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Sample records for kaapvaal craton evolution

  1. Diamond genesis, seismic structure, and evolution of the Kaapvaal-Zimbabwe craton.

    Science.gov (United States)

    Shirey, Steven B; Harris, Jeffrey W; Richardson, Stephen H; Fouch, Matthew J; James, David E; Cartigny, Pierre; Deines, Peter; Viljoen, Fanus

    2002-09-06

    The lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of southern Africa shows variations in seismic P-wave velocity at depths within the diamond stability field that correlate with differences in the composition of diamonds and their syngenetic inclusions. Middle Archean mantle depletion events initiated craton keel formation and early harzburgitic diamond formation. Late Archean accretionary events involving an oceanic lithosphere component stabilized the craton and contributed a younger Archean generation of eclogitic diamonds. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the Archean diamond suite.

  2. Petrochemical evolution of the White Mfolozi Granite pluton: Evidence for a late Palaeoarchaean A-type granite from the SE Kaapvaal Craton, South Africa

    Science.gov (United States)

    Misra, Saumitra; Reinhardt, Jürgen; Wilson, Allan H.

    2017-08-01

    One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the 3.25 Ga (U-Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous conditions, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into mid-crustal levels (c. 17 km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or + ab)] 0.48 could be 500 MPa and close to 1000 °C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg + Fe) 0.41-0.45] suggests that the final temperature of crystallization of the pluton was close to 800 °C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzo-feldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In

  3. Metasomatic control of water contents in the Kaapvaal cratonic mantle

    Science.gov (United States)

    Peslier, A. H.; Woodland, A. B.; Bell, D. R.; Lazarov, M.; Lapen, T. J.

    2012-11-01

    Water and trace element contents were measured by FTIR and laser ablation-ICPMS on minerals from peridotite xenoliths in kimberlites of the Kaapvaal craton from Finsch, Kimberley, Jagersfontein (South Africa), Letseng-La-Terae, and Liqhobong (Lesotho) mines. The peridotites record a wide range of pressure, temperature, oxygen fugacity, and metasomatic events. Correlations between water content or OH vibration bands with major, minor and trace elements in pyroxene and garnet precludes disturbance during xenolith entrainment by the host kimberlite magma and indicate preservation of mantle water contents. Clinopyroxene water contents (150-400 ppm H2O, by weight) correlate with those of orthopyroxene (40-250 ppm). Olivines (Peslier et al., 2008, 2010) and garnets have 0-86 and 0-20 ppm H2O, respectively. Relations in individual xenolith suites between the amount of water and that of incompatible elements Ti, Na, Fe3+ and rare earths in minerals suggests that metasomatism by oxidizing melts controls the water content of olivine, pyroxene and garnet. At pressures ⩽5.5 GPa, hydrous, alkaline, siliceous fluids or melts metasomatized Liqhobong and Kimberley peridotites, producing high water contents in their olivine, pyroxenes and garnet. At higher pressures, the percolation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, and the overall crystallization of garnet with lower water contents than those in the original peridotites. The upward migration of these ultramafic melts through the lithospheric mantle also increased the water content of olivines with decreasing pressure at Finsch Mine. H2O/Ce ratios of melts in equilibrium with Kaapvaal peridotites range from 100 to 20,000 and the larger values may indicate metasomatism in subduction zone settings. Metasomatic events in Kaapvaal peridotites are thought to have occurred from the Archean to the Mesozoic. However, circumstantial evidence

  4. Paleomagnetism of the early Paleoproterozoic, volcanic Hekpoort Formation (Transvaal Supergroup) of the Kaapvaal craton, South Africa

    Science.gov (United States)

    Humbert, F.; Sonnette, L.; de Kock, M. O.; Robion, P.; Horng, C. S.; Cousture, A.; Wabo, H.

    2017-02-01

    The Kaapvaal craton (South Africa) was the host of several major magmatic events during the Paleoproterozoic, including the volcanic Hekpoort and Ongeluk formations. Their possible comagmatic origin is the subject of a long debate. We performed a paleomagnetic study of the Hekpoort Formation to get a primary pole can be compared with the available paleopole of the Ongeluk Formation, but also to contribute to the apparent pole wander path of the Kaapvaal craton. Characterization of magnetic mineralogy by 3-axis thermal demagnetization of IRM and magnetic susceptibility vs temperature points out magnetite as the main remanence carrier in most samples.

  5. Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

    DEFF Research Database (Denmark)

    Schutt, Derek; Lesher, Charles

    2010-01-01

    garnet and clinopyroxene enrichment. Using the parameterization of Schutt and Lesher (2006) we show that at cratonic mantle temperatures and pressures, orthopyroxene enrichment results in little change in bulk density (ρbulk) and shear-wave velocity (VS), but decreases compressional wave velocities (VP...... and clinopyroxene enrichment possibly as a consequence of melt infiltration. More than half of the mineral mode variance among Kaapvaal Craton xenoliths can be accounted for by opx enrichment. Melt depletion effects can account for as much as 30% of the variance, while less than 20% of the variance is associated......) and VP/VS. In contrast, melt depletion has little effect on VP, but leads to an increase in VS and a decrease in ρbulk and VP/VS. Garnet (gt) and clinopyroxene (cpx) enrichment cause an increase in ρbulk, VP, VS, and VP/VS. The isolation of the major contributions to xenolith compositional variations...

  6. Evolution of the South African mantle — A case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); part 1: Inter-mineral trace element and isotopic equilibrium

    Science.gov (United States)

    Lazarov, Marina; Brey, Gerhard P.; Weyer, Stefan

    2012-12-01

    A thorough assessment of inter-mineral equilibrium is essential for the understanding of trace element partitioning and also for the interpretation of isotopic data. Here we investigated high temperature (~ 1200 °C and 6 GPa) garnet peridotite xenoliths from the Kaapvaal craton (Finsch mine, South Africa), with exceptionally well equilibrated mineral major element compositions, for their trace element and isotopic inter-mineral equilibrium. Trace element compositions for all major mineral phases, i.e. olivine, orthopyroxene (opx), clinopyroxene (cpx) and garnet, were analysed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Garnet, cpx and opx of selected samples were analysed for their Sm-Nd and Lu-Hf isotope compositions by multi collector ICP-MS. Most important mineral characteristics include: a) olivines from most samples are enriched in high-field-strength elements relative to other incompatible trace elements. Their lithium content correlates negatively with Mg#, indicating a depletion signature; b) all other minerals are depleted in heavy and middle rare earth elements (H- and M-REE) and enriched in light REE and large ion lithophile elements. This implies a complex history of depletion and metasomatic overprint for the Finsch cratonic mantle; c) orthopyroxene has similarly shaped trace element patterns as cpx, with one to two orders of magnitude lower abundances; and d) both, garnet and cpx, display variable, mostly positive ɛHf coupled with moderate variations in ɛNd. Trace element partitioning between garnet/cpx, cpx/opx and garnet/opx, displays only a weak pressure and temperature dependency. However, equilibrium partitioning of most trace elements between garnet and cpx shows a strong compositional dependency, i.e. on the Cr- (and Ca-) content of the garnets. Garnet-cpx partition coefficients follow a second grade polynomial correlation with Cr2O3 of garnet, whereby high chromium garnets (Cr2O3 > 6 wt.%) have generally higher

  7. Palaeointensity determinations on rocks from Palaeoproterozoic dykes from the Kaapvaal Craton (South Africa)

    Science.gov (United States)

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

    2014-06-01

    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'.

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

    Science.gov (United States)

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

    2012-04-01

    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

  9. Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton)

    Science.gov (United States)

    Aulbach, S.; Woodland, A. B.; Vasilyev, P.; Galvez, M. E.; Viljoen, K. S.

    2017-09-01

    Reconstructing the redox state of the mantle is critical in discussing the evolution of atmospheric composition through time. Kimberlite-borne mantle eclogite xenoliths, commonly interpreted as representing former oceanic crust, may record the chemical and physical state of Archaean and Proterozoic convecting mantle sources that generated their magmatic protoliths. However, their message is generally obscured by a range of primary (igneous differentiation) and secondary processes (seawater alteration, metamorphism, metasomatism). Here, we report the Fe3+/ΣFe ratio and δ18 O in garnet from in a suite of well-characterised mantle eclogite and pyroxenite xenoliths hosted in the Lace kimberlite (Kaapvaal craton), which originated as ca. 3 Ga-old ocean floor. Fe3+/ΣFe in garnet (0.01 to 0.063, median 0.02; n = 16) shows a negative correlation with jadeite content in clinopyroxene, suggesting increased partitioning of Fe3+ into clinopyroxene in the presence of monovalent cations with which it can form coupled substitutions. Jadeite-corrected Fe3+/ΣFe in garnet shows a broad negative trend with Eu*, consistent with incompatible behaviour of Fe3+ during olivine-plagioclase accumulation in the protoliths. This trend is partially obscured by increasing Fe3+ partitioning into garnet along a conductive cratonic geotherm. In contrast, NMORB-normalised Nd/Yb - a proxy of partial melt loss from subducting oceanic crust (1) - shows no obvious correlation with Fe3+/ΣFe, nor does garnet δ18OVSMOW (5.14 to 6.21‰) point to significant seawater alteration. Median bulk-rock Fe3+/ΣFe is roughly estimated at 0.025. This observation agrees with V/Sc systematics, which collectively point to a reduced Archaean convecting mantle source to the igneous protoliths of these eclogites compared to the modern MORB source. Oxygen fugacites (fO2) relative to the fayalite-magnetite-quartz buffer (FMQ) range from Δlog ⁡ fO2 = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility

  10. Geological evolution of the Antongil Craton, NE Madagascar

    Science.gov (United States)

    Schofield, D.I.; Thomas, Ronald J.; Goodenough, K.M.; De Waele, B.; Pitfield, P.E.J.; Key, R.M.; Bauer, W.; Walsh, G.J.; Lidke, D.J.; Ralison, A.V.; Rabarimanana, M.; Rafahatelo, J.-M.; Randriamananjara, T.

    2010-01-01

    The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U-Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ?? 14. Ma to 3231 ?? 6. Ma and 3187 ?? 2. Ma to 3154 ?? 5. Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ?? 2. Ma and subject to migmatisation at around 2597 ?? 49. Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ?? 18. Ma and 2542 ?? 5. Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ?? 13. Ma and 2513 ?? 0.4. Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ?? 6. Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ?? 130. Ma to 763 ?? 13. Ma and 553 ?? 68. Ma. These events are

  11. EVOLUTION OF SOUTHERN AFRICAN CRATONS BASED ON SEISMIC IMAGING

    DEFF Research Database (Denmark)

    Thybo, Hans; Soliman, Mohammad Youssof Ahmad; Artemieva, Irina

    2014-01-01

    present a new seismic model for the structure of the crust and lithospheric mantle of the Kalahari Craton, constrained by seismic receiver functions and finite-frequency tomography based on the seismological data from the South Africa Seismic Experiment (SASE). The combination of these two methods...

  12. Towards a Holistic Model for the Tectonic Evolution of the North China Craton

    Science.gov (United States)

    Kusky, T. M.; Polat, A.; Windley, B. F.; Wang, J.; Deng, H.

    2016-12-01

    The North China Craton (NCC) consists of distinctly different tectonic elements assembled during the late Archean - early Proterozoic. We propose a new tectonic evolution of the NCC. The Eastern Block (EB) consists of small microblocks that resemble a collage of accreted arc-rocks from a sutured archipelago similar to the SW Pacific, accreted between 2.6 and 2.7 Ga. An Atlantic-type margin developed on the western side of the EB by 2.5 Ga, and a >1,300 km long arc/accretionary prism collided with this passive margin at 2.5 Ga, obducting ophiolites and ophiolitic mélanges, and forming a foreland basin. This was followed by arc-polarity reversal, and injection of mantle wedge-derived melts. By 2.43 Ga, the ocean behind the accreted arc closed through the collision of an oceanic plateau. Rifting of the amalgamated craton followed at 2.4-2.35 Ga, with a failed rift arm preserved in the center of the craton, and two that successfully made an ocean along the northern margin. By 2.3 Ga an arc built on older cratonic material collided with this passive margin which soon converted to an Andean-type margin. Andean margin tectonics affected much of the craton from 2.3-1.9 Ga, forming a broad E-W swath of continental margin magmas, and retro-arc sedimentary basins including a superimposed basin over the passive margin on the northern margin. From 1.88-1.79 Ga the craton experienced a craton-wide granulite facies metamorphism and basement reactivation event with high-pressure granulites and eclogites in the north, and medium-pressure granulites across the craton. Early Proterozoic granulites and anatectic melts were generated by high-grade metamorphism and partial melting at mid-crustal levels beneath a collisionally-thickened plateau. This collision of the NCC on its northern margin was with the Columbia (Nuna) Continent. The NCC broke out in the period 1753-1673 Ma, as indicated by the formation of a suite of anorthosite, mangerite, charnockite, and alkali-feldspar granites

  13. At the craton edge: Geodynamic evolution of the southern Canadian Cordillera

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    DiCaprio, L.; Eaton, D. W. S.

    2016-12-01

    In the southern Canadian Cordillera, the thermal and mechanical interface with the craton may influence the geodynamic evolution of the lithosphere-asthenosphere system. Evidence including recent Rayleigh-wave tomography studies suggest that, beneath the southern Canadian Cordillera, the mantle lithosphere is virtually absent. Here, the boundary between craton and Cordillera also marks a step change in measured surface heat flux and a westward termination of magnetic anomalies. This study provides a numerical simulation of lithospheric-mantle removal by geodynamic processes that include delamination, viscous erosion, and mantle dripping. An additional constraint to the geodynamic model comes from thermochronologic data demonstrating long wavelength uplift of the cordilleran plateau in the Eocene. We have developed a suite of 2D visco-plastic models of a transect through the southern Canadian Cordillera and North American Craton. Sensitivity tests elucidate a range of geodynamic models that are consistent with tomographic results and the observed uplift history.

  14. Detrital Zircon Ages of Hanjiang River:Constraints on Evolution of Northern Yangtze Craton, South China

    Institute of Scientific and Technical Information of China (English)

    Yang Jie; Gao Shan; Yuan Honglin; Gong Hujun; Zhang Hong; Xie Shiwen

    2007-01-01

    Clastic sedimentary rocks are natural samples of the exposed continental crust over large ideal sample for studying the formation and evolution of the northern Yangtze craton. Here we report laser ablation inductively coupled plasma mass spectrometer U-Pb ages of 122 detrital zircons from one sand sample of the Hanjiang River. The 110 concordant zircons reveal four major age groups of 768,444, 212, and 124 Ma, which well correlate with known magmatic events in the northern Yangtze craton. A minor group is present at 1 536 Ma, which is less known in the study area. Only seven zircons have ages of >1 750 Ma. Our results show that the Early Paleozoic, Late Triassic, and Early Cretaceous are important episodes of zircon growth and crustal growth/reworking in addition to the previously documented Neoproterozoic event. Our results suggest very limited exposures of Paleoproterozoic and Archean rocks in the northern parts of the Yangtze craton.

  15. The extent of the Cratonic keel underneath the Southern African region: A 3D image using Finite-Frequency Tomograph

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Bezada, Max; Thybo, Hans;

    2010-01-01

    We have re-examined the P body wave data from the South Africa Seismic Experiment (Carlson et al, EOS 77, 1996) across the Kaapvaal and Zimbabwe cratons and the Bushveld complex. Using finite-frequency kernels, we inverted the P-wave delay times to obtain 3-D images of compressional velocity...... between the Archean and modified regions such as the Bushveld complex, and the mobile belts surrounding the cratons. The high velocity (+1.0%) cratonic roots extend to 220-250 km depth beneath the Kaapvaal and Zimbabwe cratons. Lower P-velocities are found under the Bushveld complex and the mobile belts...

  16. Complex evolution of the lower crust beneath the southeastern North China Craton: The Junan xenoliths and xenocrysts: Comment

    Science.gov (United States)

    Yuan, Ya-Juan; Xia, Bin

    2015-10-01

    The Junan granulite xenoliths and xenocrysts have a distinct overlap in U-Pb ages and Lu-Hf isotopic compositions with the Precambrian basement of the Yangtze craton. We thus believe that the Junan granulite cannot be derived from the North China lower crust. Moreover, broad deep seismic and magnetotelluric probing across the Sulu UHP terrane indicates the former is well coupled with the lower crust of the Yangtze craton. Therefore, we suggest that the Junan granulites are most likely to have been derived from the Yangtze Precambrian basement and their zircon U-Pb-Lu-Hf isotope systematics are more likely to indicate the complex evolution of the Precambrian lower crust along the northern margin of the Yangtze craton.

  17. A review of the tectonic evolution of the Sunsás belt, SW Amazonian Craton

    Science.gov (United States)

    Teixeira, Wilson; Geraldes, Mauro Cesar; Matos, Ramiro; Ruiz, Amarildo Salina; Saes, Gerson; Vargas-Mattos, Gabriela

    2010-01-01

    The Sunsás-Aguapeí province (1.20-0.95 Ga), SW Amazonian Craton, is a key area to study the heterogeneous effects of collisional events with Laurentia, which shows evidence of the Grenvillian and Sunsás orogens. The Sunsás orogen, characterized by an allochthonous collisional-type belt (1.11-1.00 Ga), is the youngest and southwesternmost of the events recorded along the cratonic fringe. Its evolution occurred after a period of long quiescence and erosion of the already cratonized provinces (>1.30 Ga), that led to sedimentation of the Sunsás and Vibosi groups in a passive margin setting. The passive margin stage was roughly contemporary with intraplate tectonics that produced the Nova Brasilândia proto-oceanic basin (aborted rifts that evolved to the Huanchaca-Aguapeí basin (1.17-1.15 Ga). The Sunsás belt is comprised by the metamorphosed Sunsás and Vibosi sequences, the Rincón del Tigre mafic-ultramafic sill and granitic intrusive suites. The latter rocks yield ɛNd(t) signatures (-0.5 to -4.5) and geochemistry (S, I, A-types) suggesting their origin associated with a continental arc setting. The Sunsás belt evolution is marked by "tectonic fronts" with sinistral offsets that was active from c. 1.08 to 1.05 Ga, along the southern edge of the Paraguá microcontinent where K/Ar ages (1.27-1.34 Ga) and the Huanchaca-Aguapeí flat-lying cover attest to the earliest tectonic stability at the time of the orogen. The Sunsás dynamics is coeval with inboard crustal shortening, transpression and magmatism in the Nova Brasilândia belt (1.13-1.00 Ga). Conversely, the Aguapeí aulacogen (0.96-0.91 Ga) and nearby shear zones (0.93-0.91 Ga) are the late tectonic offshoots over the cratonic margin. The post-tectonic to anorogenic stages took place after ca. 1.00 Ga, evidenced by the occurrences of intra-plate A-type granites, pegmatites, mafic dikes and sills, as well as of graben basins. Integrated interpretation of the available data related to the Sunsás orogen

  18. Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast)

    Science.gov (United States)

    Vidal, M.; Gumiaux, C.; Cagnard, F.; Pouclet, A.; Ouattara, G.; Pichon, M.

    2009-11-01

    The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of "domes and basins" geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic "weak type" orogen.

  19. Mesoproterozoic evolution of the Rio de la Plata Craton in Uruguay: at the heart of Rodinia?

    DEFF Research Database (Denmark)

    Gaucher, Claudio; Frei, Robert; Chemale, Farid

    2011-01-01

    detrital zircon ages occurring in Ediacaran sandstones of the RPC. If the RPC is fringed at both sides by Mesoproterozoic, Grenville-aged belts it is likely that it occupied a rather central position in Rodinia. A possible location between Laurentia and the Kalahari Craton, and to the south of Amazonia...

  20. Complex evolution of the lower crust beneath the southeastern North China Craton: The Junan xenoliths and xenocrysts: Reply

    Science.gov (United States)

    Tang, Huayun; Zheng, Jianping; Griffin, William L.; O‧Reilly, Suzanne Y.; Yu, Chunmei; Pearson, Norman J.; Ping, Xianquan; Xia, Bing; Yang, Huaben

    2015-10-01

    In our paper, we suggested that the Junan granulite xenoliths and xenocrysts record evolution of the Precambrian lower crust beneath the southeastern North China Craton (NCC). Yuan and Xia (2015) disagree with us. However, they have not fully considered the evolutional histories of the NCC lithosphere, and geochemical and isotopic compositions of the Junan xenoliths. We also contend that they have misinterpreted the available geophysical data. Synthesizing the geochronological characteristics of the NCC lower crust, nature of the Junan granulite xenoliths, and reinterpretation of the resistivity profile, we again emphasize that the Junan granulite xenoliths are tectonically affiliated to the NCC lower crust, and the Junan zircon data could reflect the complex evolution of the lower crust beneath the southeastern NCC.

  1. Geophysical Character and Geochemical Evolution of the Mesoproterozoic Figueira Branca Intrusive Suite, SW Amazon Craton (Brazil)

    Science.gov (United States)

    Louro, Vinicius; Cawood, Peter; Mantovani, Marta

    2016-04-01

    The Jauru Terrain hosts the Figueira Branca Intrusive Suite (FBS) in the SW of the Amazon Craton (Brazil). The FBS is a series of 1425 Ma layered mafic intrusions, previously interpreted as anorogenic. The FBS area is located in foreland to the Santa Helena orogen, formed by the subduction of the Rio Alegre Terrain under the Jauru Terrain. Potential field methods (magnetic and gravity), gamma-ray spectrometry, geochemical and isotope data were used to characterize and to model the extent of FBS magmatism, the distribution of faults and shear zones in the area, to evaluate affinities of the magmatic activity, and the relation between the FBS and the Santa Helena orogen. The geophysical methods identified three anomalies corresponding with FBS outcrops. A fourth anomaly with significantly higher amplitude was observed to the north of the three anomalies. From south to north, the anomalies were named Indiavaí, Azteca, Figueira Branca and Jauru. These anomalies were modeled and indicated a northwest-southeast trend, parallel to regional shear zones. The gamma-ray data enabled the collection of 50 samples from the FBS rocks, the Alto Jauru group that hosts the FBS, from nearby intrusive suites, and the Rio Alegre Terrain. The 30 freshest samples were analyzed by X-ray fluorescence for oxides and some trace elements, 20 by ICP-MS for Rare-Earth Elements and 10 for Nd-Sr isotope analyses. The FBS samples were gabbros and gabbro-norites with Nb/Yb-Th/Yb and TiO2/Yb-Nb/Yb ratios indicating varying degrees of crustal interaction. The TiO2/Yb-Nb/Yb data suggested a subduction related component and the ɛNd-ɛSr indicated a juvenile source. Samples from coeval adjacent intermediate magma suites displayed similar characteristics, which suggest derivation from a bimodal source probably related with the subduction of the Rio Alegre Terrain. We interpreted the tectonic setting of the FBS as a result of a roll-back of the subducted slab, which resulted in rejuvenation of the

  2. Paleoproterozoic crustal evolution of the Hengshan–Wutai–Fuping region, North China Craton

    Directory of Open Access Journals (Sweden)

    Chunjing Wei

    2014-07-01

    Full Text Available An arguable point regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton (NCC is whether the tectonic setting in the central belt during the mid-Paleoproterozoic (2.35–2.0 Ga was dominated by an extensional regime or an oceanic subduction–arc regime. A review of the mid-Paleoproterozoic magmatism and sedimentation for the Hengshan–Wutai–Fuping region suggests that a back-arc extension regime was dominant in this region. This conclusion is consistent with the observation that the 2.35–2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks mainly comprise highly-fractioned calc-alkaline to alkaline (or A-type granites, and that this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group. Although the final amalgamation of the NCC was believed to occur at ∼1.85 Ga, recent zircon U–Pb age dating for mica schist in the Wutai Group suggests a collisional event may have occurred at ∼1.95 Ga. The metamorphic ages of ∼1.85 Ga, obtained mostly from the high-grade rocks using the zircon U–Pb approach, most probably indicate uplifting and cooling of these high-grade terranes. This is because (i phase modeling suggests that newly-grown zircon grains in high-grade rocks with a melt phase cannot date the age of peak pressure and temperature stages, but the age of melt crystallization in cooling stages; (ii the metamorphic P–T paths with isobaric cooling under 6–7 kb for the Hengshan and Fuping granulites suggest their prolonged stay in the middle–lower crust; and (iii the obtained metamorphic age data show a continuous distribution from 1.95 to 1.80 Ga. Thus, an alternative tectonic scenario for the Hengshan–Wutai–Fuping region involves: (i formation of a proto-NCC at ∼2.5 Ga; (ii back-arc extension during 2.35–2.0

  3. The 1590-1520 Ma Cachoeirinha magmatic arc and its tectonic implications for the Mesoproterozoic SW Amazonian craton crustal evolution

    Directory of Open Access Journals (Sweden)

    Ruiz Amarildo S.

    2004-01-01

    Full Text Available Isotopic and chemical data of rocks from the Cachoeirinha suite provide new insights on the Proterozoic evolution of the Rio Negro/Juruena Province in SW Amazonian craton. Six U-Pb and Sm-Nd analyses in granitoid rocks of the Cachoeirinha suite yielded ages of 1587-1522 Ma and T DM model ages of 1.88-1.75 Ga (EpsilonNd values of -0.8 to +1.0. In addition, three post-tectonic plutonic rocks yielded U-Pb ages from 1485-1389 Ma (T DM of 1.77-1.74 Ga and EpsilonNd values from -1.3 to +1.7. Variations in major and trace elements of the Cachoeirinha suite rocks indicate fractional crystallization process and magmatic arc geologic setting. These results suggest the following interpretations: (1 The interval of 1590-1520 Ma represents an important magmatic activity in SW Amazonian craton. (2 T DM and arc-related chemical affinity supportthe hypothesis that the rocks are genetically associated with an east-dipping subduction zone under the older (1.79-1.74 Ga continental margin. (3 The 1590-1520 Ma age of intrusive rocks adjacent to an older crust represents similar geological framework along the southern margin of Baltica, corroborating the hypothesis of tectonic relationship at that time.

  4. DOBRE studies evolution of inverted intra-cratonic rifts in Ukraine

    Science.gov (United States)

    DOBREflection-2000 Working Groups,; DOBREfraction'99 Working Groups,

    Donbas Refraction and Reflection (DOBRE) is a multinational study of the Donbas Foldbelt (DF) of Ukraine (Figure l). The DF is the uplifted and deformed part of the more than 20-km-thick Dniepr-Donets Basin (DDB) that formed due to Late Devonian rifting of the East European Craton (EEC) in eastern Ukraine and southern Russia. The DF; especially its southern margin, was uplifted in Early Permian times in a (trans) tensional tectonic stress regime. Folding and reverse faulting occurred later, during the Triassic and Late Cretaceous.

  5. Water in the Cratonic Mantle Lithosphere

    Science.gov (United States)

    Peslier, A. H.

    2016-01-01

    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents ( 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (water contents. The olivine inclusions, however, may have been protected from metasomatism by their host diamond and record the overall low olivine water content of

  6. Tectonic Evolution of an Early Precambrian High-Pressure Granulite Belt in the North China Craton

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is located in the central collision belt between the western block and eastern block, and represents the deep crustal structural level. The typical high-pressure granulite (HPG) outcrops are distributed in the Hengshan and Chengde areas. HPGs commonly occur as mafic xenoliths within ductile shear zones, and underwent multipile deformations. To the south, the Hengshan-Chengde HPGB is juxtaposed with the Wutai greenstone belt by several strike-slip shear zones. Preliminary isotopic age dating indicates that HPGs from North China were mainly generated at the end of the Neoarchaean, assocaited with tectonic assembly of the western and esatern blocks.

  7. Geological and geochemical evolution of the Trincheira Complex, a Mesoproterozoic ophiolite in the southwestern Amazon craton, Brazil

    Science.gov (United States)

    Rizzotto, Gilmar José; Hartmann, Léo Afraneo

    2012-09-01

    amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents.

  8. Tectono-metamorphic evolution of the Paleoproterozoic ultra-high temperatures Khondalite Belt, North China Craton.

    Science.gov (United States)

    Lobjoie, Cyril; Trap, Pierre; Lin, Wei; Goncalves, Philippe; Marquer, Didier

    2016-04-01

    In the North China Craton, the Khondalite belt is a famous Paleoproterozoic domain where ultra-high temperatures (UHT) metamorphism was extensively documented over an area of 1000 square kilometers. Numerous petrological analyses argue for P-T conditions around 0.6-0.8GPa for temperature above 900°C for peak metamorphism. Unfortunately, the scarcity of available structural data prevents any discussion about thermo-mechanical behavior of the orogenic crust suffering high thermal regime. In this contribution, we present a detail structural analysis of the Khondalite belt that allowed to distinguish two main deformation events, named D1 and D2. The deformation D1 led to the formation of the S1 foliation that dips weakly toward the South-East. S1 holds a N70°E trending mineral and stretching L1 lineation that is sub-horizontal or plunges weakly to the East. The D1 fabrics is reworked by the dextral transpressional D2 deformation responsible for the development of km-scale S2-C2-C'2 system. The N30°E trending S2 foliation is sub-vertical to highly dipping toward the East. Kilometer-scale C2 and C'2 shear zones are sub-vertical and trend N70°E and N90-100°E, respectively. Petrological study and phase diagram modeling suggest that both D1 and D2 developed at UHT conditions. Garnet and spinel-bearing migmatites recording D1 fabric yield 0.7GPa for ca. 950-1015°C P-T conditions. Within D2 shear zones, numerous granitoids and mafic bodies are injected. Mafic intrusions are responsible for UHT contact metamorphism that can occur at low pressure as recorded in an olivine-bearing migmatite. This may suggest that the D2 S-C-C' system form an interconnected network of kilometer scale shear zones that act as pathways for percolation of mafic magmas from the mantle up to the base of the upper crust. Our results allow to discuss the role of localized heat advection along crustal-scale shear zones as a possible mechanism responsible for UHT metamorphism at regional scale, with

  9. Radioelemental, petrological and geochemical characterization of the Bundelkhand craton, central India: implication in the Archaean geodynamic evolution

    Science.gov (United States)

    Ray, Labani; Nagaraju, P.; Singh, S. P.; Ravi, G.; Roy, Sukanta

    2016-06-01

    We have carried out radioelemental (232Th, 238U, 40K), petrological and geochemical analyses on granitoids and gneisses covering major rock formations of the Bundelkhand craton, central India. Our data reveal that above characteristics are distinct among granitoids (i.e. pink, biotite and grey granitoids) and gneisses (i.e. potassic and sodic types). Pink granitoid is K-feldspar-rich and meta-aluminous to per-aluminous in character. Biotite granitoid is meta-aluminous in character. Grey granitoid is rich in Na-feldspar and mafic minerals, granodiorite to diorite in composition and meta-aluminous in character. Among these granitoids, radioelements (Th, U, K) are highest in pink granitoid (45.0 ± 21.7 ppm, 7.2 ± 3.4 ppm, 4.2 ± 0.4 %), intermediate in biotite granitoid (44.5 ± 28.2 ppm, 5.4 ± 2.8 ppm, 3.4 ± 0.7 %) and lowest in grey granitoid (17.7 ± 4.3 ppm, 4.4 ± 0.6 ppm, 3.0 ± 0.4 %). Among gneisses, potassic-type gneisses have higher radioelements (11.8 ± 5.3 ppm, 3.1 ± 1.2 ppm, 2.0 ± 0.5 %) than the sodic-type gneisses (5.6 ± 2.8 ppm, 1.3 ± 0.5 ppm, 1.4 ± 0.7 %). Moreover, the pink granitoid and the biotite granitoid have higher Th/U (6 and 8, respectively) compared to the grey granitoid (Th/U: 4), implying enrichment of Th in pink and biotite granitoids relative to grey granitoid. K/U among pink, biotite and grey granitoids shows little variation (0.6 × 104, 0.6 × 104, 0.7 × 104, respectively), indicating relatively similar increase in K and U. Therefore, mineralogical and petrological data along with radioelemental ratios suggest that radioelemental variations in these lithounits are mainly related to abundances of the radioactive minerals that have formed by the fractionation of LILE from different magma sources. Based on present data, the craton can be divided into three distinct zones that can be correlated with its evolution in time and space. The central part, where gneisses are associated with metavolcanics of greenstone belt, is

  10. The formation and the geochemical and thermal evolution of the lithospheric mantle beneath the Kaapvaal craton recorded by subcalcic garnets from harzburgites and by pristine eclogites and garnet-pyroxenites

    OpenAIRE

    Shu, Qiao

    2013-01-01

    The mantle xenoliths collected by kimberlites indicate that the subcratonic mantle underneath the Archean crust is mostly a residue of high degrees of partial melting which was subsequently reenriched. The majority of the xenoliths show cryptic metasomatism and only few modal metasomatism. Much effort has been put into deciphering different kinds of enrichment processes within the mantle. Here, we take the approach to look into the inventory of subcalcic garnets which stem from cpx-free ha...

  11. Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

    Science.gov (United States)

    Schmidberger, S. S.; Francis, D.

    1999-09-01

    The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet-peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet-pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m 2. Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (˜40 mW/m 2) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.

  12. Cenozoic lithospheric evolution of the Bohai Bay Basin, eastern North China Craton: Constraint from tectono-thermal modeling

    Science.gov (United States)

    Liu, Qiongying; He, Lijuan; Huang, Fang; Zhang, Linyou

    2016-01-01

    It is well established that the lithosphere beneath the eastern North China Craton (NCC) had been thinned before the Cenozoic. A 2D multi-phase extension model, in which the initial crustal and lithospheric thicknesses are variable, is presented to reconstruct the initial thicknesses of the crust and lithosphere in the early Cenozoic and to further investigate the lithospheric evolution beneath the eastern NCC through the Cenozoic. We conduct thermal modeling along three profiles from east to west in the Bohai Bay Basin, which is the center of the lithospheric destruction and thinning of the NCC. Using multiple constraints, such as tectonic subsidence, the present-day heat flow and the Moho depth, we determine the initial crustal and lithospheric thicknesses of the Bohai Bay Basin before the Cenozoic rift to be 33-36 km and 80-105 km, respectively. The model results show that the most rapid lithospheric thinning during the Cenozoic occurred in the middle Eocene for most depressions, and the thinning activity ceased at the end of the Oligocene, reaching a minimum lithospheric thickness of 53-74 km, followed by a thermal relaxation phase. Combined with previous studies, we infer that the lithosphere beneath the eastern NCC experienced two stages of alternating thinning and thickening: notable thinning in the Early Cretaceous and Paleogene, and thickening in the Late Cretaceous and late Cenozoic. We believe that thermo-chemical erosion, together with extension, was probably the major mechanism of the significant lithospheric removal during the Mesozoic, whereas the Cenozoic lithospheric thinning was mainly dominated by tectonic extension in the eastern NCC; lithospheric thickening was generally a result of thermal cooling.

  13. The evolution of the southern margin of the East European Craton based on seismic and potential field data

    Science.gov (United States)

    Kostyuchenko, S. L.; Morozov, A. F.; Stephenson, R. A.; Solodilov, L. N.; Vedrentsev, A. G.; Popolitov, K. E.; Aleshina, A. F.; Vishnevskaya, V. S.; Yegorova, T. P.

    2004-03-01

    This paper presents an integrated geophysical study of the southern margin of the East European Craton (EEC) in the Karpinksy Swell-North Caucasus area. It presents new interpretations of deep refraction and wide-angle reflection "deep seismic sounding" (DSS) data as well as conventional seismic and CDP profiling and new analyses of potential field data, including three-dimensional gravity and magnetic modelling. An integrated model of the physical properties and structure of the Earth's crust and, partially, upper mantle displays distinct features that are related to tectonic history of the study area. The Voronezh Massif (VM), the Ukrainian Shield and Rostov Dome (RD) of the EEC as well as the Donbas Foldbelt (DF), Karpinsky Swell (KS), Scythian Plate (SP) and Precaspian Basin (PCB) constitute the geodynamic ensemble that developed on the southern margin of the continent Baltica. There proposed evolutionary model comprises a stage of rifting during the middle to late Devonian, post-rift extension and subsidence during Carboniferous-early Permian times (synchronous with and related to the southward displacement of the Rostov Dome and extension in a palaeo-Scythian back-arc basin), and subsequent Mesozoic and younger evolution. A pre-Ordovician, possibly Riphean (?), mafic magmatic complex is inferred on a near vertical reflection seismic cross-section through the western portion of the Astrakhan Dome in the southwest part of the Precaspian Basin. This complex combined with evidence of a subducting slab in the upper mantle imply the presence of pre-Ordovician (Riphean?) island arc, with synchronous extension in a Precaspian back-arc basin is suggested. A middle Palaeozoic back-arc basin ensemble in what is now the western Karpinsky Swell was more than 100 km to the south from its present location. The Stavropol High migrated northwards, dislocating and moving fragments of this back-arc basin sometime thereafter. Linear positive magnetic anomalies reflect the

  14. Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

    Science.gov (United States)

    Sommer, H.; Jacob, D. E.; Stern, R. A.; Petts, D.; Mattey, D. P.; Pearson, D. G.

    2017-06-01

    Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The δ18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of δ18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic

  15. Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: Evidence for cratonic evolution and redistribution of REE during crustal anatexis

    Science.gov (United States)

    Gao, Shan; Ling, Wenli; Qiu, Yumin; Lian, Zhou; Hartmann, Gerald; Simon, Klaus

    1999-07-01

    be younger than the sampled granitic gneiss and derived from a distal source. Both Groups A and B exhibit a clear positive correlation between CIW and TDM and a negative one between CIW and Eu/Eu∗. These correlations point to the crustal evolution of the Yangtze craton towards coupled increasing CIW and Eu depletion with decreasing age. This in turn reflects the change of granitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal differentiation, stabilization and growth of the craton. Group C is restite and contains abundant sillimanite and garnet and unusually high ilmenite (7-11vol%), which can be seen to be dehydration melting products of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to strong positive (Eu/Eu∗ = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na 2O, K 2O, LREE, Rb and Ba, whereas TiO 2, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalous high Sm/Nd (0.21-0.28), 147Sm/ 144Nd (0.1361-0.1738) and 143Nd/ 144Nd (0.511589-0.511958) ratios. TDM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic TDM (3.9-4.9 Ga). The results document redistribution of REE and an open behavior of the Sm-Nd isotope system during the biotite dehydration melting of metasediments.

  16. Hypothesis of homeothermy evolution on isolated South China Craton that moved from equator to cold north latitudes 250-200Myr ago.

    Science.gov (United States)

    Kurbel, Sven

    2014-01-07

    Based on avian and mammalian fossils found in the northeastern Chinese province of Liaoning and physiological traits linked to homeothermy, a hypothesis of evolution of homeothermic animals is proposed. It is based on the importance of muscle function in cold environment, as a strong selection pressure that favors endothermic metabolism during periods of cold climates. The presented hypothesis postulates that in progressively cooling environment, animals will develop thermal insulation, increased basal metabolism if food is available, and torpor when food is scarce. Since late Permian, Triassic and Cretaceous global temperatures were high, an exceptional place that gradually became cold was needed for the homeothermy evolution. South China Craton is here proposed as a plausible candidate for that role since it drifted across the Paleo-Tethys ocean, from equator to high northern latitudes in a journey that lasted from 250 to 200Myr ago. After this small continent collided with North China Craton some 200Myr ago, the already cold-adapted animals had spread to large, mostly empty spaces on the North China Craton, due to their evolutionary advantage of making active living in the cold environment. The most advantageous early homeothermic animals went further north to the cold Liaoning to start an oasis that delivered modern birds during next 50Myr. Modern mammals possibly evolved somewhere in the cold vicinity. This made Liaoning and similarly cold places the cradles of early birds and early mammals since for the following millions of years these places remained too cold for poikilotherms to enter and warm enough for homeotherms to dwell, until the Cretaceous-Paleogene extinction event and subsequent global cooling that diminished poikilotherms. Homeothermy was probably even more important as a survival advantage in cooler climates of Paleogene, when mammals and birds became dominant animals. This interpretation is probably supported by a recent report that a small

  17. Thermodynamic evolution of lithosphere of the North China craton: Records from lower crust and upper mantle xenoliths from Hannuoba

    Institute of Scientific and Technical Information of China (English)

    LIU Yongsheng; GAO Shan; LIU Xiaoming; CHEN Xiaoming; ZHANG Wenlan; WANG Xuance

    2003-01-01

    Major element compositions of garnet, clinopyroxene, orthopyroxene andspinel in the garnet-bearing lower crust and upper mantle xenoliths from Hannuoba, North China craton are analyzed by the electron microprobe (EMP). The pressure-temperature estimates reveal the increasing temperature and pressure from core to rim for granulites. In contrast, mantle xenoliths with core temperature > 930℃ recorded a history of decrease in temperature and pressure. However, those with core temperature < 930℃ show a negligible change. The final pressures recorded by these xenoliths cluster at 0.9-1.5 GPa. The presence of high- Na2O cpx in granulite xenoliths suggests that they are products of the transition from granulite to eclogite metamorphism corresponding to the increasing temperature andpressure. Together with previous studies, it is suggested that the P-T changes preserved in the xenoliths are related to lithospheric thickening and subsequentthinning prior to their eruption in the Cenozoic.

  18. Extremely depleted lithospheric mantle and diamonds beneath the southern Zimbabwe Craton

    Science.gov (United States)

    Smith, Chris B.; Pearson, D. Graham; Bulanova, Galina P.; Beard, Andrew D.; Carlson, Richard W.; Wittig, Nadine; Sims, Keith; Chimuka, Lovemore; Muchemwa, Ellah

    2009-11-01

    Inclusion-bearing diamonds, mantle xenoliths, and kimberlite concentrates from the Cambrian-aged Murowa and Sese kimberlites have been studied to characterise the nature of the lithospheric mantle beneath the southern Zimbabwe Craton. The diamonds are mostly octahedral, moderately rich in nitrogen with moderate to high aggregation, and contain mainly dunite-harzburgite mineral inclusions. Similarly, dunite xenoliths predominate over harzburgite and lherzolite and carry olivines with Mg/Mg + Fe (Mg#) values of 0.92-0.95, spanning the average signatures for Kaapvaal Craton peridotites. Eclogitic xenoliths are extremely rare, in contrast to the Kaapvaal mantle lithosphere. The Zimbabwe mantle assemblage has been only slightly affected by later silicic metasomatism and re-fertilisation with re-introduction of pyroxenes in contrast to the Kaapvaal and many cratonic lithospheric blocks elsewhere where strong metasomatism and re-fertilisation is widespread. Pyroxene, garnet and spinel thermobarometry suggests an ambient 40 mW m - 2 geotherm, with the lithosphere extending down to 210 km at the time of kimberlite eruption. Whole rock peridotite Re-Os isotope analyses yield T RD model ages of 2.7 to 2.9 Ga, providing minimum estimates of the time of melt depletion, are slightly younger in age than the basement greenstone formation. These model ages coincide with the mean T RD age of > 200 analyses of Kaapvaal Craton peridotites, whereas the average Re-Os model age for the Zimbabwe peridotites is 3.2 Ga. The Os data and low Yb n/Lu n ratios suggest a model whereby thick lithospheric mantle was stabilised during the early stages of crustal development by shallow peridotite melting required for formation of residues with sufficiently high Cr/Al to stabilise chromite which then transforms to low Ca, high Cr garnet. Sulphide inclusions in diamond produce minimum T RD model ages of 3.4 Ga indicating that parts of the lithosphere were present at the earliest stages of crust

  19. Support for a Uniformitarian Model of Continental Mantle Lithosphere Formation from the "Near-Cratonic" Composition of Proterozoic Southern African Mantle Lithosphere

    Science.gov (United States)

    Janney, P. E.

    2014-12-01

    The transition at the end of the Archean between the generation of cratonic and mobile belt continental lithosphere is regarded as a first-order change in the mode of generation of continental lithosphere. It is widely debated whether this transition represented a fundamental change in the process by which the lithospheric mantle was generated (i.e., as melting residues of deep-seated mantle upwellings to residues of relatively shallow mantle melting at subduction zones), or whether it primarily reflected a more gradual change in the conditions (i.e., temperatures, depths and degrees of melting) of lithosphere generation in a suprasubduction zone setting. The marked contrast, in many cases, between the major element compositions of peridotite xenoliths from Archean cratons and those from adjacent post-Archean mobile belts has accentuated the significance of this transition. Peridotite xenoliths from the post-Archean mobile belt terranes surrounding the Kaapvaal craton in southern Africa are clearly Proterozoic in age from Re-Os isotope constraints, but they are unusual in that they share several key similarities in composition and mineralogy with Archean Kaapvaal peridotites (e.g., low bulk-rock Al2O3, relatively low modal olivine and high modal orthopyroxene). Although they lack the low FeO and high olivine Mg# values of the most extreme Kaapvaal samples, they show a very large degree of overlap (extending to olivine Mg# values of greater than 93 for example). These similarities support a common mode of origin for cratonic and post-cratonic lithosphere in southern Africa (although varying somewhat in the degrees and depths of melt extraction) and a similar history of post-formation modification. A comparison of the conditions of melt extraction for cratonic and post-cratonic lithosphere inferred from compatible and mildly incompatible trace elements will be presented.

  20. Physico-chemical constraints on cratonic lithosphere discontinuities

    Science.gov (United States)

    Aulbach, Sonja; Rondenay, Stéphane; Huismans, Ritske

    2014-05-01

    The origins of the mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have received much attention over the recent years. Peculiarities of cratonic lithosphere construction - compositional and rheological stratification due to thickening in collisional settings or by plume subcretion, multiple metasomatic overprints due to longevity - offer a variety of possibilities for the generation of discontinuities. Interconnected small degrees of conductive partial melt (carbonate-rich melts, such as carbonatites and kimberlites, or highly alkaline melts) at the cratonic LAB, which produce seismic discontinuities, may be generated in the presence of volatiles. These depress the peridotite solidus sufficiently to intersect the mantle adiabat at depths near the cratonic LAB at ~160-220 km, i.e. above the depth of metal saturation where carbonatite becomes unstable. The absence of agreement between the different seismic and magnetotelluric estimates for the depth of the LAB beneath Kaapvaal may be due to impingement of a plume, leading to a pervasively, but heterogeneously metasomatised ('asthenospherised') hot and deep root. Such a root and hot sublithosphere may yield conflicting seismic-thermal-geochemical depths for the LAB. The question arises whether the chemical boundary layer should be defined as above or below the asthenospherised part of the SCLM, which has preserved isotopic, compositional (non-primitive olivine forsterite content) and physical evidence (e.g. from teleseismic tomography and receiver functions) for a cratonic heritage and which therefore is still distinguishable from the asthenospheric mantle. If cratonic lithosphere overlies anomalously hot mantle for extended periods of time, the LAB may be significantly thinned, aided by penetration of relatively high-degree Fe-rich partial melts, as has occurred beneath the Tanzanian craton. Xenoliths from the deep Slave craton show little evidence for 'asthenospherisation'. Its root

  1. Petromagnetic Properties of Granulite-Facies Rocks from the Northern North China Craton:Implications for Magnetic and Evolution of the Continental Lower Crust

    Institute of Scientific and Technical Information of China (English)

    Qingsheng Liu; Hongcai Wang; Jianping Zheng; Qingli Zeng; Qingsong Liu

    2013-01-01

    This paper studies magnetic properties and composition of granulite-facies rocks of both the Neogene and Archean continental lower crust in the Neogene xenolith-bearing Hannuoba(汉诺坝) alkaline basalt and the exposed lower crustal section in the Archean Huai'an(淮安) terrain (Wayaokou (瓦窑口)-Manjinggou(蔓菁沟) profile),the northern North China Craton.It provides a unique opportunity for a comparative study of magnetic properties and composition of both the Archean and Neogene continental lower crust.We measure magnetic parameters (susceptibility kand magnetic hysteresis parameters,such as saturation magnetization Js,saturation isothermal remanent magnetization Jrs,and intrinsic coercivity Hc) of eleven Hannuoba lower crustal xenoliths and nine terrain granulites from the Archean Huai'an terrain.Results indicate that the average values of k,Js and Jrs of Archean granulites are 4 122×10-6 Sl,523.1 A/m and 74.9 A/m,respectively,which are generally higher than those of granulite-facies xenoliths (1 657×10-6 SI,163.9 A/m and 41.9 A/m,respectively).These two types of granulites contain ilmenite,(titano) magnetite,minor hematite and some "magnetic silicates" (clinopyroxene,plagioclase and biotite).The Mg-rich ilmenite in granulite-facies xenolith is relatively higher than that in terrain granulites.We observe a more evolved character as higher magnetic as well as lower Sr/Nd,Cr/Nd,Ni/Nd,Co/Nd and V/Nd ratios in terrain granulites.These differences in magnetic characteristics reflect their different origins and evolutions.The high magnetization of granulites in the Huai'an terrain represents magnetic properties of the Archean continental lower crust,and low magnetization of granulite-facies xenoliths represents magnetic properties of the Cenozoic lower crusts in the northern North China Craton.

  2. Evolution of the mantle beneath the eastern North China Craton during the Cenozoic: Linking geochemical and geophysical observations

    Science.gov (United States)

    Li, Hong-Yan; Xu, Yi-Gang; Ryan, Jeffrey G.; Whattam, Scott A.

    2017-01-01

    Recent discoveries related to the geochemistry of Cenozoic basalts and the geophysics of the deep mantle beneath eastern Eurasia make it possible to place constraints on the relationship between the seismic tomography of subcontinental mantle domains and their geochemical heterogeneities. Basalts with ocean island basalt-like trace elements erupted during (56-23 Ma) and after (≤23 Ma) rifting of the eastern North China Craton (NCC) show evidence for the mixing of an isotopically depleted source and an EMI (Enriched mantle type I) pyroxenitic mantle. NCC rifting-stage basalts exhibit anomalously low MgO and Fe2O3T and high SiO2 and Al2O3, as well as low Dy/Yb and Y/Yb and high ɛHf at a given ɛNd, as compared to the postrifting basalts. Temporal compositional variations and their association with basin subsidence indicate that heterogeneity in the eastern NCC asthenospheric mantle is the primary driver for intraplate magmatism in this region. The specific magmatic sources shifted in terms of depth, related to lithospheric thinning and thickening in the eastern NCC. The NCC EMI mantle domain most likely developed due to ancient events, is persistent through time, and is not related to dehydration of the stagnant Pacific slab in the mantle transition zone. Based on the chemical signatures of postrifting basalts, contributions from the Pacific slab are likely to be carbonatite rich. Mantle metasomatism by carbonatite melts from the Pacific slab and the interaction of these melts at shallower depths with EMI pyroxenitic mantle domains to trigger melting are contributors to the observed low P wave velocity zone beneath eastern Eurasia.

  3. The Role of Water in the Stability of Cratonic Keels

    Science.gov (United States)

    Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina

    2011-01-01

    Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

  4. Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

    Directory of Open Access Journals (Sweden)

    GILMAR J. RIZZOTTO

    2014-03-01

    Full Text Available New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma. This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton.

  5. Metamorphic evolution of the Río de la Plata Craton in the Cinco Cerros area, Buenos Aires Province, Argentina

    Science.gov (United States)

    Massonne, Hans-Joachim; Dristas, Jorge A.; Martínez, Juan Cruz

    2012-10-01

    A metapelite and an interlayered granite were studied from the Cinco Cerros area ca. 65 km WNW of the city of Mar del Plata. Garnet in these samples is slightly zoned with core and rim compositions of pyr17(gro + andr)6spes1.5alm75.5 and pyr13.5(gro + andr)5.5spes2alm80, respectively, in the metapelite. Corresponding compositions in the granite are pyr15(gro + andr)3.5spes3.5alm78 and pyr11(gro + andr)3.5spes4.5alm81. We used the PERPLE_X computer software package to calculate P-T pseudosections. From the pseudosection of the metapelite P-T conditions of 6.7 kbar and 670 °C were derived for an early metamorphic stage. Subsequently, a pressure release occurred at decreasing temperatures. The final metamorphic P-T conditions recorded by the studied rock are 4.5 kbar and 600 °C compatible with the absence of cordierite, staurolite, and an Al2SiO5-phase. Garnet in the granitoid crystallized between 715 and 690 °C at a pressure around 7.7 kbar. U-Th-Pb age dating with the electron microprobe was performed. 16 analyses of monazite in the metapelite formed three clusters resulting in ages of I: 2073 ± 11.4 (1σ) Ma, II: 1913 ± 11.0 (1σ) Ma, and III: 1805 ± 20.8 (1σ) Ma. Thus, the Paleoproterozoic metamorphic event can be related to the Trans-Amazonian cycle and was followed by slow cooling. As our study area is close to the margin of the Río de la Plata Craton, where abundant magmatic arc-derived plutonic rocks are outcropping, we interpret the derived P-T data as follows: A heating event (not recorded by the studied rocks) resulted from magmas that intruded during the Trans-Amazonian cycle. This event was followed by slow exhumation, probably caused by erosion, accompanied by thermal relaxation.

  6. Metamorphism of the northern Liaoning Complex: Implications for the tectonic evolution of Neoarchean basement of the Eastern Block, North China Craton

    Directory of Open Access Journals (Sweden)

    Kam Kuen Wu

    2013-05-01

    Full Text Available As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North China Craton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic (TTG gneisses, massive granitoids and supracrustal rocks. The supracrustal rocks, named the Qingyuan Group, consist of interbedded amphibolite, hornblende granulite, biotite granulite and BIF. Petrological evidence indicates that the amphibolites experienced the early prograde (M1, peak (M2 and post-peak (M3 metamorphism. The early prograde assemblage (M1 is preserved as mineral inclusions, represented by actinotite + hornblende + plagioclase + epidote + quartz + sphene, within garnet porphyroblasts. The peak assemblage (M2 is indicated by garnet + clinopyroxene + hornblende + plagioclase + quartz + ilmenite, which occur as major mineral phases in the rock. The post-peak assemblage (M3 is characterized by the garnet + quartz symplectite. The P–T pseudosections in the NCFMASHTO system constructed by using THERMOCALC define the P–T conditions of M1, M2 and M3 at 490–550 °C/<4.5 kbar, 780–810 °C/7.65–8.40 kbar and 630–670 °C/8.15–9.40 kbar, respectively. As a result, an anticlockwise P–T path involving isobaric cooling is inferred for the metamorphic evolution of the amphibolites. Such a P–T path suggests that the late Archean metamorphism of the northern Liaoning Complex was related to the intrusion and underplating of mantle-derived magmas. The underplating of voluminous mantle-derived magmas leading to metamorphism with an anticlockwise P–T path involving isobaric cooling may have occurred in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift environments. A mantle plume model is favored because this model can reasonably interpret many other geological features of late Archean basement rocks from the northern Liaoning Complex in the Eastern Block of

  7. Precambrian crustal evolution and Cretaceous–Palaeogene faulting in West Greenland: Evolution of Neoarchaean supracrustal belts at the northern margin of the North Atlantic Craton, West Greenland

    Directory of Open Access Journals (Sweden)

    Stensgaard, Bo Møller

    2006-12-01

    Full Text Available The Archaean North Atlantic Craton of West Greenland collided at c. 1.9 Ga with a lesser-known Archaean craton to the north, to form the Nagssugtoqidian orogen. The Palaeoproterozoic metamorphic grade and strain intensity decrease northward through the orogen, allowing investigation of the reworked Archaean components in its northern part. Two Archaean supracrustal belts in this region – the Ikamiut and Kangilinaaq belts – are investigated here using field mapping, aeromagnetic data, zircon geochronology, and geochemistry. Both belts comprise quartzo-feldspathic and pelitic metasedimentary rocks, amphibolite, and minor calc-silicate rocks, anorthosite and ultramafic rocks. Pb-Pb and U-Pb dating of detrital zircons and host orthogneisses suggest deposition at c. 2800 Ma (Kangilinaaq belt and after 2740 Ma (Ikamiut belt; both belts have zircons with Neoarchaean metamorphic rims. Metasedimentary rocks and orthogneisses at Ikamiut share similar steep REE signatures with strong LREE enrichment, consistent with local derivation of the sediment and deposition directly onto or proximal to the regional orthogneiss precursors. Zircon age data from Kangilinaaq indicate both local and distal sources for the sediment there. Geochemical data for Kangilinaaq amphibolites indicate bimodal, mixed felsic–mafic source rocks with island-arc basaltic affinities, consistent with a shelf or arc setting. Both belts experienced a similar tectono-metamorphic history involving Neoarchaean amphibolite facies peak metamorphism at c. 2740–2700 Ma, possibly due to continued emplacement of tonalitic and granodioritic magmas. Nagssugtoqidian lower amphibolite facies metamorphism at c. 1850 Ma was associated with development of the large-scale F2 folds and shear zones that control the present outcrop pattern. The observed differences in the sources of the Kangilinaaq and Ikamiut belts and their shared post-Archaean history suggest they were formed in different

  8. Geodynamic evolution of the West and Central Pilbara Craton in Western Australia : a mid-Archaean active continental margin

    NARCIS (Netherlands)

    Beintema, K.A.

    2003-01-01

    The Archaean era lasted for about one third of the Earth's history, from ca 4.0 until 2.5 billion years ago. Because the Archaean spans such a long time, knowledge about this era is for understanding the evolution of the Earth until the present day, especially because it is the time offormation of m

  9. Geodynamic evolution of the West and Central Pilbara Craton in Western Australia : a mid-Archaean active continental margin

    NARCIS (Netherlands)

    Beintema, K.A.

    2003-01-01

    The Archaean era lasted for about one third of the Earth's history, from ca 4.0 until 2.5 billion years ago. Because the Archaean spans such a long time, knowledge about this era is for understanding the evolution of the Earth until the present day, especially because it is the time offormation of m

  10. High Water Contents in the Siberian Cratonic Mantle: An FTIR Study of Udachnaya Peridotite Xenoliths

    Science.gov (United States)

    Doucet, Luc S.; Peslier, Anne H.; Ionov, Dimitri A.; Brandon, Alan D.; Golovin, Alexander V.; Ashchepkov, Igor V.

    2013-01-01

    Water is believed to be a key factor controlling the long-term stability of cratonic lithosphere, but mechanisms responsible for the water content distribution in the mantle remain poorly constrained. Water contents were obtained by FTIR in olivine, pyroxene and garnet for 20 well-characterized peridotite xenoliths from the Udachnaya kimberlite (central Siberian craton) and equilibrated at 2-7 GPa. Water contents in minerals do not appear to be related to interaction with the host kimberlite. Diffusion modeling indicates that the core of olivines preserved their original water contents. The Udachnaya peridotites show a broad range of water contents in olivine (6.5 +/- 1.1 to 323 +- 65 ppm H2O (2 sigma)), and garnet (0 - 23 +/- 6 ppm H2O). The water contents of olivine and garnet are positively correlated with modal clinopyroxene, garnet and FeO in olivine. Water-rich garnets are also rich in middle rare earth elements. This is interpreted as the result of interaction between residual peridotites and water rich-melts, consistent with modal and cryptic metasomatism evidenced in the Siberian cratonic mantle. The most water-rich Udachnaya minerals contain 2 to 3 times more water than those from the Kaapvaal craton, the only craton with an intact mantle root for which water data is available. The highest water contents in olivine and orthopyroxene in this study (>= 300 ppm) are found at the bottom of the lithosphere (> 6.5 GPa). This is in contrast with the Kaapvaal craton where the olivines of peridotites equilibrated at > 6.4 GPa have 6 GPa is lower or similar (8.4× 10(exp 16) to 8.0× 10(exp 18) Pa./s) to that of the asthenosphere (<= 3.7x10(exp 18) Pa./s ). Such lithologies would not be able to resist delamination by the convecting asthenosphere. However, seismology studies as well as the high equilibration pressures of our samples indicate that the Udachnaya cratonic lithosphere is 220-250 km thick. Consequently, the water-rich peridotites are likely not

  11. The reaction space and different P-T evolution of Xuanhua and Miyun basic granulites in North China Craton

    Institute of Scientific and Technical Information of China (English)

    马军; 王仁民

    1997-01-01

    The basic granulites from Xuanhua and Miyun have similar mineral assemblages but very different reactionary textures. Through comparison of the metamorphic reaction space between the basic granulites of the two areas, it is further confirmed that the P-T evolution of Xuanhua basic granulites and that of Miyun basic granulites are different. The former is of type CW-ITD. The latter is of ACW-IBC.

  12. Seismic imaging of Southern African cratons

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad

    Cratonic regions are the oldest stable parts of continents that hold most of Earth’s mineral resources. There are several open questions regarding their formation and evolution. In this PhD study, passive source seismic methods have been used to investigate the crustal and lithosphere structures...

  13. The Rhyacian El Cortijo suture zone:Aeromagnetic signature and insights for the geodynamic evolution of the southwestern Rio de la Plata craton, Argentina

    Institute of Scientific and Technical Information of China (English)

    Carlos J. Chernicoff; Eduardo O. Zappettini; Javier Peroni

    2014-01-01

    The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny, rather than a single one as previously envisaged, i.e. the El Cortijo suture zone and the Salado suture. We circumscribe the Tandilia terrane to the region between these two sutures. The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane, i.e. boundary between the Tandilia and Balcarce terranes. This extensive magnetic low, ca. 300 km long, and ca. 90 km wide, would be caused by demagnetization associated with shearing. At a more local scale, the trend of the El Cortijo suture zone often turns toward the EeW. At this scale, WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian El Cortijo suture zone. Spatially associated with the El Cortijo suture zone, there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation. We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean (w2650 Ma) crust occurred during Siderian times (2500e2300 Ma), causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by w2300e2200 Ma sedi-mentation over transitional econtinental to oceanice crust, and arc magmatism developed in the Tan-dilia terrane. The island arc represented by the El Cortijo Formation was also developed at this time. At late Rhyacian times, it occurred in both the closure of the narrow oceans developed previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane.

  14. Detrital zircon and apatite fission track data in the Liaoxi basins: Implication to Meso-Cenozoic thermo-tectonic evolution of the northern margin of the North China Craton

    Indian Academy of Sciences (India)

    Yi Yan; Xiaoqiong Hu; Ge Lin; Weiliang Liu; Zhengjiang Song

    2010-08-01

    Detrital zircon and apatite fission track (ZFT and AFT) data of the sandstones collected from the Liaoxi basins served as a significant probe to study the Meso-Cenozoic thermo-tectonic reactivation events in the northern margin of the North China Craton. All sandstones show wide ZFT and AFT age spectrum and most of ZFT and AFT ages are younger than depositional age of respective host rocks, which suggest widespread track resetting of the host rocks in the Liaoxi basins after deposition. This hot geothermal status in the Liaoxi basins deduced from ZFT and AFT data is temporal consistent with the lithospheric evolution of the North China Craton, which implies that the lithosphere under the northern margin of the North China Craton underwent similar thermo-tectonic destruction process as the intracratonic Bohai Sea. The young ZFT peak age, which ranges from ∼50Ma to 20 Ma, to some extend, provides a temporal constraint on the time that lithosphere significantly thinned and following reverse of the Liaoxi basins and uplift of the eastern part of the Yan-Liao Orogenic Belt. Exhumation of 1.5–2 km can be estimated in the eastern part of the Yan-Liao Orogenic Belt since ∼30Ma to 10 Ma.

  15. Low-temperature thermal evolution of the Azov Massif (Ukrainian Shield Ukraine) — Implications for interpreting (U Th)/He and fission track ages from cratons

    Science.gov (United States)

    Danišík, Martin; Sachsenhofer, Reinhard F.; Privalov, Vitaliy A.; Panova, Elena A.; Frisch, Wolfgang; Spiegel, Cornelia

    2008-08-01

    The low-temperature thermal evolution of the Azov Massif (eastern part of the Ukrainian Shield, Ukraine) is investigated by combined zircon fission track (ZFT), apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronology. The data help to better understand the geodynamic evolution of the Azov Massif and the adjacent intra-cratonic rift basin (Dniepr-Donets Basin) as follows: ZFT data reveal that the Precambrian crystalline basement of the Azov Massif was heated to temperatures close to ˜ 240 °C during the Late Palaeozoic. The heating event is interpreted in terms of burial of the basement beneath a several kilometres thick pile of Devonian and Carboniferous sedimentary deposits of the adjacent Dniepr-Donets Basin. During Permo-Triassic times, large parts of the basement were affected by a thermal event related to mantle upwelling, associated magmatic activity and increased heat flow in the adjacent rift. The major part of the basement cooled to near-surface conditions in the Early to Middle Triassic and since then was thermally stable as suggested by AFT and AHe data. Further, AFT data confirm Late Triassic magmatic activity in the Azov Massif, which, however, did not influence regional thermal pattern. The northern part of the basement and its sedimentary cover record a cooling event in the Jurassic, which was probably related to erosion. However, although Ar-Ar data of Jurassic magmatic activity in the Donbas Foldbelt are about 20 My younger than the AFT data, thermal relaxation after elevated heat flow associated with this magmatic event cannot be completely ruled out. Our results reveal apparent inconsistencies between AFT and AHe data: the AHe ages corrected for alpha ejection according to the standard procedure [Farley, K.A., Wolf, R.A., Silver, L.T., 1996. The effect of long alpha-stopping distances on (U-Th)/He ages. Geochim. Cosmochim. Acta 60(21), 4223-4229.; Farley, K.A., 2002. (U-Th)/He dating: Techniques, calibrations, and applications

  16. The Cauaburi magmatic arc: Litho-stratigraphic review and evolution of the Imeri Domain, Rio Negro Province, Amazonian Craton

    Science.gov (United States)

    Carneiro, Marcia C. R.; Nascimento, Rielva S. C.; Almeida, Marcelo E.; Salazar, Carlos A.; Trindade, Ivaldo Rodrigues da; Rodrigues, Vanisse de Oliveira; Passos, Marcel S.

    2017-08-01

    A lithostratigraphic review of the Cauaburi Complex was carried out by means of field, tectono-metamorphic and geochemical data, which were the basis for the sub-division of the Cauaburi Complex orthogneisses into the Santa Izabel do Rio Negro, Cumati and São Jorge facies. These rocks crop out between São Gabriel da Cachoeira and Santa Izabel do Rio Negro, Amazonas, Brazil. The gneisses of the Santa Izabel do Rio Negro and Cumati facies are metaluminous and of calc-alkaline affinity; in turn, the rocks of the São Jorge facies are peraluminous and of alkaline affinity. They vary from (amphibole)-biotite granodiorites/monzogranites (Cumati and Santa Izabel do Rio Negro facies) to spessartite-bearing biotite monzogranites (São Jorge facies). The Cauaburi Complex geochemical signature is compatible with that of granites generated in collisional settings (magmatic arc?) and its evolution is related to three distinct tectono-metamorphic events: D1, causing foliation S1, which developed during the Cauaburi Complex syn-tectonic emplacement in the Cauaburi Orogeny; D2/M2, causing foliation S2, which was generated under amphibolite facies conditions (717.9 °C and 5.84 kbars), and the emplacement of I- and S-type granite during the Içana Orogen, and low-temperature D3, associated with the K'Mudku Event, which caused foliation S3 and reworking via transcurrent shear zones under greenschist facies conditions.

  17. The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons

    Science.gov (United States)

    Eaton, David W.; Darbyshire, Fiona; Evans, Rob L.; Grütter, Herman; Jones, Alan G.; Yuan, Xiaohui

    2009-04-01

    The lithosphere-asthenosphere boundary (LAB) is a first-order structural discontinuity that accommodates differential motion between tectonic plates and the underlying mantle. Although it is the most extensive type of plate boundary on the planet, its definitive detection, especially beneath cratons, is proving elusive. Different proxies are used to demarcate the LAB, depending on the nature of the measurement. Here we compare interpretations of the LAB beneath three well studied Archean regions: the Kaapvaal craton, the Slave craton and the Fennoscandian Shield. For each location, xenolith and xenocryst thermobarometry define a mantle stratigraphy, as well as a steady-state conductive geotherm that constrains the minimum pressure (depth) of the base of the thermal boundary layer (TBL) to 45-65 kbar (170-245 km). High-temperature xenoliths from northern Lesotho record Fe-, Ca- and Ti-enrichment, grain-size reduction and globally unique supra-adiabatic temperatures at 53-61 kbar (200-230 km depth), all interpreted to result from efficient advection of asthenosphere-derived melts and heat into the TBL. Using a recently compiled suite of olivine creep parameters together with published geotherms, we show that beneath cratons the probable deformation mechanism near the LAB is dislocation creep, consistent with widely observed seismic and electrical anisotropy fabrics. If the LAB is dry, it is probably diffuse (> 50 km thick) and high levels of shear stress (> 2 MPa or > 20 bar) are required to accommodate plate motion. If the LAB is wet, lower shear stress is required to accommodate plate motion and the boundary may be relatively sharp (≤ 20 km thick). The seismic LAB beneath cratons is typically regarded as the base of a high-velocity mantle lid, although some workers infer its location based on a distinct change in seismic anisotropy. Surface-wave inversion studies provide depth-constrained velocity models, but are relatively insensitive to the sharpness of the LAB

  18. Early Precambrian Crustal Evolution in the Northern Margin of the North China Craton: Constraints from Zircon U-Pb and Lu-Hf Isotopes

    Institute of Scientific and Technical Information of China (English)

    LIU Shu-wen; Lü Yong-jun; LI Qiu-gen

    2008-01-01

    @@ Northern Hebei province, one of important Precambrian metamorphic areas, is located in the middle segment of northern margin of the North China Craton. Precambrian rocks in this area are subdivided into two units by the Chaiwopu-Dantazi ductile shear zone.

