期刊,Geological Society of America Bulletin 2022年134卷7/8期_国家学术搜索

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Geological Society of America Bulletin

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国际刊号：0016-7606

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Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Fernando CorfuDeta GasserTor Grenne

25页

查看更多>>摘要：Arc-continent collision,followed by subduction polarity flip,occurs during closure of oceanic basins and contributes to the growth of continental crust.Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt(MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides.Interbedded with deep-marine turbidites,pillow basalts evolve from depleted-MORB(eNdt 9.4)to enriched-MORB(eNdt 4.8)stratigraphically upward,reflecting increasingly deeper melting of asthenospheric mantle.Intercalated intermediate to felsic lava and pyroclastic units,dated at ca.474-469 Ma,are extremely enriched in incompatible trace elements(e.g.,Th)and have low eNdt(-8.0 to-6.6)and high S^(0.7089-0.7175).These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material(isotopic model ages 1.7-1.3 Ga).Detrital zircon spectra reveal a composite source for the interbedded turbidites,including Archean,Paleo-,to Neoproterozoic,and Cambro-Ordovician elements; clasts of H0londa Porphyrite provide a link to the H0londa terrane of Laurentian affinity.The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision,ophiolite obduction and subduction polarity flip.The association of MORBs and ultrapotassic rocks is appar-ently a unique feature along the Caledonian-Appalachian orogen.Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean,but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.

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Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau

Pengsheng DongGuochen DongM.Santosh

21页

查看更多>>摘要：Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history.Here we present an integrated study using penological,mineral-ogical,zircon U-Pb geochronological,whole-rock geochemical,and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau.The different lithological units of the batholith,including granodio-rite,monzogranite,and quartz diorite,with abundant mafic microgranular enclaves in the granodiorite(MME I)and monzogranite(MMEII),show identical crystallization ages of 218-215 Ma.The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions.The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source.The relatively narrow range of whole-rock chemical and Sr-Nd isotopes,as well as the zircon trace element and Hf isotopic compositions of the granodiorite,suggest a homogeneous crustal source for the magma.Our modeling suggests that the rock was produced by 20-50% of lower crustal melting.The Dao-cheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite,which are attributed to assimilation and the fractional crystallization process.This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neopro-terozoic and Paleozoic ages,a non-cotectic trend in composition,and the trend shown by the modeling of initial ~(87)Sr/~(86)Sr ratios and Sr.The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon ￡Hf(t)values and are characterized by enrichment in LI-LEs and depletion of HFSEs.They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids.The MMEs in both rocks display typical igneous texture and higher rare earth element(REE)and incompatible element concentrations than their host granites.The presence of fine-grained margins,acicular apatite,and plagioclase megacrysts suggests a magma mingling process.The overgrowth of amphibole around the pyroxene,quartz ocelli rimmed by biotite,and oscillatory zones of plagioclase are all indicative of chemical diffusion.Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites.Based on a comparison with the coeval subduction-related magma-tism,we propose that subduction and subsequent rollback of the Paleo-Tethys(Garze-Litang Ocean)oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.

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Basin response to the Jurassic geodynamic turnover from flat subduction to normal subduction in South China

Wei ZhangFeng-Qi ZhangYildirim Dilek

20页

查看更多>>摘要：Growing evidence supports that the early Mesozoic development of South China was deeply shaped by flat subduction of the paleo-Pacific plate.However,the Jurassic tectonic setting and processes remain controversial.Here,we present new detrital zircon U-Pb ages and sedimentary data from the Jurassic basin in northwestern Zhejiang to constrain the Jurassic tectonic evolution.The continuous Jurassic succession archives an abrupt sedimentary change from a high-energy coastal environment to a proximal and fast-filling terrestrial environment.This lithostratigraphic change was also accompanied by the shift of detrital provenance.Age spectra of the detrital zircons from the bottom of the Jurassic strata show strong 1.0-0.7 Ga and 500-400 Ma populations,which are inferred to be mainly derived from the Yangtze block.In contrast,samples from the overlying Lower-Middle Jurassic were dominated by age groups of 2.0-1.7 Ga and 300-170 Ma,which were probably sourced from the Cathaysia block.The switch of the sedimentary and provenance characteristics reveals that an earliest Jurassic broad sag basin in the inland shifted to an Early-Middle Jurassic retro-arc foreland basin along the coastal region.Abundant Jurassic-aged zircons are compatible with the re-initiation of"normal subduction"in the Early Jurassic.The re-initiation of"normal subduction"resulted in the generation of an accretion-ary orogeny,continental arcs,and a retroarc foreland basin along the eastern South China margin in contrast to the extensional regime in the inland.The basin response and distinct tectonic regimes of the inland and continental margin in the Early-Middle Jurassic support a geodynamic turnover from flat to normal subduction.

