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.

Key Words:

• copper–nickel ores
• mafic complex
• petrogenesis
• Qingbulake
• SHRIMP
• uranium–lead dating
• West Tianshan

Acknowledgements

This research was jointly supported by State 305 Project (No. 2006BAB07B08-01), National Natural Science Foundation of China (No. 40402012 and 40863001), Central Public-interest Scientific Institution Basal Research Fund (No. K0805), State Key Laboratory of Geological Processes and Mineral Resources (No. GPMR200627) and Geological Survey Project (No.1212010634001). We are grateful to the Section of Mineral Resource Management, Bureau of Land and Resources of Tekesi County, Xinjiang, for its great support in the fieldwork, to the Beijing Ion Microprobe Center and the Electron Microprobe Laboratory of the Institute of Mineral Resources, Chinese Academy of Geological Sciences, for their assistance in the analysis of samples and to Zhang Zhaochong, Li Jinyi, Zuo Guochao and Xie Guiqing for their valuable opinions and helps in the writing of the manuscript. FP publishes with the permission of the Executive Director of the Geological Survey of Western Australia.

Notes

*In this paper, we use early, middle and late/upper Silurian to denote Llandovery, Wenlock and Ludlow + Pridoli, respectively.