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 isoftenconsideredtobetheprimarycauseforthecrustalthickening.Inthispaper,weshowthatmagmaticcontributionisalsoimportant.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 ∼65–70 Ma to a “hard”phase at ∼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 magmaticbelt.Theseinclude(1)thesyn-collisionalLVSvolcanism(∼65–40Ma)andtheemplacementofsouthernGangdesebatholiths(a peakageof ∼50Ma); (2)aperiod(∼40Mato25Ma)thatismagmaticallyquiescent,yettectonicallydominatedbyactivecompressionand crustal shortening; and (3) the emplacements of post-collisional adakitic rocks (∼25–12 Ma), potassic–ultrapotassic volcanics (∼25– 10 Ma) and peraluminous muscovite-bearing granites (∼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 ∼65 Ma to 50 Ma with crustal thickening largely concentrated at ∼50–40Maviainputofmantle-derivedmagmas;PhaseII,crustalthickeningbytectonicshorteningat ∼40–25Ma;andPhaseIII,retaining crustal thickness, but thinning of the lithospheric mantle since ∼25 Ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle.Weemphasizethatcollision-inducedcrustalthickeningtookplacemainlyintheperiodof ∼50–40Maand ∼25Ma, 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 eNd>0, attesting to the significance of mantle input, most likely through remelting 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 ∼35 km, then the tectonic contribution would be about 20 km.