Geochronology and geochemistry of the granites from the Mengku iron deposit, Altay Mountains, northwest China: implications for its tectonic setting and metallogenesis

Abstract

SHRIMP U–Pb zircon dating of a granite in the Mengku iron district, Altay, Xinjiang, yielded an age of 400 ± 6 Ma (MSDW = 1.3), which is similar to the age (SHRIMP U–Pb zircon age 404 ± 8 Ma) of granite north of orebody No. 1 in the Mengku iron deposit. This indicates that the two plutons are of Early Devonian age. These plutons intruded the Kangbutiebao Formation, thereby constraining the age of the Kangbutiebao Formation to >404 Ma. The Mengku granite and granite north of the No. 1 orebody are characterised by high silica (SiO₂ = 72.29–78.63%), Na enrichment (Na₂O = 4.12–7.98%), elevated Th, La, Ce, Nd and LREE, depleted Nb, Ta, P and Ti, as well as moderate negative Eu anomalies (δEu = 0.38–0.65). The granites belong to the low-K (tholeiitic) series with Na₂O/K₂O = 6.36–196.25 and are metaluminous and slightly peraluminous with alumina oversaturation indices of 0.97–1.09. Sr–Nd isotopic analyses [ϵ_Nd(t) = 1.04–2.17] indicate that the sources of the Mengku pluton are predominantly mantle-derived, contaminated with small amounts of crustal components. Chemical, isotopic and structural characteristics indicate that granites in the Mengku district originated in a continental–arc setting associated with plate subduction.

KEY WORDS:

• Altay Mountains
• China
• geochemistry
• geochronology
• granite
• Mengku iron deposit

Acknowledgements

This research was jointly supported by the National Basic Research Program of China (973 Program) (2007CB411302), the National Natural Science Foundation of China Program (40672065), the National Key Technologies R&D Program (2006BAB07B02-01; 2006BAB07B02-05), and the Geological Survey of China Projects 1212010786006. We are grateful to the leaders of the State 305 Project for their great logistical and moral support. We are grateful to Peter Laznicka and Franco Pirajno for valuable suggestions and are indebted to Z. C. Zhang and T. F. Zhou for their critical and constructive reviews, which improved the paper significantly.