西昆仑造山带东段上其汗组火山岩地球化学特征、LA-ICP-MS锆石U-Pb定年及其地质意义

通过对西昆仑北带东段上其汗组玄武安山岩进行LA-ICP-MS锆石U-Pb定年,获得锆石~(206)Pb/~(238)U年龄为473.1±2.2Ma,代表玄武安山岩的结晶年龄。研究区上其汗组火山岩富集大离子亲石元素Sr、Ba、U、Th等,亏损高场强元素Nb、Zr、Ta、Ti等,为消减带火山岩的标志性特征。不相容元素Th/Ta值为9.5～20.89,与上地壳的Th/Ta值(10)较接近。岩石具有较高的La/Nb值(2.73～4.89),明显高于大陆地壳这一比值(2.2),表明有地壳物质混染。同时,岩石中Nb含量较高,为8.14×10~(-6)～12.3×10~(-6),Nb/Ta值为15.2～17.38,表明岩浆源区受到来自消减残留板片流体或熔体的交代作用。对岩石主量、微量元素特征、岩石学特征及大地构造背景综合分析认为,上其汗组火山岩形成于岛弧环境,其为库地-其曼于特洋在早古生代向北俯冲的产物。

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.