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Chao Yuan, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (China)
Min Sun, The University of Hong Kong (China)
Wenjiao Xiao, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
Simon Wilde, Curtin University of Technology (Australia)
Xianhua Li, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (China)
Xiaoping Long, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (China)
Guochun Zhao, The University of Hong Kong (China)
Dunyi Liu, Institut of Geology, Chinese Academy of Geological Sciences (China)
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Late Carboniferous to Early Permian is a crucial period for the Central Asia Orogenic Belt (CAOB), not only for being one of the major mineralization episodes, but also for the extensive underplating of mantle-derived magmas around the final closure of the Paleoasian ocean. A systematic study has been conducted for intrusions of Balikun, East Tianshan, aiming at a comprehensive understanding the final orogenic stage of the CAOB. SHRIMP zircon U-Pb dating has yielded 287±2 Ma, 288±3 Ma, 284±5 Ma and 305±4 Ma, for three granitic intrusions (DJSP, DLGP and BTB) and one gabbro pluton (SQZ), respectively. The gabbroic rocks in the area are slightly alkaline, possess high Nb content ( 9 ppm), high εNdT(+7.17 ∼ +8.74) and low initial 87Sr/86Sr ratios (0.7030 ∼ 0.7041), with negative Nb-Ta and positive Pb anomalies. These rocks consist of low-Si (LS) and high-Si (HS) members, and the HS gabbros possess HFSE contents, Nb/Y and Ta/Yb ratios higher than those of LS samples and exhibit more EMORB-like characteristics, while the LS samples have higher Ba/Th ratios than the HS samples and display stronger subduction-related signatures. Similar Mg# values (27 ∼ 30) of the HS and LS samples preclude variable degree of partial melting. Instead, a mantle upwelling regime, by which decompressional melting of the asthenosphere mantle and partial melting of the depleted mantle metasomatised by slab-derived fluid, is invoked to interpret the generation of the HS and LS gabbros. Silicic intrusions in Balikun are dominated by hornblende- and/or biotite-bearing granites, of which, the DJSP granite is characterized by Na-rich hornblende and high SiO2 (74 wt%), low Al2O3 and CaO, with high Ga/Al ratios and strong negative Eu anomalies, showing typical characteristics of A-type granites. Other granitic intrusions, though showing relatively low SiO2 (68 ∼ 72 wt%) and less evolved characteristics, also exhibit an affinity to A-type granites. Lack of coeval mafic to intermediate intrusions in the area and depleted-mantle-like Nd (εNdT = +2.49 ∼ +5.84) and variable Sr isotope compositions (0.6938 ∼ 0.7153) suggest these granites were derived from partial melting of cryptic intermediate rocks with variable assimilation of arkosic rocks. Temperature estimate using zircon saturation thermometer yielded 867 ∼ 956°C and 841 ∼ 922°C for the DJSP and other two granitic intrusions, respectively. Our data indicate that from 305 Ma to 285 Ma, magmatism in Balikun evolved from small gabbroic stock without coeval granitoids to large scale granites with A-type characteristics, suggesting a gradually increasing influence of mantle-derived magmas. Considering the relatively low temperature of granites and rock associations, neither mantle plume nor ridge-subduction models are feasible for the tectonic evolution of the East Tianshan during this period. Slab break off, assisted with large scale rotation of the major blocks, may explain the tectonic transition of the CAOB.
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