The Cenozoic Metamorphism And Tectonic Implications Of Schist And Gneiss In The Medog Area,Eastern Gangdese Magmatic Arc

The middle to high grade metamorphic rocks in the eastern Lhasa terrane were derived from the Mesozoic accretional and Cenozoic collisional orogenesis of the Gangdese magmatic arc,and therefore represent the middle-lower crust of the Gangdese magmatic arc,forming a natural experimental laboratory for studying the growth and reworking of contintntal crust.The present thesis presents a detailed study of petrology,phase equilibria modeling and monazite and zircon U-Pb geochronology of schist and gneiss from the Medog area,the eastern Gangdese magmatic arc.The results show that these rocks underwent multistages of metamorphic evolution and associated partial melting.The early prograde metamorphic mineral assemblage is garnet + biotite + plagioclase + quartz +ilmenite,the peak metamorphic mineral assemblage is garnet + biotite + sillimanite +plagioclase + quartz ± K-feldspar,and the late retrograde metamorphic mineral assemblage is garnet + biotite + muscovite + plagioclase + quartz.Phase equilibria modeling shows that these metamorphic rocks witnessed prograde metamorphic conditions of 750?780 °C and7.5?8.2kbar,peak metamorphic conditions of 800?850°C and 8.5?10kbar,and retrograde metamorphic conditions of 640?700 °C and 8.0?9.2kbar,and therefore an anti-clockwise P-T path with a prograde process of both temperature and pressure increase and associated biotite dehydration melting,and a retrograde process of near-isobaric cooling.The monazite and zircon U-Pb dating indicates that schist and gneiss underwent long-lived metamorphic process with peak metamorphic age of 44?48Ma,and retrograde age of 25?31Ma.The present study indicates that the Gangdese magmatic arc underwent a distinct crust shortening and thickening process due to the Indian continent collision with Eurasian continent and intrusion of voluminous mantle-derived magmas,which resulted in the Eocene granulite facies metamorphism and partial melting of metamorphic supracrustal rocks of the Gangdese magmatic arc.These rocks remained in the lower crust and underwent a prolonged near-isobaric cooling retrograde process that lasted to the Oligocene.