查看更多>>摘要：Serpentinites, particularly those transformed from mantle peridotites, are potential tracers for the petrogenesis and tectonic geodynamics of peridotitic protoliths and certain mantle processes (e.g., fluid/melt metasomatism). This study presents new petrographic observations, mineral chemistry, geochemistry, and Re-Os isotopic compositions of the serpentinites from the Aoyougou ophiolite, the most problematic ophiolite in the Qilian Orogenic Belt, NW China. Our data indicates that the protoliths are melting residues of oceanic lithospheric mantle in a forearc setting. The high Mg-# values (90.49-93.06) of serpentinites confirm their refractory nature, which reflects multiple episodes of partial melting and melt extraction (13.86-18.90%). Moreover, the geochemical fingerprints (e.g., U-shaped rare earth element chondrite-normalized patterns, enrichments of fluid-mobile elements, positive Eu anomalies, extremely high Re-187/Os-188 ratios, high Re abundances) and the "future" model ages (T-MA) of most of the serpentinites show that they had undergone intensive melt/fluid metasomatism. Metasomatism was probably triggered by the input of slab-derived melts/fluids in the forearc setting, which resulted in the introduction of a high abundance of Re. Moreover, the Re-Os isotopic ages record a two-stage evolutionary history of mantle residues that included Precambrian melt extraction and late-Cambrian Re input by mantle metasomatism. Combined with the geological context, an intra-oceanic subduction model was proposed to interpret the origin and evolution of the ophiolite. As a result of the intra-oceanic subduction of the North Qilian Ocean, the oceanic crust of the ophiolite was generated by decompressive melting of the previously depleted forearc mantle. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 +/- 1.1 Ma), monzogranites (194.3 +/- 1.1 Ma and 200.5 +/- 1.2 Ma), and granites (184.5 +/- 2.1 Ma, 185. 5 +/- 4.0 Ma and 185.6 +/- 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated epsilon(Hf) t) values of the diorite are - 16.7 to - 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：The Alxa Block in western China is a significant tectonic unit in the middle part of the southern Central Asian Orogenic Belt that was affected by multiple Paleozoic and Meso-Cenozoic deformation events. In this study, the results from detailed mapping and structural analysis coupled with new U-Pb zircon ages indicate that the Langshan region of the northeastern Alxa Block has experienced ten deformation events since the Late Devonian. Four separate structural domains are identified based on distinctive litho tectonic relationships, and these domains contain intrusive and structural crosscutting relationships that allow the complex deformational history to be determined. Each deformation phase can be related to regional tectonic events associated with the consolidation of Central Asia's crust and subsequent intraplate reactivation. The first three events (D1-D3) are tied to convergence between the Alxa Block, the North China and the Yangtze Cratons prior to and during closure of the Paleo-Asian Ocean in the Mid Late Permian. Subsequently, sinistral displacement (D4) occurred between the Alxa Block and the North China Craton during the Triassic. Since the late Mesozoic, reactivation of the northeastern Alxa Block occurred repeatedly as an intraplate response to subduction of the Paleo-Pacific Plate, closure of the Mongol-Okhotsk Ocean (D5), retreating subduction of the Paleo-Pacific Ocean (D6), collision between the Qiangtang and Lhasa blocks and convergence between the East Asian margin and Western Philippine Block (D7), and later collision between India and Eurasia (D8-10). The Alxa Block provides a superb case study of how a continental interior region structurally records the multi-phase transition from plate margin tectonism to intraplate reactivation. The events documented in this study invite correlation with other Paleozoic-Recent tectonic events recorded elsewhere in the southern CAOB and adjacent cratonic blocks in Central Asia. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Khukhuudei, UlambadrakhKusky, TimothyWindley, Brian F.Otgonbayar, Orolzodmaa...
33页
查看更多>>摘要：Following the breakup of Rodinia and the opening of the Paleo-Asian ocean the relative positions of geological units (now in Mongolia) remain uncertain, and in consequence relevant paleogeographic reconstructions are poorly construed. In this paper, we apply an integrated approach to the reconstruction of the Paleo-Asian ocean using preserved ophiolites and ocean plate stratigraphy (OPS) in the Mongolian Lake and Altay zones in western Mongolia, which surround the Precambrian Central Mongolian Microcontinent (CMM) in the center of Central Asia. The Lake Zone of Mongolia contains many sediments of Ocean Plate Stratigraphy (OPS) and their underlying basalt-dominated ophiolites, which occur in three main zones along the western margin of the Precambrian crystalline basement of the CMM. The ophiolites in all zones have similar isotopic ages of ca. 570 Ma. The sections of OPS, together with serpentinite melanges and other volcanogenicsedimentary assemblages, have been thrust in nappe sheets over the Zavkhan block of the CMM. In the Neoproterozoic microcontinents derived from Rodinia drifted towards Siberia, and a passive margin developed on the Zavkhan block on the eastern margin of the Paleo-Asian ocean. In mid-upper Neoproterozoic times the eastern side of the Paleo-Asian oceanic plate was thrust over the CMM, and in the early Cambrian the Khasagt and Tas Khairkhan ophiolites were obducted and a mid-ocean ridge (?) reached the CMM, preventing further obduction. In the Neoproterozoic the western margin of the Paleo-Asian ocean was subducted under the Rodinia-derived microcontinent and evolved into an active continental margin. In mid-Cambrian-early Ordovician times thick turbidites were deposited in the Altay. Marine sediments, which had accumulated in the Devonian to Carboniferous in the Paleo-Asian ocean are preserved in a remnant ocean in the Lake Zone. Locally in the Devonian, terrigenous - volcanogenic sequences were deposited in the Ulgey extensional superimposed basin, and later in Meso-Cenozoic time (?) some ophiolitic and OPS fragments (Tsagaan Nuur, Ulaankhus and Bodonch) were exposed along fault escarpments that were possibly re-activated during transpression related to the India-Eurasia collision. Our analysis of the Neoproterozoic-Middle Paleozoic tectonic development of western Mongolia leads to a new holistic model for the evolution of the western Altaids, according to which the ophiolites and OPS formed in the Mongolian Lake and Altay zones between two microcontinents within a new tectonic framework of the western Altaids. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Roots, Eric A.Hill, Graham J.Frieman, Ben M.Wannamaker, Philip E....
