Metallogeny, geochronology and tectonic setting of the Gangdese belt, southern Tibet, China

The Gangdese belt in southern Tibet forms the eastern section of the Trans- Himalayan magmatic belt. Until recently, the timing of magmatism in the Gangdese belt was poorly known due to the limited number of geochronological studies. Results of this study provide new insights into both the tectonomagmatic and metallogenic evolution of the southern Tibet. Six major magmatic events (Triassic and older, Early-Middle Jurassic, Late Jurassic, middle Cretaceous, latest Cretaceous-Eocene, and Oligocene-Miocene) are recognized within the southern Gangdese belt, each reflecting a specific paleotectonic setting. Early-Middle Jurassic magmatism occurred in a continental marginal arc tectonic setting, with magmatism related to south-dipping subduction of the Paleo-Tethys beneath the Lhasa terrane. Late Jurassic magmas were derived from a mixed magma source in an extensional magmatic arc setting, possibly during rollback of the Paleo-Tethyan slab and opening of the Neo-Tethys to the south of the Lhasa terrane. Continental arc magmatism, dominated by widespread and voluminous plutonism (Gangdese) and volcanism (Linzizong), began in Early Cretaceous time due to the onset of the north-dipping subduction of Neo-Tethys oceanic slab under the southern edge of the Lhasa terrane. This phase of subduction ended after the middle Eocene collision of the Indian and Eurasia continental plates. An apparent increase in magmatic activity around 50 Ma is attributed to the possible Neo-Tethyan slab rollback and breakoff. Cretaceous-Eocene magmatism includes a hiatus of magmatic activity from 80 Ma to 68 Ma. Miocene magmatism is post-collisional with adakite-like compositions possibly generated from partial melting of an enriched lower crust due to delamination of the thickened lithospheric mantle and asthenospheric upwelling after the collision. At least three major metallogenic epochs (Early-Middle Jurassic, Cretaceous-Eocene and Miocene) within the belt have been recognized, including the world-class Jurassic Xietongmen Cu-Au and Miocene Qulong Cu-Mo porphyry deposits. The Early-Middle Jurassic event includes formation of two major porphyry Cu-Au deposits (Xietongmen and Newtongmen) in the Xietongmen district, reflecting a major but previously unrecognized metallogenic event. This has global economic importance because the rate of new discoveries of large deposits within better explored belts elsewhere in the world has decreased significantly in the last few decades.