Building of the Deep Gangdese Arc, South Tibet: Paleocene Plutonism and Granulite-Facies Metamorphism

The Nyingchi complex, forming the eastern segment of the Gangdese magmatic arc, occurs within the southern Lhasa terrane in south Tibet, and is composed dominantly of plutons and their metamorphosed equivalents. Together with some metasedimentary units these rocks record multiple Mesozoic and Cenozoic magmatic and metamorphic events during northward subduction of Neo-Tethyan oceanic lithosphere beneath the Eurasian continent. Petrological and geochronological studies reveal that the Nyingchi complex experienced intense Paleocene subduction-related magmatism and almost synchronous granulite-facies metamorphism accompanied by the formation of S-type granites. Subduction-related I-type granitoids show geochemical features typical of continental magmatic arcs; zircon separates from them yield Pb-206/U-238 ages from c. 65 to c. 56 Ma, and commonly display positive epsilon(Hf)(t) values ranging from -1 center dot 7 to +13 center dot 0. The occurrence of magmatic epidote, as well as the syn-intrusion high-grade metamorphism, indicates that the plutons were emplaced at middle to lower crustal depths within the Lhasa terrane. Associated S-type granitoids are peraluminous and contain garnet and muscovite; their zircons yield Pb-206/U-238 ages ranging from c. 66 to c. 55 Ma, and these have distinct but mostly negative epsilon(Hf)(t) values from -18 center dot 4 to +2 center dot 1. The zircons from the associated metasedimentary rocks include both detrital and metamorphic types; the detrital zircons yielded variable inherited Pb-206/U-238 ages ranging from c. 2910 to c. 235 Ma, constraining the maximum depositional age to the Triassic. The metamorphic zircons from the metaplutonic and metasedimentary rocks yielded ages from c. 67 to 52 Ma. Phase equilibria modeling shows that the Nyingchi complex experienced peak granulite-facies metamorphism and partial melting under conditions of 800-830 degrees C and 9-10 center dot 5 kbar, and then cooled isobarically to c. 700 degrees C in the lower crust at depths of > 30 km. We argue that rollback of the flat-subducted Neo-Tethyan oceanic slab during Early Paleogene times resulted in a contractional orogeny and intrusion of voluminous mantle-derived magmas, which caused large-scale crustal heating, partial melting and granulite-facies metamorphism within the deep crust of the Gangdese arc. The Nyingchi complex represents the exposed lower crust of the Gangdese magmatic arc, and links the granulite-facies metamorphism with silicic magmatism and crustal growth during Paleocene arc accretion.