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Identification of ca.520 Ma mid-ocean-ridge-type ophiolite suite in the inner Cathaysia block,South China: Evidence from shearing-type oceanic plagiogranite

Longming LiGuangfu XingShoufa Lin

20页

查看更多>>摘要：An ophiolite suite,predominantly composed of residual mantle peridotites,mid-ocean-ridge basalt(MORB)-like ultramafic rocks,and oceanic plagiogranites,has been identified in the Zhenghe-Dapu fault zone,Cathaysia block,South China.The peridotites experienced strong serpentinization and are characterized by low ~(187)O5/~(188)Os ratios of 0.11621-0.12008 and very low ~(187)Re/~(188)Os values of 0.031-0.129,similar to those from highly refractory mantle residues.The meta-ultramafic rocks,mainly amphibolites,can be classified into two groups.Group I is characterized by high Ni and Cr and low K2O contents.Their spoon-like rare earth element(REE)patterns,along with lower concentrations of highly incompatible elements,indicate that the protolith was of cumulate origin.Group II displays depleted REE patterns and low Nb/Yb,Th/Yb,and Ti/V ratios,which are geochemically similar to normal(N)MORB.Both groups exhibit positive ε_(Nd)(t)values(1.2-4.1)and relatively high(~(87)Sr/~(86)Sr)i ratios(0.7046-0.7096),suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration.The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O(0.09-0.21 wt%)and total REE contents along with low K2O/Na2O and Rb/Sr ratios,which are consistent with those of typical oceanic plagiogranite.They exhibit strongly positive zircon ε_(Hf)(t)values(+9.5 to +15.1)and positive whole-rock ε_(Nd)(t)values(+2.8 to +3.6).Their extremely low MgO(0.6-1.65 wt%),Cr(0.22-6.26 ppm),and Ni(0.77-4.74 ppm)compositions and low Mg#(22.4-31.9)preclude their origination from mantle-derived primary magma but favor oceanic crust.Low zircon δ~(18)O values(4.02‰-5A%c)and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region.The enriched light rare earth element(LREE)patterns with strongly positive Eu anomalies,similar to the East Karm0y-type plagiogranite in western Norway,imply that the plagiogranites were derived from ana-texis of amphibolite in an active shear zone near the mid-ocean ridge.The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry(SIMS)U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma,respectively.The simultaneous mag-matism and metamorphism also signify an active high-temperature shear zone,where the plagiogranites were formed slightly later than the oceanic crust.The age of ca.520 Ma represents the formation age of the oceanic crust rather than its emplacement age.The identification of the ca.520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian,and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca.520 Ma,most likely during the late early Paleozoic.

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The Mesozoic magmatic,metamorphic,and tectonic evolution of the eastern Gangdese magmatic arc,southern Tibet

Ze-Ming ZhangHui-Xia DingXin Dong

20页

查看更多>>摘要：Magmatic arcs are natural laboratories for studying the growth of continental crusts.The Gangdese arc,southern Tibet,is an archetypal continental magmatic arc that formed due to Mesozoic subduction of the Neo-Tethyan oceanic lithosphere; however,its formation and evolution remain controversial.In this contribution,we combine newly reported and previously published geochemical and geochronological data for Mesozoic magmatic rocks in the eastern Gangdese arc to reveal its magmatic and metamorphic histories and review its growth,thickening,and fractionation and mineralization processes.Our results show that:(1)the Gangdese arc consists of multiple Mesozoic arc-type magmatic rocks and records voluminous juvenile crustal growth.(2)The Mesozoic magmatic rocks experienced Late Cretaceous granulite-facies metamorphism and partial melting,ihus producing hydrous and metallogenic element-rich migmatites that form a major component of the lower arc crust and are a potential source for the Miocene ore-hosting porphyries.@The Gangdese arc witnessed crustal thickening and reworking during the Middle to Late Jurassic and Late Cretaceous.(4)Crystallization-fractionation of mantle-derived magmas and partial melting of thickened juvenile lower crust induced intracrustal chemical differentiation during subduction.We suggest that the Gangdese arc underwent the following main tectonic,magmatic,and metamorphic evolution processes: normal subduction and associated mantle-derived magmatism during the Late Triassic to Jurassic; shallow subduction during the Early Cretaceous and an associated magmatic lull; and mid-oceanic ridge subduction,high-temperature metamorphism and an associated magmatic flare-up during the early Late Cretaceous,and flat subduction,high-temperature and high-pressure metamorphism,partial melting,and associated crust-derived magmatism during the late Late Cretaceous.Key issues for further research include the temporal and spatial distributions of Mesozoic magmatic rocks,the evolution of the components and compositions of arc crust over time,and the metallogenic processes that occur in such environments during subduction.