  19. Identification of 3.5 Ga detrital zircons from Yangtze craton in south China and the implication for Archean crust evolution

    Institute of Scientific and Technical Information of China (English)

    LIU Xiaoming; GAO Shan; LING Wenli; YUAN Honglin; HU Zhaochu

    2006-01-01

    The LA-ICP-MS U-Pb dating of hundreds of detrital zircon grains from the Sinian sandstones of Liantuo formation and tillites of Nantuo formation at Sanxia area in Yichang identified 3319-3508 Ma zircon grains. Their 207pb/206pb and 206pb/238U ages show excellent agreement (concordia degree 99 %-100 % ). Their CL images exhibit well-developed oscillatory zoning and the Th/U ratios are within 0. 46-0. 76, implying that they are igneous zircons which formed during middle-early Archean. These zircons are the oldest ones discovered in Yangtze craton until now. However, the detrital zircons with ages older than 3.3 Ga in the metamorphic rocks of Kongling group were not found by further investigation, which suggests the presence of crust older than high-grade metamorphic Kongling terrain in Yangtze craton.

  20. Speculations on the formation of cratons and cratonic basins

    Science.gov (United States)

    McKenzie, Dan; Priestley, Keith

    2016-02-01

    Surface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of similar thickness to that beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P, T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that it has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins.

  1. Possible magmatic underplating beneath the west coast of India and adjoining Dharwar craton: Imprint from Archean crustal evolution to breakup of India and Madagascar

    Science.gov (United States)

    Saikia, Utpal; Das, Ritima; Rai, S. S.

    2017-03-01

    The shear wave velocity of the crust along a ∼660 km profile from the west to the east coast of South India is mapped through the joint inversion of receiver functions and Rayleigh wave group velocity. The profile, consisting of 38 broadband seismic stations, covers the Archean Dharwar craton, Proterozoic Cuddapah basin, and rifted margin and escarpment. The Moho is mapped at a depth of ∼40 km beneath the mid-Archean Western Dharwar Craton (WDC), Cuddapah Basin (CB), and the west and east coasts formed through the rifting process. This is in contrast with a thin (∼35 km) crust beneath the late-Archean Eastern Dharwar Craton (EDC). Along the profile, the average thickness of the upper, middle and lower crust is ∼4 km, 12 ± 4 km and 24 ± 4 km respectively. Above the Moho, we observe a high-velocity layer (HVL, Vs > 4 km/s) of variable thickness increasing from 3 ± 1 km beneath the EDC to 11 ± 3 km beneath the WDC and the CB, and 18 ± 2 km beneath the west coast of India. The seismic wave velocity in this layer is greater than typical oceanic lower crust. We interpret the high-velocity layer as a signature of magmatic underplating due to past tectonic processes. Its significant thinning beneath the EDC may be attributed to crustal delamination or relamination at 2.5 Ga. These results demonstrate the dual signature of the Archean Dharwar crust. The change in the geochemical character of the crust possibly occurred at the end of Archean when Komatiite volcanism ceased. The unusually thick HVL beneath the west coast of India and the adjoining region may represent underplated material formed due to India-Madagascar rifting, which is supported by the presence of seaward dipping reflectors and a 85-90 Ma mafic dyke in the adjoining island.

  2. Nature and evolution of Neoproterozoic ocean-continent transition: Evidence from the passive margin of the West African craton in NE Mali

    Science.gov (United States)

    Renaud, Caby

    2014-03-01

    The Timétrine massif exposed west of the Pan-African suture zone in northeastern Mali belongs to the passive margin of the West African craton facing to the east intra-oceanic arc assemblages and 730 Ma old pre-collisional calc-alkaline plutons. The Timétrine lithologic succession includes from the base to the top Mesoproterozoic cratonic to passive margin formations overlain by deep-sea Fe-Mg schists. Submarine metabasalts and two ultramafic massifs of serpentinized mantle peridotites are inserted as olistoliths towards the top whereas turbidites of continental origin represent the younger unit. Field and petrological data have revealed a distinct metasedimentary sequence attached to the serpentinized peridotites. It essentially consists of impure carbonates, Fe jaspers and polymictic breccias containing altered blocks of mantle peridotites, most rocks being enriched in detrital chromite. This association is interpreted as reworked chemical and detrital sediments derived from the alteration of mafic-ultramafic rocks. It is argued that mantle exhumation above sea floor took place during the Neoproterozoic rifting and crustal thinning period under possible tropical conditions, as suggested by the large volume of silicified serpentinites. In spite of greenschist facies metamorphic overprint characterized by widespread Fe-rich blue amphiboles that are not diagnostic of high-pressure conditions, it is possible to reconstruct a former ocean-continent transition similar to that evidenced for the Mesozoic period, followed by the deposition of syn-to post rift terrigeneous turbidites roughly coeval with ocean spreading some time before 800 Ma. It is concluded that the serpentinite massifs were tectonically emplaced first in an extensional setting, then incorporated within deep-sea sediments as olistoliths and finally transported westward during late Neoproterozoic collisional tectonics onto the West African craton.

  3. Archean crustal evolution in the Southern São Francisco craton, Brazil: Constraints from U-Pb, Lu-Hf and O isotope analyses

    Science.gov (United States)

    Albert, Capucine; Farina, Federico; Lana, Cristiano; Stevens, Gary; Storey, Craig; Gerdes, Axel; Dopico, Carmen Martínez

    2016-12-01

    In this study we present U-Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920-2850 Ma), Rio das Velhas II (2800-2760 Ma) and Mamona (2750-2680 Ma). The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct εHf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium- to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ18O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ18O values (down to 2.50‰) throughout the Meso- and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism. We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65 Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9 Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision.

  4. 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

    2004-01-01

    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

  5. 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

    2004-01-01

    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

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

    Science.gov (United States)

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

    2015-11-01

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

  7. Structure and extent of the southern African cratons: Integrated images from receiver functions and teleseimic tomography

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Levander, Alan; Bezada, Max

    2011-01-01

    components we find that most of the strong anisotropy is confined to the lower crust. Using finite-frequency kernels, we inverted the P- and S- wave delay times to obtain 3-D images of com- pressional and shear velocity perturbations in the mantle by use of three frequency bands: 1, 0.5 and 0.25 Hz for P......4◦ checkerboards show moderately good recovery. To isolate the depth extent of anomalies in the model we ran two suites of squeezing tests: 1) For maximum depth of the model being 1000, 700 and 410 km. 2) For the 1000 km deep model, we increased the damping parameter in the deeper layers....... The Receiver Functions show a thin crust with a flat and sharp Moho discontinuity throughout the entire Kaapvaal and Zimbabwe cratons. These results are consistent with expectations for Archean areas. The lowest Vp/Vs value sites are found around the locations of diamondiferous kimberlite pipes at flat Moho...

  8. Electrical conductivity images across the Namibian passive margin: Tectonic implications for the evolution of the Kaoko Belt, the western Kongo Craton and the Walvis Ridge

    Science.gov (United States)

    Weckmann, U.; Meqbel, N. M.; Kapinos, G.; Ritter, O.; Jegen, M. D.

    2014-12-01

    High-quality magnetotelluric (MT) data were collected at 167 sites onshore in Northwestern Namibia within the framework of a project (SAMPLE) to investigate processes related to the breakup of Gondwana. The entire study area extents from the Walvis Ridge in the Atlantic Ocean and crosses onshore the Kaoko Belt and the western boundary of the Kongo Craton. The surface expressions of prominent faults such as the Purros Shear Zone and the Three Palm Mylonite Zone of the Kaoko Belt are expressed in 2D models as zones of high electrical conductivity. A region where the Etendeka flood basalts outcrop in the Western Kaoko Zone appears as a 10-15 km deep reaching highly resistive feature. However, dimensionality and strike direction analysis of the data exhibit 3D effects. The 3D inversion models reproduce the main structures obtained by 2D modelling but the spatial coherency of structures is better resolved. The Purros Shear Zone and the Three Palm Mylonite Zone appear in the 3D inversion models as zones of high electrical conductivity striking in NW-SE direction in the middle crust. Where the Walvis Ridge intersects with the continent, an upwelling of resistive structures indicates a modification of the middle crust, which seems be caused by a deeper anomaly which is not resolved with MT. The spatial extent and possible emplacement of the highly resistive material associated with the Etendeka flood basalts is more enigmatic as only a relatively confined region at upper- to mid-crustal levels seems to be affected. Most importantly, these Cretaceous flood basalt events did not affect or alter the high conductivity structures associated with the preexisting Proterozoic shear zones. Compared with other cratonic boundaries in Southern Africa and elsewhere the transition from the Kaoko Belt into the Kongo Craton is unusual as it is associated with an area of high conductivity at upper to mid crustal levels. As MT site coverage in this region is sparse, however, this

  9. 华北克拉通的组成及其变质演化%Constituents and Evolution of the Metamorphic Basement of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    沈其韩; 耿元生; 宋会侠

    2016-01-01

    Precambrian metamorphic basement of North China Craton (NCC) is composed of five sets of different types of metamorphic rocks. In the formation process, the NCC experienced multiple tectonic activities, multiple magma emplacement, multiple metamorphism, different degrees of migmatization and anatexis. The rocks suffered multiple superposition of different geological processes and, therefore, the NCC has a complicated evolution history. From Archean to Late Paleoproterozoic, the NCC mainly underwent five stages of regional metamorphism. Anshan area experienced amphibolite facies metamorphism in Paleo/Meso-Archean. However, no metamorphic ages have been obtained except 3560 Ma and 3000~3300 Ma metamorphic ages obtained from TTG series. In Lushan Taihua complex of Henan, 2776~2792 Ma and 2671~2651 Ma metamorphic ages are obtained from Mesoarchean amphibolites, which represent metamorphism in early Neoarchean. Granulites-TTG series and granite-greenstone belt of Neoarchean both experienced metamorphic transformation in late Neoarchean and early Paleoproterozoic. In Paleoproterozoic, on the northern margin of the NCC, L-MP/HP granulite facies metamorphism occurred between 1965~1900 Ma and UHT metamorphism appeared locally. This metamorphism is considered to be related to the continental collision and subsequent mantle upwelling. In Late Paleoproterozoic (1890~1800 Ma), in the central and eastern NCC, high-pressure granulite facies to amphibolite facies metamorphism occurred in Jiao–Liao–Ji belt, which indicates the collision and collage of continental blocks. Different types of metamorphic rock series experienced different kinds of metamorphism, which reflect different tectonic settings. A large number of late Archean TTG rock series and planar distribution M-LP granulites are mainly located in the central-northern part of NCC. They generally have counterclockwisep-T paths, which reflect mantle plume underplating tectonic setting. Granite-greenstone series of

  10. Metamorphic P-T-t path retrieved from metapelites in the southeastern Taihua metamorphic complex, and the Paleoproterozoic tectonic evolution of the southern North China Craton

    Science.gov (United States)

    Lu, Jun-Sheng; Zhai, Ming-Guo; Lu, Lin-Sheng; Wang, Hao Y. C.; Chen, Hong-Xu; Peng, Tao; Wu, Chun-Ming; Zhao, Tai-Ping

    2017-02-01

    The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9 Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite + plagioclase + quartz + ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite + biotite + plagioclase + quartz + K-feldspar + ilmenite + rutile + pyrite. The retrograde mineral assemblage (M3), biotite + plagioclase + quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P-T conditions of 12.2 kbar and 830 °C, whereas garnet rims record P-T conditions of 10.2 kbar and 840 °C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P-T path can be retrieved. High resolution SIMS U-Pb dating and LA-ICP-MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92 Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83 Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous

  11. Charts of episodes and chronometric duration have implications for provinces such as the Kaapvaal

    Directory of Open Access Journals (Sweden)

    H. de la R. Winter

    1997-07-01

    Full Text Available An accurate basin analysis and knowledge of its evolutionary development being a prerequisite framework for metallogenic model­ling, a procedure was developed for the Kaapvaal Province to maximise quantitative observational data and constrain assumptions and interpretations. Employing an episodes chart to display various events interactively and its conversion to a geochronometric chart as the geological model, the method was applied to existing stratigraphic studies.

  12. Geochronology, Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of the São Luís Craton, northern Brazil: Implications for tectonic setting and crustal evolution

    Science.gov (United States)

    Klein, Evandro L.; Luzardo, Renê; Moura, Candido A. V.; Lobato, Denise C.; Brito, Reinaldo S. C.; Armstrong, Richard

    2009-02-01

    New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high- K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium- K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium- K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.

  13. U-Pb zircon ages and geochemistry of the Wuguan complex in the Qinling orogen, central China: Implications for the late Paleozoic tectonic evolution between the Sino-Korean and Yangtze cratons

    Science.gov (United States)

    Chen, Longyao; Liu, Xiaochun; Qu, Wei; Hu, Juan

    2014-04-01

    The tectonic evolution of the Qinling orogen, central China, is the key to understanding the assembly of the Sino-Korean and Yangtze cratons. The Wuguan complex, between the early Paleozoic North Qinling and Mesozoic South Qinling tectonic belts, can provide important constraints on the late Paleozoic evolutionary processes in the Qinling orogen. U-Pb zircon analyses, using laser ablation-multicollector-inductively coupled plasma-mass spectrometry, reveal protolith ages of 446 ± 2 Ma for a garnet amphibolite, 368 ± 3 Ma for a meta-andesite, and 351 ± 2 Ma for a mylonitized granitic dike from the Wuguan complex. Elemental geochemistry indicates typical island arc affinities for all the above rocks, but some amphibolites of unknown age have E-MORB signatures. Detrital zircons from a metaquartzite have an age spectrum with a major peak at 462 Ma, two subordinate peaks at 828 and 446 Ma, and a youngest weighted mean age of 423 ± 5 Ma. This suggests that at least some of metasedimentary rocks from the Wuguan complex belong to the part of the Devonian turbidite sequence of the Liuling Group, which was deposited in a fore-arc basin along the southern accreted margin of the Sino-Korean craton, whereas the late Ordovician precursors of the amphibolite might be derived from the Danfeng Group. The occurrence of late Devonian-early Carboniferous arc-related rocks in the Wuguan complex implies penecontemporaneous oceanic subduction, and therefore the Paleo-Qinling Ocean was not finally closed until the early Carboniferous. On the other hand, metamorphic zircon grains from two amphibolites yielded ages of 321 ± 2 and 318 ± 3 Ma. Hence, the Wuguan complex in the Qinling orogen and the Guishan complex in the Tongbai orogen constitute a medium-pressure Carboniferous metamorphic belt that is more than 500 km long, and which was formed in the hanging wall of a subduction zone.

  14. Geochemical constraints on komatiite volcanism from Sargur Group Nagamangala greenstone belt, western Dharwar craton, southern India: Implications for Mesoarchean mantle evolution and continental growth

    Directory of Open Access Journals (Sweden)

    Tushipokla

    2013-05-01

    Full Text Available We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites (northern extension of Nagamangala belt and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca. 3.2–3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination. About 1/3 of studied komatiites show Al-depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, (Gd/YbN ratios show low CaO/Al2O3 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaO/Al2O3 ratios. The elemental characteristics of Al-depleted komatiites such as higher (Gd/YbN (>1.0, CaO/Al2O3 (>1.0, Al2O3/TiO2 (18 together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle (depleted to primitive mantle at

  15. Water and Metasomatism in the Slave Cratonic Lithosphere (Canada): An FTIR Study

    Science.gov (United States)

    Kilgore, McKensie; Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; Pearson, D. Graham; O'Reilly, Suzanne Yvette; Kopylova, Maya G.; Griffin, William L.

    2017-01-01

    Water in the mantle influences melting, viscosity, seismic velocity, and electrical conductivity. The role played by water in the long-term stabilization of cratonic roots is currently being debated. This study focuses on water contents of mantle minerals (olivine, pyroxene and garnet) from xenoliths found in kimberlites of the Archean Slave craton. 19 mantle xenoliths from central Lac de Gras, and 10 from northern Jericho were analyzed by FTIR for water, and their equilibration depths span the several compositional layers identified beneath the region. At both locations, the shallow peridotites have lower water contents in their olivines (11-30 ppm H2O) than those from the deeper layers (28-300 ppm H2O). The driest olivines, however, are not at the base of the cratonic lithosphere (>6 GPa) as in the Kaapvaal craton. Instead, the deepest olivines are hydrous (31-72 ppm H2O at Lac de Gras and 275 ppm H2O at Jericho). Correlations of water in clinopyroxene and garnet with their other trace element contents are consistent with water being added by metasomatism by melts resembling kimberlite precursors in the mantle approx.0.35 Ga ago beneath Lac de Gras. The northern Jericho xenoliths are derived from a region of the Slave craton that is even more chemically stratified, and was affected at depth by the 1.27 Ga Mackenzie igneous events. Metasomatism at Jericho may be responsible for the particularly high olivine water contents (up to 300 ppm H2O) compared to those at Lac de Gras, which will be investigated by acquiring trace-element data on these xenoliths. These data indicate that several episodes of metasomatic rehydration occurred in the deep part of the Slave craton mantle lithosphere, with the process being more intense in the northern part beneath Jericho, likely related to a translithospheric suture serving as a channel to introduce fluids and/or melts in the northern region. Consequently, rehydration of the lithosphere does not necessarily cause cratonic root

  16. Olivine water contents in the continental lithosphere and the longevity of cratons.

    Science.gov (United States)

    Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

    2010-09-02

    Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

  17. Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: Implications for early crustal evolution of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    WU Fuyuan; YANG Jinhui; LIU Xiaoming; LI Tiesheng; XIE Liewen; YANG Yueheng

    2005-01-01

    Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6-3.8 Ga granitic rocks. In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The ~3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks (granitic rocks) were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before ~3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.

  18. Geology, petrology, U-Pb (SHRIMP) geochronology of the Morrinhos granite - Paragua terrane, SW Amazonian craton: implications for the magmatic evolution of the San Ignacio orogeny

    Energy Technology Data Exchange (ETDEWEB)

    Franca, Ohana; Ruiz, Amarildo Salina; Sousa, Maria Zelia Aguiar de, E-mail: ohana.geo@gmail.com, E-mail: asruiz@gmail.com, E-mail: mzaguiar@terra.com.br [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Instituto de Ciencias Exatas e da Terra. Dept. de Geologia Geral; Batata, Maria Elisa Froes, E-mail: elisabatata@bol.com.br [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Grupo de Pesquisa em Evolucao Crustal e Tectonica; Lafon, Jean-Michel [Universidade Federal do Para (GEOCIAM/UFPA), Belem, PR (Brazil). Inst. Nacional de Cencia e Tecnologia de Geociencias da Amazonia

    2014-09-15

    Morrinhos granite is a batholith body that is slightly elongated in the NNW direction and approximately 1,140 km{sup 2} long; it is located in the municipality of Vila Bela da Santissima Trindade of the state of Mato Grosso, Brazil, in the Paragua Terrane, Rondonian-San Ignacio Province, in the SW portion of the Amazonian Craton. This intrusion displays a compositional variation from tonalite to monzogranite, has a medium to coarse inequigranular texture and is locally porphyritic; biotite is the predominant mafic in one of the facies, and hornblende is predominant in the other, with both metamorphosed into the green schist facies. The studied rocks characterize an intermediate to acidic sequence that was formed by a subalkaline magmatism; the series is alkali-calcic to metaluminous to slightly peraluminous, and the rocks evolved through fractioned crystallization mechanisms. The structural data show two deformation phases represented by penetrative foliation (S{sub 1}) and open folds (D{sub 2}), and both phases were most likely related to the San Ignacio Orogeny. The geochronological (U-Pb SHRIMP) and isotopic (Sm-Nd) investigations of these rocks indicated a crystallization age of 1350±12Ma, T{sub DM} of approximately 1.77 Ga and εNd{sub (1.35}) with a negative value of -2.57, suggesting that their generation was related to a partial melting process of a Paleoproterozoic (Statherian) continental crust. The results herein indicate that the Morrinhos granite was generated in a continental magmatic arc in a late- to post-orogenic stage of the San Ignacio Orogeny, and it can be recognized as belonging to the Pensamiento Intrusive Suite. (author)

  19. LA-ICP-MS zircon U-Pb dating and phenocryst EPMA of dikes, Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

    Institute of Scientific and Technical Information of China (English)

    2008-01-01

    Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the al- tered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes. Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr2O3 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GS1 and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously pub- lished data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula. These also imply that the intensive crust-mantle interaction and asthenospheric underplating had oc- curred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).

  20. LA-ICP-MS zircon U-Pb dating and phenocryst of dikes, Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

    Institute of Scientific and Technical Information of China (English)

    TAN Jun; WEI JunHao; GUO LingLi; ZHANG KeQing; YAO ChunLiang; LU JianPei; LI HongMei

    2008-01-01

    Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the altered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes.Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst's mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr203 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GSl and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously published data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula.These also imply that the intensive crust-mantle interaction and asthenospheric underplating had occurred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).

  1. Re-Os isotope evidence from Mesozoic and Cenozoic basalts for secular evolution of the mantle beneath the North China Craton

    Science.gov (United States)

    Huang, Feng; Xu, Ji-Feng; Liu, Yong-Sheng; Li, Jie; Chen, Jian-Lin; Li, Xi-Yao

    2017-05-01

    The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re-Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re-Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial 187Os/188Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γOs(t) values (+20 to +202). In contrast, the Re-Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial 187Os/188Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γOs(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re-Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.

  2. New insight on the paleoproterozoic evolution of the São Francisco Craton: Reinterpretation of the geology, the suture zones and the thicknesses of the crustal blocks using geophysical and geological data

    Science.gov (United States)

    Sampaio, Edson E. S.; Barbosa, Johildo S. F.; Correa-Gomes, Luiz C.

    2017-07-01

    The Archean-Paleoproterozoic Jequié (JB) and Itabuna-Salvador-Curaçá (ISCB) blocks and their tectonic transition zone in the Valença region, Bahia, Brazil are potentially important for ore deposits, but the geological knowledge of the area is still meager. The paucity of geological information restricts the knowledge of the position and of the field characteristics of the tectonic suture zone between these two crustal segments JB and ISCB. Therefore, interpretation of geophysical data is necessary to supplement the regional structural and petrological knowledge of the area as well as to assist mining exploration programs. The analysis of the airborne radiometric and magnetic data of the region has established, respectively, five radiometric domains and five magnetic zones. Modeling of a gravity profile has defined the major density contrasts of the deep structures. The integrated interpretation of the geophysical data fitted to the known geological information substantially improved the suture zone (lower plate JB versus upper plate ISCB) delimitation, the geological map of the area and allowed to estimate the thicknesses of these two blocks, and raised key questions about the São Francisco Craton tectonic evolution.

  3. METASOMATIC AND MAGMATIC PROCESSES IN THE MANTLE LITHOSPHERE OF THE BIREKTE TERRAIN OF THE SIBERIAN CRATON AND THEIR EFFECT ON THE LITHOSPHERE EVOLUTION

    Directory of Open Access Journals (Sweden)

    Lidia V. Solov’eva

    2015-10-01

    Full Text Available The area of studies covers the north-eastern part of the Siberian craton (the Birekte terrain, Russia. The influence of metasomatic and magmatic processes on the mantle lithosphere is studied based on results of analyses of phlogopite- and phlogopite-amphibole-containing deep-seated xenoliths from kimberlites of the Kuoika field. In the kimberlitic pipes, deep-seated xenoliths with mantle phlogopite- and phlogopite-amphibole mineralization are developed in two genetically different rock series: magnesian (Mg pyroxenite-peridotite series (with magnesian composition of rocks and minerals and phlogopite-ilmenite (Phl-Ilm hyperbasite series (with ferrous types of rocks and minerals. This paper is focused on issues of petrography and mineralogy of the xenoliths and describes the evidence of metasomatic / magmatic genesis of phlogopite and amphibole. We report here the first data set of 40Ar/39Ar age determinations for phlogopite from the rocks of the magnesian pyroxenite-peridotite series and the ferrous Phl-Ilm hyperbasite series.The Mg series is represented by a continuous transition of rocks from Sp, Sp-Grt, Grt clinopyroxenite and ortopyroxenite to websterite and lherzolite. Many researchers consider it as a layered intrusion in the mantle [Ukhanov et al., 1988; Solov’eva et al., 1994]. The mantle metasomatic phlogopite and amphibole are revealed in all petrographic types of the rocks in this series and compose transverse veins and irregular patchs at grain boundaries of primary minerals. At contacts of xenolith and its host kimberlite, grains of phlogopite and amphibole are often cut off, which gives an evidence of the development of metasomatic phlogopite-amphibole mineralization in the rocks before its’ entraiment into the kimberlite. In the xenoliths with exsolution pyroxene megacrystalls, comprising parallel plates of clino- and orthopyroxene ± garnet ± spinel (former high-temperature pigeonite [Solov’eva et al., 1994], the

  4. Geochemistry of komatiites and basalts from the Rio das Velhas and Pitangui greenstone belts, São Francisco Craton, Brazil: Implications for the origin, evolution, and tectonic setting

    Science.gov (United States)

    Verma, Sanjeet K.; Oliveira, Elson P.; Silva, Paola M.; Moreno, Juan A.; Amaral, Wagner S.

    2017-07-01

    The Neoarchean Rio das Velhas and Pitangui greenstone belts are situated in the southern São Francisco Craton, Minas Gerais, Brazil. These greenstone belts were formed between ca. 2.79-2.73 Ga, and consist mostly of mafic to ultramafic volcanics and clastic sediments, with minor chemical sediments and felsic volcanics that were metamorphosed under greenschist facies. Komatiites are found only in the Rio das Velhas greenstone belt, which is composed of high-MgO volcanic rocks that have been identified as komatiites and high-Mg basalts, based on their distinctive geochemical characteristics. The Rio das Velhas komatiites are composed of tremolite + actinolite + serpentine + albite with a relict spinifex-texture. The Rio das Velhas komatiites have a high magnesium content ((MgO)adj ≥ 28 wt.%), an Al-undepleted Munro-type [(Al2O3/TiO2)adj and (CaO/Al2O3)adj] ratio ranging from 27 to 47 and 0.48 to 0.89, relatively low abundances of incompatible elements, a depletion of light rare earth elements (LREE), a pattern of non-fractionated heavy rare- earth elements (HREE), and a low (Gd/Yb)PM ratio (≤ 1.0). Negative Ce anomalies suggest that alteration occurred during greenschist facies metamorphism for the komatiites and high-Mg basalts. The low [(Gd/Yb)PM 18] and high HREE, Y, and Zr content suggest that the Rio das Velhas komatiites were derived from the shallow upper mantle without garnet involvement in the residue. The chemical compositions [(Al2O3/TiO2)adj, (FeO)adj, (MgO)adj, (CaO/Al2O3)adj, Na, Th, Ta, Ni, Cr, Zr, Y, Hf, and REE] indicate that the formation of the komatiites, high-Mg basalts and basalts occurred at different depths and temperatures in a heterogeneous mantle. The komatiites and high-Mg basalts melted at liquidus temperatures of 1450-1550 °C. The Pitangui basalts are enriched in the highly incompatible LILE (large-ion lithophile elements) relative to the moderately incompatible HFS (high field strength) elements. The Zr/Th ratio ranging from 76 to

  5. Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton, southern India: Implications for the antiquity of the eastern Dharwar craton

    Indian Academy of Sciences (India)

    B Maibam; J N Goswami; R Srinivasan

    2011-08-01

    207Pb–206Pb ages of zircons in samples of metasediments as well as ortho- and para-gneisses from both the western and the eastern parts of the Dharwar craton have been determined using an ion microprobe. Detrital zircons in metasedimentary rocks from both yielded ages ranging from 3.2 to 3.5 Ga. Zircons from orthogneisses from the two parts also yielded similar ages. Imprints of younger events have been discerned in the ages of overgrowths on older zircon cores in samples collected throughout the craton. Our data show that the evolution of the southwestern part of eastern Dharwar craton involved a significant amount of older crust (< 3.0 Ga). This would suggest that crust formation in both the western and eastern parts of the Dharwar craton took place over similar time interval starting in the Mesoarchaean at ca. 3.5 Ga and continuing until 2.5 Ga. Our data coupled with geological features and geodynamic setting of the Dharwar craton tend to suggest that the eastern Dharwar craton and the western Dharwar craton formed part of a single terrane.

  6. Mineral chemistry and magnetic petrology of the Archean Planalto Suite, Carajás Province - Amazonian Craton: Implications for the evolution of ferroan Archean granites

    Science.gov (United States)

    Cunha, Ingrid Roberta Viana da; Dall'Agnol, Roberto; Feio, Gilmara Regina Lima

    2016-04-01

    The Planalto Suite is located in the Canaã dos Carajás subdomain of the Carajás Province in the southeastern part of the Amazonian Craton. The suite is of Neoarchean age (∼2.73 Ga), ferroan character, and A-type affinity. Magnetic petrology studies allowed for the distinction of two groups: (1) ilmenite granites showing low magnetic susceptibility (MS) values between 0.6247×10-3 and 0.0102 × 10-3 SI and (2) magnetite-ilmenite-bearing granites with comparatively higher but still moderate MS values between 15.700×10-3 and 0.8036 × 10-3 SI. Textural evidence indicates that amphibole, ilmenite, titanite, and, in the rocks of Group 2, magnetite also formed during magmatic crystallization. However, compositional zoning suggests that titanite was partially re-equilibrated by subsolidus processes. The amphibole varies from potassian-hastingsite to chloro-potassian-hastingsite and shows Fe/(Fe + Mg) > 0.8. Biotite also shows high Fe/(Fe + Mg) ratios and is classified as annite. Plagioclase porphyroclasts are oligoclase (An25-10), and the grains of the recrystallized matrix show a similar composition or are albitic (An9-2). The dominant Group 1 granites of the Planalto Suite were formed under reduced conditions below the FMQ buffer. The Group 2 granites crystallized under more oxidizing conditions on or slightly above the FMQ buffer. Pressures of 900-700 MPa for the origin and of 500-300 MPa for the emplacement were estimated for the Planalto magmas. Geothermometers suggest initial crystallization temperatures between 900 °C and 830 °C, and the water content in the magma is estimated to be higher than 4 wt%. The Neoarchean Planalto Suite and the Estrela Granite of the Carajás Province reveal strong mineralogical analogies, and their amphibole and biotite compositions have high total Al contents. The latter characteristic is also observed in the same minerals of the Neoarchean Matok Pluton of the Limpopo Belt but not in those of the Proterozoic rapakivi A

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2002-12-15

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

  8. What garnet, clinopyroxene, and diamond potential can tell us about the evolution of sub-cratonic mantle sections: a case study of the Zagadochnaya kimberlite (Yakutia)

    OpenAIRE

    Ziberna, Luca

    2013-01-01

    This PhD provides major and trace element compositions for a new suite of Cr-rich garnet xenocrysts and associated minerals from the diamond-free Zagadochnaya kimberlite, Daldyn Field, Yakutia (Russia). Interpreting the nature and evolution of the underlying lithospheric mantle from these samples requires a good understanding of relationships between pressures and temperatures of formation, upper mantle phase relations and trace element signatures of petrochemical processes. I addressed these...

  9. Evolution of the North China Craton and Early Plate Tectonics%华北克拉通的形成以及早期板块构造

    Institute of Scientific and Technical Information of China (English)

    翟明国

    2012-01-01

    The oldset rock discovered on the Earth is the TTG gneiss but whether there was oldest oceanic crust and how continental crust formed deal with all aspects of continental dynamics. Among them is when the plate tectonics started , "which has been a front scientific question for decades. The popular answer is from Neoproterozoic, others suggest Paleoproterozoic or Neoarchean, or even some believe the plate tectonics started from the occurrence of water. In various marks identifying the plate tectonics, ophiolite and high-pressure metamorphic belt are no doubt the most important issues. The former implies that the old oceanic crust slab was involved in orogenic belt, and the latter probably indicates that supracrustal rock unit was subducted under deep crust or mantle and can be lithological evidence of subduction, denudation and collision. Based on the discussion and comparison between Archean greenstone belt and ophiolite and between UH-HP/UT-UHT granulites and Phanerozoic HP metamorphic belt, authors come to a primitive conclusion that these two can not be used as the convincing evidence to support plate tectonics. The paper also discussed Archean continental formation and rift-subdution-collision tectonic process of Paleoproterozoic mobile belts of the NCC. It is proposed that the Neoarchean tectonic pattern of greenstone belt-high grade region in the NCC probably indicates a dominant heat tectonic regime (mantle plume) with limited transverse movement. The micro-blocks were welded by greenstone belts, followed by metamorphism and granitization, completing craton process of stable continent. The tectonic regime is likely controlled by frequent moderate-scale mantle plumbs, accompanied by small-scale horizontal tectonic movement. The Paleoproterozoic supracrustal rocks in the NCC occur as a linear mobile belt with middle-grade metamorphism, multi-stage deformation, intruded by nearly synchronous or little later granitic intrusion and associated by Cu

  10. Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications

    Science.gov (United States)

    Ranganai, Rubeni T.; Whaler, Kathryn A.; Ebinger, Cynthia J.

    2016-11-01

    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

  11. The Rhyacian El Cortijo suture zone: Aeromagnetic signature and insights for the geodynamic evolution of the southwestern Rio de la Plata craton, Argentina

    Directory of Open Access Journals (Sweden)

    Carlos J. Chernicoff

    2014-01-01

    We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean (∼2650 Ma crust occurred during Siderian times (2500–2300 Ma, causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by ∼2300–2200 Ma sedimentation over transitional –continental to oceanic– crust, and arc magmatism developed in the Tandilia terrane. The island arc represented by the El Cortijo Formation was also developed at this time. At late Rhyacian times, it occurred in both the closure of the narrow oceans developed previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane.

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

    Directory of Open Access Journals (Sweden)

    ARILDO H. OLIVEIRA

    2001-09-01

    Full Text Available Systematic geological studies performed in the study area allowed the characterization of six lithodemic units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The mineral assemblage and the structural record of these lithodemic units indicate that the study area was affected by five tectonothermal events. The structural pattern of the first and oldest event occurred under granulite facies conditions and reveals essentially a sinistral kinematic pattern. The second event, showing dominant extensional characteristics, is related to the generation of an ensialic basin filled by the volcano-sedimentary sequence of the supracrustal lithodemic unit. The third event, which is the most expressive in the study region, is characterized by a vigorous regional migmatization process and by the generation of the Claudio Shear Zone, presenting dextral kinematic movement. The fourth event is represented by a fissure mafic magmatism (probably two different mafic dike swarms and finally, the fifth event is a regional metamorphic re-equilibration that reached the greenschist facies, closing the main processes of the tectonic evolution of the Campo Belo Metamorphic Complex.Estudos geológicos sistemáticos permitiram a caracterização de seis unidades litodêmicas na área estudada: três gnáissicas, uma anfibolítica, uma supracrustal e uma máfica fissural. A assembléia mineral e os registros estruturais dessas unidades litodêmicas mostraram que a área estudada foi afetada por cinco eventos tectonotermais. O padrão estrutural do primeiro e último evento ocorreu em condições de fácies granulito e revelaram uma cinemática essencialmente sinistral. O segundo evento mostrou uma tectônica extensional relacionado à abertura da bacia ensiálica onde se alojou a seqüência vulcanosedimentar da unidade supracrustal. O terceiro evento, que é o mais expressivo na região estudada, é o responsável por um intenso processo de

  13. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

    Science.gov (United States)

    Pilet, Sebastien; Guex, Jean; Muntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Schaltegger, Urs

    2016-04-01

    studies of the composition of the Kaapvaal craton have shown that sulfide minerals are enclosed in the basal part of the cratonic lithosphere. The formation of these sulfide minerals are linked to multiple refertilization/metasomatic events, which affected the base of the subcontinental lithospheric mantle from the Archean to the Proterozoic. We suggest that the transitions from an initial cool period to greenhouse conditions recorded by T-J and Pl-To sedimentary sections result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or thermal internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere causing global cooling and eustatic regression, which was followed by warming/transgression associated with the progressive increase of CO2 in the atmosphere associated to LIPs emission and metamorphic reactions in sedimentary basins. We suggest that the nature of the underlying lithosphere during large LIP eruption potentially exerts an important control on the consequences at the Earth's surface. This model offers an explanation for why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

  14. Nd-Sr Isotopic Geochemistry and U-Pb Geochronology of the Fé Granitic Gneiss and Lajedo Granodiorite: Implications for Paleoproterozoic Evolution of the Mineiro Belt, Southern São Francisco Craton, Brazil

    Directory of Open Access Journals (Sweden)

    Wilson Teixeira

    2008-04-01

    Full Text Available The Fé granitic gneiss and Lajedo granodiorite belong to a voluminous felsic-mafic plutonism, tectonically linked toPaleoproterozoic magmatic evolution of the Mineiro Belt, southern portion of the São Francisco Craton, central-easternBrazil. The Fé pluton is located north of the Lenheiros shear zone and is intrusive with respect to the Rio das Mortesgreenstone belt and pyroxenite - gabbroic bodies, as indicated by xenoliths of gneiss and amphibolite, in the first case, andpyroxenite in the latter. The Lajedo granodiorite is located south of the Lenheiros shear zone and cuts the metamafic rocks ofthe Forro peridotite – pyroxenite and mafic and intermediate rocks of the Nazareno greenstone belt, as evidenced by xenolithsfrom the latter unit. The modal composition of the Fé granitic gneiss lies within the ranges of monzogranite and syenogranite.It is peraluminous and shows a large variation in K2O content, which implies a middle-K calc-alkaline to high-K calc-alkalinetendency. The Lajedo modal composition is consistent with granodioritic and tonalitic compositions. It indicates a predominantlyperaluminous composition and calc-alkaline character. The U-Pb zircon crystallization age of the Fé granitic gneiss is 2191± 9 Ma, whereas the Lajedo granodiorite yields 2208 ± 26 Ma. The Nd/Sr characteristics of the Fé and Lajedo plutons areconsistent with mixtures of enriched mantle (EMI-type, DMM and crustal components during magma genesis in a plutonicarc setting, while the low 87Sr/86Sri ratios point to contribution of mafic rock protoliths during magma genesis. This is also inaccordance with the characteristic xenoliths observed within the investigated plutons from the Nazareno and Rio das Mortesgreenstone belts. The Fé granitic gneiss and Lajedo granodiorite show tectonic characteristics which are comparable to thoseof nearby coeval plutons: Brito quartz-diorite (2221 ± 2 Ma, Brumado de Cima granodiorite (2219 ± 2 Ma, Brumado de

  15. Nd-Sr isotopic geochemistry and U-Pb geochronology of the Fe granitic gneiss and Lajedo Granodiorite: implications for paleoproterozoic evolution of the Mineiro Belt, southern Sao Francisco craton, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Teixeira, Wilson [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas]. E-mail: wteixeir@usp.br; Avila, Ciro Alexandre [Universidade Federal do Rio de Janeiro (UFRJ), RJ (Brazil). Museu Nacional. Dept. de Geologia e Paleontologia]. E-mail: avila@mn.ufrj.br; Nunes, Luciana Cabral [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias]. E-mail: luciana@igc.usp.br

    2008-07-01

    The Fe granitic gneiss and Lajedo granodiorite belong to a voluminous felsic-mafic plutonism, tectonically linked to Paleoproterozoic magmatic evolution of the Mineiro Belt, southern portion of the Sao Francisco Craton, central-eastern Brazil. The Fe pluton is located north of the Lenheiros shear zone and is intrusive with respect to the Rio das Mortes greenstone belt and pyroxenite - gabbroic bodies, as indicated by xenoliths of gneiss and amphibolite, in the first case, and pyroxenite in the latter. The Lajedo granodiorite is located south of the Lenheiros shear zone and cuts the metamafic rocks of the Forro peridotite - pyroxenite and mafic and intermediate rocks of the Nazareno greenstone belt, as evidenced by xenoliths from the latter unit. The modal composition of the Fe granitic gneiss lies within the ranges of monzogranite and syenogranite. It is peraluminous and shows a large variation in K{sub 2}O content, which implies a middle-K calc-alkaline to high-K calc-alkaline tendency. The Lajedo modal composition is consistent with granodioritic and tonalitic compositions. It indicates a predominantly peraluminous composition and calc-alkaline character. The U-Pb zircon crystallization age of the Fe granitic gneiss is 2191 {+-} 9 Ma, whereas the Lajedo granodiorite yields 2208 {+-} 26 Ma. The Nd/Sr characteristics of the Fe and Lajedo plutons are consistent with mixtures of enriched mantle (EMI-type), DMM and crustal components during magma genesis in a plutonic arc setting, while the low {sup 87}Sr/{sup 86}Sri ratios point to contribution of mafic rock protoliths during magma genesis. This is also in accordance with the characteristic xenoliths observed within the investigated plutons from the Nazareno and Rio das Mortes greenstone belts. The Fe granitic gneiss and Lajedo granodiorite show tectonic characteristics which are comparable to those of nearby coeval plutons: Brito quartz-diorite (2221 +- 2 Ma), Brumado de Cima granodiorite (2219 {+-} 2 Ma), Brumado

  16. Metasomatism and the Weakening of Cratons: A Mechanism to Rift Cratons

    Science.gov (United States)

    Wenker, Stefanie; Beaumont, Christopher

    2016-04-01

    The preservation of cratons is a demonstration of their strength and resistance to deformation. However, several cratons are rifting now (e.g. Tanzania and North China Craton) or have rifted in the past (e.g. North Atlantic Craton). To explain this paradox, we suggest that widespread metasomatism of the originally cold depleted dehydrated craton mantle lithosphere root can act as a potential weakening mechanism. This process, particularly melt metasomatism, increases root density through a melt-peridotite reaction, and reduces root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. Using 2D numerical models, we model silicate-melt metasomatism and rehydration of cold cratonic mantle lithosphere that is positioned beside standard Phanerozoic lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by the initially weaker adjacent standard Phanerozoic lithosphere. Melt is added to specified layers in the cratonic mantle lithosphere at a uniform volumetric rate determined by the duration of metasomatism (3 Myr, 10 Myr or 30 Myr), until a total of ~30% by volume of melt has been added. During melt addition heat and mass are properly conserved and the density and volume increase by the respective amounts required by the reaction with the peridotite. No extensional boundary conditions are applied to the models during the metasomatism process. As expected, significant refertilization leads to removal and thinning of progressively more gravitationally unstable cratonic mantle lithosphere. We show that the duration of metasomatism dictates the final temperature in the cratonic upper mantle lithosphere. Consequently, when extensional boundary conditions are applied in our rifting tests in most cases the Phanerozoic lithosphere rifts. The craton rifts only in the models with the hottest cratonic upper mantle lithosphere. Our results indicate rifting of cratons

  17. Proterozoic granitoids of the Amazonian craton

    Science.gov (United States)

    dalĺAgnol, R.; Costi, H. T.; Lamarão, C. N.; Teixeira, N. P.; Bettencourt, J. S.; Fraga, L. M.

    2003-04-01

    Proterozoic granitoids are widespread in all provinces of the Amazonian craton. In the Maroni-Itacaiunas Province, granitoids associated with the Trans-Amazonian event include: subduction related, 2.16 to 2.14 Ga, calc-alkaline tonalites and trondhjemites; 2.10 to 2.08 Ga, syncolisional potassic granites; 2.05 Ga, charnockites. In the Tapajós Province, ˜2.01 Ga, tonalites are followed by ˜2.0 Ga volcanic sequences and ˜1.98 to 1.96 Ga calc-alkaline granitoids. A reappraisal of magmatic activity occurred at ˜1.88 Ga when calc-alkaline granitoids, as well as subalkaline, A-type granites, associated with felsic volcanic sequences were formed. A similar picture is observed in the northern Roraima region, where post-collisional 2.0 to 1.96 Ga calc-alkaline granitoids and associated volcanic sequences are followed by 1.92 Ga A-type granites. The remarkable 1.88 Ga magmatic event has a continental scale and is related to an extensional tectonism. It affected also the Archean Carajás Province, where, at this time, within-plate, shallow-level, A-type granites were emplaced. Coeval intermediate to felsic volcanic sequences are widespread in the Central Amazonian Province. In the Pitinga region, these sequences are intruded by ˜1.82 Ga, tin-mineralized granites. In the Central Guiana Belt and in the northwestern domains of the Guiana shield ˜1.55 Ga rapakivi complexes, locally with associated anorthosites and mangerites, are common. In the Rio Negro Province, 1.8 to 1.60 calc-alkaline (?) granitoids and gneisses are dominant. They are followed by 1.55 to 1.52 Ga, oxidized, titanite-bearing A-type granites and S-type, two-mica granites. The evolution of the southwestern part of the Amazonian craton is characterized by the occurrence of successive tectonic events extending from ˜1.75 Ga to ˜1.0 Ga. The oldest granitoids are dominantly calc-alkaline tonalites, trondhjemites and granodiorites. However, the Rondonia region is marked by the occurrence of 1.6 to 1.0 Ga old

  18. Intra-cratonic melting as a result of delamination of mantle lithosphere - insight from numerical modelling

    Science.gov (United States)

    Gorczyk, W.; Vogt, K.; Gerya, T.; Hobbs, B. E.

    2012-12-01

    It is becoming increasingly apparent that intense deformation, metamorphism and metasomatism occur within continental cratonic blocks far removed form subducting margins Such changes may occur intra-cratonically arising from lithospheric thickening and the development of gravitational instabilities, but mostly occur at the boundary of cratonic blocks. The contact of two cratons is characterized by rheological lateral variations within mantle-lithosphere and overlying crust. Tectonic stresses acting on craton/craton boundaries may lead to thinning or thickening due to delamination of the mantle lithosphere. This is reflected in tectonic deformation, topography evolution, melting and crustal metamorphism. To understand the controls on these processes a number of 2D, coupled petrological thermo-mechanical numerical experiments has been performed to test the response of a laterally weakened zone to a compressional regime. The results indicate that the presence of water-bearing minerals in the lithosphere and lower crust is essential to initiate melting, which in the later stages may expand to dry melting of crust and mantle. In the case of anhydrous crust and lithosphere, no melting occurs. Thus a variety of instabilities, melting behaviour and topographic responses occurs at the base of the lithosphere as well as intensive faulting and buckling in the crust dependent on the strength and "water" content of the lithosphere.

  19. On the Origin of Cratonic Sag Basins: Did They Sag?

    Science.gov (United States)

    Morgan, Jason P.

    2015-04-01

    Cratonic sag basins are regions of long-lived, extremely slow (~20-30 m/Myr) shallow water and terrestrial sediment accumulation that have no striking signs of tectonic activity (cf. Allen and Armitage, 2012). In their evolution, hundreds of Myr-long periods of slow sediment accumulation are separated by unconformities. The mechanisms for their formation resist geodynamic characterization by other common hypotheses for basin subsidence because of their extremely slow subsidence and lack of evident tectonic activity. I propose their dynamics are better understood within the geodynamic context of continental cratons that ride over a ~250km-deep sub-asthenospheric mantle with lateral temperature variations between a few wide and persistent 1000s-km broad ~1400C 'superplume' upwelling mantle structures (e.g. currently beneath S. African Atlantic and French Polynesia) and prevalent surrounding ~1150C average temperature sub-asthenospheric mantle. When continents pass over typical mantle plumes, buoyant plume material tends to drain beneath the continent along junctions between cratons where the lithosphere is relatively thin, keeping the lithosphere over regions where plume material drains hotter than the average temperature of ~250km-deep mantle. (e.g., the Cameroon Line.) Regions where melting of plume material occurs during decompression associated with either plume ascent or lateral drainage beneath continents are associated with the addition of a buoyant rind of more depleted mantle to the continent. In addition, regions where plume material can pond in a relatively thin sub-lithospheric 'anti-basin' beneath a continent, or that stay stationary for long times over super plumes will heat to a lithospheric basal temperature of ~1400C instead of ~1150C, with ~700m of associated uplift. (e.g., Southern Africa). In this scenario (cf. Yamamoto, Morgan, and Morgan in "Plumes, Plates, and Paradigms"), it is the relative plume-passage-induced uplift of arches between

  20. Electrical conductivity of continental lithospheric mantle from integrated geophysical and petrological modeling: Application to the Kaapvaal Craton and Rehoboth Terrane, southern Africa

    OpenAIRE

    Fullea, J.; Muller, M.R.; Jones, A. G.

    2011-01-01

    The electrical conductivity of mantle minerals is highly sensitive to parameters that characterize the structure and state of the lithosphere and sublithospheric mantle, and mapping its lateral and vertical variations gives insights into formation and deformation processes. We review state-of-the-art conductivity models based on laboratory studies for the most relevant upper mantle minerals and define a bulk conductivity model for the upper mantle that accounts for temperature, pressure, and ...

  1. Deep vs. shallow expressions of continental cratons: Can cratonic roots be destroyed by subduction?

    Science.gov (United States)

    Perry-Houts, J.; Calo, M.; Eddy, C. L.; Guerri, M.; Holt, A.; Hopper, E.; Tesoniero, A.; Romanowicz, B. A.; Becker, T. W.; Wagner, L. S.

    2013-12-01

    Cratons are parts of continents that have remained tectonically quiescent over billion-year timescales. Although cratonic lithosphere has the stabilizing properties of chemical buoyancy and high viscosity, it can still be destroyed. The best known example of a missing cratonic root is beneath the eastern North China Craton (NCC). Despite strong evidence for the past existence of a craton in northern China, high heat flow, Mesozoic basin formation, extensive seismicity, and the lack of a fast seismic root imply that the deep cratonic lithosphere is missing. The mechanism for the lithospheric root loss is a source of much debate. Many mechanisms have been proposed, among them: shearing of the lithospheric root by asthenospheric flow induced by the Indo-Eurasian collision; ponding of the Pacific slab in the transition zone acting as a source of fluids that enable hydrous weakening; and thermal erosion due to the corner-flow upwelling of hot, deep material. It is generally agreed that the influence of subduction is key, both from the temporal coincidence of subduction with increased tectonomagmatic activity on the craton and from the spatial correlation of lithospheric loss adjacent to the Pacific trench. We investigate how cratons extend to depth through comparison between seismic signatures of the cratonic lithosphere in the upper mantle and surficial evidence of cratonic boundaries. We examine global and regional tomography, as well as receiver-function constraints on lithospheric thickness in the NCC. We define craton boundaries at the surface through analyses on crust and lithospheric mantle ages and kimberlite locations. We aim to identify regions where the fast cratonic root has been lost or altered beneath Archean and Proterozoic crust and in particular place constraints on the extent of the remaining cratonic root beneath North China. Given the common emphasis on the role of subduction as a driving force for the root loss beneath the eastern NCC, we focus on

  2. Reassessment of the geologic evolution of selected precambrian terranes in Brazil, based on SHRIMP U-Pb data, part 2: mineiro and Aracuai orogens and Southern Sao Francisco craton; Reavaliacao da evolucao geologica em terrenos pre-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte 2: orogeno Aracuai, cinturao mineiro e craton Sao Francisco Meridional

    Energy Technology Data Exchange (ETDEWEB)

    Silva, Luiz Carlos da; Pimentel, Marcio [Brasilia Univ., DF (Brazil). Inst. de Geociencias]. E-mail: luizcarlos@aneel.gov.br; Leite, Carlos Augusto; Vieira, Valter Salino; Silva, Marcio Antonio da; Paes, Vinicius Jose de Castro; Cardoso Filho, Joao Moraes [Companhia de Pesquisas de Recursos Minerais (CPRM), Belo Horizonte, MG (Brazil); Armstrong, Richard [Australian National Univ., Canberra (Australia). Research School of Earth Sciences; Noce, Carlos Mauricio; Pedrosa-Soares, Antonio Carlos [Minas Gerais Univ., Belo Horizonte (Brazil). Inst. de Geociencias. Centro de Pesquisa Manuel Teixeira da Costa; Carneiro, Mauricio Antonio [Ouro Preto Univ., MG (Brazil). Dept. de Geologia

    2002-12-15

    This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W tran sects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca. 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca. 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca. 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca. 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the

  3. An in situ zircon Hf isotopic,U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt:Tracking the early evolution of the lower crust in the North China craton

    Institute of Scientific and Technical Information of China (English)

    ZHENG Jianping; LU Fengxiang; YU Chunmei; TANG Huayun

    2004-01-01

    Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842±40 Ma, except few ages of 3097-2824 Ma and 2489-2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5-2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHf of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHf up to +9.2-+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.

  4. Seismic anisotropy of Precambrian lithosphere: Insights from Rayleigh wave tomography of the eastern Superior Craton

    Science.gov (United States)

    Petrescu, Laura; Darbyshire, Fiona; Bastow, Ian; Totten, Eoghan; Gilligan, Amy

    2017-05-01

    The thick, seismically fast lithospheric keels underlying continental cores (cratons) are thought to have formed in the Precambrian and resisted subsequent tectonic destruction. A consensus is emerging from a variety of disciplines that keels are vertically stratified, but the processes that led to their development remain uncertain. Eastern Canada is a natural laboratory to study Precambrian lithospheric formation and evolution. It comprises the largest Archean craton in the world, the Superior Craton, surrounded by multiple Proterozoic orogenic belts. To investigate its lithospheric structure, we construct a frequency-dependent anisotropic seismic model of the region using Rayleigh waves from teleseismic earthquakes recorded at broadband seismic stations across eastern Canada. The joint interpretation of phase velocity heterogeneity and azimuthal anisotropy patterns reveals a seismically fast and anisotropically complex Superior Craton. The upper lithosphere records fossilized Archean tectonic deformation: anisotropic patterns align with the orientation of the main tectonic boundaries at periods ≤110 s. This implies that cratonic blocks were strong enough to sustain plate-scale deformation during collision at 2.5 Ga. Cratonic lithosphere with fossil anisotropy partially extends beneath adjacent Proterozoic belts. At periods sensitive to the lower lithosphere, we detect fast, more homogenous, and weakly anisotropic material, documenting postassembly lithospheric growth, possibly in a slow or stagnant convection regime. A heterogeneous, anisotropic transitional zone may also be present at the base of the keel. The detection of multiple lithospheric fabrics at different periods with distinct tectonic origins supports growing evidence that cratonization processes may be episodic and are not exclusively an Archean phenomenon.

  5. Destruction geodynamics of the North China Craton and its Paleoproterozoic plate tectonics

    Institute of Scientific and Technical Information of China (English)

    ZHU RiXiang; ZHENG TianYu

    2009-01-01

    Much attention has been paid in the last two decades to the physical and chemical processes as well as temporal-spatial variations of the lithospheric mantle beneath the North China Craton. In order to provide insights into the geodynamics of this variation, it is necessary to thoroughly study the state and structure of the lithospheric crust and mantle of the North China Craton and its adjacent regions as an integrated unit. Based on the velocity structure of the crust and upper mantle constrained from seismological studies, this paper presents various available geophysical results regarding the lithosphere thickness, the nature of crust-mantle boundary, the upper mantle structure and deformation characteristics as well as their tectonic features and evolution systematics. Combined with the obtained data from petrology and geochemistry, a mantle flow model is proposed for the tectonic evolution of the North China Craton during the Mesozoic-Cenozoic. We suggest that subduction of the Pacific plate made the mantle underneath the eastern Asian continent unstable and able to flow faster. Such a regional mantle flow system would cause an elevation of melt/fluid content in the upper mantle of the North China Craton and the lithospheric softening, which, subsequently resulted in destruction of the North China Craton in different ways of delamination and thermal erosion in Yanshan, Taihang Mountains and the Tan-Lu Fault zone. Multiple lines of evidence recorded in the crust of the North China Craton, such as the amalgamation of the Archean eastern and western blocks, the subduction of Paleo-oceanic crust and Paleo-continental residue, indicate that the Earth in the Paleoproterozoic had already evolved into the plate tectonic system similar to the present plate tectonics.

  6. Density heterogeneity of the cratonic lithosphere

    DEFF Research Database (Denmark)

    Cherepanova, Yulia; Artemieva, Irina

    2015-01-01

    Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean-Proterozoic Siberian craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2...... the base of the CBL is at a 180 km depth. The uncertainty of density model is density structure of the Siberian lithospheric mantle with a strong...... correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. 'Pristine' cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8-3.0% (and SPT density of 3.29-3.33 t/m3...

  7. The Acasta Gneiss - a Hadean cratonic nucleus

    Science.gov (United States)

    Sprung, P.; Scherer, E. E.; Maltese, A.; Bast, R.; Bleeker, W.; Mezger, K.

    2016-12-01

    The known terrestrial rock record lacks undisputed, chemically intact Hadean crust. Direct evidence from this eon has been restricted to zircon grains within younger rocks [1]. The Acasta Gneiss Complex (AGC; NT, CA) has yielded zircon with Hadean domains [e.g., 2,3], but the time at which AGC rocks became closed chemical systems is unclear [4,5]. Determining this `time of last disturbance' (tld) would provide a minimum protolith age, and is crucial for using radiogenic isotope compositions of bulk rocks to trace crust-mantle evolution. Recent studies mostly focused on the `low-strain' eastern AGC [e.g., 6, 7], which records an evolving, early-mid Archean cratonic nucleus [7]. We also studied the `high-strain' banded gneiss in the western AGC, which hosts >4 Ga zircon domains [2,3], too. Our focusing lay on adjoining, lithologically distinct bands [8] of two distinct chemical groups: A) Mafic, chondrite-normalized LaN/YbN ≦20, slightly HFSE- depleted, and B) TTG-like, LaN/YbN up to 145, markedly HFSE-depleted. Six adjacent bands yield a well-defined 4 Ga Sm-Nd isochron with a ɛNd4Ga of +2 and ɛHf4Ga values from +1 to +6. Within-band Sm-Nd and Lu-Hf systematics imply younger mineral re-equilibration [9]. We interpret the 4 Ga Sm-Nd isochron to date the physical juxtaposition of bands in the gneiss unit and to define tld among bands for elements less mobile and diffusive than Sm and Nd. Contrasting Sm-Nd results from the same unit [10] likely are due to sampling at too fine a scale. Digestion of metamict pre-tld zircon likely caused the scatter in Lu-Hf. Both decay systems hint at the existence of a possibly local, strongly depleted Hadean mantle domain. The TTG-like bands are 0.4 Gyr older than similar rocks in the `low-strain' eastern AGC [7]. The AGC was thus an evolved cratonic nucleus already at 4 Ga, possibly with a depleted lithospheric keel. [1] Cavosie et al. (2004) Prec. Res. 135, 251-279 [2] Bowring & Williams (1999) CMP 134, 3-16 [3] Iizuka et al

  8. Cratonic lithospheric mantle: Is anything subducted?

    Institute of Scientific and Technical Information of China (English)

    William L. Griffin; Suzanne Y. O'ReiUy

    2007-01-01

    @@ If the subcontinental lithospheric mantle (SCLM) formed through the repeated underthrusting of oceanic slabs, peridotitic SCLM should resemble oceanic peridotites, and mafic rocks (eclogites, s.l.) should be distributed throughout the SCLM. However, cratonic peridotites (both exposed massifs and xenoliths) differ markedly from oceanic and ophiolitic peridotites in their Fe-Cr-Al relationships and abundances of trace elements (Li and B) diagnostic of subduction. "Typical"cratonic peridotites have experienced extensive metaso matism; modelling of their refractory protoliths indicates high-degree melting at high P, perhaps a uniquely Archean process.

  9. Cyclic Cratonic Carbonates and Phanerozoic Calcite Seas.

    Science.gov (United States)

    Wilkinson, Bruce H.

    1982-01-01

    Discusses causes of cyclicity in cratonic carbonate sequences and evidence for and potential significance of postulated primary calcite sediment components in past Paleozoic seas, outlining problems, focusing on models explaining existing data, and identifying background. Future sedimentary geologists will need to address these and related areas…

  10. Geology, petrology, U-Pb (shrimp geochronology of the Morrinhos granite -Paraguá terrane, SW Amazonian craton: implications for the magmatic evolution of the San Ignácio orogeny

    Directory of Open Access Journals (Sweden)

    Ohana França

    Full Text Available Morrinhos granite is a batholith body that is slightly elongated in the NNW direction and approximately 1,140 km2 long; it is located in the municipality of Vila Bela da Santíssima Trindade of the state of Mato Grosso, Brazil, in the Paraguá Terrane, Rondonian-San Ignácio Province, in the SW portion of the Amazonian Craton. This intrusion displays a compositional variation from tonalite to monzogranite, has a medium to coarse inequigranular texture and is locally porphyritic; biotite is the predominant mafic in one of the facies, and hornblende is predominant in the other, with both metamorphosed into the greenschist facies. The studied rocks characterize an intermediate to acidic sequence that was formed by a subalkaline magmatism; the series is alkali-calcic to metaluminous to slightly peraluminous, and the rocks evolved through fractioned crystallization mechanisms. The structural data show two deformation phases represented by penetrative foliation (S1 and open folds (D2, and both phases were most likely related to the San Ignácio Orogeny. The geochronological (U-Pb SHRIMP and isotopic (Sm-Nd investigations of these rocks indicated a crystallization age of 1350 ± 12 Ma, TDMof approximately 1.77 Ga and εNd(1.35with a negative value of -2.57, suggesting that their generation was related to a partial melting process of a Paleoproterozoic (Statherian continental crust. The results herein indicate that the Morrinhos granite was generated in a continental magmatic arc in a late- to post-orogenic stage of the San Ignácio Orogeny, and it can be recognized as belonging to the Pensamiento Intrusive Suite.

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

    Science.gov (United States)

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

    2016-07-01

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

  12. Paleo-climatic and paleo-environmental evolution of the Neoproterozoic basal sedimentary cover on the Río de La Plata Craton, Argentina: Insights from the δ13C chemostratigraphy

    Science.gov (United States)

    Gómez-Peral, Lucía E.; Sial, Alcides N.; Arrouy, M. Julia; Richiano, Sebastián; Ferreira, Valderez P.; Kaufman, Alan J.; Poiré, Daniel G.

    2017-05-01

    The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover of the Río de La Plata Craton in Argentina that accumulated amid the breakup of the Rodinia supercontinent and subsequent assembly of Gondwanaland. Evidence for glaciation in the Villa Mónica Formation (VMF) at the base of the succession comes in the form of iron-rich laminated sediments containing dropstones composed predominantly of basement crystalline rocks and quartzites that, are sequentially overlain by a phosphatic mudstone and a 40 m thick stromatolitic dolomite. Subtidal facies preserve columnar forms similar to post-glacial tubestone stromatolites seen in the Neoproterozoic records. These morphologies suggest rapid growth associated with elevated seawater alkalinity and high rates of carbonate accumulation records. The VMF dolomites in our four studied sections near Olavarría-Sierras Bayas area reveal a pronounced negative-to-positive δ13C up section that is similarly to these cap carbonates and others worldwide. Our sedimentological and geochemical study of the VMF sections reveal consistent carbon and oxygen isotope trends that may be useful for detailed intra-basinal correlations. Samples of the VMF fabric-retentive dolomite preserve an unusually narrow range of non-radiogenic strontium isotopic compositions (0.7068 to 0.7070) that are consistent with Cryogenian limestone facies in the potential Namibian and Brazilian equivalents. Exceptional preservation of 87Sr/86Sr compositions suggest the possibility of primary dolomite precipitation in post-glacial seawater, and furthermore that REE patterns and distributions may yield similar insights to redox conditions in the depositional basin. In particular, the VMF dolomites reveal depleted LREE abundances, a negative Ce anomaly, positive La and Gd anomalies, and low Y/Ho values. As a whole, these observations suggest oxidizing post-glacial seawater conditions associated with significant freshwater inputs

  13. The crustal growth and evolution of North China Craton: Revealed by Hf isotopes in detrital zircons from modern rivers%华北克拉通地壳生长和演化:来自现代河流碎屑锆石Hf同位素组成的启示

    Institute of Scientific and Technical Information of China (English)

    第五春荣; 孙勇; 王倩

    2012-01-01

    The outstanding questions is how to quantitatively estimate the rate of growth of the continental crust and the proportion of juvenile material added to the crust at any given time during its evolution. In recent years, the U-Pb and Hf isotopic composition of detrital zircons provide an effective and simple approach to constrain the periods and rate of continental growth. In order to characterize the crustal growth history of North China Craton, 187 detrital zircons in two sand samples from the lower reach of Jing River and the Luo River were measured for Lu-Hf isotopic compositions by LA-MC-ICPMS. Compared with the global continental crust, the North China Craton shows a characteristic of episodic continental growth rate. For instance, about 60% of the present crustal volume of the North China Craton was generated in the between Mesoarchean and Late Neoarchean (3. 0 to 2. 5Ga). Since then, the continental crust keeps a stable rate of growth and completely formed at the end of the Neoproterozoic (600Ma). It indicates that the present crustal of the North China Craton was mainly growth during Archean and Proterozoic, and the juvenile additions to the continental crust are almost negligible during Phanerozoic. Combined with the available studies and the early Precambrian crust growth curves of based on U-Pb ages, two-stage Hf crust formation ages(tDM2Cand tNO2C)and εHf(t) , our results also reveal that — 2. 7Ga and ~2. 5Ga are the most prominent time of magmatism and the period of continental rapidly growth, respectively. Indicating that the North China Craton has been experienced obviously twice crustal growth.%定量地给出大陆地壳生长速率以及生长量随时间的演化是研究大陆地壳形成与演化的核心问题之一.近年来,利用沉积岩和沉积物中碎屑锆石的U-Pb年龄和Hf同位素组成已成为目前研究大陆地壳生长和演化最为简捷、有效的工具.本文对来自华北克拉通西

  14. Petroleum Exploration of Craton Basins in China

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    Craton basins are a significant petroliferous provenance. Having undergone multiple openclose tectonic cycles and strong reworking of the late Cenozoic tectonic movement, the craton basins in China are highly broken. This has resulted in multi-source and multiphase hydrocarbon generation and later hydrocarbon accumulation so that a complicated spatial assemblage of primary, paraprimary and secondary oil-gas pools has been formed. The primary factors controlling hydrocarbon accumulation include hydrocarbon-generating depressions, paleouplifts, paleoslopes, unconformity surfaces, paleo-karst, faults and fissure systems as well as the later conservation conditions. In consequence, the strategy of exploration for China's craton basins is to identify the effective source rocks, pay attention to the different effects of paleohighs and late reworking, enhance studies of the secondary storage space, attach importance to the exploration of lithologic oil-gas reservoirs and natural gas pools, and approach consciously from the secondary oil pools to the targets near the source rocks. At the same time, a complete system of technologies and techniques must be built up.

  15. Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr-Nd-Hf-Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

    Science.gov (United States)

    Tappe, Sebastian; Smart, Katie A.; Stracke, Andreas; Romer, Rolf L.; Prelević, Dejan; van den Bogaard, Paul

    2016-01-01

    amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286 Ma lamprophyres from Scania (87Sr/86Sri = 0.7040-0.7054; εNdi = +2.0 to +3.1; εHfi = +6.1 to +9.0; 206Pb/204Pbi = 17.8-18.2). Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the 'lid effect' in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.

  16. How craton margins are preserved: Insights from geodynamic models

    Science.gov (United States)

    Currie, Claire A.; van Wijk, Jolante

    2016-10-01

    Lateral variations in lithosphere thickness are observed in many continental regions, especially at the boundary between the ancient cratonic core and the adjacent more juvenile lithosphere. In some places, such as the North America craton margin in western Canada and the Sorgenfrei-Tornquist Zone in northern Europe, the transition in lithosphere thickness has a steep gradient (>45°) and it appears to be a long-lived feature (at least 50 Ma). We use thermal-mechanical numerical models to address the dynamics of lithospheric thickness changes on timescales of 100 Ma. Models start with the juxtaposition of 60 km thick lithosphere ("mobile belt") and 160 km thick lithosphere ("craton"). In the reference model, all mantle materials have a damp olivine rheology and a density comparable to primitive mantle. With this configuration, edge-driven mantle convection occurs at the craton boundary, resulting in a lateral smoothing of the thickness transition. The density and rheology of the craton mantle lithosphere are then varied to approximate changes in composition and water content. For all densities, a steep transition is maintained only if the craton strength is 5-50 times stronger than the reference damp olivine. If dry olivine is an upper limit on strength, only cratonic mantle with moderate compositional buoyancy (20-40 kg/m3 less dense than primitive mantle) remains stable. At higher densities, the thick lithosphere is eroded through downwellings, and the craton margin migrates inboard. Conversely, a compositionally buoyant craton destabilises through lateral spreading below the mobile belt.

