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Early Permian post-collisional magmatic events, East Junggar: Constraints from zircon SHRIMP U-Pb age, geochemistry and Hf isotope of rhyolite in the Yundukala area

LI Di1, HE DengFa1**, FAN Chun1, XIANG Kui2 and JIN LuYing3 1. Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China 2. Xinjiang Exploration and Development Center, Shengli Oil field Company, SINOPEC, Dongying 257017, China 3. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China  
The magma source, petrogenesis, tectonic setting and its geochronology of the Middle-Late Paleozoic felsic volcanic rocks, which widely exposed in the Zhaheba area, East Junggar, remain not restriction well so far. A better understanding of these issues above will help us to reveal the magmatic processes and the continental growth of Central Asia. The field investigation showed significant structural characteristic with NE-SW trending compression and the Batamayineishan Formation is represented by typical basic and intermediate-felsic volcanic rocks and pyroclastic rocks. This paper reports zircon SHRIMP U-Pb age, whole-rock geochemistry and zircon Hf isotope analysis results from the Yundukala rhyolite, East Junggar. Three zircon SHRIMP U-Pb ages indicate that the rhyolite in the Yundukala area were formed at 276.0~279.8Ma. The rhyolites are rich in silica (SiO2=60.61%~78.07%), alkali (K2O+Na2O=6.46%~9.28%) and have high Fe2O3T (0.70%~3.30%) contents and Ga/Al ratios, low CaO (0.18%~2.79%) and MgO (0.13%~1.31%) contents, and it is a typical high-medium-K calc-alkaline A-type rhyolite with character of enrichment of LREE and HSFE (Zr, Hf) and depletion of Ba, Sr, Eu, Nb and Ta. They have high values of εHf(t) (10.3~14.6)and young crustal model ages (348~557Ma), suggesting that they were generated by partial melting of juvenile lower crust. Integrating regional geological data of the eastern Junggar and previous research on the nearest ophiolite belt, it is concluded that the Yundukala region was in an extensional setting in the Early Permian and the rhyolites may be the products of felsic magma during its ascend and emplacement processes when upwelling asthenosphere triggered partial melting of juvenile lower crust in an arc-arc or arc-continent post-collisional extensional setting, indicating that the vertical growth play an important role in the continental crust of Central Asia in the Early Permian.
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