Petrogenesis And Magma Mixing Of Nyemo Post-collisional Granitic Magmatisms From The Lhasa Terrane,Southern Tibet

The Lhasa terrane,located at the southernmost of Asian continent,has the geological records of both the subduction of the Neo-Tethyan oceanic lithosphere beneath the Asian continent and the subsequent India-Asia collisional process,and it is now underlain with one of the thickest continental crust on the Earth.However,the changing process of crustal thickness of the Lhasa terrane has been less studied,which restricts our understanding of the formation of the Tibetan plateau.The post-collisional adakitic magmatisms from southern Lhasa subterrane show systematic variations in space,age and the characteristics of composition and magmatic evolution.Although all of them were ordinarily believed to be generated by partial melting of thickened lower crust,their distinct light-heavy rare earth element fractionation can speculate the variation of crustal thickness in this region,containing the information of deep lower crust.In this study,we present mineralogical,geochronological,geochemical and zircon Hf isotopic data for two stages of post-collisional adakitic rocks?granodiorite and monzogranite,respectively?from the Nyemo area,southern Lhasa subterrane.Disequilibrium textures occurred in both granodiorite and monzogranite,such as some plagioclase grains are zoned with Ca-spikes,resorption boundaries or sieved core domains,rapakivi texture in K-feldspar megacrysts and biotite mantled by amphibole,indicate the existence of magma mixing in this region.Zircon U-Pb dating yielded crystallization ages of ca.15 and 30 Ma for the granodiorite and monzogranite,respectively.Their zircon Hf isotopic compositions suggest that the magmas were dominantly derived from partial melting of Lhasa thickened lower crust with residual amphibole and garnet.Furthermore,zircons from enclaves in ca.15 Ma granodiorite show a bit more positive?_Hf?t?values showing some contributions of depleted mantle,while zircons from enclaves in ca.30 Ma monzogranite possess more negative?_Hf?t?values showing some contributions of enriched mantle.The obviously lower?La/Yb?_N ratios and higher Yb values in ca.15 Ma granodiorite,indicating less residual garnet in its source region,suggests a shallower depth to generate the granodioritic magma.In addition,an obvious decline of La/Yb ratios of ca.15 Ma granodiorite compared with those of ca.30 Ma monzogranite show a thinner crust at ca.15 Ma.Therefore,we suggest that the crust of Lhasa terrane in southern Tibet was not continually thickened after India–Asia collision but experienced a thinning event at about 25 Ma.