  17. Petrogenesis of Early Cretaceous basaltic lavas from the North China Craton: Implications for cratonic destruction

    Science.gov (United States)

    Qian, Sheng-Ping; Ren, Zhong-Yuan; Richard, Wysoczanski; Zhang, Le; Zhang, Yin-Hui; Hong, Lu-Bing; Ding, Xiang-Li; Wu, Ya-Dong

    2017-03-01

    The North China Craton (NCC) is believed to be the best example of cratonic destruction. However, the processes leading to cratonic destruction remain unclear, largely due to a lack of knowledge of the nature of the Mesozoic NCC lithospheric mantle. Here we report new petrological and geochemical data for Early Cretaceous NCC basalts, which provide insights into the nature of the underlying lithospheric mantle. The Early Cretaceous basalts (all tholeiites) show a limited variation in geochemical composition. In contrast, olivine-hosted melt inclusions from these basalts display a wide range in compositional variation and include both alkalic and tholeiitic basaltic compositions. This result provides the direct evidence of the contribution of silica-undersaturated alkali basaltic melts in the petrogenesis of the Early Cretaceous NCC basalts. In addition, the compositions of olivine phenocrysts and reconstructed primary melts indicate that the Early Cretaceous basalts are derived from a mixed peridotite and refertilized peridotite source. The Pb isotopic compositions of melt inclusions in high fugacity of oxygen (fo) olivines combined with trace element characteristics of these basalts reveal that heterogeneous lithospheric mantle sources for Early Cretaceous basalts were metasomatized by carbonate-bearing eclogite-derived melts. The Pb isotopic variations of the melt inclusions and clinopyroxene and plagioclase phenocrysts demonstrate that the mantle-derived magmas were variably contaminated by lower continental crust. We propose that multiple subduction events during the Phanerozoic, combined with mantle-plume activity, likely play a vital role in the generation of the Early Cretaceous voluminous magmatism and cratonic destruction.

  18. Seismic Structure of Southern African Cratons

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Artemieva, Irina; Levander, Alan

    2014-01-01

    Cratons are extremely stable continental crustal areas above thick depleted lithosphere. These regions have remained largely unchanged for more than 2.5 Ga. This study presents a new seismic model of the seismic structure of the crust and lithospheric mantle constrained by seismic receiver...... functions and finite-frequency tomography based on data from the South Africa Seismic Experiment (SASE). Combining the two methods provides high vertical and lateral resolution. The main results obtained are (1) the presence of a highly heterogeneous crustal structure, in terms of thickness, composition (as...

  19. Integrated Seismic Arrays for Imaging the North China Craton: the ¡°Destruction of the North China Craton¡± Project

    Science.gov (United States)

    Chen, Y. J.; Chen, L.; Zheng, T.; Zhou, S.

    2007-12-01

    It has been known, mostly according to the petrological and geochemical studies particularly the xenolith's data, that the North China craton (NCC), which is part of the Archaean Sino-Korean craton, had been reactivated since Mesozoic, and experienced widespread extension and volcanism through much of the Cenozoic. Currently the NCC is characterized by a thin lithosphere (as thin as 80 km according to published studies) and strong internal deformation, where over half of the eastern China's earthquakes occurred while the two major plate boundaries about China are thousands kilometers away. Although it is seismically quite active this region encompasses the China's capital and several mega cities which together hosts a large population and are very important to China's growing economy. Supported by the Chinese earth science community, the Chinese NSF recently started a major research program, the ¡°Destruction of the North China craton¡± (DNCC). About 150 million RMB (~ 20 million US dollars) will be allocated for this 5-year multi-disciplinary research program which is open for competition for all the earth scientists in China. Here we report one major seismic observation project of ¡°Integrated Seismic Arrays of DNCC¡± just funded during the first phase funding of DNCC. This observation-driven project integrates two groups at the Institute of Geology and Geophysics, Chinese Academy of Sciences and Peking University, and both institutions have their own broadband seismometers and have recently conducted pilot portable seismic array studies in North China. Up to seven linear broadband seismic arrays, each consists of 60-100 stations, are planned within the NCC. The principle objectives are to quantify the range and degree of the craton destruction in spatial domain with major focus on the east-west variation from the previously proposed intact craton in the west to the rejuvenated region in the east and the transition zone in the middle. With the expected

  20. Main controlling factors for hydrocarbon reservoir formation and petroleum distribution in Cratonic Area of Tarim Basin

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    The Cratonic Area of the Tarim Basin is located in the central part of the basin, developing primarily with Cambrian marine source rocks and secondly Middle to Upper Ordovician marine and Carboniferous-Permian transitional facies source rocks. The source rocks were matured in the changeable period and space, forming multiple hydrocarbon generating centers during the periods. The Cratonic Area experienced multiple tectonic orogenies, forming several palaeouplifts. The matching condition between effective hydrocarbon generating centers and the palaeouplifts in various periods is the main control factor for the formation and distribution of hydrocarbon reservoirs. The palaeouplifts have experienced multiple hydrocarbon-filling phases, several periods of modifications and even breakdown. The palaeouplifts and the adjacent slopes around the effective hydrocarbon generating center compose the most favorable places for hydrocarbon accumulation. The hydrocarbon phase is related with the evolution of the hydrocarbon generating center. In the Tarim Basin's Cratonic Area, reservoirs were mostly formed during late Hercynian. The originally formed hydrocarbon reservoirs which are adjacent to source kitchens and in the good preservation condition are the most favorable prospecting targets. Hydrocarbon is richly accumulated under the regional caprock, surrounding the faulted trends, and over and below the unconformity surfaces. Reservoirs in the Carboniferous sandstone, Ordovician karstic weathered crust and carbonate rock inside the buried hill compose the main intervals for hydrocarbon accumulation. Carboniferous and Silurian sandstone pinchout reservoirs and carbonate lithologic reservoirs with rich fractures and pores are the main targets for further prospecting.

  1. Discovery of Late Paleozoic retrograded eclogites from the middle part of the northern margin of North China Craton

    Institute of Scientific and Technical Information of China (English)

    NI Zhiyao; ZHAI Mingguo; WANG Renmin; TONG Ying; SHU Guiming; HAl Xiuling

    2004-01-01

    The retrograded eciogites have been discovered in the middle part of the northern margin of the North China Craton, which occur as lens or boudin within biotite-plagioclase gneisses in Paleoproterozoic Hongqiyingzi Group. The peak eclogite facies (P > 1.40-1.50 GPa, T = 680-730℃) mineral assemblage is composed of garnet, omphacite and rutile (+ quartz), which was overprinted by the granulite facies mineral assemblage of vermicular symplectite of sodic clinopyroxene and plagioclase which replaced the precursory omphacite, and then amphibolite facies retrograded minerals with characterization of Amp+Pl kelyphitic rim and symplectite, and amphibole replaced clinopyroxene. The protolith of retrograded eclogites is oceanic basalt formed at 438 + 11 Ma. The peak eclogite facies metamorphic age of the retrograded eclogite is 325 + 4 Ma. These relict eclogites may be formed by the subduction of Paleo-Asian oceanic crust beneath the North China Craton during Late Paleozoic. The discovery of relict eclogite in this paper provides a new insight into farther understanding of tectonic evolution of the northern margin of the North China Craton, and the relationship between the Paleo-Asian Ocean and the North China Craton.

  2. Architecture of the Sulu crustal suture between the North China Craton and Yangtze Craton: Constraints from Mesozoic granitoids

    Science.gov (United States)

    Zhao, Rui; Wang, Qingfei; Liu, Xuefei; Wang, Wei; Pan, Ruiguang

    2016-12-01

    The Yangtze Craton (YC) and the North China Craton (NCC) collided in the Triassic, producing the prominent NNE-trending Sulu high-ultrahigh pressure metamorphic belt and associated crustal thickening. Late Jurassic-Early Cretaceous granitic plutons in the Sulu orogenic belt and the Jiaobei terrane to the west were used to investigate the crustal architecture across the suture. Our new data show that the granitoids from these two regions have similar chemical and isotope compositions. They are all characterized by very high Sr and low Y-Yb contents, high Sr/Y and (La/Yb)N ratios, similar ƐNd(t) values from - 18.2 to - 21.4, and similar initial 87Sr/86Sr ratios from 0.7076 to 0.7119. The calculated Nd two-stage model ages (TDM2) based on whole rock data vary from 2415 to 2662 Ma. Co-magmatic zircon crystals from the granitoids have variably negative ƐHf(t) values from - 26.8 to - 12.8, with the calculated Hf TDM2 from 2008 to 2892 Ma. The inherited zircon crystals from these rocks are dominated by Neoproterozoic (800-600 Ma) and Triassic-Early Jurassic ( 220 Ma and 180 Ma) ages. The ƐHf(t) values of the inherited zircon crystals with U-Pb ages between 180 Ma and 800 Ma from Sulu and Jiaobei range from - 21.6 to 4.2 and from - 23 to - 1.9, respectively. They all plot within the field of crustal evolution between 1385 and 2583 Ma. The similar whole rock geochemical signatures and similar inherited zircon data indicate a similar source for the granitoids in these two regions. We propose that the source regions across the suture all belong to the YC. The occurrence of the YC crust beneath the NCC at this location is thought to have resulted from the westward subduction of the YC beneath the NCC and subsequent continental collision in the Triassic. In this model, the abundant 800 to 230 Ma inherited zircon crystals in the granitoids are interpreted to have been derived from the source region whereas the rare older inherited zircon crystals are thought to have been

  3. Construction and destruction of some North American cratons

    Science.gov (United States)

    Snyder, D. B.; Humphreys, G.

    2015-12-01

    Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of, by definition, its rarity. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slabs similar to modern oceanic lithosphere in these construction histories whereas underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities. Archean continental building blocks may resemble the modern lithosphere of Ontong-Java-Hikurangi oceanic plateau. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons comprise smaller continental terranes that 'cratonized' during a granitic bloom at 2.61-2.55 ga. Cratonization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and conductive by introducing or concentrating sulfides or graphite throughout the lithosphere at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. The arrival of the subducted Shatsky Rise conjugate at the Wyoming craton at 65-75 Ma appears to have eroded and displaced the thus weakened base of the craton below 140-160 km. This replaced old refertilized continental mantle with new depleted oceanic mantle. Is this the same craton?

  4. The record of the Amazon craton in two supercontinents: Paleomagnetic and geological constraints for Mesoproterozoic to Paleozoic times

    Science.gov (United States)

    Tohver, E.

    2008-05-01

    The Amazon craton plays a fundamental role in the evolution of two supercontinents, the late Mesoproterozoic Rodinia, the break-up of which led to thethe late Neoproterozoic-early Paleozoic formation of Gondwana. A recent review of the paleomagnetic database for South America and Africa highlights the different role of the principal elements of western Gondwana elements Amazonia, conjoined with West Africa, versus the more centrally- located pieces of Gondwana; -Congo-São Francisco, Kalahari, the Rio de Plata, and the accreted terranes of the Arabia-Nubia shield. Whereas the Amazon-West Africa conjoined craton appears to have been alongside Laurentia within the framework of Rodinia, the other "central Gondwana" cratons were not part of Rodinia. New paleomagnetic data from the SW Amazon craton demonstrate the transpressive evolution of the Grenvillian collision, which resulted in thousands of kilometers of along-strike between the Amazon and Laurentia cratons. Portions of Amazonian crust stranded within the North American craton, notably the Blue Ridge province of the southern Appalachians, is evidence for this long-lived motion. An extensive review of recent thermochronological data from the North American Grenville Province and new data from the SW Amazon belts of "Grenvillian" age reveals the effects of differential post-orogenic exhumation. Restoration of this exhumation gives us a crustal-scale cross-section of the synorogenic structure, marked by thrust-related imbrication on the North American side, and large-scale, strike-slip faults on the Amazon side. It is this asymmetric structure that accounts for the differences in tectonic style between the two cratons. The timing of the break-up of Rodinia is still mostly unconstrained by geochronological data from rift-related sediments from the Amazon side. The Paraguai belt that marks the SE margin of the Amazon craton is a curved, fold-and-thrust belt that affected the late Neoproterozoic-Cambrian sediments that

  5. A lithospheric perspective on structure and evolution of Precambrian cratons

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2012-01-01

    the roots of the continents, and moves together with continental plates. Depending on geophysical techniques (and physical properties measured), the lithosphere has different practical definitions. Most of them (i.e., seismic, electrical) are on the basis of a sharp change in temperature-dependent physical...

  6. A lithospheric perspective on structure and evolution of Precambrian cratons

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2012-01-01

    the roots of the continents, and moves together with continental plates. Depending on geophysical techniques (and physical properties measured), the lithosphere has different practical definitions. Most of them (i.e., seismic, electrical) are on the basis of a sharp change in temperature-dependent physical...

  7. Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

    Science.gov (United States)

    Tappe, Sebastian; Romer, Rolf L.; Stracke, Andreas; Steenfelt, Agnete; Smart, Katie A.; Muehlenbachs, Karlis; Torsvik, Trond H.

    2017-05-01

    Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with > + 5 ‰ δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (-3.6 to - 2.4 ‰ δ13C for carbonatites versus -5.7 to - 3.6 ‰ δ13C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly

  8. An infrared investigation of inclusion-bearing diamonds from the Venetia kimberlite, Northern Province, South Africa : implications for diamonds from craton-margin settings

    Science.gov (United States)

    Viljoen, (Fanus)

    2002-08-01

    The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal-Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite

  9. Thermal modeling and geomorphology of the south border of the Sao Francisco Craton: thermochronology by fission tracks in apatites;Modelagem termica e geomorfologia da borda sul do Craton do Sao Francisco: termocronologia por tracos de fissao em apatita

    Energy Technology Data Exchange (ETDEWEB)

    Hackspacher, Peter Christian [UNESP, Rio Claro, SP (Brazil). Inst. de Geociencias e Ciencias Exatas; Godoy, Daniel Francoso de; Franco, Ana Olivia Barufi [UNESP, Rio Claro, SP (Brazil). Pos-Graduacao em Geologia Regional; Ribeiro, Luiz Felipe Brandini [NUCLEARGEO, Rio Claro, SP (Brazil); Hadler Neto, Julio Cesar [Universidade Estadual de Campinas (IFGW/UNICAMP), SP (Brazil). Inst. de Fisica Gleb Wataghin

    2007-12-15

    Recent developments in Fission Track thermochronology associated to mesozoic-cenozoic erosion and tectonic presented trough thematic maps (isotemperature), permit to model the landscape evolution in the southern border of the Sao Francisco craton, southeastern Brazil. Paleotemperature, obtained by fission track analysis in apatite, is closely related to geomorphologic interpretations. The area suffered a complex imprint of endogenous and exogenous processes resulting diversified and differentiated relieves. The landscape is strongly controlled by exhumation between Jurassic and Lower Cretaceous, uplift with tectonic denudation related to crustal heating at the Upper Cretaceous and reactivation of faults until the Miocene. This scenario is a result of reactivations of different brittle structures that accommodate the deformation in the southern border of the Sao Francisco craton. The landscape reflects denudations of up to 3 km with preserved remains of erosive surfaces in the topographical tops and chronocorrelates deposits in the basins of the region. (author)

  10. A trace element and Pb isotopic investigation into the provenance and deposition of stromatolitic carbonates, ironstones and associated shales of the ∼3.0 Ga Pongola Supergroup, Kaapvaal Craton

    Science.gov (United States)

    Bolhar, Robert; Hofmann, Axel; Siahi, Mehrnaz; Feng, Yue-xing; Delvigne, Camille

    2015-06-01

    Major and trace element, and Pb isotopic data for chemical and clastic sedimentary rocks of the Mesoarchaean Pongola Supergroup are employed to infer aspects of the provenance and depositional environment, including ambient seawater composition. Stromatolitic carbonates of the Nsuze Group were formed in a tidal-flat setting, whereas ironstones of the Mozaan Group were deposited in an outer-shelf setting during marine transgression. Geochemical criteria, employed to test for crustal contamination and diagenetic/metamorphic overprinting, demonstrate that carbonates and ironstones preserved their primary chemical signature. In comparison to other documented Precambrian stromatolites, shale-normalised REE+Y patterns for Nsuze carbonates show pronounced enrichment in middle REE, but lack strong elemental anomalies (La, Gd, Y) that are diagnostic for derivation from open marine waters. In contrast, normalised REE+Y for ironstones exhibit distinct positive La, Gd and Y anomalies. Both rock types are devoid of normalised Ce anomalies and show only minor enrichment in Eu, suggesting deposition in anoxic environments (with respect to the Ce3+/Ce4+ redox couple) accompanied by minor high-temperature hydrothermal input. Trace element geochemical data are most consistent with deposition of Nsuze carbonates in a shallow-water epicontinental basin with restricted but variable exchange to the open-ocean and dominant fluvial input, whereas ironstone precipitated in a deeper-water, epicontinental sea. Estuarine fractionation and organic complexation due to microbial activity is possibly indicated by MREE enrichment of the carbonates, also consistent with a restricted environment. Shales belonging to the Mozaan Group are characterised by high concentrations of Al and K relative to Ca, Na and Sr, indicative of pronounced in-situ weathering, coupled with K-metasomatism. The provenance is mixed, comprising (ultra)mafic and granitic source rocks. Pb isotope regression for Nsuze carbonates documents a widespread, tectono-thermal or fluid percolation event at around 2.4 Ga. Two-stage modelling of Pb isotope data, in association with published Sr and Nd isotope data, requires a source for Nzuse carbonates that was derived from evolved continental crust with an elevated U/Pb ratio (μ-value) and an approximate crustal residence time of ∼100-600 Ma.

  11. Pull-Apart Basins Within the North China Craton: Structural Pattern and Evolution of Huanghua Depression in Bohai Bay Basin%华北克拉通内部的拉分盆地:渤海湾盆地黄骅坳陷结构构造与演化

    Institute of Scientific and Technical Information of China (English)

    李三忠; 吴奇; 焦倩; 索艳慧; 周立宏; 戴黎明; 周均太; 赵峰梅; 卢异; 蒲秀刚; 楼达

    2011-01-01

    There are many pull apart basins such as Shanxi graben, Jiao-Lai basin and some sags in the Bohai Bay basin within the North China craton. The Huanghua depression is such a pull apart basin located at the center of the Bohai Bay basin. It is NNE-trending depression separating the NNE-trending Jizhong depression from the NWW-trending Jiyang depression. It developed as a pull apart basin in a large-scale strike-slipping fault system. The western margin of the depression is the Cangdong fault system, which is the Lan - Liao - Yanshan - Yangerzhuang fault system. The "Great Qikou sag" is the biggest sag with sedimentary thickness of more than 11 km and under intensive extensional region of the Bohai Bay basin area. The Huanghua depression has sharp western and eastern boundary faults controlling pull apart basin. The "Great Qikou sag" is the major component of the Huanghua depression, its inner structural pattern is controlled by E - W trending structures rather than the previously known Cangdong fault. The single sag in the Huanghua depression is characterized by graben in the lower part and the half graben in the upper part. The heaves in the Huanghua depression developed in the Shayi sedimentary stage, separating "the Great Qikou sag" into several sub-sags such as the present-day Qikou sag, Banqiao sag, Qibei sag and Qinan sag. The Huanghua depression has four major faults including NNE-trending strike-slipping boundary faults, NEE-trending regional extensional faults, E - W trending secondary extensional faults and N - S trending transfer zones such as the coastal transfer zone and the Kongdian heave. The different extensional domains are controlled by the single detachment fault system in the basement. The architecture of the "Great Qikou sag" is a result of the late destruction of the North China craton under regional NNW-oriented extension. The basin underwent four stages of evolutions including Eocene propagating rifting called Kongdian event, Early Oligocene

  12. The 3-dimensional construction of the Rae craton, central Canada

    Science.gov (United States)

    Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

    2015-10-01

    Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

  13. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

    Science.gov (United States)

    Stevenson, Ross K.; Patchett, P. Jonathan

    1990-01-01

    Results from the fractionation of zircon by sedimentary processes into continental margin sandstone yield information on the preservation of preexisting continental crust in the form of zircon, making it possible to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time. In this work, Hf-176/Hf-177 ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Gyr old sandstones from the Canadian-Shield, the North-Atlantic, the Wyoming, and the Kaapvaal Cratons. Results pointed to small amounts of continental crust prior to 3.0 Gyr ago and a rapid addition of continental crust between 2.5 and 3.0 Gyr ago, consistent with the gradual growth of continental crust, and giving evidence against no-growth histories.

  14. Multi-stage crustal growth and cratonization of the North China Craton

    Directory of Open Access Journals (Sweden)

    Mingguo Zhai

    2014-07-01

    The ∼2.5 Ga metamorphic-magmatic event is stronger than in most other cratons in the world. How to understand the geological significance of the 2.5 Ga event? The following points are emphasized: (1 nearly all old rocks >2.5 Ga underwent metamorphism at ∼2.52–2.5 Ga; (2 Archean basement rocks in the NCC experienced strong partial melting and migmatization; (3 granitoid rocks derived from partial melting include potassium granites, TTG granites and monzonites. These granitoids rocks intruded both the Archean greenstone belts and micro-blocks; (4 ∼2.5 Ga mafic dikes (amphibolites, granitic dikes (veins and syenitic-ultramafic dykes are also developed. Therefore, we suggest an assembly model that all micro-blocks in the NCC were welded together by late Archean greenstone belts at the end of the late Neoarchean. We also propose that the various micro-blocks were surrounded by small ocean basins, and the old continental crust and the oceanic crust were hotter than today. Subduction and collision were on much smaller scales as compared to the Phanerozoic plate tectonic regime, although the tectonic style and mechanisms were more or less similar. The formation of crustal melt granites is one of the processes of cratonization, inducing generation of stable upper and lower crustal layers. This process also generated an upper crust of more felsic composition and a lower crust of more mafic composition, due to molten residual materials and some underplated gabbros.

  15. Eclogite-High-Pressure Granulite Belt in Northern Edge of the Archean North China Craton

    Institute of Scientific and Technical Information of China (English)

    1998-01-01

    The discovery of retrograded eclogites and high-pressure basic granulites in the joining region of Hebei-Shanxi-Inner Mongolia (HSIM) abandon the old thoughts that Archean granulites in the North China craton are of middle or low pressure facies and promote the reconsideration of Early Precambrian cratonization tectonic process, and reveal the geological fact that the scale, rigid behavior and geological structure of Archean cratonic blocks have strong similarities to the present fundamental plate tectonics, which suggest new tectonic mechanism to understand the early continental evolution of the North China craton. (1) The retrograded eclogites and high-pressure granulites constitute a ENE-NE-striking structure-rock zone termed as the Sanggan structural belt. (2) The retrograded eclogites are closely associated with high-pressure granulites. We can call this belt a transitional eclogite-granulite facies metamorphic belt. Petrographically three metamorphic stages, at least, in the retrograded eclogite can be distinguished. ① The main mineral assemblage is composed of garnet+clinopyroxene+quartz+rutile. The mineral inclusions in garnet are fine-grained quartz, rutile and small inclusions of fine-grained second stage mineral aggregate. This aggregate consists of hypersthene+albite, and has the typical texture of small hypersthene core surrounded by albite micro-grained grains. ② The second mineral assemblage is represented by corona of garnet and symplectite of clinopyroxene. The corona of garnet is composed of hypersthene+plagioclase+clinopyroxene+a minor amount of quartz and magnetite. The symplectite of clinopyroxene is composed of hypersthene + albite+clinopyroxene. The secondary mineral assemblage along boundaries between quartz and garnet (or clinopyroxene) is fine-grained aggregate of hypersthene and clinopyroxene. ③ The third retrograded metamorphic minerals are mainly amphiboles replacing pyroxenes and plagioclases replacing garnets. The estimated

  16. Geochemistry of khondalites from the central portion of North China craton (NCC): implications for the continental cratonization in the Neoarchean

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    Within the high-grade metamorphic basement, the central portion of North China Craton (NCC), a group of Neoarchean khondalites (KS) is identified. They are characterized by large ion lithophile elements (LILE) enrichment, lower abundances of Zr, Hf and Sr. Their rare earth element (REE) distribution has significant LREE enrichment and negative Eu anomalies. The protoliths of KS are interpreted as feldspathic quartzite, shale or pelite and carbonite, deposited in a shallow sea upon cratonic shelf distant from the land. KS's source region might be dominated by granitic rocks, with a minor amount of TTG, underwent comparatively severe chemical weathering. Considering relevent tectonic constraints, we suggest that khondalites from central portion of NCC, an important metamophosed sedimentary cover, are the most significant exogenetic marker of Neoarchean continental cratonization for NCC.

  17. The story of a craton from heart to margins: illuminating cratonic lithosphere with Rayleigh wave phase velocities in Eastern Canada

    Science.gov (United States)

    Petrescu, L.; Darbyshire, F. A.; Gilligan, A.; Bastow, I. D.; Totten, E. J.

    2015-12-01

    Cratons are Precambrian continental nuclei that are geologically distinct from modern continental regions and are typically underlain by seismically fast lithospheric roots (keels) to at least 200 km depth. Both plate and non-plate tectonic origin theories such as stacking of subducted slabs or multiple mantle plume underplating have been proposed to explain keel growth.Eastern Canada is an ideal continental region to investigate cratonization processes and the onset of plate tectonics. It comprises part of the largest Archean craton in the world, the Superior Province, flanked by a ~1.1 Ga Himalayan-scale orogenic belt, the Grenville Province, and the 500-300 Ma old Appalachian orogenic province, following the same general SW-NE axial trend. The region is also cross-cut by the Great Meteor Hotspot track, providing an excellent opportunity to study the interaction of hotspot tectonism with progressively younger lithospheric domains.We investigate the lithospheric structure of Precambrian Eastern Canada using teleseismic earthquake data recorded at permanent and temporary networks. We measure interstation dispersion curves of Rayleigh wave phase velocities between ~15 and 220 s, and compare the results to standard continental and cratonic reference models. We combine the dispersion curves via a tomographic inversion which solves for isotropic phase velocity heterogeneity and azimuthal anisotropy across the region at a range of periods. The phase velocity maps indicate variations in lithospheric properties from the heart of the Superior craton to the SE Canadian coast.The new regional-scale models will help to understand the processes that generated, stabilized and reworked the cratonic roots through their billion-year tectonic history. We investigate how surface tectonic boundaries relate to deeper lithospheric structural changes, and consider the effects of the multiple Wilson cycles that affected Laurentia.

  18. Comparison of mantle-derived matierals from different spatiotemporal settings: Implications for destructive and accretional processes of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    ZHENG JianPing

    2009-01-01

    Cratonic destruction or lithospheric thinning beneath North China makes it as one of the most ideal areas for the studying on the formation and evolution of continent. However, the mechanism, time, range and dynamic setting of the destruction, even the lithospheric status before the destruction, are contentious. The comparison among mantle xenoliths in the volcanic rocks from different captured times (e.g. Paleozoic, Mesozoic and Cenozoic) and locations (e.g. Intra-plate or its rim, the translithospheric Tanlu fault or the North-South Gravity Line), and peridotitic massifs within the Sulu-Dabie ultrahigh-pressure metamorphism belt along the southern margin of the North China Craton, indicates that (1) the cratonic lithosphere is heterogeneous in structure and composition, and contains mantle weak zones; and (2) the Mesozoic-Cenozoic lithospheric thinning process is complex, including lateral spreading of lithosphere, interaction between melt and peridotite, non-even asthenospheric erosion (huge lithospheric thinning), and the limited lithospheric accretion and thus thickening, which resulted in the final replacement of the refractory cratonic lithosphere by juvenile fertile mantle. In early Mesozoic, the integrity of the North China Craton was interrupted, even destroyed by subduction and collision of the Yangtze block. The mantle wedge of the North China Craton was also metasomatized and modified by melt/fluids revealed from the subducted Yangtze continent. Lithospheric mantle extension and tectonic intrusion of the North China Craton also occurred, accompanied by the asthenospheric upwelling that due to the detachement of the subducted Yangtze continent (orogenic root). During early Cretaceous-early Tertiary, the huge thinning of lithosphere was triggered by the upwelling asthenosphere due to the subduction of the Pacific plate. Since late Tertiary, the cooling of the upwelling asthenosphere resulted in the replacement of the mantle in existence by the newly

  19. Lithological and age structure of the lower crust beneath the northern edge of the North China Craton: Xenolith evidence

    Science.gov (United States)

    Wei, Ying; Zheng, Jianping; Su, Yuping; Ma, Qiang; Griffin, William L.

    2015-02-01

    Deep-seated xenoliths in volcanic rocks offer direct glimpses into the nature and evolution of the lower continental crust. In this contribution, new data on the U-Pb ages and Hf isotopes of zircons in six felsic granulite xenoliths and one pyroxenite xenolith from the Hannuoba Cenozoic basalts, combined with published data from mafic to felsic xenoliths, are used to constrain the lithological and age structure of the lower crust beneath the northern edge of the North China Craton. Two newly-reported felsic granulites contain Precambrian zircons with positive (+ 7.5-+ 10.6) and negative εHf values (- 10.1 to - 3.7) corresponding to upper intercept ages of 2449 ± 62 Ma and 1880 ± 54 Ma, respectively, indicating crustal accretion in the late Archean and reworking in Paleoproterozoic time. Zircons in another four felsic xenoliths give Phanerozoic ages from 142 Ma to 73 Ma and zircons from one pyroxenite xenolith give a concordant age of 158 Ma. The zircon εHf values of these four felsic xenoliths range between - 23.3 and - 19.1, reflecting re-melting of the pre-existing lower crust. Integration of geothermobarometric, and geochronological data on the Hannuoba xenoliths with seismic refraction studies shows that the lower crust beneath the northern edge of the North China Craton is temporally and compositionally zoned: the upper lower crust (24-33 km) consists dominantly of Archean (~ 2.5 Ga with minor 2.7 Ga) felsic granulites with subordinate felsic granulites that reworked at 140-120 Ma; both Precambrian and late Mesozoic mafic granulites are important constituents of the middle lower crust (33-38 km); major late Mesozoic (140-120 Ma) and less Cenozoic (45-47 Ma) granulites and pyroxenites are presented in the lowermost crust (38-42 km). The zoned architecture of the lower crust beneath Hannuoba suggests a complex evolution beneath the northern margin of the craton, including late Neoarchean (~ 2.5 Ga) accretion and subsequent episodic accretion and/or reworking

  20. Paleoproterozoic, High-Metamorphic, Metasedimentary Units of Siberian Craton

    Institute of Scientific and Technical Information of China (English)

    Lena URMANTSEVA; Olga TURKINA

    2009-01-01

    Sensitive, high-resointion ion microprobe zircon U-Pb ages of Paleoproterozoic, high-grade,metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet-biotite, hypersthene-biotite, and cordierite-bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4-3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show core-rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ≥2.7, -2.3, and 1.95-2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga;therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenons sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the AIdan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton.

  1. The structure of the Amazonian craton: Available geophysical evidence

    Science.gov (United States)

    Rosa, João Willy Corrêa; Rosa, José Wilson Corrêa; Fuck, Reinhardt A.

    2016-10-01

    The Amazonian craton, which covers a large area of South America, and is thought to have been stable since the end of the Mesoproterozoic, has recently benefited from a series of regional geophysical surveys. The Amazonian craton comprises the northern Guyana shield and the southern Central Brazil shield. It has become the main subject of seismological studies aiming to determine crustal thickness. Moho thickness maps that cover a large part of the South American continent summarize these studies. Receiver function studies, aided by surface wave dispersion tomography, were also useful tools applied in the region over the past decade. These have been improved by the addition of temporary and permanent regional seismological arrays and stations. An interesting NNW-SSE Moho depth anomaly, pointing to crustal thickening of up to 60 km in the central Guyana shield and a 50 km thick anomaly of the southern Central Brazil shield were recently identified. Areas with crustal thickening correspond to Paleoproterozoic magmatic arcs. The upper mantle seismic anisotropy in part of the region has been determined from SKS splitting studies. The currently available seismic anisotropy information shows that the orientation of the determined anisotropic axis is related to the frozen in anisotropy hypothesis for the Amazonian craton. The orientation of the anisotropic axis shows no relation to the current South American plate motion in the Amazonian craton. Most recently, detailed information for the two shields has benefited from a series of high-resolution, regional aerogeophysical surveys, made available by CPRM, the Brazilian Geological Survey. In addition to the mentioned contribution from seismology for imaging deeper crustal structures, regional gravity surveys have been expanded, adding to previous Bouguer anomaly maps, and deep drilling information from early exploration efforts have been compiled for the Amazon basin, which covers the Amazonian craton separating the Guyana

  2. Multi-stage crustal growth and cratonization of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    Mingguo Zhai

    2014-01-01

    The North China Craton (NCC) has a complicated evolutionary history with multi-stage crustal growth, recording nearly all important geological events in the early geotectonic history of the Earth. Our studies propose that the NCC can be divided into six micro-blocks with >w3.0e3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts (GRB). The micro-blocks are also termed as high-grade regions (HGR) and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses, all of which underwent strong deformation and metamorphism of granu-lite-to high-grade amphibolite-facies. The micro-blocks are, in turn, from east to west, the Jiaoliao (JL), Qianhuai (QH), Ordos (ODS), Ji’ning (JN) and Alashan (ALS) blocks, and Xuchang (XCH) in the south. Recent studies led to a consensus that the basement of the NCC was composed of different blocks/ter-ranes that were finally amalgamated to form a coherent craton at the end of Neoarchean. Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca. 2.9e2.7 and 2.6e2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC. Hafnium isotopic model ages range from ca. 3.8 to 2.5 Ga and mostly are in the range of 3.0e2.6 Ga with a peak at 2.82 Ga. Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered, with a dominant ca. 2.7 Ga magmatic zircon ages. Most of the ca. 2.7 Ga TTG gneisses un-derwent metamorphism in 2.6e2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks. Abundant ca. 2.6e2.5 Ga orthogneisses have Hf-in-zircon and Nd whole-rock model ages mostly around 2.9e2.7 Ga and some around 2.6e2.5 Ga, indicating the timing of pro-tolith formation or extraction of the protolith magma was from the mantle. Therefore, it is suggested that the 2.6e2.5 Ga TTGs probably represent a coherent event of continental accretion and major

  3. Petrogenesis and Tectonic Implications of Paleoproterozoic Metapelitic Rocks in the Archean Kongling Complex from the Northern Yangtze Craton, South China

    Science.gov (United States)

    Li, Y.; Zheng, J.; Wang, W.; Xiong, Q.

    2015-12-01

    The Archean Kongling Complex in the northern Yangtze Craton is an ideal target to investigate the Precambrian accretion and evolution of continental crust in South China. This study aims to unravel the crustal evolution and tectonic setting of the Yangtze Craton during the Paleoproterozoic time, using integrated studies of petrography, zircon U-Pb and Hf isotopes and whole-rock geochemistry of Paleoproterozoic metapelitic rocks in the Kongling Complex. These rocks contain garnet, sillimanite, biotite, plagioclase, minor graphite and ilmenite. Zircons from the samples show nebulous sector-zoning and rim-core structure, suggesting both metamorphic origin and detrital origin with metamorphic overprints. The metamorphic zircons and metamorphic overprints have concordant 207Pb/206Pb age at ~2.0 Ga, while detrital grains yield three distinct concordant-age populations of >2.5 Ga, 2.4-2.2 Ga and 2.2-2.1 Ga. The age patterns indicate that the depositional age of the metasedimentary rocks was 2.1-2.0 Ga. Those 2.2-2.1 Ga detrital zircons with variable ɛHf(t) values (-7.28 to 2.97) suggest the addition of juvenile materials from depleted mantle to the crust during 2.2-2.1 Ga. The 2.4-2.2 Ga zircons have Hf model ages (TDM2) of ~2.6-3.5 Ga and >2.5 Ga zircons have TDM2 ages varying from 2.9 Ga to 3.3 Ga. The new data suggest that the Kongling Complex was originally a Paleoarchean (old up to 3.5 Ga) continental nucleus, which experienced multiple episodes of growth and reworking events at 3.3-3.2 Ga, 2.9 Ga, 2.7-2.6 Ga, 2.4-2.2 Ga and 2.2-2.1 Ga. In combination with available data, the new results in this study suggest a continent-arc-continent evolution model to explain the tectonic evolution of the Yangtze Craton during the Paleoproterozoic time: the western margin of Yangtze Craton was originally an individual continent, which underwent a reworking event during 2.4-2.2 Ga and a crust growth event caused by continent-arc collision during 2.2-2.1 Ga; it subsequently collided

  4. Reprocessing Coincident Refraction and Reflection Data to Constrain the Moho Depth in the Slave Craton, Northwest Territories, Canada.

    Science.gov (United States)

    Aristimuno, J.

    2005-12-01

    The Slave geological province, a relatively small area in the Canadian Northwest Territories incluiding the oldest rocks on Earth, is one of five cratons that form the Archean continental core of North America. Its tectonic evolution differs from the classical "life-raft model" proposed for most Archean terranes. In the Slave Craton, radiometric dating has revealed an east-west disparity between the various bedrock units exposed. The underlying lithospheric mantle, on the other hand, exhibits a NW-SE zonation, comprising three regions with distinctive geochemical and geophysical characteristics. This complex superposition suggests that crust-mantle coupling and stabilization occurred late in the orogenic development of the craton. Previous reflection and refraction studies in the Slave Craton have shown coincident reflection and refraction Moho depths at 33-35 km, which remain relatively constant beneath the Archean Slave Province and the Proterozoic domains located to the west. This is contrary to average values of crustal thickness from global compilations that would suggest a thicker Proterozoic crust of approximately 45 km. This observation implies that the Moho probably acted as a zone of detachment (mechanical boundary) during the Proterozoic Hottah-Slave collision. The Slave Craton has been extensively surveyed using seismic methods as part of the SNORCLE transect of the Canadian Lithoprobe project. The available data for this study are from a 600 km long seismic refraction/deep wide-angle reflection 2D profile acquired in 1997. In this paper, we present results from reprocessing both refraction and reflection seismic data. First, the refraction data is inverted and the resulting synthetic shot gathers are compared to observed data to validate the refraction velocity model. Then, the new velocity model is used as input to migrate the wide-angle reflection data. In addition, the depth to the Moho derived from inverted data will be compared to the interpreted

  5. Density structure of the cratonic mantle in southern Africa

    DEFF Research Database (Denmark)

    Artemieva, Irina; Vinnik, Lev P.

    2016-01-01

    . An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative...... contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa...... and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37-3.41g/cm3 in the Cape Fold and Namaqua...

  6. Construction and destruction of some North American cratons

    Science.gov (United States)

    Snyder, David B.; Humphreys, Eugene; Pearson, D. Graham

    2017-01-01

    Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of its rarity, but metasomatic weakening is an essential precursor. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slab-like geometries similar to modern oceanic lithosphere in these construction histories. Underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities, emphasizing the role of lateral accretion. Archean continental building blocks may resemble the modern lithosphere of oceanic plateau, but they better match the sort of refractory crust expected to have formed at Archean ocean spreading centres. Radiometric dating of mantle xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences, and these ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons stabilized during a granitic bloom at 2.61-2.55 Ga. This stabilization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and more conductive by introducing or concentrating sulfides or graphite at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. Late Cretaceous flattening of Farallon lithosphere that included the Shatsky Rise conjugate appears to have weakened, eroded and displaced the base of the Wyoming craton below 140-160 km. This

  7. Cretaceous mantle of the Congo craton: Evidence from mineral and fluid inclusions in Kasai alluvial diamonds

    Science.gov (United States)

    Kosman, Charles W.; Kopylova, Maya G.; Stern, Richard A.; Hagadorn, James W.; Hurlbut, James F.

    2016-11-01

    Alluvial diamonds from the Kasai River, Democratic Republic of the Congo (DRC) are sourced from Cretaceous kimberlites of the Lucapa graben in Angola. Analysis of 40 inclusion-bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the Congo craton. Silicate inclusions permitted us to classify diamonds as peridotitic, containing Fo91-95 and En92-94, (23 diamonds, 70% of the suite), and eclogitic, containing Cr-poor pyrope and omphacite with 11-27% jadeite (6 diamonds, 18% of the suite). Fluid inclusion compositions of fibrous diamonds are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. In situ cathodoluminescence, secondary ion mass spectrometry and Fourier transform infrared spectroscopy reveal large heterogeneities in N, N aggregation into B-centers (NB), and δ13C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962 ppm N, show 0-88% NB, and have δ13C isotopic compositions from - 12.5‰ to - 1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14-1432 ppm N, NB spanning 29%-68%, and wider and lighter δ13C isotopic compositions of - 17.8‰ to - 3.4‰. Fibrous diamonds on average contain more N (up to 2976 ppm) and are restricted in δ13C from - 4.1‰ to - 9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350-1375 °C at 5.8 to 6.3 GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280 °C, if the assumed mantle residence time is 0.9-3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41-44 mW/m2 during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190-210 km. The hotter

  8. THE STRUCTURE OF THE LITHOSPHERIC MANTLE OF THE SIBERAIN CRATON AND SEISMODYNAMICS OF DEFORMATION WAVES IN THE BAIKAL SEISMIC ZONE

    Directory of Open Access Journals (Sweden)

    A. A. Stepashko

    2015-09-01

    Full Text Available  The evolution and specific features of seismogynamics of the Baikal zones are reviewed in the context of interactions between deep deformation waves and the regional structure of the lithospheric mantle. The study is based on a model of the mantle structure with reference to chemical compositions of mantle peridotites from ophiolotic series located in the south-western framing of the Siberian craton (Fig. 1. The chemical zonation of the lithospheric mantle at the regional scale is determined from results of analyses of the heterogeneity of compositions of peridotites (Fig. 2, Table 1 and variations of contents of whole rock major components, such as iron, magnesium and silica (Fig. 3. According to spatial variations of the compositions of peridotites, the mantle has the concentric zonal structure, and the content of SiO2 is regularly decreasing, while concentrations of FeO∑ and MgO are increasing towards the centre of such structure (Fig. 4. This structure belongs to the mantle of the Siberian craton, which deep edge extends beyond the surface contour of the craton and underlies the north-western segment of the Central Asian orogenic belt.Results of the studies of peridotites of the Baikal region are consistent with modern concepts [Snyder, 2002; O’Reilly, Griffin, 2006; Chen et al., 2009] that suggest that large mantle lenses underlie the Archaean cratons (Fig. 5. The lenses are distinguished by high-density ultrabasic rocks and compose high-velocity roots of cratons which have remained isolated from technic processes. Edges of the mantle lenses may extend a few hundred kilometers beyond the limits of the cratons and underlie orogenic belts that frame the cratons, and this takes place in the south-western segment of the Siberian craton.The revealed structure of the lithospheric mantle is consistent with independent results of seismic and magmatectonical studies of the region. The Angara geoblock is located above the central part of the

  9. Latest Jurassic-early Cretaceous regressive facies, northeast Africa craton

    Energy Technology Data Exchange (ETDEWEB)

    van Houten, F.B.

    1980-06-01

    Nonmarine to paralic detrital deposits accumulated in six large basins between Algeria and the Arabo-Nubian shield during major regression in latest Jurassic and Early Cretaceous time. The Ghadames Sirte (north-central Libya), and Northern (Egypt) basins lay along the cratonic margin of northeastern Africa. The Murzuk, Kufra, and Southern (Egypt) basins lay in the south within the craton. Data for reconstructing distribution, facies, and thickness of relevant sequences are adequate for the three northern basins only. High detrital influx near the end of Jurassic time and in mid-Cretaceous time produced regressive nubian facies composed largely of low-sinuosity stream and fahdelta deposits. In the west and southwest the Ghadames, Murzuk, and Kufra basins were filled with a few hundred meters of detritus after long-continued earlier Mesozoic aggradation. In northern Egypt the regressive sequence succeeded earlier Mesozoic marine sedimentation; in the Sirte and Southern basins correlative deposits accumulated on Precambrian and Variscan terranes after earlier Mesozoic uplift and erosion. Waning of detrital influx into southern Tunisia and adjacent Libya in the west and into Israel in the east initiated an Albian to early Cenomanian transgression of Tethys. By late Cenomanian time it had flooded the entire cratonic margin, and spread southward into the Murzuk and Southern basins, as well as onto the Arabo-Nubian shield. Latest Jurassic-earliest Cretaceous, mid-Cretaceous, and Late Cretaceous transgressions across northeastern Africa recorded in these sequences may reflect worldwide eustatic sea-level rises. In contrast, renewed large supply of detritus during each regression and a comparable subsidence history of intracratonic and marginal basins imply regional tectonic control. 6 figures.

  10. Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

    Science.gov (United States)

    Jacob; Viljoen; Grassineau; Jagoutz

    2000-08-18

    Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere.

  11. U/Pb (SHRIMP), {sup 207}Pb/{sup 206}Pb, Rb/Sr, Sm/Nd e K/Ar geochronology of granite-greenstone terrains of Gaviao Block: implications for the Proterozoic and Archean evolution of Sao Francisco Craton, Brazil; Geocronologia U/Pb (SHRIMP), {sup 207}Pb/{sup 206}Pb, Rb/Sr, Sm/Nd e K/Ar dos terrenos granito-greenstone do Bloco do Gaviao: implicacoes para a evolucao arqueana e proterozoica do craton do Sao Francisco, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Leal, Luiz Rogerio Bastos

    1998-07-01

    The Gaviao Block (GB) in the northern portion of the Sao Francisco Craton-Northeast of Brazil, constitutes one of the oldest Archean fragments of the South American Platform Archean crust. GB underwent several events of juvenile accretion and reworking of continental crust along its evolutionary history, notably between the Archean and the Paleoproterozoic. {sup 207}Pb/{sup 206}Pb isotopic analyses were carried out in two zircons populations from strongly migmatized TTG terranes found in the proximity of Brumado: the first population (7 crystals) is taken as representative of the crystallization period of the TTG terranes at 3300 {+-} 45 Ma; the second (2 crystals) represents the age of the first even of metamorphism/migmatization at 2910 {+-} 10 Ma. {sup 207} Pb/{sup 206} Pb analyses in zircons from an outcrop of non-migmatized TTG in the area yielded a 3202 {+-} 15 Ma age (4 crystals), interpreted to be the crystallization period of the gneiss protolith. Sm/Nd analyses on the TTG rocks of the Brumado region yielded T{sub DM} model ages varying between 3.26 and 3.36 Ga and {epsilon}{sub Nd}{sup (t)} between -3.5 and +0.7. These data suggest the occurrence of juvenile accretions to the continental crust during the Archean, with differential involvement of crustal materials. The geochemical data of rare earth elements corresponding to the TTG terranes revealed moderate LRRE contents (La{sub N}=83,5), low HREE contents (La{sub N}=2,5) and a fairly fractionated pattern (La/Yb){sub N}=34, besides lack of negative Eu anomaly, showing that these rocks have similar compositions to those TTG terranes of cratonic continents, as well as some Archean rocks from CSF (e.g. Sete Voltas, Boa Vista). Finally, the youngest ages present in GB rocks (ca. 1.2-0.45 Ga) represent the role played by tectono thermal events, which produced partial or total rejuvenation of the Rb/Sr and K/Ar isotopic systems during the Espinhaco and Brasiliano cycles. In particular, K/Ar ages illustrate the

  12. Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India

    DEFF Research Database (Denmark)

    Sinha, R.; Kettanah, Y.; Gibling, M.R.

    2009-01-01

    of the Yamuna. This gray cratonic sediment was probably deposited in part by the Chambal River, which transports high-grade metamorphic minerals from the Banded Gneiss Complex of the Aravalli belt. Cratonic sediment appears to interfinger with Himalayan detritus farther north below the Ganga-Yamuna Interfluve...

  13. Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: Occurrence of Archean oceanic plateau

    Science.gov (United States)

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

    2015-04-01

    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

  14. Crustal and uppermost mantle structure of the eastern margin of the Yilgarn Craton (Australia) from passive seismic data

    Science.gov (United States)

    Sippl, Christian; Tkalčić, Hrvoje; Kennett, Brian; Spaggiari, Catherine; Gessner, Klaus

    2016-04-01

    very fast even in the shallowest crust (around or even above 3.5 km/s), with the AFO featuring even higher velocities than the Yilgarn Craton. The most notable feature in the S-velocity map are clearly elevated velocities in the Fraser Zone, a 450 km long elongated region of exhumed lower crustal metagabbros within the AFO. The footprint of the array limits the resolvable depth to about 40 km, hence we image mantle S velocities only in regions with relatively thin crust, i.e. for the Yilgarn Craton and the eastern and southernmost parts of the AFO. Due to the use of Bayesian inversion methods, the obtained results come with robust uncertainty estimates, which is an advantage in interpretation. We combine our results with evidence from active seismic experiments and potential field data to derive a three-dimensional model of the present structure of the region. This is ongoing work and most likely still in a preliminary stage at the time of the presentation. This model will be used to inform existing evolution models of the AFO and processes of modification of Archean crust.

  15. Paleoproterozoic Greenstone-Granite Belts in Northern Brazil and the Former Guyana Shield - West African Craton Province

    Directory of Open Access Journals (Sweden)

    Ian McReath

    2006-03-01

    Full Text Available The mainly meta-volcano-sedimentary Vila Nova Group and associated granites constitute separate belts, which formpart of a large paleoproterozoic (mainly rhyacian province in the Guyana Shield of which northern Brazil forms a part, andthe West African Craton. In Brazil the southwestern Serra do Ipitinga and Serra Tumucumaque-Serra do Navio belts have noobvious geometrical extensions in the Guyanas or Venezuela, and may represent deposits formed at penecontemporaneouspassive continental margins and ocean floor spreading centres. To the Northeast the Serra Lombarda-Tartarugalzinho andOiapoque belts are continuations of Guianese belts. In the former the igneous rocks have geochemical characteristics ofsuprasubduction environments. Belts in the Guyana Shield and West African Craton have many similar features. Themegaprovince evolved in a number of stages, which may have started at about 2.3 Ga, and continued with diminished activityafter 2.11 Ga. Both major juvenile additions and (possibly minor reworking of earlier crust contributed to the growth of theprovince. The major transcurrent deformation, responsible for the present structure of the belts, probably occurred duringthe later stages of evolution of the province, but metamorphism and deformation are registered even in the oldest rocks.

  16. Electrical conductivity images across the Namibian passive margin: Implications for tectonic processes along the Kaoko Belt, the western Kongo Craton and the Walvis Ridge

    Science.gov (United States)

    Weckmann, Ute; Meqbel, Naser; Kapinos, Gerhard; Jegen-Kulcsar, Marion; Ritter, Oliver

    2014-05-01

    The Special Priority Programme SAMPLE of the German Science Foundation DFG is focussed on investigating processes related to the breakup of supercontinent Gondwana and the post breakup evolution of the passive continental margins of Africa and South America. Within this framework an amphibian magnetotelluric (MT) experiment was conducted at the Southern African passive continental margin, starting at the Walvis Ridge in the Atlantic Ocean and crossing onshore the entire Kaoko Belt and the western boundary of the Kongo Craton in Northern Namibia. High-quality MT data at 167 onshore and xx offshore sites show a strong variability within short distances and indicate complex subsurface structures in parts of the Kaoko Belt and along some of the major thrust and fault zones. To identify the main conductivity features and resolve their properties in more spatial detail we started our modelling procedure with 2D inversion for a sub-set of the data where the 3D effects are less dominant along the amphibian profile. However, to account for 3D effects in the MT data and to assess robustness of conductivity anomalies revealed in the 2D model we used the entire data set for the 3D inversion using ModEM. 2D and 3D inversion models show zones of high electrical conductivity that correlate with surface expressions of prominent faults such as the Purros Mylonite Zone and the Three Palm Mylonite Zone of the Kaoko Belt. Outcropping Etendeka flood basalts in the Western Kaoko Zones are imaged by 10-15km deep reaching zones of high resistivity. Additionally, the inversion models reveal a spatial correlation of resistive zones with the cratonic Northern Platform; however, the geologically defined onset of the Kongo Craton appears as an area of high conductivity. Compared with other craton boundaries in Southern Africa this is very untypical.

  17. 3-D Structure of the Slave and Rae Cratons Provides Clues to Their Construction

    Science.gov (United States)

    Snyder, D. B.

    2013-12-01

    Deep geologic structures within cratons that make up continental cores were long neglected. Recently acquired geophysical data from large observational arrays and geochemical data resulting from exploration for diamond has now made possible co-registration of large-scale (400-km depth), truly 3-dimensional data sets. P-waves, surface waves and magnetotelluric observations provide 3-D wavespeed and conductivity models. Multi-azimuthal receiver functions map seismic discontinuity surfaces in 3-D. Xenolith suites erupted in kimberlites provide rock samples at key lithospheric depths, albeit at sparsely distributed locations. These multi-disciplinary models are becoming available for several key cratons worldwide; here the deep structure of the Slave and Rae cratons of the Canadian Shield is described. Lithospheric layers with tapered, wedge-shaped margins are common. Slave craton layers are sub-horizontal and indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. The central Rae craton has predominantly dipping discontinuities that indicate construction at 1.9 Ga by thrusting similar to that observed in crustal ';thick-skinned' fold-and-thrust belts. 3-D mapping of conductivity and metasomatism, the latter via mineral recrystallization and resetting of isotopic ages, overprints primary structures in both cratons. Distribution of more conductivitve mantle suggests that assumed causative pervasive metasomatism occurs at 100-200 km depths with ';chimneys' reaching to shallower depths, typically in locations where kimberlites or mineralization has occurred.

  18. Metallogenic Districts of Yangtze Cratonic Rim at the Edge of Chaos

    Institute of Scientific and Technical Information of China (English)

    1999-01-01

    Combining the science of complexity with ore geology, the author puts forward a new theory of metallogenesis: "complexity and self-organized criticality of metallogenic dynamic systems", and three fundamental theories are raised for it. The ore genesis and regularity of ore formation of four metallogenic districts around the Yangtze craton in China are studied with this theory. It is found that"metallogenic districts of Yangtze cratonic rim are all at the edge of chaos". This proposition is expounded by four determinative criteria of the edge of chaos for metallogenic districts of Yangtze cratonic rim.

  19. Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

    Science.gov (United States)

    Aulbach, Sonja; Massuyeau, Malcolm; Gaillard, Fabrice

    2017-01-01

    Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities. LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to 200-250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton). Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past

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

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    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.

  1. Metallogenic Systems on the Paleocontinental Margin of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    ZHAI Yusheng; DENG Jun; TANG Zhongli; XIAO Rongge; SONG Honglin; PENG Runmin; SUN Zhongshi; WANG Jianping

    2004-01-01

    The North China Craton (NCC) is one of the largest blocks composing the continent. Different types of continental margins well developed around the NCC, along with lots of metallogenic systems of different metals and different times. Based on the study on the structural evolution of the NCC, the authors made a new division of tectonic units of the NCC. Through an analysis of the data of 1:25000 geochemical survey on stream sediments, regional geochemical features of main ore-forming elements including Au, Ag, Cu, Pb, Zn, W, Ni, Co and Mo of the NCC are discussed in the paper. Then different metallogenic systems and their forming processes and geodynamics are discussed in detail. At last, temporal and spatial distribution regularities are summarized and ten favorable ore-control factors on the paleocontinental margins are put forward, including (1) abundance of ore sources; (2) rendezvous of ore-forming fluids; (3) high thermo-dynamic anomaly; (4) remarkable Earth crust-mantle interaction; (5) cluster of macroscopic structures and their long activities; (6) diversity of ore-forming environments; (7) long geohistory; (8) multiforms of critical transitional ore-forming mechanisms; (9) multi-staged and superimposed ore-formation; and (10) suitable preservation condition.

  2. Plume tectonics and cratons formation in the early Earth

    Science.gov (United States)

    Gerya, T.; Stern, R. J.; Baes, M.; Fischer, R.; Sizova, E.; Sobolev, S. V.; Whattam, S. A.

    2015-12-01

    Modern geodynamics and continental growth are critically driven by subduction and plate tectonics, however how this tectonic regime started and what geodynamic regime was before remains controversial. Most present-day subduction initiation mechanisms require acting plate forces and/or pre-existing zones of lithospheric weakness, which are themselves the consequence of plate tectonics. Here, we focus on plume-lithosphere interactions and spontaneous plume-induced subduction initiation, which does not require pre-existing lithospheric fabric and is viable for both stagnant lid and mobile/deformable lid conditions. We present results of 2D and 3D numerical modeling of plume-induced deformation and associated crustal growth resulting from tectono-magmatic interaction of ascending mantle plumes with oceanic-type lithosphere. We demonstrate that weakening of the lithosphere by plume-induced magmatism is the key factor allowing for its internal deformation and differentiation resulting in continental crust growth. We also show that plume-lithosphere interaction can enable subduction and rudimentary plate tectonics initiation at the margins of a crustal plateau growing above the plume head. We argue that frequent plume-arc interactions recorded in Archean crust could reflect either short-term plume-induced subduction or plume-induced episodic lithospheric drips. We furthermore suggest a distinct plume-tectonics regime operated on Earth before plate tectonics, which was associated with widespread tectono-magmatic heat and mass exchange between the crust and the mantle. This regime was characterized by weak deformable plates with low topography, massive juvenile crust production from mantle derived melts, mantle-flows-driven crustal deformation, magma-assisted crustal convection and widespread development of lithospheric delamination and crustal drips. Plume tectonics also resulted in growth of hot depleted chemically buoyant subcrustal proto-cratonic mantle layer. Later

  3. Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

    Institute of Scientific and Technical Information of China (English)

    GUO; Jinghui

    2001-01-01

    [1]Lu, L. Z., Jin, S. Q., P-T-t paths and tectonic history of an early Precambrian granulite facies terrane, Jining District, south-eastern Inner Mongolia, China, J. Metamorphic Geol., 1993, 11: 483-498.[2]Liu, F. L., Shen, Q. H., Retrogressive textures and metamorphic reaction features of Al-rich gneisses in the granulite facies belt from northwestern Hebei province, Acta Petrologia Sinica (in Chinese with English abstract), 1999, 15(4): 505-517.[3]Zhai, M. G., Guo, J. H., Yan, Y. H. et al., Discovery of high-pressure basic granulite terrain in North China Archaean craton and preliminary study, Science in China, Ser. B, 1993, 36(11): 1402-1408.[4]Guo, J. H., Zhai, M. G., Zhang, Y. G. et al., Early Precambrian Manjinggou high-pressure granulite melange belt on the south edge of the Huaian complex, North China craton: geological features, petrology and isotopic geochronology, Acta Petrologica Sinica (in Chinese with English abstract), 1993, 9(4): 329-341.[5]Liu, S. W., Shen, Q. H., Geng, Y. S., Metamorphic evolution of two types of garnet-granulites in Northwestern Hebei province and analyses by Gibbs method, Acta Petrologica Sinica (in Chinese with English abstract), 1996, 12(2): 261-275.[6]Wang, R. M., Some evidence of the late Archaean collision zone in the northwestern Hebei Province, in Geological Evolution of the Granulite Terrane in North Part of the North China Craton (eds. Qian, X., Wang, R.), Beijing: Seismolgical Press. 1994, 7-20.[7]Liu, D. Y., Geng, Y. S., Song, B., Late Archean crustal accretion and reworking in northwest Hebei Province: geochronological evidence, Acta Geoscientia Sinica (in Chinese with English abstract), 1997, 18(3): 226-232.[8]Geng, Y. S., Liu, D. Y., Song, B., Chronological framework of the early Precambrian important events of the north-western Hebei granulite terrain, Acta Geologica Sinica (in Chinese with English abstract), 1997, 71:316-327.[9]Guo, J. H., Zhai, M. G., Sm-Nd age dating of high

  4. The amalgamation of the supercontinent of North China Craton at the end of Neo-Archaean and its breakup during late Palaeoproterozoic and Meso-Proterozoic

    Institute of Scientific and Technical Information of China (English)

    翟明国; 卞爱国; 赵太平

    2000-01-01

    The most important geological events in the formation and evolution of the North China Craton concentrate at two stages: 2 600-2 400 Ma and 2 000-1 700 Ma (briefly, we call them 2.5 Ga event and 1.8 Ga event respectively in this paper). We propose that the essences of these two events are: Several Archaean micro-continents amalgamated to form one supercontinent according to the plate tectonic principle with a small scale at about 2.5 Ga, and the supercontinent broke down by upwelling of an ancient mantle plume at about 1.8 Ga.

  5. 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

    2001-01-01

    [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

  6. Geochemistry of khondalites from the central portion of North China craton (NCC):implications for the continental cratonization in the Neoarchean

    Institute of Scientific and Technical Information of China (English)

    李江海; 钱祥麟; 刘树文

    2000-01-01

    Within the high-grade metamorphic basement, the central portion of North China Craton (NCC), a group of Neoarchean khondalites (KS) is identified. They are characterized by large ion lithophile elements (LILE) enrichment, lower abundances of Zr, Hf and Sr. Their rare earth element (REE) distribution has significant LREE enrichment and negative Eu anomalies. The protoliths of KS are interpreted as feldspathic quartzite, shale or petite and carbonite, deposited in a shallow sea upon cratonic shelf distant from the land. KS’s source region might be dominated by granitic rocks, with a minor amount of TTG, underwent comparatively severe chemical weathering. Considering relevent tectonic constraints, we suggest that khondalites from central portion of NCC, an important metamophosed sedimentary cover, are the most significant exogenetic marker of Neoarchean continental cratonization for NCC.

  7. Major tectonic units of the North China Craton and their Paleoproterozoic assembly

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    The basement of the North China Craton can be divided into the Eastern and Western Blocks and the Central Zone (Trans-North China Orogen). The West Block formed by the amalgamation of the Ordos Block in the south and the Yinshan Block in the north 1.9-2.0 Ga ago. In 1.8-1.9 Ga, the Eastern and Western Blocks were amalgamated along the Central Zone to form the North China Craton.

  8. Crustal and upper mantle structure of the Slave craton from P- and S- Receiver Functions

    Science.gov (United States)

    Barantseva, Olga; Vinnik, Lev; Artemieva, Irina

    2017-04-01

    Teleseismic events recorded by POLARIS array in NW Canada (Slave craton) and Yellowknife station were used to calculate a sufficient number of receiver functions for P (PRF) and S (SRF) waves. Velocity (Vp and Vs) and Vp/Vs profiles from the Earth's surface down to 300 km are obtained through the simultaneous inversion of PRF and SRF with teleseismic travel time residuals for the crust and upper mantle. We observe highly heterogeneous structure of the cratonic upper mantle. The Lehman discontinuity (the bottom of the low velocity zone) is found in the western Slave craton, whereas it is not observed in the eastern part of the Slave craton. At stations located in the southern part of the craton, we observe an increase of S-wave velocities (as compared to IASP91 values) at the depths 45-150 km which is typical for depleted cratonic mantle. Low Vp/Vs ratio, obtained for the uppermost mantle (1.65-1.70) can be explained by a high fraction of Opx. A comparison of our results with available xenoliths data shows a good agreement between seismic velocity change at a depth of ca. 160 km and a decrease in mantle depletion at about the same depth.

  9. Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons

    Science.gov (United States)

    Bolhar, Robert; Hofmann, Axel; Kemp, Anthony I. S.; Whitehouse, Martin J.; Wind, Sandra; Kamber, Balz S.

    2017-10-01

    Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8-3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral

  10. SHRIMP single zircon U-Pb dating of the Kongling high-grade metamorphic terrain: Evidence for >3.2 Ga old continental crust in the Yangtze craton

    Institute of Scientific and Technical Information of China (English)

    GAO; Shan; (

    2001-01-01

    [1]Liu, G. L., New progress in the geochronology of the Kongling terrain, Regional Geology of China, 1987, 1: 95.[2]Zheng, W. Z., Liu, G. L., Wang, X. W., Geochronology of the Archean Kongling terrain, Bull. Yichang Inst. Geol. Miner. Resour. (in Chinese), 1991, 16: 97-105.[3]Yuan, H. H., Zhang, Z. L., Liu, W. et al., Dating of zircons by evaporation method and its application, Mineral. Petrol. (in Chinese), 1991, 11: 72.-79[4]Ling, W. L., Gao, S., Zheng, H. F. et al., Sm-Nd isotopic dating of Kongling terrain, Chinese. Sci. Bull., 1998, 43(1): 86-89.[5]Gao, S., Ling, W. L., Qiu, Y. et al., Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: evidence for cratonic evolution and redistribution of REE during crustal anatexis, Geochim. Cosmochim. Acta, 1999, 63: 2071-2088.[6]Gao, S., Zhang, B. R., The discovery of Archean TTG gneisses in northern Yangtze craton and their implications, Earth Sci. (in Chinese, with English abstract), 1990, 15: 675-679.[7]Dong, S. B., Metamorphism and Its Relation to the Crustal Evolution in China (in Chinese), Beijing: Geological Publishing House, 1986. [8]Composton, W., Williams, I. S., Meyer, C., U-Pb geochronology of zircons from lunar breccia 73217 using sensitive high mass-resolution ion microprobe, J. Geophys. Res., 1984, 89(B): 252-534.[9]Williams, I. S., Composton, W., Black, L. P et al., Unsupported radiogenic Pb in zircon: a case of anomalously high Pb-Pb, U-Pb and Th-Pb ages, Contrib. Mineral. Petrol., 1984, 88: 322-327.[10] Nelson, D. R., Evolution of the Archean granite-greenstone terrains of the Eastern Goldfileds, Western Australia: SHRIMP U-Pb zircon constraints, Precambrian Res., 1997, 83: 57-81.[11] Ling, W. L., Geochronology and crustal growth of the Paleoproterozoic basements along the northern margin of the Yangzte craton, Earth Sci., 1996, 21(5): 491—493.

  11. Paleoproterozoic postcollisional magmatic belt of the southern Siberian craton

    Science.gov (United States)

    Salnikova, E. B.; Larin, A. M.; Kotov, A. B.; Levitsky, V. I.; Reznitsky, L. Z.; Kovach, V. P.; Yakovleva, S. Z.

    2003-04-01

    Paleoproterozoic time is characterized by large-scale collisional and postcollisional magmatic activity evidenced in the most of ancient cratons. This global event is related to the Arctic supercontinent assemblage (Rogers, 1996). The formation of postcollisional magmatic belt at 1.9-1.8 Ga within the south-west flanking of the Siberian craton is a remarkable example of these processes happened during overall lithospheric convergence. This belt is extends for about 3000 km from the southern Enisey ridge at the west to the central Aldan shield at the east. Within the central Aldan shield the latest collisional event occurred 1925+/-5 Ma (Kotov et al, 2003) and post-collisional subalkaline S- and I-type granites emplaced at 1916+/-10 Ma (Bibikova et al., 1989), 1901+/-1 Ma (Frost et al., 1998) and 1899+/-6 Ma (Kotov et al, 2003). However at the southern Olekma terrain (western Aldan shield) the syncollisional granites have formed at 1906+/-4 Ma and postcollisional Kodar granitoids were generated at 1876-1873 Ma (Larin et al., 2000). The is a tendency outlined in timing of postcollision processes to the west (in modern coordinates). Ages of postcollisional Kevakta granitoid plutons (1846+/-8 Ma) and volcanics of North-Baikal volcano-plutonic belt (1869+/-6 Ma 1856+/-3 Ma), Baikal folded area, support this tendency. Emplacement of the Primorsky complex postcollisional rapakivi-type granitoids (southern Baikal lake) occurred at 1859+/-16 Ma (Donskaya et al., 2002). New results of U-Pb single zircon and baddeleyite dating demonstrate that formation of late-synkinematic syenite, charnockite and pegmatitic veins (1856±12 Ma, 1853±20 Ma) and calciphyre (1868±2 Ma) within the eastern part of the Sharyzhalgay block (southern Baikal lake) occurred virtually within the same episode. Postcollisional intrusive charnockite of the Shumikha complex from the western Sharyzhalgay block dated at 1861±1 Ma (Donskaya et al., 2001) and 1871±17 Ma (Levitsky et al., 2002). Subalcaline

  12. Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data

    Science.gov (United States)

    Roy, Corinna; Calo, Marco; Bodin, Thomas; Romanowicz, Barbara

    2016-04-01

    Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. On the other hand, several recent studies documented significant changes in the direction of azimuthal anisotropy with depth that suggest layering in the anisotropic structure of the stable part of the North American continent. In particular, Yuan and Romanowicz (2010) combined long period surface wave and overtone data with core refracted shear wave (SKS) splitting measurements in a joint tomographic inversion. A question that arises is whether the anisotropic layering observed coincides with that obtained from receiver function studies. To address this question, we use a trans-dimensional Markov-chain Monte Carlo (MCMC) algorithm to generate probabilistic 1D radially and azimuthal anisotropic shear wave velocity profiles for selected stations in North America. In the algorithm we jointly invert short period (Ps Receiver Functions, surface wave dispersion for Love and Rayleigh waves) and long period data (SKS waveforms). By including three different data types, which sample different volumes of the Earth and have different sensitivities to 
structure, we overcome the problem of

  13. Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: What Is the Message?

