Geochemical signatures and geodynamic significance of Gejiu alkaline complex in southwestern South China Block

To investigate the evolution of the continental lithosphere of the southwestern South China Block, this study compiled previously whole-rock major and trace elements (n=76), whole-rock Sr-Nd-Pb isotopes (n=54) and in-situ zircon Hf isotopes data (n=113) of late Cretaceous Gejiu alkaline complex in South China Block, to reconstruct the geochemical signature and petrological model.According to its geochemical characteristics of the alkaline complex, which can be divided into three groups. Group one, shoshonitic nepheline syenites that show notably high total alkali (12.76%~17.57%) and total REE contents (134×10^-6~1 764×10^-6), low Ba/La ratios, and enriched Sr-Nd-Pb-Hf, indicating the enriched lithospheric mantle metasomatized by subduction-related melts. Group two, shoshonitic monzogabbro-monzonite suite has strongly high Ba (1 619×10^-6~4 640×10^-6) and Sr (970×10^-6~1 690×10^-6)contents, low Th/Yb ratios, similar to Sr-Nd-Pb-Hf isotopic components in nepheline syenite, indicating the enriched lithospheric mantle metasomatized by subduction-related fluids.Group three, monzogranite-K-feldspar granite suite shows high Th (30.7×10^-6~68.3×10^-6)and U (4.11×10^-6~23.2×10^-6) contents, and consistently Nb/U ratios of the lower crust and its trace element characteristics, indicating that they could be derived from the lower crust at variable depth. The MME and mantle-derived geochemical signatures are very developed in monzogranites, which indicate that their sources have experienced mantle-derived magma injection and crust-mantle magma mixing.During the Late Cretaceous, the South China Block occurred the lithospheric extension in the east-west direction that was influenced by the Neo-Tethys tectonic system, then the upwelling of asthenosphere material resulted into the reworking of the lithosphere, where a large amount of mantle-derived magma was injected into the lower crust and supported continental crust growth.