Geochemical and Isotopic Characterization of Middle Eocene Hybrid Magmatism in the Gangdese Belt (Tibet) and Its Ancient Indian Crustal Fingerprint Juan He, 1,2,3 Yildirim Dilek, 3 Yalin Li, 1, * Chengshan Wang, 1 Yushuai Wei, 1 Xi Chen, 1 Nuoya Sun, 4 and Yunling Hou 5 1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China; 2. Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China; 3. Department of Geology and Environmental Earth Science, Miami University, Oxford, Ohio 45056, USA; 4. Special Technology Exploration Center of China Coal Geology Bureau, China Corporation of Coal Geology Engineering, Beijing 100040, China; 5. Sichuan Institute of Geological Survey, Chengdu 610081, China ABSTRACT We present new geochemistry, geochronology, whole-rock Sr-Nd-Pb, and zircon Hf isotopic data from the Zhongba pluton in the western part of the Gangdese magmatic belt (GMB) and discuss its melt evolution within the collisional tectonic framework of southern Tibet. Our U-Pb zircon dating of the Zhongba plutonic rocks has constrained the timing of their emplacement at 38 Ma. The Zhongba granitoids are enriched in light rare earth elements, large-ion lithophile elements, and Pb and are depleted in high-field-strength elements. They show enriched Sr isotopes, low ε Nd (t) values, and fairly radiogenic Pb isotopic signatures. They have negative zircon ε Hf (t) values and ancient crustal model ages of 1483–1180 Ma. These geochemical and isotopic characteristics, combined with the literature data, suggest mixing of different magma types, derived from partial melting of juvenile southern Lhasa crust and ancient Indian continental crust, to produce hybrid magmas of the Zhongba granitoids. We propose that asthenospheric upwelling caused by the breakoff of the northward- subducting Neotethyan oceanic lithosphere provided the heat and material flux to trigger partial melting of the overlying Lhasa arc lithosphere and subducted sediments of the ancient Indian crust. Similar geochemical and isotopic signatures of the coeval Kailas and Zedong granitoids within the GMB indicate that this slab breakoff–induced mid- to late Eocene magmatism was a syncollisional and orogen-parallel magmatic event. The Zhongba pluton and its contemporaneous equivalents thus provide the geochronological and isotopic evidence for the earliest melt flux derived from the sub- ducted continental crust and sediments of India into the melt regime of Cenozoic magmatism in southern Tibet. Online enhancements: supplemental tables. Introduction The Tibetan-Himalayan orogenic belt is an arche- type of continental-collision tectonics and displays the topographically highest elevation and the thick- est continental crust on Earth (Molnar and Tappon- nier 1975; Dewey et al. 1989; Harrison et al. 1992; Le Pichon et al. 1992; Yin et al. 1999). Geophysical data indicate that the crustal thickness varies between 60 and 80 km discontinuously in the E-W direction throughout the collision zone. Much of this topo- graphic and crustal buildup has resulted from ∼N-S- oriented shortening, caused by the collision of In- dia with Asia in the early Cenozoic (Hodges 2000; Guillot et al. 2003; Liebke et al. 2010; Jagoutz et al. 2015), and the subsequent response of the Himala- yan and Tibetan lithosphere to the continental con- vergence, the ongoing underplating of India beneath Eurasia, crustal-mantle flow processes, the interplay and feedback mechanisms among uplift, erosion, de- position, and climate processes, extension, and syn- to postcollisional magmatism (Burchfiel and Royden 1985; Copeland et al. 1987; Kutzbach et al. 1989; Manuscript received November 7, 2016; accepted August 6, 2018; electronically published October 2, 2018. * Author for correspondence; email: liyalin@cugb.edu.cn. 601 [The Journal of Geology, 2018, volume 126, p. 601–620] q 2018 by The University of Chicago. All rights reserved. 0022-1376/2018/12606-0003$15.00. DOI: 10.1086/700071