Metamorphic P-T path and tectonic implication of the sillimanite-garnet-biotite schist in the eastern Gangdese magmatic arc

The Mesozoic to Cenozoic metamorphic rocks in the eastern Gangdese arc provided a window into investigating the deep components, formation and evolution of the arc. The present paper conducts a petrological and geochronological study of the sillimanitegarnet-biotite schist. The petrography and mineral composition show that the rock experienced three stages of metamorphism. The early prograde metamorphic mineral assemblage is Grt + PI + Bt + Ms + Qz + Ep + Rt + Ilm. These minerals are included within the core of porphyroblastic garnet and plagioclase. The peak metamorphic mineral assemblage is Grt + PI + Bt + Ms + Qz + Rt + Ilm, represented by the mantle of porphyroblastic garnet and plagioclase, and also matrix minerals. The late retrograde mineral assemblage is Grt + PI + Bt + Sil + Qz + Ilm, which is represented by the minerals in the symplectitic corona after the garnet. Phase equilibria modelling shows that the prograde, peak and retrograde metamorphic conditions of the sillimanite-garnet-biotite schist are 600 similar to 650 degrees C and 9 similar to 12. 5kbar, similar to 820 degrees C and similar to 16kbar, and similar to 680 degrees C and similar to 7kbar, respectively. This indicates that the sillimanite-garnet-biotite schist has experienced a clockwise P-T path with the peak metamorphism of high-pressure and high-temperature granulite-facies and significantly partial melting. The zircon U-Pb dating data shows that the sillimanite-garnet-biotite schist was metamorphosed at the Late Cretaceous of similar to 89Ma. The present study and the previous results indicate that the underplating and accretion of voluminous mantle derived magma resulted in significant crustal thickening of the eastern Gangdese magmatic arc, as well as the high-pressure and high temperature metamorphism and associated partial melting of the thickened lower crust, including the meta-gabbros and the hosted metamorphic supracrustal rocks.