P-T-t Path And Tectonic Implication Of Metapelite In The Gangdese Arc,Southeastern Lhasa Terrane

The south Lhasa terrane in southern Tibet was formed during the Mesozoic Andean-type accretionary and Cenozoic collision orogenesis.In comparison with numerous studies of the Gangdese magmatic rocks in the south Lhasa terrane,the middle-to high-grade metamorphic rocks from the middle and lower crust of the Gangdese arc have received much less attention.In this paper,a combined study of petrology,phase equilibrium modelling and geochronology of the schists from the eastern Gangdese arc is carried out.The results show that the schists consist of biotite,muscovite,garnet,sillimanite,plagioclase,quartz and minor ilmenite and rutile(±kyanite),and have three-stage mineral assemblages:the prograde mineral assemblage is garnet+biotite+sillimanite+plagioclase+quartz+ilmenite,the peak mineral assemblage is garnet+biotite+muscovite+plagioclase+quartz+rutile±kyanite and the retrograde mineral assemblage is garnet+biotite+sillimanite+plagioclase+quartz+ilmenite.Phase equilibrium modelling shows that the schists experienced early medium-pressure and high-temperature metamorphism under the P–T conditions of?780–790°C and?6–7 kbar,peak granulite-facies metamorphism and partial melting under P–T conditions of?760–800 ~oC and?10–11 kbar with a melt production of at?6–8 vol.%,and the retrograde amphibolite-facies metamorphism under P–T conditions of?640–725°C and?5–8 kbar.U–Pb dating of zircon shows that the retrograde metamorphism and melt crystallization occurred between c.34 and c.24 Ma.Therefore,the schists recored an anticlockwise P–T–t path which includes a pragrade process of of isothermal burial and a retrograde process of cooling and decompression,typical of metamorphism in magmatic arcs.The present results suggest that the collision and ongoing convergence of the Indian and Asian continents and mantle-derived magmatic accretion and loading induced the crustal shorting and thickening of the Gandese magmatic arc,which in turn resulted in the high-temperature and high-pressure metamorphism and partial melting of the supracrustal rocks.