Zircon U-Pb age and Hf isotopic compositions of Mesozoic granitoids in southern Qiangtang, Tibet: Implications for the subduction of the Bangong-Nujiang Tethyan Ocean

The southern Qiangtang magmatic belt was formed by the north-dipping subduction of the Bangong-Nujiang Tethyan Ocean during Mesozoic. To better understand the petrogenesis, time-space distribution along the length of this belt, 21 samples of several granitoid bodies, from west to east, in the Bangong Co, Gaize, Dongqiao and Amdo areas were selected for in-situ zircon U-Pb dating, Hf isotopic and whole-rock chemical analyses. The results suggest a prolonged period of magmatic activity (185-84 Ma) with two major stages during the Jurassic (185-150 Ma) and the Early Cretaceous (126-100 Ma). Both the Jurassic and Cretaceous granitoids are high-K calc-alkaline I-type rocks, except the Cretaceous two-mica granite from Amdo in the east, which belongs to S-type. The granitoids are generated from different source materials as indicated by zircon Hf isotopic compositions. The Bangong Co and Dongqiao granitoids show high zircon epsilon(Hf)(t) values of -1.3-13.6 with younger T-DM(C) ages of 293-1263 Ma, suggesting a relatively juvenile source; whereas the Gaize and Amdo granitoids have low epsilon(Hf)(t) values of -16.1-2.9 with older T-DM(C) ages of 999-2024 Ma, indicating an old crustal contribution. These source rocks melt at different P-T conditions as suggested by Sr/Y ratio and T-Zr. The Sr/Y ratio of both stage granitoids increases with decreasing age. However, the T-Zr of the Jurassic granitoids decreases, whereas the T-Zr of the Cretaceous granitoids increases with decreasing age. The contrasting geochemical signatures of these granitoids may be controlled by the varying contribution of slab-derived fluids involved in the generation of the Jurassic and Cretaceous granitic magmas; i.e. increasing amount of fluids in the Jurassic, whereas decreasing amount of fluids in the Cretaceous. Therefore, it is proposed that the Jurassic and Cretaceous magmatism may be related to subduction and closure of the Bangong-Nujiang Tethyan Ocean, respectively. The age pattern of the Jurassic and Cretaceous granitoids suggests an oblique subduction of the Bangong-Nujiang Tethyan Ocean and a diachronous collision between the Lhasa and Qiangtang blocks. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.