three generations of flows are delineated by cone and lava-flow morphology, degree of vegetation and cultivation, and radiocarbon dating. The flows lie in the behind-the-arc portion of the northeastern part of the MVB and show major- and trace-element chemical patterns transitional between intraplate and subduction zone environments. Flows of the oldest group originated from La Joya cinder cone (radiocarbon ages ˜42 000 yr BP) at the eastern end of the CPVC. This cone fed an olivine-basaltic flow field of ˜20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of Jalapa, the capital city of the state of Veracruz. The Central Cone Group (CCG), of intermediate age, consists of four morphologically youthful cinder cones and associated vents that were the source of a lava field>27 km 2 of late-Pleistocene or Holocene age. The youngest group includes the westernmost flow, from Cerro Colorado, and a lava flow ˜2980 BP from the Rincón de Chapultepec scoria cone of the NVF. The latest eruption, from the compound El Volcancillo scoria cone, occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in México and resemble paired aa-pahoehoe flows from Mauna Loa vol