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Precambrian Research
Elsevier Pub. Co.
Precambrian Research

Elsevier Pub. Co.

0301-9268

Precambrian Research/Journal Precambrian ResearchSCIISTP
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    Delineation of sulfide mineralization over a part of western margin of greenstone belt of the Dhanjori Basin, Singhbhum Craton from petrophysical, petrographic and geophysical studies

    Gupta, Ashok KumarSrivastava, Shalivahan
    20页
    查看更多>>摘要：The Archean greenstone belt occupying the Dhanjori basin is a potential target for exploring resources of various precious metals and sulfide minerals like other Precambrian greenstone belts. Thick alluvium cover and limited outcrops in the survey area pose challenges in mineral exploration strategies. This work presents an integrated study of petrophysics, petrography and geophysics (gravity and magnetic survey) over an identified conductor in the western margin of the Dhanjori basin, which aims to delineate anomalous sulfide-bearing zones. The present analysis of petrophysical parameters (bulk density, and volume magnetic susceptibility) of various litho-units including dolerite, metabasalt, magnetite, chert and metasediments (quartzite and quartz pebble conglomerate (QPC)) indicate the occurrences of sulfide minerals. Petrographic studies of the smoky and fuchsitic quartzites also suggest sulfide mineral phases (chalcopyrite, and pyrite). EPMA studies on these metasediments reveal the presence of sulfide minerals (pyrite, chalcopyrite, millerite, bornite and pyrrhotite) and sulfosalts (gersdorffite, and cobaltite), along with traces of gold and silver. The 3D inversion of gravity and magnetic data identified significant geophysical signatures corresponding to possible subsurface sulfide mineralization and have helped in the reduction of the target area. This work also demonstrates the utility of integrated investigations in identifying deep-seated mineralization, which may be helpful in exploring analogous prospective Precambrian greenstone belts.
      原文链接:
    • NSTL
    • Elsevier

    Paleoproterozoic tectonic evolution from subduction to collision of the Khondalite Belt in North China: Evidence from multiple magmatism in the Qianlishan Complex

    Li, WenxuanYin, ChangqingLin, ShoufaLi, Wenjing...
    14页
    查看更多>>摘要：This study presents detailed petrology, major and trace element chemistry, Sr-Nd isotope, and in-situ zircon U-Pb dating and Hf-O isotopes on granites from the Qianlishan Complex of the Khondalite Belt in North China. Zircon SIMS U-Pb data indicate three episodes of Paleoproterozoic magmatism at ca. 2.06 Ga, 1.95 Ga and 1.92 Ga. Of those, the ca. 2.06 Ga hornblende monzogranites and fine-grained potassic granites display typical features of A-type granite, represented by metaluminous affinity, high FeOT/MgO ratios (3.29-5.63), and high Ga/Al and Zr + Nb + Ce + Y values with high zircon saturation temperatures (916-1000 degrees C). Moreover, they have positive zircon epsilon Hf values of +0.29 to +5.01 and homogenous delta O-18 values of 5.11-5.87 parts per thousand, probably derived from the partial melting of talc-alkaline granitoids without addition of supracrustal rocks. The ca. 1.95 Ga garnet-bearing granites contain peraluminous minerals of garnet and muscovite and have high ASI index (>1.1) and delta O-18 values (7.14-9.18 parts per thousand), indicating S-type granite. Their negative whole-rock epsilon Nd(t) values (-2.87 to -2.81) and zircon epsilon Hf(t) values (-4.06 to +1.29) suggest a source of ancient supracrustal rocks. The ca. 1.92 Ga coarse-grained potassic granites present metaluminous and magnesian signatures of I-type granite, including low A/CNK and FeOT/MgO ratios. Their negative epsilon Hf(t) values (-5.00 to -1.64) and homogenous delta O-18 values (5.20-6.00%) suggest that the original magma was derived primarily from enriched lower crust. The results, combined with regional geology and previous studies, establish a prolonged magmatic-metamorphic evolution of the Khondalite Belt from subduction to collision, involving ca. 2.3-2.0 Ga subduction-related arc magmatism, ca. 1.95 Ga syncollisional high-pressure high-temperature crustal anatexis, and ca. 1.92 Ga post-collisional magmatism and synchronous ultrahigh temperature metamorphism.
      原文链接:
    • NSTL
    • Elsevier

