西天山乌孙山地区大哈拉军山组碎屑锆石U-Pb定年及其地质意义

Detrital zircon U-Pb dating of Dahalajunshan Formation in Wusun Mountain region, western Tianshan, and its geological implications

西天山伊犁地区广泛出露的大哈拉军山组火山-沉积岩系,是研究西天山早石炭世古地理格局和天山古生代造山作用演化过程最为直接的载体。文章对西天山乌孙山地区大哈拉军山组砂岩样品进行了LA-ICP-MS锆石U-Pb测年。结果显示,其碎屑锆石206Pb/238U表面年龄分布范围较宽((321±2)^(435±2)Ma),按年龄及频率分布特征大致可以划分为2组:321~372 Ma和395~435 Ma。结合锆石的矿物学特征、CL图像特点及乌孙山区域地质资料,初步获得以下认识:(1)西天山乌孙山地区大哈拉军山组沉积时代不晚于早石炭世晚期;(2)所研究砂岩的碎屑物质主要来源于乌孙山及南部那拉提山相关的火山岩浆岩;(3)西天山造山带在中—晚泥盆世期间经历了一次重要的洋陆转换事件。早石炭世以后,进入后碰撞裂谷伸展演化阶段,因此大哈拉军山组火山-沉积岩系形成后碰撞伸展构造环境。

Dalaalajunshan Formation volcanic-sedimentary rocks, widespread in Yili area, westem Tianshan, are the most direct cartier in the study of Early Carboniferous palaeogeography of western Tianshan and Paleozoic orogenic evolution of the Tianshan. The age distribution of detrital zircons of sandstone in Dahalajunshan Formation was studied by using LA-ICP-MS U-Pb isotopic ages of zircons. The results show that the detrital zircons have a wide range of zircon ages from （321 ±2） Ma to （435±2） Ma, whichcan be divided into 2 groups： 321-372 Ma and 395-435 Ma, according to the age and frequency distribution characteristics. Based on these dating data, combined with the mineralogical characteristics and CL images of zircons as well as the regional geological data of the Wusun Mountain, the authors have reached the following conclusions： （1） The age of the deposition of Dahalajunshan Formation is not later than the later Early Carboniferous; （2） the detrital materials of the studied sandstone were mainly derived from volcanic and magmatic rocks related to the Wusun Mountain and Nalati Mountain; （3） during middle-late Devonian, western Tianshan orogenic belt experienced significant ocean-land transition events and entered into the post-collision rift extensional stage. Therefore, the formation of volcanic- sedimentary rocks of Dahalajunshan Formation was in a post- collision extensional setting.