Melt components derived from a subducted slab in late orogenic ore-bearing porphyries in the Gangdese copper belt,southern Tibetan plateau

摘要

A porphyry-type Cu (Mo,Au) polymetallic ore belt extends in an E-W direction for>400 km along the Gangdese magma arc in the southern Tibetan plateau.Extensive field investigations and systematic geochemical study,combined with S and Pb isotopic tracing,indicate that Cu polymetallic mineralization of the copper belt is genetically related to late orogenic granitic porphyries formed in a late Himalayan crustal extension stage(18-14 Ma).Geochemistry of the porphyries shows the essential characteristics of adakite and suggests a dominant role of subduction-related components in their genesis.The genesis of these ore-bearing porphyries is mainly related to melt components derived from subducted slabs and has little relationship with fluids released from them.Pb isotopes show a clear linear array in the plumbotectonics model and are manifested by a progressive variation from orogenic Pb in the east segment of the copper belt to mantle Pb in the west segment,forming a mixing line of Indian Oceanic MORB and Indian Oceanic sediments.This suggests that the porphyry magma dominantly originated from partial melting of subducted oceanic crust and was mixed with a minor melts of sediments and mantle wedge components.