India's hidden inputs to Tibetan orogeny revealed by Hf isotopes of Transhimalayan zircons and host rocks

To better understand Tibetan orogenesis, which involves complex tectonic processes, here we report the first integrated analysis of Hf isotopes of zircon separates and their host rocks from the Transhimalayan batholiths. The rocks studied include 19 granitoids from different parts of the Gangdese batholith, the largest Transhimalayan intrusive complex, and five postcollisional adakites that occurred as plugs or dikes cutting the Gangdese batholith, in the southern Lhasa terrane, southern Tibet. Published zircon Hf isotope data from other Transhimalayan granitoids and the Linzizong volcanic rocks are also synthesized. The magmatic zircons, crystallizing between ca. 200 and 15. Ma, show depleted mantle-type Hf isotopic characteristics throughout the Mesozoic but shift markedly in the Paleogene. The Hf isotopic shift is interpreted as tracking the evolving progress of Himalayan sediment subduction driven by the approaching Indian continent, and thus signals the initiation of the India-Asia collision that we infer to have occurred by 55. Ma. Our data furthermore indicate that southern Tibet underwent significant crustal thickening during ca. 45 and 30. Ma, before emplacement of the postcollisional adakites that exhibit Hf-Nd isotopic systematics suggesting binary mixing of melts from the juvenile Gangdese mafic crust and incorporated Himalayan sediments in the petrogenesis.