Timing of initiation of extension in the Tianshan, based on structural, geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China)

International audienceThis paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan. The main lithotectonic units consist of olistostrome, volcanic rocks and turbidite. The olistostrome is tectonically underlain by Upper Carboniferous limestone and sandstone along a NEE-trending detachment fault. Paleo-growth fault is locally observed. The olistostrome unit includes plenty of blocks of limestone, sandstone, rhyolite and volcaniclastic rocks, and a matrix of graywacke. Limestone blocks are dated as Pennsylvanian-Bashkirian in age by the coral and brachiopod fossils that are extensively recognized in the Upper Carboniferous strata. The volcanic unit consists of pillowed and massive basalt and rhyolite, the latter occur as an 8- to 10-meter-thick layer above the olistostrome unit. The turbidite unit is mainly composed of chert, siliceous mudstone and sandstone, within which the Bouma sequence can be locally recognized. Meter-wide gabbro and diabase dykes intrude these three units. Geochemically, rhyolites are characterized by high ACNK value of >1.1, depletion of Ba, Nb and Sm, and enrichment in Rb, Th and Zr. Basaltic rocks are rich in K2O, they show a LREE-enriched pattern and depletion in Ba, Nb and Zr, and enrichment in Ti, Ce and Hf, similar to continental rift-type tholeiite series. A gabbro porphyrite intruding the olistostrome was dated at 288 ± 3 Ma by a sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb method, and a rhyolite at 297 ± 2 Ma by a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) zircon U-Pb method. The Baiyanggou olistostrome and accompanying bimodal volcanic series are linked to an extensional setting that developed in the south of the Bogda Shan. Several lines of evidence, e.g. occurrence of large-scale strike-slip shear zones, large number of mantle-derived magmatic rocks and available geochronological data, demonstrate a significant geodynamic change from convergence to extension in the Chinese Tianshan belt, even in the whole Central Asian Orogenic Belt. The extension in the Chinese Tianshan belt is initiated at ca. 300 Ma, i.e. around Carboniferous-Permian boundary times, and the peak period of intra-plate magmatism occurred in the interval of 300-250 Ma

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

We acknowledge the financial support by the Major State Research Development Program of China (Grant No. 2016YFC0600202), National Natural Science Foundation of China (Nos. 41330208 and 41572200) and State Key Laboratory for Mineral Deposits Research (Nanjing University) (ZZKT-201603).The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860 ± 6 Ma and 435 ± 5 Ma, along with variably disconcordant ages with lower intercept ages of 442 ± 41 Ma. The zircon core ages range from 3015 Ma to 851 Ma, with three major age populations at 930–865 Ma, 1850–1200 Ma and 2650–2400 Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736 Ma, 1835–1775 Ma and 2720–2500 Ma. Metamorphic ages of ca. 860 Ma and ca. 435 Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860 Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736 Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction -collision process within the Cathaysia Block at around 860 Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440 Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440 Ma.PostprintPeer reviewe

Paleozoic structural and geodynamic evolution of eastern Tianshan (NW China): welding of the Tarim and Junggar plates

to cite the paper EPISODES Volume: 30 Issue: 3 Pages: 162-186 Published: September, 2007International audienceChinese East Tianshan is a key area for understanding the Paleozoic accretion of the southern Central Asian Orogenic Belt. A first accretion-collision stage, before the Visean, developed the Eo-Tianshan range, which exhibits north-verging structures. The geodynamic evolution included: i) Ordovician-Early Devonian southward subduction of a Central Tianshan ocean beneath a Central Tianshan arc; ii) Devonian oceanic closure and collision between Central Tianshan arc and Yili-North Tianshan block, along the Central Tianshan Suture Zone; iii) Late Devonian-earliest Carboniferous closure of a South Tianshan back-arc basin, and subsequent Central Tianshan-Tarim active margin collision along the South Tianshan Suture Zone. A second stage involved: i) Late Devonian-Carboniferous southward subduction of North Tianshan ocean beneath the Eo-Tianshan active margin (Yili-North Tianshan arc); ii) Late Carboniferous-Early Permian North Tianshan-Junggar collision. The Harlike range, unit of Mongolian Fold Belt, collided with Junggar at Mid- Carboniferous, ending a north-dipping subduction. The last CAOB oceanic suture is likely the North Tianshan Suture Zone, between Yili-North Tianshan and Junggar. During the Permian, all the already welded units suffered from a major wrenching, dextral in Tianshan, sinistral in Mongolian Fold Belt, due to opposite motion of Siberia and Tarim

Mesozoic magmatism and granitic dome in the Wugongshan Massif, Jiangxi province and their genetical relationship to the tectonic events in southeast China