    Mantle source heterogeneity in a Neoproterozoic back-arc basin: Geochemical and thermodynamic modeling of the volcanic section of Wadi Ghadir ophiolite, Egypt

    Zoheir, BasemDiab, AliaaKoutsovitis, PetrosFeigenson, Mark...
    20页
    查看更多>>摘要：Wadi Ghadir ophiolite in the Egyptian Eastern Desert, which forms the northern part of the Nubian Shield, is considered as one of the best-preserved segments of the Neoproterozoic oceanic lithosphere on Earth. Primary melt calculations and thermodynamic modeling of new geochemical data for the Wadi Ghadir pillow lavas and dike complexes, integrated with comprehensive petrographic investigations, are here employed to unravel the geodynamic evolution of this ophiolitic section.& nbsp;Whole-rock geochemical and clinopyroxene data indicate that the pillow lavas and sheeted dike complexes preserve geochemical signatures of N-MORB, E-MORB and OIB-like basalts. Less abundant, discrete dikes have a distinctly LILE-enriched island arc tholeiite (IAT) composition. The pillow lavas and sheeted dike complexes replicate mixed liquid lines of descent, whereas the discrete dikes exhibit a calc-alkaline differentiation trend. The pillow lavas were produced by limited (5-7%) mantle partial melting as a result of isothermal decompression over a wide pressure range (5 - 19 kbar) and restricted mantle potential temperatures (Tp & AP;1260-1300 C), suggesting ponding of rising melts from depths of ~60 to 15 km in dry mantle. Primary melts of the sheeted dike complexes were produced by 9-11% partial melting at P & AP;9.7 & PLUSMN; 2.5 kbar, Tp & AP;1290 C, and P & AP;14.7 & PLUSMN; 1.4 kbar, Tp & AP;1325 C, suggesting a role of varying hydrostatic pressure. The magma source for the discrete dikes occurred at 10.7 & PLUSMN; 0.6 kbar and Tp of 1230-1300 C by 11-13% partial melting of a metasomatized mantle region.& nbsp;The calculated Tp ranges for the different melts are consistently lower than temperatures of sub-ridge ambient mantle. Together with the variable concentrations of K2O and other incompatible elements, the estimated low temperatures of melt generation emphasize mixed sources and wet peridotite melting. The modeled primary melts designate crustal growth during back-arc basin opening and closure. During the basin opening, decompression melting of mildly enriched mantle produced high-Ti tholeiitic (pillow lavas) and transitional (sheeted dike complexes) melts, whereas subduction initiation during basin closure promoted melt-peridotite interaction. Lowering of the solidus by subduction-related components triggered melting of a spinel lherzolitic mantle and produced low-Ti calc-alkaline melts that sourced the discrete dikes. The mixed MORB and SSZ geochemical characteristics of the studied volcanic rocks, coupled with the various modeled melts and lack of significant thermal anomalies in the mantle conditions, are here interpreted as manifestations of mantle source heterogeneity in a marginal oceanic basin during the accretionary stages of the Arabian-Nubian Shield.
      原文链接:
    • NSTL
    • Elsevier

    Crustal structure around the margins of the eastern Superior craton, Canada, from receiver function analysis