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Short-lived intra-oceanic arc-trench system in the North Qaidam belt(NW China)reveals complex evolution of the Proto-Tethyan Ocean

Changlei FuZhen YanJonathan C.Aitchison

19页

查看更多>>摘要：Recognition of any intra-oceanic arc-trench system(IOAS)could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin.The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean.Four lithotec-tonic units,including island arc,ophiolite,forearc basin,and accretionary complex,are recognized based on detailed field investigation.They rest on the northern margin of the Qaidam block and occur as alloch-thons in fault contact with underlying high-grade metamorphic rocks.The ophiolite unit mainly consists of ultramafic rocks,527-506 Ma gabbro,515-506 Ma plagiogranite,dolerite,and massive lava.High-Cr spinels in serpentinite,dolerite with forearc basalt affinity,and boninitic lava collectively indicate a forearc setting.The accretionary complex,exposed to the south of the ophiolite complex and island arc,is highly disrupted and contains repeated slices of basalt,495-486 Ma tuff,chert,limestone,and melange.luffs with positive zircon e^t)values indicate derivation from a nearby juvenile island arc.These lithotectonic units,as well as the back-arc basin,are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean.Combined with regional geology,we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean.The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.;

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Linking sediment supply variations and tectonic evolution in deep time,source-to-sink systems-The Triassic Greater Barents Sea Basin

Albina GilmullinaTore Grane KlausenAnthony George Dore

21页

查看更多>>摘要：Triassic strata in the Greater Barents Sea Basin are important records of geodynamic activity in the surrounding catchments and sediment transport in the Arctic basins.This study is the first attempt to investigate the evolution of these source areas through time.Our analysis of sediment budgets from subsurface data in the Greater Barents Sea Basin and application of the BQART approach to estimate catchment properties shows that(1)during the Lower Triassic,sediment supply was at its peak in the basin and comparable to that of the biggest modern-day river systems,which are supplied by tectonically active orogens;(2)the Middle Triassic sediment load was significantly lower but still comparable to that of the top 10 largest modern rivers;@during the Upper Triassic,sediment load increased again in the Carnian; and(4)there is a large mismatch(70%)between the modeled and estimated sediment load of the Carnian.These results are consistent with the Triassic Greater Barents Sea Basin succession being deposited under the influence of the largest volcanic event ever at the Perm-ian-Triassic boundary(Siberian Traps)and concurrent with the climatic changes of the Carnian Pluvial Event and the final stages of the Northern Ural orogeny.They also pro-vide a better understanding of geodynamic impacts on sedimentary systems and improve our knowledge of continental-scale sediment transport.Finally,the study demonstrates bypass of sediment from the Ural Mountains and West Siberia into the adjacent Arctic Sverdrup,Chukotka,and Alaska Basins in Late Carnian and Late Norian time.

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Effects of sea-level variation and sedimentary noise variation on the development of biogenic reefs since the Pliocene among the Xisha Islands,South China Sea

Ke ZhaoXuebin DuJixin Jia

12页

查看更多>>摘要：Carbonate strata are a vital and favorable reservoir for global oil and gas exploration,and carbonate sedimentary systems record ancient oceanic and paleoclimatic conditions,including paleoenvironmen-tal variations throughout geologic periods.Carbonate platforms are widely distributed among the Xisha Islands in the South China Sea and contain large amounts of oil and gas resources.Biogenic reefs are the dominant parts of the carbonate platforms in the Xisha Islands; however,research on the factors that control and affect biogenic reef development is lacking.In this study,a core from well XK-1,which is located on Yongxing Island in the Xisha Islands,a sedimentary noise model,and time-series analyses were used to determine the effects of sea-level fluctuations from 5.3 Ma to present.The results show that coral reefs in the Xisha Islands are sensitive to eu-3tatic fluctuations and that a decrease in sea level essentially corresponds to an increase in sedimentation rate.Indexes of the East Asian monsoon and other environmental indexes show that the Pleistocene and Holocene were suitable for coral growth; however,the trends shown by these indexes and the sea-level variation indicate that the future growth of coral reefs will be at a disadvantage.Research on he controlling factors of biogenic reefs is of significance for understanding reef growth,performing global reef comparisons,and encouraging the future protection of coral reefs.