12页
查看更多>>摘要：Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. These delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of orebearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.
查看更多>>摘要：Massif-type anorthosites, mainly Proterozoic in age, have long been recognized as a signature of crust mantle interactions. The Daxigou Anorthosite Complex (DAC) is one such massif situated in the Kuluketage block, a tectonically important domain between the Tarim Craton and the Central Asian Orogenic Belt. In this contribution, a combination of geochronological and geochemical analyses of fine-grained DAC diabase are used to constrain the characteristics of the parental magma and the initiation time of DAC magmatism. We then pinpoint the DAC's closure time by in-situ petrographic thin section U-Pb dating of zircon coronas around Fe-Ti oxides identified in a gabbroic anorthosite. Within data uncertainty, we find that the closing age of 1813 +/- 9 Ma, is indistinguishable from the 1804 +/- 7 Ma crystallization age of its last episode of parental magma. MELTS simulations of the parental diabase magma suggest a pressure of 8 kbar at a fayalite-magnetite-quartz oxygen fugacity leads to broadly observed mineral compositions and mineral phases. The bulk rock major and trace elements, integrated with the whole-rock Sr-Nd and Hf isotopes in zircon, indicate that the DAC was formed by partially melting of a metasomatized sub-continental lithospheric mantle, which was likely induced by a postcollisional slab break-off with minor crustal assimilation. Finally, we propose that the DAC magmatism can be ascribed to a series of Paleoproterozoic tectono-thermal events along the northern and southern margins of the Tarim Craton, which could have provided important implications for the Tarim Craton and the North China Craton associated with the assembly of Columbia supercontinent. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
V. Skoblenko, AnfisaDegtyarev, Kirill E.Kanygina, Nadezhda A.Tretyakov, Andrey A....
26页
查看更多>>摘要：Precambrian metamagmatic complexes of the Zhingeldy and Kendyktas blocks of the Zheltau and ChuKendyktas terranes of the SW part of the Central Asian Orogenic Belt (CAOB) consist of predominant Neoproterozoic orthogneisses with the estimated protolith ages of ca. 790-800 Ma, and subordinate epidote and garnet amphibolites, metabasalts and metadolerites of the lower metamorphic grades. The orthogneisses are characterized by negative epsilon Nd(t) values of-12 to-4 with Nd model ages of 2.3-1.7 Ga, indicating their protoliths could have formed by a reworking of the Palaeoproterozoic crustal materials. Precambrian metavolcanic rocks in the studied region are rhyolites with the estimated crystallization ages of ca. 800-830 Ma, and are associated with metabasalts. Metasedimentary complexes in the studied region consist of intensively retrogressed garnet-mica schists interlayered with marmorized limestones, metasilicic rocks, epidote and garnet amphibolites and strongly altered eclogites. The inferred protoliths of the garnet-mica schists are terrigenous rocks accumulated during the latest Ediacaran - middle Cambrian and formed from Mesoproterozoic (ca. 1.0 Ga) and to a lesser extent Palaeoproterozoic felsic source rocks (ca. 1.6-1.7 Ga and ca. 2.5 Ga). The garnet-mica schists of the Kendyktas block have negative epsilon Nd(t) of-13.4 and a model age tNd(DM) of 2.2 Ga, implying formation of their protolith from Palaeoproterozoic crustal source. The studied Zheltau and Chu-Kendyktas terranes from the SW part of the CAOB have much in common in terms of Precambrian and Early Palaeozoic evolution and chemical compositions of the key rock types. The presence of the Early Precambrian rocks (Zheltau terrane), an abundance of the Late Mesoproterozoic (1000-1100 Ma) detrital zircons in the Ediacaran-Cambrian metasedimentary complexes and a wide distribution of the Late Neoproterozoic magmatism, are distinctive features of the studied terranes. We refer such Precambrian terranes of the western CAOB to the Ulutau-Moyunkum group where tectono-magmatic evolution is consistent with that observed from the northern part of the Tarim craton. Our results also aid comprehensive correlation of metamorphic complexes of the Precambrian terranes from the SW CAOB. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Wu, LeiHuang, WentingLiang, HuayingMurphy, J. Brendan...