    Vervaet, FrancoisDarbyshire, Fiona
    17页
    查看更多>>摘要：The Precambrian domains of eastern Canada preserve a remarkable record of continental crust formation and evolution spanning at least 3 billion years, and thus present an excellent opportunity to investigate the imprints of early tectonic processes on the present-day continental crust. Archean domains in the region include the eastern Superior craton, part of one of the largest cratons on Earth, the western portion of the North Atlantic craton, and a collage of continental fragments. The Paleoproterozoic Trans-Hudson Orogen and its eastern branches played a fundamental role in the assembly of Laurentia. In the southeast, a succession of orogenic events culminated in the Mesoproterozoic Grenville Orogen and assembly of supercontinent Rodinia. To better understand crustal formation and evolution in this complex region, we use teleseismic P-wave receiver functions recorded by a network of broadband seismographs distributed across Precambrian eastern Canada. At each station, back-azimuthal variations in receiver function waveforms indicate significant lateral crustal heterogeneity, leading us to model different representative directions separately. The stations situated on Archean terranes present a relatively simple crustal structure with a well-defined Moho at-33-46 km depth. In contrast, those on Proterozoic domains show a more complex structure, higher velocities in the lower crust and a deeper, more diffuse Moho at-46-55 km depth. Across the entire region, bulk crustal composition is largely felsic to intermediate (Vp/Vs-1.66-1.76), except for a station situated in an area dominated by anorthosite massifs whose composition is significantly more mafic (Vp/Vs-1.85). We combine our new models with those from previous natural-and active-source seismic studies of eastern Canada, and compare our results to those from other Precambrian regions. We suggest that the present-day crustal structure of Precambrian eastern Canada is more strongly influenced by the tectonic processes that assembled Laurentia than secular variations in initial crustal formation.
      原文链接:
    • NSTL
    • Elsevier

    Timing and duration of discrete tectono-metamorphic events of the polymetamorphic high-grade Central zone of the Limpopo Belt (South Africa): Insight from in situ geochronology of monazite and zircon

    Klemd, ReinerBrandt, SoenkeLiu, YuLi, Qiu-Li...
    20页
    查看更多>>摘要：The Central Zone of the Limpopo Belt in southern Africa is a complex polymetamorphic terrane that has expe-rienced three metamorphic events, at 3.22 Ga (M1), 2.71-2.56 Ga (M-2) and 2.02 Ga (M3). On the basis of a previous detailed petrological and geothermobarometrical study of a newly discovered supracrustal enclave within the Bulai Pluton (ca. 2.61 Ga granitoid gneiss in the Central Zone), we conducted a geochronological in situ study of texturally and chemically different monazite and zircon grains, which occur respectively as in-clusions in garnet, in decompressional cordierite coronas around garnet and as matrix grains in polymetamorphic granulite-facies metapelites. Secondary-ion mass spectrometry (SIMS) in situ analyses of the monazite and zircon grains in thin sections, complemented by analyses of separated zircon grains, allowed the texturally-controlled direct determination of U-Pb and( 207)Pb/Pb-206 ages at a scale of < 10 mu m. A concordant age of ca. 2.71 Ga derived from zircon included in garnet is interpreted to reflect the age of initial garnet growth during the pro-grade M2 metamorphism. Zircons in the cordierite coronas around garnet revealed a slightly younger concordant age of ca. 2.56 Ga. Zircon growth in the coronas is related to the retrograde breakdown of garnet (Zr-bearing) through to the decompression reaction garnet + sillimanite + quartz & RARR; cordierite + zircon and hence the ca. 2.56 Ga zircon age is interpreted to reflect the timing of post-peak decompressional uplift. The zircon ages constrain the duration of M2 metamorphism to approximately 145 Myr. A concordant 2.71 Ga age of large monazite inclusions in garnet cores corroborates the 2.71 Ga zircon age of initial garnet growth during the prograde M(2 )metamorphism. Core domains of the largest monazite in cordierite corona revealed ages between 2.5 and 2.0 Ga that are interpreted to reflect (multiple) Pb-loss during the M-3 granulite-facies metamorphism. Other monazites in cordierite coronas and matrix showed an age of ca. 2.0 Ga, reflecting complete re-equilibration or new growth during M3 overprinting. Owing to the preservation of the original micro-textures related to the growth of zircon and monazite we were able to link the age data with the P-T record, and refine the timing and duration of the M2 granulite-facies metamorphism (approximately 2.71-2.56 Ga) in the Limpopo Belt that was previously dated at ca. 2.62 Ga by zircon separation techniques. The ca. 145 Ma duration of granulite-facies metamorphism is consistent with a tectonic model of long-lasting asthenosphere upwelling and subsequent continent-continent collision in the late Neoarchean. Decompression triggered by post-collisional extension occurred at 2.56 Ga. Paleoproterozoic M3 granulite-facies metamorphism at ca. 2.0 Ga caused the widespread re-setting of the U-Pb and Pb-Pb data that also affected most zircon and monazite grains. This study reveals that precise dating of the timing and duration of distinct tectono-metamorphic events by texturally-controlled in situ geochronology allows a much better insight into the complex geodynamic evolution of high-grade polymetamorphic terranes.
      原文链接:
    • NSTL
    • Elsevier

