Formation and evolution of the Gangdese magmatic arc, southern Tibet

The Gangdese magmatic arc, southern Tibet, was products of the Mesozoic and Cenozoic magmatism during the longlasting subduction of the Neo-Tethyan oceanic lithosphere and the subsequent India-Asia collision, respectively, and therefore, it is a natural laboratory for studying accretionary and collisional orogenesis, as well as growth and reworking of the continental crust. Based on a synthesis of available results of magmatism, metamorphism and mineralization in this region, the formation and evolution history of the Gangdese arc is divided into five stages, namely, the early subduction of the Neo-Tethyan lithosphere, the subduction of the NeoTethyan mid-oceanic ridge, the late subduction of remnant Neo-Tethyan lithosphere, the collision of India and Asia, and the postcollisional stage. The first stage, lasting from Late Triassic to Middle Cretaceous, is characterized by the normal subduction of the NeoTethyan oceanic lithosphere and the formation of subduction-related arc magmatic rocks, and during this stage, the long-term mantle derived magmatism resulted in the significantly growth of juvenile crust throughout the Gangdese arc, together with the generation of a giant porphyry Cu deposit in the western segment of the Gangdese arc. The second stage, happened in Late Cretaceous, was related to the subduction of the active Neo-Tethyan mid-oceanic ridge. In this stage, the upwell of the asthenosphere along the slab window resulted in extensive partial melting of upwelling asthenosphere, subduction slab and hinging-wall mantle wedge, which in turn, resulted in formation of diversity magmatic rocks and high-temperature metamorphic rocks. At the same time, underplating and accretion of the voluminous mantle-derived magma induced in the significantly growth and thickening of the Gangdese arc crust, and high-temperature and high-pressure metamorphism and partial melting of the thickened lower crust. The third stage, the latest Late Cretaceous, is characterized by the subduction of remnant Neo-Tethyan oceanic lithosphere and the normal arc magmatism. The fourth stage is represented by Paleocene to Middle Eocene magmatic flare-up, which was induced by the roll-back and breakoff of the subducted Neo-Tethyan slab during the Indo-Asian collision. This stage is characterized by the significantly thickening and partial melting of juvenile and old crusts, and extensive mixing of the mantle- and crust-derived magmas. The generated I-type granites inherited the chemical compositions of arc-type magmatic rocks, and also have the geochemical features of adakites. A series of large and giant Pb-Zn deposits related to the old crust-derived granites formed in the northern part of the Gangdese arc. The latest postcollisional stage is characterized by the continuous thickening and formation of the thickened lower crust-derived adakitic rocks during the Late Oligocene to Middle Miocene and many large and giant adakitic rock-related porphyry Cu-Au-Mo deposits formed in the eastern Gangdese arc in this stage. The multistage magmatic, metamorphic and mineralization processes provide excellent constraints for the Gangdese tectonic evolution from the Neo-Tethyan ocean subduction to India-Asia continental collision.