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Constraining the timing of Arabia-Eurasia collision in the Zagros orogen by sandstone provenance(Neyriz,Iran)

Farisa GholamiZadehXiumian HuEduardo Garzanti

18页

查看更多>>摘要：The Zagros orogen,formed by the collision of the Arabian and Eurasian continental margins,represents one of the largest and richest oil and gas provinces in the world.The Zagros fold-thrust belt records collision and convergence along the Neotethys suture z one.By coupling field observations,sandstone modal analysis,U-Pb zircon dating,and Hf isotopic data from the Upper Cretaceous to Pliocene sedimentary succession of the Neyriz region,this paper documents several major provenance changes that allow us to propose a refined scenario for the Zagros orogeny.An ophiolitic complex ciated by detrital-zircon U-Pb geochronol-ogy as ca.95 Ma provided detritus to Upper Cretaceous-Paleocene strata deposited along the northeastern margin of the Arabian lower plate(ophiolite provenance).Yet,on the southwestern margin of the Eurasian upper plate,upper Paleocene-lower Eocene strata indicate provenance from Mesozoic magmatic rocks yielding zircons dated as ca.140 Ma and 170 Ma as well as the recycling of clastic rocks.Since the early Miocene,the sedimentary basin located on the Arabian plate received both ophiolitic detritus and magmatic-arc,recycled clastic,and axial-belt metamorphic detritus from Eurasia.U-Pb ages of detrital zircons reflect polyphase mag-r latism at 170 Ma,95 Ma,and 40 Ma on the Eurasian active margin.Our results indicate that progressive accretion,uplift,and exhumation of the Zagros orogen was well under way by the beginning of the Miocene in the Neyriz region.Literature data from adjacent regions suggest that the Arabia/Eurasia collision may have occurred diachronously and later in the Kermanshah and Lurestan areas to the north.

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Age,material source,and formation mechanism of bedding-parallel calcite beef veins: Case from the mature Eocene lacustrine shales in the Biyang Sag,Nanxiang Basin,China

Ao SuPaul D.BonsHonghan Chen

23页

查看更多>>摘要：The mechanisms leading to the formation of bedding-parallel calcite beef veins have been widely debated,with discussions centered on timing or burial depth of vein generation,source of the vein material,driving forces for vein initiation and widening,and growth direction and mechanism.To address these issues,a comprehensive study of drill core samples containing beef veins in the mature Eocene lacustrine Hetaoyuan Formation in the Biyang Sag,Nanxiang Basin,China was undertaken through a combination of microstructural observation,isotopic geochronological,geochemical,and fluid inclusion analyses,as well as basin modeling.X-ray diffraction and total organic carbon content analyses indicate that most of the beef veins accumulated in calcite-rich laminated shales with high organic matter contents.These beef veins yielded an absolute laser ablation-multi-collector-inductively coupled plasma-mass spectrometry U-Pb age of ca.41.02 ± 0.44 Ma,which corresponds to a burial depth of 500-800 m.Such a shallow burial depth suggests that the full compaction and consolidation of sediments would not yet have been achieved,which is compatible with the following observations:(1)plastic deformation of shale laminae adjacent to the veins,and(2)a beef vein formation temperature of ~59 ℃ derived from fluid-inclusion microthermometry.The radio-isotopic age of the beef veins is ~1-3 m.y.younger than the stratigraphic age of the host rock(ca.43.1 Ma)but earlier than the model-derived timing of oil generation(ca.35.8 Ma)and tectonic extrusion(ca.23.0-13.0 Ma).The beef vein formation predated bacterial sulfate reduction,as evidenced by crosscutting relationships with carbonate concretions,pyrite framboids,and apatite pellets.A two-stage formation model for these beef veins is proposed.When burial depth of laminated shales rich in organic matter and calcite reaches the methanogenic zone,overpressure triggered by biogenetic gas generation results in horizontal hydro-fracturing,initiating cracks that act as gas expulsion pathways.Once all the generated gas has migrated,the opened fractures close again due to overburden load.The materials fed by pressure solution of host-rock calcite fractions then mobilized into the unhealed horizontal fractures by diffusion.Subsequently,by a force of crystallization,antitaxial,displacive growth of calcite fibers commenced,contemporaneous with fracture dilation,eventually leading to the formation of bedding-parallel beef veins.

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