    Barite in the Ediacaran Doushantuo Formation and its implications for marine carbon cycling during the largest negative carbon isotope excursion in Earth's history

    Lei, LidanOuyang, GuangZhang, ZihuDodd, Matthew S....
    13页
    查看更多>>摘要：The Ediacaran Period (-635-539 Ma) witnessed the largest negative inorganic carbon isotope (delta C-13(carb)) excursion in Earth's history (i.e., the Shuram Excursion), which is characterized by decoupling from the organic carbon isotope (delta C-13(org)) record. The cause(s) of this event remains highly debated. Here, we report a major (-8-9-Myr-long) episode of strong barium (Ba) accumulation during the Shuram Excursion in the form of barite, as recorded in the Ediacaran Doushantuo Formation (-635-551 Ma) of South China. The inner-shelf Zhangcunping section exhibits minimal Ba enrichment, while the slope Siduping section shows maximal Ba enrichment with the intrashelf basinal Jiulongwan section in the middle. The Siduping section contains-5 mu m-diameter, ellipsoidal barite crystals of marine origin; and the Jiulongwan section contains large (> 50 mu m), euhedral barite crystals and cements that are partly replaced by pyrite, pointing to a diagenetic origin with barite formation within the sulfate-methane transition zone. The barite delta S-34 is-10%o higher than delta S-34 of carbonate-associated sulfate at Jiulongwan in contrast to similar values of these two components at Siduping, suggesting a limited influence of methane oxidation, if any, on the formation of Shuram Excursion at Jiulongwan. The Ce/Ce* exhibits a lateral gradient among study sections which is reverse to the Ba enrichment, supporting the hypothesis that local surface-water productivity controlled dissolved oxygen levels in the Ediacaran surface ocean. Based on these findings, we attribute the highest Ba enrichment at Siduping to oceanic upwelling which enhanced local marine productivity. Furthermore, we propose that episodic oceanic upwelling in the Ediacaran shelf regions likely transported phosphorus and dissolved organic carbon (DOC) to shallow waters, increasing their productivity and facilitating the oxidation of DOC, contributing to the largest negative carbon isotope excursion in Earth's history.
      原文链接:
    • NSTL
    • Elsevier

    Cyanobacterial spheroids and other biosignatures from microdigitate stromatolites of Mesoproterozoic Wumishan Formation in Jixian, North China

