LA‐ICP‐MS Zircon U‐Pb Geochronology and Petrology of the Muchang Alkali Granite, Zhenkang County, Western Yunnan Province, China

Abstract:The Muchang composite intrusion is located about 14 km southeast of the Fengwei town and south of the Baoshan‐Zhenkang block. The rift‐related intrusion consists of the early‐stage riebeckite nordmarkite in the east and west sides and the discontinuous marginal zone, the late‐stage main body of dominant riebeckite granite, and minor aegirine granite. Studies on petrological geochemistry and LA‐ICP‐MS zircon U‐Pb dating of the late‐stage riebeckite granite, origin and evolution of the Muchang alkali granite and the relationship between the granite and the associated skarn‐type Luziyuan Pb‐Zn deposit are discussed in this paper. The results show that the Muchang alkali granites belong to A‐type granites, which are characterized by enrichment in Al2O3, SiO2, total alkali and Fe, depletion in MgO and CaO contents with high FeOT/MgO ratios. The REE concentrations are relatively high, exhibiting highly fractionated LREE patterns with significantly Eu negative anomaly. The Muchang granites are obviously enriched in lithophile elements (e.g., Rb, K, U and Th) and high field strength elements (e.g., Zr, Hf, Nb, Y and Ga) with high 10000×Ga/Al ratios and depleted in Sr, Ba, Ti, Cr and Ni, which are similar to those of the A‐type granites and quite different from those of S‐type and I‐type granites. The LA‐ICP‐MS zircon dating results of the Muchang granite gave a weighted mean age of 266.2±5.4 Ma (2σ), suggesting that they were formed in the stage of extension at the end of post‐collision at Middle Permian and the consumption of Paleo‐Tethys ocean took place before 266 Ma. It is suggested that the unexposed intermediate‐acid intrusive rocks in the Luziyuan ore district, which is the “sister” rocks material of the Muchang granites and related closely to Luziyuan Pb‐Zn mineralization, were the product of Middle Permian.