In SE China, a Mesozoic granitic dome coeval to extensional tectonics is exposed in the Wugongshan massif. Its center is occupied by Mesozoic granitoids and granitic gneisses, and the E–W-trending Late-Paleozoic–Mesozoic Pingxiang and Anfu basins are located along its northern and southern sides, respectively. In this paper, the petrological, geochemical (major, incompatible, and rare earth elements) and isotopic characteristics of the granitoids and granitic gneisses of this dome allow us to constrain their geodynamic evolution. The Wugongshan Mesozoic K-feldspar-enriched granitoids consist of six granitic bodies and lie within a Paleozoic metamorphic basement. The Caledonian Shanzhuang granodiorite, containing some dioritic enclaves, occurs in the study area and is a calc-alkaline type granodiorite characterised by lower SiO2 and composed of andesine, biotite, quartz, hornblende, magnetite and sphene. The Mesozoic granitoids, either Indosinian plutons or Yanshanian bodies, show a geochemical affinity of peraluminous S-type granite, having higher SiO2, K2O contents, Al>K+Na+2Ca, enriched in Rb, Th and depleted in Eu. They also exhibit LREE-enriched patterns and marked negative Eu anomalies. The Yanshanian plutons also contain characteristic minerals such as sillimanite, garnet and monazite. The Wugongshan Mesozoic granitoids have higher ISr values (0.70981–0.72885) and lower var epsilonNd(T) (−10.6 to −14.7; an average of −12.6) than those of the early Paleozoic Shanzhuang granodiorite. The Mesozoic granitoids were likely derived from ancient sedimentary rocks by partial melting. The Wugongshan Mesozoic granitoids contain three petrological zones, that is, an Early Cretaceous massive granitic zone in the core (Zone I), a foliated granitic zone (Zone II) and a Triassic granitic gneissic zone in the outer-rim (Zone III). The petrological and geochemical features of massive granitoids (Zone I) are similar to those of foliated granites (Zone II) and of granitic gneisses (Zone III) in this Mesozoic dome. Moreover, geometric and kinematic features within the ductile parts (Units 2 and 3) are also similar to those within the brittle part (Unit 1) of the dome. All these features show that the geologic evolution of the Wugongshan doming is a continuous process lasting from Triassic to Early Cretaceous time. Published 40Ar/39Ar and K–Ar ages in the Wugongshan Mesozoic granitic rocks fall into two major age fields, corresponding to two geodynamic events of intracontinental deformation and magmatic activity. The first tectonothermal event, dated at 226–259 Ma on the granitic gneisses in the outer-rim of dome, can be linked to Indosinian collision between the South China plate and the North China plate during Triassic time. The second event took place during the Early Yanshanian, dated at 180–120 Ma on the granites of the core of dome, and are responsible for final doming of the Wugongshan plutons. This event may be related to a westward subduction of the Paleo-Pacific oceanic plate under the SE-China continent during Late Jurassic–Early Cretaceous time