    Science.gov (United States)

    El Atrassi, F.; Debaille, V.; Mattielli, N. D. C.; Berger, J.

    2014-12-01

    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. Our main objectives aim to the identification of the mafic lithology origin and a better understanding of their role in the continental crust emplacement. Our petrological observations show that these amphibolites have fine to medium granoblastic and nematoblastic textures. The amphibolites are dominated by amphibolite-facies mineral assemblages (mainly amphibole and plagioclase), but garnet and clinopyroxene occur in a few samples. Two groups are distinct in their geochemical characteristics (major and trace elements), although both have tholeiitic basalt composition. The first group show LREE-enriched patterns and negative Nb-Ta anomalies. The second group is characterized by near-flat LREE patterns and flat HREE patterns. This second group clearly shows no Nb-Ta anomalies. The first group could be related to arc-like basalts, as it is many similarities with some Archean amphibolites probably formed in a supra-subduction zone, for instance the volcanic rocks from the southern edge of the Isua Supracrustal Belt. On the contrary, the second group has a MORB-like signature which is more unusual during the Archean. Different scenarios will be discussed regards to the Archean geodynamics.

  14. Peridotite-melt interaction: A key point for the destruction of cratonic lithospheric mantle

    Institute of Scientific and Technical Information of China (English)

    ZHANG HongFu

    2009-01-01

    This paper presents an overview of recent studies dealing with different ages of mantle peridotitic xenoliths and xenocrysts from the North China Craton, with aim to provide new ideas for further study on the destruction of the North China Craton. Re-Os isotopic studies suggest that the lithospheric mantle of the North China Craton is of Archean age prior to its thinning. The key reason why such a low density and highly refractory Archean lithospheric mantle would be thinned is changes in composition, thermal regime, and physical properties of the lithospheric mantle due to interaction of peridotites with melts of different origins. Inward subducUon of circum craton plates and collision with the North China Craton provided not only the driving force for the destruction of the craton, but also continuous melts derived from partial melting of subducted continental or oceanic crustal materials that resulted in the compositional change of the lithospheric mantle. Regional thermal anomaly at ca. 120 Ma led to the melting of highly modified iithospheric mantle. At the same time or subsequently lithospheric extension and asthenospheric upwelling further reinforced the melting and thinning of the lithospheric mantle. Therefore, the destruction and thinning of the North China Craton is a combined result of peridotite-melt interaction (addition of volatile), enhanced regional thermal anomaly (temperature increase) and lithospheric extension (decompression). Such a complex geological process finally produced a "mixed" lithospheric mantle of highly chemical heterogeneity during the Mesozoic and Cenozoic. It also resulted in significant difference in the composition of mantle peridotitic xenoliths between different regions and times.

  15. Paleoproterozoic andesitic volcanism in the southern Amazonian craton (northern Brazil); lithofacies analysis and geodynamic setting

    Science.gov (United States)

    Roverato, Matteo; Juliani, Caetano; Capra, Lucia; Dias Fernandes, Carlos Marcelo

    2016-04-01

    Precambrian volcanism played an important role in geological evolution and formation of new crust. Most of the literature on Precambrian volcanic rocks describes settings belonging to subaqueous volcanic systems. This is likely because subaerial volcanic rocks in Proterozoic and Archean volcano-sedimentary succession are poorly preserved due to erosive/weathering processes. The late Paleoproterozoic Sobreiro Formation (SF) here described, seems to be one of the rare exceptions to the rule and deserves particular attention. SF represents the subaerial expression of an andesitic magmatism that, linked with the upper felsic Santa Rosa F., composes the Uatumã Group. Uatumã Group is an extensive magmatic event located in the Xingú region, southwestern of Pará state, Amazonian Craton (northern Brazil). The Sobreiro volcanism is thought to be related to an ocean-continent convergent margin. It is characterized by ~1880 Ma well-preserved calc-alkaline basaltic/andesitic to andesitic lava flows, pyroclastic rocks and associated reworked successions. The superb preservation of its rock-textures allowed us to describe in detail a large variety of volcaniclastic deposits. We divided them into primary and secondary, depending if they result from a direct volcanic activity (pyroclastic) or reworked processes. Our study reinforces the importance of ancient volcanic arcs and rocks contribution to the terrestrial volcaniclastic sedimentation and evolution of plate tectonics. The volcanic activity that produced pyroclastic rocks influenced the amount of detritus shed into sedimentary basins and played a major role in the control of sedimentary dispersal patterns. This study aims to provide, for the first time, an analysis of the physical volcanic processes for the subaerial SF, based in field observation, lithofacies analysis, thin section petrography and less geochemical data. The modern volcanological approach here used can serve as a model about the evolution of Precambrian

  16. Triassic magmatism and its relation to decratonization in the eastern North China Craton

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    Lithospheric removal and destruction of the North China Craton have been hotly discussed recently. It has been confirmed that the timing of a strong lithospheric removal took place in Late Mesozoic; however, little is known about when the lithospheric removal was initiated and how the Late Permian to Early Triassic deep subduction of Yangtze continental crust affected the cratonic destruction. This paper presents an overview on the temporal and spatial framework and geochemical characteristics of the Triassic intrusive rocks in the eastern North China Craton and use these data to trace their sources and petrogenetic processes, in order to constrain the tectonic setting in which they evolved. It is concluded that the destruction of the North China Craton was initiated in the Late Triassic and induced by delamination of the thickened continental crust by deep subduction of Yangtze continental crust and continent-continent collision. This suggests that the subduction of the continental crust and continent-continent collision are possibly interpreted as the inducement of Late Mesozoic decratonization of the North China Craton.

  17. Triassic magmatism and its relation to decratonization in the eastern North China Craton

    Institute of Scientific and Technical Information of China (English)

    YANG JinHui; WU FuYuan

    2009-01-01

    Lithospheric removal and destruction of the North China Craton have been hotly discussed recently.It has been confirmed that the timing of a strong lithospheric removal took place in Late Mesozoic;however,little is known about when the lithospheric removal was initiated and how the Late Permian to Early Triassic deep subduction of Yangtze continental crust affected the cratonic destruction.This paper presents an overview on the temporal and spatial framework and geochemical characteristics of the Triassic intrusive rocks in the eastern North China Craton and use these data to trace their sources and petrogenetic processes,in order to constrain the tectonic setting in which they evolved.It is concluded that the destruction of the North China Craton was initiated in the Late Triassic and induced by delamination of the thickened continental crust by deep subduction of Yangtze continental crust and continent-continent collision.This suggests that the subduction of the continental crust and continent-continent collision are possibly interpreted as the inducement of Late Mesozoic decratonization of the North China Craton.

  18. Composition and Thermal History of the Lower Crust Beneath the Tanzania Craton and the Adjacent Mozambique Belt

    Science.gov (United States)

    Mansur, A. T.; Manya, S.; Rudnick, R. L.

    2006-05-01

    The Tanzanian craton has undergone little deformation since its formation 2.6 Ga ago, but Archaean crust of the adjacent Mozambique Belt (MB) has been reworked by at least two high-grade metamorphic events. An old, thick (~~200-km) lithospheric keel appears to have stabilized the craton during these deformational events. Although a thick keel appears to be absent beneath the MB today, ancient mantle lithosphere is preserved beneath much of the MB and the original thickness of this lithosphere is uncertain. Studies of the present-day lower crust can help to constrain the compositional and thermal evolution of this region. Granulite xenoliths from the Labait volcano (craton margin) are exclusively mafic and are mostly two pyroxene granulites, but also include gt-opx granulites and a gt-cpx-spinel-corundum anorthosite. Most samples also have orthoclase as a major phase. Two-pyroxene thermometry yields temperatures of 710 to 810°C for an estimated lower crustal pressure of 1 GPa; whereas the anorthosite appears to have equilibrated at a somewhat hotter temperature (gt-cpx T = 970°C). All Labait xenoliths exhibit high K2O (0.8 to 2.6 wt., excluding the anorthosite), Ba (530 to 6730 ppm), Sr (440 to 1040 ppm) and Ni contents (100 to 400 ppm) and relatively high Mg# (47 to 63). The combined high Ni, Mg# and alkali and alkaline-earth elements may reflect an unusual igneous protolith (e.g., adakitic magma) or mafic cumulates that have been metasomatically enriched in the lower crust. Granulite xenoliths from Lashaine (MB) are also exclusively mafic and form two groups: 1) anorthositic, high Al2O3 (17 to 23 wt. %) and Mg#, plag-ky-cpx-gt granulites, which are enriched in Sr and have positive Eu anomalies and 2) lower Al2O3 (13 wt. %), two pyroxene ± gt granulites, which are enriched in Ti, K, P and Ni. The latter may be meta-cumulates from alkaline magmas. Temperatures for Lashaine granulites range from 770 to 980°C. Unlike Labait and Lashaine, the Naibor Soito

  19. The evolution of the lithosphere in southern Africa: A perspective on the basic granulite xenoliths from kimberlites in South Africa

    Science.gov (United States)

    Huang, Yi-Ming; Van Calsteren, Peter; Hawkesworth, Chris I.

    1995-12-01

    Major, trace element, and strontium, neodymium, and lead isotope data are presented on mafic granulite xenoliths from the margins of the Kaapvaal Craton in southern Africa. Those from the Markt pipe in the central Cape province preserve good correlations between Cr and Mg#, negative correlations between Co, Sc, Y, and V with Al 2O 3, and positive correlations between Sr, Rb/Sr, and Eu/Eu ∗ with Al 2O 3 Such trends reflect magmatic differentiation and crystal cumulation, and they have not been affected significantly by subsequent metamorphism. Present-day strontium and neodymium isotope ratios range from 0.70411-0.71179 and from 0.51149-0.51294. The lead isotope data from the Markt granulites yield a bulk rock PbPb age of 3.19 ± 0.07 Ga and those from the other Cape province granulites have similar lead isotope ages (3.2 ˜ 3.4 Ga). These ages are supported by two PbPb mineral-whole ages of ˜2.9 Ga, indicating their Archaean origin, even though both SmNd whole rock and mineral errorchrons yield Proterozoic ages. A PbPb errorchron for the granulites from northern Lesotho yields an age of 1.49 ± 0.36 which is indistinguishable from their SmNd errorchron age of 1.24 ± 0.17 Ga. Thus, it is inferred that the granulite xenoliths from northern Lesotho are from Proterozoic lower crust, whereas the granulite xenoliths from the central Cape province are from Archaean lower crust. Most lead isotope data from the granulite xenoliths plot to the left of the Geochron and indicate relatively low U/Pb ratios. Lead isotope ratios in the granulites from Markt correlate with A1 2O 3, and such correlations are considered to be largely due to magmatic processes. The lead isotope trends displayed by the Archaean granulite xenoliths represent μ ( 238U /204Pb ) fractionation during extraction of the Archaean crust from the mantle, as do those preserved in greenstone belt rocks from the southern Kaapvaal Craton. Group 2 kimberlites, mantle xenoliths from Kimberley, and

  20. Passive margin asymmetry and its polarity in the presence of a craton

    Science.gov (United States)

    Andres-Martinez, Miguel; Perez-Gussinye, Marta; Neto-Araujo, Mario; Morgan, Jason

    2016-04-01

    When continental lithosphere is extended to break-up it forms two conjugate passive margins. In many instances these margins are asymmetric: while one is wide and extensively faulted, the conjugate thins more abruptly and exhibits little faulting. Recent observational studies have suggested that this asymmetry results from the formation of an oceanward-younging sequential normal fault array on the future wide margin. Numerical models have shown that fault sequentiality arises as a result of asymmetric uplift of the hot mantle towards the hanging wall of the active fault, which weakens this area and promotes the formation of a new oceanward fault. In numerical models the polarity of the asymmetry is random. It results from spontaneous preferential localization of strain in a given fault, a process reinforced by strain weakening effects. Slight changes in the experiments initial grid result in an opposite polarity of the asymmetry. However, along a long stretch of the South Atlantic margins, from the Camamu-Gabon to the North Santos-South Kwanza conjugates, the polarity is not random and is very well correlated with the distance of the rift to nearby cratons. Here, we use numerical experiments to show that the presence of a thick cratonic root inhibits asthenospheric flow from underneath the craton towards the adjacent fold belt, while flow from underneath the fold belt towards the craton is favoured. This enhances and promotes sequential faulting towards the craton and results in a wide faulted margin located in the fold belt and a narrow conjugate margin in the craton side, thereby determining the polarity of the asymmetry, as observed in nature.

  1. Small-scale upper mantle flow during the initiation of craton destruction

    Science.gov (United States)

    Zhao, Liang; Wang, Kun; Xu, Xiaobing

    2017-04-01

    The North China Craton (NCC) is an old craton which has experienced multi-episodic tectonism with surrounding plates. Bordered to the north by Xing'an-Mongolian Orogenic Belt, to the south by Qingling-Dabie-Sulu Orogen and to the far-east by (Paleo-) Pacific plate, the NCC has lost the cratonic properties within its eastern part. Evidently, the initiation and mechanism of craton destruction attract tremendous attention and remain hot debated. During the Mesozoic to the Cenozoic, the northeastern part of the NCC has been intensively revoked, along with the transition from NE shortening to NW-SE extension. The subduction of Paleo-Pacific plate becomes the prime suspect due to the same kinematic direction. Here we present a hybrid shear wave splitting measurement to investigate the mantle deformation of the NCC, and intend to constrain geodynamic process during the initiation of craton destruction. The SKS waveform data is recorded from 60 broadband stations with an average spacing of 15 km. We employ the traditional routine method to obtain fast polarization directions (FPDs, Φ) and delay times (δt) for the teleseismic events with epicenter range in 85°-115°. One may often have troubles in delimiting SKS and S wave with regard to the events at distances NCC, the east-end nearly E-W FPD is possibly owing to the fossil anisotropy in the lithosphere during the N-S shortening in Jurassic. The other is located in the Solonker suture zone beneath where the Moho and lithosphere and asthenosphere boundary (LAB) have sharp variation in depth. It suggests that the subduction of Pacific plate apparently reactivates the upper mantle of the north edge of the NCC but has minor effects westwards. The inconsistency in FPDs may result from small-scale mantle flow in the upper mantle, which could be the dominant operating mode of the Pacific subduction during the initiation of cratonic destruction.

  2. Magnetotelluric characterization of the northern margin of the Yilgarn Craton (Western Australia)

    Science.gov (United States)

    Piña-Varas, Perla; Dentith, Michael

    2017-04-01

    The northern margin of the Yilgarn Craton (Western Australia) was deformed during the convergence and collision with the Pilbara Craton and the intervening Glenburgh Terrain that created the Capricorn Orogen. The Yilgarn Craton is one of the most intensively mineralised areas of continental crust with world class deposits of gold and nickel. However, the region to its north has surprisingly few deposits. Cratonic margins are considered to be key indicators of prospectivity at a regional scale. The northern limit of the Yilgarn Craton within the Capricorn Orogen is not well resolved at date because of overlying Proterozoic sedimentary basins. We present here some of the results of an extensive magnetotelluric (MT) study that is being performed in the area. This study is a component of large multi-disciplinary geoscience project on the 'Distal Footprints of Giant Ore Systems' in the Capricorn Orogen. The MT dataset consists of a total of 240 broadband magnetotelluric stations (BBMT) and 84 long period stations (LMT). Analysis of the dataset reveals a clear 3-D geoelectrical behaviour and extreme complexity for most of the sites, including an extremely high number of sites with phases out-of-quadrant at long periods. 3-D inverse modelling of the MT data shows high resistivity Archean units and low resistivity Paleoproterozoic basins, including very low resistivity structures at depth. These strong resistivity contrasts allow us to successfully map northern margin of the Yilgarn Craton beneath basin cover, as well as identifying major lateral conductivity changes in the deep crust suggestive of different tectonic blocks. Upper crustal conductive zones can be correlated with faults on seismic reflection data. Our results suggest MT surveys are a useful tool for regional-scale exploration in the study area and in area of thick cover in general.

  3. Sulfide Mineralization In The Marble Bar Greenstone Belt Around Mount Edger Batholith, Pilbara Craton, Western Australia

    Science.gov (United States)

    Kitazono, S.; Nedachi, M.; Taguchi, S.

    2004-12-01

    Pilbara Craton is one of the most important regions in the world to understand the evolution of early Earth, because the geological history is well preserved through the metamorphism of low grade. Pilbara Craton, Western Australia, consists of two different tectonic components formed 3.6 and 2.8 Ga; an older Archean granite occupying the east Pilbara and greenstone belt. In the east Pilbara, the most conspicuous structures are broad domal granitoid complex separated by narrow synformal greenstone belts, and a model of continuous lithostratigraphy in the greenstones in which the dominant structures were produced by multi-stage granitoid diapirism. The Marble Bar greenstone belt is distributed around the Mount Edgar of granitoid pluton, and numerous hydrothermal gold veins are distributed in the greenstone near the boundary of pluton. Also base-metal veins and volcanogenic sedimentary type deposits are located in the same area. In this study, we examined the hydrothermal mineralization observed in the core samples of the Marble Bar greenstone belt, drilled at the Salgash area by the Archean Biosphere Drilling Project (ABDP). The Salgash drill hole is composed of tuff breccias with numerous fragments of black shale of 100 m in thickness, alternation of sandstone and shale of 40 m in thickness, basaltic lava and tuff of 30 m in thickness, and shale and sandstone of 110m in thickness with some sills of basalt and ultramafic rock. The rocks had been metamorphosed, and the grade is near the boundary between green schist and amphibolite facies. Low REE content (43 to 88 ppm), low La/Yb ratio (6.3 to 14.3), and high Eu/Eu* ratio (0.9 to 1.3) of the volcanic rocks are ordinal as the basaltic rock in Archean greenstone belts. On the other hand, these rocks show extremely high values of Cr (1500ppm), Ni (700ppm), Co (70 ppm), and Zn (600 ppm). The C isotopic ratios of carbonate in the volcanic rocks are around -3.8 permil. The clastic sediment sandstone and black shale show

  4. THE SOUTHERN FRAGMENT OF THE SIBERIAN CRATON: “LANDSCAPE” HISTORY OVER TWO BILLION YEARS

    Directory of Open Access Journals (Sweden)

    Arkady M. Stanevich

    2015-09-01

    Full Text Available In the state-of-the-art geology, concepts of evolution of interrelated geodynamic and biotic events throughout the history of the Earth have been developed (Fig. 1. Research results on sediments, bio-stratigraphy and geodynamics of the southern fragment of the Siberian craton (SSC, Fig. 2 provide for more or less reliable assessments of the status and evolution of ancient landscapes and biotas from the Lower Proterozoic to the Cenozoic.In the Lower Proterozoic, the geodynamic regime of the Urik-Iyskiy graben was similar to those of the westernpacific island-arc systems, which resulted in the orogen formation and established post-orogen granitoids of 1.86 bln years of age. At the beginning of the Early Riphean, volcano-sedimentary masses were accumulated in continental basins (Fig. 2, 3A. Collision orogenesis also resulted in the occurrence of the terrigeno-volcanogenic complex of the Akitkanskaya suite in the Western Pribaikalie and the transecting Irelskiy granitoids, aged 1.86 bln years, at the edge of the craton. Later on, most probably before the Riphean, peneplanation took place, and a shallow peripheral sea was formed with highly-mature sediments of the Purpolskaya suite. Different environments are reconstructed in the KodarUdokan zone. Sediments of the Udokanskaya suite, varying in thicknesses from 11 to 14 km, suggest a complicated evolution of sedimentation in the peripheral marine basin. Dozens of radiochronological datings of granitoids of the Chuiskiy and Kodarskiy complex which transect the Udokanskaya suite are within the range from 1.7 to 2.0 bln years. From the deposit composition and texture, it can be suggested that the middle, Chineiskaya sub-suite was formed under island-arc conditions; and glacial phenomena occurred in the late Udokan time.Further geological history of the SSC can be described only within the period after the Late Riphean sedimentations (see Fig. 3Б, В. The SSC evolution in the Neo-Proterozoic began with

  5. BRYANSK-KURSK-VORONEZH INTRA-CONTINENTAL COLLISIONAL OROGEN (EAST EUROPEAN CRATON

    Directory of Open Access Journals (Sweden)

    M. V. Mints

    2015-09-01

    Full Text Available The article presents new data on the deep crustal structure, origin and evolution of the Bryansk-Kursk-Voronezh orogen in the south-eastern segment of the East European craton; it is composed of the Paleoproterozoic formations and areas of reworked Archean crust. The purpose of this paper is the development and improvement of ideas on intra-continental orogens. The deep structure of the orogen is presented by the 3D model based on results of geological mapping of the Precambrian basement and interpretations of geophysical fields and seismic images of the crust along geotraverse 1-EB and profile DOBRE. It is established that the orogen originated with riftogenic extension of the crust at 2.6–2.5 Ga, that was repeated at 2.2–2.1 Ga, and formation of wide depressions that were efficiently filled in with volcanosedimentary layers including BIF, which accumulation was followed by high-temperature up to granulite facies metamorphism. Suprasubduction magmatism took place at 2.10–2.08 Ga and resulted in formation of the Lipetsk-Losevka volcano-plutonic complex. The active margin was completely formed at about 2.05 Ga. The short duration of subduction-related magmatic activity and the lack of relics of the oceanic lithosphere suggest short-term and spatially limited developing of the oceanic structure. The tectonothermal activity of collisional and postcollision stages was expressed in emplacement of alkaline ultramafic (2.1–2.0 Ga and gabbro-syenite (1.8–1.7 Ga complexes. It is difficult or impossible to explain specific features of the structure and evolution of the orogen in framework of the model of the Cordilleras type  accretionary orogen. Mafic-ultramafic magmatism and indications of intensive heating of the crust suggest a special role of plume type processes that provided for influx of mantle heat and juvenile mantle derived matter. 

  6. Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

    Science.gov (United States)

    Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

    2016-07-01

    This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some

  7. Thermo-chemical structure of the North China Craton from multi-observable probabilistic inversion: extent and causes of cratonic lithosphere modification

    Science.gov (United States)

    Guo, Z.; Afonso, J. C.; Qashqai, M.; Yang, Y.; Chen, J.

    2016-12-01

    Although the North China Craton (NCC) is one of the best documented cases of cratonic lithosphere modification, the actual causes, processes, and extent of lithospheric modification still are a matter of debate. Here, we present the first thermo-chemical model of the NCC from the surface down to 350 km by jointly inverting surface wave phase velocity data, geoid height, surface heat flow and absolute elevation with a novel multi-observable probabilistic inversion method. Our model reveals a thin ( 65-100 km) and chemically fertile lithosphere (8790) lithospheric mantle is imaged beneath the central TNCO and Ordos Block, reaching depths > 260 km. This lithospheric "keel" is surrounded to the east by a high-temperature sublithospheric anomaly that originates at depths > 280 km. The spatial distribution of this anomaly and its correlation with the location of recent volcanism in the region suggest that the anomaly represents a deep mantle upwelling being diverted by the cratonic keel and spreading onto regions of shallow lithosphere. Our results indicate that the present-day thermochemical structure beneath the NCC is the result of a complex interaction between a large-scale return flow associated with the subduction of the Pacific slab and the shallow lithospheric structure.

  8. Rb–Sr and Sm–Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India

    Indian Academy of Sciences (India)

    M Rajamanickam; S Balakrishnan; R Bhutani

    2014-07-01

    Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (< 3.3 Ga) as evident from their initial Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

  9. The thin-skinned fold-and-thrust belt of Irecê Basin, São Francisco Craton: main structural setting and physical analog modeling

    Directory of Open Access Journals (Sweden)

    Humberto Luis Siqueira Reis

    2013-12-01

    Full Text Available Located in the central portion of Bahia state, Irecê Basin displays the best exposures of neoproterozoic sedimentary cover at Northern São Francisco Craton. Despite of the large amount of geological studies performed there, some questions remain unsolved, especially concerning the tectonic evolution of the thin-skinned fold-and-thrust belt that involves the rocks of the basin. In order to contribute to the understanding of such evolution, the present study reviews the main structural elements of the basin and surroundings, and present new data acquired through sandbox physical analog modeling. The Thin-skinned Fold-and-thrust Belt of Irecê Basin is a great curved feature, confined in the homonymous syncline, whose genesis is related to the development of orogenic belts north of São Francisco Craton. Its evolution was conditioned by a N-S tectonic vector, responsible by the nucleation of E-W folds and thrusts. At basin boundaries, the deformation is accommodated by strike-slip faults, which locally rotated early structures. Towards south, the belt gradually loses its expression, only remaining structures related to the Chapada Diamantina thrust-and-fold system. The sandbox analog model successfully simulated the development of the Thin-skinned Fold-and-thrust Belt of Irecê Basin, and indicates that its map-view curve results from the interaction with the syncline borders, as well as substrate geometry of the foreland belt. The propagation was made through a low-friction detachment, probably conditioned by the rheological contrast between the Una Group carbonates and the underlying Espinhaço Supergroup siliciclastic rocks.

  10. The nature of cratonic lithosphere: Combining constraints from seismology, mineral physics, and petrology

    Science.gov (United States)

    Dalton, Colleen; Faul, Ulrich; Hirsch, Aaron

    2014-05-01

    In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (~200-300 km) with a very depleted composition and temperature structure controlled by steady-state conductive cooling has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth in the uppermost mantle and anomalously high attenuation, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected in the thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. We have used a forward-modeling approach to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves traversing cratons in North America, Africa, and Australia. We have also calculated the range of lithospheric temperatures and compositions that are consistent with the elastic and anelastic seismological models, using laboratory measurements on the sensitivity of velocity and attenuation to temperature, major-element composition, and mineralogy. Finally, we consider the implications of the models for the long-term stability of cratons.

  11. Lateral heterogeneity and vertical stratification of cratonic lithospheric keels: examples from Europe, Siberia, and North America

    DEFF Research Database (Denmark)

    Artemieva, Irina; Cherepanova, Yulia; Herceg, Matija;

    The presentation summarizes geophysical models for Precambrian cratons, including the structure of the crust and the lithospheric mantle. A particular focus is on thermo-compositional heterogeneity of the lithospheric mantle as constrained by different geophysical data sets: (i) thermal structure...... by an increase in mantle density as compared to light and strongly depleted lithospheric mantle of the Archean nuclei....

  12. Major element composition of the lithospheric mantle under the North Atlantic craton

    DEFF Research Database (Denmark)

    Bizzarro, Martin; Stevenson, R.K.

    2003-01-01

    The composition and thermal state of the lithospheric mantle under the North Atlantic craton was investigated using a suite of peridotite xenoliths from the diamond-bearing Sarfartoq kimberlite dike swarm of southwestern Greenland. Elevated olivine and whole-rock Mg# (>0.9) attest to the refracto...

  13. Seismic structure of a late-Archean microcontinent in the middle of the Western Australian Craton

    Science.gov (United States)

    Yuan, Huaiyu; Johnson, Simon; Dentith, Mike; Murdie, Ruth; Gessner, Klaus; Korhonen, Fawna; Bodin, Thomas

    2017-04-01

    The Capricorn Orogen recorded the Paleoproterozoic amalgamation of the Archean Pilbara and Yilgarn cratons to form the Western Australian Craton. Regional surveys involving geological mapping, geochemistry, and geophysics reveal a prolonged tectonic history in craton assembly and subsequent intracratonic reworking, which have significantly re-shaped the orogenic crust. A high-density earthquake seismology deployment targeted the Glenburgh Terrane, an exotic late-Archean to Paleoproterozoic crustal block previously inferred from distinct structural and isotopic characters in the core region of the terrane. Prominent Moho and intracrustal discontinuities are present, replicating the overall trend and depth range found in the previous high-resolution deep crustal reflection image. Significant lateral variations in the seismic signal are found across the terrane boundary, showing a relatively thin crust (40km) crust with elevated Vp/Vs ratios (>1.76) in the margin. The small Vp/Vs ratios ( 1.70) are mapped terrane-wide, indicating a felsic bulk crustal composition. Considering the available constraints from isotopic age, magnetotelluric models and absolute shear wave velocities from ambient noise tomography, the Glenburgh Terrane is interpreted as a microcontinent made in the Archean, which however may have been altered during the WAC assembly and cratonization, as well as subsequent intracratonic reworking/magmatic differentiation processes. Our results illustrate that multi-disciplinary datasets bring complementary resolution and therefore may put tighter constraints on the tectonic processes that have affected the crust.

  14. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Science.gov (United States)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  15. Dating Metasomatism in the Lithosphere Beneath North China Craton

    Science.gov (United States)

    Chen, L.; Zhou, X.

    2005-12-01

    Dating of mantle metasomatism had been carried out using zircons in metasomatized mantle xenoliths entrained in kimberlites (Kinny and Dawson, 1992; Rudnick et al., 1999; Konzett et al., 1998, 2000; Liati et al., 2004), because the U-Pb system in zircon can remain closed at high temperature (>900-)(Lee et al., 1997). Here we report a SHRIMP U-Pb dating analysis of zircons from a unique dunite-orthopyroxenite xenolith entrained in Cretaceous high-Mg diorite of Shandong province, which provides a timing constraint for the multi-stage metasomatism in the lithosphere beneath North China craton (NCC). Abundant ultramafic xenoliths had been found in the Tietonggou intrusion, one of the Cretaceous high-Mg diorite-dominated plutons in North China (Chen and Zhou, 2004). The lithology, mineral chemistry, equilibrium temperature (690-790A), and metasomatic characteristics of the ultramafic xenoliths indicate that they might be derived from the shallow lithosphere (the crust-mantle transitional zone or the uppermost lithospheric mantle) and had suffered multi-stage metasomatism (Chen and Zhou, 2004, 2005). Xenolith LW0006 is the most extremely metasomatized sample found so far in the xenolith suite of the Tietonggou pluton. The petrography, mineral chemistry, and major element compositions provide a clear metasomatic record of the composite xenolith: K (and/or Ca) metasomatism, and Si (Na) metasomatism (Chen and Zhou). We found seven zircons range from 100-170 Im in longest dimension, which is reflected in the unusually high Zr content of the bulk rock (49 ppm) of this sample. SHRIMP U-Pb dating reveals that these zircons might be grouped three kinds: Mesozoic (concordia age of 127-A3 Ma, 5 zircons), Paleozoic (430-470 Ma, 1 zircon only) and Mesoproterozoic (1310-1540 Ma, 1 zircon only). Cathodoluminescence (CL) images reveal that a few Mesozoic zircons and the Paleozoic zircons retain oscillatory zoning. The Mesozoic zircons are characterized with high Th, U contents and high

  16. Constraints on Shear Velocity in the Cratonic Upper Mantle From Rayleigh Wave Phase Velocity

    Science.gov (United States)

    Hirsch, A. C.; Dalton, C. A.

    2014-12-01

    In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (200-300 km), defined by a depleted composition and a steady-state conductively cooled temperature structure, has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth and anomalously high attenuation in the uppermost mantle, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected with a thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. Here, we used forward-modeling to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves that primarily traversed cratons in North America, South America, Africa, and Australia. We considered two approaches; with the first, depth profiles of shear velocity were predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the crust and upper mantle. With the second approach, depth profiles of shear velocity were randomly generated. In both cases, Rayleigh wave phase velocity was calculated from the Earth models and compared to the observed values. We show that it is very difficult to match the observations with an Earth model containing a low-velocity zone in the upper mantle; instead, the best-fit models contain a flat or positive velocity gradient with depth. We explore the implications of this result for the thermal and

  17. Crustal evolution and the eclogite to granulite phase transition in xenoliths from the West African Craton

    Science.gov (United States)

    Haggerty, S. E.; Hills, D. V.; Toft, P. B.

    1988-01-01

    A suite of eclogite and granulite facies xenoliths from kimberlite pipes in the Archean Man Shield of West Africa is described. The xenoliths include lithologies ranging in composition from komatiite to anorthosite and appear to be geochemically, petrologically, and geophysically related. The suite may represent fractionation of felsic material separated from ancient mantle and added to early Archean crust. The samples can be used to define a xenolith geotherm, which may represent an ancient episode of high heat flow. The samples also imply that the crust-mantle boundary is a gradational and possibly interlayered geochemical, mineralogical, and seismic transition. It is speculated that the depleted subcontinental mantle required by diamond bearing coalescence of smaller depletion cells formed by extraction of ancient crustal components. These depleted zones are surrounded by fertile asthenospheric mantle, which may have given rise to later flood basalts such as the Karroo and Parana Provinces.

  18. The Siberian Craton and its evolution in terms of the Rodinia hypothesis

    Institute of Scientific and Technical Information of China (English)

    Dmitry Gladkochub; Sergei Pisarevsky; Tatiana Donskaya; Lev Natapov; Anatoliy Mazukabzov; Arkadiy Stanevich; Eugene Sklyarov

    2006-01-01

    @@ Recent geochronological studies in southern Siberia support a Siberian assembly between 2.1 and 1.8 Ga. This broadly coincides with major orogenic events in most other Precambrian continents including Laurentia.

  19. Geochemical characteristics and petrogenesis of Mesozoic basalts from the North China Craton: A case study in Fuxin, Liaoning Province

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILE, but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel lherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic-late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.

  20. Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin

    Science.gov (United States)

    Aulbach, S.; Jacob, D. E.; Cartigny, P.; Stern, R. A.; Simonetti, S. S.; Wörner, G.; Viljoen, K. S.

    2017-09-01

    Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and δ18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy δ18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu∗, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like δ18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ∼3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with

  1. Spectral characteristics of banded iron formations in Singhbhum craton, eastern India: Implications for hematite deposits on Mars

    Directory of Open Access Journals (Sweden)

    Mahima Singh

    2016-11-01

    Full Text Available Banded iron formations (BIFs are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior (Canada and Carajas (Brazil have been studied by planetary scientists to trace the evolution of hematite deposits on Mars. Hematite deposits are extensively identified in Meridiani region on Mars. Many hypotheses have been proposed to decipher the mechanism for the formation of these deposits. On the basis of geomorphological and mineralogical studies, aqueous environment of deposition is found to be the most supportive mechanism for its secondary iron rich deposits. In the present study, we examined the spectral characteristics of banded iron formations of Joda and Daitari located in Singhbhum craton in eastern India to check its potentiality as an analog to the aqueous/marine environment on Mars. The prominent banding feature of banded iron formations is in the range of few millimeters to few centimeters in thickness. Fe rich bands are darker (gray in color compared to the light reddish jaspilitic chert bands. Thin quartz veins (<4 mm are occasionally observed in the hand-specimens of banded iron formations. Spectral investigations have been conducted in VIS/NIR region of electromagnetic spectrum in the laboratory conditions. Optimum absorption bands identified include 0.65, 0.86, 1.4 and 1.9 μm, in which 0.56 and 0.86 μm absorption bands are due to ferric iron and 1.4 and 1.9 μm bands are due to OH/H2O. To validate the mineralogical results obtained from VIS/NIR spectral radiometry, laser Raman and Fourier transform infrared spectroscopic techniques were utilized and the results were found to be similar. Goethite-hematite association in banded iron formation in Singhbhum craton suggests dehydration activity, which has

  2. Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

    Science.gov (United States)

    Yinshuang, A.; Zhang, Y.; Chen, L.

    2016-12-01

    The central and western NCC(CWNCC) only experienced localized lithospheric modification and has remained relatively stable since the Pre-Cambrian in contrast to the fundamental destruction in the east. For better unraveling the tectonic evolution and dynamics of CWNCC, detailed knowledge of lithospheric structure is thus important. However, most of the available seismological observations are dominated by regional seismic tomography and the resolutions are rather low due to the limited data coverage or intrinsic limitation of the methods. S receiver function(RF) contains information from deep velocity discontinuities and is free from the interference of crustal multiples, so it is widely used in subcontinental lithospheric structural studies. We collected teleseismic data from 340 broadband stations in CWNCC, and adopted 2-D wave equation-based poststack migration method to do S-receiver function CCP imaging. Finally, we get 8 migrated profile images in CWNCC and adjacent areas and integrate them for an overview. The most prominent feature of the LAB beneath central NCC is an sudden subsidence to 160km in the southern portion, and the dimension and extension of this deep anomaly is correlated to the lithosphere in Ordos, so we interpret it as a remnant cratonic mantle root. The LAB beneath western NCC can extend to the depth of 150-180 km but appears laterally variable. Western Ordos becomes shallower than its eastern counterpart and there are two obvious deep anomalies beneath the eastern Ordos, divided by a geological boundary at 37°N, which reflects that the lithosphere of Ordos is not so homogeneous or rigid as people thought before. Furthermore, a negative velocity discontinuity is widely identified at the depth of 80- 110 km within the thick lithosphere of CWNCC, and the location is spatially coincide with the modified LAB in ENCC. Although the cause of this mid-lithospheric discontinuity(MLD) is still controversial, mechanically, it may indicate an ancient

  3. Spectral characteristics of banded iron formations in Singhbhum craton, eastern India:Implications for hematite deposits on Mars

    Institute of Scientific and Technical Information of China (English)

    Mahima Singh; Jayant Singhal; K. Arun Prasad; V.J. Rajesh; Dwijesh Ray; Priyadarshi Sahoo

    2016-01-01

    Banded iron formations (BIFs) are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior (Canada) and Carajas (Brazil) have been studied by planetary scientists to trace the evo-lution of hematite deposits on Mars. Hematite deposits are extensively identified in Meridiani region on Mars. Many hypotheses have been proposed to decipher the mechanism for the formation of these deposits. On the basis of geomorphological and mineralogical studies, aqueous environment of deposi-tion is found to be the most supportive mechanism for its secondary iron rich deposits. In the present study, we examined the spectral characteristics of banded iron formations of Joda and Daitari located in Singhbhum craton in eastern India to check its potentiality as an analog to the aqueous/marine envi-ronment on Mars. The prominent banding feature of banded iron formations is in the range of few millimeters to few centimeters in thickness. Fe rich bands are darker (gray) in color compared to the light reddish jaspilitic chert bands. Thin quartz veins (<4 mm) are occasionally observed in the hand-specimens of banded iron formations. Spectral investigations have been conducted in VIS/NIR region of electromagnetic spectrum in the laboratory conditions. Optimum absorption bands identified include 0.65, 0.86, 1.4 and 1.9 mm, in which 0.56 and 0.86 mm absorption bands are due to ferric iron and 1.4 and 1.9 mm bands are due to OH/H2O. To validate the mineralogical results obtained from VIS/NIR spectral radiometry, laser Raman and Fourier transform infrared spectroscopic techniques were utilized and the results were found to be similar. Goethite-hematite association in banded iron formation in Singhbhum craton suggests dehydration activity, which has altered the primary

  4. New Paleomagnetic Constraints on the Proterozoic Supercontinent Evolution: A view from the South

    Science.gov (United States)

    Trindade, R. I.

    2013-05-01

    The assembly and disruption of supercontinents is thought to have impacted the long-term evolution of different envelopes of the Earth throughout Precambrian times, from mantle convection dynamics to feedback mechanisms leading to the stepwise change in atmospheric oxygenation. But the timing, duration, the size and the paleogeographic configuration of Precambrian supercontinents is still a matter of discussion. Large South American cratonic units (>30,000 km2) such as the Amazon, Rio de la Plata and São Francisco are usually represented as key pieces of different supercontinental assemblies but their paleomagnetic database is still scarce. The most important advances in the Precambrian paleomagnetic database concerns the Amazon Craton. Recent paleomagnetic studies allows one to track the participation of the Amazon Craton in several supercontinent assemblies from 2.0 Ga up to the end of the Proterozoic era. Amazonia was definitely part of the Columbia Supercontinent as attested by 1.78-1.79 Ga key poles. This supercontinent also comprised Laurentia, Baltica, North China, and Amazonia, forming a long and continuous landmass, linked by Paleo- to Mesoproterozoic mobile belts. Paleomagnetic data for Amazonia support a long-lived connection between Laurentia and Baltica at least until 1.26 Ga ago. However, new paleomagnetic poles from the same craton suggest that Columbia was in fact ephemerous, indicating a changing configuration between Amazonia and Baltica between 1.78 and 1.44 Ma. At the end of the Mesoproterozoic, the Amazon craton is part of Rodinia based on its record of Grenvillian events with overlapping ages with similar orogenic belts in eastern Laurentia. But its relative position in that supercontinent is still intensively debated. Presently only four poles for the Amazonian craton are available for the 1,200-900 Ma interval. Based on these results a dynamic model for the Amazonian craton was envisaged, which considers its oblique collision with southern

  5. Evolution tectonique de la Montagne noire: un modèle en transpression

    Science.gov (United States)

    Démange, Michel

    1999-12-01

    Formation of the French Variscan belt results from the punching of several blocks that form at present the internal zones, by the Catalan Craton. Montagne Noire is located at the edge of this Catalan block, where transpressional stresses predominate. The whole orogenic evolution of Montagne Noire, from deep seated metamorphic nappes to doming and late wrenching, may be explained in this transpressional context.

  6. Time constraints on the tectonic evolution of the eastern Sierras Pampeanas (Central Argentina)

    DEFF Research Database (Denmark)

    Siegesmund, Siegfried; Steenken, A; Martino, R D

    2010-01-01

    cycle in the neighbouring Sierra de San Luis and has not affected the titanite ages. The PTt evolution can be correlated with the plate tectonic processes responsible for the formation of the Pampean orogene, i.e., the accretion of the Pampean basement to the Río de La Plata craton (M2) and the later...

  7. Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, Eastern Dharwar Craton, southern India

    DEFF Research Database (Denmark)

    Chalapathi Rao, N.V.; Paton, Chad; Lehmann, B.

    2012-01-01

    The Mesoproterozoic Narayanpet Kimberlite Field (NKF) is located ~200km north of the well-known Wajrakarur Kimberlite Field (WKF) in the Eastern Dharwar Craton, southern India. Whereas a majority of the WKF occurrences are diamondiferous and contain mantle xenoliths and xenocrysts, their paucity ......-wide, indicates that redox conditions were favourable for diamond prospectivity, and that magmatic emplacement could, instead, have played a major role in their low diamond potential. © 2011 John Wiley & Sons, Ltd.....

  8. Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

    Science.gov (United States)

    Spratt, Jessica E.; Skulski, Thomas; Craven, James A.; Jones, Alan G.; Snyder, David B.; Kiyan, Duygu

    2014-03-01

    New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to 10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of 500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

  9. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: from subduction magmatism to cratonization

    Energy Technology Data Exchange (ETDEWEB)

    Johnson, Simon P.; Korhonen, Fawna; Kirkland, Christopher; Cliff, John B.; Belousova, Elena; Sheppard, Stephen

    2017-01-03

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  10. Magnetic anomalies across Bastar craton and Pranhita–Godavari basin in south of central India

    Indian Academy of Sciences (India)

    I V Radhakrishna Murthy; S Bangaru Babu

    2009-02-01

    Aeromagnetic anomalies over Bastar craton and Pranhita –Godavari (P –G)basin in the south of central India could be attributed to NW –SE striking mafic intrusives in both the areas at variable depths.Such intrusions can be explained considering the collision of the Bastar and Dharwar cratons by the end of the Archaean and the development of tensile regimes that followed in the Paleoproterozoic,facilitating intrusions of mafic dykes into the continental crust.The P –G basin area,being a zone of crustal weakness along the contact of the Bastar and Dharwar cratons, also experienced extensional tectonics.The inferred remanent magnetization of these dykes dips upwards and it is such that the dykes are oriented towards the east of the magnetic north at the time of their formation compared to their present NW –SE strike.Assuming that there was no imprint of magnetization of a later date,it is concluded that the Indian plate was located in the southern hemisphere,either independently or as part of a supercontinent,for some span of time during Paleoproterozoic and was involved in complex path of movement and rotation subsequently. The paper presents a case study of the utility of aeromagnetic anomalies in qualitatively deducing the palaeopositions of the landmasses from the interpreted remanent magnetism of buried intrusive bodies.

  11. Geology and geochemistry of giant quartz veins from the Bundelkhand Craton, central India and their implications

    Indian Academy of Sciences (India)

    J K Pati; S C Patel; K L Pruseth; V P Malviya; M Arima; S Raju; P Pati; K Prakash

    2007-12-01

    Giant quartz veins (GQVs; earlier referred to as `quartz reefs’) occurring in the Archean Bundelkhand Craton (29, 000 km2) represent a gigantic Precambrian (∼2.15 Ga) silica-rich fluid activity in the central Indian shield. These veins form a striking curvilinear feature with positive relief having a preferred orientation NE–SW to NNE–SSW in the Bundelkhand Craton. Their outcrop widths vary from ≤ 1 to 70m and pervasively extend over tens of kilometers along the strike over the entire craton. Numerous younger thin quartz veins with somewhat similar orientation cut across the giant quartz veins. They show imprints of strong brittle to ductile–brittle deformation, and in places are associated with base metal and gold incidences, and pyrophyllite-diaspore mineralization. The geochemistry of giant quartz veins were studied. Apart from presenting new data on the geology and geochemistry of these veins, an attempt has been made to resolve the long standing debate on their origin, in favour of an emplacement due to tectonically controlled polyphase hydrothermal fluid activity.

  12. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction.

    Science.gov (United States)

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-03-24

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

  13. Archean komatiite volcanism controlled by the evolution of early continents.

    Science.gov (United States)

    Mole, David R; Fiorentini, Marco L; Thebaud, Nicolas; Cassidy, Kevin F; McCuaig, T Campbell; Kirkland, Christopher L; Romano, Sandra S; Doublier, Michael P; Belousova, Elena A; Barnes, Stephen J; Miller, John

    2014-07-15

    The generation and evolution of Earth's continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50-30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean-Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

  14. U-Pb ases and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    DIWU ChunRong; SUN Yong; YUAN HongLin; WANG HongLiang; ZHONG XingPing; LIU XiaoMing

    2008-01-01

    In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton (NCC). The re-sults provide further constraints on the crustal formation and evolution history of NCC. Four 207Pb/206Pb age populations were obtained from 99 analyses, with clusters at ~3.40 Ga, 2.77-2.80 Ga, ~2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Ar-chean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ga detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77-2.80 Ga zircons make up a relatively small proportion and have the highest ε (t) values (up to 6.1±1.6), consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The ~2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at ~2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±23 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.

  15. Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics

    Science.gov (United States)

    Luguet, Ambre; Behrens, Melanie; Pearson, D. Graham; König, Stephan; Herwartz, Daniel

    2015-09-01

    The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the "mantle" perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS ± PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS ± PGM did not disturb

  16. Moho Depth Variations in the Northeastern North China Craton Revealed by Receiver Function Imaging

    Science.gov (United States)

    Zhang, P.; Chen, L.; Yao, H.; Fang, L.

    2016-12-01

    The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of the region. In this study, we used two-year teleseismic receiver function data from the North China Seismic Array consisting of 200 broadband stations deployed in the northeastern NCC to image the Moho undulation of the region. A 2-D wave equation-based poststack depth migration method was employed to construct the structural images along 19 profiles, and a pseudo 3D crustal velocity model of the region based on previous ambient noise tomography and receiver function study was adopted in the migration. We considered both the Ps and PpPs phases, but in some cases we also conducted PpSs+PsPs migration using different back azimuth ranges of the data, and calculated the travel times of all the considered phases to constrain the Moho depths. By combining the structure images along the 19 profiles, we got a high-resolution Moho depth map beneath the northeastern NCC. Our results broadly consist with the results of previous active source studies [http://www.craton.cn/data], and show a good correlation of the Moho depths with geological and tectonic features. Generally, the Moho depths are distinctly different on the opposite sides of the North-South Gravity Lineament. The Moho in the west are deeper than 40 km and shows a rapid uplift from 40 km to 30 km beneath the Taihang Mountain Range in the middle. To the east in the Bohai Bay Basin, the Moho further shallows to 30-26 km depth and undulates by 3 km, coinciding well with the depressions and uplifts inside the basin. The Moho depth beneath the Yin-Yan Mountains in the north gradually decreases from 42 km in the west to 25 km in the east, varying much smoother than that to the south.

  17. Tectonics of Precambrian basement of the Tarim craton

    Institute of Scientific and Technical Information of China (English)

    GUO; Zhaojie; (

    2001-01-01

    [1]Jia Chengzao, Tectonic Characteristics and Petroleum, Tarim Basin, China (in Chinese), Beijing: Petroleum Industry Press, 1997, 29?/FONT>92.[2]Huang, T. K., Ren, J. S., Jiang, C. F. et al., The Geologic Evolution of China (in Chinese), Beijing: Geological Publishing House, 1980.[3]Che, Z. C., Liu, L., Liu, H. F. et al., Discovery and occurrence of high-pressure meta-pelitic rocks from Altun Mountain areas, Xinjiang Uygur Autonomous Region, Chinese Science Bulletin, 1995, 40(23): 1988.[4]An Yin, Nie Shangyou, A phanerozoic palinspastic reconstruction of China and its neighboring regions, in The Tectonic Evolution of Asia (eds. An Yin, Harrison, T. M.), London: Cambridge University Press, 1996, 442-485.[5]Sobel, E. R., Arnaud, N., A possible middle Paleozoic suture in the Altyn Tagh, NW China, Tectonics, 1999, 18(1): 67.[6]Xu, Z. Q., Yang, J. S., Zhang, J. X. et al., A comparison between the tectonic units on the two sides of the Altun sinistral strike-slip fault and the mechanism of lithospheric shearing, Acta Geologica Sinica (in Chinese with English abstract), 1999, 73(3): 193.[7]Guo, Z., J., Zhang, Z. C., Wang, J. J., Sm-Nd isochron age of ophiolite along northern margin of Altun Tagh Mountain and its significance, Chinese Science Bulletin, 1999, 44(5): 456.[8]Liu, L., Che, Z. C., Wang, Y. et al., The evidence of Sm-Nd isochron age for the early Paleozoic ophiolite in Mangya area, Altun Mountains, Chinese Science Bulletin, 1998, 43(15): 754.[9]Xinjiang Bureau of Geology and Mineral Resources, Regional Geology of Xinjiang Uygur Autonomous Region (in Chinese with English abstract), Beijing: Geological Publishing House, 1993, 555-557.[10]Gansu Bureau of Geology and Mineral Resources, Regional Geology of Gansu Province (in Chinese with English abstract), Beijing: Geological Publishing House, 1989.[11]Hu, A. Q., Rogers, G., Discovery of 3.3 Ga Archean rocks in North Tarim Block of Xinjiang, Western China, Chinese Science

  18. Shear zone evolution and timing of deformation in the Neoproterozoic transpressional Dom Feliciano Belt, Uruguay

    Science.gov (United States)

    Oriolo, Sebastián; Oyhantçabal, Pedro; Wemmer, Klaus; Heidelbach, Florian; Pfänder, Jörg; Basei, Miguel A. S.; Hueck, Mathias; Hannich, Felix; Sperner, Blanka; Siegesmund, Siegfried

    2016-11-01

    New structural, microstructural and geochronological (U-Pb LA-ICP-MS, Ar/Ar, K-Ar, Rb-Sr) data were obtained for the Dom Feliciano Belt in Uruguay. The main phase of crustal shortening, metamorphism and associated exhumation is recorded between 630 and 600 Ma. This stage is related to the collision of the Río de la Plata and Congo cratons at ca. 630 Ma, which also involved crustal reworking of minor crustal blocks such as the Nico Pérez Terrane and voluminous post-collisional magmatism. Subsequent orogen-parallel sinistral shearing gave rise to further deformation up to ca. 584 Ma and resulted from the onset of the convergence of the Kalahari Craton and the Río de la Plata-Congo cratons. Sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation. Dextral ENE-striking shear zones were subsequently active at ca. 550 Ma, coeval with further sinistral shearing along N- to NNE-striking shear zones. The tectonothermal evolution of the Dom Feliciano Belt thus recorded the collision of the Río de la Plata and Congo cratons, which comprised one of the first amalgamated nuclei of Gondwana, and the subsequent incorporation of the Kalahari Craton into Western Gondwana.

  19. Flood-basalt magmatism of the Vodlozero Block of the Karelian Craton: relations between high- and low-Cr Varieties

    Science.gov (United States)

    Bogina, Maria; Zlobin, Valeriy; Sharkov, Evgenii; Chistyakov, Alexii

    2016-04-01

    The early Paleoproterozoic (2.5-2.3 Ga) volcanic rocks of the Karelian Craton are ascribed to the large igneous province of the eastern Fennoscandian Shield. They are mainly represented by calc-alkaline low-Ti basalts and basaltic andesites with relatively high SiO2 and clearly pronounced continental trace element signatures. The compositions of the rocks vary in the different domains of the Karelian craton. In particular, basalts developed in the Central Domain are represented by strongly fractionated varieties (Mg # Mg rocks similar to the fractionated varieties developed in the Central Domain. They are characterized by high contents of Zr, Y, and REE, and LILE, fractionated REE patterns with (La/Yb)n = 5.44-12.34, (La/Sm)n = 4.4-2.03, and (Gd/Yb)n = 1.36-2.71), and demonstrate negative Ti and Nb anomalies. The second group is represented by more primitive high Cr (up to 1000 ppm) high Mg# (up to 68) basalts with high Ni contents. Such composition is close to the primary non-fractionated mantle-derived magmas and may be used to provide insight into parental melts of continental flood basalts and their crustal evolution. In the spidergrams they demonstrate weak positive Ti anomaly at positive or absent Zr anomaly and negative Nb anomaly. The rocks of the second group are also characterized (with rare exception) by LREE enriched but less fractionated patterns than the first group: ((La/Yb)n up to 7.5, (La/Sm)n = up to 2.8, (Gd/Yb)n = up to 2.0). High Cr and low Y contents are indicative of relatively high degree of partial melting of a depleted mantle source. These rocks are simulated by sequential fractionation of uncontaminated continental flood basalts leaving Ol residue and lower crustal contamination (rocks with low values of eNd). The percentage of crustal contamination is controlled by Nb/Th ratio. Examination of Nd isotope data revealed that both these types have negative eNd, but high-Cr rocks have slightly more radiogenic Nd isotope composition. A high

  20. Dobreflection: An Exceptional Example of Basin Inversion In The East-european Craton

    Science.gov (United States)

    Bayer, U.; Stovba, S.; Maystrenko, Y.; Stephenson, R.; Tolkunov, A.; Dobreflection Working Group

    DOBREflection is a joint project of Ukrainian organisations (Ukrgeofisika and the Institute of Geophysics of the National Academy of Sciences) and an international European consortium that included acquisition of some 250 km of deep seismic re- flection data in 2000 and 2001. The aim was to study the south-eastern continuation of the Dnieper-Donets Basin (DDB) into the coal mining areas of the Donbas Foldbelt (DF), including the sedimentary fill and its deformation as well as the structure of the deeper crust. The DF is the strongly inverted and compressionally deformed part of the DDB, which is a Late Devonian rift basin located on the south-western part of the East-European Craton (EEC) between the Ukrainian Shield (UkS) to the south-west and the Voronezh Massif (VM) to the north-east. DOBREflection reveals that the base of the predominantly Devonian and Carboniferous sedimentary succession in the DF reaches a maximum of about 20-km in its axial part. However, the Donbas area has been much more seriously affected by secondary tectonic events than those parts of the DDB further north. In particular, significant shortening in response to Late Cretaceous Eo-Alpine compression is in evidence. While syn- and post-rift faults with offsets up to more than 3-km disrupt the basement horizon, the shortening (basin inversion) is displayed mainly as folding within the sedimentary succession. The fold patterns sug- gest a detachment surface most likely located within the Upper Devonian sequence, perhaps indicating the presence of salt rich layers. Shortening is also accommodated on two important crustal-scale structures. The first of these is a slightly inclined listric shear zone, or thrust, cutting the entire crust - including the UkS Moho - south of the DF, upwards through the complete sedimentary package ending in the northern part of the DF within an area of reverse faults exposed at the surface. The latter have vertical offsets up to a few kilometres whereas the

  1. Petrogenesis and tectonic implications of the Neoarchean North Liaoning tonalitic-trondhjemitic gneisses of the North China Craton, North China

    Science.gov (United States)

    Wang, Maojiang; Liu, Shuwen; Wang, Wei; Wang, Kang; Yan, Ming; Guo, Boran; Bai, Xiang; Guo, Rongrong

    2016-12-01

    Tonalitic-trondhjemitic-granodioritic (TTG) gneisses are dominant lithological assemblages in Archean high grade metamorphic terranes in the world. These TTG gneisses preserve important information in formation and evolution of Archean continental crust. Tangtu-Majuanzi microblock in North Liaoning Province (NLP) is one of the major Neoarchean metamorphic basement terranes in the northeastern margin of the North China Craton (NCC). The Tangtu-Majuanzi microblock is composed mainly of Neoarchean tonalitic-trondhjemitic (TT) gneisses, subordinate granodioritic to monzogranitic association (GMA) and minor supracrustal rocks. The tonalitic-trondhjemitic gneisses are divided into high MgO Group (HMG) and low MgO Group (LMG) based on their chemical compositions. Detailed petrogenetic investigations suggest that the magmatic precursors of the HMG samples were derived from partial melting of subducted slabs and contaminated by the overlying mantle wedge during its ascent; whereas, magmatic precursors of the LMG samples were derived from the juvenile lower crust. LA-ICPMS zircon U-Pb isotopic dating analyses reveal that the magmatic precursors of the HMG samples were formed at 2553-2531 Ma. An older HMG tonalitic gneiss sample was discovered at Sandaoguan in the southmost of the studied area, with its magmatic precursor emplaced at 2680 Ma. The magmatic precursors of the LMG samples emplaced at 2595-2583 Ma. Combined with previous credible chronological data, our newly obtained zircon U-Pb dating and Lu-Hf isotopic data indicate that three episodes of magmatism at ∼2700-2680 Ma, ∼2600-2570 Ma and ∼2550-2510 Ma occurred in the Tangtu-Majuanzi microblock, and the TT gneisses in this microblock were subjected to generally amphibolite-facies metamorphism at ∼2520-2470 Ma. Based on the above Neoarchean crust-mantle thermal-dynamic processes, we propose that the Neoarchean magmatism and metamorphism in the Tangtu-Majuanzi microblock of North Liaoning Province occurred in

  2. Petrology of Gameleira potassic lamprophyres, São Francisco Craton

    Directory of Open Access Journals (Sweden)

    Jorge Plá Cid

    2012-06-01

    Full Text Available Gameleira lamprophyres are dykes and mafic microgranular enclaves associated with the shoshonitic Gameleira monzonite. This association belongs to the Paleoproterozoic alkaline magmatism from Serrinha nucleus, northeast Brazil. The liquidus paragenesis is diopside, pargasite, apatite and mica. Reverse zoning was identified in the groundmass alkali feldspar and was related to the undercooling of lamprophyric magma during the emplacement, with high growth rate of pargasite/edenite inducing disequilibrium between feldspars and liquid. Chemical data indicate that the lamprophyres are basic rocks (SiO2 3 wt% and potassic signature (K2O/Na2O ≈ 2. High contents of MgO and Cr are consistent with a signature of a primary liquid, and such concentrations, as well as Al, K, P, Ba, Ni- and light rare earth elements, are consistent with an olivine-free metasomatic mantle source enriched in amphibole, clinopyroxene and apatite. By contrast, the ultrapotassic lamprophyres from Morro do Afonso, contemporaneous alkaline ultrapotassic magmatism in Serrinha nucleus, were probably produced by melting of a clinopyroxene-phlogopite-apatite enriched-source. The identification of different mineral paragenesis in the source of potassic and ultrapotassic lamprophyres from Serrinha nucleus can contribute to the understanding of the mantle heterogeneities and tectonic evolution of this region.Os lamprófiros Gameleira são formados por diques e enclaves máficos microgranulares associados ao monzonito shoshonítico Gameleira. Esta associação pertence ao magmatismo alcalino Paleoproterozóico do núcleo Serrinha, nordeste do Brasil. A paragênese no liquidus é formada por diopsídio, pargasita, apatite e mica. Zonamento inverso foi identificado no feldspato alcalino da matriz e relacionado ao super-resfriamento do magma lamprofírico durante sua colocação, com alta taxa de crescimento da pargasita/edenita induzindo desequilíbrio entre os feldspatos e o l

  3. Cretaceous Apparent Polar Wander Relative to the Major Cratons and Displacement Estimates of Baja British Columbia

    Science.gov (United States)

    Enkin, R. J.

    2004-12-01

    When paleogeographic interpretations derived from independent observations conflict, the methods and results from each discipline come under careful scrutiny, as illustrated by the Baja British Columbia controversy. Cretaceous paleomagnetic data from a large region of the Canadian Cordillera render paleopoles which are far-sided with respect to cratonic North American poles, suggesting this region, designated Baja British Columbia, translated northward during Late Cretaceous - Paleogene time. Criticism of this interpretation based on other geological reasoning prompted me to perform new reviews of Cretaceous to Eocene paleomagnetic results from the Cordillera and from the major cratons of the globe. The global review follows the method of Besse and Courtillot (1991; 2002). One difference between our methods is that I compiled paleomagnetic results from highly studied rock units to single results to balance data weightings spatially and temporally, thus reducing the number of individual results. For the period 160 to 40 Ma, 51 poles were included compared to 92 poles by Besse and Courtillot (2002). Differences between apparent polar wander paths in their and my analyses are never significant at 95% confidence, however mean pole positions differ by up to 500 km, which is important for paleogeographic analysis. The global distribution of sampling localities and the tight clustering of the paleomagnetic poles after plate reconstruction provide invaluable confirmation of plate tectonically derived Euler rotations, the reliability of paleomagnetic remanence directions, and the geocentric dipole geometry of the geomagnetic field. My Cordilleran review shows that paleolatitudes derived from plutons and remagnetized rocks are significantly more scattered than those derived from bedded rocks. Using bedded rocks only, the paleomagnetic record shows that Baja British Columbia sat 2100 ± 500 km south of its present position with respect to cratonic North America during the

  4. Hydrothermal alteration at the Panorama Formation, North Pole Dome, Pilbara Craton, Western Australia

    CERN Document Server

    Brown, Adrian J; Walter, Malcolm R

    2014-01-01

    An airborne hyperspectral remote sensing dataset was obtained of the North Pole Dome region of the Pilbara Craton in October 2002. It has been analyzed for indications of hydrothermal minerals. Here we report on the identification and mapping of hydrothermal minerals in the 3.459 Ga Panorama Formation and surrounding strata. The spatial distribution of a pattern of subvertical pyrophyllite rich veins connected to a pyrophyllite rich palaeohorizontal layer is interpreted to represent the base of an acid-sulfate epithermal system that is unconformably overlain by the stromatolitic 3.42 Ga Strelley Pool Chert.

  5. Compositional variations in the Mesoarchean chromites of the Nuggihalli schist belt, Western Dharwar Craton (India)

    DEFF Research Database (Denmark)

    Mukherjee, Ria; Mondal, Sisir Kanti; Rosing, Minik Thorleif

    2010-01-01

    -98)) and pyroxene grains (Mg-numbers = 97-99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified...... has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination...

  6. Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton

    Science.gov (United States)

    He, Xiao-Fang; Santosh, M.; Bockmann, Kiara; Kelsey, David E.; Hand, Martin; Hu, Jianmin; Wan, Yusheng

    2016-10-01

    Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet-sillimanite-biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930-1940 Ma. The (U + Pb)-Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P-T conditions ca. 7-9 kbar and 775-825 °C, followed by decompression and evolution along a clockwise P-T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

  7. P and S Wave Finite-frequency Imaging of the Cordillera-craton Boundary Zone in Western Canada

    Science.gov (United States)

    Gu, Y. J.; Chen, Y.; Hung, S. H.

    2016-12-01

    The Western Canada Sedimentary Basin (WCSB) marks a boundary zone and a strong seismic lateral velocity gradient in the upper mantle between the Precambrian North American craton and the Phanerozoic Cordillera. While much of the tectonic imprints are buried below thick sediments, seismic data coverage in this region has been greatly improved to unravel the underlying structure, owing to a growing number of regional broadband arrays. In this study we conduct a high-resolution survey of the mantle P- and S-wave velocities in the WCSB using a multi-scale parameterization and the finite-frequency theory. Our models suggest respective increases of 4% and 6% of Vp and Vs velocities across the Cordilleran Deformation Front (CDF) toward the craton interior. This sharp gradient resides to the west of the CDF, which may imply the over-thrusting of the crustal terranes of the Cordillera onto the craton edge. Significant along-strike variation in the lithospheric basal geometry and dip could reflect secular tectonic modifications such as episodic shortening/extension and convective erosion. The results of our finite-frequency tomography indicate vertically continuous high velocities at least down to 200 km beneath the crustal domains of the Precambrian Buffalo Head Terrane (BHT), Hearne craton and Medicine Hat Block (MHB). The lithosphere beneath the southern Hearne province could extend down to 280 km, nearly 70 km deeper than those of its neighboring cratons of similar ages. The velocity anomalies and their intricate variations/reversals within the mantle lithosphere may be evidence for a multi-stage formation of western Laurentia during Precambrian plate convergence. On the other hand, destruction processes since formation, e.g., convective removal and thermomagmatic erosion, may also have played key roles in shaping the mantle lithosphere beneath the western margin of the North American craton.

  8. Les organismes du Précambrien terminal du craton ouest africain

    Directory of Open Access Journals (Sweden)

    E. Eboureau

    1983-12-01

    Full Text Available THE ORGANISMS OF THE TERMINAL PRECAMBRIAN OF THE WESTERN AFRICAN CRATON The terminal Precamhrian (Lipalian of the western African Craton, in the Adrar of Mauritania, has provided, especially in the Guelb er Richát, many organisms with an archaic structure, often very simple. It concerns some coccoides, often grouped in more or less complex spheroides according to an evolutionary pattern that could be accurately established. The understanding of fossil forms from the Precambrian is, to a great extent, the result of an interpretation made of an included ‘black spot', which one adopted for the emissions of substance produced by these forms. Compared to the present species, the Precambrian organisms recall the coccoid Bacteria and the Cyanophyceae. These microorganisms are oolithes in origin and are by no means mineral, since they contain stain-sensitive glucides with Schiff’s reagent following the action of periodic acid. These forms, usually spherical, are classified amongst the Oncolithes. These organisms are also responsible for the massive calcareous or dolomitic reefs reaching considerable thickness and containing some known stromatolites in many parts of the world. Stromatolites are Conophyton. Collenia . . . They are spherical, lamellate or columnar. These reef formations of Africa reach a thickness of 3 000 m in the Adoudounian of Collenia of the Anti-Atlas, of 600 m in the Zemmour, of 150 m in the Adrar of Mauritania, of 100 m in the cliff of Hank in the north of the Taoudeni Basin.

  9. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    Science.gov (United States)

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-03-04

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

  10. On the Origin of High Shear Wave Velocities in the Deep Roots of Cratons

    Science.gov (United States)

    Zeng, L.; Duncan, M. S.; Garber, J. M.; Hernandez, J. A.; Maurya, S.; Zhang, H.; Faul, U.; McCammon, C. A.; Montagner, J. P.; Moresi, L. N.; Romanowicz, B. A.; Rudnick, R. L.; Stixrude, L. P.

    2016-12-01

    Some seismic models derived from tomographic studies indicate very high shear wave velocities around 150 km depth, which cannot be explained by standard cratonic peridotite compositions derived from kimberlites, even under the assumption of very cold geotherms (i.e. 28mW/m3 surface heat flux). We present the results of a multi-disciplinary study conducted at the CIDER Summer 2016 program in Santa Barbara (CA), in which we have reviewed various geophysical and petrological constraints on the nature of cratonic roots (seismic velocities, electrical conductivity, gravity, lithologies) and explored a range of possible solutions. We find that matching the high shear wave velocities requires a large proportion of eclogite that is not matched by observed eclogite proportions in kimberlite samples. The high shear velocity of diamond makes it a viable candidate to account for such high velocities, in a proportion that is compatible with the global carbon budget. Our most recent results will be presented as well as suggestions for possible mechanisms for diamond formation and emplacement.