    Chen, Zhong-QiangPapineau, DominicYang, Hao
    17页
    查看更多>>摘要：Stromatolites have been widely reported from the Archean and Paleoproterozoic successions worldwide, and they could represent one of oldest life forms on Earth. Of these, a group of small stromatolites occur as micro-digitate low-relief columns, and are also conspicuous in the field. However, biogenicity of these microdigitate stromatolites (MDSs) has long been disputed due to to the abundance of radial-fibrous texture and a lack of convincing microfossils. New examples of MDS are documented from the Mesoproterozoic Wumishan Formation of the Jixian area, North China. Vertically oriented fibrous fabrics are conspicuous and penetrate laminae as well as microscopic spheroids, which point to an abiotic genesis for this specific fabric. Stromatolite laminae contain abundant spheroids, typically 15-30 mu m in diameter, with single or double outlines and they occur as solitary coccoid-like microfossils or in small aggregated colonies. Spheroids show strong fluorescence under both green and purple exciting lights, consistent with their composition of organic matter. Spheroids are abundant in the Wumishan stromatolites and they are categorized into two types. The first kind comprises micrite nuclei sur-rounded by sparitic sheaths, without nano-particle coatings. A smooth to grainy spheroidal surface defines the first kind of spheroids that also has a distinct rounded opening, which is often broken probably due to diagenesis and silicification. The second kind of spheroids is usually covered with nano-particles and lacks circular opening on surface. These spheroids possess large nuclei of single sparitic calcite coated with thin sparry sheaths. Overall, the Wumishan spheroids resemble coccoidal microorganisms reported from other Archean-Paleoproterozoic strata worldwide, but they are also better preserved. The rounded opening on spheroid surface is interpreted as division point of unicells during reproduction of the life cycle of bacteria akin to Myxococcoides grandis. Clump-like micro-particle aggregates in nuclei could represent daughter cells released from the parental enve-lope, similar to the reproduction process and life cycle suggested for similar spheroidal microfossils from other similar Precambrian occurrences. The Wumishan spheroids therefore may represent fossilized prokaryotes that could have contributed to construct the MDS. Moreover, filamentous microfossils are occasionally present in the coloumns of stromatolites, and they resemble filamentous cyanobacteria, but may not be major constructors of MDS due to their rarity in the buildups. Three types of nano-particles are also conspicuous: (1) putative organic relics, such as fragmented filaments and mucuslike biofilms (purported EPS), (2) organominerals, including nanoglobules, polyhedrons, and their aggregates, and (3) dumbbell-shaped nano-particle aggregates. All of these nano-particles are interpreted to be likely biogenic in origin, and many of them were found from the radial-fibrous fabrics of carbonate precipitates in the MDS, implying that some heterotrophic bacteria may have affi-cliated the precipitation of radial fibers in deep-time radial-fibrous carbonate precipitates. Therefore, abundant and diverse biosignatures (spheroids, tubular filaments, and nano-particles) are identified in the Wumishan MDSs, and we conclude that diverse filamentous and coccoidal micro-organismscontributed to the formation of the Wumishan stromatolites.
      原文链接:
    • NSTL
    • Elsevier

    Zircon xenocryst Hf-O isotopic compositions in the Qiyugou Au orefield: A record of Paleoproterozoic oceanic slab subduction in the Trans-North China Orogen

    Wang, PengMao, JingwenYe, HuishouZhao, Guochun...
    8页
    查看更多>>摘要：The Trans-North China Orogen (TNCO) is suggested to form under a transition from subduction to collision from 2500 Ma to ~ 1850 Ma, and the Taihua TTG gneiss in the Xiong'ershan district is thought to have formed in an Andean-type continental arc setting or island arc setting during this geological event. However, to date, the geochemical record of the subducted oceanic slab is still lacking. This study focuses on Hf-O isotopic analyses of zircon xenocrysts from the Qi189 hornblende monzogranite, Qiyugou Au orefield, TNCO. The results reveal that zircon xenocrysts with ages from 2304 Ma to 1934 Ma generally have low-delta O-18 values from 2.66 parts per thousand & nbsp; to 3.83 parts per thousand & nbsp; and high-epsilon Hf(t) values from 2.7 to 7.8. These zircon grains were generated from remelting of high-temperature altered gabbroic rocks of the subducted oceanic slab during the transition from compression to extension. In this paper, we present new evidence for Paleoproterozoic oceanic slab subduction in the TNCO.
      原文链接:
    • NSTL
    • Elsevier

    Tectonic evolution of Neoproterozoic rocks, eastern Cameroon: Implication for gold mineralization in the Betare Oya and Woumbou-Colomine-Kette districts

