Geochronology and geochemistry of the Chuanwulu complex in the South Tianshan, western Xinjiang, NW China: Implications for petrogenesis and Phanerozoic continental growth

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• 作者：He Huang^a ; Zhaochong Zhang^a ; ^{zczhang@cugb.edu.cn} ; Timothy Kusky^b ; Dongyang Zhang^a ; Tong Hou^a ; Junlai Liu^a ; Zhidan Zhao^a
• 关键词：Mafic&ndash ; felsic complex ; Magma mixing ; Recycling of ancient lithosphere ; Permian ; South Tianshan Collisional Belt
• 刊名：Lithos
• 出版年：2012
• 期刊代码：54_00244937
• 类别：ear
• 出版时间：May, 2012
• 卷：140-141
• 期：Complete
• 页码：66-85
• 文件大小：2917 K

摘要

The South Tianshan Collisional Belt (STCB), formed by the collision between the Tarim and Central Tianshan blocks, is pivotal for understanding the complex and prolonged tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Chuanwulu complex is located in the eastern part of the STCB, Xinjiang province, NW China. It consists of a gabbro-diorite unit (unit I) and a monzonite-syenite unit (unit II), which are both cut by some granite dykes (unit III). Based on LA-ICP-MS U-Pb zircon dating, the units I and II were emplaced at 287.8 卤 4.3 Ma and 286.4 卤 2.5 Ma, respectively. The SiO₂ contents of the samples from the complex display a wide range from 50.52 wt.%to 70.64 wt.%, and most samples are of alkaline affinities. The magma mixing process during the formation of the complex is suggested by considerable petrographic and geochemical evidence such as mafic microgranular enclaves (MEEs) that occur within unit II, disequilibrium textures, linear negative correlations between SiO₂ and some major elements, and a wide range of (⁸⁷Sr/⁸⁶Sr)_t ratios and 未¹⁸O_SMOW values. Unit I is characterized by relatively low SiO₂ contents (50.52-55.05 wt.%), high MgO contents (3.29-5.43 wt.%with Mg^# = 0.50-0.56), high Sr contents (1646-3101 ppm), pronounced negative Nb-Ta anomalies and light rare earth element (LREE) enrichment. In combination with their isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_t = 0.70543-0.70751, 蔚_Nd (t) = 鈭?#xA0;2.3 to 鈭?#xA0;1.8, 未¹⁸O_SMOW = 5.7-9.4鈥? 蔚_Hf (t) = 0.2-4.9], these features indicate that they are derived from an incompatible element-enriched lithospheric mantle. The SiO₂ contents of units II and III range from 58.88 wt.%to 65.91 wt.%and 67.10 wt.%to 70.64 wt.%, respectively. They exhibit pronounced positive Zr-Hf anomalies and negative P and Ti anomalies, and relatively elevated Sr and O isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_t = 0.70543-0.70751, 未¹⁸O_SMOW = 8.9-9.1鈥癩. In addition, samples of unit III have high Sr (600 ppm to 1201 ppm) contents and Sr/Yb (373 to 1905) and low Y/Yb (8 to 12) ratios. The felsic rocks (units II and III) have a 鈥淐-type鈥?adakite-like geochemical signature and are comparable with those of rocks derived from an ancient garnet-bearing amphibolite facies lower crust. Our study, complied with other geological evidence, indicate that the collision between the Central Tianshan and Tarim blocks and the final amalgamation of the CAOB should have occurred during Late Carboniferous. On the regional scale, we propose that the recycling of ancient lithosphere is the predominant mechanism which is able to account for Phanerozoic continental evolution in STCB, differing from other tectonic units in CAOB.