Geochemical characteristics and LA-ICP-MS zircon U-Pb dating of volcanic rocks in Shangqihan Formation along eastern orogenic belt in West Kunlun Mountains

Through a study of LA-ICP-MS zircon U-Pb isotopes of basaltic andesite in Shangqihan Formation at the east segment in the northern belt of Western Kunlun Mountains, zircon ²⁰⁶Pb/²³⁸U isotopic age was obtained as 473.1±2.2Ma, which represents crystallization age of basaltic andesite. Shangqihan Formation volcanic rocks in the study area were rich in large ion lithophile elements such as Sr, Ba, U and Th, but were depleted in high field strength elements such as Nb, Zr, Ta and Ti, and these features are symbolic features of volcanic rocks in the subduction belt. Ratio of incompatible elements Th/Ta is within 9.5~20.89, relatively approximate to Th/Ta ratio (10) of the upper crust. Rocks have higher La/Nb ratios (2.73~4.89), obviously higher than the ratio (2.2) in continental crust, and this indicates hybridization of crust materials. In the meantime, Nb content in rocks is high (8.14×10^-6~12.3×10^-6)and Nb/Ta ratio is 15.2~17.38, which indicates magmatic source region was subjected to metasomatism from subducted residual sheet bar fluid or melt. According to comprehensive analysis of the major and trace element features, petrologic features and geotectonic background, it is believed that volcanic rocks in Shangqihan Formation were formed in an island arc environment and were products of Kudi-Qimanyute Ocean which pitched down in the northward direction in Early Paleozoic.