Early Cenozoic Metamorphism Of The Sedimentary Rocks From The Eastern Gangdese Magmatic Arc And Its Tectonic Implications

The Gangdese magmatic arc,south Tibet,was products of the Mesozoic and Cenozoic magmatism during the longlasting subduction of the Neo-Tethyan oceanic lithosphere and the subsequent India-Asia collision,respectively.The Mesozoic to Cenozoic metamorphic rocks in the eastern Gangdese arc provide a window into investigating the growth and reworking of continental crust.The present paper conducts a petrological and geochronological study of the metasedimentary rock(garnet-sillimanite-mica schist and garnet-biotite gneiss)from the eastern Gangdese arc.The petrographical study shows that the garnet-sillimanite mica schists have a mineral assemblage of Grt + Bt + Ms + Pl + Kfs + Sil + Q + Rt.Phase equilibria modelling shows that the peak metamorphic conditions of the schists are 730?750 °C and 7.8?8.1 Kbar.The petrographical study shows that the garnet-biotite gneiss have a mineral assemblage of Grt + Bt + Ms + Pl + Sil + Q + Rt.Phase equilibria modelling shows that the peak metamorphic conditions of the gneiss are ?730 °C and ?8.0 Kbar.These indicate that the schists and associated gneisses experienced the upper amphibolite-facies metamorphism and partial melting.The zircons from the schists and gneisses have an inherited detrital core and a metamorphic overgrowth rim.The U-Pb dating results show that the zircon metamorphic rims have an age range of 51.0?71.8 Ma,while the detrital cores have a wide ages ranging from 314 Ma to 3286 Ma.These results indicate that the sedimentary rocks were probably deposited at the Late Carboniferous,and underwent the Early Cenozoic metamorphism and partial melting.We suggest that the Late Paleozoic sedimentary rocks were buried into the middle to lower crust during the Early Cenozoic collisional orogeny,and in trun resulted in significant changes of the component of lower crust,and the chemical composition of syn-collisional magmatic rocks of the Gangdese magmatic arc.