    Ndonfack, Kevin Igor AzeudaXie, YulingZhong, RichenYomeun, Bovari Syprien...
    19页
    查看更多>>摘要：The Betare Oya and Woumbou-Colomine-Kette gold districts, located in the East Metallogenic Province of Cameroon, are characterized by a complex Neoproterozoic history of metamorphism, polyphase deformation, and widespread crustal anatexis and magmatism. Controversy exists on the timing of the metamorphic and magmatic events, particularly within the area of the Lom Belt, and accurate absolute ages and even geodynamic chronology pertaining to the tectono-magmatism are yet to be well constrained. New LA-ICP-MS U-Pb zircon ages, Lu-Hf isotope data, and whole-rock geochemistry of granitoids and metavolcanic-metasedimentary rocks from the Woumbou-Colomine-Kette and Betare Oya districts better constrain the tectonic evolution of eastern Cameroon. The U-Pb age data of 646.5 +/- 2.0 Ma, 643.7 +/- 1.8 Ma, 641.3 +/- 4.4 Ma, 641.4 +/- 3.5 Ma, and 620.9 +/- 2.6 Ma revealed that the granitoids from the Woumbou-Colomine-Kette district were emplaced during two main magmatic events at ca. 650-635 Ma and ca. 620 Ma. The zircon Hf(t) values between -13.80 and -6.20 with Hf-TDM model ages of 1.9-2.4 Ga, coupled with geochemical signatures show that granitoids were generated by partial melting of Paleoproterozoic supracrustal metasedimentary units and meta-igneous rocks during subduction and orogenic convergence. Metamorphic zircons from the actinolite-chlorite schist of the Lom Belt gave ages between ca. 655 and 585 Ma, implying that the metavolcanic-metasedimentary rocks of the belt were deposited prior to 655 Ma, then metamorphosed and deformed between ca. 655 and 585 Ma. Two age peaks of detrital zircons were measured at ca. 1035 Ma and ca. 784 Ma with the epsilon Hf-(t) values of -14.60 and -18.37 and +2.97 and +8.60, respectively. These data, coupled with those from previous studies, suggest that the Lom Belt may represent a suture zone between the Central and Southern Cameroon domains with an early rifting at ca. 784 Ma. In general, our U-Pb ages indicate compressional to transpressional deformational phases in the two districts at ca. 660-635 Ma (D1), 630-610 Ma (D2), and 600-580 Ma (D3). The latter D3 transpressional event was also associated with the formation of the widespread occurrences of orogenic gold, although the precise timing of gold deposition needs to be constrained.
      原文链接:
    • NSTL
    • Elsevier

    Evidence for change in crust formation process during the Paleoarchean in the Sao Francisco Craton (Gaviao Block): Coupled zircon Lu-Hf and U-Pb isotopic analyses and tectonic implications<br>

    dos Santos, ClaudiaZincone, Stefano A.Queiroga, Glaucia N.Bersan, Samuel M....
    14页
    查看更多>>摘要：The continental crust growth/evolution processes and the tectonic regime through Eo-and Paleoarchean times are enigmatic due to the scarcity of preserved crust. The Gavia & SIM;o Block, Sa & SIM;o Francisco Craton (Brazil), contains exposed and well-preserved Eo-Paleoarchean crust remnants, providing a rare opportunity to investigate these issues. Here, we describe new U-Pb ages and Hf isotope data of 3.51-3.4 Ga tonalites and diorites from the Gavia & SIM;o Block and compare these with previously published Lu-Hf data from Hadean/Eo-Paleoarquean zircons from the Gavia & SIM;o Block and other primitive cratons. The Eo-to Paleoarchean evolution of the Gavia & SIM;o Block is registered by ca. 360 Myr of continuous magmatic events from ca. 3.66 Ga to 3.30 Ga. From the available Hf data, we interpret that each of the events younger than 3.6 Ga registers a new juvenile addition that assimilated older crust, whereas the rocks older than 3.6 Ga are exclusively formed through the reworking of a Hadean, and to a less extent early Eoarchean crust. The shift in the crust generation process with the input of juvenile material into the Gavia & SIM;o Block has been documented within the -3.8-3.5 Ga time in other primitive cratonic complexes including the Wyoming, Pilbara, Kaapvaal, Slave, Singhbhum, and Yilgarn. As documented in these other cratons, our data suggest that a shift in the Hf isotope record to rocks younger than 3.6 Ga reflects a transition from stagnant-lid to mobile-lid tectonics in the crust formation process of the Sa & SIM;o Francisco Craton. This change in the geodynamic regime appears to have been global at ca. -3.8-3.5 Ga and facilitated the extraction of juvenile melts, crustal reworking, evolved magmatism, and the production of stabilizing melt-depleted lithospheric mantle.
      原文链接:
    • NSTL
    • Elsevier