Geochemistry and petrogenesis of Jurassic granites in Nyemo area, South Lhasa terrane, Tibet

Abstract

冈底斯岩基是新特提斯洋北向俯冲和印度-欧亚大陆碰撞的重要岩浆记录，尼木地区位于冈底斯岩基中部。本文报道了尼木地区寄主二长花岗岩和辉长质包体的锆石U-Pb年代学、元素地球化学和锆石Hf同位素。锆石U-Pb定年结果表明，寄主花岗岩（197~190Ma）和包体（195Ma）同期侵位，形成于早侏罗纪。寄主二长花岗岩为弱过铝质高钾钙碱性系列岩石，富集轻稀土和K、U、Sr等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素，锆石ε_Hf（t）值为+13.9~+16.0，其可能来源于初生地壳的部分熔融；辉长质包体为准铝质钙碱性系列岩石，具有与寄主岩相似的稀土和微量元素分配特征，锆石ε_Hf（t）值为+13.8~+16.0，其可能代表了侏罗纪形成的新生下地壳，即寄主岩石的岩浆源区；结合前人研究，本文认为尼木地区的寄主花岗岩和暗色包体可能与新特提斯洋北向俯冲引起的地幔和初生地壳的部分熔融有关。;The Gangdese batholith is an important magmatic record of the northward subduction of the Neo-Tethys Ocean and the Indian-Eurasian collision, and Nyemo is located in the middle of the Gangdese batholith. In this paper, we report the zircon U-Pb geochronology, element geochemistry and zircon Hf isotopes of the host granites and gabbroic enclaves from Nyemo area. Zircon U-Pb dating results show that both the host granites (197~190Ma) and enclaves (195Ma) were formed in the Early Jurassic, and they should be emplaced at the same time. The host rocks are slightly peraluminous and high K calc-alkaline granites, which enriched light rare earth elements such as K, U and Sr, and depleted high field strength elements such as Nb, Ta and Ti. Their zircon ε_Hf(t) values varied within the range of +13.9~+16.0, indicating they may have been derived from partial melting of the juvenile crust. The gabbroic enclaves are metaluminous calc-alkaline rocks with rare earth elements and trace elements distribution characteristics similar to the host rocks, and their zircon ε_Hf(t) values are within +13.8~+16.0, showing that they may represent the juvenile lower crust formed in the Jurassic, which is the magmatic source of the host rocks. Based on previous studies, we suggest that the host granites and enclaves in Nyemo may be related to partial melting of mantle and juvenile crust caused by the northward subduction of the Neo-Tethys Ocean.