  11. Correlations between the North China Craton and the Indian Shield: Constraints from regional metallogeny

    Directory of Open Access Journals (Sweden)

    Caifeng Li

    2015-11-01

    Full Text Available The correlation between the North China Craton (NCC and the Indian Shield (IND has been a hot topic in recent years. On the basis of ore deposit databases, the NCC and IND have shown broad similarity in metallogenesis from the middle Archaean to the Mesoproterozoic. The two blocks both have three major metallogenic systems: (1 the Archaean BIF metallogenic system; (2 the Paleoproterozoic Cu-Pb-Zn metallogenic system; and (3 the Mesoproterozoic Fe-Pb-Zn system. In the north margin of the NCC and the west margin of the IND, the Archaean BIF-Au-Cu-Pb-Zn deposits had the same petrogenesis and host rocks, the Paleoproterozoic Cu-Pb-Zn deposits were controlled by active belts, and the Mesoproterozoic Fe-Pb-Zn deposits were mainly related to multi-stage rifting. Matching regional mineralization patterns and geological features has established the continental assembly referred to as “NCWI”, an acronym for the north margin of the NCC (NC and the west margin of the IND (WI during the middle Archaean to the Mesoproterozoic. In this assembly, the available geological and metallogenic data from the Eastern Block and active belts of NC fit those from the Dharwar craton and the Aravalli–Delhi–Vindhyan belt of WI, respectively. Moreover, the depositional model and environment of Paleoproterozoic metasedimentary manganese deposits in NCWI implied that the assembly may be located at low latitudes, where the conditions were favorable for dissolving ice and precipitating manganese deposits.

  12. Les organismes du Précambrien terminal du craton ouest africain

    Directory of Open Access Journals (Sweden)

    E. Eboureau

    1983-12-01

    Full Text Available THE ORGANISMS OF THE TERMINAL PRECAMBRIAN OF THE WESTERN AFRICAN CRATON The terminal Precamhrian (Lipalian of the western African Craton, in the Adrar of Mauritania, has provided, especially in the Guelb er Richát, many organisms with an archaic structure, often very simple. It concerns some coccoides, often grouped in more or less complex spheroides according to an evolutionary pattern that could be accurately established. The understanding of fossil forms from the Precambrian is, to a great extent, the result of an interpretation made of an included ‘black spot', which one adopted for the emissions of substance produced by these forms. Compared to the present species, the Precambrian organisms recall the coccoid Bacteria and the Cyanophyceae. These microorganisms are oolithes in origin and are by no means mineral, since they contain stain-sensitive glucides with Schiff’s reagent following the action of periodic acid. These forms, usually spherical, are classified amongst the Oncolithes. These organisms are also responsible for the massive calcareous or dolomitic reefs reaching considerable thickness and containing some known stromatolites in many parts of the world. Stromatolites are Conophyton. Collenia . . . They are spherical, lamellate or columnar. These reef formations of Africa reach a thickness of 3 000 m in the Adoudounian of Collenia of the Anti-Atlas, of 600 m in the Zemmour, of 150 m in the Adrar of Mauritania, of 100 m in the cliff of Hank in the north of the Taoudeni Basin.

  13. Organic matter in the Neoproterozoic cap carbonate from the Amazonian Craton, Brazil

    Science.gov (United States)

    Sousa Júnior, Gustavo R.; Nogueira, Afonso C. R.; Santos Neto, Eugênio V.; Moura, Candido A. V.; Araújo, Bruno Q.; Reis, Francisco de A. M.

    2016-12-01

    Bitumen found in Neoproterozoic carbonates from the southern Amazonian Craton, Brazil, represents a great challenge for its geochemical characterization (origin, thermal maturity and the degree of preservation) within a context of petroleum system. This organic material occurs in the basal Araras Group, considered as a Neoproterozoic cap carbonate, composed of dolostones (Mirassol d'Oeste Formation) overlaid by limestones and shales (Guia Formation). Geochemical analyses in samples of carbonate with bitumen from two open pits (Terconi and Tangará quarries) have shown low to very low total organic carbon content. Analyses of representative samples of Guia and Mirassol d'Oeste formations allowed us to obtain Gas chromatography (GC) traces and diagnostic biomarkers. n-C14 to n-C37 alkane distribution patterns in all samples suggests a major contribution of marine algae. Mid-chain monomethyl alkanes (C14sbnd C25) identified in both sets of samples were also reported in all mid to late Proterozoic oils and source rocks. However, there are significant differences among terpane distribution between the Mirassol d'Oeste and Tangará da Serra regions. The integration of organic geochemistry data and geological information suggests an indigenous origin for studied bitumen, primarily accumulated as hydrocarbon fluids migrated to carbonate rocks with higher porosity and permeability, and afterwards, altered to bitumen or migrabitumen. Although further investigations are required, this work provides a significant contribution to the knowledge about the remnant of this hypothetical Neoproterozoic petroleum system developed in the Southern Amazonian Craton.

  14. Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

    Science.gov (United States)

    Bauer, W.; Walsh, G.J.; De Waele, B.; Thomas, Ronald J.; Horstwood, M.S.A.; Bracciali, L.; Schofield, D.I.; Wollenberg, U.; Lidke, D.J.; Rasaona, I.T.; Rabarimanana, M.H.

    2011-01-01

    The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian

  15. The 3.1 Ga Nuggihalli chromite deposits, Western Dhawar craton (India)

    DEFF Research Database (Denmark)

    Mukherjee, Ria; Mondal, Sisir K.; Frei, Robert

    2012-01-01

    , therefore melting and mixing of the eclogite component with depleted mantle melts resulted in distinct HFSE enrichment in the Nuggihalli rocks. Alternatively, melting of a HFSE-enriched eclogitic slab and the surrounding depleted mantle within an active subduction zone is another possible mechanism, however......The Nuggihalli greenstone belt is part of the older greenstone belts (3.4 - 3.0 Ga) in the Western Dharwar Craton, southern India. This greenstone sequence consists of conformable metavolcanic and metasedimentary supracrustal rock assemblages that belong to the Sargur Group. Sill-like ultramafic......-mafic plutonic bodies are present within these supracrustal rocks (schist rocks) which are in turn enclosed by tonalite-trondhjemite-granodiorite gneiss (TTG). The sill-like ultramafic-mafic rocks are cumulates derived from a high-Mg parental magma that are represented by chromitite-hosted serpentinite...

  16. The Montesbelos mass-flow (southern Amazonian craton, Brazil): a Paleoproterozoic volcanic debris avalanche deposit?

    Science.gov (United States)

    Roverato, M.

    2016-07-01

    The present contribution documents the extremely well-preserved Paleoproterozoic architecture of the Montesbelos breccia (named here for the first time), which is interpreted as a rare example of a subaerial paleoproterozoic (>1.85 Ga) granular-dominated mass-flow deposit, few of which are recorded in the literature. Montesbelos deposit is part of the andesitic Sobreiro Formation located in the São Felix do Xingu region, southern Amazonian craton, northern Brazil. The large volume, high variability of textural features, presence of broken clasts, angular low sphericity fragments, mono- to heterolithic character, and the size of the outcrops point to a volcanic debris avalanche flow. Fluviatile sandy material and debris flows are associated with the deposit as a result of post-depositional reworking processes.

  17. Neotectonic activity and parity in movements of Udaipur block of the Arvalli Craton and Indian Plate

    Indian Academy of Sciences (India)

    Harsh Bhu; Ritesh Purohit; Joga Ram; Pankaj Sharma; S R Jakhar

    2014-03-01

    We report site motion of a permanent GPS site at Udaipur (udai), Rajasthan on the Udaipur block of Aravalli Craton. The GPS measurements of 2007–2011 suggest that the site moves at a rate of about 49 mm/year towards northeast. As the site motion is consistent with the predicted plate motion using the estimated euler pole of rotation for the Indian Plate, it implies that there is insignificant internal deformation/strain in the region. Such a deformation is consistent with very low seismic activity in the region. The epicenters of the infrequent low magnitude earthquakes are located on the Precambrian lineaments on the west of Udaipur Block, and on the NW–SE striking younger lineament in the south of the block.

  18. Discovery of the Early Mesozoic granulite xenoliths in North China Craton

    Institute of Scientific and Technical Information of China (English)

    邵济安[1; 韩庆军[2; 李惠民[3

    2000-01-01

    The discovery of the Early Mesozoic basic granulite xenoliths in the Harqin area of the Inner Mongolia Autonomous Region (for short Inner Mongolia) is reported for the first time in this paper. According to the mineral assemblage the xenoliths include two-pyroxene granulite, clinopyroxene granulite, and hypersthene granulite. Their protolytes are mainly gabbroite rocks. The zircon U-Pb age of the granulite xenoliths is 251 Ma, and K-Ar age of the hypersthene is 229 Ma. They represent the times of metamorphism and cooling of the granulite facies respectively. The host rock of the xenoliths is Early Mesozoic biotite-quartz diorites, whose whole-rock K-Ar age is 219 Ma. This discovery confirms existence of an Early Mesozoic underplating in the North China Craton, which is of much importance in research on the Early Mesozoic mantle-crust interaction in the concerned area.

  19. Strong crustal seismic anisotropy in the Kalahari Craton based on Receiver Functions

    DEFF Research Database (Denmark)

    Thybo, Hans; Soliman, Mohammad Youssof Ahmad; Artemieva, Irina

    2015-01-01

    Earlier seismic studies of the Kalahari Craton in southern Africa infer deformation of upper mantle by flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We present evidence for very strong seismic...... is uniform within tectonic units and parallel to orogenic strike in the Limpopo and Cape fold belts. It is further parallel to the strike of major dyke swarms which indicates that a large part of the observed anisotropy is controlled by lithosphere fabrics and macroscopic effects. The directions of the fast...... that the crust and lithospheric mantle may have been coupled since cratonisation. If so, the apparent match between mantle anisotropy and the present plate motion is coincidental....

  20. 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)

    2001-01-01

    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.

  1. Nitrogen isotope and content record of Mesozoic orogenic gold deposits surrounding the North China craton

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    As an effective tracer, nitrogen isotopes have been used to determine the source of ore materials in recent years. In this study, the nitrogen isotopes and contents were measured on K- feldspar and sericite of gold deposits and some related granitic intrusions in Jiaodong, Xiaoqinling-Xiong'ershan, west Qinling, the west part of North Qilian and the Zhangjiakou-Xuanhua district around the North China craton (NCC). Although the gold deposits around the NCC are hosted in Precambrian metamorphic rocks, Phanerozoic sedimentary rocks, mafic volcanic rocks or granite, comparison of which with the nitrogen contents and isotope data of previous studies on mantle-derived rocks, granites, metamorphic rocks and gold deposits indicates that those deposits are closely related to granitic rocks. In addition, mantle-derived materials may have been involved in the ore-forming processes to a certain degree. This conclusion is consistent with the result of previous hydrogen, oxygen and carbon isotopic studies of those gold deposits.

  2. Mid-lithosphere discontinuities beneath the western and central North China Craton

    Science.gov (United States)

    Sun, Weijia; Kennett, B. L. N.

    2017-02-01

    By analyzing P reflectivity extracted from stacked autocorrelograms for teleseismic events on a dense seismic profile, we obtain a detailed image of the mid-lithosphere discontinuity (MLD) beneath western and central North China Craton (NCC). This seismic daylight imaging exploits a broad high-frequency band (0.5-4 Hz) to reveal the fine-scale component of multi-scale lithospheric heterogeneity. The depth of the MLD beneath the western and central parts of the NCC ranges 80-120 km, with a good match to the transition to negative S velocity gradient with depth from Rayleigh wave tomography. The MLD inferred from seismic daylight imaging also has good correspondence with the transition from conductive to convective regimes estimated from heat flow data indicating likely thermal control within the seismological lithosphere.

  3. Strong crustal seismic anisotropy in the Kalahari Craton based on Receiver Functions

    DEFF Research Database (Denmark)

    Thybo, Hans; Soliman, Mohammad Youssof Ahmad; Artemieva, Irina

    2015-01-01

    Earlier seismic studies of the Kalahari Craton in southern Africa infer deformation of upper mantle by flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We present evidence for very strong seismic...... is uniform within tectonic units and parallel to orogenic strike in the Limpopo and Cape fold belts. It is further parallel to the strike of major dyke swarms which indicates that a large part of the observed anisotropy is controlled by lithosphere fabrics and macroscopic effects. The directions of the fast...... that the crust and lithospheric mantle may have been coupled since cratonisation. If so, the apparent match between mantle anisotropy and the present plate motion is coincidental....

  4. High frequency peritidal cycles of the upper Araras Group: Implications for disappearance of the neoproterozoic carbonate platform in southern Amazon Craton

    Science.gov (United States)

    Rudnitzki, Isaac Daniel; Romero, Guilherme Raffaeli; Hidalgo, Renata; Nogueira, Afonso Cesar Rodrigues

    2016-01-01

    The Araras Group is an extensive carbonate platform developed at the southeastern margin of the Amazon Craton during the Neoproterozoic. The Nobres Formation corresponds to the upper unit of the Neoproterozoic Araras Group. It is exposed in road cuts and quarries in the Northern Paraguay Belt, and is characterized by meter-scale shallowing upward cycles. Forty-four fourth-to fifth-order parasequence cycles are enclosed into three third order sequences/megacycles, unconformably overlain by siliciclastic deposits of the Alto Paraguay Group. The cycles are generally of peritidal type, limited by exposure surfaces composed of asymmetrical tidal flat/sabkha lithofacies in the basal Nobres Formation. They consist of fine dolostone, intraclastic dolostones with megaripples, stromatolites biostrome, sandy dolostone with enterolithic structures and silicified evaporite molds. Upsection, the cycles progressively become symmetrical, comprising arid tidal flat deposits with abundant stromatolite biostrome, fine-grained sandstone and rare evaporitic molds. The stacking patterns for hundreds of meters indicate continuous and recurrent generation of accommodation space, probably triggered by subsidence concomitant with relative sea-level changes. Palynomorphs found in the upper part of Nobres Formation comprehend spheroidal forms, such as Leiospharidia, rare filamentous and acanthomorphous acritarchs, mostly Tanarium correlated to the Ediacaran Complex Acantomorph Palynoflora of ˜580-570 Ma. Previous data of carbon isotopes and paleogeographic reconstructions, and also the presence of evaporites and storm-influenced deposits in the Araras Group, suggest a wet to tropical setting for Amazonia during the Mid-Ediacaran, which is incompatible with previous claims for Gaskiers-related glacial sedimentation in the region. During the final stages of evolution of the Araras carbonate platform, a progressive input of terrigenous has occurred in the peritidal setting likely due tectonic

  5. Bi-directional subduction of the South Tianshan Ocean during the Late Silurian: Magmatic records from both the southern Central Tianshan Block and northern Tarim Craton

    Science.gov (United States)

    Wang, Meng; Zhang, Jinjiang; Zhang, Bo; Liu, Kai; Ge, Maohui

    2016-10-01

    The subduction polarity of the South Tianshan Ocean (STO) is a matter of debate, primarily in that the Paleozoic structures of Tianshan orogenic belt have been strongly overprinted by the Cenozoic intra-continental deformation. Indentifying the arc-related magmatic rocks may provide a convincible clue for understanding the closure process of the STO. In this study, whole-rock geochemistry, zircon U-Pb dating and Hf isotope were presented on the andesite and monzonite from the Bayanbulak area of the southern Central Tianshan Block (CTB) and on the quartz diorite from the Ouxidaban area of the northern Tarim Craton. Geochemically, all the samples are Na-rich, enriched in light rare earth elements and large ion lithophile elements (Rb, Ba, U, K and Pb), and depleted in high strength field elements (Nb, Ta and Ti), like most arc-type igneous rocks. The Bayanbulak andesite samples display high MgO, Fe2O3T, TiO2 and Mg# values, and positive εHf(t) values, indicating magma source from the wedge mantle. But the existence of xenocrystic zircons implies that continental crust material were involved during magma ascend, suggesting a continental arc setting for the Bayanbulak andesite. The Bayanbulak monzonite and the Ouxidaban quartz diorite samples display relatively higher SiO2 contents, and lower MgO, Fe2O3T and TiO2 concentrations, indicating crustal sources. But the Mg# values of the Bayanbulak monzonite and the Ouxidaban quartz diorite are 48.76-51.85 and 50.31-53.73, and the εHf(t) values are -2.5 to 8.7 and -1.7 to 4.1, indicating that their magma sources were also mixed by mantle-derived components. LA-ICP-MS zircon U-Pb dating results reveal that the Bayanbulak andesite, the Bayanbulak monzonite and the Ouxidaban quartz diorite were formed at 423, 424 Ma, and 421 Ma, respectively. The age and geochemical data indicate that both the southern CTB and northern Tarim Craton were active continental margins during the Late Silurian, favoring a bi-directional subduction

  6. The Archean kalsilite-nepheline syenites of the Awsard intrusive massif (Reguibat Shield, West African Craton, Morocco) and its relationship to the alkaline magmatism of Africa

    Science.gov (United States)

    Haissen, Faouziya; Cambeses, Aitor; Montero, Pilar; Bea, Fernando; Dilek, Yildirim; Mouttaqi, Abdellah

    2017-03-01

    More than 40% of the known alkaline complexes are reported from Africa. Most are ring complexes composed of syenites and associated or not, lithotypes as carbonatites, granites and mafic rocks. Radiometric dating indicates the presence of alkaline complexes with ages spanning from Precambrian to the present. In terms of outcrops, alkaline complexes are reported from cratonic zones and from belts embedded between cratonic areas. Because of the high economic potential for associated REE deposits, these alkaline complexes have received much attention from Earth scientists. These studies aim mainly to constrain the role of the mantle and the crust (and the interaction between them) in the genesis of this peculiar magmatism, and also to explain the variability observed in lithotypes and geotectonic settings. Among those alkaline complexes, Precambrian occurrences are rare. Up-to-date only a few Proterozoic examples were cited in Africa. The recently studied Awsard complex in Southern Morocco is a peculiar one with a crystallization age of 2.46 Ga and an unusual rock assemblages. This paper is a first approximation to a comparison of geochemical and isotopic fingerprints of the Awsard magmatism (as the oldest one) with other known different ages African complexes from different geotectonic settings, aiming to detect if there is any evolution in this alkaline magmatism through time. A first conclusion is that magma sources for this alkaline magmatism has been probably evaluating over geological time, from parental magmas compositions close to that of primitive mantle in these early geological time to compositions holding more and more depleted mantle and continental crust components. However, to go further in this debate more modern isotopic, geochemical and geochronological data from all these complexes are needed. Nevertheless, this comparison highlighted the peculiar character of the Awsard magmatism with an isotopic composition very close to that of Primitive mantle

  7. Contrasting geochemistry of the Cretaceous volcanic suites in Shandong province and its implications for the Mesozoic lower crust delamination in the eastern North China craton

    Science.gov (United States)

    Ling, Wen-Li; Duan, Rui-Chun; Xie, Xian-Jun; Zhang, Yong-Qing; Zhang, Jun-Bo; Cheng, Jian-Ping; Liu, Xiao-Ming; Yang, Hong-Mei

    2009-12-01

    The Qingshan volcanic sequences occur in the Mengyin and Jiaozhou basins, west and east of the Tan-Lu fault zone, respectively, were formed at 128 ± 2 and 106 - 98 Ma. Lithologically the Mengyin succession comprises unimodal andesite (53 - 65 wt.% SiO 2), whereas the Jiaozhou succession consists of bimodal andesite (56-59 wt.% SiO 2) and rhyolite (69-77 wt.% SiO 2). High-Mg (Mg# >60) and low-Mg (Mg# andesitic suites are recognized in the Mengyin volcanic rocks. They show tholeiitic and calc-alkaline trends, respectively. Though the two suites share common trace element features of LILE enrichment and HFSE depletion, higher incompatible element contents in the high-Mg andesite relative to the low-Mg andesite rule out their genetic connection by magmatic differentiation. Similarly, the Jiaozhou andesites also possess higher incompatible element contents compared to their interbedded rhyolites, suggesting their distinct source rocks. The Qingshan mafic volcanic suites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions, of which the Mengyin high-Mg andesitic rocks display the lowest ɛ Nd(t) and highest initial Sr ratio. By contrast, the Jiaozhou rhyolites possess a positive correlation between Sr-Nd isotopes and relatively unradiogenic Sr isotopic ratios. The Qingshan mafic rocks exhibit geochemical characteristics distinct from those of mantle peridotite- or pyroxenite-derived melt as well as from Fangcheng basalts, the only basalts documented in the Cretaceous volcanic suites in Shandong province. They are inferred to be crust-derived melts. However, a clear decreasing trend in ACNK with Mg# increasing and the elevated MgO, Cr and Ni contents relative to basalt-derived melts indicate assimilation with mantle peridotite during their pristine magmatic evolution. Such a two-step process can be best explained by the Archean lower crust delamination of the North China craton with a ~35% partial melting followed by a variable extent of

  8. Paleo-Mesoproterozoic arc-accretion along the southwestern margin of the Amazonian craton: The Juruena accretionary orogen and possible implications for Columbia supercontinent

    Science.gov (United States)

    Scandolara, J. E.; Correa, R. T.; Fuck, R. A.; Souza, V. S.; Rodrigues, J. B.; Ribeiro, P. S. E.; Frasca, A. A. S.; Saboia, A. M.; Lacerda Filho, J. V.

    2017-01-01

    peak of the Rio Negro-Juruena orogeny and the original structural architecture of the orogen. The main structures of the Rio Negro-Juruena orogeny were almost completely obliterated between 1.42 and 1.37 Ga, due to the superimposed Rondonian-San Ignácio orogeny. Later on, between 1.20 and 1.12 Ga, these structures were affected by a ductile event represented by the large Ji-Paraná transpressive shear system and by the Tarilândia transcurrent shear zone developed under low-T amphibolite facies conditions, both related to the Sunsás orogeny. The evolution of Jamari and Juruena terranes as a Paleoproterozoic/Mesoproterozoic orogen developed between 1.81 and 1.51 Ga and their thorough reworking in Mesoproterozoic times is instrumental to establishing the position of the Amazonian craton in the proposed Paleo-Mesoproterozoic Columbia (Nuna) supercontinent, as well as in the Rodinia reconstruction at the end of the Mesoproterozoic.

  9. Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities

    Science.gov (United States)

    Snyder, David B.

    2008-08-01

    A 20-station seismic array in NW Canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the Mohorovicic and Lehman discontinuities. Following wavefield decomposition, groups of seismograms were source-normalized through simultaneous deconvolution to estimate the near-receiver impulse response and thus detect major discontinuities beneath each seismic station. Stations within the Lac de Gras kimberlite field display an unusually strong negative impulse on the radial component within the NW quadrant and two moderate impulses on the transverse component. Forward modeling of these impulses suggests a mantle layer dipping at 22° to the southeast with a mildly anisotropic (2%) upper discontinuity at 120-135 km depth and another mildly anisotropic (2%) discontinuity at about 170 km depth. Superimposed on these layers is another, stronger anisotropic (4%) layer between 110 and 180 km depths that dips to the west. Stations outside of the Lac de Gras field, but within the southeastern Slave craton, display more numerous, but weaker, impulses. The most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north. The prominent negative impulse observed on the radial component is interpreted to arise from structural-preferred orientation in the form of a stockwork of wehrlite dykes beneath the Lac de Gras field. Interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000-2900 Ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 Ma. The layer beneath the SE Slave craton has a similar, but distinct, tectonic history of NW-verging underthrusting associated with the 2635-2615 Ma Defeat Suite of

  10. Mineral chemistry of paleoproterozoic volcano-plutonism from the São Félix do Xingu (PA region, Amazonian Craton

    Directory of Open Access Journals (Sweden)

    Raquel Souza da Cruz

    2014-03-01

    Full Text Available The Sobreiro and Santa Rosa formations are result of large paleoproterozoic volcanic activities in the São Félix do Xingu (PA region, SE of Amazonian Craton. The Sobreiro Formation is composed of andesitic lava flow facies, with subordinate dacite and rhyodacite, and autoclastic volcaniclastic facies characterized by tuff, lapilli-tuff, and massive polymictic breccia. These rocks exhibit clinopyroxene, amphibole, and plagioclase phenocrysts in a microlytic or trachytic groundmass. The clinopyroxene is augite with subordinate diopside, and reveals chemical variations typical of minerals generated by arc-related magmatism. The amphibole is magnesiohastingsite, has oscillatory oxidizing conditions, and reveals breakdown rim textures linked to degassing during magma ascent. The Santa Rosa Formation has polyphase evolution controlled by large NE-SW crustal fissures, materialized by rhyolitic and dacitic lava flow facies. Volcaniclastic facies of ignimbrites, lapilli-tuffs, felsic crystal tuffs, and massive polymict breccias represents an explosive cycle in this unit. Metric dikes and stocks of granitic porphyries and equigranular granitoids complete this suite. K-feldspar, plagioclase, and quartz phenocrysts surrounded by quartz and K-feldspar integrowth occur in these rocks. Electron microprobe pressure and temperature estimates in clinopyroxene phenocrysts reveal formation depth between 58 and 17.5 km (17.5 – 4.5 kbar at temperatures between 1,249 and 1,082 ºC; and between 28 and 15 km (7.8 – 4.1 kbar for amphibole grains of the Sobreiro Formation, suggesting polybaric evolution. A model with generation of hydrated basaltic magma from partial melting of mantle wedge and accumulation in a hot zone of the lower crust, from which the andesitic and dacitic magmas are formed by the assimilation of continental crust and following fractional crystallization is proposed.

  11. Sm-Nd age dating of high- pressure granulites and amphi- bolite from Sanggan area, North China craton

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856 ± 26) Ma for HP amphibolite. The Sm-Nd whole rock isochron of HP granulites give out an age of (1 870±150) Ma with Nd deplet-ed mantle model age of (2 402-2 482) Ma. Considering the Nd isotope homogenization during the peak metamorphism of the HP granulite, Sm-Nd closure temperature and the retention of Nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we pro-posed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assem-bly of the North China craton.

  12. Heat flow, heat generation and crustal thermal structure of the northern block of the South Indian Craton

    Science.gov (United States)

    Gupta, Mohan L.; Sharma, S. R.; Sundar, A.

    1988-01-01

    Heat flow values and heat generation data calculated from the concentration of heat producing radioactive elements, U, Th and K in surface rocks were analyzed. The South Indian Craton according to Drury et al., can be divided into various blocks, separated by late Proterozoic shear belts. The northern block comprises Eastern and Western Dharwar Cratons of Rogers (1986), Naqvi and Rogers (1987) and a part of the South Indian granulite terrain up to a shear system occupying the Palghat-Cauvery low lands. The geothermal data analysis clearly demonstrates that the present thermal characteristics of the above two Archaean terrains of the Indian and Australian Shields are quite similar. Their crustal thermal structures are likely to be similar also.

  13. The Zimbabwe Craton in Mozambique: A brief review of its geochronological pattern and its relation to the Mozambique Belt

    Science.gov (United States)

    Chaúque, F. R.; Cordani, U. G.; Jamal, D. L.; Onoe, A. T.

    2017-05-01

    The eastern margin of the Zimbabwe Craton, along the Mozambique-Zimbabwe border, includes the oldest rocks of west-central Mozambique constituting a large terrain of granite-greenstone type dated between 3000 and 2500 Ma. These rocks consist mainly of gneisses and granitoid rocks of tonalitic-trondhjemitic-granodioritic composition belonging to the Mudzi Metamorphic Complex in the northern part and to the Mavonde Complex in the southern part. The latter is associated with a granite-greenstone terrain, which includes the eastern part of Mutare-Odzi-Manica greenstone belt. A volcano-sedimentary sequences cover, belonging to the apparently Mesoproterozoic and Paleoproterozoic Umkondo and Gairezi groups respectively was deposited along the eastern margin of the craton and is exposed in the territory of Mozambique. The Umkondo minimum age is marked by intrusive dolerite in Zimbabwe dated at 1100 Ma while for the Ghairezi it is still not well established. The Gairezi Group was subjected to progressive metamorphism of Pan-African age. At the margin of the Zimbabwe Craton, in its northern part, metasedimentary units occur representing a passive margin of Neoproterozoic age. They make up the Rushinga Group, which includes felsic metavolcanic rocks dated at ca.800 Ma. Granulites and medium- to high-grade paragneisses, and migmatites of the Chimoio, Macossa and Mungari Groups of Neoproterozoic metamorphic age, overly the ortho-metamorphic pre-existing rock of ca. 1100 Ma, which belongs to the Báruè Magmatic Arc. They characterize the N-S trend Mozambique Belt, which appears to the east of the craton tectonically juxtaposed on the Archean rocks. The maximum age of deposition of these rocks, indicated by U-Pb dating of detrital zircons, is ca. 700 Ma and their minimum age is limited by a few monzonitic Cambrian intrusions dated at ca. 500 Ma. The Neoproterozoic bimodal Guro Suite, dated at ca. 850 Ma and composed of felsic and mafic members characterizes the east

  14. Crustal uplift of the Precambrian cratons due to metamorphism in crustal rocks under infiltration of mantle fluids

    Science.gov (United States)

    Artyushkov, Eugene; Chekhovich, Peter; Korikovsky, Sergey; Massonne, Hans-Joachim

    2017-04-01

    Precambrian cratons cover about 70% of the total area of the continents. During the last several million years cratonic areas underwent rapid uplift, from 100-200 m in East Europe to 1000-1500 m Southern Africa. Shortening of the Precambrian crust terminated half a billion years ago or earlier and this popular mechanism cannot be applied to its recent uplift. Large thickness of cratonic mantle lithosphere, 100-200 km in most regions, together with its low density precludes delamination of this layer and magmatic underplating as possible causes of recent uplift. It cannot be precluded that in some cratonic regions recent uplift occurred due to delamination of the lower part of mantle lithosphere with the density increased by metasomatism. Even a small uplift of ≥ 100-200 m would require delamination of a thick layer of mantle lithosphere. As a result a temperature drop of > 200 C would arise at the base of the lithosphere producing a shear wave velocities drop of > 2%. According to the seismic tomography data such a drop in VS is observed only in some regions with the Precambrian lithosphere, e.g., in Northeastern Africa. Spatial distribution of the Precambrian cratons is quite different from that predicted by the main models of dynamic topography in the mantle. Moreover, many uplifted blocks are bounded by steep slopes hundreds of meters to one kilometer high and only tens of kilometers wide. Such slopes could not have been formed by bending of thick cratonic lithosphere under the forces acting from below. Their recent formation indicates rock expansion within the crust at shallow depth comparable with the slope width. Rocks formed at the pressure P ˜ 0.5-1.0 GPa are widespread on the Precambrian cratons. This indicates that during their lifetime a layer of rocks ˜ 15-30 km thick has been removed from the crustal surface by denudation. As a result rocks which were initially located in the lower crust emerged to the middle or upper crust. Due to metamorphic

  15. Evolution of the West Siberian Basin

    Energy Technology Data Exchange (ETDEWEB)

    Vyssotski, A.V. [Chevron, 1500 Louisiana Street, Houston (United States); Vyssotski, V.N. [TNK-BP, 1 Arbat St, Moscow 119019 (Russian Federation); Nezhdanov, A.A. [OOO TyumenNIIgiprogas, 2 Vorovskogo Str., Tyumen 625019 (Russian Federation)

    2006-01-01

    The West Siberian Basin is one of the largest intra-cratonic basins of the world and an important hydrocarbon province of Russia. Perhaps the most important geologic event in Siberia was the emplacement of basalts around {approx} 250Ma (i.e. Permo-Triassic boundary) covering an area of about 5x10{sup 6} km{sup 2}. This volcanism may be responsible for a mass extinction that occurred around Permian-Triassic time. The pre-basaltic rifting event was limited to the north-northeastern sector of the basin. Initial basin wide subsidence took place in the Jurassic as a result of which the western part of Siberia became the West Siberian Basin bounded by uplifts to the east and to the west. One of the surprising aspects of the West Siberian Basin is the abundance of sub-vertical faults believed to be result of strike-slip movement. While intra-plate inversions and fault reactivation structures have been observed in many cratons, sub-vertical faults observed in the West Siberian Basin are unique because of their geometries and abundance. The differentiation between the effects of tectonics and eustasy in cratonic basins is simple-the global eustatic signal is basin-wide with regional and local tectonics playing an overprinting role. Thus, the Middle Jurassic-Turonian 1st, 2nd, and 3rd order cycles in the West Siberian Basin were primarily driven by eustasy. The Middle Jurassic-Turonian series can be subdivided into two second-order and 16 third-order transgressive-regressive cycles (within dataset extent). Fourth-order cycles appear to be controlled by delta shifting. Although extensively studied, a number of fundamental questions regarding the origin and evolution of the West Siberian Basin remain unresolved or poorly documented in the literature. [Author].

  16. Tectonic setting of the Helong Block: Implications for the northern boundary of the eastern North China Craton

    Institute of Scientific and Technical Information of China (English)

    ZHANG Yanbin; WU Fuyuan; ZHAI Mingguo; LU Xiaoping

    2005-01-01

    LA-ICP-MS zircon U-Pb dating indicates that the Bailiping granitic pluton was emplaced during Late Paleozoic to Early Cretaceous (285―116 Ma) by four distinct magmatic episodes, arguing against the previous thought that the Bailiping granites were emplaced during Archean, Proterozoic or Early Paleozoic. It is suggested that the so-called Archean Jinchengdong granite-greenstone belt, mainly composed of Bailiping granites, should be broken down. Sr-Nd isotopic compositions of the Bailiping granites are similar to those of the Phanerozoic granites in the Xing'an-Mongolian orogenic belt, suggesting that their parental magmas were mainly derived from a juvenile crust with some contamination of ancient crustal materials during magma rising and/or emplacement. Therefore, the basement of the Helong Block is similar to that in the northern Xing'an-Mongolian Orogenic Belt, but different from that of the North China Craton. It is proposed that the Archean Jinchengdong complex in this area is probably an exotic slice from the North China Craton, which indicates that the Fu'erhe-Gudonghe Fault, located in northern part of the Helong area, is not the boundary between the North China Craton and the Xing'an-Mongolian Orogenic Belt. The possible boundary should be located further southward more than 50 km and near the Baitoushan volcano.

  17. The new occurrence of Marinoan cap carbonate in Brazil: The expansion of snowball Earth events to the southwesternmost Amazon Craton

    Science.gov (United States)

    Gaia, Valber do Carmo de Souza; Nogueira, Afonso César Rodrigues; Domingos, Fábio Henrique Garcia; Sans-Jofre, Pierre; Bandeira, José Cavalcante da Silva; Oliveira, José Guilherme Ferreira de; Sial, Alcides Nóbrega

    2017-07-01

    Carbonate deposits exposed in the border of the Pimenta Bueno and Colorado grabens, western part of Parecis Basin, southwestern Amazon Craton, Brazil, have been previously considered as Paleozoic record. These deposits lying unconformably on Mesoproterozoic crystalline rocks, the basement of the grabens, and consist predominantly by pinkish dolomite overlying glacial diamictites, with average negative values of δ13C of -3,10‰VPDB. The contact between the dolostone and diamictites is sharp and deformed similarly with others Neoproterozoic cap carbonates occurrences in the Amazon Craton, also related to the Marinoan Glaciation (635 Ma). This new occurrence of Marinoan cap carbonate is composed by two facies associations. Facies Association 1 consists of pinkish peloidal dolostone with even parallel and quasi-planar laminations, wavy and megarriple bedding, macropeloid lenses associated with low-angle truncations, interpreted as fairwhether- and storm-influenced shallow platform deposits. Facies association 2 consists in dolostone rhythmically interbedded with shale underlaid by 5 m-thick laminated siltstones, interpreted as moderately deep platform deposits. This retrogradational succession is overlaid in angular unconformity by Early Paleozoic diamictites and locally by Mesozoic volcanic rocks. This cap carbonate precedes the Paleozoic deposits of Parecis Basin and represents a post-glacial event linked to the Marinoan glaciation, extending to the southwesternmost Amazon Craton the phenomena of the Snowball Earth hypothesis.

  18. Emplacement and deformation of the A-type Madeira granite (Amazonian Craton, Brazil)

    Science.gov (United States)

    Siachoque, Astrid; Salazar, Carlos Alejandro; Trindade, Ricardo

    2017-04-01

    The Madeira granite is one of the Paleoproterozoic (1.82 Ga) A-type granite intrusions in the Amazonian Craton. It is elongated in the NE-SW direction and is composed of four facies. Classical structural techniques and the anisotropy of magnetic susceptibility (AMS) method were applied to the study of its internal fabric. Magnetic susceptibility measurements, thermomagnetic curves, remanent coercivity spectra, optical microscopy and SEM (scanning electron microscopy) analyses were carried out on the earlier and later facies of the Madeira granite: the rapakivi granite (RG) and the albite granite (AG) respectively. The last one is subdivided into the border albite granite (BAG) and the core albite granite (CAG) subfacies. AMS fabric pattern is controlled by pure magnetite in all facies, despite significant amounts of hematite in the BAG subfacies. Microstructural observations show that in almost all sites, magnetic fabric correlates to magmatic state fabrics that are defined by a weak NE-SW orientation of mafic and felsic silicates. However, strain mechanisms in both subfacies of AG also exhibit evidence for solid-state deformation at high to moderate temperatures. Pegmatite dyke, strike slip fault (SFA-B-C), hydrothermal vein, normal fault (F1-2) and joint (J) structures were observed and their orientation and kinematics is consistent with the magmatic and solid-state structures. Dykes, SFA-C and F1, are usually orientated along the N70°E/40°N plane, which is nearly parallel to the strike of AMS and magmatic foliations. In contrast, veins, SFB, F2 and some J are oriented perpendicular to the N70°E trend. Kinematic analysis in these structures shows evidence for a dextral sense of movement in the system in the brittle regime. The coherent structural pattern for the three facies of Madeira granite suggests that the different facies form a nested pluton. The coherence in orientation and kinematics from magmatic to high-temperature solid-state, and into the brittle

  19. A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

    Science.gov (United States)

    Ikenne, Moha; Souhassou, Mustapha; Arai, Shoji; Soulaimani, Abderrahmane

    2017-03-01

    Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760-700 Ma), syn-collisional Bou-Azzer magmatism (680-640 Ma), followed by widespread Ediacaran magmatism (620-555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma. Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330-300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins. Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in

  20. The thermal structure of cratonic lithosphere from global Rayleigh wave attenuation

    Science.gov (United States)

    Dalton, Colleen A.; Bao, Xueyang; Ma, Zhitu

    2017-01-01

    The resolution of and level of agreement between different attenuation models have historically been limited by complexities associated with extracting attenuation from seismic-wave amplitudes, which are also affected by the source, the receiver, and propagation through velocity heterogeneities. For intermediate- and long-period Rayleigh waves, removing the amplitude signal due to focusing and defocusing effects is the greatest challenge. In this paper, three independent data sets of fundamental-mode Rayleigh wave amplitude are analyzed to investigate how three factors contribute to discrepancies between the attenuation models: uncertainties in the amplitude measurements themselves, variable path coverage, and the treatment of focusing effects. Regionalized pure-path and fully two-dimensional attenuation models are derived and compared. The approach for determining attenuation models from real data is guided by an analysis of amplitudes measured from synthetic spectral-element waveforms, for which the input Earth model is perfectly known. The results show that differences in the amplitude measurements introduce only very minor differences between the attenuation models; path coverage and the removal of focusing effects are more important. The pure-path attenuation values exhibit a clear dependence on tectonic region at shorter periods that disappears at long periods, in agreement with pure-path phase-velocity results obtained by inverting Rayleigh wave phase delays. The 2-D attenuation maps are highly correlated with each other to spherical-harmonic degree 16 and can resolve smaller features than the previous generation of global attenuation models. Anomalously low attenuation is nearly perfectly associated with continental cratons. Variations in lithospheric thickness are determined by forward modeling the global attenuation variations as a thermal boundary layer of variable thickness. Temperature profiles that satisfy the attenuation values systematically

  1. Magnesium isotopic heterogeneity across the cratonic lithosphere in eastern China and its origins

    Science.gov (United States)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Ke, Shan; Liu, Yi-Can; Li, Shu-Guang

    2016-10-01

    Available data in the literature have demonstrated a broad magnesium (Mg) isotope range for mantle and lower continental crustal rocks, implying an isotopically heterogeneous continental lithosphere, but its origin has not been thoroughly understood. Here, to investigate the primary cause of lithospheric Mg isotopic heterogeneity, we report major-trace elements, Sr and Mg isotope data for thirty deep-seated mafic xenoliths, which sampled different lithospheric depths in the southeastern North China Craton (NCC). The xenoliths are classified into three types based upon petrology and mineralogy, sampling from middle continental crust (Group I), lower continental crust (Group II) and lithospheric mantle (Group III), respectively. The Group I xenoliths have mantle-like to slightly high δ26Mg values (- 0.32 ‰ to + 0.01 ‰), whereas some of the Group II xenoliths have very low δ26Mg values (- 0.93 ‰ to - 0.07 ‰), reflecting substantial reaction with intracrustal carbonate-derived fluids. Combined with data in the literature, the results suggest that the Mg isotopic composition of the lower continental crust is much more heterogeneous and lighter on average relative to the middle continental crust. Except for one sample, the Group III xenoliths have extremely low δ26Mg values (- 1.23 ‰ to - 0.73 ‰), the lightest among values already reported for mantle-derived rocks including peridotites and basalts. They also have highly variable 87Sr/86Sr ratios, of 0.70387 to 0.71675. The covariation of Mg and Sr isotopes in Group III xenoliths can be explained by Mg and Sr isotopic exchange reactions during mantle metasomatism, implying that the sub-continental mantle has been significantly modified by fluids derived from recycled carbonate-pelite bearing oceanic crust. Together with the metasomatism age of ∼400 Ma obtained for one Group III xenolith, the results provide new evidence for the presence of extremely low-δ26Mg rocks in the lithosphere and indicate ancient

  2. Synsedimentary deformation and the paleoseismic record in Marinoan cap carbonate of the southern Amazon Craton, Brazil

    Science.gov (United States)

    Soares, Joelson Lima; Nogueira, Afonso César Rodrigues; Domingos, Fábio; Riccomini, Claudio

    2013-12-01

    Event Layers in Neoproterozoic cap carbonates of Brazil's southwestern Amazon Craton record post-Marinoan synsedimentary seismicity. The 35 m-thick cap carbonates overlie glaciogenic sediments related to the Marinoan glaciation (635 Ma) and are comprised of two units: the lower cap consists of dolomite (˜15 m thick) and the upper cap is limestone (˜25 m thick). The cap dolomite includes pinkish crystalline dolostone with even parallel lamination, stratiform stromatolites, eventual tube structures and megaripple bedded peloidal dolostone interpreted as shallow (euphotic) platform deposits. The cap limestone onlaps the cap dolomite and consists of red marl, gray to black bituminous lime mudstone, bituminous shale with abundant calcite crystal fans (pseudomorphs after aragonite) and even parallel lamination interpreted as moderately deep to deep platform deposits. Five successive events of synsedimentary deformation were recognized in the cap carbonates exposed at Mirassol d'Oeste and Tangará da Serra, in Central Brazil: Event 1 - large to small-scale load cast structures in the contact between dolostones and glaciogenic sediments; Event 2 - stromatolitic lamination truncated by tube structures; Event 3 - vertical to subvertical fractures and faults, and large-scale synclines and anticlines with chevron folds; Event 4 - conglomerate and breccia filling neptunian dykes limited by undeformed beds; and Event 5 - slump and sliding deposits found only in the upper part of the cap limestone. Event 1 was produced by hydroplastic dynamics likely induced by isostatic rebound during ice cap melting in the final stages of the Marinoan glaciation. Events 2 and 5 are autocyclic in nature, and related to depositional processes. Event 2 is linked to fluid and methane escape from organic degradation of microbial mats and domes that formed tubestones; Event 5 is associated to collapse and sliding/slumping in the platform and slope. The reliable orientations of synsedimentary faults

  3. Petrography and geochronology (U/Pb-Sm/Nd) the Passagem Granite, Pensamiento Granitoid Complex, Paragua Terrane, SW Amazon Craton, Mato Grosso, Brazil; Petrologia e geocronologia (U/Pb-Sm/Nd) do Granito Passagem, Complexo Granitoide Pensamiento, SW do Craton Amazonico (MT)

    Energy Technology Data Exchange (ETDEWEB)

    Jesus, Gisely Carmo de, E-mail: giselycarmo@hotmail.co [Universidade Federal de Mato Grosso (ICET/UFMT), Cuiaba, MT (Brazil). Inst. de Ciencias Exatas e da Terra. Programa de Pos-Graduacao em Geociencias; Sousa, Maria Zelia Aguiar de, E-mail: mzaguiar@terra.com.b [Universidade Federal de Mato Grosso(ICET/UFMT), Cuiaba, MT (Brazil). Inst. de Ciencias Exatas e da Terra. Dept. de Recursos Minerais; Ruiz, Amarildo Salina; Matos, Joao Batista de, E-mail: asruiz@gmail.co, E-mail: jmatos@cpd.ufmt.b [Universidade Federal de Mato Grosso (ICET/UFMT), Cuiaba, MT (Brazil). Inst. de Ciencias Exatas e da Terra. Dept. de Geologia Geral

    2010-09-15

    The Passagem granite includes stocks, plugs and dikes located in the Ricardo Franco hill - Vila Bela da Santissima Trindade region - state of Mato Grosso, central Brazil. The Passagem Granite is included in the Paragua terrane - SW Amazonian Craton. It consists of isotropic monzogranite, sienogranite and more rarely granodiorites with leucocratic dark gray to white color. These rocks range from hypidomorphic inequigranular to xenomorphic texture, fine to medium grained. Biotite is the only primary mafic present as essential phase and characterize an expanded slightly acid sequence formed by a sub-alkaline magmatism of high-potassium calc-alkaline, slightly peraluminous composition from arc magmatic tectonic environment during a post-collisional period. Mechanism of fractional crystallization of plagioclase, biotite, titanite, apatite and zircon associated with simultaneous crustal assimilation are suggested for the evolution of these rocks. The results support the hypothesis of a post-collisional magmatism in the Paragua terrane at 1284 +- 20 Ma corresponding to the crystallization age of the Passagem granite. This paper propose that Passagem Granite represents as an extension in Brazilian terrane of the Pensamiento Granitoid Complex. (author)

  4. The Guerrero suspect terrane (western Mexico) and coeval arc terranes (the Greater Antilles and the Western Cordillera of Colombia): a late Mesozoic intra-oceanic arc accreted to cratonal America during the Cretaceous

    Science.gov (United States)

    Tardy, M.; Lapierre, H.; Freydier, C.; Coulon, C.; Gill, J.-B.; de Lepinay, B. Mercier; Beck, C.; Martinez R., J.; O. Talavera, M.; E. Ortiz, H.; Stein, G.; Bourdier, J.-L.; Yta, M.

    1994-02-01

    The Guerrero suspect terrane, composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California to Acapulco and is considered to be coeval with the late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, the West Indies, Venezuela and the Western Cordillera of Colombia. These sequences represent the remnants of an arc which accreted to the North American and northern South American cratons at the end of the Cretaceous. In western Mexico, the arc sequences built on continental crust consist of high-K calc-alkaline basalts, andesites and rhyolites enriched in LREE with abundant siliceous pyroclastic rocks interbedded either with Aptian-Albian reefal limestones or red beds. They do not show magmatic changes during the arc development. In contrast, the arc sequences built on oceanic crust show an evolution with time. Arc activity began with the development of depleted low K-tholeiitic mafic suite (Guanajuato igneous sequence), followed first by mature tholeiitic basalts and then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age. At the end of the arc growth, during Aptian-Albian times, calc-alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc. The tholeiitic and shoshonitic mafic rocks as well as the calc-alkaline lavas are mildly enriched in LREE, Y and Nb and show high ɛNd ratios, typical of oceanic arcs. In contrast, the calc-alkaline mafic suite enriched in LREE, Y and Nb exhibits lower ɛNd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by Paleozoic subducted sediments or old source enrichments (OIB). The Cretaceous arc rocks of the Greater Antilles, interbedded with and/or capped by Aptian-Albian limestones, the Cretaceous andesites of northern Colombia, the Cretaceous tholeiitic and calc-alkaline volcanic rocks of Venezuela, and

  5. Eclogite-melt/peridotite reaction: Experimental constrains on the destruction mechanism of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    To study the mechanism of melt-peridotite reaction pertinent to the destruction of the North China Craton (NCC) lithosphere, a series of experiments were performed at a pressure of 2.0 GPa and temperatures from 1250 to 1400°C using Bixiling eclogite and Damaping peridotite as starting materials. The experimental results show that the reaction between eclogite melt and peridotite causes dissolution of olivine and orthopyroxene and precipitation of clinopyroxene in the melt. The experimental run products, characterized by a lherzolite/pyroxenite/garnet-pyroxenite sequence, are consistent with the mantle xenoliths in the Neogene Hannuoba basalt of the NCC found by Liu et al. (2005). It suggests that the mafic lower continental crust was probably recycled into the mantle during the Mesozoic Era. In the experiments conducted at 1300 and 1350°C, the resulting melts have a high Mg# andesite signature, indicating that the melt-peridotite reaction may have played a major role in the generation of high Mg# andesite. Our experimental results support the hypothesis that melts derived from foundered eclogite in the asthenosphere will consume the lithospheric peridotites. Therefore, melt-peridotite reaction is an important mechanism for the destruction/thinning of the lithosphere.

  6. Lithospheric thermal structure of the North China Craton and its geodynamic implications

    Science.gov (United States)

    Liu, Qiongying; Zhang, Linyou; Zhang, Chao; He, Lijuan

    2016-12-01

    We conduct 2-D numerical modeling of the lithospheric thermal structure of the North China Craton (NCC) on basis of twenty-four crustal velocity structure profiles. About five hundred heat flow data constitute the principal constraints for our modeling. The modeling results demonstrate marked lateral variations in thermal regime of the crust-lithosphere system in the NCC. The average mantle heat flow decreases from 38 ± 5 mW m-2 under the Bohai Bay Basin in the eastern NCC to 27 ± 4 mW m-2 under the Ordos Basin in the western NCC, characterized by a 'cold crust but hot mantle' structure and a 'hot crust but cold mantle' structure, respectively. Thermal lithospheric thickness varies from ∼65 km beneath the Tan-Lu Fault zone to ∼160 km beneath the western and northern Ordos Basin, with similar trend to the seismic lithosphere. However, the disparities in thickness between the thermal and seismic lithosphere are within 20 km beneath the Bohai Bay Basin, but 30-90 km beneath the Shanxi-Weihe Graben and 50-120 km beneath the Ordos Basin. This may imply a westward thickening trend of the rheological boundary layer, which might be attributed to the reducing of asthenosphere viscosity due to hydrous fluid released by dehydration of the subducting Pacific Plate under the eastern NCC. Combined with other pieces of evidence, we suggest that vigorous mantle processes may occur beneath the eastern NCC, whereas the western NCC is relatively stable.

  7. Is the outcrop topology of dolerite dikes of the Precambrian Singhbhum Craton fractal?

    Indian Academy of Sciences (India)

    Nibir Mandal; Atin Kumar Mitra; Santanu Misra; Chandan Chakraborty

    2006-12-01

    In the Precambrian Singhbhum Craton of eastern India, newer dolerite dikes occur profusely with varying outcrop lengths. We have analysed the nature of their length-size and orientation distributions in relation to the theory of fractals. Two orientational sets of dikes (NW–SE and NE–SW) are present. Both the sets show strongly non-power-law size distributions, as reflected in nonlinear variations in logarithmic space. We analyzed thousands of data, revealing that polynomial functions with a degree of 3 to 4 are the best representatives of the non-linear variations. Orientation analysis shows that the degree of dispersions from the mean trend tends to decrease with increasing dike length. The length-size distributions were studied by simulating fractures in physical models. Experimental fractures also show a non-power-law distribution, which grossly conforms to those of the dolerite dikes. This type of complex size distributions results from the combined effects of nucleation, propagation and coalescence of fractures.

  8. On the nature and origin of highly-refractory Archean lithosphere: Petrological and geophysical constraints from the Tanzanian craton

    Science.gov (United States)

    Gibson, S. A.; McMahon, S. C.; Day, J. A.; Dawson, J. B.

    2012-12-01

    The nature and timescales of garnet formation are important to understanding how subcontinental lithospheric mantle (SCLM) has evolved since the Archean, and also to mantle dynamics, because the presence of garnet greatly influences the density of the lower lithosphere and hence the long-term stability of thick (150 to 220 km) subcratonic lithosphere. Nevertheless, the widespread occurrence of garnet in the SCLM remains one of the 'holy grails' of mantle petrology. Garnets found in mantle xenoliths from the eastern margin of the Tanzanian Craton (Lashaine) have diverse compositions and provide major constraints on how the underlying deep (120 to 160 km) mantle evolved during the last 3 billion years. Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element patterns similar to hypothetical garnets proposed to have formed in the Earth's SCLM during the Archean, prior to metasomatism [Stachel et al., 2004]. These rare ultradepleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global mantle harzburgites and diamond inclusions. The ultradepleted garnets form interconnecting networks around grains of orthopyroxene which give the rocks a banded appearance: we propose that the increase in pressure associated with cratonization may have caused isochemical exsolution of ultradepleted garnet from orthopyroxene. These unique garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. We believe they are rare because their low concentrations of trace elements make them readily susceptible to geochemical overprinting. This highly-refractory low-density peridotite may be common in the 'shallow' SCLM but not normally brought to the

  9. Neoarchean (2.5–2.8 Ga crustal growth of the North China Craton revealed by zircon Hf isotope: A synthesis

    Directory of Open Access Journals (Sweden)

    Andong Wang

    2012-03-01

    Full Text Available The crustal growth of the North China Craton (NCC during the Neoarchean time (2.5–2.8 Ga is a hotly controversial topic, with some proposing that the main crustal growth occurred in the late Neoarchean (2.5–2.6 Ga, in agreement with the time of the magmatism, whereas others suggest that the main crustal accretion took place during early Neoarchean time (2.7–2.8 Ga, consistent with the time of crustal-formation of other cratons in the world. Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC. In this contribution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochronology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC. The results suggest that both 2.7–2.8 Ga and 2.5–2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested. The Eastern block is characterized by the main 2.7–2.8 Ga crustal growth with local new crustal-formation at 2.5–2.6 Ga, and the Yinshan block is characterized by ∼2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group. Detrital zircon data of the Khondalite Belt indicate that the main crustal growth period of the Western block is Paleoproterozoic involving some ∼2.6 Ga and minor Early- to Middle-Archean crustal components, and the crustal accretion in the Trans-North China Orogen (TNCO has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy. Zircon Hf isotope compositions, coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO, and the evolution and tectonic division of the NCC is more complex than previously proposed, probably involving multi-stage crustal growth and subduction processes

  10. 克拉通盆地构造枢纽带类型及其在碳酸盐岩油气成藏中的作用%Types of tectonic hinge zone and its effect on hydrocarbon accumulation in carbonate reservoirs of cratonic basins in China

    Institute of Scientific and Technical Information of China (English)

    汪泽成; 姜华; 刘伟; 谷志东; 王铜山; 江青春

    2012-01-01

    与北美等大型克拉通盆地相比,中国小型克拉通的构造演化更易受周缘构造活动影响.受构造分异的影响,克拉通坳陷盆地存在2大类构造枢纽带,即同沉积期构造枢纽带和跨构造期构造枢纽带.同沉积期构造枢纽带的形成和演化受同沉积断裂活动的影响,控制了碳酸盐台地边缘礁滩体的分布,与相邻的盆地相烃源岩共同构成良好的源-储配置,具备有利的油气成藏条件.跨构造期构造枢纽带主要有3种类型,即“跷跷板”运动枢纽带、克拉通古隆起一前陆隆起叠合枢纽带、多期古隆起继承性叠合枢纽带,发育岩溶型储层和地层-岩性型圈闭,是油气运聚的有利指向区.构造枢纽带是碳酸盐岩油气富集的有利区带,油气勘探前景广阔.%Compared to large cratonic basins in northern America, small cratonic basins in China are more susceptible to peripheral tectonic activities in terms of tectonic evolution. Due to tectonic differentiation, there are commonly two types of tectonic hinge zone in cratonic depression basins, I. E. Synsedimentary tectonic hinge zone and cross-structure tectonic hinge zone. The formation and e-volution of synsedimentary tectonic hinge zones are affected by activity of synsedimentary faults. These controlled distribution of reefs and shoals on carbonate platform margin and formed an excellent source-storage configuration with adjacent source rocks, providing favorable conditions for hydrocarbon accumulation. The cross-structure tectonic hinge zones can be divided into three types, I. E. , "seesaw" movement hinge zone, cratonic palaeouplift-foreland uplift superimposed hinge zone, and multi-phase palaeouplift inheritance superimposed hinge zone. They develop karst reservoirs and stratigraphic-lithologic traps, and are favorable oriented regions of hydrocarbon migration and accumulation. The tectonic hinge zones are favorable for hydrocarbon enrichment in carbonates with broad

  11. Southern Brasilia Belt (SE Brazil): tectonic discontinuities, K-Ar data and evolution during the Neoproterozoic Brasiliano orogeny

    Energy Technology Data Exchange (ETDEWEB)

    Valeriano, Claudio Morrison de [Universidade do Estado, Rio de Janeiro, RJ (Brazil)]. E-mail: cmval@uerj.br; Teixeira, Wilson [Sao Paulo Univ., SP (Brazil); Simoes, Luiz Sergio Amarante [UNESP, Sao Paulo, SP (Brazil); Heilbron, Monica [Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brasilia, DF (Brazil)

    2000-03-01

    This paper focuses the tectonic evolution of the southern brasilia belt, with emphasis on the Furnas segment, along the 21 deg C S parallel. The uppermost structural unit (Passos Nappe - PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western Sao francisco craton. An inverted metamorphic gradient ranging from greensvhits to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain display a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic gaywacke succession and a calc-alkaline granitoid suite) with undated siliciclast low-grade metasedimentary rocks. The Sao Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambui group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of Sao Francisco Craton, coeval with the exhumation of the External Domain thrust sheets. New K-Ar determinations on mineral separates are presented an interpreted among previous data. The SFC basement rocks display Paleo-to Meesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a cold Sao Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their-K-ar systems reset during the allochthony. (author)

  12. Dynamic contribution of recycled components from the subducted Pacific slab: Oxygen isotopic composition of the basalts from 106 Ma to 60 Ma in North China Craton

    Science.gov (United States)

    Liu, Jia; Wang, Zi-Zhen; Yu, Hao-Ran; Xia, Qun-Ke; Deloule, Etienne; Feng, Min

    2017-02-01

    How the materials derived from the stagnant Pacific slab contributed to the mantle sources of the mafic rocks in east China is still in hot debate. In this work, δ18O (Vienna standard mean ocean water) values of clinopyroxene phenocrysts in the oceanic island basalts (OIB)-type mafic rocks from 106 Ma to 60 Ma in the east North China Craton (NCC) were measured by secondary ion mass spectrometry. Our data show that for all of the samples, the δ18Ocpx are dominantly higher than that of the clinopyroxene from normal mid-oceanic ridge basalt (5.4-5.8‰), which confirms the role of recycled oceanic crust (ROC) in their mantle sources. Combining the δ18O data of basalts and the lithospheric mantle in the literature, we found that in the southeast NCC, upper and lower ROC components alternately appeared in the mantle sources of basalts, but these ROC components are consistently different from that in the lithospheric mantle, while in the northern NCC, the recycled components in the sources seem to be persistently from upper ROC. These observations suggest that (1) these mafic OIB-type rocks are most likely derived from the convective asthenosphere and (2) the contribution of components from the Pacific oceanic slab into the NCC upper mantle was dynamic, without a simple temporal trend. This new knowledge calls for the reconsideration of the existing models of the thinning process of the NCC lithospheric mantle, and it warns against simply using the chemical composition of basalts to infer the evolution of lithosphere.

  13. High-pressure pelitic granulites from the Jiao-Liao-Ji Belt, North China Craton: A complete P-T path and its tectonic implications

    Science.gov (United States)

    Zou, Yi; Zhai, Mingguo; Santosh, M.; Zhou, Ligang; Zhao, Lei; Lu, Junsheng; Shan, Houxiang

    2017-02-01

    The Paleoproterozoic khondalite series rocks were widely distributed along the Jiao-Liao-Ji Belt (JLJB), North China Craton (NCC). Metamorphic investigations on these rocks can definitely shed light on the tectonic evolution of the JLJB and provide some clues for Paleoproterozoic history of the NCC. Here we present a comprehensive metamorphic study of high-pressure (HP) pelitic granulites from the Jiaobei terrane in the southern part of the JLJB and combine microstructure, mineral chemistry, Raman spectroscopy, thermobarometry, and P-T-X pseudosections in NCKFMASHTO to trace the metamorphic evolution of these rocks. Staurolite inclusion in kyanite was firstly reported using Raman spectroscopy and four successive mineral assemblages (M1-M4) were identified. Integrated method of thermobarometry and pseudosection modeling constrains P-T conditions of 6-8 kbar at below 660 °C, 15-16.6 kbar at 850-860 °C, 8.8-10.5 kbar at 830-870 °C, 5.8-7.8 kbar at 660-700 °C, for M1-M4, respectively. By employing the melt-reintegrated method, a prograde P-T trajectory is evaluated and hence a complete P-T path is defined that involves a prograde trajectory with marked increase both in pressure and temperature to peak metamorphism, followed by isothermal decompression and subsequent near-isobaric cooling. Such a P-T path implies that a subduction process before collision followed by subsequent extension and cooling processes, was involved in the formation the JLJB, consistent with an orogenic event. Available geochronological data suggests that HP granulites-facies metamorphism probably initiated at 1.90-1.95 Ga and that exhumation and cooling of the metamorphic terrane occurred at 1.80-1.85 Ga, indicating a long-lived hot orogeny during Paleoproterozoic. This contribution provides mineralogical evidence for low-pressure prograde metamorphism in HP pelitic granulites and hence gives a complete P-T path, placing a more rigorous metamorphic constraint on evolution of the JLJB.

  14. Mechanisms for strain localization within Archaean craton: A structural study from the Bundelkhand Tectonic Zone, north-central India

    Science.gov (United States)

    Sarkar, Saheli; Patole, Vishal; Saha, Lopamudra; Pati, Jayanta Kumar; Nasipuri, Pritam

    2015-04-01

    The transformation of palaeo-continents involve breakup, dispersal and reassembly of cratonic blocks by collisional suturing that develop a network of orogenic (mobile) belts around the periphery of the stable cratons. The nature of deformation in the orogenic belt depends on the complex interaction of fracturing, plastic deformation and diffusive mass transfer. Additionally, the degree and amount of melting during regional deformation is critical as the presence of melt facilitates the rate of diffusive mass transfer and weakens the rock by reducing the effective viscosity of the deformed zone. The nature of strain localization and formation of ductile shear zones surrounding the cratonic blocks have been correlated with Proterozoic-Palaeozoic supercontinent assembly (Columbia, Rodinia and Gondwana reconstruction). Although, a pre-Columbia supercontinent termed as Kenorland has been postulated, there is no evidence that supports the notion due to lack of the presence of shear zones within the Archaean cratonic blocks. In this contribution, we present the detailed structural analysis of ductile shear zones within the Bundelkhand craton. The ductlile shear zone is termed as Bundelkhand Tectonic Zone (BTZ) that extends east-west for nearly 300 km throughout the craton with a width of two-three kilometer . In the north-central India, the Bundelkhand craton is exposed over an area of 26,000 sq. The craton is bounded by Central Indian Tectonic zone in the south, the Great Boundary fault in the west and by the rocks of Lesser Himalaya in the north. A series of tonalite-trondjhemite-granodiorite gneiss are the oldest rocks of the Bundelkhand craton that also contains a succession of metamorphosed supracrustal rocks comprising of banded iron formation, quartzite, calc-silicate and ultramafic rocks. K-feldspar bearing granites intrude the tonalite-trondjhemite-granodiorite and the supracrustal rocks during the time span of 2.1 to 2.5 Ga. The TTGs near Babina, in central

  15. Neoproterozoic extension in the greater dharwar craton: A reevaluation of the "betsimisaraka suture" in madagascar

    Science.gov (United States)

    Tucker, R.D.; Roig, J.-Y.; Delor, C.; Amlin, Y.; Goncalves, P.; Rabarimanana, M.H.; Ralison, A.V.; Belcher, R.W.

    2011-01-01

    The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the "Betsimisaraka suture" is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.

  16. Chemical transfers along slowly eroding catenas developed on granitic cratons in southern Africa

    Science.gov (United States)

    Khomo, Lesego; Bern, Carleton R.; Hartshorn, Anthony S.; Rogers, Kevin H.; Chadwick, Oliver A.

    2013-01-01

    A catena is a series of distinct but co-evolving soils arrayed along a slope. On low-slope, slowly eroding catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~ 5 mm ky− 1) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr− 1) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr− 1) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscapes.

  17. 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

    2016-01-01

    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.

  18. Ultrahigh-temperature metamorphism under isobaric heating: New evidence from the North China Craton

    Science.gov (United States)

    Yang, Qiong-Yan; Santosh, M.; Tsunogae, Toshiaki

    2014-12-01

    The Khondalite Belt within Inner Mongolia Suture Zone (IMSZ) in the North China Craton (NCC) preserves evidence for extreme crustal metamorphism under ultra-high temperature (UHT) conditions at ca. 1.92 Ga, associated with the subduction-collision tectonics between the Yinshan and Ordos Blocks. Here we report a new locality in Hongsigou where cordierite- and spinel-bearing granulites record UHT metamorphism. The prograde, peak, and retrograde mineral assemblages in these pelitic granulites have been identified based on petrography and mineral chemistry as: Bt1 + Grt1 + Sil1 + Kfs1 + Pl1 + Ilm + Qtz1, Grt1 + Sil2 + Kfs2 + Pl2 + Spl + Ilm + Qtz2 + Liq, and Crd + Grt2 + Sil3 + Kfs2 + Pl2 + Ilm + Qtz2 respectively. The peak metamorphic conditions of the pelitic granulite were estimated as 930-1050 °C and 6.5-7.5 kbar based on pseudosection analysis in the system NCKFMASHTO, suggesting extreme thermal metamorphism. We report LA-ICPMS zircon U-Pb data from the granulite which show weighted mean 207Pb/206Pb age of 1881 ± 6.6 Ma, marking the timing of UHT metamorphism. Lu-Hf analyses of the zircons show εHf(t) values within a restricted range of -4.2 to 0.3 and together with Hf model ages, a Paleoproterozoic arc magmatic source is inferred for the detrital zircons. The estimated P-T path for the UHT granulite suggests isobaric heating followed by cooling and decompression along a clockwise trajectory, different from the anti-clockwise P-T paths defined in earlier studies for the 1.92 Ga UHT rocks from the IMSZ. The younger age and the isobaric heating trajectory suggest that the Hongsigou UHT rocks are related to heat input from underplated mafic magmas following continental collision.

  19. Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

    Science.gov (United States)

    Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

    2002-01-01

    We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

  20. Assessment of Undiscovered Petroleum Resources of the North and East Margins of the Siberian Craton, Russian Federation

    Science.gov (United States)

    Klett, T.R.

    2008-01-01

    Four geologic provinces located along the north and east margins of the Siberian craton were assessed for undiscovered crude oil, natural gas, and natural gas liquids/condensates resources as part of the U.S. Geological Survey's (USGS) Circum-Arctic Oil and Gas Resource Appraisal. Using a geology-based methodology, the USGS estimated the mean undiscovered, conventional petroleum resources in these provinces to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 106 trillion cubic feet of natural gas, and 3 billion barrels of natural gas liquids.

  1. REE geochemistry of auriferous quartz carbonate veins of Neoarchean Ajjanahalli gold deposit, Chitradurga schist belt, Dharwar Craton, India

    Directory of Open Access Journals (Sweden)

    S. Sarangi

    2013-03-01

    Full Text Available REE composition of the carbonates of the auriferous quartz carbonate veins (QCVs of the Neoarchean Ajjanahalli gold deposit, Chitradurga schist belt, Dharwar Craton, is characterized by U-shaped chondrite normalized REE patterns with both LREE and HREE enrichment and a distinct positive Eu anomaly. As positive Eu anomaly is associated with low oxygen fugacity, we propose that the auriferous fluids responsible for gold mineralization at Ajjanahalli could be from an oxygen depleted fluid. The observed positive Eu anomaly is interpreted to suggest the derivation of the auriferous fluids from a mantle reservoir. The location of Ajjanahalli gold deposit in a crustal scale shear zone is consistent with this interpretation.

  2. Discussion: The timing of gold mineralization across the eastern Yilgarn Craton using U-Pb geochronology of hydrothermal phosphate minerals

    Science.gov (United States)

    Bateman, Roger; Jones, Sarah

    2015-10-01

    The presentation of recent geochronological work on orogenic gold deposits in the Eastern Goldfields of the Yilgarn Craton, Western Australia, claims to prove that there is a single broad event of gold mineralization and that structural work demonstrating that there are a number of discrete gold mineralization events is wrong. This new data demonstrates no such thing, as this data, no doubt the best that can currently be produced, shows a very wide and inconsistent range in ages. Geochronology is not yet able to reliably separate these events, which appear to be spread over an interval of perhaps 30 Ma, up to ˜2635 Ma.

