Geochemistry And Petrogenesis Of Miocene Basalt In Maguan, Southeastern Yunnan

Post-collisional magmatic rocks widely distributed in Sanjiang region in western Yunnan region which is caught between Pacific tectonic domain and South-China sea tectonic domain（plume mantle）provide significant information for comprehensive understanding of mantle evolution in these three domains.In this study,we focus on the systematic dataset of whole-rock major,trace element,Sr-Nd-Mg isotopes and EPMA analyses of Maguan basalt.There are various mantle olivines and mantle xenoliths in the basalt.According to EPMA analyses,three types of the mantle olivine xenocrysts(L_Ca-Ol,H_Mg-Ol and H_Ca-Ol)have been noticed.Olivine xenocrysts(H_Ca-Ol)with high CaO content and Ca/Al ratios indicate that the lithospheric mantle might be replaced by carbonate melt.H_Mg-Ol is characterized by high Fo value and has spinel inclusions of high Cr^#,which are interpreted as ancient lithospheric mantle residuals.The geological thermobarometer indicating the refractory lithospheric mantle in Maguan area might be replaced by upwelling asthenospheric mantle.Maguan basalt is characterized by the enrichment of trace element and positive anomaly of Nb-Ta.The result of geological thermometer reveals a temperature of1456¹552°C for melt and depth of 68.7¹29 km,indicate that magma was derived from asthenospheric mantle.The low Mg isotopic composition(?²⁶Mg,-0.6^-0.4‰)and significant negative Ti anomaly indicate that the asthenospheric mantle beneath Maguan have been reshaped by carbonate melt which are mainly dolomite.Our study indicates that two periods of metasomatism events recorded by deep mantle beneath Maguan.In late Proterozoic,the lithospheric mantle in west of South China block was reshaped by fluids released from subducted oceanic plate.After that,under the stress of collision of India and Eurasia plates,the asthenospheric material beneath Tibetan plateau migrated eastward and metasomatized the lithospheric mantle in Maguan during the late Cretaceous to Miocene.Based on the experimental petrology and geophysical data,the mantle-derived magmatism lags behind the neo-tectonic extension activities in Sanjiang,southeast China and South China Sea area in Miocene.This kind of magmatic activity is related to the southeastward lateral migration and extrusion of asthenospheric mantle of the Tibetan plateau.