17页
查看更多>>摘要：The Devonian position of the South China Block (SCB) remains debated because apparently contradictory Devonian paleopoles have been documented. Here we report two characteristic remanent magnetization components from the mildly deformed Guanyang Devonian successions (Lochkovian-Famennian) within the cratonic interior of the SCB. Both components pass reversal tests and occur in different samples across the successions. Component A and associated paleopole DA (228.9 degrees E, 37.7 degrees N, A95 = 3.7 degrees) are comparable with published Devonian data and were likely acquired before Permian-Triassic folding and likely reflect syn-depositional magnetization. Component B and its associated paleopole DB (204.6 degrees E, 4.5 degrees N, A95 = 5.5 degrees) are distinct from published results and were probably acquired during minor tilting associ-ated with the Late Devonian Liujiang orogeny. If paleopole DA was acquired prior to the Late Devonian Liujiang orogeny, the paleolatitude of the Guanyang successions varied little throughout the Devonian, but the azimuthal orientation of recorded declinations exhibits a 40 degrees difference. The distinct positions of the two paleopoles are broadly consistent with existing enigmatic Devonian field records. Alternatively, given the uncertainty in the acquisition ages of both components in the Guanyang succes-sions, a case can be made for component A being a secondary magnetization acquired during the Permian-Triassic Indosinian orogeny. Nonetheless, the similar distribution of Devonian paleopoles from Gondwana to those of cratonic interior of the SCB suggests a Devonian connection between the two con-tinents, and adds further support for a non-uniformitarian geomagnetic field during this enigmatic time interval.(c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：The paradigm for hydrous high-strain zones that cut dry host rocks is for fluid-rock interaction to have involved aqueous fluids. However, the role of silicate melt is increasingly recognised. This contribution examines the formation of glimmerite (biotitite) bands during melt migration in the Gough Dam shear zone, a high-strain zone in central Australia that was active during the Alice Springs Orogeny (c. 450- 300 Ma). The glimmerite bands cut and replace a range of quartzo-feldspathic protoliths, including granitic gneiss and quartzite. Melt that migrated through the high-strain zone is interpreted to have penetrated relict layers along a network of fractures, enhancing dissolution of the precursor rock and causing replacement by glimmerite crystallisation. Microstructures indicative of the former presence of melt in the high-strain zone include: pseudomorphs of former melt pockets of granitic composition; small dihedral angles of interstitial phases; elongate grain boundary melt pseudomorphs; neighbourhoods of grains connected in three dimensions; and localised static grain growth and recovery. Other microstructures indicative of melt-present deformation include randomly oriented neosome grains, and evidence of activation of multiple slip systems during deformation. The degree of quartzite modification to glimmerite is recorded by an increase in biotite mode, and correlated with higher Ti concentrations in biotite (higher apparent temperature) and changes to trace element and REE compositions. Meltassisted coupled dissolution-precipitation reactions during melt flux are interpreted to partially reset Proterozoic monazite U-Pb ages inherited from the protolith (> 1630 Ma) to younger Palaeozoic ages, with a complex age pattern partially congruent with the Alice Springs Orogeny (apparent ages range from c. 606-371 Ma, with a dominant age peak at c. 451 Ma). We propose that the glimmerite formed during dynamic melt migration of an externally-derived hydrous peraluminous melt, driving reaction replacement of various felsic protoliths during this orogenic event.(c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：Combining detrital zircon (DZ) U-Pb age and Lu-Hf isotope data is important in revealing the tectonic evolution of ancient orogens. Here, we conducted DZ U-Pb and Lu-Hf isotope research on 7 modern river sand samples collected from the northern Chinese West Tianshan Orogen (CWTO). The DZs from modern river sands yield ages 340 to 320 Ma and reflect Mesozoic magmatism in the northern Yili Block (YB). In addition, the DZs reflected disproportionate contributions from the source area, which might be affected by the zircon fertility and erodibility of the source rocks. These newly obtained data, together with compiled DZ U-Pb and Lu-Hf isotope data from other modern river sands and ancient sedimentary rocks in the CWTO, are used to constrain the subduction and closure history of the southwestern part of the Paleo-Asian Ocean. The eHf(t) values of 500 to 450 Ma zircons from the southern YB and Central Tianshan Block display a fanning trend (changing from a narrow to a wide range over time), indicating a crustal thickening event at ti 450 Ma. The closure of the North Tianshan Ocean was completed when the YB and an immature/nascent island arc collided in the latest Carboniferous. The South Tianshan Ocean (STO) subducted bidirectionally beneath the Yili-Central Tianshan Block (YCTB) and the Tarim Craton starting at 460 Ma. Slab rollback or retreat of the STO in the northern Tarim Craton starting at 440 Ma caused the opening of several small diachronous back-arc basins. The fanning trend of eHf(t) values indicates that collision between the YCTB and the Tarim Craton occurred at 320 to 310 Ma. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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