Petrogenesis And Tectonic Implications Of The Triassic High-silica Granitic Magmatism In The Central Tibetan Plateau

High-silica granitic magmatism is not only considered as one of the key indexes to measure the maturity of continental crust,but also can provide essential information for understanding the growth of continental crust and the rare-mental mineralization.However,the origin,evolution,and geodynamic environment of the high-silica granitic magmatism remains highly controversial.This paper presents a comprehensive study of zircon U-Pb ages,bulk-rock geochemistry,and Sr-Nd isotopes for the studied high-silica granites in the central Tibetan Plateau.Zircon U-Pb dating shows that Duocai granite yield ages of 219±2 Ma and Longbao granitic porphyry yield ages of 217±3 Ma.In particular,the zircons in Yushu granite were subdivided into two groups with different²⁰⁶Pb/²³⁸U ages.The first group of zircons with light-CL rims,yield ages of 214±2 Ma.The second group of zircons with dark-CL rims,yield ages of 200±2 Ma.All the studied granite samples show(⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7090 to 0.7126 and depletedε_Nd（t）values of-8.58 to-7.58,together with their old two-stage Nd model ages（1.6 to 1.7 Ga）,the involvement of ancient lower crust materials should be considered.The genetic type and melting condition were constrained based on the P-T diagram,the Ti-in-zircon thermometry,petrography and bulk-rock geochemistry.The Duocai granite originated from partial melting under a low temperature and medium to low pressure condition（～778℃;～1 GPa）.The Longbao granitic porphyry originated from partial melting under a high temperature and medium pressure（～904℃;～1.2 GPa）condition.And the Yushu granite is formed by partial melting under a low temperature and medium to low pressure（～842℃;<1 GPa）.Combined with experimental petrology,the studied granites are mainly from a source consist of meta-graywacke,produced by dehydration melting of biotite and/or muscovite shist,and they had experienced a relatively degree of fractional crystallization.The zircons in the Yushu granite indicate a magmatic mush spanned a～13Ma period with prolonged evolution history beneath,and magmatic-hydrothermal activity enriched volatiles（F,Cl,and H₂O）remobilized this mush zone combined with rare metals enrichment（Li,Sn,and Cs）.Combining with the other regional studies,the Late Triassic magmatism in the north margin of the North Qiangtang（NQ）terrane was induced by slab rollback of the Garzê-Litang Palaeo-Tethyan ocean.