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

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

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

Compression to extension switch during the Middle Triassic orogeny of Eastern China: the case study of the Jiulingshan massif in the southern foreland of the Dabieshan

The Jiulingshan massif is an E–W trending anticlinorium in the South China Block (SCB) that forms the southern foreland of the Qinling–Dabie orogen. The Jiulingshan consists of Middle Proterozoic (ca. 1 Ga–800 Ma) metamorphic and plutonic rocks unconformably overlain by Sinian to Triassic rocks. In these cover rocks, two episodes of deformation of pre-Late Triassic age are recognized. Namely, a N–S compressional phase characterized by south verging thrusts, upright folds and vertical cleavage followed by an extensional phase characterized by layer parallel slip, collapse folds and extensional allochthons of Proterozoic slate on top of Carboniferous to Middle Triassic carbonates. The extensional tectonics appears to be a mechanical consequence of the compressional tectonics. On a regional scale, the Jiulingshan massif forms one of the domed crustal scale shear zones of the SCB that accommodated the continuing convergence that immediately followed the Qinling–Dabie collision

Growth of the Tian Shan drives migration of the conglomerate-sandstone transition in the southern Junggar foreland basin

International audienceIn an orogenic belt-foreland basin setting, sediments from the mountain are transported downstream and accumulate in foreland basins. Sediments routing through the network of rivers display downstream grain size fining due to sorting and abrasion (Paola et al., 1992). A grain size transition from gravel to sand, termed the gravel-sand transition (GST; Ferguson et al., 1996), occurs in a short downstream distance from the sediment source. The GST is preserved in the stratigraphy of a sedimentary basin as the conglomerate-sandstone transition (CST; e.g., Dubille & Lavé, 2015). The position of the CST in a foreland basin succession is determined by basin subsidence, sediment supply, and grain size (Allen et al., 2013; Armitage et al., 2011), and all these factors depend on the interactions of tectonics in the adjacent mountains and regional climate (Dingle et al., 2016, 2017; Duller et al., 2010; Quick et al., 2020). The propagation of the orogenic wedge toward to the foreland drives the forelandward migration of the coupled foreland basin system as well as sedimentary facies (Flemings & Jordan, 198