Jiangnan Orogen, South China : a ~970–820 Ma Rodinia margin accretionary belt

Authors thank the Major State Research Development Program of China (Grant No. 2016YFC0600202) for financial support, as well as financial support from National Natural Science Foundation of China (Nos. 41330208, 41572200 and 41190070) and State Key Laboratory for Mineral Deposits Research (Nanjing University) (ZZKT–201603). Peter A. Cawood acknowledges support from Australian Research Council grant FL160100168.The Neoproterozoic Jiangnan Orogen in South China records a succession of arc-trench-basin assemblages culminating in accretion of the bounding Yangtze and Cathaysia blocks to form the stabilized South China Craton. The orogen can be traced over some 1500 km and extends up to 100 km across strike. It is divisible into three domains: the northeast domain (also referred to as the Huaiyu or Shuangxiwu Terrane), the central domain (Jiuling Terrane), and an undifferentiated southwest domain. The northeast domain contains arc type volcanic suites and I-type granitoids dated at ca. 970–850 Ma. It is interpreted as an intra–oceanic terrane based on the juvenile radiogenic isotopic signature of the igneous rocks, the absence of older detritus and inherited xenocrysts, and the presence of ophiolites along its southwestern and western margins. The central and southwest domains contain trench-arc-basin assemblages of clastic sedimentary units, mappable magmatic arc suites and ophiolitic mélanges (Sibao and equivalent groups) that range in age from ca. 880 to 820–815 Ma. The presence of old zircon grains within these two domains, both as detritus within sedimentary units and as inherited zircon in arc basalt, suggest they formed at convergent continental margins. S-type granites dated at 845–815 Ma are a distinctive element of the central and southwest domains. The ages of these granites overlap with convergent plate magmatism in the two domains, arguing against previous models for plume-rift and post-collisional geodynamic settings. Instead, these bodies likely formed in an accretionary orogenic margin setting in which granitic magmatism occurred in an extensional regime triggered by slab rollback. The slab-rollback process triggered mantle-sourced thermal input and partial melting of the older and buried arc-bounding basin sediments. Early Paleozoic S-type granites in the Lachlan and New England belts in eastern Australia and Jurassic ones in the Cordillera belt of the western US provide analogous geodynamic environments. Isotopic data indicate that the central Jiangnan domain experienced significant crustal growth, whereas in the southwest domain there was a greater degree of crustal reworking. The character and distribution of the early Neoproterozoic sedimentary and igneous succession in the orogen suggests it represents a ca. 970–820 Ma accretionary orogen. Upper age limits on the Jiangnan Orogen are provided by a regional angular unconformity in the central and southwest domains at ca. 810–805 Ma, and in the northeast domain at ca. 825 Ma, along with the overlying bimodal volcanic and clastic sedimentary successions mostly dated at ca. 810–730 Ma. Thus, timing of final assembly of South China displays variations across the Jiangnan Orogen, from ca. 825 Ma in the northeast to ca. 820–805 Ma in the central and southwest of the orogen. Post-assembly successions are parts of the Nanhua Basin and are interpreted to have formed during regional lithospheric extension across the eastern and central South China Craton. The age patterns across the South China Craton are indicative of northwest directed accretion of fragments and suggest an external rather than an internal position of the craton within the assembled Rodinia supercontinent. Paleomagnetic data, regional correlations and sedimentary records are consistent with a position along the northern margin of Rodinia, adjacent to India and Australia. The Jiangnan Orogen recorded the accretion of trench-arc assemblages and ultimately the Yangtze Block to the Cathaysia Block that was already located on the margin of Rodinia. The Panxi-Hanan belt, which lies along the western and northwestern margin of the Yangtze Block, formed on the upper plate to a subduction system that both overlaps with, and is younger than, the Jiangnan Orogen. The belt provides a record of ongoing accretion on the Rodinia margin until the mid-Neoproterozoic.PostprintPeer reviewe

Delineating and characterizing the boundary of the Cathaysia Block and the Jiangnan orogenic belt in South China

The authors acknowledge the financial support provided by the National Basic Research Program of China (973 Program, No. 2012CB416701), the National Natural Science Foundation of China (Nos. 41330208, 41572200, 41272226) and the Bureau of China Geological Survey (No. 1212011121064-01).The Jiangshan-Shaoxing fault zone lies along the SE margin of the Jiangnan belt, and delineates the northeastern margin of the Cathaysia Block of the South China Craton. At Shijiao in NE Zhejiang, the fault zone consists of hornblende schist intruded by migmatized quartz diorite. It constitutes a shear zone delineating the welded boundary between the Neoproterozoic Shuangxiwu Group of Jiangnan belt to the north and the Chencai Complex of Cathaysia Block to the south. The Shuangxiwu Group is composed mainly of basalt, andesite and flysch, whereas the Chencai Complex contains magmatic and sedimentary rocks that experienced amphibolite facies metamorphism. Zircons from quartz diorite and gabbro from the fault zone at Shijiao yield ages of 854 ± 6 Ma, 857 ± 5 Ma and 860 ± 5 Ma, with positive ɛHf(t) values of 7.81∼11.8 and 4.57∼10.39. The quartz diorite and mafic-ultramafic rock samples display minor LREE enriched pattern with obvious depletion of Nb, Ta, Rb, Ba and Ti, compared to their neighboring elements and plot in the volcanic arc field on geochemical diagrams, similar to that of volcanic rocks from Shuangxiwu Group. Overall relationships within the Jiangshan-Shaoxing fault zone at Shijiao suggest the ca. 860-850 Ma rock suites were generated in a convergent plate margin and are part of the Jiangnan belt, and not the Cathaysia Block, thus constraining the location of the suture between the two lithotectonic units in NE Zhejiang area during Neoproterozoic.PostprintPeer reviewe

Geochronological and geochemical features of the Cathaysia block (South China): new evidence for the Neoproterozoic breakup of Rodinia

