青藏高原东南缘后碰撞埃达克质岩石的地球化学特征及古高程指示OA

Due to the lack of detailed characterization of the Cenozoic uplift process along the southeastern Tibetan Plateau,the post-collision uplift history of the Tibetan Plateau remains poorly understood.A series of C-type Adakites have formed in the post-collisional setting of the Tibetan Plateau.Those rocks are character-ized by high K2O/Na2O ratios,strong rare earth element(REE)fractionation,and low Yb contents,which are generally associated with crustal thickening and thus could provide key insights into the uplift history of the pla-teau.In this study,zircon U-Pb dating,zircon Hf isotopes,whole-rock geochemistry,and Nd isotopes are reported for the Adakitic granites and quartz monzodiorites from the Weixi area to explore their genesis and their relationship with crustal thickening and uplift in the southeastern Tibetan Plateau.The results show that the Weixi granites were crystallized at～40 Ma,while the quartz monzodiorites were crystallized during 37-33 Ma,indicating their formation during the Middle to Late Eocene.These rocks exhibit high SiO2(67.2%-72.29%)and Al2O3(15.13%-16.37%)contents.They are characterized by high K2O/Na2O ratios(0.75-2.16),elevated Sr concentrations(852×10-6-1725×10-6),low Y(7.19×10-6-21.22×10-6)and Yb(0.53×10-6-1.81×10-6)contents,high Sr/Y ratios(42-143),and with strong rare earth element(REE)fractionation,which is consistent with C-type adakitic geochemical signatures.Petrogenetic analyses suggest that the Weixi granites was derived from partial melting of the thickened lower crust,while the quartz monzodiorites also origi-nated from partial melting of the thickened lower crust but subsequently mixed with regional mafic magmas.The crustal thickness and paleo-elevation of the southeastern Tibetan Plateau were reconstructed using the(La/Yb)N ratios of these C-type dakite rocks from two sections in Weixi area.Results show that during the Middle to Late Eocene,the crustal thickness in the Weixi-Jianchuan area was(50±9)km and(53±17)km,respectively,with paleo-elevations of(2,678±1480)m and(2,947±641)m.These results suggest that crustal thickening and uplift of the southeastern Tibetan Plateau had already taken place before the Oligocene,reaching its current state.This process was possibly driven by regional lithospheric-scale extrusion during the post-collision tec-tonic setting.