Metamorphism process and SHRIMP dating of granulite at Yushugou,northern margin of South Tianshan

Granulite facies terrane along northern boundary of South Tianshan is mainly composed of basic granulites,divided into various slabs of different extents of metamorphism.Careful petrographical observation recognized following metamorphic stages: Progressive metamorphism facies of amphibolite,low-pressure granulite,medium-pressure granulite,high-pressure granulite and retrogressive metamorphism facies of medium-pressure granulite,amphibolite and green schist.Based on a reliable geothermobarometer(Grt-Opx-Cpx-Plg-Qtz mineral assemblage),the pressure-temperature conditions are estimated for two medium-pressure proxene granulite samples,with the P-T results of 0.64~0.88GPa and 724~826℃,in accordance with the medium-pressure conditions as indicated by mineral assemblage.According to the zircon morphology and SHRIMP dating from the granulite whose protolith was volcanic rock,the cores of zircon grains are variable,and the age of rims of zircon grains is concentrative,ranging between 390~401Ma,showing the age of progressive medium-pressure granulite facies metamorphism.