西天山博罗科努地区志留系碎屑锆石U-Pb定年及其地质意义

Detrital zircon U-Pb dating of Silurian sandstone in Borohoro aren, Western Tianshan Mountains, and its geological implications

西天山伊犁地块北缘博罗科努地区志留系出露齐全,是研究西天山早古生代构造演化的重要载体。对尼勒克县北水泥厂沟志留系库茹尔组砂岩开展了沉积特征和岩石学、地球化学研究及LA-ICP-MS锆石U-Pb测年。结果显示,库茹尔组形成于潮坪环境。砂岩样品LREE/HREE值为5.21～8.42,δEu值为0.74～0.90,球粒陨石标准化稀土元素配分模式呈现轻稀土元素富集、重稀土元素平坦及轻微的负Eu异常,地球化学特征指示,库茹尔组砂岩物源区构造背景为大陆岛弧。砂岩碎屑锆石U-Pb年龄分为3期,主要为425～510Ma,其次是698～796Ma和1240～1641Ma,表明库茹尔组物源区锆石年龄主要为早古生代,其次为中、新元古代。综合分析认为,库茹尔组形成于弧后盆地,沉积时代为晚志留世,其物源主要来自北侧岛弧带,其次是南侧伊犁地块。

The Silurian strata in the Borohoro area on the northern margin of the Yili block in the Western Tianshan Mountains are well exposed and serve as an important carrier for the study of the Early Paleozoic tectonic evolution of the Western Tianshan Moun.tains.In this paper,sedimentary petrology,geochemistry and LA-ICP-MS zircon U-Pb geochronology were investigated for the Si.lurian Kuruer Formation sandstone in Shuinichanggou,Nileke County.The result shows that Kuruer Formation was formed in tidalswamp facies.The sandstone in Kuruer Formation is characterized by high LREE,smooth HREE and rather weak negative Eu anom.aly.The geochemical characteristics indicate that the sandstone source of Kuruer Formation belongs to the continental arc.According to the U-Pb ages of LA-ICP-MS of 80 detrital zircon grains,the source rocks of Kuruer Formation detrital rocks came from ter.rains with three different geological periods,mostly 425~510Ma,and subordinately 698~796Ma and 1240~1641Ma,which respective.ly correspond to the Early Paleozoic,Neoproterozoic and Mesoproterozoic.According to the comprehensive analysis,the Kuruer Formation was formed in Late Silurian and was deposited on the back-arc basin formed by the southward subduction of the North Tianshan Ocean.The material of Kuruer Formation was mainly derived from the northern island arc zone,with the addition of a small amount of materials from the south side of the Yili block.