Hot Paleocene-Eocene Gangdese arc: Growth of continental crust in southern Tibet

The 1600 km-long Gangdese magmatic belt features extensive Paleocene-Eocene I-type intrusive rocks and coeval volcanic successions, which can be divided into Group I (-69-53 Ma), Group II (-53-49 Ma), and Group Ill (-49-43 Ma), corresponding to Neo-Tethyan slab rollback, Neo-Tethyan slab breakoff, and ongoing Indian Asian collision, respectively. The magmas from these three groups show significant variations in geochemical and isotopic compositions, which provide the information of the growth of continental crust in southern Tibet. The most voluminous magmatism in the Gangdese belt occurred during-53-49 Ma. High zircon saturation temperature (up to 800 C) and Ti in zircon temperature (up to 980 C) estimations suggest there is a period of thermal anomaly during-53-49 Ma. Starting from-53 Ma, magmas have increased K20 contents, and their zircons have decreased Th/U ratios, and Y and Yb contents. Zircons from Group 11 have the most heterogeneous lif isotopic compositions (epsilon Hf-(t) = 5.3 to 15.1). These are evident of ingress of asthenosphere mantle in the arc, extensive crustal melting, and magma mixing. Magma underplating during this time is the main mechanism for the growth of continental crust. With the Indian-Asian collision going on, the magmas in Group Ill show high Th/Y and La/Yb ratios and IC20 contents, but significantly low Tzr and T(ti-zr) values (mostly below 750 CC). These features suggest the water-fluxed melting of early arc residues occurred in the late stage of growth of continental crust. The crust has been thickened and nearly mature at this stage. This study has great implication on understanding of growth of continental crust in orogenic belts. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.