东昆仑志留纪火山岩锆石U-Pb年龄及其对陆-陆碰撞时限的制约

Zircon U-Pb ages of Silurian volcanic rock in East Kunlun and its constraint on the duration of continent-continent collision

东昆仑造山带早古生代经历了完整的洋壳形成、俯冲消减、陆-陆碰撞造山和造山后垮塌演变过程,目前对陆-陆初始碰撞时间及碰撞时限还存在较大争议。周缘前陆盆地启动引发的沉积环境突变或不整合形成时间是用来约束大陆初始碰撞时间最直接和最有效的方法之一。本文以东昆仑水泥厂地区角度不整合覆盖于石灰厂组之上的志留纪周缘前陆盆地沉积哈拉巴依沟组为研究对象,开展火山岩夹层锆石U-Pb年代学研究,为约束早古生代陆-陆初始碰撞时间与碰撞造山时限提供沉积记录证据。结果表明,水泥厂东和雪水河东哈拉巴依沟组下部流纹质凝灰岩锆石U-Pb年龄分别为443.0±3.9 Ma和441.8±1.3 Ma,结合已报道的石灰厂组火山岩锆石U-Pb年龄(450.4±4.3 Ma),可以确定东昆仑陆-陆初始碰撞发生在450~443 Ma之间。综合区域上古生代岩浆活动、变质作用、构造变形与相关沉积记录证据,认为东昆仑地区至少从450 Ma左右开始进入陆壳深俯冲及陆-陆碰撞阶段,在425 Ma左右进入碰撞后伸展阶段,碰撞造山作用至少持续了25 Ma。

The East Kunlun orogenic belt experienced a complete process of oceanic crust formation,subduction erosion,collision orogeny and post-orogenic collapse evolution in the early Paleozoic era.But the initial continent-continent collision time and the duration of continent-continent collision are still controversial.One of the most direct and effective methods to constrain the initial continental collision time is the age of formation of the depositional environment mutation or unconformity caused by the start-up of peripheral foreland basin.In this paper,we take the Halabayigou Formation of the Silurian peripheral foreland basin,which is angular unconformity covered by the Shihuichang Formation in Shuinichang area of East Kunlun,as the object of study.The zircon U-Pb chronology of the intercalated volcanic rocks is carried out to provide the sedimentary record evidence to constrain the initial continent-continent collision time and the collision orogeny time of the early Paleozoic.The results show that the U-Pb ages of the rhyolite tuff in the lower Halabayigou Formation in the east of Shuinichang and Xueshui River are 443.0±3.9 Ma and 441.8±1.3 Ma,respectively.Combined with the reported U-Pb age of the volcanic rock of Shihuichang Formation(450.4±4.3 Ma),it can be determined that the initial continent-continent collision of East Kunlun occurred during 450~443 Ma.Based on the evidence of Paleozoic magmatism,metamorphism,tectonic deformation and related sedimentary records,it is concluded that the East Kunlun area entered the stage of continental crust subduction and continent-continent collision at least from about 450 Ma,and entered the stage of post-collision extension at about 425 Ma.The collision orogeny lasted about 25 Ma.