  3. Hyperspectral imaging spectroscopy of a Mars analogue environment at the North Pole Dome, Pilbara Craton, Western Australia

    CERN Document Server

    Brown, Adrian J; Cudahy, Thomas

    2014-01-01

    A visible and near infrared (VNIR) to shortwave infrared (SWIR) hyperspectral dataset of the Early Archaean North Pole Dome, Pilbara Craton, Western Australia, has been analysed for indications of hydrothermal alteration. Occurrence maps of hydrothermal alteration minerals were produced. It was found that using a spatial resolution on the ground of approximately 5 m and spectral coverage from 0.4 to 2.5 mm was sufficient to delineate several hydrothermal alteration zones and associated veins, including phyllic, serpentinitic and chloritic alteration. These results suggest this level of spectral and spatial resolution would be ideal for localising shallow epithermal activity, should such activity have existed, on the surface of Mars.

  4. Modern-style Subduction Processes in the Archean:Evidence from the Shangyi Complex in North China Craton

    Institute of Scientific and Technical Information of China (English)

    WANG Renmin; WAN Yusheng; CHENG Suhua; FENG Yonggang

    2009-01-01

    Three fragments of the Arehean oceanic crust have been found between the Archean granulite belt and the Paleo-Proterozoic Hongqiyingzi group in North China craton,which spread and geochronology evidence of the ancient oceanic fragments.The magma crystallizing age of the tonalite in the Shangyi complex is 2512+19 Ma and the geochemical characteristics suggest that the Nb-enriched basalts may be related to crustal contamination and formed in the intra-oceanic arc of the supra subduction zone setting.

  5. Brittle Deformation in the Ordos Basin in response to the Mesozoic destruction of the North China Craton

    Science.gov (United States)

    Wang, Q.; Jiang, L.

    2012-12-01

    Craton is continental block that has been tectonically stable since at least Proterozoic. Some cratons, however, become unstable for some geodynamic reasons. The North China Craton (NCC) is an example. Structure geological, geochemical, and geophysical works have revealed that the NCC was destructed in Cretaceous and that lithosphere thickness beneath the eastern NCC were thinned by 120 km. The present study will focus on deformation of the western NCC, and to understand the effect of the Mesozoic destruction of the North China Craton (NCC). Structural partitioning of the Ordos Basin, which is located in the western NCC, from the eastern NCC occurred during the Mesozoic. Unlike the eastern NCC where many Cretaceous metamorphic core complexes developed, sedimentary cover of the NCC remains nearly horizontal and deformation is manifested by joint. We visited 216 sites of outcrops and got 1928 joints measurements, among which 270 from Jurassic sandstones, 1378 from the Upper Triassic sandstones, 124 from the Middle and Lower Triassic sandstones, and 156 from Paleozoic sandstones. In the interior of the Ordos Basin, joints developed quite well in the Triassic strata, while joints in the Jurassic stata developed weakly and no joint in the Cretaceous strata. The Mesozoic stratigraphic thickness are: 1000 meters for the Lower Triassic, the Middle Triassic sandstone with thickness of 800 meters, 3000 meters for the Upper Triassic, 4000 meters for the Jurassic, and 1100 meters for the Lower Cretaceous. The vertical difference in joint development might be related to the burying depth of the strata: the higher the strata, the smaller the lithostatic stress, and then the weaker the joint. Joints in all stratigraphic levels showed a similar strain direction with the sigma 1 (the maximum pressure stress) vertical and the sigma 3 (the minimum pressure stress) horizontal and running N-S. The unconformity below the Cretaceous further indicates that joints in Jurassic and Triassic

  6. Petrogenesis and geochemistry of circa 2.5 Ga granitoids in the Zanhuang Massif: Implications for magmatic source and Neoarchean metamorphism of the North China Craton

    Science.gov (United States)

    Wang, Junpeng; Kusky, Timothy; Wang, Lu; Polat, Ali; Wang, Songjie; Deng, Hao; Fu, Jianmin; Fu, Dong

    2017-01-01

    The tectonic framework of the North China Craton (NCC) during late Archean to early Paleoproterozoic (circa 2.5 Ga) is still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. Circa 2.5 Ga magmatic and metamorphic activities are widely spread throughout the NCC, which can be used as an efficient target to better understand the tectonic evolution at this period. In this study, based on a detailed field, structural, geochemical, geochronological and Sm-Nd isotopic study, we focus our work on the Haozhuang granitoids in the Zanhuang Massif located at the eastern margin of the Central Orogenic Belt of the NCC. The granitoids mainly include undeformed pegmatite and granodiorite. One pegmatite and two granodiorite samples yield zircon 207Pb/206Pb ages of 2513 ± 29 Ma, 2511 ± 36 Ma and 2528 ± 18 Ma, respectively. The granodiorites show metaluminous and shoshonitic to high-K calc-alkaline series characteristics with A-type granite affinity. The circa 2.5 Ga granodiorites have highly negative εNd(t) values (- 29.22 - 33.12) and TDM model ages between 2671 Ma and 3151 Ma. This work shows clearly, from whole-rock major and trace elements and Sm-Nd isotopic studies, that the Haozhuang granodiorites were derived from partial melting of old and thickened TTG crust rather than mantle sources, and formed in a subduction-related tectonic setting. With geochemical comparison studies to other similar-aged granitic rocks in the Zanhuang Massif, we suggest that these granitic rocks possibly have a certain correlation during the magma evolution. Coupled with our previous geochemical and isotopic studies on circa 2.5 Ga mafic dike swarms, we propose that the similar-aged granitic rocks and mafic dike swarms were produced by an east-dipping subduction polarity reversal event following an arc-continent collision between the Fuping/Wutai island arc and Eastern Block of the NCC above a west-dipping slab. The east

  7. Seismic velocity model of the crust and upper mantle along profile PANCAKE across the Carpathians between the Pannonian Basin and the East European Craton

    DEFF Research Database (Denmark)

    Starostenko, V.; Janik, T.; Kolomiyets, K.

    2013-01-01

    Results are presented of a seismic wide-angle reflection/refraction survey along a profile between the Pannonian Basin (PB) and the East European Craton (EEC) called PANCAKE. The P- and S-wave velocity model derived can be divided into three sectors: the PB; the Carpathians, including the Transca......Results are presented of a seismic wide-angle reflection/refraction survey along a profile between the Pannonian Basin (PB) and the East European Craton (EEC) called PANCAKE. The P- and S-wave velocity model derived can be divided into three sectors: the PB; the Carpathians, including...

  8. Crustal evolution in Asia: Correlations and connections

    Science.gov (United States)

    Tsunogae, Toshiaki; Kwon, Sanghoon; Santosh, M.

    2016-11-01

    The Asian region records multiple subduction, accretion and collision processes related to the breakup of Gondwana and Pangea, and the ongoing formation of the future supercontinent Amasia. The oldest geological record of Asia is preserved in Archean crustal fragments which were welded together by later collisional events related to the assembly of several supercontinents. The Asian region also records recent geological events such as volcanic activities and mega-earthquakes related to subduction of oceanic plates along active continental margins and collision of microplates. This region is thus regarded as an excellent field laboratory for examining the evolution of continental crust and cratons, formation and destruction of continents and supercontinents, and related metallogenic and surface environmental processes.

  9. Multifractal spatial organisation in hydrothermal gold systems of the Archaean Yilgarn craton, Western Australia

    Science.gov (United States)

    Munro, Mark; Ord, Alison; Hobbs, Bruce

    2015-04-01

    A range of factors controls the location of hydrothermal alteration and gold mineralisation in the Earth's crust. These include the broad-scale lithospheric architecture, availability of fluid sources, fluid composition and pH, pressure-temperature conditions, microscopic to macroscopic structural development, the distribution of primary lithologies, and the extent of fluid-rock interactions. Consequently, the spatial distribution of alteration and mineralization in hydrothermal systems is complex and often considered highly irregular. However, despite this, do they organize themselves in a configuration that can be documented and quantified? Wavelets, mathematical functions representing wave-like oscillations, are commonly used in digital signals analysis. Wavelet-based multifractal analysis involves incrementally scanning a wavelet across the dataset multiple times (varying its scale) and recording its degree of fit to the signal at each interval. This approach (the wavelet transform modulus maxima method) highlights patterns of self-similarity present in the dataset and addresses the range of scales over which these patterns replicate themselves (expressed by their range in 'fractal dimension'). Focusing on seven gold ore bodies in the Archaean Yilgarn craton of Western Australia, this study investigates whether different aspects of hydrothermal gold systems evolve to organize themselves spatially as multifractals. Four ore bodies were selected from the Sunrise Dam deposit (situated in the Laverton tectonic zone of the Kurnalpi terrane) in addition to the Imperial, Majestic and Salt Creek gold prospects, situated in the Yindarlgooda dome of the Mount Monger goldfield (approximately 40km due east of Kalgoorlie). The Vogue, GQ, Cosmo East and Astro ore bodies at Sunrise Dam were chosen because they exhibit different structural geometries and relationships between gold and associated host-rock alteration styles. Wavelet-based analysis was conducted on 0.5m and 1m

  10. Large Calcium Isotopic Variation in Peridotitic Xenoliths from North China Craton

    Science.gov (United States)

    Huang, S.; Zhao, X.; Zhang, Z.

    2016-12-01

    Calcium is the fifth most abundant element in the Earth. The Ca isotopic composition of the Earth is important in many aspects, ranging from tracing the Ca cycle on the Earth to comparing the Earth to other terrestrial planets. There is large mass-dependent Ca isotopic variation, measured as δ44/40Ca relative to a standard sample, in terrestrial igneous rocks: about 2 per mil in silicate rocks, compared to 3 per mil in carbonates. Therefore, a good understanding of the Ca isotopic variation in igneous rocks is necessary. Here we report Ca isotopic data on a series of peridotitic xenoliths from North China Craton (NCC). There is about 1 per mil δ44/40Ca variation in these NCC peridotites: The highest δ44/40Ca is close to typical mantle values, and the lowest δ44/40Ca is found in an Fe-rich peridotite, -1.13 relative to normal mantle (or -0.08 on the SRM 915a scale). This represents the lowest δ44/40Ca value ever reported for igneous rocks. Combined with published Fe isotopic data on the same samples, our data show a positive linear correlation between δ44/40Ca and δ57/54Fe in NCC peridotites. This trend is inconsistent with mixing a low-δ44/40Ca and -δ57/54Fe sedimentary component with a normal mantle component. Rather, it is best explained as the result of kinetic isotopic effect caused by melt-peridotite reaction on a time scale of several hundreds of years. In detail, basaltic melt reacts with peridotite, replaces orthopyroxene with clinopyroxene, and increases the Fo number of olivine. Consistent with this interpretation, our on-going Mg isotopic study shows that low-δ44/40Ca and -δ57/54Fe NCC peridotites also have heavier Mg isotopes compared to normal mantle. Our study shows that mantle metasomatism plays an important role generating stable isotopic variations within the Earth's mantle.

  11. Fluid-induced martitization of magnetite in BIFs from the Dharwar Craton, India.

    Science.gov (United States)

    Wagner, Christiane; Orberger, Beate; Tudryn, Alina; Wirth, Richard; Morgan, Rachael

    2013-04-01

    Banded iron formations (BIFs) represent the largest iron deposits on Earth, which mainly formed in the Late Archean and Early Proterozoic. The complex geological history of BIFs makes it difficult to reconstruct the primary mineralogy and thus the initial depositional environment. Magnetite and hematite are the most important iron oxide minerals in BIFs. Magnetite (FeOFe2O3) comprising of both ferrous and ferric iron, easily undergoes transformation at low temperature. Hematite (α-Fe2O3) is often a result of the pseudomorphic replacement of magnetite, in the processus called martitisation. Despite the process of martitization having been widely studied, in both synthetic and natural magnetites, the mechanics of the transformation are poorly understood. What is generally agreed is that the transformation from magnetite to hematite occurs via a maghemite (g-Fe2O3) intermediate. The 2.9 Ga BIF from the Western Dharwar Craton, Southern India (a 500 m thick Archean BIF), is characterized by millimetric to centrimetric alternating white quartz and grey Fe-oxide bands. The Fe-oxide bands consist of martite crystals (~20µm) which represent the hematitisation of euhedral magnetite. The hematite crystals are in part euhedral, cubic shaped pointing to the replacement of magnetite. The crystals show a trellis pattern. Magnetite patches occur within the hematite. Raman spectroscopy, X-Ray diffraction, Curie balance and magnetic hysteresis analyses and FIB-TEM investigation indicate the presence of maghemite, and the presence of subhedral magnetite and interstitial hematite crystal. The latter are characterized by dislocation with fluid inclusions and high porosity zones. The magnetite grains contain lamellae and the interfaces between magnetite-maghemite and hematite are curved suggesting grain boundary migrations with the growth of hematite at the expense of magnetite and maghemite. It is thus suggested that martite result from low-T exsolutions along cleavage resulting in

  12. Provenance of zircon of the lowermost sedimentary cover, Estonia, East-European Craton

    Directory of Open Access Journals (Sweden)

    Konsa, M.

    1999-12-01

    Cambrian sequence, zircons resembling those of local basement sources are very rare or absent. Obviously, basal Vendian/Cambrian sedimentary rocks sealed off the basement as a source of zircon. Therefore a distant source, probably outside the Svecofennian Domain, could be supposed for the bulk clastic minerals and zircons of the upperpart of the Vendian and the lower part of the Cambrian. Probably, studies of isotopic ages of different typological varieties of zircons, both of obviously local and distant origin, could provide new information on respective source rock ages and areas, and on the general palaeogeographic pattern of the Vendian and Cambrian epicratonic sedimentary basins of the East-European Craton.

  13. Over 400 m.y. metamorphic history of the Fennoscandian lithospheric segment in the Proterozoic (the East European Craton)

    Science.gov (United States)

    Skridlaite, G.; Bogdanova, S.; Taran, L.; Baginski, B.; Krzeminska, E.; Wiszniewska, J.; Whitehouse, M.

    2009-04-01

    Several Palaeoproterozoic terranes in the Fennoscandian lithospheric segment of the East European Craton (EEC) evolved differently prior to their final amalgamation at c. 1.8 Ga. South-westward younging of the major tectono-thermal events characterizes the Baltic -Belarus region between the Baltic and Ukrainian Shields of the EEC. While at c.1.89-1.87 Ga and 1.85-1.84 Ga rocks of some northern and eastern terranes (Estonia, Belarus and eastern Lithuania) experienced syncollisional, moderate P metamorphism, subduction-related volcanic island arc magmatism still dominated southwestern terranes in Lithuania and Poland. The available age determinations of metamorphic zircon (SIMS/NORDSIM and TIMS methods, Stockholm, SHRIMP method, RSES, ANU, Canberra) and metamorphic monazite (TIMS, Stockholm and EPMA method, Warsaw University) allow to distinguish several metamorphic events related to major orogenic processes: - 1.90-1.87 Ga amphibolite-facies H/MP metamorphism occurred along with emplacements of juvenile TTG-type granitoids in the North Estonian and Lithuanian-Belarus terranes. They are coeval with the main accretionary growth of the crust in the Svecofennian Domain in the Baltic Shield (e.g. Lahtinen et al., 2005). - 1.84-1.79 Ga high-grade metamorphism affected sedimentary and igneous rocks in almost all the terranes and is assumed to have been related to the major aggregation of the EEC (Bogdanova et al, 2006, 2008). In the metasedimentary granulites of western Lithuania, a prograde metamorphism commenced with monazite growth prior garnet at 1.84-1.83 Ga. The sediments and mafic igneous rocks in Lithuania, felsic igneous rocks in NE Poland underwent peak metamorphism and deformation at 1.81-1.79 Ga (zircon and monazite ages). The 1.83-1.79 Ga metamorphism has the same age as a metamorphic imprint and strong shearing of the crust in central Sweden (Andersson et al., 2004). The postcollisional granulite metamorphism of mafic intrusions at 1.80-1.79 Ga in Belarus

  14. Deciphering relative timing of fabric development in granitoids with similar absolute ages based on AMS study (Dharwar Craton, South India)

    Science.gov (United States)

    Bhatt, Sandeep; Rana, Virendra; Mamtani, Manish A.

    2017-01-01

    Anisotropy of Magnetic Susceptibility (AMS) data are presented from the Koppal Granitoid (Dharwar Craton, South India) that has U-Pb zircon age of 2528 ± 9 Ma. The magnetic fabric is oriented in NNE-SSW direction. This is parallel to the planar structures that developed during regional D3 deformation, but oblique to the NNW-SSE oriented magnetic foliation as well as field foliation (D1/D2 deformation) recorded in the country rock Peninsular Gneiss. Variation in the intensity of fabric within the granitoid is mapped. It is inferred that the emplacement of Koppal Granitoid took place by ballooning and fabric development within the pluton was syntectonic with regional D3. These results are compared with the time-relationship between emplacement/fabric development and regional deformation reported from the Mulgund Granite (2555 ± 6 Ma; U-Pb zircon), which is also located in the Dharwar Craton and is equivalent to the Koppal Granitoid in age. This granite is known to have emplaced syntectonically with regional D1/D2 deformation, and is thus not related to the same deformation event as the Koppal Granitoid, despite their similar absolute ages. It is argued that in the study area, D3 is ≤2537 Ma, while D1/D2 is ≥2549 Ma in age. Thus, this study highlights the use of AMS in (a) deciphering the relative timing of regional deformation and emplacement of granitoids of equivalent age and (b) constraining the timing of regional superposed deformation events.

  15. Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent.

    Science.gov (United States)

    Wan, Bo; Windley, Brian F; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

    2015-09-21

    The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic.

  16. No coincidence? Exploring the connection between the Great Oxidation Event and craton stabilization during the Archean-Proterozoic transition

    Science.gov (United States)

    Kump, L. R.

    2014-12-01

    As geochronological constraints on the timing of the Great Oxidation Event (here defined as the passage of atmospheric oxygen levels through the proposed upper limit of 10-5 of present) have improved, it has become increasingly clear that this event is somehow tied to the tectonic factors that have defined the Archean-Proterozoic boundary for decades, namely the stabilization of continental cratons allowing for the growth of large continents. We have proposed two connections in the past: 1) elevated late Archean mantle plume activity brought oxidized material from the lithospheric graveyard to the upper mantle, reducing the oxygen fugacity of post-Archean volcanism, and 2) that the stabilization of the cratons allowed for a proportional increase in less-reducing, subaerial volcanism at the expense of more reducing, submarine volcanism. Critiques of these two proposals will be addressed in the context of subsequent work by the geosciences community on the geodynamics and geochemistry of the Archean-Proterozoic transition, and a synthetic hypothesis for a tectonic driver for atmospheric oxygenation will be presented.

  17. Ultra-hot Mesoproterozoic evolution of intracontinental central Australia

    Directory of Open Access Journals (Sweden)

    Weronika Gorczyk

    2015-01-01

    Full Text Available The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature (UHT metamorphism from ca. 1220 to ca. 1120 Ma. This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism. The tectonic setting for this thermal event (the Musgrave Orogeny is thought to have been intracontinental and the lithospheric architecture of the region is suggested to have had a major influence on the thermal evolution. We use a series of two dimensional, fully coupled thermo-mechanical-petrological numerical models to investigate the plausibility of initiating and prolonging UHT conditions under model setup conditions appropriate to the inferred tectonic setting and lithospheric architecture of the Musgrave Province. The results support the inferred tectonic framework for the Musgrave Orogeny, predicting periods of UHT metamorphism of up to 70 Ma, accompanied by thin crust and extensive magmatism derived from both crustal and mantle sources. The results also appear to be critically dependent upon the specific location of the Musgrave Province, constrained between thicker cratonic masses.

  18. Ultra-hot Mesoproterozoic evolution of intracontinental central Australia

    Institute of Scientific and Technical Information of China (English)

    Weronika Gorczyk a; Hugh Smithies b; Fawna Korhonen b; Heather Howard b; Raphael Quentin De Gromard b

    2015-01-01

    The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature (UHT) meta-morphism from ca. 1220 to ca. 1120 Ma. This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism. The tectonic setting for this thermal event (the Musgrave Orogeny) is thought to have been intracontinental and the lithospheric architecture of the region is suggested to have had a major influence on the thermal evolution. We use a series of two dimensional, fully coupled thermo-mechanical-petrological numerical models to investi-gate the plausibility of initiating and prolonging UHT conditions under model setup conditions appro-priate to the inferred tectonic setting and lithospheric architecture of the Musgrave Province. The results support the inferred tectonic framework for the Musgrave Orogeny, predicting periods of UHT meta-morphism of up to 70 Ma, accompanied by thin crust and extensive magmatism derived from both crustal and mantle sources. The results also appear to be critically dependent upon the specific location of the Musgrave Province, constrained between thicker cratonic masses.

  19. Dynamics of Pre-3 Ga Crust-Mantle Evolution

    Science.gov (United States)

    Patchett, P. J.; Chase, C. G.; Vervoort, J. D.

    2004-05-01

    During 3.0 to 2.7 Ga, the Earth's crust underwent a non-uniformitarian change from a pre-3.0 Ga environment where long-term preservation of cratons was rare and difficult, to post-2.7 Ga conditions where cratons were established and new continental crust generation took place largely at craton margins. Many models view the Earth's surface during pre-3 Ga time as broadly equivalent to the post 2.7 Ga regime. Any such uniformitarian or gradual evolution cannot explain the conundrum that only a tiny amount of pre-3 Ga crust is preserved today coupled with the fact that very little pre-3 Ga crust was incorporated into the large amount of new craton that came into existence during 3.0-2.7 Ga. If large volumes of pre-3 Ga continental crust existed, it disappeared either just prior to 3 Ga, or during 3.0-2.7 Ga. To explain sudden appearance of surviving but dominantly juvenile continental crust in a model where continents were large prior to 3 Ga, it would be necessary either that pre-3 Ga continent was recycled into the mantle at sites systematically different from those where new 3.0-2.7 Ga crust was made, or that widespread continent destruction preceded the 3.0-2.7 Ga crustal genesis. From expected mantle overturn in response to the heat budget, it is likely that most pre-3 Ga crust was both more mafic and shorter-lived than after 3 Ga. Although Nd and Hf ratios for pre-3 Ga rocks are uncertain due to polymetamorphism, it appears that depleted upper mantle was widespread by 2.7 Ga, even pre-3 Ga. Depletion may have been largely achieved by formation, subduction and storage of mafic crust for periods of 200-500 m.y. The rapid change to large surviving continents during 3.0-2.7 Ga was due to declining mantle overturn, and particularly to development of the ability to maintain subduction in one zone of the earth's surface for the time needed to allow evolution to felsic igneous rock compositions. In as much as storage of subducted slabs is probably occurring today, and

  20. Is subduction really in the plate tectonics driving seat, or do two other global mechanisms do the driving? A review in the 'deep-keeled cratons' frame for global dynamics

    Science.gov (United States)

    Osmaston, M. F.

    2012-04-01

    mantle boundary layer habitually assumed, but its LVZ content gives it (i) residual heat content, (ii) corresponding buoyancy, and (iii) a flexural strength which demands a reconsideration of its mode of downbend, hitherto widely regarded as flexural, but still be able to explain outer rises and their differences. Solutions for (ii) and (iii) are convincingly supported by widespread exposure of the resulting rocks in the Alps, telling us how they and other UHP metamorphic mountain belts have been built [11]. I will illustrate the essential points. In particular, the buoyancy (ii) provides the upward mechanical contact essential for the shallow basal subduction tectonic erosion of the upper plate as preparation of thin imbricate crustal slices to subduct to UHP. And a seismologically supported through-plate step-faulting mode of downbend copes with the flexure problem (iii) and provides the tectonic erosion mechanism. In tackling these matters, important intrinsic properties of the materials are, notably:- (1) the thermal conductivity of non-migrating interstitial melt is >20 times less than its parent rock, so the LVZ heat is effectively trapped during the plate's journey across the ocean, only to be released when subduction raises the pressure and the melt freezes; (2) the garnet-to-spinel peridotite phase change, typically at 50 to 90 km depth, gives some 50 times more volume change per joule than pure expansivity, and it does so with the big force of solid-state recrystallization. This force is the crack-wall push-apart force provided by our thick-plate MOR model, which thereby develops at least an order more ridge push than the divergent flow model. We now consider the post-downbend evolution of the subducting plate, recognizing both the heat content of its ex-LVZ material and that, within the 2-layer mantle picture established by the plate dynamics of 'deep-keeled cratons' [2 - 5], there is no substantial mantle transport across the 660 km level. Examination of

  1. 华北前寒武纪成矿系统与重大地质事件的联系%Secular changes of metallogenic systems link with continental evolving of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    翟明国

    2013-01-01

    Precambrian period is an oldest and longest eon from 5420Ma to about 4500Ma,taking over ~ 90% of the Earth' s history.The 80% ~ 90% continental crust in the Earth generated in Precambrian,which records complicated geotectonic processes.The most important geological events in Precambrian tectonic evolution include Neoarchean enormous crustal growth,tectonic regime inversion from pre-plate tectonics to plate tectonics,and great oxygen event (GOE) in Paleoproterozoic.The North China Craton (NCC) is one of oldest cratons in the world and records almost all the important geological events of the Earth.The NCC also demonstrates some special characteristics,such as multi-stage cratonization,Paleoproterozoic rift-subduction-collisional event,Earth' s paleo-climate and paleo-environment change,and Late-Paleoproterozoic-Neoproterozoic multi-stage rifting event.These important geological events controlled mineralization with tectonic evolution and formed various and abundant ore-deposits in the NCC.%前寒武纪是指显生宙最古老的地质时代——寒武纪之前的地质时代,它占了地球演化历史的近90%.地球陆壳的80%~90%以上是在前寒武纪形成的,记录了复杂和惊心动魄的地质构造过程,还赋存着丰富的矿产资源.前寒武纪最重要的地质事件有陆壳的巨量增生、前板块机制/板块机制的构造转折、由缺氧到富氧的地球环境的剧变.华北克拉通是全球最古老陆块之一,前寒武纪各阶段全球性重大地质事件几乎都被记录下来,并表现出一些特殊性.与全球其它克拉通相比,华北陆壳生长-稳定化过程具有多阶段特征,太古宙末-古元古代环境剧变记录复杂多样,古元古代与板块体制建立和超大陆演化相关的俯冲碰撞和伸展裂解等地质记录丰富,中-新元古代经历持续伸展并接受巨量裂谷沉积.这些重大地质事件都伴随大规模成矿作用,形成了华北克拉通丰富的矿产资源和独特的优势矿种.

  2. Relationship between Precambrian North Korean Peninsula and the North China Craton: Evidence from LA-ICP-MS U-Pb ages of detrital zircons from Neoproterozoic tillites of North Korea and Southern North China Craton

    Science.gov (United States)

    Hu, B.; Zhai, M.; Peng, P.; Zhang, Y.; Wu, J.; Jia, X.; Zhang, H.; Lei, W.; Zhuang, G.

    2016-12-01

    Relationship between Precambrian Korean Peninsula and the North China Craton (NCC) is focus of attention. There are Neoproterozoic tillites in Phyongnam Basin, Nangrim massif, North Korea (NK) and Southern NCC. Nangrim massif was regarded as a part of the NCC according to similar Precambrian basements between Nangrim massif and Longgang massif in the Northeast NCC. But the comparation of Neoproterozoic rocks is lacked between NK and NCC. Detrital zircon LA-ICP-MS U-Pb ages of 2 pebbly phyllite samples of Pirangdong Series in Phyongnam Basin and 2 argillaceous cemented mix-conglomeate samples of Luoquan Series in Southern NCC was analyzed in this research. Detrital zircon ages of pebbly phyllites of Pirangdong Series distribute mainly at 1.85 Ga, 1.8 Ga, 1.6 Ga, 1.4 Ga and 1.2 Ga. A small number of them are at 3.2 Ga, 2.6 - 2.5 Ga, 2.3 Ga, 2.1 Ga and 900 - 860 Ma. Detrital zircon ages of mix-conglomeates of Luoquan Series mainly focus on 2.5 Ga, 2.2 Ga, 2.0 Ga, 1.8 Ga and 1.6 Ga. Minor of them distribute at 1.12 Ga. The similar age distribution of Pirangdong and Luoquan Series of 2.6 - 2.5 Ga, 2.1 - 2.0 Ga, 1.85 - 1.8 Ga and 1.6 Ga corresponds to Precambrian significant tectonic- magmatic- thermal events of the NCC, which indicates that the Precambrian basement rocks of the NCC are main provenances of both Pirangdong and Luoquan Series. This also confirm that the Phyongnam Basin is a part of Neoproterozoic sedimentary covers of the NCC. It is worth to mention that 1.2 - 1.0 Ga and 900 - 850 Ma magmatic rocks in the NCC are seldom reported which relate to the assemblage and breakup of Rodinia Supercontinent. whereas they crop out widely in the South China Craton (SCC) and was always regarded as a mark distingusing the two craton. 1.2 - 1.0 Ga and 900 - 850 Ma zircon ages preserved in sedimentary rocks not only in North Korea and Southern NCC but also in Northeast NCC and East NCC provide data to compare Neoproterozoic strata between NCC and SCC and important clues to

  3. Chapter 27: Geology and petroleum potential of the north and east margins of the Siberian Craton, north of the Arctic Circle

    Science.gov (United States)

    Klett, T.R.; Wandrey, C.J.; Pitman, J.K.

    2011-01-01

    The Siberian Craton consists of crystalline rocks and superimposed Precambrian sedimentary rocks deposited in rift basins. Palaeozoic rocks, mainly carbonates, were deposited along the margins of the craton to form an outwardly younger concentric pattern that underlies an outward-thickening Mesozoic sedimentary section. The north and east margins of the Siberian Craton subsequently became foreland basins created by compressional deformation during collision with other tectonic plates. The Tunguska Basin developed as a Palaeozoic rift/sag basin over Proterozoic rifts. The geological provinces along the north and east margins of the Siberian Craton are immature with respect to exploration, so exploration-history analysis alone cannot be used for assessing undiscovered petroleum resources. Therefore, other areas from around the world having greater petroleum exploration maturity and similar geological characteristics, and which have been previously assessed, were used as analogues to aid in this assessment. The analogues included those of foreland basins and rift/sag basins that were later subjected to compression. The US Geological Survey estimated the mean undiscovered, technically recoverable conventional petroleum resources to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 103 trillion cubic feet of natural gas and 3 billion barrels of natural gas liquids. ?? 2011 The Geological Society of London.

  4. An integrated petrological, geochemical and Re-Os isotope study of peridotite xenoliths from the Argyle lamproite, Western Australia and implications for cratonic diamond occurrences

    Science.gov (United States)

    Luguet, A.; Jaques, A. L.; Pearson, D. G.; Smith, C. B.; Bulanova, G. P.; Roffey, S. L.; Rayner, M. J.; Lorand, J.-P.

    2009-11-01

    An integrated study of the petrology and Re-Os geochemistry of a suite of peridotite xenoliths, some carrying abundant diamonds, from the richly diamondiferous Argyle AK1 lamproite pipe provides definitive evidence for a depleted lithospheric root of Neoarchean age (T RD eruption ˜ 2.2-3.1 Ga) beneath the Proterozoic Halls Creek Orogen at the margin of the Kimberley Craton, Western Australia. The microdiamonds from the peridotitic xenoliths are similar in their properties to the minor population of small, commercial sized, peridotitic diamonds from Argyle, both formed in the Archean from isotopically mantle-like carbon. The major element bulk chemistry and mineral chemistry of the Argyle peridotites are slightly less depleted than Archean cratonic peridotites as a whole but similar to those reported from Neoarchean-Paleoproterozoic cratonic provinces. The Argyle peridotite xenoliths were derived from within the diamond stability field (1050-1300 °C and 4.9-5.9 GPa) near the base of the lithosphere (typically 160-200 km depth) with a geothermal gradient of 41.5 mW/m 2. This thick diamondiferous lithosphere, estimated at up to 225 km thick from present day seismic S-wave tomography, appears to have persisted since the time of eruption of the Argyle lamproite (˜ 1180 Ma). The existence of late Archean age lithosphere beneath the Argyle diamond pipe, in a region where no crustal rocks of Archean age are known, suggests a decoupling of the crust and mantle in the region of the Halls Creek Orogen, perhaps as a consequence of Paleoproterozoic (˜ 1.85 Ga) reworking and/or subduction at the margin of the Kimberley Craton. The confirmation of an Archean lithospheric root beneath the Argyle pipe at the margin of the Kimberley Craton seemingly conforms with "Clifford's Rule", regarding the restriction of economic diamond deposits to those underlain by Archean cratons. However, Argyle owes its rich diamond grades not to its Neoarchean mantle roots but to the presence of

  5. EarthScope in Midcontinent North America: Investigating the Architecture and Tectonic History of Cratonic-Platform Lithosphere

    Science.gov (United States)

    Marshak, S.; Larson, T.; Hamburger, M. W.; Pavlis, G. L.; Gilbert, H. J.; Parke, M.

    2010-12-01

    The transportable array of EarthScope will sweep across the Midcontinent of North America during 2011 and 2012. The central portion of this swath, between latitudes 36°N and 38°N, covers a "type example" of cratonic-platform lithosphere, where a veneer of Paleozoic sedimentary strata overlies Precambrian crystalline basement. In anticipating this scientific opportunity, we have compiled a unique suite of geologic, geophysical, subsurface, and topographic data sets for this area. The maps emphasize that, in spite of low topographic relief, the region has large subsurface structural relief. Specifically, its western portion includes a large intracratonic uplift (the Ozark Plateau), whereas its central portion includes a major intracratonic basin (the Illinois Basin). The elevation difference between the Cambrian-Precambrian unconformity at the crest of the Ozark Plateau and the same horizon at the base of the Illinois Basin (< 100 km to the east) is over 7.5 km. The region also includes the northern end of the Mississippi embayment (an anomalous depression), three major Proterozoic lithosphere accretionary boundaries (borders of the Yavapai, Mazatzal, and Grenville belts), one of the world's largest anorogenic igneous provinces (the Eastern Granite-Rhyolite Province), pronounced gravity and magnetic anomalies, and numerous fault-and-fold zones. Many of the zones remain active, both within and outside the notorious New Madrid seismic zone, making the central Midcontinent one of the most seismically active examples of cratonic platform lithosphere anywhere. As part of the USArray deployment in this region, a number of research groups (some of whom met at an EarthScope Workshop held in Urbana) have proposed dense, Flex-Array networks that would densify the sparser Transportable Array network. We propose an experiment that would span the Ozark Dome and the Illinois Basin, the Rough Creek Graben and other fault zones including the Wabash Valley seismic zone. This

  6. Aeromagnetic signatures reveal a back-arc basin imposed upon the inherited rifted margin of the East Antarctic craton

    Science.gov (United States)

    Armadillo, E.; Ferraccioli, F.; Jordan, T. A.; Bozzo, E.

    2009-12-01

    The Wilkes Subglacial Basin (WSB) represents a largely unexplored, approximately 1400 km-long and up to 600 km-wide subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. During the 2005-06 austral summer an extensive aerogeophysical survey was flown to investigate the WSB adjacent to northern Victoria Land (NVL), and included the acquisition of new airborne radar, aeromagnetic and aerogravity data. Several contrasting models for the origin of the basin have been previously proposed, and are based primarily on relatively sparse gravity data. These range from Cenozoic flexure, to distributed crustal extension of unknown age (possibly Mesozoic to Cenozoic), and even compression along the margin of craton. Our recent aeromagnetic data reveal that the basin is structurally controlled and has a tectonic origin, at least adjacent to NVL. The eastern margin of the basin is imposed upon an Early Paleozoic thrust fault belt, which can be traced under the ice using aeromagnetic signatures from exposures in Oates Land and the Ross Sea coast. Aeromagnetic patterns reveal that the western margin of the basin is imposed upon a Proterozoic-age shear zone mapped in the Mertz Glacier, and that is interpreted from geological studies to represent the continuation of a coeval shear zone in Australia. The broad aeromagnetic and satellite magnetic low over the WSB contrasts with the high over the un-reworked Proterozoic craton to the west of the basin, and is interpreted to reflect Neoproterozoic-age sediments deposited along the rifted margin of the craton. Magnetic intrusions within the WSB are interpreted as back-arc plutons that formed later in response to Cambrian-Ordovician age subduction along the paleo-Pacific margin of Gondwana. The aeromagnetic interpretation for a former broad back-arc basin in the WSB is supported by the occurrence of low-grade metasedimentary rocks of back-arc affinity in Oates Land, and also by the similarity in long

  7. The Rondonian-San Ignacio Province in the SW Amazonian Craton: An overview

    Science.gov (United States)

    Bettencourt, Jorge Silva; Leite, Washington Barbosa; Ruiz, Amarildo Salina; Matos, Ramiro; Payolla, Bruno Leonelo; Tosdal, Richard M.

    2010-01-01

    .50 Ga), Santo Antonio Intrusive Suite (1.40-1.36 Ga), and the Teotônio Intrusive Suite (1.38 Ga). Magmatism of these types also occur at the end of the Rondonian-San Ignacio Orogeny, and are represented by the Alto Candeias Intrusive Suite (1.34-1.36 Ga), and the São Lourenço-Caripunas Intrusive Suite (1.31-1.30 Ga). The cratonization of the province occurred between 1.30 and 1.25 Ga.

  8. Boron isotopes reveal multiple metasomatic events in the mantle beneath the eastern North China Craton

    Science.gov (United States)

    Li, Hong-Yan; Zhou, Zhou; Ryan, Jeffrey G.; Wei, Gang-Jian; Xu, Yi-Gang

    2016-12-01

    Linkages inferred between the geochemical heterogeneity of the mantle beneath eastern Eurasia and the stagnant Pacific slab documented geophysically in its mantle transition zone are as yet not clearly characterized. In this paper we report new elemental and isotopic data for boron (B) on a suite of well-characterized Cenozoic basalts (alkali basalts, basanites and nephelinites), with ocean island basalt (OIB)-like trace element signatures from western Shandong of the eastern North China Craton (NCC). Correlations between major elements (e.g., FeOT versus SiO2), trace elements (e.g., CeN/PbN versus BaN/ThN) and radiogenic isotopes (e.g., 206Pb/204Pb versus 87Sr/86Sr) suggest these basalts are derived via the mixing of melts from two mantle components: a fluid mobile element (FME; such as Ba, K, Pb and Sr) enriched component, which is most evident in the alkali basalts, and a FME depleted mantle component that is more evident in the basanites and nephelinites. The alkali basalts in this study have lower B concentrations (1.4-2.2 μg/g) but higher δ11B (-4.9 to -1.4) values than the basanites and nephelinites (B = 2.1-5.0 μg/g; δ11B = -6.9 to -3.9), and all the samples have nearly constant B/Nb ratios between 0.03 and 0.07, similar to the observed range in B/Nb for intraplate lavas. Our high-SiO2 samples have higher δ11B than that of our low SiO2 samples, indicating that the B isotopic differences among our samples do not result from the addition of a continental crustal component in the mantle source, or direct crustal assimilation during the eruption process. The positive B versus Nb correlation suggests the B isotopic compositions of the western Shandong basalts primarily reflect the pre-eruptive compositions of their mantle sources. Correlations among B, Nd and Sr isotope signatures of the western Shandong basalts differ from those among basalts from plume settings (e.g., Azores and Hawaii), and are inconsistent with models suggesting single-step metasomatic

  9. Gold distribution in Archean continental crust: Evaluating the effects of intracrustal differentiation in the Tanzanian Craton

    Science.gov (United States)

    Long, K.; Rudnick, R. L.; McDonough, W. F.; Manya, S.

    2013-12-01

    We have evaluated the vertical distribution of gold in variably metamorphosed igneous rocks in the Tanzanian Craton: 2.6 Ga upper-crustal greenschist-facies greenstone belt basalts and andesites from the Lake Victoria Gold Field of northern Tanzania, and compositionally similar 2.6 Ga lower-crustal mafic granulite-facies xenoliths that were carried in rift-related basalts that erupted nearby. We implemented the preconcentration method of Pitcairn et al. (2006), which utilizes chromatographic separation of gold from acid-digested rocks using diisobutyl ketone (DIBK), followed by standard addition ICP-MS to determine the distribution of gold in the crust. Repeat analyses of the certified reference material TDB-1, a whole-rock powder diabase dike from Tremblay Lake, Saskatchewan, Canada (certified gold concentration = 6.3 × 1.0 ng/g), yielded an average gold concentration of 6.5 × 1.1 ng/g. Results were reproducible to within 17% for rock powder aliquots between 200-600 mg (n=38), where 400 mg sample aliquots were reproducible to within 6% (n=9), and 600 mg aliquots were reproducible to within 4.5% (n=4). Better reproducibility for the greater sample aliquots likely reflects the 'nugget' effect. Rock samples in the 0.1-0.8 ng/g gold concentration range reproduced to within 27% for 400-600 mg sample aliquots. Although the lavas come from an area containing gold deposits, all were more than 5 km from any gold mine. The Tanzanian greenstone belt basalts have the highest gold concentrations (9 ng/g to 62 μg/g, ave. = 40 (+68/-25) ng/g, 1σ (n=10)), followed by the greenstone belt andesites (0.4 to 120 ng/g, ave. = 1.1 (+0.9/-0.5) ng/g, 1σ (n=14)). The lowest concentrations were observed in the granulite-facies lower-crustal xenoliths (0.1 to 3.3 ng/g, ave. = 0.3 (+0.3/-0.1) ng/g, 1σ (n=21)). Gold is incompatible in silicates and can partition into hydrothermal and/or magmatic fluid or vapour during high-grade metamorphic dehydration reactions or partial melting

  10. DOBRE studies the evolution of inverted intracratonic rifts in Ukraine

    Energy Technology Data Exchange (ETDEWEB)

    Grad, M.; Gryn, D.; Guterch, A. [and others

    2003-07-01

    DOBRE is a multinational study of the Donbas Foldbelt (DF) of Ukraine. The DF is the uplifted and deformed part of the more than 20-km thick Dniepr-Donets Basin (DDB) that formed as the result of rifting of the Eastern European Craton (EEC) in the late Devonian in what is now eastern Ukraine and southern Russia. The DF, especially its southern margin, was uplifted in Early Permian times, in a (trans)tensional tectonic stress regime while folding and reverse faulting occurred later, during the Triassic and late Cretaceous. In order to investigate this classic example of the tectonic inversion of a continental rift zone, DOBRE includes seismic refraction and reflection profiling as well as new geological mapping and geochemical studies. DOBRE is aimed at the study in general of the processes governing intracratonic rifting and inversion (uplift and shortening) of intracratonic rift basins and the DF displays exceptional characteristics for the study of the destabilisation of cratonic interiors, both extensionally and compressionally. Thus, DOBRE is expected to elucidate the evolution (destruction/replacement or deformation) of the Moho as well as other lower crustal/upper mantle processes that occur during rifting and rift reactivation and basin uplift and inversion. Furthermore, DOBRE should clarify the relationship between the craton and accreted terranes to the south, and evolutionary connection of these to central Europe. Finally, DOBRE can provide fundamental background information in support of further oil and gas exploration in Ukraine and the definition of environmental problems within the coal-mining province of the Donbas region. In this regard, preliminary findings of the highly complementary DOBRE datasets indicate that the basic crustal structure of the DF and the tectonic processes involved in its formation require fundamental revision. (BA)

  11. Contributions to the petrography, geochemistry and geochronology (U-Pb and Sm-Nd) of the Paleoproterozoic effusive rocks from Iricoume Group, Amazonian Craton, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Marques, Suelen Nonata de Souza; Nascimento, Rielva Solimairy Campelo do, E-mail: suelen-marques@hotmail.com, E-mail: rielva@ufam.edu.br [Universidade Federal do Amazonas (UFAM), Manaus, AM (Brazil). Inst. de Geociencias; Souza, Valmir da Silva; Dantas, Elton Luiz, E-mail: vsouza@unb.br, E-mail: elton@unb.br [Universidade de Brasilia (UnB), DF (Brazil). Inst. de Geociencias; Valerio, Cristovao da Silva, E-mail: cristovao@igeo.ufrr.br [Universidade Federal de Roraima (UFRR), Boa Vista, RR (Brazil). Inst. de Geociencias

    2014-07-01

    The southernmost region of the Guyana shield, Amazonian craton, hosts large record of Paleoproterozoic effusive rocks of the Iricoume Group. They present remarkably well-preserved igneous textures and structures. The SiO{sub 2} contents reveal a bimodal association marked by a compositional gap between acid (SiO{sub 2} > 67 wt%) and intermediate (SiO{sub 2} < 57.7 wt%) rocks. The acid effusive rocks are rhyolites to rhyodacites with high SiO{sub 2}, alkali, Rb, Zr, Nb + Ta, La + Ce and 104 Ga/Al content and low Fe{sub 2}O{sub 3tot}, TiO{sub 2}, CaO, Sr and Co content. They exhibit subalkaline, metaluminous-to-peraluminous compositions, and geochemically compatible to A-type magmatism emplaced in post-collisional to within-plate tectonic settings. The intermediate rocks are andesitic/basalt to andesite relatively high contents of TiO{sub 2}, Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3total}, MgO, CaO, Sr and Co; low SiO{sub 2}, K{sub 2}O, Rb, Zr, Nb + Ta, La + Ce. They have subalkaline and metaluminous geochemical composition and plot on within-plate basalt field. The acid rocks crystallized at 1882 ± 11 Ma in U-Pb analyses for LA-MC-ICPMS zircon data. The Sm-Nd isotopic data on all rocks reveal a Nd TDM model ages between 2.59 and 2.16 Ga and ε{sub Nd}(t) values between -5.78 and 0.03, indicate that the magmatic evolution was related to the reworking of older Paleoproterozoic at the Rhyacian-Siderian period, continental crust (Transamazonian crust-forming event) with some mixing with a limited amount mantle-derived magmas or with contamination by Archean crust. The petrographic, geochemical and geochronological data presented in this paper suggest a within-plate to post-collisional tectonic setting for the Iricoume volcanism, involving lower crust uplift and generation of basalt magma in an extensional regime. (author)

  12. Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton, India: Implications for early mantle heterogeneity

    Directory of Open Access Journals (Sweden)

    J.M. Maya

    2017-05-01

    Full Text Available Spinifex-textured, magnesian (MgO >25 wt.% komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaO/Al2O3, Al2O3/TiO2, Gd/Yb, La/Nb and Nb/Y suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial ɛNd(t value of +3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high ɛNd(t value of +3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ∼4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio. The source of Isua greenstone rocks with positive 142Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred

  13. Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data

    Science.gov (United States)

    Yegorova, T. P.; Stephenson, R. A.; Kostyuchenko, S. L.; Baranova, E. P.; Starostenko, V. I.; Popolitov, K. E.

    2004-03-01

    The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)-Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC-the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic-Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF-KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF-KS with amplitudes of 100-140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and

  14. Complex metasomatism of lithospheric mantle by asthenosphere-derived melts: Evidence from peridotite xenoliths in Weichang at the northern margin of the North China Craton

    Science.gov (United States)

    Zou, Dongya; Zhang, Hongfu; Hu, Zhaochu; Santosh, M.

    2016-11-01

    The petrology, in situ analyses of major and trace elements as well as Sr isotopic compositions of spinel-facies lherzolite and harzburgite xenoliths from Weichang within the northern margin of the North China Craton (NCC) are reported for the first time in this study to evaluate the nature and evolution of the lithospheric mantle. These peridotite xenoliths display porphyroclastic texture and can be subdivided into two groups. Group I peridotites have slightly higher forsterite contents (Fo) (90.6-91.2) and 87Sr/86Sr ratios (0.7025-0.7043) in the cores than those in the rims (89.8-90.8; 0.7025-0.7038) of olivines and clinopyroxenes, respectively. The clinopyroxenes in these rocks exhibit uniform LREE-depleted patterns. These geochemical features suggest that the Group I peridotites were weakly metasomatized by recent asthenospheric melts. In contrast, Group II peridotites show sieve-texture and clear compositional zoning in minerals. The olivines and clinopyroxenes from these rocks have higher Fo (86.9-91.3) and 87Sr/86Sr ratios (0.7035-0.7049) in the cores than those in the rims (76.9-90.6; 0.7021-0.7046). The clinopyroxenes show three types of REE patterns: LREE-enriched, convex-upward and spoon-shaped. The LREE-enriched clinopyroxenes have the highest (La/Yb)N and lowest Ti/Eu and those with spoon-shaped REE patterns show an increase in LREE, Ba, Th and U contents from the cores to the rims. These features indicate that the Group II peridotites witnessed a high degree of refertilization by recent asthenosphere-derived silicate and carbonatite melts or their mixture. Compared with the data of the Mesozoic NCC lithospheric mantle, the heterogeneous and low 87Sr/86Sr ratios (0.7025-0.7049) in the LREE-depleted clinopyroxenes reveal that the ancient lithospheric mantle could have been modified by asthenospheric melts before the recent metasomatism event. We conclude that the lithospheric mantle beneath Weichang underwent multiple modifications through asthenosphere

  15. Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005±9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921 ±1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892±10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.

  16. Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts

    Directory of Open Access Journals (Sweden)

    I.V. Ashchepkov

    2016-07-01

    Kharamai mantle clinopyroxenes represent three geochemical types: (1 harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; (2 lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; (3 eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary-Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and Sr, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe#Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian–Triassic superplume, but with no signs of delamination.

  17. Geochronology of middle Neoproterozoic volcanic deposits in Yangtze Craton interior of South China and its implications to tectonic settings

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    Here we report new SHRIMP dating results of the crystal tuff in Ejiaao Formation of middle Banxi Group in northern Guizhou. The results indicate that the volcanic deposition occurred at ~780 Ma. We also suggests that the igneous activities across Yangtze Craton with diabase dyke swarms and basal volcanic rocks during middle Banxi time are indicative of the episodic, extensive and vigorous great bimodal igneous events during middle Neoproterozoic (825-720 Ma) in South China. The characteristics of the igneous series are contrary to the model claiming they are of island-arc origin, but suggest that they are the records of rifting process and may be related to the episodic plume activities leading to the break-up of Rodinia.

  18. Magnetic fabrics and rock magnetism of the Xiong'er volcanic rocks and their implications for tectonic correlation of the North China Craton with other crustal blocks in the Nuna/Columbia supercontinent

    Science.gov (United States)

    Xu, Huiru; Yang, Zhenyu; Peng, Peng; Ge, Kunpeng; Jin, Zhenmin; Zhu, Rixiang

    2017-08-01

    The tectonic background of the Paleoproterozoic Xiong'er volcanic rocks (XVR) is important for understanding the tectonic evolution of the North China Craton (NCC), as well as its paleogeographic position during the assembly of the Nuna/Columbia supercontinent. Here we report the results of the first anisotropy of magnetic susceptibility (AMS) study of the XVR, and use the interpreted flow directions to constrain the emplacement mechanism and to assess its geological significance for the reconstruction of the Nuna/Columbia supercontinent. Thirty lavas were sampled from three sections in western Henan Province. Detailed rock magnetic analyses, including measurements of hysteresis loops, magnetization versus temperature curves and first order reverse curves, were performed to identify the main magnetic phases and grain sizes. The inferred directions from the AMS results reveal a radial flow pattern with an eruption center probably located near Xiong'er Mountain. Our data suggest that the XVR may have been emplaced in a triple-conjugated continental rift on the south margin of the NCC, probably initiated from a paleoplume. Based on this interpretation, a comparison of geological and paleomagnetic results among the proposed crustal blocks in the Nuna/Columbia supercontinent suggests a close linkage of the NCC with São Francisco-Congo, Rio de la Plate and Siberia.

  19. Traces of the crustal units and the upper mantle structure in the southwestern part of the East European Craton

    Directory of Open Access Journals (Sweden)

    I. Janutyte

    2014-04-01

    Full Text Available The presented study is a part of the passive seismic experiment PASSEQ 2006–2008 which took place around the Trans-European Suture Zone (TESZ from May 2006 to June 2008. The dataset of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs recorded in the PASSEQ seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT corrections. As a result, we obtained a model of P wave velocity variations in the upper mantle beneath the TESZ and the EEC. In the study area beneath the craton we observed 5 to 6.5% higher and beneath the TESZ about 4% lower seismic velocities compared to the IASP91 velocity model. We found the seismic lithosphere-asthenosphere boundary (LAB beneath the TESZ at a depth of about 180 km, while we observed no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic datasets indicated a ramp shape of the LAB in the northern TESZ where we observed values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we possibly found an upper mantle dome. In our results the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related with the crustal units in the study area. On the other hand, at a depth of 120–150 km we possibly found a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL and the West Lithuanian Granulite Domain (WLG. Also, in our results we may have identified two anorogenic granitoid plutons.

  20. Study on the genesis of Yishui banded iron formation (BIF) in the North China Craton: geochemical characteristics and tectonic environment

    Science.gov (United States)

    Moon, I.; Lee, I.; Yang, X.

    2015-12-01

    The Yishui BIFs are located in the Taishan Group, Shandong province of Eastern Block of North China Craton. The iron ore samples were collected from the mine pits. Major elements were analyzed by X-ray Fluoresence Spectromemter (XRF). Trace elements and REY (REE + Y) were analyzed by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Petrological, mineralogical and geochemical analyses of iron ores and their wall rock (amphibolite) were conducted to trace the genesis of Yishui BIF. Iron ores of Yishui BIF are mainly composed of SiO2 and Fe2O3T (SiO2+ Fe2O3T= 85.8 to 98.8 wt%) and consistent with major mineral components which are quartz and iron oxide such as magnetite and hematite. Low contents of TiO2 (0.01 to 0.09 wt%) , Al2O3 (0.42 to 1.18 wt%) and HFSE indicate no or little effect of detrital contamination. Iron ores have positive La, Eu, Gd, Er and Y anomalies with enriched HREE in PAAS normalized REY graph. The REY patterns of iron ores were used as a fingerprint to trace the source of iron and silica. Distinctive positive Eu anomalies (Eu/Eu*= 2.44-4.19), Y anomalies (Y/Y*=0.97 - 4.19), slightly negative Ce anomalies (Ce/Ce*= 0.87-0.97) and enriched HREE ((La/Yb)SN= 0.17-0.32) indicate that mixture of seawater and high-temperature hydrothermal fluid (>250 ◦C). Depositional environment in North China Craton implies that Yishui BIFs were formed at Neoarchean and associated arc-related tectonic setting. All these data suggest that Yishui BIFs belong to typical Algoma-type BIF.

  1. The Ufa indenter: stratigraphic and geophysic evidences for an actual indentation of the Southern Urals by the East European craton

    Science.gov (United States)

    Lefort, Jean-Pierre; Danukalova, Guzel

    2014-07-01

    Study of the altitudes of the lowest part of the Upper Cretaceous-Eocene and Aktschagylian-Quaternary stratigraphic ensembles known on the western slope of the Southern Urals evidences the existence of an East-West elongated dome which follows the N53° latitude. This ridge is superimposed at depth with the remnants of the Sernovodsk-Abdulino Aulacogen and with the Belaya tear fault, which support the existence of a recent rejuvenation of these old structures. North of these disruptions the Southern Urals display a clear bent towards the East. Detailed microstructural studies show that this curvature is associated with a typical stress pattern which suggests the existence of an indentation of the fold belt by the East European craton. The hypothesis of an Ufa indenter is not supported by an equivalent East-West deep fault north of the bend. However, a long N100° magnetic anomaly, interpreted as a shear zone, suggests that the indenter is a reality. Quaternary uplift and crustal thickening at its front as well as seismological data support our interpretation. It is not stressed that the curvature of the Urals observed at 56° latitude results solely from this recent indentation. It is only assumed that the actual indentation is rejuvenating a former unevenness which existed before in the East European craton. Study of the inner part of the indenter shows that this type of structure is not necessarily rigid and undeformed. Some of the structures described on the URSEIS deep seismic line could be much younger than previously expected.

  2. Tectonic evolution of the Dom Feliciano Belt in Southern Brazil: Geological relationships and U-Pb geochronology

    Directory of Open Access Journals (Sweden)

    Ruy Paulo Philipp

    Full Text Available ABSTRACT: The Dom Feliciano Belt is an important Neoproterozoic to Cambrian orogenic complex, extending from eastern Uruguay to southern Brazil. It comprises a collage of oceanic domains and continental fragments developed between 900 and 540 Ma between the Rio de La Plata, Congo and Kalahari cratons. The integration of field and structural data with recent isotopic results has introduced new insights on the sources of the magmatism and sedimentary processes. This paper presents a review of the geochronological results combined with stratigraphic, structural and geochemical data. The evolution of the Dom Feliciano Belt involved three orogenic events known as the Passinho (0.89 - 0.86 Ga, São Gabriel (0.77 - 0.68 Ga and Dom Feliciano (0.65 - 0.54 Ga. The first two events involved the closure of the Charrua Ocean generating an intra-oceanic arc (Passinho and, subsequently, an active continental margin arc (São Gabriel. This ocean separated the continental areas represented by the Rio de la Plata Craton and the Nico Perez continental microplate. Closure of the Adamastor ocean resulted in an important collisional event between the Nico Perez Microplate/Rio de La Plata Craton and Kalahari and Congo cratons between 650 and 620 Ma, involving high T/intermediate P metamorphism. At this time of crustal thickening, the partition of the deformation controled the final evolution of the belt with important escape tectonics, responsible for nucleating crustal-scale transcurrent shear zones. These structures were deep and promoted the rise of mafic magmas, which, associated with high regional thermal gradient, lead to an important event of crustal reworking, responsible for the formation of the Pelotas Batholith. The orogenic collapse is represented by late magmatism of Pelotas Batholith and deposition of upper section of the Camaquã Basin.

  3. Petrologic, Geochemical and Isotopic Study of 3.1Ga Peridotite-Chromitite Suite from the Western Dharwar Craton, India: Evidence for Recycling of Oceanic Crust in the Mesoarchean

    Science.gov (United States)

    Mondal, S. K.; Mukherjee, R.; Rosing, M. T.; Frei, R.; Waight, T.

    2008-12-01

    Sill-like ultramafic intrusions with massive chromitite bodies are common in Archean greenstone belts such as in the Zimbabwe craton and in the Singhbhum and Dharwar cratons of the Indian shield. In the western Dharwar craton deformed massive chromitites are hosted within dissected peridotitic rocks of the Nuggihulli schist belt and part of early to mid-Archean greenstone belts. The linear ultramafic belts are often associated with gabbroic rocks containing Ti-V-bearing magnetite bands and surrounded by the tonalite-trondhjemite- granodioritic (TTG) suites of rocks. Detailed electron microprobe study reveals high Cr-numbers (0.72- 0.87) and moderate Mg-numbers (0.47-0.59) for chromite and very high Fo content (~ Fo97) for interstitial olivine in massive chromitites from the seams. Our study suggests that the original igneous compositions of chromite grains are preserved in some of the massive chromitites, whereas accessory chromites in serpentinized peridotites are extensively altered to ferritchromit. The primitive composition of chromites along with high Fo content of olivine suggest parental melts produced by high degrees of partial melting of the source mantle. The tectonic discrimination plots plus parental melt calculations indicate derivation from a high-Mg komatiitic magma having a liquid Al2O3 ~ 9.64 wt% and liquid (FeO/MgO)wt% ~ 0.2 which is similar to the compositions recorded for komatiitic rocks within the schist belt. A Pb-isotope study of fourteen samples from this suite including peridotite, chromitite, gabbro and ultramafic-mafic schist rocks define an isochron yielding an age of 3159±180 Ma which is similar to the 207Pb/206Pb zircon age of ~ 3.1 Ga of the surrounding TTG-suite (Bidyananda et al. 2003). Ten samples from this suite have strongly scattering Sm-Nd model ages that range from 2.34 to 3.96 Ga (average 3.1 Ga) with respect to a depleted mantle source. The average initial Nd ratio, calculated at the presumed emplacement age of 3.1 Ga, is

  4. PdS and SdP Receiver Functions Image of the Lithosphere underneath the Southern African Regions

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Thybo, Hans; Levander, A

    2009-01-01

    One of the unique spots in our Earth is the Kaapvaal craton; it preserves a rich record of the early earth and is underlain by unusually thick, ~ 200 km deep, chemically depleted lithospheric mantle of early Archean age. The internal structure of this unusual upper mantle, termed the tectosphere...... of a significant difference in anisotropy between the craton and the surrounding tectonic belts. The evidence for this variation arises from the good azimuthal coverage, which provides an obvious polarity reversal of signals versus back azimuth coinciding spatially with the Kaapvaal boundaries. To confirm...

  5. Electrical Conductivity Model of the Mantle Lithosphere of the Slave Craton (NW Canada) and its tectonic interpretation in the context of Geochemical Results

    Science.gov (United States)

    Lezaeta, P.; Chave, A.; Evans, R.; Jones, A. G.; Ferguson, I.

    2002-12-01

    The Slave Craton, northwestern Canada, contains the oldest known rocks on Earth, with exposed outcrop over an area of about 600x400 km2. The discovery of economic diamondiferous kimberlite pipes during the early 1990s motivated extensive research in the region. Over the last six years, four types of deep-probing magnetotelluric (MT) surveys were conducted within the framework of diverse geoscientific programs, aimed at determining the regional-scale electrical structures of the craton. Two of the surveys involved novel acquisition; one through frozen lake ice along ice roads during winter, and the second deploying ocean-bottom instrumentation from float planes during summer. The latter surveys required one year of recording between summers, thus allowing long period transfer functions that lead to mantle penetration depths of over 300 km. Two-dimensional modeling of the MT data from along the winter road showed the existence of a high conductivity zone at depths of 80-120 km beneath the central Slave craton. This anomalous region is spatially coincident with an ultradepleted harzburgitic layer in the upper mantle that was interpreted by others to be related to a subducted slab emplaced during the mid-Archean. A 3-D electrical conductivity model of the Slave lithosphere has been obtained, by trial and error, to fit the magnetic transfer and MT response functions from the lake experiments. This 3-D model traces the central Slave conductor as a NE-SW oriented mantle structure. Its NE-SW orientation coincides with that of a late fold belt system, with the first phase of craton-wide plutonism at ca 2630-2590 Ma, three-part subdivision of the craton based on SKS results, and with a G10 (garnet) geochemical mantle boundaries. All of these highlight a NE-SW structural grain to the lithospheric mantle of the craton, in sharp contrast to the N-S grain of the crust. Constraints on the depth range and lateral extension of the electrical conductive structure are obtained

  6. Phanerozoic tectonic evolution of the Circum-North Pacific

    Science.gov (United States)

    Nokleberg, Warren J.; Parfenov, Leonid M.; Monger, James W.H.; Norton, Ian O.; Khanchuk, Alexander I.; Stone, David B.; Scotese, Christopher R.; Scholl, David W.; Fujita, Kazuya

    2000-01-01

    The Phanerozoic tectonic evolution of the Circum-North Pacific is recorded mainly in the orogenic collages of the Circum-North Pacific mountain belts that separate the North Pacific from the eastern part of the North Asian Craton and the western part of the North American Craton. These collages consist of tectonostratigraphic terranes that are composed of fragments of igneous arcs, accretionary-wedge and subduction-zone complexes, passive continental margins, and cratons; they are overlapped by continental-margin-arc and sedimentary-basin assemblages. The geologic history of the terranes and overlap assemblages is highly complex because of postaccretionary dismemberment and translation during strike-slip faulting that occurred subparallel to continental margins.We analyze the complex tectonics of this region by the following steps. (1) We assign tectonic environments for the orogenic collages from regional compilation and synthesis of stratigraphic and faunal data. The types of tectonic environments include cratonal, passive continental margin, metamorphosed continental margin, continental-margin arc, island arc, oceanic crust, seamount, ophiolite, accretionary wedge, subduction zone, turbidite basin, and metamorphic. (2) We make correlations between terranes. (3) We group coeval terranes into a single tectonic origin, for example, a single island arc or subduction zone. (4) We group igneous-arc and subduction- zone terranes, which are interpreted as being tectonically linked, into coeval, curvilinear arc/subduction-zone complexes. (5) We interpret the original positions of terranes, using geologic, faunal, and paleomagnetic data. (6) We construct the paths of tectonic migration. Six processes overlapping in time were responsible for most of the complexities of the collage of terranes and overlap assemblages around the Circum-North Pacific, as follows. (1) During the Late Proterozoic, Late Devonian, and Early Carboniferous, major periods of rifting occurred along

  7. Major, trace and platinum group element (PGE) geochemistry of Archean Iron Ore Group and Proterozoic Malangtoli metavolcanic rocks of Singhbhum Craton, Eastern India: Inferences on mantle melting and sulphur saturation history

    Digital Repository Service at National Institute of Oceanography (India)

    Singh, M.R.; Manikyamba, C.; Ray, J.; Ganguly, S.; Santosh, M.; Saha, A.; Rambabu, S.; Sawant, S.S.

    The geological and metallogenic history of the Singhbhum Craton of eastern India is marked by several episodes of volcanism, plutonism, sedimentation and mineralization spanning from Paleoarchean to Mesoproterozoic in a dynamic tectonic milieu...

  8. New paleomagnetic data from Bornholm granitoids testing whether the East-European Craton rotated during the 1.50-1.45 Ga Danopolonian orogeny

    Science.gov (United States)

    Lubnina, N.; Bogdanova, S.; Cecys, A.

    2009-04-01

    -temperature/coercivity component points to the NE moderate positive inclination. The paleomagnetic pole recalculated from this component (Plat=52.7 Plong=160.7 dp=5.4 dm=3.2) corresponds to ca. 250 Ma poles of the EEC. The second, high temperature/coercivity component points to the NE with shallow negative inclination. The mean direction of this component is Dec=13.9 Inc=-38.5 K=27.6 a95=6.7. The paleomagnetic pole is close to the poles obtained from the Mesoproterozoic Lake Ladoga and Dalarna mafic rocks. The mafic dyke cutting the Ronne granitoids demonstrates mainly one stable high-temperature/coercivity component with the NE moderate positive inclination. The mean direction of this component (Dec=21.7 Inc=41.9 K=87.3 a95=5.2) is similar to that of the low-temperature component of the granitoids. Therefore it correlates with the Permo-Triassic poles of the EEC. The baked granitoids show the same stable component as the mafic dyke. Thus, the positive indirect contact test evidences a primary origin of the high-temperature component in the host granitoids, which is also supported by the close coincidence of the obtained Mesoproterozoic pole with the coeval poles of the Dalarna and Lake Ladoga rocks. The newly obtained paleomagnetic data from the 1.46 Ga Bornholm granitoids favour an anticlockwise rotation of the entire East-European Craton as a probable consequence of the Danopolonian orogeny. However, some local rotation of different blocks within the EEC like those in the Pasha-Ladoga graben cannot be ruled out entirely. This is a contribution to the project "The Precambrian structure of Baltica as a control of its recent environment and evolution" of the Visby Programme (the Swedish Institute) and RFBR project 07-05-01140, a. References Bogdanova, S. et al., 2008. Precambrian Research 160, 23-45. Condie, K.C., 2002. Gondwana Research, 5(1): 41-43. Hoffman, P.F., 1997. In: Earth Structure: an Introduction to Structural Geology and Tectonics, New York, pp. 459-464. Lubnina, N. et al., 2005. In

  9. The 1.33-1.30 Ga Yanliao large igneous province in the North China Craton: Implications for reconstruction of the Nuna (Columbia) supercontinent, and specifically with the North Australian Craton

    Science.gov (United States)

    Zhang, Shuan-Hong; Zhao, Yue; Li, Xian-Hua; Ernst, Richard E.; Yang, Zhen-Yu

    2017-05-01

    The Yanliao rift zone in the northern North China Craton (NCC) is the location of the standard section for late Paleoproterozoic-Mesoproterozoic stratigraphy in China and is associated with the emplacement of large volumes of diabase sills. Detailed field investigations show that the sills are distributed over a region that is >600 km long and >200 km wide, with areal extent > 1.2 ×105 km2 and cumulative thickness of the sills in any one area ranging from 50 m to >1800 m. High-resolution secondary ion mass spectrometry (SIMS) baddeleyite dating shows that emplacement of these sills occurred between about 1330 and 1305 Ma with a peak age of 1323 Ma. Emplacement of these diabase sills was accompanied by pre-magmatic uplift that started at about 1.35-1.34 Ga as indicated by the disconformity between the Changlongshan and Xiamaling formations and absence of sedimentation after the Xiamaling Formation in some areas. All the diabase sills exhibit similar geochemical features of tholeiitic compositions with intraplate characteristics. Given a relatively short duration of emplacement at 1.33-1.30 Ga, along with the large areal extent and volume, as well as intraplate character, this magmatic province constitutes a large igneous province (LIP). This Yanliao LIP and the accompanying pre-magmatic uplift were related either to a mantle plume and/or continental rifting during breakup of the NCC from the Nuna (Columbia) supercontinent. Paleomagnetic, ash bed and LIP data and other geological constraints suggest that the NCC had a close connection with Siberia, Laurentia, Baltica, North Australia and India crustal blocks. In particular, the most direct age match between the 1.33-1.30 Ga Yanliao LIP and the 1.33-1.30 Ga Derim Derim-Galiwinku LIP of the North Australian Craton (NAC), as well as the similarities between the late Paleoproterozoic-Mesoproterozoic stratigraphic units of the Yanliao rift in the NCC with the southeastern McArthur Basin in the NAC, indicate that the

  10. Combined teleseismic imaging of the structure of southern African cratons using P-receiver functions and P-and S-finite-frequency tomography

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Thybo, Hans; Levander, Alan;

    2011-01-01

    bands (1, 0.5 and 0.25 Hz for P and 0.1, 0.05 and 0.02 Hz for S) to obtain 3-D P- and S-wave perturbation models for the upper mantle. Crustal corrections are based on the RF models. Tests showed that our dataset is able to resolve structure of 3°x3° up to 400 km depth. The high-velocity cratonic roots...... extend to 300-350 km depth. Lower velocities are detected below the Bushveld complex and the mobile belts. The model also suggests a stratified structure, since we found a low velocity zone (LVZ) at about 170 km depth in the cratonic areas. SdP RFs and surface-wave tomography are in progress and should...

