Mantle contributions to crustal thickening during continental collision: Evidence from Cenozoic igneous rocks in southern Tibet

The Tibetan crust is twice as thick as average continental crust. Crustal compression and shortening as a result of Indian-Asian collision is often considered to be the primary cause for the crustal thickening. In this paper, we show that magmatic contribution is also important. We come to this conclusion by documenting the Paleogene Linzizong volcanic succession (LVS), its coeval granitoid batholiths and the Miocene adakitic rocks along the Gangdese magmatic belt in southern Tibet. It has been widely accepted that the Indian-Asian collision proceeded from a "soft" phase at similar to 65-70 Ma to a "hard" phase at similar to 45-40 Ma, followed by continued post-collisional convergence to the present. In response to the collision and post-collision convergence are a series of tectono-magmatic events recorded in the Gangdese magmatic belt. These include (1) the syn-collisional LVS volcanism (similar to 65-40 Ma) and the emplacement of southern Gangdese batholiths (a peak age of similar to 50 Ma); (2) a period (similar to 40 Ma to 25 Ma) that is magmatically quiescent, yet tectonically dominated by active compression and crustal shortening; and (3) the emplacements of post-collisional adakitic rocks (similar to 25-12 Ma), potassic-ultrapotassic volcanics (similar to 25-10 Ma) and peraluminous muscovite-bearing granites (similar to between 24 and 18 Ma). These three major events contribute in different ways to the crustal thickness. Phase I, formation of the lower juvenile crust from similar to 65 Ma to 50 Ma with crustal thickening largely concentrated at similar to 50-40 Ma via input of mantle-derived magmas; Phase II, crustal thickening by tectonic shortening at similar to 40-25 Ma; and Phase III, retaining crustal thickness, but thinning of the lithospheric mantle since similar to 25 Ma in response to crustal extension and upwelling and lateral flow of asthenospheric mantle. We emphasize that collision-induced crustal thickening took place mainly in the period of similar to 50-40 Ma and similar to 25 Ma, i.e., the period between the late stage of the LVS volcanism and the beginning of the adakitic rock emplacement. Most of the LVS rocks and the collision related granitoids in southern Gangdese have epsilon(Nd)>0, attesting to the significance of mantle input, most likely through re-melting of mantle-derived basaltic rocks, including the subducted Neo-Tethyan ocean crust. The petrologic and geochemical characteristics of the Miocene potassic adakitic rocks support the idea that the lower portion of the thickened Tibetan crust is mafic and is genetically associated with the earlier LVS magmatism. We estimate that the mantle material input contributed about 30% of the total thickness of the present-day Tibetan crust. By assuming a pre-collision crustal thickness of similar to 35 km, then the tectonic contribution would be about 20 km. (C) 2006 Elsevier B.V All rights reserved.