International audienceThe Cathaysia block is an important element for the reconstruction of the Proterozoic tectonic evolution of South China within the Rodinia supercontinent. The Pre-Devonian Cathaysia comprises two litho-tectonic units: a low-grade metamorphic unit and a basement unit; the former was a late Neoproterozoic-Ordovician sandy and muddy sedimentary sequence, the latter consists essentially of metamorphosed Neoproterozoic marine facies sedimentary and basaltic rocks, and a subordinate amount of Paleoproterozoic granites and amphibolites. This block has undergone several tectono-magmatic events. The first event occurred in the late Paleoproterozoic, at ca. 1.9-1.8 Ga, and the tectonic-magmatic event dated at 0.45-0.40 Ga was resulted from the early Paleozoic orogeny that made the Pre-Devonian rocks to undergo a regional lower greenschist to amphibolite facies metamorphism. The Neoproterozoic geodynamic event is poorly understood. In this paper, new U-Pb zircon age, whole-rock chemical and zircon Hf isotopic data for mafic and felsic igneous rocks are used to constrain the tectonic evolution of Cathaysia. Zircon SHRIMP U-Pb analyses on four mafic samples yielded rather similar Neoprotorozoic ages of 836 ± 7 Ma (gabbro), 841 ± 12 Ma (gabbro), 847 ± 8 Ma (gabbro) and 857 ± 7 Ma (basalt). Combined with the published isotopic age data, most of the mafic samples dated at 800-860Ma show geochemical characteristics of continental rift basalt. By contrast, rhyolitic samples with an age of 970 Ma have a volcanic arc affinity. All mafic samples have LREE-enriched REE patterns, and non-ophiolitic trace element characteristics. However, the zircon Hf isotopic data of mafic samples show positive epsilon var epsilonHf(t) values (+4.1 to +10.5), suggesting that they were originated from a long-term depleted mantle source. All the available ages indicate that the Cathaysia block has registered two stages of Neoproterozoic magmatism. The younger stage corresponds to a continental rifting phase with emplacement of mafic rocks during the period of 860-800 Ma, whereas the older stage represents an eruption of volcanic arc rocks at about 970 Ma. These two magmatic stages correspond to two distinct tectonic settings within the framework of the geodynamic evolution of Cathaysia. Such a similar Neoproterozoic stratigraphy and magmatism between the Cathaysia, Yangtze and Australian blocks provide a significant line of evidence for placing the Cathaysia block within the Rodinia supercontinent

Late Paleozoic tectonic and magmatic evolution of the Chinese West Tianshan

In Xinjiang Province of NW China, the Tianshan Belt belongs to the Late Paleozoic Variscan orogens that shaped up the Eurasian continent. Most of geologists drew to a conclusion that the Tianshan Belt was built during Paleozoic times through oceanic subduction, accretion and collision between the main Precambrian cratons such as Tarim, Junggar and Kazakhstan and some intervening microcontinents such as the Yili Block. However, there are still some controversies on the geodynamics of Paleozoic Tianshan, especially the West Tianshan (WTS), which was less studied than the East Tianshan. In particular, the tectonic significance in terms of heat transfer and crustal rheology of the huge magmatic rocks that develop in WTS is rarely taken into account in the lithosphere-scale evolution models

Geochemical constraints on carboniferous volcanic rocks of Yili Block (Xinjiang, NW China) ; implication for the tectonic evolution of western Tianshan.

The Yili Block is important for understanding the Late Paleozoic geodynamic evolution of Central Asia. It is bounded to the north by the Northern Tianshan Carboniferous flysch and ophiolitic mélange. The center of the Block is dominated by Carboniferous sedimentary rocks with intercalation of volcanic rocks. Petrological and geochemical features of these Carboniferous volcanic rocks show that: (1) they belong to the calc-alkaline series, (2) they display prominent Nb–Ta negative anomalies consistent with subduction-related magmas, and (3) HFSE-based discriminations place these volcanic rocks in the field of continental arcs. The depositional evolution of the sedimentary series shows evidence for Carboniferous sedimentation in a basin instead of rifting as previously proposed. All these evidences, together with the occurrence of contemporaneous turbidites and ophiolitic mélange along the northern boundary of the Yili Block, allow us to infer that the northern border of the Yili Block was a continental active margin during the Carboniferous. The Late Carboniferous southward subduction that finally closed the Late Devonian to Early Carboniferous North Tianshan oceanic basin was followed by Permian–Mesozoic polyphase transcurrent faulting

Tectonic evolution of the northern part of Western Tianshan (Xinjiang, NW China).

The Yili Block is important for understanding the Late Paleozoic geodynamic evolution of Central Asia. It is bounded to the north by the Northern Tianshan Carboniferous flysch and ophiolitic mélange. The center of the Block is dominated by Carboniferous sedimentary rocks with intercalation of volcanic rocks. Petrological and geochemical features of these Carboniferous volcanic rocks show that: (1) they belong to the calc-alkaline series, (2) they display prominent Nb-Ta negative anomalies consistent with subduction-related magmas, and (3) HFSE-based discriminations place these volcanic rocks in the field of continental arcs. The depositional evolution of the sedimentary series shows evidence for Carboniferous sedimentation in a basin instead of rifting as previously proposed. All these evidences, together with the occurrence of contemporaneous turbidites and ophiolitic mélange along the northern boundary of the Yili Block, allow us to infer that the northern border of the Yili Block was a continental active margin during the Carboniferous. The Late Carboniferous southward subduction that finally closed the Late Devonian to Early Carboniferous North Tianshan oceanic basin was followed by Permian-Mesozoic polyphase transcurrent faulting