  11. Neogene marine isotopic evolution and the erosion of Lesser Himalayan strata: Implications for Cenozoic tectonic history

    Science.gov (United States)

    Myrow, Paul M.; Hughes, Nigel C.; Derry, Louis A.; Ryan McKenzie, N.; Jiang, Ganqing; Webb, A. Alexander G.; Banerjee, Dhiraj M.; Paulsen, Timothy S.; Singh, Birendra P.

    2015-05-01

    An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this "upper Lesser Himalayan" succession, which include deposits of the "outer" Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼ 16 Ma. First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.

  12. Petrological and Sr-Nd-Os isotopic constraints on the origin of the Fanshan ultrapotassic complex from the North China Craton

    Science.gov (United States)

    Niu, Xiaolu; Chen, Bin; Liu, Ankun; Suzuki, K.; Ma, Xu

    2012-09-01

    The Fanshan ultramafic-syenitic complex is located on the northern margin of North China Craton, with zircon U-Pb ages of 220 Ma. It is a concentrically zoned complex, with syenite in the core, surrounded by ultramafic rocks (clinopyroxenite, glimmerite) and garnet-clinopyroxene syenite, respectively, towards the rim. Diopside, biotite, orthoclase, melanite, magnetite and apatite are the major minerals, with subordinate sphene and calcite. Mineralogy, petrology and geochemical studies point to the formation of the complex through fractional crystallization and accumulation from a SiO2-undersaturated ultrapotassic alkaline-peralkaline parent magma that is characterized by high CaO, Fe2O3, K2O and fluid compositions (P2O5, CO2, H2O, F), and by high temperature and high oxygen fugacity. Rocks from the complex are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE; e.g., Rb, Ba, Sr), depleted in high field strength elements (HFSE; e.g., Nb, Ta, Ti), and show relatively homogeneous Sr-Nd isotopic compositions with initial 87Sr/86Sr = 0.70513-0.70597 and ɛNd(t) = - 5.8 to - 5.3. The data suggest that the Fanshan complex originated from a phlogopite-clinopyroxenite-rich lithosphere mantle source that had previously been metasomatized by melts from carbonated oceanic crust above a subduction zone. Highly radiogenic Os isotope compositions (initial 187Os/188Os = 0.299-2.449) suggest that the parent magma to the Fanshan complex has been contaminated by Precambrian mafic crustal rocks during magma emplacement in crustal levels. The occurrence of the Fanshan complex, together with many other ultramafic/alkaline plutons of similar ages, on the northern margin of the North China Craton suggests that the northern margin of the craton entered into a large-scale extensional regime in late Triassic after the final amalgamation between Mongolian microcontinent and the craton.

  13. Chemical deposits associated to metavolcanosedimentary sequences of the central portion of the São Francisco craton the state of Bahia, Brazil: a review

    OpenAIRE

    Carvalho, Ilson Guimarães

    2000-01-01

    p. 279-284 Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the São Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMP) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported...

  14. 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

    2004-12-01

    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.

  15. Multi-phase tectonic structures in the collision zone of the Kolyma-Omolon microcontinent and the eastern margin of the North Asian craton, Northeastern Russia

    OpenAIRE

    Prokopiev, A. V.; Oxman, V. S.

    2009-01-01

    The sequence of formation of structures is established in the zone of junction of the eastern margin of the North Asian craton and the northeastern flank of the Kolyma-Omolon microcontinent, in the area of bend of the Kolyma structural loop. Detailed structural studies revealed two phases in the formation of Mesozoic structures – an early thrust phase and a late strike-slip phase. Structures formed during each of the phases are described. Thrust structures are represented...

  16. Sequence stratigraphy of the Upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: Transgressing assumptions of cratonic flooding

    Science.gov (United States)

    Eoff, Jennifer D.

    2014-01-01

    New data from detailed measured sections permit comprehensive analysis of the sequence framework of the Furongian (Upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. The sequence-stratigraphic architecture of the lower part of the Sunwaptan Stage at the base of the Tunnel City Group, at the contact between the Wonewoc Formation and Lone Rock Formation, records the first part of complex polyphase flooding (Sauk III) of the Laurentian craton, at a scale smaller than most events recorded by global sea-level curves. Flat-pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation. Thinly-bedded, fossiliferous sandstone represents the most distal setting during earliest highstand. Subsequent deposition of sandstone characterized by hummocky or trough cross-stratification records progradational pulses of shallower, storm- and wave-dominated environments across the craton before final flooding of Sauk III commenced with carbonate deposition during the middle part of the Sunwaptan Stage. Comparison of early Sunwaptan flooding of the inner Laurentian craton to published interpretations from other parts of North America suggests that Sauk III was not a single, long-term accommodation event as previously proposed.

  17. SHRIMP zircon U-Pb ages for the Paleoproterozoic metamorphic-magmatic events in the southeast margin of the North China Craton

    Institute of Scientific and Technical Information of China (English)

    GUO SuShu; LI ShuGuang

    2009-01-01

    A garnet-pyroxene bearing amphibolite as a xenolith hosted by the Mesozoic igneous rocks from Xuzhou-Suzhou area was dated by zircon SHRIMP U-Pb method, which yields a metamorphic age of Archean metamorphic complex named Wuhe group in the Bengbu uplift give a metamorphic U-Pb age Bengbu uplift give a magma crystallization U-Pb age of 2054 ± 22 Ma. Both the Xuzhou-Suzhou area and Bengbu uplift are located in the southeastern margin of the North China Craton. Therefore, these ages indicate that there is a Paleoproterozoic tectonic zone in the southeastern margin of the North China Craton, and its metamorphic and magmatic ages are consistent with those of the other three Paleoproterozoic tectonic zones in the North China Craton. In view of the large scale sinistral strike-slip movement occurred at the Mesozoic along the Tan-Lu fault zone, the position of the eastern Shandong area, which is a south section of the Paleoproterozoic Jiao-Liao-Ji Belt, was correlated to Xuzhou-Suzhou-Bengbu area prior to movement of the Tan-Lu fault zone. This suggests that the Xuzhou-Suzhou-Bengbu Paleoproterozoic tectonic zone might be a southwest extension of the Paleoproterozoic Jiao-Liao-Ji Belt.

  18. Tectono-magmatic evolution of the Hutti-Maski Greenstone Belt, India: Constrained using geochemical and geochronological data

    Science.gov (United States)

    Rogers, A. J.; Kolb, J.; Meyer, F. M.; Armstrong, R. A.

    2007-08-01

    The Hutti-Maski Greenstone Belt (HMGB), situated in the eastern block of the Dharwar Craton, India is dominated by bimodal volcanics with a minimum magmatic age of 2586 ± 59 Ma. Two phases of granitoid intruded into the belt, the syn-tectonic Kavital granitoid, homogeneous, medium-grained porphyritic granodiorite, with an intrusion age of 2543 ± 9 Ma, followed by the post-tectonic Yelagatti granitoid, heterogeneous, fine- to medium-grained granite to granodiorite. The extensively altered zircons from the Yelagatti granitoid have significant enrichments of U, Th (>1%) and common-Pb (up to 47%). Only two of the analyses were reproducible, providing a minimum 207Pb/ 206Pb age of 2221 ± 99 Ma, this may indicate an approximate magmatic age or more realistically a subsequent event. Felsic metavolcanic rocks contain cross-cutting veinlets, which formed during the Mesoproterozoic (ca. 1180 Ma), the final closure of the Pb system occurred between the Neoproterozoic and Ordovician, possibly related to the Pan-African orogeny. The tectono-magmatic evolution of the HMGB can be correlated with the collision between the eastern and western blocks of the Dharwar Craton subsequent to 2658 Ma and the craton wide magmatism from 2613 to 2513 Ma. These events can be accounted for by combining uniformitarian and non-uniformitarian models.

  19. Research on lithospheric density distributions beneath North China Craton and its destruction mechanism by gravity and seismic observations

    Science.gov (United States)

    Wang, X.; Fang, J.; Hsu, H.

    2011-12-01

    North China Craton (NCC) has been a research hotspot for geoscientists all over the world. Partial North China Craton (NCC) has lost its lithospheric keel since Mesozoic. Researchers have reached a consensus on destruction of NCC' lithosphere, however, the destruction mechanism and dynamic processes still remain controversy. In this study, a three-dimensional density distribution of lithosphere beneath NCC is determined using gravity datum combined with P-wave travel times by sequential inversion method. After the analyses and discussions on our density results referred to other geophysical and geochemical researches and then gave our viewpoint about destruction mechanisms of NCC lithosphere from the standpoint of density distribution. A linear velocity-density relationship is used to achieve mutual transformations and constraints between density and velocity. As we know, the gravity anomalies measured on the ground surface are the integrated reflection of the interface undulations and underground density inhomogeneous. In order to invert the lithospheric density structures, we firstly separated the gravity effects of lithospheric density inhomogeneous by removing the effects of other contributions to the gravity field from the observed integrated gravity filed before density inversion. The method of Zhao et al.,(1994) is used for seismic tomography, while Algebraic Reconstruction Technique (ART) is applied in density inversion, which highly improved the calculation velocity compared to common least squares method. The inversion results indicate that, the lithospheric density beneath NCC is extremely inhomogeneous and its distributions are coherent with surface regional tectonics; Low density anomalies exist in lower crust beneath rift basins around Ordos block. High poisson' ratios are found in these regions (about 3.0), which may indicate partial melting occurred. Receive function studies prevailed thinned ( 8.2km/s) is also found in this region. The prominent

  20. Gold deposits of the northern margin of the North China craton: Multiple late Paleozoic-Mesozoic mineralizing events

    Science.gov (United States)

    Hart, C.J.R.; Goldfarb, R.J.; Qiu, Y.; Snee, L.; Miller, L.D.; Miller, M.L.

    2002-01-01

    The northern margin of the North China craton is well-endowed with lode gold deposits hosting a resource of approximately 900 tonnes (t) of gold. The ???1,500-km-long region is characterized by east-trending blocks of metamorphosed Archean and Proterozoic strata that were episodically uplifted during Variscan, Indosinian, and Yanshanian deformational and magmatic events. At least 12 gold deposits from the Daqinshan, Yan-Liao (includes the Zhangjiakou, Yanshan, and Chifeng gold districts), and Changbaishan gold provinces contain resources of 20-100 t Au each. Most deposits are hosted in uplifted blocks of Precambrian metamorphic rocks, although felsic Paleozoic and Mesozoic plutons are typically proximal and host ???30% of the deposits. The lodes are characterized by sulfide-poor quartz veins in brittle structures with low base metal values and high Au:Ag ratios. Although phyllic alteration is most common, intensive alkali feldspar metasomatism characterizes the Wulashan, Dongping, and Zhongshangou deposits, but is apparently coeval with Variscan alkalic magmatism only at Wulashan. Stepwise 40Ar-39Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U-Pb dates on associated granitoids, suggest that gold mineralizing events occured during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129 Ma. However, widespread Permo-Triassic (???250 Ma) and Early Jurassic (???180 Ma) thermal events caused variable resetting of most of the white mica and K-feldspar argon spectra, as well as previously reported K-Ar determinations. Compiled and new stable isotope and fluid inclusion data show that most ??18O values for ore-stage veins range from 8 to 14???, indicating a fluid in equilibrium with the Precambrian metamorphic basement rocks; ??D values from fluid inclysions range widely from -64 to -154???, which is indicative of a local meteoric component in some veins; and highly variable ??34S data

  1. Petrogenesis of the middle Jurassic appinite and coeval granitoids in the Eastern Hebei area of North China Craton

    Science.gov (United States)

    Fan, Wenbo; Jiang, Neng; Xu, Xiyang; Hu, Jun; Zong, Keqing

    2017-05-01

    An integrated study of zircon U-Pb ages and Hf-O isotopic compositions, whole rock elemental and Sr-Nd isotope geochemistry was conducted on three lithologically diverse middle Jurassic plutons from the Eastern Hebei area of the North China Craton (NCC), in order to reveal both their petrogenesis and possible tectonic affinity. The three plutons have consistent magmatic zircon U-Pb ages from 167 ± 1 Ma to 173 ± 1 Ma. The Nianziyu pluton has typical characteristics of appinite with low SiO2 (43.7-52.6%), high Ca, Mg, Fe and H2O contents. It possesses subduction-related trace element patterns, enriched Nd-Hf isotopic signatures as well as elevated zircon δ18O values (6.2-7.2‰), arguing for an enriched mantle source metasomatized by fluids related to subduction. The Shuihutong monzogranites have high silica (SiO2 = 75.4-75.9%) and alkali contents, low Ca contents and striking negative Ba, Sr and Eu anomalies. Samples from the pluton have more evolved Nd-Hf isotopic values and are considered to be most likely derived from anatexis of ancient lower continental crust. Hybridization between mantle- and ancient lower crust-derived magmas is proposed for the mafic microgranular enclave-bearing Baijiadian granitoids, which are characterized by variable εNd (t) and εHf(t) values. Integrated with the regional geologic history, we suggest that the formation of the three middle Jurassic plutons were related to the subduction of the Paleo-Pacific ocean plate beneath the NCC. Their petrogenetic differences reflect complex magmatic processes in subduction settings involving melting of multiple sources, possible partly facilitated by fluid metasomatism and water-rich magma injection, accompanied with various degrees of magma mixing. The appearance of middle Jurassic appinitic rocks leads us to propose that the NCC destruction and lithosphere thinning were facilitated and controlled by the weakening of the lithospheric mantle after hydration because of the subduction of the

  2. Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

    Science.gov (United States)

    Mandal, Prantik

    2017-02-01

    In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (∼2.5-3.6 Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (∼1.0-2.5 Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7-70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6 Ga Dalma and ∼117 Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (Vs) of 3.0 km/s and a 26-km thick lower-crust (LC) of average Vs of 3.6 km/s. The present study detects a sharp drop in Vs (∼-2 to 3%) at 120-260 km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120 km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88-103 km depths, suggesting a 88-103 km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2-3% drop in Vs could be attributed to the presence of partial melts in the

  3. Compositions of Upper Mantle Fluids Beneath Eastern China:Implications for Mantle Evolution

    Institute of Scientific and Technical Information of China (English)

    ZHANG Mingjie; WANG Xianbin; LIU Gang; ZHANG Tongwei; BO Wenrui

    2004-01-01

    The composition of gases trapped in olivine, orthopyroxene and clinopyroxene in lherzolite xenoliths collected from different locations in eastern China has been measured by the vacuum stepped-heating mass spectrometry.These xenoliths are hosted in alkali basalts and considered as residues of partial melting of the upper mantle, and may contain evidence of mantle evolution. The results show that various kinds of fluid inclusions in lherzolite xenoliths have been released at distinct times, which could be related to different stages of mantle evolution. In general, primitive fluids of the upper mantle (PFUM) beneath eastern China are dominated by H2, CO2 and CO, and are characterized by high contents of H2 and reduced gases. The compositions of PFUM are highly variable and related to tectonic settings. CO, CO2 and H2 are the main components of the PFUM beneath cratons; the PFUM in the mantle enriched in potassic metasomatism in the northern part of northeastern China has a high content of H2, while CO2 and SO2 are the dominant components of the PFUM in the Su-Lu-Wan (Jiangsu-Shandong-Anhui) region, where recycled crustal fluids were mixed with deeper mantle components. There are several fluids with distinct compositions beneath eastern China, such as primitive fluids of upper mantle (CO, CO2 and H2), partial melting fluids (CO2 and CO) and metasomatic fluids mixed with recycled crustal fluids (CO2, N2, 8O2 and CH4) etc. Fluids of the upper mantle beneath the North China craton are different from that of the South China craton in total gases and chemical compositions: the contents of the reduced gases of the PFUM in the NCC are higher than those in the SCC.

  4. Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton:Implication for replacement process of lithospheric mantle

    Institute of Scientific and Technical Information of China (English)

    ZHANG Hongfu; YING Jifeng; XU Ping; MA Yuguang

    2004-01-01

    Mesoxzoic(125 Ma) Fangcheng basalts from Shandong Province contain clearly zoned olivines that are rare in terrestrial samples and provide first evidence for the replacement of lithospheric mantle from high-Mg peridotites to low-Mg peridotites through peridotite-melt reaction. Zoned olivines have compostions in the core(Mg#=87.2-90.7)similar to those olivines from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the North China craton and in the rim (Mg#=76.8-83.9)close to olivine phenocrysts of the host basalts (75.7-79.0).These compositional features as well as rounded crystal shapes and smaller grain sizes (300-800μm)demonstrate that these zoned olivines are mantle xenocrysts ,I.e.an important type of the replacement of lithospheric mantle.The reaction resulted in the transformation of the Paleozoic refractory (high-Mg)peridotites to the late Mesozoic fertile (low-Mg) and radiogenic isotope-enriched peridotites,leading to the loss of old lithospheric mantle.

  5. Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton, southern India

    Indian Academy of Sciences (India)

    R Anand; S Balakrishnan

    2011-04-01

    Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.

  6. Mantle lithosphere transition from the East European Craton to the Variscan Bohemian Massif imaged by shear-wave splitting

    Directory of Open Access Journals (Sweden)

    L. Vecsey

    2014-01-01

    Full Text Available We analyse splitting of teleseismic shear-wave recorded during the PASSEQ passive experiment (2006–2008 focussed on the upper mantle structure across the Trans-European Suture Zone (TESZ. 1009 pairs of the delay times of the slow split-shear waves and orientations of the polarized fast-shear waves exhibit lateral variations across the array, as well as backazimuth dependences of measurements at individual stations. While a distinct regionalization of the splitting parameters exists in the Phanerozoic part of Europe, a correlation with the large-scale tectonics around the TESZ and in the East European Craton (EEC is less evident. No general and abrupt change in the splitting parameters (anisotropic structure can be related to the Teisseyre–Tornquist Zone (TTZ, marking the edge of the Precambrian province on the surface. Instead, regional variations of anisotropic structure were found along the TESZ/TTZ. We suggest a south-westward continuation of the Precambrian mantle lithosphere beneath the TESZ and the adjacent Phanerozoic part of Europe, probably as far as towards the Bohemian Massif.

  7. Ichnologic evidence of a Cambrian age in the southern Amazon Craton: Implications for the onset of the Western Gondwana history

    Science.gov (United States)

    Santos, Hudson P.; Mángano, M. Gabriela; Soares, Joelson L.; Nogueira, Afonso C. R.; Bandeira, José; Rudnitzki, Isaac D.

    2017-07-01

    Colonization of the infaunal ecospace by burrowing bilaterians is one of the most important behavioral innovations during the Ediacaran-Cambrian transition. The establishment of vertical burrows by suspension feeders in high-energy nearshore settings during Cambrian Age 2 is reflected by the appearance of the Skolithos Ichnofacies. For the first time, unquestionable vertical burrows typical of the Skolithos Ichnofacies, such as Skolithos linearis, Diplocraterion parallelum and Arenicolites isp., are recorded from nearshore siliciclastic deposits of the Raizama Formation, southeastern Amazon Craton, Brazil. Integration of ichnologic and sedimentologic datasets suggests that these trace fossils record colonization of high-energy and well-oxygenated nearshore sandy environments. Chronostratigraphically, the presence of these vertical burrows indicates an age not older than early Cambrian for the Raizama Formation, which traditionally has been regarded as Ediacaran. Therefore, the Raizama ichnofauna illustrates the advent of modern Phanerozoic ecology marked by the Agronomic Revolution. The discovery of the Skolithos Ichnofacies in these shallow-marine strata suggests possible connections between some central Western Gondwana basins.

  8. Big insights from tiny peridotites: Evidence for persistence of Precambrian lithosphere beneath the eastern North China Craton

    Science.gov (United States)

    Liu, Jingao; Rudnick, Roberta L.; Walker, Richard J.; Xu, Wen-liang; Gao, Shan; Wu, Fu-yuan

    2015-05-01

    Previous studies have shown that the eastern North China Craton (NCC) lost its ancient lithospheric mantle root during the Phanerozoic. The temporal sequence, spatial extent, and cause of the lithospheric thinning, however, continue to be debated. Here we report olivine compositions, whole-rock Re-Os isotopic systematics, and platinum-group element abundances of small ( 92) lithospheric mantle is largely absent. Osmium isotopic data suggest the Wudi peridotites experienced melt depletion primarily during the Paleoproterozoic (~ 1.8 Ga), although an Archean Os model age for one xenolith indicates incorporation of a minor component of Archean lithospheric mantle. These data suggest that a previously unrecognized Paleoproterozoic orogenic event removed and replaced the original Archean lithospheric mantle beneath the sedimentary basin at the southern edge of the Bohai Sea. By contrast, the Fuxin peridotites, entrained in Cretaceous basalts that crop out along the northern edge of the eastern NCC, document the coexistence of both ancient (≥ 2.3 Ga) and modern lithospheric mantle components. Here, the original Late Archean-Early Paleoproterozoic lithospheric mantle was, at least partially, removed and replaced prior to 100 Ma. Combined with literature data, our results show that removal of the original Archean lithosphere occurred within Proterozoic collisional orogens, and that replacement of Precambrian lithosphere during the Mesozoic may have been spatially associated with the collisional boundaries and the strike-slip Tan-Lu fault, as well as the onset of Paleo-Pacific plate subduction.

  9. Geochemical signatures of copper redistribution in IOCG-type mineralisation, Gawler Craton, South Australia

    Science.gov (United States)

    Uvarova, Yulia A.; Pearce, Mark A.; Liu, Weihua; Cleverley, James S.; Hough, Robert M.

    2017-07-01

    The Emmie Bluff iron oxide, copper, gold (IOCG) prospect is located in the Olympic Dam district, South Australia, and hosts sub-economic 150-m-thick Cu-Au mineralisation associated with the hematite-chlorite-sericite alteration with chalcopyrite commonly replacing pre-existing pyrite at a depth of 800 m. With the use of cutting-edge synchrotron X-ray fluorescence microscopy and field emission gun-scanning electron microscopy, it is shown for the first time that sub-economic IOCG mineralisation in the Olympic Dam district was affected by a late fluid event, which resulted in partial dissolution of Cu mineralisation and transport of Cu in the form of chloride complexes. The porous chlorite-sericite matrix associated with the late alteration of chalcopyrite hosts a Cu-Cl-OH phase previously undescribed in IOCG rocks, which was identified as one of the polymorphs of the atacamite group of minerals, Cu2Cl(OH)3. Thermodynamic modelling shows that "atacamite" is produced during dissolution of chalcopyrite by an oxidised, Cl-bearing fluid. An acidic environment is produced within millimetres of the chalcopyrite grains during oxidation. This process drives chlorite recrystallisation that is recorded by compositional variation of chlorite proximal to chalcopyrite. The existence of the atacamite is discussed in the context of fluid evolution and interaction with IOCG-type mineralisation and its implications to ore preservation versus destruction and remobilisation.

  10. Lithosphere structure of the Donbas Fodbelt and Karpinsky Swell region (the southern margin of the East-European Craton), Ukraine and Russia, from seismic and gravity data

    Science.gov (United States)

    Yegorova, T.; Baranova, E.; Starostenko, V.

    2003-04-01

    LITHOSPHERE STRUCTURE OF THE DONBAS FOLDBELT AND KARPINSKI SWELL REGION (THE SOUTHERN MARGIN OF THE EAST-EUROPEAN CRATON), UKRAINE AND RUSSIA, FROM SEISMIC AND GRAVITY DATA T.Yegorova (1), E.Baranova (1), V.Starostenko (1) (1) Institute of Geophysics, National Academy of Sciences of Ukraine egorova@igph.kiev.ua Along the southern margin of the East-European platform (EEC) super deep Late Devonian rift basins Dnieper-Donets Basin (DDB) and Peri-Caspian Basin (PCB) are located. The structures are adjacent to a zone along which crust was reworked and/or accreted to the EEC during Late Palaeozoic-Triassic times. The objective of the present study is deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the EEC. The study area includes the inverted southernmost part of the intracratonic DDB Donbas Foldbelt (DF), its south-eastern prolongation along the margin of the EEC the sedimentary succession of the Karpinsky Swell (KS), the south-western part of the Peri-Caspian Basin (PCB) and the Scythian Plate. According to the structure of the sedimentary basin, the DF and the KS form a single linear structure, represented by the uplift of Palaeozoic rocks, with the exposure of Carboniferous coal-bearing rocks in the DF, and by deep trough (down to the depth of 20 km and more) on the top of the crystalline Precambrian basement. The 3D gravity back-stripping analysis, implemented to test the sediment structure, reveals a distinct elongate zone of positive sediment corrected anomalies along the axis of the DF-KS and strong positive anomaly in the PCB. This is caused by heterogeneous lithosphere structure below the basin: Moho topography and/or the existence of a high density material in the crystalline crust and uppermost mantle. Our previous investigations have supported the existence of high-density body in the crystalline crust along the DDB axis. The

  11. Stellar evolution

    CERN Document Server

    Meadows, A J

    2013-01-01

    Stellar Evolution, Second Edition covers the significant advances in the understanding of birth, life, and death of stars.This book is divided into nine chapters and begins with a description of the characteristics of stars according to their brightness, distance, size, mass, age, and chemical composition. The next chapters deal with the families, structure, and birth of stars. These topics are followed by discussions of the chemical composition and the evolution of main-sequence stars. A chapter focuses on the unique features of the sun as a star, including its evolution, magnetic fields, act

  12. Crustal evolution of South American Platform based on Sm-Nd isotope geochemistry; Evolucao crustal da plataforma sul americana com base na geoquimica isotopica Sm-Nd

    Energy Technology Data Exchange (ETDEWEB)

    Sato, Kei

    1998-07-01

    Sm-Nd isotopic systematics is relevant to the topics of origin and evolution the of continental crust, where model ages refer to the time when crustal material was differentiated from the upper mantle. Alternative interpretations are due to a lack of adequate information on crustal processes and the variable composition of the mantle sources. The Sm-Nd methods are presented, and applied on rock materials from the South American Platform. The main conclusions indicate juvenile accretion with higher growth rates (peaks), around 3.7-3.5 Ga ({approx} 0.5% in volume), 3.1 - 2.9 Ga ({approx}16%), 2.7 - 2.6 ({approx} 9%), 2.2 - 1.9 (35%) and 1.3-1.0 (7%). The continental growth curve indicates that about 35 % of the crust was formed by 2.5 Ga, 88% by 1.8 Ga and 99% by 1.0 Ga, and the remaining {approx} 1 % was added in the Phanerozoic. Rapid crustal growth occurred between 2.2 and 1.9 Ga. The main period of continental crust formation occurred during the Paleoproterozoic, corresponding to 54 % in volume. Sm-Nd model ages, when compared with the crystallisation ages of granitoid rocks, furnish a rough estimate of juvenile vs. reworked material. Within the South American Platform about 45% of juvenile continental crust is still preserved within tectonic provinces of different ages. The remainder represents continental crust reworked in younger tectono-thermal events. In particular crustal reworking was predominating over juvenile accretion during Meso-Neoproterozoic. The Transbrasiliano Lineament is a megasuture, active in the Neoproterozoic, which separates a large northwestern mass, including the Amazonian and Sao Luis Cratons, from a southeastern mass, formed by a collage of cratonic fragments, of which the Sao Francisco and Rio de La Plata are the largest. The crustal evolutions of these two large continental masses are considered individually, and can be resumed following form: I - Old Archean rocks (>3.4 Ga) are found only within the south-eastern part (Gaviao Block

  13. Neoproterozoic diamictite-bearing sedimentary rocks in the northern Yili Block and their constraints on the Precambrian evolution of microcontinents in the Western Central Asian Orogenic Belt

    Science.gov (United States)

    He, Jingwen; Zhu, Wenbin; Zheng, Bihai; Wu, Hailin; Cui, Xiang; Lu, Yuanzhi

    2015-12-01

    The origin and tectonic setting of Precambrian sequences in the Central Asian Orogenic Belt (CAOB) have been debated due to a lack of high resolution geochronological data. Answering this question is essential for the understanding of the tectonic framework and Precambrian evolution of the blocks within the CAOB. Here we reported LA-ICP-MS detrital zircon U-Pb ages and in-situ Hf isotopic data for Neoproterozoic sedimentary cover in the northern Yili Block, an important component of the CAOB, in order to provide information on possible provenance and regional tectonic evolution. A total of 271 concordant U-Pb zircon ages from Neoproterozoic sedimentary cover in the northern Yili Block define three major age populations of 1900-1400 Ma, 1300-1150 Ma and 700-580 Ma, which are quite different from cratons and microcontinents involved in the CAOB. Although it is not completely consistent with the local basement ages, an autochthonous provenance interpretation is more suitable. Some zircon grains show significant old Hf model ages (TDMC; 3.9-2.4 Ga) and reveal continental crust as old as Paleoarchean probably existed. Continuous Mesoproterozoic zircon age populations exhibit large variations in the εHf(t) ratios, suggesting the long-time involvement of both reworked ancient crust and juvenile material. Similar Mesoproterozoic evolution pattern is identified in many continental terranes involved in the CAOB that surround the Tarim Craton. Based on our analysis and published research, we postulate that the northern Yili Block, together with Chinese Central Tianshan, Kyrgyz North Tianshan and some other microcontinents surrounding the Tarim Craton, once constituted the continental margin of the Tarim Craton in the Mesoproterozoic, formed by long-lived accretionary processes. Most of the late Neoproterozoic zircons exhibit significant positive εHf(t) ratios, suggesting the addition of juvenile crust. It is consistent with the tectonic event related to the East Africa

  14. U-Pb evidence for late Neoarchean crustal reworking in the Southern São Francisco Craton (Minas Gerais, Brazil

    Directory of Open Access Journals (Sweden)

    José C.S. Campos

    2003-12-01

    Full Text Available The Passa Tempo Metamorphic Complex is one of several metamorphic complexes that form the Archean sialic crust of the southern São Francisco Craton. It encompasses hypersthene-bearing gneissic rocks, with subordinateNW- or EW-trending mafic-ultramafic bodies and granodioritic to alkali-granitic, weakly foliated, and light-colored granitoids. These granitoids are the product of generalized migmatization that followed granulite-facies metamorphism. To determine the ages of the granulite-facies metamorphism and granitoid genesis, we obtained U-Pb ages on zircon extracted from the mesosome and leucosome of the migmatitic gneisses. For the mesosome, a discordia that intercepts Concordia at 2622 ± 18 Ma is interpreted as a minimum age for granulite-facies metamorphism. For the leucosome, the upper intercept of discordia at 2599 ± 45 Ma corresponds to migmatization and granitoid genesis. Contemporaneous metamorphism and magmatism have been documented elsewhere in the São Francisco Craton, especially in the southern portion, demonstrating vast and vigorous reworking of sialic crust by the end of the Neoarchean.O Complexo Metamórfico Passa Tempo é um dos diversos complexos metamórficos que constituem a crosta siálica arqueana da porção meridional do Cráton São Francisco. Ele engloba rochas gnáissicas portadoras de hiperstênio, com corpos máfico-ultramáficos orientados NW ou EW subordinados e granitóides granodioríticos a sienograníticos, fracamente foliados e de coloração clara. Corpos de granitóides isolados de dimensões decamétricas a quilométricas são o produto de generalizada migmatização que seguiu metamorfismo de facies granulito. Para determinar as idades do metamorfismo de fácies granulito e da gênese dos granitos, nós obtivemos idades U-Pb em zircões extraídos de mesossoma e leucossoma de gnaisses migmatíticos. Para o mesossoma, a discórdia que intercepta a Concórdia em 2622 ±18 Ma é interpretada como idade

  15. Mafic dyke swarms of the Bastar Craton, central India: geochemistry, Sr-Nd isotopes and tectonic implications

    Science.gov (United States)

    Liao, Chien-Yi; Shellnutt, J. Gregory; Raghvan Hari, Kosiyathu; Viswakarma, Neeraj

    2017-04-01

    The Archean Bastar Craton of central India which is demarcated by Godivari rift in the west, Mahanadi rift in the east, Narmada-Son rift in the north and the Eastern Ghats Mobile Belt in the south was intruded by several generations of mafic dyke swarms during the Precambrian, with most of the dykes NW-SE to WNW-ESE trending. The dyke swarms can be subdivided into three main groups: the Meso-Neoarchean sub-alkaline mafic dykes (BD1), Neoarchean to Paleoproterozoic boninite-norite dykes (BN) and Paleoproterozoic sub-alkaline mafic dykes (BD2). In the present work, an attempt has been made to constrain the petrogenesis and tectonic implications of the dykes in the Bhanupratapur area of central Bastar Craton. Petrographically, the dykes are metabasites / metadolerites that experienced hydrothermal alteration, as most of the samples are moderately to highly altered. Primary minerals were replaced but still retain the original igneous texture (ophitic). The least-altered samples contain orthopyroxene, plagioclase, secondary amphibole, secondary quartz and some euhedral Fe-Ti oxides. The dykes can be classified into two groups according to major element data. Petrographically, the two groups cannot be clearly distinguished. Group 1 is sub-alkaline basalt (SiO2 = 51 wt%) with low magnesium (MgO = 4.6 to 7.2 wt%), low Mg-number (Mg# ≤ 53) and higher titanium (TiO2 ≥ 0.83 wt%). Group 2 is sub-alkaline basaltic andesite and has boninitic characteristics with higher silica (SiO2 = 51 to 56 wt%), higher magnesium (MgO = 5.9 to 19.1 wt%), higher Mg-number (Mg# = 50 to 79) and lower titanium (TiO2 ≤ 0.8 wt%). The chondrite-normalized REE patterns of Group 1 show variability in the light rare earth elements (La/SmN = 1.2 to 2.4). Group 2 exhibit light rare earth element enrichment (La/YbN = 2.2 to 10.4). The Sm-Nd isochron age of Group 1 is approximately 1.9 Ga, with the initial 87Sr/86Sr ratios range from 0.7029 to 0.7058 and ɛNd(t) values range from-0.9 to +1.8. Group 1

  16. Platinum Group Elements (PGE) geochemistry of komatiites and boninites from Dharwar Craton, India: Implications for mantle melting processes

    Science.gov (United States)

    Saha, Abhishek; Manikyamba, C.; Santosh, M.; Ganguly, Sohini; Khelen, Arubam C.; Subramanyam, K. S. V.

    2015-06-01

    High MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for platinum group elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. Though their higher PGE abundance is economically significant in mineral exploration studies, their lower concentrations are also valuable geochemical tools to evaluate petrogenetic processes. In this paper an attempt has been made to evaluate the PGE geochemistry of high MgO volcanic rocks from two greenstone belts of western and eastern Dharwar Craton and to discuss different mantle processes operative at diverse geodynamic settings during the Neoarchean time. The Bababudan greenstone belt of western and Gadwal greenstone belt of eastern Dharwar Cratons are dominantly composed of high MgO volcanic rocks which, based on distinct geochemical characteristics, have been identified as komatiites and boninites respectively. The Bababudan komatiites are essentially composed of olivine and clinopyroxene with rare plagioclase tending towards komatiitic basalts. The Gadwal boninites contain clinopyroxene, recrystallized hornblende with minor orthopyroxene, plagioclase and sulphide minerals. The Bababudan komatiites are Al-undepleted type (Al2O3/TiO2 = 23-59) with distinctly high MgO (27.4-35.8 wt.%), Ni (509-1066 ppm) and Cr (136-3036 ppm) contents. These rocks have low ΣPGE (9-42 ppb) contents with 0.2-2.4 ppb Iridium (Ir), 0.2-1.4 ppb Osmium (Os) and 0.4-4.4 ppb Ruthenium (Ru) among Iridium group PGE (IPGE); and 1.4-16.2 ppb Platinum (Pt), 2.8-19 ppb Palladium (Pd) and 0.2-9.8 ppb Rhodium (Rh) among Platinum group PGE (PPGE). The Gadwal boninites are high-Ca boninites with CaO/Al2O3 ratios varying between 0.8 and 1.0, with 12-24 wt.% MgO, 821-1168 ppm Ni and 2307-2765 ppm Cr. They show higher concentration of total PGE (82-207 ppb) with Pt concentration ranging from 13 to 19 ppb, Pd between 65 and 180 ppb and Rh in the range of 1.4-3 ppb compared to the Bababudan komatiites. Ir

  17. Simulating Evolution

    Science.gov (United States)

    Stebbins, Robert C.; Allen, Brockenbrough

    1975-01-01

    Described are simulations that can be used to illustrate evolution by natural selection. Suggestions for simulating phenomena such as adaptive radiation, color match to background and vision of predators are offered. (BR)

  18. Schumpeter's Evolution

    DEFF Research Database (Denmark)

    Andersen, Esben Sloth

    reworking of his basic theory of economic evolution in Development from 1934, and this reworking was continued in Cycles from 1939. Here Schumpeter also tried to handle the statistical and historical evidence on the waveform evolution of the capitalist economy. Capitalism from 1942 modified the model...... of economic evolution and added evolutionary contributions to other social sciences. History, which was published by his widow, was based on his evolutionary theory of the history of economic analysis. This sequential analysis of Schumpeter's six books demonstrates the progress he within his research......This draft of a book on Schumpeter is distributed for commenting. It is a stylised intellectual biography that focus on the emergence and extension of the Schumpeterian vision and analysis of economic and social evolution. The draft provides novel interpretations of Schumpeter's six major books. He...

  19. Schumpeter's Evolution

    DEFF Research Database (Denmark)

    Andersen, Esben Sloth

    This draft of a book on Schumpeter is distributed for commenting. It is a stylised intellectual biography that focus on the emergence and extension of the Schumpeterian vision and analysis of economic and social evolution. The draft provides novel interpretations of Schumpeter's six major books. He...... originally developed his evolutionary research programme in Wesen from 1908 by studying the inherent limitations of Neoclassical Economics. He presented core results on economic evolution and sketched an extension evolutionary analysis to all social sciences in Entwicklung from 1912. He made a partial...... reworking of his basic theory of economic evolution in Development from 1934, and this reworking was continued in Cycles from 1939. Here Schumpeter also tried to handle the statistical and historical evidence on the waveform evolution of the capitalist economy. Capitalism from 1942 modified the model...

  20. Geology of the Early Archean Mid-Ocean Ridge Hydrothermal System in the North Pole Dome, Pilbara Craton, Western Australia

    Science.gov (United States)

    Kitajima, K.; Maruyama, S.

    2007-12-01

    An Archean hydrothermal system in the North Pole Dome, Pilbara Craton is associated with extensive fluid circulation driven by numerous extensional fracture systems and the underlying heat source. The fracture system is now occupied by abundant fine-grained quartz aggregate, hence we call this as silica dikes. Some of the fracture system extends deeper structural levels as listric normal faults down to 1000 m depth in the MORB crust. Barite-bearing fine-grained quartz predominant mineralogy indicates the extensive development of fracturing and quenching in a short time. Accompanying the fluid circulation, the extensive metasomatism proceeded to form the four different chemical courses, (1) silicification, (2) carbonation, (3) potassium-enrichment, and (4) Fe- enrichment. Silicification occurs along the silica dikes, carbonated greenstones are distributed relatively shallower level. Potassium-enriched (mica-rich) greenstones occur at the top of the greenstone sequence, and Fe-enriched (chlorite-rich) greenstones are distributed at lower part of the basaltic greenstones. The down going fluid precipitated carbonate-rich layer at shallow levels, whereas depleted in SiO2. Then, the fluid went down to more deeper level, and was dissolved SiO2 at high temperature (~350°C) and chlorite-rich greenstone was formed by water-rock interaction. The upwelling fluid precipitated dominantly SiO2 and formed silica dikes. Silica dikes cement the fractures formed by extensional faulting at earliest stage of development of oceanic crust. Therefore, the hydrothermal system must have related to normal fault system simultaneously with MORB volcanism. Particularly the greenish breccia with cherty matrix (oregano chert) was formed at positions by upwelling near ridge axis. After the horizontal removal of MORB crust from the ridge-axis with time, the propagating fracture into deeper levels, transports hydrothermal fluids into 500-1000 m depth range where metasomatic element exchange between

  1. Geochemistry of Archean metasedimentary rocks of the Aravalli craton, NW India: Implications for provenance, paleoweathering and supercontinent reconstruction

    Science.gov (United States)

    Ahmad, Iftikhar; Mondal, M. E. A.; Satyanarayanan, M.

    2016-08-01

    Basement complex of the Aravalli craton (NW India) known as the Banded Gneissic Complex (BGC) is classified into two domains viz. Archean BGC-I and Proterozoic BGC-II. We present first comprehensive geochemical study of the Archean metasedimentary rocks occurring within the BGC-I. These rocks occur associated with intrusive amphibolites in a linear belt within the basement gneisses. The association is only concentrated on the western margin of the BGC-I. The samples are highly mature (MSm) to very immature (MSi), along with highly variable geochemistry. Their major (SiO2/Al2O3, Na2O/K2O and Al2O3/TiO2) and trace (Th/Sc, Cr/Th, Th/Co, La/Sc, Zr/Sc) element ratios, and rare earth element (REE) patterns are consistent with derivation of detritus from the basement gneisses and its mafic enclaves, with major contribution from the former. Variable mixing between the two end members and closed system recycling (cannibalism) resulted in the compositional heterogeneity. Chemical index of alteration (CIA) of the samples indicate low to moderate weathering of the source terrain in a sub-tropical environment. In A-CN-K ternary diagram, some samples deceptively appear to have undergone post-depositional K-metasomatism. Nevertheless, their petrography and geochemistry (low K2O and Rb) preclude the post-depositional alteration. We propose non-preferential leaching of elements during cannibalism as the cause of the deceptive K-metasomatism as well as enigmatic low CIA values of some highly mature samples. The Archean metasedimentary rocks were deposited on stable basement gneisses, making the BGC-I a plausible participant in the Archean Ur supercontinent.

  2. Out-of-sequence thrusting in polycyclic thrust belts: An example from the Mesozoic Yanshan belt, North China Craton

    Science.gov (United States)

    Li, Chengming; Zhang, Changhou; Cope, Tim D.; Lin, Yi

    2016-09-01

    The EW trending Yanshan belt, an intraplate fold-thrust belt located in the northern North China Craton that has experienced several episodes of deformation widely separated in time, is characterized by out-of-sequence thrusts. According to detailed mapping in the central Yanshan belt, five geometric and stratigraphic criteria used to aid in determining whether a thrust has an out-of-sequence geometry or not can be recognized. They are (1) unconformable relationships, (2) inclination of fault surfaces, (3) irregular changes in apparent offset along strike, (4) short fault length relative to apparent offset, and (5) in-sequence geometry. With the help of these criteria, two generations of out-of-sequence thrusts that postdate the original in-sequence thrusting in the central Yanshan belt are recognized. The ancestral southward verging fold-and-thrust belt that formed prior to 180 Ma was deformed and cut by two younger generations of faults that are probably more deeply rooted and are constrained to between 172-165 Ma and 152-135 Ma. A series of thrusts with opposite vergence formed during the last period, resulting in abundant abnormal field relationships such as younger-on-older thrust relations, fold truncation, and cutting down-section. The nature and occurrence of faults in the Yanshan belt implies that superimposed deformation, a common feature in polycyclic orogenic belts, is a mechanism for the generation of out-of-sequence thrusting. This adds to mechanisms already described in the literature, such as maintaining constant critical taper at an orogenic scale, inhibition of the deformation front, and lateral changes in the nature of the décollement horizons.

  3. Layering of the lithospheric mantle beneath the Siberian Craton: Modeling using thermobarometry of mantle xenolith and xenocrysts

    Science.gov (United States)

    Ashchepkov, I. V.; Vladykin, N. N.; Ntaflos, T.; Kostrovitsky, S. I.; Prokopiev, S. A.; Downes, H.; Smelov, A. P.; Agashev, A. M.; Logvinova, A. M.; Kuligin, S. S.; Tychkov, N. S.; Salikhov, R. F.; Stegnitsky, Yu. B.; Alymova, N. V.; Vavilov, M. A.; Minin, V. A.; Babushkina, S. A.; Ovchinnikov, Yu. I.; Karpenko, M. A.; Tolstov, A. V.; Shmarov, G. P.

    2014-11-01

    Single-grain thermobarometric studies of xenocrysts were used to compile local SCLM transects through the major regions of kimberlite magmatism in Siberia and longer transects through the subcontinental mantle lithosphere (SCLM) beneath the Siberian craton. The mantle structure was obtained using P-Fe#, Ca in garnets, oxygen fugacity values fO2 and calculated temperatures T°C. The most detail transect obtained for the Daldyn field on the Udachnaya-Zarnitsa reveals layering showing an inclination of > 35° to Udachnaya. Mantle layering beneath the Alakit field determined from the Krasnopresnenskaya-Sytykanskaya transect shows a moderate inclination from N to S. The inflection near Yubileinaya-Aykhal is also supported by the extreme depletion in peridotites with low-Fe sub-Ca garnets. Beneath the Malo-Botuobinsky field the sharply layered mantle section starts from 5.5 GPa and reveals step-like P-Fe#Ol trends for garnets and ilmenites. The deeper part of SCLM in this field was originally highly depleted but has been regenerated by percolation of protokimberlites and hybrid melts especially beneath Internationalnaya pipe. The three global transects reveal flat layering in granite-greenstone terranes and fluctuations in the granulite-orthogneiss Daldyn collision terranes. The mantle layering beneath the Daldyn - Alakite region may have been created by marginal accretion. Most of southern fields including the Malo-Botuobinsky field reveal flat layering. The primary subduction layering is smoothed beneath the Alakit field. Lower Jurassic kimberlites from the Kharamai-Anabar kimberlite fields reveal a small decrease of the thickness of the SCLM and heating of its base. The Jurassic Kuoyka field shows an uneven base of the SCLM inclined from west to east. SCLM sequences sampled at this time started mainly from depths of 130 km, but some pipes still showed mantle roots to 250 km. The garnet series demonstrates an inclined straight line pyroxenite P-Fe# trend due to

  4. Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

    Science.gov (United States)

    Downes, H.; Macdonald, R.; Upton, B.G.J.; Cox, K.G.; Bodinier, J.-L.; Mason, P.R.D.; James, D.; Hill, P.G.; Hearn, B.C.

    2004-01-01

    Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzbugites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0??5121 (close to the host minette values) to 0??5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd

  5. Modes, tempo and spatial variability of Cenozoic cratonic denudation: morphoclimatic constraints from West Africa

    Science.gov (United States)

    Beauvais, Anicet; Chardon, Dominique

    2010-05-01

    After the onset of Gondwana break-up in the Early Mesozoic, the emerged part of the African plate underwent long Greenhouse effect climatic periods and epeirogeny. The last Greenhouse effect period in the Early Cenozoic and the alternation of wet and dry climatic periods since the Eocene enhanced episodes of rock chemical weathering and laterite production, forming bauxites and ferricretes, interrupted by drier periods of dominantly mechanical denudation, shaping glacis [1]. In Sub-Saharan West Africa, this evolution resulted in pulsate and essentially climatically-forced denudation that has shaped an ubiquitous sequence of five stepped lateritic paleosurfaces that synchronously developed over Cenozoic times. The modes, timing and spatial variability of continental denudation of the region are investigated by combining geomorphologic and geochronological data sets. The geomorphologic data set comprises the altitudinal distribution of the lateritic paleosurfaces relicts and their differential elevation from 42 locations in Sub-Saharan West Africa where the sequence (or part of it) has been documented. The geochronological data set consists in the age ranges of each paleosurface tackled by radiometric 39Ar-40Ar dating of the neoformed oxy-hydroxides (i.e., cryptomelane, K1-2Mn8O16, nH2O, [4]) carried by their laterites at the Tambao reference site, Burkina Faso [1, 3]. Five groups of 39Ar-40Ar ages, ~ 59 - 45 Ma, ~ 29 - 24 Ma, ~ 18 - 11.5 Ma, ~ 7.2 - 5.8 Ma, and ~ 3.4 - 2.9 Ma, characterize periods of chemical weathering whereas the time laps between these groups of ages correspond to episodes of mechanical denudation that reflect physical shaping of the paleosurfaces. For the last 45 Ma, the denudation rate estimates (3 to 8 m Ma-1) are comparable with those derived on shorter time scale (103 to 106 y.) in the same region by the cosmogenic radionuclide method [2]. Combined with the geomorphologic data set, these age ranges allow the visualization of the regional

  6. Numerical validation of the 'Pop-Down tectonics' as a structural frame for hot lithospheres with particular reference to the Hearne craton (Canadian Shield)

    Science.gov (United States)

    Poh, Jonathan; Yamato, Philippe; Gapais, Denis; Duretz, Thibault; Ledru, Patrick

    2017-04-01

    The formation of the architecture of the main cratons of the Canadian Shield has been debated over the past three decades. Understanding the role of tangential Vs. vertical tectonics in the Rae craton is of great interest as the role of inherited structure is fundamental for the subsequent drainage of fluids and the formation of high to ultra-high grade unconformity-type uranium deposits. These deposits are located in the vicinity of the intersection between the unconformity at the base of the Paleoproterozoic Athabasca sedimentary basin (1.75-1.5 Ga) and the graphite-rich metasediments of the Wollaston-Mudjatik transition zone, one of the main fault system of the Rae Craton related to the Trans-Hudson orogeny (1.82-1.78 Ga). A new tectonic model, Pop-down tectonics, was proposed as the primary driving process to concentrate supracrustal materials, strains, fluid transfers and metal transport in permeability enhanced deformation zones. The sub-vertical structural patterns with regional horizontal shortening seen in the tectonic model appear to be consistent with field evidences and has the potential for sustaining strong fluid-rock interactions. In the light of previous analogue modelling studies, we test the viability of the Pop-down tectonics model in a thermo-mechanical framework. The numerical experiments are based on a series of 2D visco-elasto-plastic thermo-mechanical models. We employ various thermal and rheological parameters derived from laboratory experiments. The geometry, thermicity and kinematics of the models are further constrained by applying existing geophysical and geological data. We impose a fixed upper mantle temperature of 1330 °C and a thin crust ranging from 30 - 40 km. The outcome of the models will provide insights into the mechanical processes controlling the deformation of hot lithospheres in convergent settings.

  7. 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

    2015-07-01

    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).

  8. Highly Silica-Undersaturated Sapphirine Granulites from the Daqingshan Area of the Western Block, North China Craton: Palaeoproterozoic UHT Metamorphism and Tectonic Implications

    Institute of Scientific and Technical Information of China (English)

    GUO Jing-hui; ZHAO Guo-chun; Chen Yi; Peng Peng; B. F. Windley; SUN Min

    2008-01-01

    @@ Sapphirine-bearing granulites have recently been found in the Daqingshan and Jining areas, both of which are located in a Palaeoproterozoic collisional belt, named the Khondalite Belt, along which the Yinshan Block in the north and the Ordos Block in the south were amalgamated to form the Western Block at~1.95 Ga, and then the Western Block collided with the Eastern Block along the Trans-North China Orogen to form the coherent basement of the North China Craton at~1.85 Ga.

  9. Tourmaline from the Archean G.R.Halli gold deposit, Chitradurga greenstone belt, Dharwar craton (India): Implications for the gold metallogeny

    OpenAIRE

    Susmita Gupta; Jayananda, M.; Fareeduddin

    2014-01-01

    Tourmaline occurs as a minor but important mineral in the alteration zone of the Archean orogenic gold deposit of Guddadarangavanahalli (G.R.Halli) in the Chitradurga greenstone belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli gold deposit as (a) clusters of very fine grained aggregates which form a minor constituent in the matrix of the altered metabasalt (AMB tourmaline) and (b) in quartz-carbonate veins (vein tourmaline). The vein...

  10. The Rae craton of Laurentia/Nuna: a tectonically unique entity providing critical insights into the concept of Precambrian supercontinental cyclicity

    Science.gov (United States)

    Bethune, K. M.

    2015-12-01

    Forming the nucleus of Laurentia/Nuna, the Rae craton contains rocks and structures ranging from Paleo/Mesoarchean to Mesoproterozoic in age and has long been known for a high degree of tectonic complexity. Recent work strongly supports the notion that the Rae developed independently from the Hearne; however, while the Hearne appears to have been affiliated with the Superior craton and related blocks of 'Superia', the genealogy of Rae is far less clear. A diagnostic feature of the Rae, setting it apart from both Hearne and Slave, is the high degree of late Neoarchean to early Paleoproterozoic reworking. Indeed, following a widespread 2.62-2.58 Ga granite bloom, the margins of Rae were subjected to seemingly continuous tectonism, with 2.55-2.50 Ga MacQuoid orogenesis in the east superseded by 2.50 to 2.28 Ga Arrowsmith orogenesis in the west. A recent wide-ranging survey of Hf isotopic ratios in detrital and magmatic zircons across Rae has demonstrated significant juvenile, subduction-related crustal production in this period. Following break-up at ca. 2.1 Ga, the Rae later became a tectonic aggregation point as the western and eastern margins transitioned back to convergent plate boundaries (Thelon-Taltson and Snowbird orogens) marking onset of the 2.0-1.8 Ga assembly of Nuna. The distinctive features of Rae, including orogenic imprints of MacQuoid and Arrowsmith vintage have now been identified in about two dozen cratonic blocks world-wide, substantiating the idea that the Rae cratonic family spawned from an independent earliest Paleoproterozoic landmass before its incorportation in Nuna. While critical tests remain to be made, including more reliable ground-truthing of proposed global correlations, these relationships strongly support the notion of supercontinental cyclicity in the Precambrian, including the Archean. They also challenge the idea of a globally quiescent period in the early Paleoproterozoic (2.45-2.2 Ga) in which plate tectonics slowed or shut down.

  11. 从变质作用观看板块构造何时在华北克拉通开始%WHEN DID PLATE TECTONICS BEGIN ON THE NORTH CHINA CRATON? INSIGHTS FROM METAMORPHISM

    Institute of Scientific and Technical Information of China (English)

    赵国春

    2007-01-01

    Understanding when and why plate tectonics began on Earth is one of the most important unresolved problems in earth sciences. As a final result of plate tectonics, continental collisional orogens are formed, and the identification of such orogens is regarded as a milestone in recognizing the operation of plate tectonics in Earths history. Metamorphism of continental collisional belts is generally characterized by clockwise p - T paths, especially for those involving isothermal decompression following the peak metamorphism. Therefore, clockwise p - T paths involving isothermal decompression are considered one of the important indicators for recognition of plate tectonics, especially in the early history of the earth. As one of the oldest continental blocks in the world, the North China Craton is considered a promising area for applying the large-scale synthesis approach of metamorphic p - T paths to understand tectonic settings or processes since extensive investigations have been undertaken on the p - T - t evolution of the basement rocks over the craton. Tectonically, the North China Craton is composed of three small continental blocks (Eastern Block, Yinshan Block, and Ordos Block) and three Paleoproterozoic mobile belts (Trans-North China Orogen, Khondalite Belt, and Jiao - Liao - Ji Belt). Metamorphism of Late Archean basement rocks in the Eastern and Yinshan Blocks is characterized by anticlockwise p - T paths, mostly involving isobaric cooling, suggesting that the heating source of the metamorphism was related to the underplating or intrusion of large volumes of mantle-derived magmas. Although theoretically such mantle-derived magmas form under continental magmatic arc, mantle plume or continental rift environments, only a mantle plume model can reasonably explain spatial and temporal distribution, lithological association, and structural features of the Late Archean basement rocks in the Eastern and Yinshan Blocks. Therefore, mantle plumes may be the

  12. Overview of geology and tectonic evolution of the Baikal-Tuva area.

    Science.gov (United States)

    Gladkochub, Dmitry; Donskaya, Tatiana

    2009-01-01

    This chapter provides the results of geological investigations of the main tectonic units of the Baikal-Tuva region (southwestern part of Siberia) during the last decades: the ancient Siberian craton and adjacent areas of the Central Asian Orogenic belt. In the framework of these main units we describe small-scale blocks (terranes) with focus on details of their inner structure and evolution through time. As well as describing the geology and tectonics of the area studied, we give an overview of underwater sediments, neotectonics, and some phenomena of history and development of the Baikal, Khubsugul, Chargytai, and Tore-Chol Lakes basins of the Baikal-Tuva region. It is suggested that these lakes' evolution was controlled by neotectonic processes, modern seismic activity, and global climate changes.

  13. Galaxy Evolution

    Science.gov (United States)

    Matteucci, F.

    We review both the observational and theoretical constraints on the evolution of the abundances of heavy elements in gas and stars in galaxies of different morphological type. The main aim of this work is to document the progress made in our understanding of the physical processes regulating the chemical evolution of galaxies during the last sixteen years since the appearance, in this same journal (volume 5, page 287), of the well know review of Beatrice Tinsley, to whom I dedicate this paper. Finally, this article is addressed particularly to readers who do not actively work on galactic chemical evolution and who might use it as a cook book where the main ingredients are discussed and useful recipes can be found.

  14. Neotectonics and geomorphic evolution of the northwestern arm of the Yellowstone Tectonic Parabola: Controls on intra-cratonic extensional regimes, southwest Montana

    Science.gov (United States)

    Ruleman, Chester A.; Larsen, Mort; Stickney, Michael C.

    2014-01-01

    The catastrophic Hebgen Lake earthquake of 18 August 1959 (MW 7.3) led many geoscientists to develop new methods to better understand active tectonics in extensional tectonic regimes that address seismic hazards. The Madison Range fault system and adjacent Hebgen Lake–Red Canyon fault system provide an intermountain active tectonic analog for regional analyses of extensional crustal deformation. The Madison Range fault system comprises fault zones (~100 km in length) that have multiple salients and embayments marked by preexisting structures exposed in the footwall. Quaternary tectonic activity rates differ along the length of the fault system, with less displacement to the north. Within the Hebgen Lake basin, the 1959 earthquake is the latest slip event in the Hebgen Lake–Red Canyon fault system and southern Madison Range fault system. Geomorphic and paleoseismic investigations indicate previous faulting events on both fault systems. Surficial geologic mapping and historic seismicity support a coseismic structural linkage between the Madison Range and Hebgen Lake–Red Canyon fault systems. On this trip, we will look at Quaternary surface ruptures that characterize prehistoric earthquake magnitudes. The one-day field trip begins and ends in Bozeman, and includes an overview of the active tectonics within the Madison Valley and Hebgen Lake basin, southwestern Montana. We will also review geologic evidence, which includes new geologic maps and geomorphic analyses that demonstrate preexisting structural controls on surface rupture patterns along the Madison Range and Hebgen Lake–Red Canyon fault systems.

  15. Geochemistry and petrogenesis of the late Archaean high-K granites in the southern Musoma-Mara Greenstone Belt: Their influence in evolution of Archaean Tanzania Craton

    Science.gov (United States)

    Mshiu, Elisante Elisaimon; Maboko, Makenya A. H.

    2012-05-01

    Musoma-Mara Greenstone Belt (MMGB) is abundantly occupied by the post-orogenic high-K granites which also they mark the end of magmatism in the area. The granites are characterized by high SiO2 and Al2O3 contents that average 74.42% and 13.08% by weight respectively. They have low Na2O content (mean = 3.36 wt.%) and high K2O contents (mean = 4.95 wt.%) which resulted to relatively high K2O/Na2O ratios (mean = 1.50). They also characterized by low Mg# (mean = 33) as well as low contents of transition elements such as Cr and Ni which are below detection limit (<20 ppm). Negative anomalies in Eu (Eu/Eu*, mean = 0.56), Nd, Ta and Ti elements as shown in the chondrite and primitive mantle normalized diagrams indicate MMGB high-K granites originated from a subduction related environments. These high-K granites also characterized by relative enrichment of the LREE compared to HREE as revealed by their high (La/Yb)CN ratio ranging from 8.71 to 50.93 (mean = 26.32). They have relatively flat HREE pattern with (Tb/Yb)CN ratio varying between 0.81 and 2.12 (mean = 1.55). Their linear trend in the variation diagrams of both major and trace elements indicate magmatic differentiation was also an important process during their formation. Conclusively, the geochemical characteristics as well as experimental evidences suggests MMGB high-K granites were formed from partial melting of pre-existing TTG rocks, under low pressure at 15 km depth or less and temperature around 950 °C in which plagioclase minerals were the stable phases in the melt.

  16. Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)

    Science.gov (United States)

    Bouougri, E.; Porada, H.

    2002-11-01

    Proterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or "syneresis". Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation

  17. Iron-rich condition maintained throughout the mid-Proterozoic ocean: new evidence from the North China Craton

    Science.gov (United States)

    Zhao, H.; Zhang, S.; Shi, X.; Zhang, C.; Huang, Y.; Li, H.

    2015-12-01

    The redox characteristics of ocean during Earth's middle age (1.7-0.75 Ga) are less well known, but it is conventionally assumed that the mid-Proterozoic was a period of a globally sulphidic (euxinic) in the deep ocean when the surface oceans were mildly oxygenated. Clay rich sedimentary rocks in Paleo-Mesoproterozoic strata deposited stably in the Yanshan-Taihangshan aulacogen (YTA), North China Craton (NCC), which created a unique opportunity to investigate the marine redox chemistry.Our Fe speciation analysis involved totally 48 samples from the black shale of the Chuanlinggou Formation (~1650 Ma), the mud shale of the Gaoyuzhuang Formation (~1560 Ma) and the black shale of the Xiamaling Formation (~1380 Ma) cropped out in Hebei Province, North China. Of all the 48 samples, 19 of them from Chuanlinggou Formation have FePy/FeHR ratios well below 0.8, while 14 of those have FeHR/FeT values above 0.15. For the 9 samples from Gaoyuzhuang Formation, their FeHR/FeT values are generally above 0.15, while 8 of them have FePy/FeHR ratios well below 0.8. For the 21 samples from Xiamaling Formation, 19 of them have FePy/FeHR values below 0.8, among which 13 samples have FeHR/FeT values above 0.15. Although a few samples analysed are more or less weathered, these data can still point to widespread ferruginous instead of sulfidic conditions over these three stratigraphic units.Abundant Fe containing carbonate concretions have been found in the shale of the Xiamaling Formation, which are significant in revealing the early burial and diagenetic marine environment. Our X-ray diffraction and rock magnetism studies reveal that the major minerals in the concretions are ferron dolomite and siderite, without any signs of sulphide of iron. These results imply that the concretions in the Xiamaling Formation were formed in ferruginous.Our analysis suggests a ferruginous condition was maintained throughout the Paleo-Mesoproterozoic (~1650 Ma to ~1380 Ma) in the YTA, a typical

  18. How unique is the Udachnaya-East kimberlite? Comparison with kimberlites from the Slave Craton (Canada) and SW Greenland

    Science.gov (United States)

    Kamenetsky, Vadim S.; Kamenetsky, Maya B.; Weiss, Yakov; Navon, Oded; Nielsen, Troels F. D.; Mernagh, Terrence P.

    2009-11-01

    The origin of alkali carbonates and chlorides in the groundmass of unaltered Udachnaya-East kimberlites in Siberia is still controversial. Contrary to existing dogma that the Udachnaya-East kimberlite was either contaminated by the crustal sediments or platform brines, magmatic origin of the groundmass assemblage has been proposed on the basis of melt immiscibility textures, melt inclusion studies, and strontium and neon isotope compositions. We further tested the idea of alkali- and chlorine enrichment of the kimberlite parental melt by studying olivine-hosted melt inclusions and secondary serpentine in kimberlites from the Slave Craton, Canada (Gahcho Kué, Jericho, Aaron and Leslie pipes) and southern West Greenland (Majuagaa dyke). Host olivine phenocrysts closely resemble groundmass olivine from the Udachnaya-East kimberlite in morphology, compositions (high-Fo, low-Ca), complex zoning with cores of varying shapes and compositions and rims of constant Fo. Melt inclusions in olivine consist of several translucent and opaque daughter phases and vapour bubble(s). The daughter crystals studied in unexposed inclusions by laser Raman spectroscopy and in carefully exposed inclusions by WDS-EDS are represented by Na-K chlorides, calcite, dolomite, magnesite, Ca-Na, Ca-Na-K and Ca-Mg-Ba carbonates, bradleyite Na 3 Mg(CO 3)(PO 4), K-bearing nahpoite Na 2(HPO 4), apatite, phlogopite and tetraferriphlogopite, unidentified sulphates, Fe sulphides, djerfisherite, pyrochlore (Na,Ca) 2Nb 2O 6(OH,F), monticellite, Cr-spinel and Fe-Ti oxides. High abundances of Na, K (e.g., (Na + K)/Ca = 0.15-0.85) and incompatible trace elements in the melt inclusions are confirmed by LA-ICPMS analysis of individual inclusions. Heating experiments show that melting of daughter minerals starts and completes at low temperatures (~ 100 °C and 600 °C, respectively), further reinforcing the similarity with the Udachnaya-East kimberlite. Serpentine minerals replacing olivine in some of the studied

  19. Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

    Science.gov (United States)

    Pandit, M. K.; Carter, L. M.; Ashwal, L. D.; Tucker, R. D.; Torsvik, T. H.; Jamtveit, B.; Bhushan, S. K.

    2003-12-01

    We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (˜15 km 2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An 7-20) and microcline (Or 92-98), with lesser biotite (Fe ∗=0.8-0.9) and accessory muscovite (Fe ∗=0.7-0.8). U-Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/ 206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb-Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U-Pb age. There is evidence for open-system behaviour in the Rb-Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm-Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids ( ISr=0.7110±14; ɛNd=-3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An 45-68) and magnesio-hornblende (Fe ∗=0.3-0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios ( ISr=0

  20. Metallogenetic systems associated with granitoid magmatism in the Amazonian Craton: An overview of the present level of understanding and exploration significance

    Science.gov (United States)

    Bettencourt, Jorge Silva; Juliani, Caetano; Xavier, Roberto P.; Monteiro, Lena V. S.; Bastos Neto, Artur C.; Klein, Evandro L.; Assis, Rafael R.; Leite, Washington Barbosa, Jr.; Moreto, Carolina P. N.; Fernandes, Carlos Marcello Dias; Pereira, Vitor Paulo

    2016-07-01

    The Amazonian Craton hosts world-class metallogenic provinces with a wide range of styles of primary precious, rare, base metal, and placer deposits. This paper provides a synthesis of the geological database with regard to granitoid magmatic suites, spatio temporal distribution, tectonic settings, and the nature of selected mineral deposits. The Archean Carajás Mineral Province comprises greenstone belts (3.04-2.97 Ga), metavolcanic-sedimentary units (2.76-2.74 Ga), granitoids (3.07-2.84 Ga) formed in a magmatic arc and syn-collisional setting, post-orogenic A2-type granites as well as gabbros (ca. 2.74 Ga), and anorogenic granites (1.88 Ga). Archean iron oxide-Cu-Au (IOCG) deposits were synchronous or later than bimodal magmatism (2.74-2.70 Ga). Paleoproterozoic IOCG deposits, emplaced at shallow-crustal levels, are enriched with Nb-Y-Sn-Be-U. The latter, as well as Sn-W and Au-EGP deposits are coeval with ca. 1.88 Ga A2-type granites. The Tapajós Mineral Province includes a low-grade meta-volcano-sedimentary sequence (2.01 Ga), tonalites to granites (2.0-1.87 Ga), two calc-alkaline volcanic sequences (2.0-1.95 Ga to 1.89-1.87 Ga) and A-type rhyolites and granites (1.88 Ga). The calc-alkaline volcanic rocks host epithermal Au and base metal mineralization, whereas Cu-Au and Cu-Mo ± Au porphyry-type mineralization is associated with sub-volcanic felsic rocks, formed in two continental magmatic arcs related to an accretionary event, resulting from an Andean-type northwards subduction. The Alta Floresta Gold Province consists of Paleoproterozoic plutono-volcanic sequences (1.98-1.75 Ga), generated in ocean-ocean orogenies. Disseminated and vein-type Au ± Cu and Au + base metal deposits are hosted by calc-alkaline I-type granitic intrusions (1.98 Ga, 1.90 Ga, and 1.87 Ga) and quartz-feldspar porphyries (ca. 1.77 Ga). Timing of the gold deposits has been constrained between 1.78 Ga and 1.77 Ga and linked to post-collisional Juruena arc felsic magmatism (e.g., Col

  1. Paleoproterozoic volcanic centers of the São Félix do Xingu region, Amazonian craton, Brazil: Hydrothermal alteration and metallogenetic potential

    Science.gov (United States)

    da Cruz, Raquel Souza; Fernandes, Carlos Marcello Dias; Villas, Raimundo Netuno Nobre; Juliani, Caetano; Monteiro, Lena Virgínia Soares; Lagler, Bruno; Misas, Carlos Mario Echeverri

    2016-06-01

    Geological, petrographic, scanning electron microscopy, and X-ray diffraction studies revealed hydrothermalized lithotypes evidenced by overprinted zones of potassic, propylitic, sericitic, and intermediate argillic alterations types, with pervasive and fracture-controlled styles, in Paleoproterozoic volcano-plutonic units of the São Félix do Xingu region, Amazonian craton, northern Brazil. The Sobreiro Formation presents propylitic (epidote + chlorite + carbonate + clinozoisite + sericite + quartz ± albite ± hematite ± pyrite), sericitic (sericite + quartz + carbonate), and potassic (potassic feldspar + hematite) alterations. The prehnite-pumpellyite pair that is common in geothermal fields also occurs in this unit. The Santa Rosa Formation shows mainly potassic (biotite + microcline ± magnetite), sericitic (sericite + quartz + carbonate ± chlorite ± gold), and intermediate argillic (montmorillonite + kaolinite/halloysite + illite) alterations. These findings strongly suggest the involvement of magma-sourced and meteoric fluids and draw attention to the metallogenetic potential of these volcanic units for Paleoproterozoic epithermal and rare and base metal porphyry-type mineralizations, similar to those already identified in other portions of the Amazonian craton.

