The epilogue of Paleo-Tethyan tectonics in the South China Block: Insights from the Triassic aluminous A-type granitic and bimodal magmatism

Abstract

Triassic aluminous A-type granitoids, mostly peraluminous biotite granites, occur in various areas in the South China Block. Here, we focus biotite granites from three plutons and one granitic dike that provide new insights into the Triassic tectono-magmatic evolution of South China Block. The peraluminous biotite granites from Zhulanbu, Xiaotao and Wengshan plutons all emplaced at ca. 225 Ma and contain Fe-rich biotites. Their high FeOT/(FeOT + MgO) and Ga/Al ratios and HFSE contents imply an aluminous A-type affinity. The geochemical features and heterogeneous Sr-Nd-Hf isotopic compositions of these A-type granitoids, together with previous results, indicate their derivation from disequilibrium partial melting of a sedimentary source or fractional crystallization of coeval monzonitic melts. The Xiamao granitic porphyry dike emplaced synchronously with the nearby Xiamao diabase dike at ca. 225 Ma. This granitic dike is highly siliceous and its geochemical characteristics imply derivation from crustal assimilation and fractional crystallization (AFC) of mafic melt. Closure of Paleo-Tethyan oceanic basin and final collision of the South China Block with the Indochina and the North China Blocks were completed by ca. 230 Ma. The onset of A-type granitic magmatism occurred near Triassic dextral shear zones or surrounded by Triassic NE-SW strike-slip shear zones that generated localized extension under an overall compressional regime until 230 Ma. The subsequent formation of abundant A-type granitoids, bimodal dikes and hybrid granitoids hosting mafic microgranular enclaves implies the tectonic regime to a within-plate post-collision extensional setting with more intense mafic magmas underplating, marking the epilogue of Paleo-Tethyan tectonics in the South China Block.