Geochemical Features and Tectonic Significance of Volcanic Rock in Qijiaojing Group of Middle Section of Bogda, Xinjiang

A large number of bimodal volcanic rocks are formed in the Carboniferous Qijiaojing group in Mulei areas, Bogda tectonic zone, The SiO₂ content of the volcanic rocks is obviously interrupted between 48% and 65%, with a discontinuous Daly component. Its basalt is characterized by depleted potassium,and rich in sodium, Ti, Al and Mg. The ratio of K₂O/TiO₂ and K₂O/P₂O₅ is high and the variation range is large. As a whole, there is mild crustal contamination in the later stage of basaltic magma. Meanwhile, dacite has the characteristics of depleted potassium, rich sodium, and low Ti and P, which is different from basalt. The basalt light rare earth is moderately enriched, and the light and heavy rare earth elements are moderately different; the dacite is higher than the basalt, the total amount of rare earth elements is 2~3 times higher, and the light and heavy rare earths are distinctly different. It has a strong negative anomaly (δEu=0.89~0.93). The basaltic magma may be formed by partial melting of the depleted mantle, and the dacitic magma may be formed by remelting the crustal material. Based on the comprehensive analysis, the Early Carboniferous in Bogda is in the post-arc rift basin environment.