  2. The significance of 24-norcholestanes, triaromatic steroids and dinosteroids in oils and Cambrian-Ordovician source rocks from the cratonic region of the Tarim Basin, NW China

    Science.gov (United States)

    Li, Meijun; Wang, T.-G.; Lillis, Paul G.; Wang, Chunjiang; Shi, Shengbao

    2012-01-01

    Two oil families in Ordovician reservoirs from the cratonic region of the Tarim Basin are distinguished by the distribution of regular steranes, triaromatic steroids, norcholestanes and dinosteroids. Oils with relatively lower contents of C28 regular steranes, C26 20S, C26 20R + C27 20S and C27 20R regular triaromatic steroids, dinosteranes, 24-norcholestanes and triaromatic dinosteroids originated from Middle–Upper Ordovician source rocks. In contrast, oils with abnormally high abundances of the above compounds are derived from Cambrian and Lower Ordovician source rocks. Only a few oils have previously been reported to be of Cambrian and Lower Ordovician origin, especially in the east region of the Tarim Basin. This study further reports the discovery of oil accumulations of Cambrian and Lower Ordovician origin in the Tabei and Tazhong Uplifts, which indicates a potential for further discoveries involving Cambrian and Lower Ordovician sourced oils in the Tarim Basin. Dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24-norcholestanes for dinoflagellates and diatoms. Therefore, the abnormally high abundance of these compounds in extracts from the organic-rich sediments in the Cambrian and Lower Ordovician and related oils in the cratonic region of the Tarim Basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the Tarim Basin during the Cambrian Period. Steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms, as well as define petroleum systems.

  3. Petrology of Teofilândia granitoids: An example of 2.1 Ga crustal accretion in the São Francisco Craton (Bahia, Brazil)

    Science.gov (United States)

    Nascimento, H. S.; Nédélec, Anne; Bouchez, Jean-Luc

    2017-07-01

    Teofilândia granitoids are representative of the Paleoproterozoic plutonic rocks, which intruded the Serrinha block, an Archean crustal fragment of the Sao Francisco Craton (Bahia, Brazil). Three plutons were emplaced, the Teofilândia granodiorite, the Barrocas trondhjemite and the Santa Rosa granite, respectively dated at 2130, 2127 and 2073 Ma. The two first plutons are calc-alkaline rocks following a trondhjemitic trend. They resemble Archean TTGs (tonalites-trondhjemites-granodiorites) by their major and trace element compositions and especially by their fractionated REE patterns, with very low HREE contents. These juvenile magmas resulted from partial melting of a young mafic protolith, likely represented by the nearby Rio Itapicuru greenstone belt. Barrocas trondhjemite and Teofilândia granodiorite derive from similar sources, possibly at different depths and with a different degree of melting. The rocks were deformed at high temperature during the Trans-Amazonian collision and are therefore pre-collisional and ascribed to a subduction stage. The younger Santa Rosa pluton is a small, syn-to post-collisional granite that derived from anatexis of the Archean crust. It is representative of a second, volumetrically minor, plutonic episode of potassic, shoshonitic or alkaline affinities. The large amount of 2.1 Ga granitoids emplaced in Brazil as well as in the West African craton, suggests that, at that time, a global event of possible mantle origin was responsible for the intense magmatic activity that involved both crustal accretion and crustal reworking in many places of the world.

  4. SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton

    Institute of Scientific and Technical Information of China (English)

    Yusheng Wan; Runlong Fan; Huiyi Sun; Xianzheng Zhao; Zejiu Wang; Dunyi Liu; Alfred Kröner; Chunyan Dong; Hangqian Xie; Yuansheng Geng; Yuhai Zhang

    2014-01-01

    The Central Hebei Basin (CHB) is one of the largest sedimentary basins in the North China Craton, extending in a northeastesouthwest direction with an area of>350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi-mentary rocks recovered from drill holes that penetrated into the basement of the CHB. Two samples of gneissic granodiorite (XG1-1) and gneissic quartz diorite (J48-1) have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41e2.51 and w2.5 Ga, respec-tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher SREE contents and lower Eu/Eu* and (La/Yb)n values. Two metasedimentary samples (MG1, H5) mainly contain w2.5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins haveεHf (2.5 Ga) values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively. Therefore, w2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.

  5. Animal evolution

    DEFF Research Database (Denmark)

    Nielsen, Claus

    This book provides a comprehensive analysis of evolution in the animal kingdom. It reviews the classical, morphological information from structure and embryology, as well as the new data gained from studies using immune stainings of nerves and muscles and blastomere markings, which makes...

  6. Representing Evolution

    DEFF Research Database (Denmark)

    Hedin, Gry

    2012-01-01

    . This article discusses Willumsen's etching in the context of evolutionary theory, arguing that Willumsen is a rare example of an artist who not only let the theory of evolution fuel his artistic imagination, but also concerned himself with a core issue of the theory, namely to what extent it could be applied...

  7. Metamorphic and age constraints on tectono-thermal reworking in the western H.U. Sverdrupfjella: A new crustal evolution model for Western Dronning Maud Land, Antarctica

    Science.gov (United States)

    Grosch, Eugene; Frimmel, Hartwig; Abu-Alam, Tamer; Košler, Jan

    2014-05-01

    Western Dronning Maud Land (WDML) of East Antarctica is argued to consist of two major crustal domains, namely the low-grade Archaean Kalahari-Grunehogna Craton and the high-grade Maud belt (e.g. Grantham et al., 1995; Jacobs et al. 2008). The geodynamic and tectono-thermal crustal evolution histories of these two proposed domains remain a debated topic in Rodinia and Gondwana reconstructions. In this study we conducted a petrological and metamorphic comparison of Mesoproterozoic metabasic rocks on the eastern margin of the Archaean Grunehogna Craton and the adjacent westernmost Maud Belt, across a major structural discontinuity known as the Pencksökket-Jutulstraumen Discontinuity (PJD). As such we evaluate to what extent the two domains of WDML represent independent crustal growth and metamorphic histories. Metamorphic constraints on low-grade rocks on the eastern Grunehogna craton record greenschist facies conditions of T = 340 ± 25oC and P = 2.9 ± 0.8 kbar. The high-grade PT-constraint of T =700 ± 30oC and P = 9.0 ± 2 kbar for the western extreme of the Maud Belt, derived from garnet-hornblende-plagioclase-quartz geothermobarometry and phase diagram modeling in PERPLEX, is very similar to that reported for the eastern Maud Belt and thus, does not support the concept of a westward decreasing metamorphic field gradient within the Maud Belt as previously proposed. Laser-ablation-ICP-MS U-Pb dating of titanite in a hornblende-plagioclase-quartz symplectite (after garnet breakdown), yielded a Pan-African age for high-grade metamorphism in the westernmost Maud belt, which overlaps with the age of tectonic decompression in the eastern Maud Belt. The new U-Pb age data argues against previous models that invoke only late-Mesoproterozoic high-grade metamorphism in the western Maud Belt. The new petrological data indicate that the inferred sub-glacial boundary (PJD) between the Grunehogna Craton and the Maud Belt, represents a major metamorphic hiatus as a Pan

  8. Assessment of undiscovered oil and gas resources of the southern Siberian craton (Baykit High, Nepa--Botuoba High, Angara--Lena Terrace, and Cis--Patom Foredeep Provinces), Russia, 2011

    Science.gov (United States)

    Klett, T.R.; Schenk, Christopher J.; Wandrey, Craig J.; Charpentier, Ronald R.; Brownfield, Michael E.; Pitman, Janet K.; Pollastro, Richard M.; Cook, Troy A.; Tennyson, Marilyn E.

    2012-01-01

    Using a geology-based assessment methodology, the U.S. Geological Survey estimated volumes of undiscovered, technically recoverable, conventional petroleum resources for the southern Siberian craton provinces of Russia. The mean volumes were estimated at 3.0 billion barrels of crude oil, 63.3 trillion cubic feet of natural gas, and 1.2 billion barrels of natural gas liquids.

  9. The peculiar case of Marosticano xenoliths: a cratonic mantle fragment affected by carbonatite metasomatism in the Veneto Volcanic Province (Northern Italy)

    Science.gov (United States)

    Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo; Florencia Fahnestock, M.; Bryce, Julia G.; Marzoli, Andrea

    2017-04-01

    The Tertiary Magmatic Province of Veneto, known as Veneto Volcanic Province (VVP), in the Northern Italy, represents one of the most important volcanic provinces of the Adria Plate. It is composed by five volcanic districts: Val d'Adige, Marosticano, Mts. Lessini, Berici Hills and Euganean Hills. Most of the volcanic products are relatively undifferentiated lavas, from nephelinites to tholeiites in composition. Commonly VVP nephelinites and basanites carry mantle xenoliths. This study presents a petrological characterization of the new xenolith occurrence of Marosticano and comparison with previously studied VVP xenolith populations (i.e. from the Lessinean and Val d'Adige areas), which represent off-craton lithospheric mantle fragment affected by Na-alkaline silicate metasomatism (Siena & Coltorti 1989; Beccaluva et al., 2001; Gasperini et al., 2006). Marosticano (MA) peridotites are anhydrous spinel-bearing lherzolites and harzburgites, which are geochemically well distinguishible from the other VVP mantle xenoliths. Primary minerals record the "most restitic" composition of the VVP sampled mantle, even calling the geochemical features of a sub-cratonic mantle. Olivines in both lherzolites and harzburgites show high Ni contents compared with the Fo values (Ni→ lherzolite: 2600-3620 ppm; harzburgite: 2600-3540 ppm; Fo → lh: 91-92; hz: 90-93) that follow the trend of olivine from a cratonic area (Kelemen, 1998). Orthopyroxenes have mg# values with 1:1 ratio with coexisting olivines and Al2O3 contents always 0.5 wt%) contents are also the chemical characteristics of the clinopyroxenes. On the whole both MA pyroxenes show major element contents that recall the characteristics of those from cratonic (sp-bearing) peridotites (e.g. from Greenland, South Africa and Tanzania; Downes et al., 2004). In addition, the relationship between the high Fo content of olivine and the high chromium contents (cr#=(Cr/(Cr+Al)X100); lh: 30-53; hz: 38-67) in coexisting spinel, out of

  10. Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton

    Science.gov (United States)

    Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei

    2017-08-01

    There is a wide support that the Inner Mongolia Palaeo-uplift on the northern margin of the North China Craton has undergone an uplifting history. However, when and how did the uplift occurred keeps controversial. Extensive field-based structural, metamorphic, geochemical, geochronological and geophysical investigations on the Inner Mongolia Palaeo-uplift, which suggested that the Inner Mongolia Palaeo-uplift was an uplifted region since the Early Precambrian or range from Late Carboniferous-Early Jurassic. The geochemical characteristics of the Late Paleozoic to Early Mesozoic intrusive rocks indicated that the Inner Mongolia Palaeo-uplift was an Andean-type continental margin that is the extensional tectonic setting. To address the spatial and temporal development of the Inner Mongolia Palaeo-uplift, we have carried out provenance analysis of Permian sedimentary rocks which collected from the Panyangshan basin along the northern margin of the North China Craton. The QFL diagram revealed a dissected arc-recycled orogenic tectonic setting. Moreover, the framework grains are abundant with feldspar (36-50%), indicating the short transport distance and unstable tectonic setting. Detrital zircon U-Pb analysis ascertained possible provenance information: the Precambrian basement ( 2490 and 1840 Ma) and continental arc magmatic action ( 279 and 295 Ma) along the northern margin of the North China Craton. The projection in rose diagrams of the mean palaeocurrent direction, revealing the SSW and SSE palaeoflow direction, also shows the provenance of the Panyangshan basin sources mainly from the Inner Mongolia Palaeo-uplift. The andesite overlying the Naobaogou Formation has yielded U-Pb age of 277.3 ± 1.4 Ma. The additional dioritic porphyry dike intruded the Naobaogou and Laowopu Formations, which has an emplacement age of 236 ± 1 Ma. The above data identify that the basin formed ranges from Early Permian to Middle Triassic (277-236 Ma). Accordingly, the Inner Mongolia

  11. Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

    Science.gov (United States)

    Carlson, R.W.; Irving, A.J.; Schulze, D.J.; Hearn, B.C.

    2004-01-01

    Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm-Nd and Lu-Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic-a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous-early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm-Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger "ages" suggesting

  12. Paleoproterozoic Cordilleran-style accretion along the south eastern margin of the eastern Dharwar craton: Evidence from the Vinjamuru arc terrane of the Krishna orogen, India

    Science.gov (United States)

    Chatterjee, Chiranjeeb; Vadlamani, Ravikant; Kaptan, Om Prakash

    2016-10-01

    Accretion along continental or island arcs at cratonic margins was responsible for most Paleoproterozoic crustal growth. For the development of the Krishna orogen, India, at the southeastern margin of the Eastern Dharwar craton (EDC), two contrasting models, one by long-lived accretion between ~ 1.85 Ga and 1.33 Ga terminating in continental collision with the Napier Complex and the other involving continental collision with the Napier Complex at ~ 1.6 Ga have been proposed. Here we report the geology and geochemistry of the granitoid rocks grouping them into the Vinjamuru arc terrane. These comprise biotite ± hornblende high-silica granite which are mostly calc-alkaline, weakly metaluminous to peraluminous with normalized trace and rare earth element plots resembling those derived from arc sources as seen by negative Nb, Ti, Zr anomalies, enriched LREE and moderate Eu anomalies. On (La/Yb)CN vs YbCN and Sr/Y vs Y discrimination diagrams these rocks plot in the field of liquids from mantle-derived melts resembling Cordilleran type granitoids. Petrography, major oxide and trace element concentrations suggest formation in an arc tectonic setting during convergent tectonics at the active continental margin of the EDC with evidence for crustal assimilation. To generate the observed high-silica granite, using selected trace and REE, we modeled 10% aggregate continuous melting of a lower crustal hydrous, high K2O-bearing gabbro yielding a granodiorite magma that underwent fractional crystallization at mid-to lower crust followed by mixing with country rock tonalite and minor assimilation with metasedimentary crustal rocks resulting in the observed heterogeneity in trace elements from the granite. We interpret Paleoproterozoic paleopostions of component Indian cratons leading to their Mesoproterozoic assembly and in that context relate the crustal growth along the southeastern margin of the EDC. In contrast to the existing two models, we propose an alternative

  13. Titanite-scale insights into multi-stage magma mixing in Early Cretaceous of NW Jiaodong terrane, North China Craton

    Science.gov (United States)

    Jiang, Peng; Yang, Kui-Feng; Fan, Hong-Rui; Liu, Xuan; Cai, Ya-Chun; Yang, Yue-Heng

    2016-08-01

    The Early Cretaceous Guojialing-type granodiorites in northwestern Jiaodong terrane carry significant records for strong mantle-crust interaction during the destruction of North China Craton (NCC); however, the definite petrogenetic mechanism and detailed magmatic process remain an enigma. Titanite in igneous rocks can serve as an effective petrogenetic indicator. Here, we present integrated geochronological and geochemical studies on titanites from Guojialing-type granodiorites and their dioritic enclaves to constrain their petrogenesis. Titanites from granodiorites (G-type) and plagioclase-rich dioritic enclaves (E-type-I) present an identical U-Pb age (~ 130 Ma) and an indistinguishable wide range of Zr and total REEs contents, and Th/U ratios. However, these two types of titanites exhibit distinct micro-scale textures and geochemical compositions. G-type titanites are characterized by oscillatory zonings with two Light BSE zones (LBZ) and two or three dark BSE zones, whereas E-type-I titanites are marked by core-mantle-rim zonings. Drastic increase of LREEs, Zr, Hf, and Fe and decrease of Nb, Ta, Al, and F contents are observed in LBZ of G-type titanites, whereas remarkable reduction of LREEs, Zr, and Hf and elevation of F contents are observed from the cores to the mantles of E-type-I titanites. Based on Zr-in-titanite thermometry, G-type titanites are interpreted to have experienced twice notable temperature increase, while E-type-I titanites are inferred to have undergone a rapid cooling process. Furthermore, we suggest that the drastic chemical changes in G-type and E-type-I titanites are ascribed to early-stage magma mixing between a colder felsic magma and a Fe-, REE-rich hotter dioritic magma. Compared to G-type and E-type-I titanites, titanites from plagioclase-poor dioritic enclaves (E-type-II) are characterized by their occurrence in interstitial space and present a relatively younger U-Pb age (~ 128 Ma) and much narrower and lower range of Zr, total

  14. Microstructural analysis of calcite-filled fractures inherited from basement structures, southern Ontario, Canada: long term instability of the craton?

    Science.gov (United States)

    Spalding, Jennifer; Schneider, David

    2016-04-01

    Intra-cratonic regions are generally characterized by tectonic stability and low seismicity. In southern Ontario, Canada, moderate levels of seismicity have been recorded over the last few decades reaching magnitudes of 5 MN, indicating that the geosphere is not as stable as predicted. The stratigraphy of the region consists of Ordovician limestone with a thickness of ~200 m that unconformably overlays the Mesoproterozoic crystalline Grenville Province. Subsequent tectonism including repeated Paleozoic orogenies and rifting along the east coast of North America has reactivated Proterozoic structures that have propagated into the overlying carbonate platform forming mesoscopic-scale brittle structures. Exposed along the shores of Lake Ontario are decameter-scale fracture zones, with a fracture spacing of 0.5 to 10 meters. The dominant fracture set trends E-W, and often forms conjugate sets with less prominent NNE-oriented fractures. More locally, an older NW-oriented fracture set is cross cut by the E-W and NNE oriented fractures. Regionally, there have been six directions of maximum horizontal stress in southern Ontario since the Precambrian, with the current orientation of maximum stress oriented ENE as a consequence of far field Atlantic ridge-push forces generated at distant plate boundaries. Calcite mineralization along fractured surfaces locally form sub-horizontal slickenside fabrics which are covered by a layer of euhedral calcite crystals, suggesting that fracture dilation (and fluid flow) occurred after fracture slip to allow the growth of calcite crystals. Due to the proximity of the carbonate units to the crystalline basement, we expect the calcitic veins to be enriched in rare earth elements and are presently conducting geochemical analyses. The calcite veins and surfaces vary from 2.5 cm to 1 mm thicknesses, often with larger calcite crystals in the center of the vein and smaller crystals at the vein boundaries, likely representing nucleation on small

  15. Trans-Amazonian U-Th-Pb monazite ages and P-T-d exhumation paths of garnet-bearing leucogranite and migmatitic country rock of the southeastern Tandilia belt, Rio de la Plata craton in Argentina

    Science.gov (United States)

    Martínez, Juan Cruz; Massonne, Hans-Joachim; Frisicale, María Cristina; Dristas, Jorge A.

    2017-03-01

    A garnet-bearing leucogranite and two country rocks from the Transamazonian Tandilia belt of the Rio de la Plata craton were studied in detail. The leucogranite contains garnet with homogeneous composition of pyr6(gros + andr)2spes5alm87. In a garnet-biotite migmatite, the core and rim compositions of garnet are pyr1.7(gros + andr)5spes5.6alm87.7 and pyr1.2(gros + andr)5.5spes6.7alm86.6, respectively. These compositions in a sillimanite-garnet-muscovite migmatite are pyr4(gros + andr)2.7spes2.7alm90.6 and pyr2.7(gros + andr)4spes3.2alm90.1, respectively. We used this information to decipher the P-T evolution of the rocks applying P-T and T-H2O pseudosections with the PERPLE_X computer software package taking into consideration deformational microstructures. The leucogranite records an isothermal decompression from 5.3 to 3.8 kbar at 665 °C. The garnet-biotite migmatite was exhumed from 5.5 kbar at 630 °C to 4.3 kbar at 615 °C and the sillimanite-garnet-muscovite migmatite from supersolidus conditions of 670 °C and 3.6 kbar to 625 °C at 2.4 kbar. Late andalusite formed in this rock. Seventy four analyses of 28 monazite grains of the country rocks yielded three groups of U-Th-Pb ages which were related to a collisional event (I: ca. 2.13-2.14 Ga.), a postcollisional thermal overprint (II: ca. 2.01 Ga) and slow cooling of the orogen (III: 1.80-1.90 Ga). Inherited ages of 2.28 and 2.25 Ga could refer to an early accretionary stage of the orogen. An age of 2.41 Ga indicates the presence of recycled Siderian continental crust. Synkinematic crystallization of melts and the subsolidus development of an S2-foliation, demonstrated by deformational microstructures, occurred during the exhumation of the studied area from depths of 18 km to 8 km in the time interval 2.01-1.90 Ga.

  16. Representing Evolution

    DEFF Research Database (Denmark)

    Hedin, Gry

    2012-01-01

    A scandal shook Danish art when Jens Ferdinand Willumsen exhibited his etching Fertility in 1891. A heavily pregnant woman—Willumsen's wife, Juliette—and a short text about a new language of art flank a grain, which sprouts in a way that looks less like a plant than some sort of diagram....... This article discusses Willumsen's etching in the context of evolutionary theory, arguing that Willumsen is a rare example of an artist who not only let the theory of evolution fuel his artistic imagination, but also concerned himself with a core issue of the theory, namely to what extent it could be applied...

  17. Crust structure of the Northern Margin of North China Craton and adjacent region from Sinoprobe-02 North China seismic WAR/R experiment

    Science.gov (United States)

    Li, W.; Gao, R.; Keller, G. R.; Li, Q.; Cox, C. M.; Hou, H.; Guan, Y.

    2011-12-01

    The Central Asian Orogen Belt (CAOB) or Altaids, situated between the Siberian craton(SC) to the north and north China craton (NCC) with tarim to the south, is one of the world's largest accretionary orogens formed by subduction and accretion of juvenile material from the Neoproterozoic through the Paleozoic. The NCC is the oldest craton in China, which suffered Yanshan intercontinental orogenic process and lithosphere thinning in Mesozoic. In the past 20 years, remarkable studies about this region have been carried out and different tectonic models were proposed, however, some crucial geologic problems remain controversial. In order to obtain better knowledge of deep structure and properties of crust on the northern margin of north China craton, a 450 km long WAR/R section was completed jointly by Institute of Geology, CAGS and University of Oklahoma. Our 450 km long NW-SE WAR/R line extends from west end of the Yanshan orogen, across the Bainaimiao arc, Ondor sum subduction accretion complex to the Solonker suture zone. The recording of seismic waves from 8 explorations was conducted in 4 deployments of 300 reftek-125A records and single-channel 4.5Hz geophones with station spacing of 1km. The shooting procedure was employ 500 or 1500kg explosives in 4-5 or 15-23 boreholes at 40-45m depth. The sampling rate was 100 HZ, and recording time window was 1200s. The P wave field on the sections got high quality data for most part of the profile, but have low signal-to-noise for the south end, where closed to Beijing with a lot of ambient noise from traffic, industry and human activity. Arrivals from of refracted and reflected waves from sediments and basement (Pg), intracrust (Pcp, Plp) and Moho (Pmp) were typically observed, but Pn phase through the upper most mantle was only observed for 2 shots. Identification and correlation of seismic phases was done manually on computer screen Zplot software. Each trace has been bandpass filtered (1-20Hz) and normalized with AGC

  18. Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

    Science.gov (United States)

    Haggerty, Stephen E.; Toft, Paul B.

    1988-01-01

    Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

  19. Carbonatite magmatism of the southern Siberian Craton 1 Ga ago: Evidence for the beginning of breakup of Laurasia in the early Neoproterozoic

    Science.gov (United States)

    Savelieva, V. B.; Danilova, Yu. V.; Bazarova, E. P.; Ivanov, A. V.; Kamenetsky, V. S.

    2016-11-01

    Apatite and biotite from dolomite‒ankerite and calcite‒dolomite carbonatite dikes emplaced into the Paleoproterozoic metamorphic rock complex in the southern part of the Siberian Craton are dated by the U-Pb (LA-ICP-MS) and 40Ar-39Ar methods, respectively. Proceeding from the lower intercept of discordia with concordia, the age of apatite from calcite‒dolomite carbonatite is estimated to be 972 ± 21 Ma and that for apatite from dolomite‒ankerite carbonatite, as 929 ± 37 Ma. Values derived from their upper intercept have no geological sense. The ages obtained for biotite by the 40Ar-39Ar method are 965 ± 9 and 975 ± 14 Ma. It means that the formation of carbonatites reflects the earliest phases of the Neoproterozoic stage in extension of the continental lithosphere.

  20. Rb-Sr Geochronology and Geochemical Characteristics of Mafic Dikes in the Nova Lacerda and Conquista D´Oeste Region, Mato Grosso, SW Amazonian Craton

    Directory of Open Access Journals (Sweden)

    Amarildo Salina Ruiz

    2009-01-01

    Full Text Available In the Nova Lacerda and Conquista D’Oeste regions, Mato Grosso State, SW part of the Amazonian Craton, mafi c dikestrending NNW intrude the Nova Lacerda Granite (1462 ± 12 Ma, within the Jauru Domain, in the Rondonia-San IgnacioProvince (1.55 - 1.3 Ga. The mafi c swarm comprises diabases, metadiabases and amphibolites. Metadiabases originatedfrom uralitization of diabases. These rocks have tholeiitic affi nity 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.

  1. THE RECENT STRUCTURE AND THE ASSUMED HISTORY OF FORMATION OF THE CRUST IN THE SOUTH-EASTERN SEGMENT OF THE NORTH ASIAN CRATON ALONG REFERENCE PROFILE 3-DV

    Directory of Open Access Journals (Sweden)

    E. Yu. Goshko

    2015-09-01

    Full Text Available The article presents results of specialized processing of the deep seismic profile along a part of Reference Profile 3-DV which crosses the Aldan-Stanovoi shield in the meridian direction and goes across its buried northern slope. The study is aimed at determining frequency-energy characteristics of the seismic wave field which are related to physical conditions of geological features of the crust. Based on analysis and interpretation of the dynamic profiles, it is possible to reveal and contour the Archean cores of consolidation of the Aldan shield and its buried continuation that is covered by sediments of the Middle Lena monocline and to input new facts in the proposed geodynamic model showing formation of the crust in the south-eastern segment of the North Asian craton.

  2. Micro-scale characterization of iron ores from a banded iron formation in Yishui county, western Shandong province of North China Craton

    Science.gov (United States)

    Moon, I.; Lee, I.; Yang, X.

    2016-12-01

    Banded iron formations (BIFs) are widely distributed in North China Craton (NCC). Yishui BIF is located in Yishui county, western Shandong Province of NCC and is categorized as Algoma-type. The origin of iron and silica of BIFs in this region have been studied extensively for decades. The trace elemental concentrations of magnetite and hematite in iron ores from Yishui BIF are focused in this study to better understand the origins of BIF. To discuss micro-structural signatures of iron ores, X-ray diffraction (XRD), Raman spectrometer, electron microprobe (EPMA) and laser ablation inductively-coupled plasma mass spectrometer (LA-ICP-MS) were utilized. Overall geochemical data represents abundance of trace elements, oxygen fugacity (fO2) condition during the formation of iron oxides, depositional environment of Yishui BIF, implying the involvement of volcanic eruption and hydrothermal exhalation during the chemical deposition.

  3. Tourmaline from the Archean G.R.Halli gold deposit, Chitradurga greenstone belt, Dharwar craton (India):Implications for the gold metallogeny

    Institute of Scientific and Technical Information of China (English)

    Susmita Gupta; M. Jayananda; Fareeduddin

    2014-01-01

    Tourmaline occurs as a minor but important mineral in the alteration zone of the Archean orogenic gold deposit of Guddadarangavanahalli (G.R.Halli) in the Chitradurga greenstone belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli gold deposit as (a) clusters of very fine grained aggregates which form a minor constituent in the matrix of the altered metabasalt (AMB tourmaline) and (b) in quartz-carbonate veins (vein tourmaline). The vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as (i) albite-tourmaline-ankerite-quartz veins (vein-1 tourmaline) and (ii) albite-tourmaline-calcite-quartz veins (vein-2 tourmaline). Both the AMB tourmaline and the vein tourmalines (vein-1 and vein-2) belong to the alkali group and are clas-sified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tour-malines are unzoned. Mineral chemistry and discrimination diagrams reveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the vein tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tour-malines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-à-vis gold mineralization is spatially linked to the juvenile magmatic accretion (2.56e2.50 Ga) east of the studied area in the western part of the eastern Dharwar craton.

  4. First SHRIMP zircon U-Pb ages for Hutuo Group in Wutaishan:Further evidence for Palaeoproterozoic amalgamation of North China Craton

    Institute of Scientific and Technical Information of China (English)

    S.A.Wilde; ZHAO Guochun; WANG Kaiyi; SUN Min

    2004-01-01

    A felsic tuffaceous rock, obtained from a metamorphosed sequence of volcanics and sediments of the Hutuo Group, 8 km south of Taihuai in Wutaishan, contains two zircon populations. These record SHRIMP 207Pb/206Pb weighted mean ages of 2180 ± 5 Ma and 2087±9 Ma, respectively. The older date is within error of the age of the Dawaliang Granite in Wutaishan and is considered to be derived from a similar crustal magmatic source. The younger date is within error of reported ages from metasediments and meta-volcanics of the Wanzi supracrustal rocks and the Nanying granitic gneisses in the adjacent Fuping Complex and is interpreted to be the age of volcanism in the Hutuo Group. These data establish that: (1) the Hutuo Group is Paleoproterozoic and not Archean in age and (2) the volcanism and sedimentation were coeval in the Wutai and Fuping complexes. Sedimentation was therefore widespread at this time and possibly reflects deposition along an evolving continental margin, most likely the western margin of the Eastern Block of the North China Craton. The age of 2087 ± 9 Ma for volcanism in the Hutuo Group means that it must have been deformed and metamorphosed after this time. This further supports the evidence, obtained from other recent studies, that the main tectonism in the Wutaishan area occurred at ~1.8 Ga during the Lüliang orogeny. This orogeny resulted in the collision of the Eastern and Western blocks of the North China Craton and formed part of a global supercontinental amalgamation event.

  5. SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton

    Directory of Open Access Journals (Sweden)

    Yusheng Wan

    2014-07-01

    Full Text Available The Central Hebei Basin (CHB is one of the largest sedimentary basins in the North China Craton, extending in a northeast–southwest direction with an area of >350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedimentary rocks recovered from drill holes that penetrated into the basement of the CHB. Two samples of gneissic granodiorite (XG1-1 and gneissic quartz diorite (J48-1 have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41–2.51 and ∼2.5 Ga, respectively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher ΣREE contents and lower Eu/Eu* and (La/Ybn values. Two metasedimentary samples (MG1, H5 mainly contain ∼2.5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have εHf (2.5 Ga values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively. Therefore, ∼2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.

  6. CHEMICAL EVOLUTION

    Energy Technology Data Exchange (ETDEWEB)

    Calvin, Melvin

    1965-06-01

    How did life come to be on the surface of the earth? Darwin himself recognized that his basic idea of evolution by variation and natural selection must be a continuous process extending backward in time through that period in which the first living things arose and into the period of 'Chemical Evolution' which preceded it. We are approaching the examination of these events by two routes. One is to seek for evidence in the ancient rocks of the earth which were laid down prior to that time in which organisms capable of leaving their skeletons in the rocks to be fossilized were in existence. This period is sometime prior to approximately 600 million years ago. The earth is believed to have taken its present form approximately 4700 million years ago. We have found in rocks whose age is about 1000 million years certain organic molecules which are closely related to the green pigment of plants, chlorophyll. This seems to establish that green plants were already fluorishing prior to that time. We have now found in rocks of still greater age, namely, 2500 million years, the same kinds of molecules mentioned above which can be attributed to the presence of living organisms. If these molecules are as old as the rocks, we have thus shortened the time available for the generation of the complex biosynthetic sequences which give rise to these specific hydrocarbons (polyisoprenoids) to less than 2000 million years.

  7. Thought Evolution

    Directory of Open Access Journals (Sweden)

    Shadrikov V.D.

    2015-08-01

    Full Text Available The thought evolution is studied by historical reconstruction method that is based on the propositions of the theory of culturalhistorical determination of the psyche development, and the data of the morphological analysis and child development, and the conception of the psyche neuroontogenesis. The grounds for advisability of protothinking are presented. The protothinking is understood as the use of objective thought in cases of awareness absence. It is shown that protothinking is a form of transition from animal thinking to human speech. The particular attention is paid to the process of the word producing and thought generation in that process. The conditions of word producing as cooccurring acoustic pattern served for though expression are discussed. It is emphasized that a word is produced by a particular person. The historical development of the language and the specificity of this development are pointed out

  8. Community Evolution

    CERN Document Server

    Bródka, Piotr; Kazienko, Przemysław

    2016-01-01

    The continuous interest in the social network area contributes to the fast development of this field. The new possibilities of obtaining and storing data facilitate deeper analysis of the entire social network, extracted social groups and single individuals as well. One of the most interesting research topic is the network dynamics and dynamics of social groups in particular, it means analysis of group evolution over time. It is the natural step forward after social community extraction. Having communities extracted, appropriate knowledge and methods for dynamic analysis may be applied in order to identify changes as well as to predict the future of all or some selected groups. Furthermore, knowing the most probably change of a given group some additional steps may be performed in order to change this predicted future according to specific needs. Such ability would be a powerful tool in the hands of human resource managers, personnel recruitment, marketing, telecommunication companies, etc.

  9. Spectral characteristics of banded iron formations in Singhbhum craton, eastern India: Implications for hematite deposits on Mars

    OpenAIRE

    Singh, Mahima; Singhal, Jayant; K.arun Prasad; Rajesh, V.J.; Ray, Dwijesh; Sahoo, Priyadarshi

    2016-01-01

    Banded iron formations (BIFs) are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior (Canada) and Carajas (Brazil) have been studied by planetary scientists to trace the evolution of hematite deposits on Mars. Hematite deposits are extensively identified in Mer...

  10. Magma mixing in the Kalaqin core complex, northern North China Craton: Linking deep lithospheric destruction and shallow extension

    Science.gov (United States)

    Fu, Lebing; Wei, Junhao; Tan, Jun; Santosh, M.; Zhang, Daohan; Chen, Jiajie; Li, Yanjun; Zhao, Shaoqing; Peng, Lina

    2016-09-01

    The widespread Mesozoic magmatism in the North China Craton (NCC) has received considerable attention as a trigger for large scale lithospheric destruction. Here we investigate the Early Cretaceous Jiguanzi adamellite from the northern part of the NCC which is contemporaneous with shallow extensional deformation and deep lithospheric destruction. This intrusion emplaced at ca. 133 Ma is located in the foot wall of the Kalaqin metamorphic core complex (MCC), and occurs as a synextensional ring complex with numerous magmatic equigranular (Group 1) and porphyritic (Group 2) enclaves. Hornblende and plagioclase from the host adamellite and xenocrysts of Group 2 enclaves show distinct inverse zoning with Mg- and Ca-rich mantle. The Group 2 enclaves are characterized by plagioclase xenocrysts hosting hornblende, biotite and apatite inclusions, quartz ocelli with fine-grained rim enriched in biotite and hornblende, and poikilitic biotite surrounded by hornblende. Geochemically, the host intrusion is calc-alkaline to alkaline and metaluminous with variable contents of SiO2 (60.70-72.20 wt.%), Al2O3 (14.19-17.22 wt.%), Na2O + K2O (6.16-9.42 wt.%), and Mg# values (28.0-47.7), whereas the Group 2 enclaves exhibit low SiO2 (54.05-55.55 wt.%), high Fe2O3 (8.18-8.64 wt.%) and TiO2 (2.08-2.28 wt.%), and moderate Mg# (44.0-44.1). Both the host intrusion and Group 2 enclaves are enriched in large-ion lithophile and light rare earth elements, and depleted in high field strength elements and heavy rare earth elements except that the latter has lower Ba and high Nb, Ta and Ti contents. The major and trace element contents of the Group 1 enclaves are broadly similar to those of the host intrusion. Analyses of Sr-Nd-Hf isotopes in the host intrusion, and in Group 1 and Group 2 enclaves show (87Sr/86Sr)ihost = 0.70600-0.70618, εNd(t)host = - 8.2 to - 9.6, T2DM(Nd)host = 1592-1706 Ma, εHf(t)host = - 9.2 to - 12.0, (87Sr/86Sr)iGroup 1 = 0.70590-0.70635, εNd(t)Group 1 = - 9.6 to - 10

  11. Om religion og evolution

    DEFF Research Database (Denmark)

    Geertz, Armin W.

    2011-01-01

    for kulturens kausale virkning på den menneskelige kognition og ikke mindst den hominine evolution. Ud fra, hvad vi ved om den menneskelige evolution, ses det, at den hominine evolution har en dybde, som sjældent medtænkes i teorier og hypoteser om den menneskelige evolution. Den menneskelige evolution er...

  12. Groupware requirements evolution patterns

    NARCIS (Netherlands)

    Pumareja, Dulce Trinidad

    2013-01-01

    Requirements evolution is a generally known problem in software development. Requirements are known to change all throughout a system's lifecycle. Nevertheless, requirements evolution is a poorly understood phenomenon. Most studies on requirements evolution focus on changes to written specifications

  13. Has Human Evolution Stopped?

    OpenAIRE

    TEMPLETON, Alan R

    2010-01-01

    It has been argued that human evolution has stopped because humans now adapt to their environment via cultural evolution and not biological evolution. However, all organisms adapt to their environment, and humans are no exception. Culture defines much of the human environment, so cultural evolution has actually led to adaptive evolution in humans. Examples are given to illustrate the rapid pace of adaptive evolution in response to cultural innovations. These adaptive responses have important ...

  14. Lower Devonian paleomagnetic dating of a large mafic sill along the western border of the Murzuq cratonic basin (Saharan metacraton, SE Algeria).

    Science.gov (United States)

    El-M. Derder, Mohamed; Maouche, Said; Liégeois, Jean-Paul; Henry, Bernard; Amenna, Mohamed; Ouabadi, Aziouz; Bellon, Hervé; Bruguier, Olivier; Bayou, Boualem; Bestandji, Rafik; Nouar, Omar; Bouabdallah, Hamza; Ayache, Mohamed; Beddiaf, Mohamed

    2017-04-01

    The Murzuq basin located in central North Africa, in Algeria, Libya and Niger is a key area, delineating a relictual cratonic area within the Saharan metacraton (Liégeois et al., 2013). On its western border, we discovered a very large sill ("Arrikine" sill), with a thickness up to 250m and a minimum length of 35 km. It is made of mafic rocks and is interbedded within the Silurian sediments of the Tassilis series. In the vicinity, the only known post-Pan-African magmatism is the Cenozoic volcanism in the In Ezzane area. Further south in Niger, also along the SW border of the Murzuq basin, large Paleozoic dolerite (Carte géologique du Sahara central, 1962) are probably related to the "Arrikine" sill magmatism, as they are in the same stratigraphical position. Several hundred kilometers westward and southwestward of Arrikine, Paleozoic magmatic products are known: Carboniferous basic intrusives (346 Ma; Djellit et al., 2006) are located in the Tin Serririne basin and Devonian ring complexes (407 Ma; Moreau et al, 1994) in the Aïr Mountains. For the Arrikine sill, K/Ar data gave a rejuvenation age (326 Ma) related to a K-rich aplitic phase and the LA-ICP-MS U-Pb method on zircon showed that only inherited zircons are present (0.6 to 0.7, 2.0 and 2.7 Ga ages), pointing to ages from the underlying basement corresponding to the Murzuq craton covered by Pan-African sediments (Derder et al., 2016). By contrast, a well-defined paleomagnetic pole yielded an age of 410-400 Ma by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age, similar to that reported for the Aïr complexes (Moreau et al., 1994), can be correlated with the deep phreatic eruption before Pragian time thought to be at the origin of sand injections, which gave circular structures observed on different borders of the Murzuq basin (Moreau et al,. 2012). This Lower Devonian magmatism had therefore a regional extension and can be related to a "Caledonian" transtensive reactivation of the

  15. A review of structural patterns and melting processes in the Archean craton of West Greenland: Evidence for crustal growth at convergent plate margins as opposed to non-uniformitarian models

    Science.gov (United States)

    Polat, Ali; Wang, Lu; Appel, Peter W. U.

    2015-11-01

    The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite-trondhjemite-granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West

  16. Nudging Evolution?

    Directory of Open Access Journals (Sweden)

    Katharine N. Farrell

    2013-12-01

    Full Text Available This Special Feature, "Nudging Evolution? Critical Exploration of the Potential and Limitations of the Concept of Institutional Fit for the Study and Adaptive Management of Social-Ecological Systems," aims to contribute toward the development of social theory and social research methods for the study of social-ecological system dynamics. Our objective is to help strengthen the academic discourse concerning if, and if so, how, to what extent, and in what concrete ways the concept of institutional "fit" might play a role in helping to develop better understanding of the social components of interlinkages between the socioeconomic-cultural and ecological dynamics of social-ecological systems. Two clearly discernible patterns provide a map of this Special Feature: (1 One pattern is the authors' positions regarding the place and role of normativity within their studies and assessment of institutional fit. Some place this at the center of their studies, exploring phenomena endogenous to the process of defining what constitutes institutional fit, whereas others take the formation of norms as a phenomenon exogenous to their study. (2 Another pattern is the type of studies presented: critiques and elaborations of the theory, methods for judging qualities of fit, and/or applied case studies using the concept. As a body of work, these contributions highlight that self-understanding of social-ecological place, whether explicit or implicit, constitutes an important part of the study object, i.e., the role of institutions in social-ecological systems, and that this is, at the same time, a crucial point of reference for the scholar wishing to evaluate what constitutes institutional fit and how it might be brought into being.

  17. U-Pb SHRIMP and {sup 40}Ar/{sup 39}Ar constraints on the timing of mineralization in the Paleoproterozoic Caxias orogenic gold deposit, Sao Luis cratonic fragment, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Klein, Evandro Luiz, E-mail: evandro.klein@cprm.gov.br [Servico Geologico do Brasil (CPRM), Belem, PA (Brazil); Tassinari, Colombo Celso Gaeta, E-mail: ccgtassi@usp.br [Universidade de Sao Paulo (USP), SP (Brazil). Instituto de Geociencias. Centro de Pesquisas Geocronologicas; Vasconcelos, Paulo Marcos, E-mail: paulo@earth.uq.edu.au [University of Queensland, School of Earth Sciences, Brisbane (Australia)

    2014-07-01

    Caxias is an orogenic gold deposit in the Sao Luis cratonic fragment, which is correlated with the Rhyacian terranes of the West-African Craton. The deposit postdates peak metamorphism (estimated at 2100 ± 15 Ma) and is hosted in a shear zone that cuts across schists of the Aurizona Group (2240 ± 5 Ma) and the Caxias Microtonalite. The emplacement age of the microtonalite, as determined in this work by SHRIMP U-Pb zircon dating, is 2009 ± 11 Ma and represents a latest age magmatic event in the Sao Luis cratonic fragment. Older zircon age of 2139 ± 10 Ma is interpreted as due to inheritance from the older granitoid or volcanic suites (magmatic sources?) or to contamination during emplacement. Lead isotope compositions indicate that the Pb incorporated in ore-related pyrite was probably sourced from regional, orogenic calc-alkaline granitoids of ca. 2160 Ma. Hydrothermal sericite from Caxias yielded a {sup 40}Ar/{sup 39}Ar plateau age of 1990 ± 30 Ma, which combined with the emplacement age of the Caxias Microtonalite brackets the age of gold mineralization between 2009 ± 11 and 1990 ± 30 Ma. (author)

  18. Micron-scale coupled carbon isotope and nitrogen abundance variations in diamonds: Evidence for episodic diamond formation beneath the Siberian Craton

    Science.gov (United States)

    Wiggers de Vries, D. F.; Bulanova, G. P.; De Corte, K.; Pearson, D. G.; Craven, J. A.; Davies, G. R.

    2013-01-01

    diamond-forming fluids within the cratonic mantle beneath the Siberian Craton record significant variations in composition and volume and include both oxidised and reduced sources. These observations suggest that dating diamond inclusions using an isochron approach will best provide geologically meaningful ages if inclusions can be shown to be genetically (spatially) related.

  19. High-grade metamorphism during Archean-Paleoproterozoic transition associated with microblock amalgamation in the North China Craton: Mineral phase equilibria and zircon geochronology

    Science.gov (United States)

    Yang, Qiong-Yan; Santosh, M.; Tsunogae, Toshiaki

    2016-10-01

    Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth. The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recent models propose that the cratonic architecture was built through the assembly of several Archean microcontinental blocks into larger crustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery of the Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblage of garnet + clinopyroxene + orthopyroxene + magnetite + plagioclase + quartz ± biotite ± ilmenite. Mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak P-T condition of 800-830 °C and 7-8 kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in another sample containing the peak mineral assemblage of garnet + orthopyroxene + quartz + magnetite ± fluid yield peak P-T conditions of 860-920 °C and 11-14 kbar. Geochemical data show tonalitic to granodioritic composition and arc-related tectonic setting for the magmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562 ± 20 Ma (MSWD = 0.94) and 2539 ± 21 Ma (MSWD = 0.59) which is correlated with the timing of emplacement of the magmatic protolith. A younger group of zircons with upper intercept ages of 2449 ± 41 Ma (MSWD = 0.83); N = 6 as 2449 ± 41 Ma (MSWD = 0.83; N = 6) and 2480 ± 44 Ma (MSWD = 1.2; N = 9) constrains the timing of metamorphism. Zircon Lu-Hf data show dominantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC) in the range of 2529 to 2884 Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction-collision tectonics associated with microblock amalgamation in the NCC at

  20. Crustal high-velocity anomaly at the East European Craton margin in SE Poland (TESZ) modelled by 3-D seismic tomography of refracted and reflected arrivals

    Science.gov (United States)

    Środa, Piotr; Dec, Monika

    2016-04-01

    The area of Trans-European Suture Zone in SE Poland represents a contact of major tectonic units of different consolidation age - from the Precambrian East European Craton, through Palaeozoic West European Platform to Cenozoic Carpathian orogen. The region was built by several phases of crustal accretion, which resulted in a complex collage of tectonic blocks. In 2000, this region was studied by several seismic wide-angle profiles of CELEBRATION 2000 experiment, providing a dense coverage of seismic data in SE Poland and allowing for detailed investigations of the crustal structure and properties in this area. Beneath the marginal part of the EEC, the 2-D modelling of in-line data form several CELEBRATION profiles revealed a prominent high P-wave velocity anomaly in the upper crust, with Vp of 6.7-7.1 km/s, starting at 10-16 km depth (e.g., Środa et al., 2006). Anomalously high velocities are observed in the area located approximately beneath Lublin trough, to the NE of Teisseyre-Tornquist Zone. Based on 3-D tomography of first arrivals of in- and off-line CELEBRATION 2000 recordings (Malinowski et al., 2008), elevated velocities are also reported in the same area and seem to continue to the SW, off the craton margin. Gravimetric modelling also revealed anomalously high density in the same region at similar depths. High seismic velocities and densities are interpreted as indicative for a pronounced mafic intrusion, possibly related to extensional processes at the EEC margin. Previous 3-D models of the high-velocity intrusion were based on first arrivals (crustal refractions) only. In this study, also off-line reflections (not modelled up to now) are used, in order to enlarge the data set and to better constrain the geometry and properties of the velocity anomaly. A code for 3-D joint tomographic inversion of refracted and reflected arrivals, with model parametrization allowing for velocity discontinuities was used (Rawlinson, 2007). With this approach, besides the

  1. Geologia e Geoquímica do Batólito Rapakivi Rio Branco, SW do Craton Amazônico - MT

    Directory of Open Access Journals (Sweden)

    Larissa Marques Barbosa de Araújo-Ruiz

    2007-04-01

    Full Text Available O Batólito Rapakivi Rio Branco está inserido no Domínio Tectônico Cachoeirinha, parte da Província GeocronológicaRio Negro-Juruena, localizado na porção sudoeste do Craton Amazônico em Mato Grosso. Com base no mapeamentogeológico sistemático na escala 1:100.000 e nos estudos petrográficos e geoquímicos dos litotipos que constituem o conjuntoplutônico, foi possível redefinir a área de ocorrência desta unidade, sua constituição petrográfica e faciológica e caracterizar alitogeoquímica do magmatismo granítico. O batólito é constituído por duas suítes plutônicas principais, a primeira, formadapela suíte básica constituída por litotipos meso- a melanocráticos, cor cinza a negra, equigranulares finos, podendo exibirvariedades porfiríticas com matriz fina, estrutura maciça, de distribuição descontínua e localizada nas bordas da intrusão e asegunda, pela suíte ácida/intermediária, constituída por granitos porfiríticos, granofíricos (com textura rapakivi, isotrópicos,de cor vermelha, com a presença de fenocristais de feldspato potássico, de até 4 cm, envoltos em matriz fina a média. A suíteácida é composta por três fácies petrográficas: 1. monzogranitos equi-inequigranulares a pegmatóides; 2. leuco-monzogranitovermelho rapakivi e 3. monzogranitos a quartzo-monzonitos vermelhos escuros rapakivi. As rochas apresentam valores deSiO2 entre 67% a 73%, são peraluminosas a metaluminosas e definem um magmatismo da série cálcio-alcalina alto potássio ashoshonítica, em ambiente entre os tipos I e A de caráter pós-orogênico a anorogênico, gerados em ambiente intra-placa. Estesprocessos magmáticos encontram-se relacionados ao final do evento colisional, alcançando ambientes mais estáveis deconsolidação e estabilização tectônica do SW do Craton Amazônico.

  2. Monazite geochronology and geothermobarometry in polymetamorphic host rocks of volcanic-hosted massive sulphide mineralizations in the Mesoproterozoic Areachap Terrane, South Africa

    Science.gov (United States)

    Bachmann, Kai; Schulz, Bernhard; Bailie, Russell; Gutzmer, Jens

    2015-11-01

    The Areachap Terrane represents the medium-to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1240-1300 Ma) volcanic arc bound to the margin of the Archean Kaapvaal Craton in the east, and the polydeformed and metamorphosed Proterozoic Namaqua Province in the west. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of this terrane, adjacent to an important Mesoproterozoic crustal suture between the Archean Kaapvaal Craton and the Namaqua Province, which developed during the ∼1200-1000 Ma Namaquan Orogeny. The Areachap Terrane comprises highly deformed bimodal volcanic and volcano sedimentary successions that host a number of massive sulphide base metal orebodies. Samples from three of these orebodies at different locations were analysed to determine the age and P-T conditions of metamorphism along the Areachap Terrane. Metamorphic ages were determined by electron microprobe chemical dating of monazite. Garnet- and amphibole-bearing mineral assemblages were used for geothermobarometry at the Areachap Mine, located in the northern sector of the Areachap Terrane, monazite geochronology yields evidence for two populations of Th-U-Pb-ages at 1432 ± 30 Ma - a possible protolith age - and a metamorphic age of 1153 ± 21 Ma. Kantienpan and Copperton, representing the central and southern sector of the Areachap Terrane respectively, yield monazite ages for a younger metamorphic event with U-Th-Pb-ages of 1108 ± 19 Ma and 1104 ± 17 Ma, respectively. Geothermobarometric data give a differentiated view on the metamorphic evolution of the Areachap Terrane. An arc consistent clockwise P-T evolution path and upper amphibolite-facies peak metamorphic conditions are consistent at the three locations. The Areachap site shows a short prograde development with 8.0 kbar maximum pressure at circa 700 °C maximum temperature and a subsequent retrograde metamorphism. At Kantienpan, on the other hand, maximum metamorphic

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2009-07-01

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

  4. Evolution prediction from tomography

    Science.gov (United States)

    Dominy, Jason M.; Venuti, Lorenzo Campos; Shabani, Alireza; Lidar, Daniel A.

    2017-03-01

    Quantum process tomography provides a means of measuring the evolution operator for a system at a fixed measurement time t. The problem of using that tomographic snapshot to predict the evolution operator at other times is generally ill-posed since there are, in general, infinitely many distinct and compatible solutions. We describe the prediction, in some "maximal ignorance" sense, of the evolution of a quantum system based on knowledge only of the evolution operator for finitely many times 0evolution at times away from the measurement times. Even if the original evolution is unitary, the predicted evolution is described by a non-unitary, completely positive map.

  5. Contribuição ao estudo do magmatismo Paleo e Mesoproterozóico do SW do Craton Amazônico através da aplicação de isótopos estáveis de O, H e S Contribution to the study of the Paleo and Mesoproterozoic magmatism of the SW Amazonian craton based on stable isotopes of O, H and S

    Directory of Open Access Journals (Sweden)

    Mauro Cesar Geraldes

    2008-01-01

    Full Text Available Os isótopos estáveis de O, H e S foram utilizados para investigar a origem das rochas magmáticas nos Terrenos Jauru e Pontes e Lacerda do SW do Craton Amazônico, estado de Mato Grosso, Brasil. No Terreno Jauru as rochas granitóides do Greenstone belt Alto Jauru e da Suíte Cachoeirinha apresentam valores de δ18O entre +9,0‰ e +6,3‰ que indicam derivação a partir de magmas juvenis. Na Suíte Intrusiva Rio Branco valores de δ18O para rochas básicas estão entre +5,4‰ e +5,8‰ e para rochas félsicas entre +8,7‰ e +9,0‰; rochas intermediárias apresentam valores entre +7,3‰ e +8,3‰. Os valores mais baixos de δ18O, obtidos nas rochas básicas, são compatíveis com derivação mantélica, porém as rochas félsicas apresentam valores de δ18O compatíveis com origem crustais. Análises de isótopos estáveis de H (rocha total forneceram valores de δD entre - 83‰ e -92‰, diferente das assinaturas de rochas metamórficas e de águas meteóricas. Resultados em sulfetos para isótopos estáveis de S em rochas básicas e intermediárias desta suíte apresentam valores de δ34S coerentes com uma fonte mantélica (entre + 0,7‰ e +3,8‰, enquanto os valores de δ34S (entre +5,2‰ e +6,1‰ obtidos nas rochas félsicas sugerem participação crustal na sua gênese. Na Suíte Santa Helena (Terreno Pontes e Lacerda os resultados obtidos para δ18O se agrupam entre +4,4‰ e +8,9‰ indicando uma origem mantélica. O presente estudo confirma a importância da aplicação de isótopos estáveis para a compreensão de processos magmáticos e evolução crustal.Stable isotopes of O, H and S were used to investigate the origin of magmatic rocks of the Jauru and the Pontes e Lacerda Terrains, SW portion of the Amazonian craton, Mato Grosso state, Brazil. Granitic rocks of the Alto Jauru Greenstone belt and Cachoeirinha Suite, Jauru terrain, present δ18O values between +9.0‰ and +6.3‰, indicative of derivation from juvenile

  6. Melting of a subduction-modified mantle source: A case study from the Archean Marda Volcanic Complex, central Yilgarn Craton, Western Australia

    Science.gov (United States)

    Morris, P. A.; Kirkland, C. L.

    2014-03-01

    Subduction processes on early earth are controversial, with some suggestions that tectonics did not operate until the earth cooled to a sufficient point around the Archean-Proterozoic boundary. One way of addressing this issue is to examine well-preserved successions of Archean supracrustal rocks. Here we discuss petrography, whole-rock chemical and isotopic data combined with zircon Hf isotopes from andesites, high-magnesium andesites (HMA), dacites, high-magnesium dacites (HMD), rhyolites and coeval felsic intrusive rocks of the c. 2730 Ma Marda Volcanic Complex (MVC) in the central Yilgarn Craton of Western Australia. We demonstrate that these rocks result from melting of a metasomatized mantle source, followed by fractional crystallization in a crustal magma chamber. Contamination of komatiite by Archean crust, to produce the Marda Volcanic Complex andesites, is not feasible, as most of these crustal sources are too radiogenic to act as viable contaminants. The ɛNd(2730) of MVC andesites can be produced by mixing 10% Narryer semi-pelite with komatiite, consistent with modelling using Hf isotopes, but to achieve the required trace element concentrations, the mixture needs to be melted by about 25%. The most likely scenario is the modification of a mantle wedge above a subducting plate, coeval with partial melting, producing volcanic rocks with subduction signatures and variable Mg, Cr and Ni contents. Subsequent fractionation of cognate phases can account for the chemistry of dacites and rhyolites.

  7. Depositional environment and origin of the Lilaozhuang Neoarchean BIF-hosted iron-magnesite deposit on the southern margin of the North China Craton

    Science.gov (United States)

    Huang, Hua; Zhang, LianChang; Fabre, Sébastien; Wang, ChangLe; Zhai, MingGuo

    2017-07-01

    The Neoarchean Lilaozhuang iron-magnesite deposit is located in the middle of the Huoqiu banded iron formation (BIF) ore belt in Anhui Province on the southern margin of the North China Craton. The Huoqiu BIF is the unique one that simultaneously develops quartz-type, silicate-type, and carbonate-type magnetite in the region. The Lilaozhuang deposit is characterized by magnesium-rich carbonate (magnesite) in magnetite ores. The BIF-hosted iron ores include mainly of silicate type and carbonate type, with a small amount of quartz type, which chiefly exhibit banded and massive structure, with minor disseminated structure. The magnesite ores occur as crystal-like bright white and exhibits massive structure. The Y/Ho ratio and REY pattern of both iron and magnesite ores are similar to that of seawater, while Eu shows positive anomaly, which is the sign of seafloor hydrothermal mixture. These features suggest that ore-forming materials of iron and magnesium in the Lilaozhuang deposit are mainly from the mixture of seafloor hydrothermal and seawater. Both ores do not exhibit negative Ce anomaly, which indicates that the deposit was formed in an environment showing a lack of oxygen. C-O isotopic compositions indicate that magnesite ore has been reformed by metamorphism of low amphibolite facies and later hydrothermal alteration. Based on the comprehensive analysis, authors suggest that iron and magnesite ores in the Lilaozhuang deposits formed in a confined sea basin on continental margin and was influenced by later complex geological processes.

  8. Geological features and the Paleoproterozoic collision of four Archean crustal segments of the São Francisco Craton, Bahia, Brazil: a synthesis

    Directory of Open Access Journals (Sweden)

    BARBOSA JOHILDO S.F.

    2002-01-01

    Full Text Available Recent geological, geochronological and isotopic research has identified four important Archean crustal segments in the basement of the São Francisco Craton in the State of Bahia. The oldest Gavião Block occurs in the WSW part, composed essentially of granitic, granodioritic and migmatitic rocks. It includes remnants of TTG suites, considered to represent the oldest rocks in the South American continent (~ 3,4Ga and associated Archean greenstone belt sequences. The youngest segment, termed the Itabuna-Salvador-Curaçá Belt is exposed along the Atlantic Coast, from the SE part of Bahia up to Salvador and then along a NE trend. It is mainly composed of tonalite/trondhjemites, but also includes stripes of intercalated metasediments and ocean-floor/back-arc gabbros and basalts. The Jequié Block, the third segment, is exposed in the SE-SSW area, being characterized by Archean granulitic migmatites with supracrustal inclusions and several charnockitic intrusions. The Serrinha Block (fourth segment occurs to the NE, composed of orthogneisses and migmatites, which represent the basement of Paleoproterozoic greenstone belts sequences. During the Paleoproterozoic Transamazonian Orogeny, these four crustal segments collided, resulting in the formation of an important mountain belt. Geochronological constrains indicate that the regional metamorphism resulting from crustal thickening associated with the collision process took place around 2.0 Ga.

  9. Calymmian magmatism in the basement of the Jauru Terrain (Rondonian - San Ignacio Province), Amazon Craton: U-Pb and Sm-Nd geochemistry and geochronology

    Energy Technology Data Exchange (ETDEWEB)

    Fachetti, Frankie James Serrano; Costa, Ana Claudia Dantas da; Silva, Carlos Humberto da, E-mail: frankiefachetti@hotmail.com, E-mail: acdcosta@ufmt.br, E-mail: chsilva@ufmt.br [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Instituto de Ciencias Exatas e da Terra

    2016-11-01

    The Taquarussu Orthogneiss and the Guadalupe Granodiorite, part of the Rondonian-San Ignacio Province basement, southwest of the Amazonian Craton, correspond to oriented bodies with a NW trend. The rocks show granodiorite composition with minor occurrences of coarse grained monzogranites consisting essentially of plagioclase, quartz, microcline, orthoclase and biotite. The accessory minerals are amphibole, titanite, garnet, apatite, epidote, zircon and opaque. The geochemical data indicate that the rocks are classified as granodiorites and monzogranites, with an intermediate to acid magmatism, sub-alkaline character, from the calc-alkaline to the high-K calc-alkaline series, with alumina ratios ranging from metaluminous to lightly peraluminous. The rocks were classified as generated in volcanic islands arc environment and the U-Pb data (SHRIMP zircon) show a concord age 1575 ± 6 Ma. The Sm-Nd model age (T{sub DM}) is 1.63 Ga with εNd (t = 1.57 Ga) ranging from -1.52 to +0.78. These data indicate that these rocks are probably a juvenile crust with a possible contamination of crustal rocks. (author)

  10. Geochronology and Hf–Fe isotopic geochemistry of the Phanerozoic mafic–ultramafic intrusions in the Damiao area, northern North China Craton: Implications for lithospheric destruction

    Indian Academy of Sciences (India)

    Jiulong Zhou; Zhaohua Luo

    2014-06-01

    Timing and source of several Fe-mineralized mafic–ultramafic intrusions in the Damiao area are investigated here by coupling new geochronological and Hf–Fe isotopic data with previous results. Although regarded as a Late Paleoproterozoic assemblage previously, two ∼140 Ma intrusions are recognized by zircon U–Pb dating, indicating emplacement of these intrusions from Middle Devonian to Early Cretaceous times. Both Hf and Fe isotopic features lead to the conclusion that distinct mantle components contributed to their magma generation. As the first magmatic phase, the ∼395 Ma intrusions were mainly derived from the slightly-enriched SCLM that was prevalent during the Paleozoic. However, asthenospheric material was strongly involved in the formation of the ∼215 Ma Gaositai intrusion. Therefore, the initiation of lithospheric destruction in the northern NCC is inferred to have occurred in Late Triassic time, triggered by post-orogenic extension following the ∼250 Ma collision between the Siberian Craton and the NCC. The ∼140 Ma intrusions originated from a significantly-enriched mantle component probably resided in the predominant slightly-enriched SCLM. This mantle source would have melted in the Late Mesozoic, when the thin lithosphere enabled enhanced heat transfer from the asthenosphere. In summary, these distinct mantle sources of mafic–ultramafic magmatism provide a record of mantle heterogeneity and the gradual upward migration of the lithosphere–asthenosphere boundary during lithospheric destruction.

  11. Devonian alkaline magmatism in the northern North China Craton: Geochemistry, SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

    Directory of Open Access Journals (Sweden)

    Dingling Huang

    2017-01-01

    Full Text Available The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton (NCC. The intrusion is mainly composed of quartz-monzonite. Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca. 381.5 Ma. The rock is metaluminous with high (Na2O + K2O values ranging from 8.46 to 9.66 wt.%. The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies. The Wulanhada rocks exhibit high initial values of (87Sr/86Srt = 0.70762–0.70809, low ɛNd(t = −12.76 to −12.15 values and negative values of ɛHf(t = −23.49 to −17.02 with small variations in (176Hf/177Hft (0.281873–0.282049. These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC. The Wulanhada rocks, together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions, constitute a post-collisional magmatic belt along the northern NCC.

  12. The 40Ar/39Ar dating of magmatic activity in the Donbas Fold Belt and the Scythian Platform (Eastern European Craton)

    Science.gov (United States)

    Alexandre, P.; Chalot-Prat, F.; Saintot, A.; Wijbrans, J.; Stephenson, R.; Wilson, M.; Kitchka, A.; Stovba, S.

    2004-09-01

    The Donbas Fold Belt is the compressionally deformed southeasternmost part of the intracratonic late Paleozoic Dniepr-Donets rift basin. It is situated in an intracratonic setting but close to the southern margin of the East European Craton, south of which lies the Scythian Platform. A range of igneous rocks from the Donbas Fold Belt and the Scythian Platform were dated by the 40Ar/39Ar method in order to constrain the ages of magmatic activity in these areas, and compare them. The plateau ages from the south margin of the Donbas Fold Belt vary from 151.4 +/- 4.7 Ma to 278.1 +/- 5.3 Ma, and define three main age groups: Middle-Late Jurassic, Middle-Late Triassic, and Early Permian. The age spectra obtained from the Scythian Platform samples are often disturbed as a result of limited alteration. The proposed ages (plateau and pseudoplateau) vary from 174.4 +/- 2.1 Ma to 243.7 +/- 1.4 Ma, and two major age groups are defined, in Early Carboniferous and Triassic/Jurassic times. The Early Permian (285-270 Ma) and Early Triassic (245-250 Ma) ages of magmatic activity are the same in both areas; in the Late Triassic, the ages of magmatic activity are slightly different (220 and 205 Ma), and they are entirely different thereafter. These data can be interpreted as indicating a mantle plume as common deep magmatic source.

  13. Paragenesis of Cr-rich muscovite and chlorite in green-mica quartzites of Saigaon–Palasgaon area, Western Bastar Craton, India

    Indian Academy of Sciences (India)

    K R Randive; M M Korakoppa; S V Muley; A M Varade; H W Khandare; S G Lanjewar; R R Tiwari; K K Aradhi

    2015-02-01

    Green mica (fuchsite or chromian-muscovite) is reported worldwide in the Archaean metasedimentary rocks, especially quartzites. They are generally associated with a suite of heavy minerals and a range of phyllosilicates. We report the occurrence of green-mica quartzites in the Saigaon–Palasgaon area within Bastar Craton in central India. Mineralogical study has shown that there are two types of muscovites; the chromium-containing muscovite (Cr2O3 0.84–1.84%) and muscovite (Cr2O3 0.00–0.22%). Chlorites are chromium-containing chlorites (Cr2O3 3.66–5.39%) and low-chromium-containing chlorites (Cr2O3 0.56–2.62%), and as such represent ripidolite–brunsvigite varieties. Back scattered electron images and EPMA data has revealed that chlorite occurs in two forms, viz., parallel to subparallel stacks in the form of intergrowth with muscovite and independent crystals within the matrix. The present study indicates that the replacement of chromium-containing chlorite by chromium-containing muscovite is found to be due to increasing grade of metamorphism of chromium-rich sediments. However, the absence of significant compositional gap between aforementioned varieties indicates disparate substitution of cations, especially chromium, within matrix chlorites. The chromium-containing muscovite and muscovite are two separate varieties having distinct paragenesis.

  14. Tectonic insights of the southwest Amazon Craton from geophysical, geochemical and mineralogical data of Figueira Branca mafic-ultramafic suite, Brazil

    Science.gov (United States)

    Louro, Vinicius H. A.; Cawood, Peter A.; Mantovani, Marta S. M.; Ribeiro, Vanessa B.

    2017-0