Geochemical and SHRIMP U–Pb age constraints on the origin of the Qingbulake mafic–ultramafic complex in the West Tianshan Mountains, Xinjiang, northwest China

Abstract

The Qingbulake mafic complex hosting a copper–nickel sulfide deposit, Tekesi County, West Tianshan (Xinjiang Uygur Autonomous Region) consists of pyroxene diorite–gabbro, olivine gabbro–pyroxenite, pyroxenite–olivine gabbro, and clinopyroxene peridotite. The geochemical study presented in this paper indicates that the complex is characterised by low Ti, depletion in Nb and Ta, enrichment in LILEs and slight enrichment in light REEs. These geochemical characteristics, combined with isotope systematics, suggest that the formation of the complex may be related to the subduction of the South Tianshan oceanic plate beneath the Central Tianshan plate and their collision and that the magma could be derived from a depleted MORB-source mantle. The magma underwent weak hybridisation with the crust and country rocks during its emplacement. SHRIMP U–Pb dating of zircons in pyroxene diorite samples from the Qingbulake complex yielded a concordia age of 434.4 ± 6.2 Ma, indicating that the complex was emplaced in the early Silurian (Llandovery). The features of island-arc magma shown by the Qingbulake complex were probably induced by the subduction of oceanic crust beneath the depleted mantle.