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    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) [查看] LiangshuShu、BoWang、WenbinZhu、ZhaojieGuo、JacquesCharvet、YuanZhang
    This 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.
    Geochemistry and SHRIMP U-Pb zircon geochronology of the Korla mafic dykes Constrains on the Neoproterozoic continental breakup in the Tarim Block, northwest China [查看] WenbinZhu、BihaiZheng、LiangshuShu、DongshengMa、JinglinWan、DewenZheng、ZhiyongZhang、XiaoqingZhu
    The Kuruktag uplift is located to the north of the Tarim Block, NW China. At the western end of the Kuruktag uplift, mafic dykes crop out in the Korla area, which were rarely subjected to deformation and metamorphism in contrast to their metamorphic wall-rocks that were strongly deformed and metamorphosed to amphibolite facies. A SHRIMP U–Pb zircon age of 634 Ma was obtained for a spessartite dyke, documenting the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic.Most samples of the Korla mafic dykes show clear enrichments in Th, La, and variable depletions in Nb, Ta and Ti, except for samples 08T-14 and 08T-19 that display incompatible element distribution patterns similar to those of OIB without obvious depletions in Nb, Ta and Ti. The geochemical signatures suggest that the Korla mafic dykes were formed in an intra-plate setting and their primary magama was possibly produced by partial melting of a sub-continental lithospheric mantle that has been metasomatized by previous subduction processes, and then heated by a rising mantle plume. In combination with previous geochronological data of Neoproterozoic igneous rocks throughout the Tarim Block, at least three pulses of magmatic activity, from ca. 830 to 800 Ma, from ca. 790 to 740 Ma and from ca.650 to 615 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a longlasting process.
    SHRIMP U-Pb zircon geochronology of Neoproterozoic Korla mafic dykes in the northern Tarim Block,NW China implications for the long-lasting breakup process of Rodinia [查看] WENBINZHU、ZHIYONGZHANG、LIANGSHUSHU、HUAFULU、JINBAOSU、WEIYANG
    Mafic dykes are observed in the Korla region along the northern Tarim Block, NW China.Our sensitive high-resolution ion microprobe U–Pb zircon ages, the first determined for these dykes, indicate that the mafic dykes were mainly formed at 650–630 Ma, and thus document the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic. Combined with previous geochronological data, at least three pulses of magmatic activity, from c. 830 to 800 Ma, from c. 790 to 740 Ma and from c. 650 to 630 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a long-lasting process.
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