Early Cenozoic partial melting of meta-sedimentary rocks of the eastern Gangdese arc, southern Tibet, and its contribution to syn-collisional magmatism

Continental magmatic arcs are characterized by the accretion of voluminous mantle-derived magmatic rocks and the growth of juvenile crust. However, significant volumes of meta-sedimentary rocks occur in the middle and lower arc crust, and the contributions of these rocks to the evolution of arc crust remain unclear. In this paper, we conduct a systematic study of petrology, geochronology, and geochemistry of migmatitic paragneisses from the eastern Gangdese magmatic arc, southern Tibet. The results show that the paragneisses were derived from late Carboniferous greywacke, and underwent an early Cenozoic (69-41 Ma) upper amphibolite-facies metamorphism and partial melting at pressure-temperature conditions of similar to 11 kbar and similar to 740 degrees C, and generated granitic melts with enriched Hf isotopic compositions (anatectic zircon epsilon(Hf)(t) = -10.57 to +0.78). Combined with the existing results, we conclude that the widely distributed meta-sedimentary rocks in the eastern Gangdese arc deep crust have the same protolith ages of late Carboniferous, and record northwestward-decreasing metamorphic conditions. We consider that the deeply buried sedimentary rocks resulted in the compositional change of juvenile lower crust from mafic to felsic and the formation of syn-collisional S -type granitoids. The mixing of melts derived from mantle, juvenile lower crust, and ancient crustal materials resulted in the isotopic enrichment of the syn- collisional arc-type magmatic rocks of the Gangdese arc. We suggest that crustal shortening and underthrusting, and the accretion of mantle-derived magma during the Indo-Asian collision transported the supracrustal rocks to the deep crust of the Gangdese arc.

Automated summary

This study of migmatitic paragneisses in the eastern Gangdese magmatic arc in southern Tibet reveals that the meta-sedimentary rocks in the deep crust have Late Carboniferous protolith ages and record northwestward-decreasing metamorphic conditions. The mixing of mantle-derived melts, juvenile lower crust, and ancient crustal materials has enriched the isotopic composition of the syn-collisional magmatic rocks, suggesting crustal shortening and underthrusting during the Indo-Asian collision transported supracrustal rocks to the deep crust.