Late Carboniferous high 蔚_Nd(t)-蔚_Hf(t) granitoids, enclaves and dikes in western Junggar, NW China: Ridge-subduction-related magmatism and crustal growth

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• 作者：Gong-Jian Tang^a ; ^b ; Qiang Wang^a ; ^{wqiang@gig.ac.cn} ; Derek A. Wyman^c ; Zheng-Xiang Li^d ; Zhen-Hua Zhao^a ; Yue-Heng Yang^e
• 关键词：Granitoid ; Enclave ; Dike ; Crustal growth ; Ridge subduction ; Central Asian Orogenic 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：86-102
• 文件大小：3110 K

摘要

We report results of petrologic, geochronological and geochemical investigation of the Late Carboniferous diorites, granodiorites, amphibole (Am)-bearing granites, and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the Keramay area, of the western Junggar region of Central Asian Orogenic Belt (CAOB). Zircon U-Pb dating suggests that they were generated in the Late Carboniferous (316-304 Ma). The diorite and granodiorite compositions extend over a wide range of SiO₂ (53-70 wt.%), Sr (240-602 ppm), and Mg^# (41-58) values, and are characterized by moderately fractionated rare earth element (REE) patterns, Nb-Ta depletion and relatively low Y and Yb contents. The mafic dikes consist of dolerites, diorite porphyries and minor granodiorite porphyries, and have variable SiO₂ (51-59 wt.%) and high Mg^#, Cr and Ni values. With the exception of two samples with relatively high heavy REE (HREE) contents, the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites. The Am-bearing granites and a granite porphyry dike sample have high levels of SiO₂ (73-77 wt.%), HREEs (e.g., Yb = 3.46-15.7 ppm) and low Mg^#, Cr and Ni contents, along with clearly negative Eu, Ba and Sr anomalies, similar to typical A-type granites. All granitoids, enclaves and dikes in this region have high positive 蔚_Nd(t) (+ 7.13 to + 9.74) and zircon 蔚_Hf(t) (+ 10 to + 16) values and moderate initial ⁸⁷Sr/⁸⁷Sr ratios (0.7004-0.7049). Mineral composition data suggest that the parental magmas for mafic dikes are similar to Cenozoic sanukitoids in the Setouchi arc area (Japan) and were possibly generated under water-rich and high oxygen fugacity (NNO + 1.5 to NNO + 2.7) conditions. They most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust-derived melts and minor fluids and subsequently underwent fractional crystallization. The diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust-derived magmas coupled with minor crystal fractionation. The Am-bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels. Taking into account widespread contemporaneous magmatism including 鈥淢ORB-type鈥?basalts and slab-derived adakites in western Junggar, we suggest that the Keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window, which played an important role in the crustal growth of the CAOB.