Metamorphism of pelites in the eastern Gangdese magmatic arc and its tectonic implications

The Nyingchi area, eastern Gangdese magmatic arc, exposed middle-to high-grade metamorphic rocks, representing the middle and lower crustal components of the arc, and therefore, it provides a window into investigating the deep components, formation, and evolution of the arc. In this paper, we conduct a study of petrology and zircon U-Pb geochronology of pelitic schist from the west of the Bujiu, the Nyingchi area. The results show that the sillimanite-bearing garnet mica schist has the peak assemblage of garnet + biotite + plagioclase + muscovite + sillimanite + quartz + rutile. Phase equilibrium modeling and geothermobarometry reveal that the schists experienced amphibolite-facies metamorphism and partial melting under peak metamorphic conditions of similar to 7. 4kbar and similar to 715 degrees C. The prograde process and associated partial melting of these schists probably initiated at similar to 70Ma, and the retrograde process and melt crystallization occurred at 61 similar to 48 Ma. Combined available results, we conclude that the metamorphic conditions of metamorphic rocks from the eastern Gangdese arc have obvious spatial variation, and amphibolite-facies and granulite-facies metamorphic belt correspond to the component of arc middle crust and lower crust, respectively. Moreover, the Gangdese arc lower crust is composed mostly of meta-mafic and felsic igneous rocks, including minor meta-sedimentary rocks, and meta-granitic igneous rocks and meta-sedimentary rocks are the main components of the middle crust. We suggest that the continental collision of Indo and Asia and the roll-back and breakoff of the subducted Neo-Tethyan slab during Late Mesozoic-Early Cenozoic, resulted in the shortening and thickening of the Gangdese arc crust and the accretion of voluminous mantle-derived magma, which in turn induced the sedimentary rocks from upper crust transported into the middle and lower crust and experienced long-lasting high-temperature metamorphism and partial melting. The present study does not only reveal that the eastern Gangdese arc experienced Late Mesozoic-Early Cenozoic metamorphism, but also provides a key constraint on the component and spatial variation of the arc crust.