藏北羌塘盆地基底高级变质岩LA-ICP-MS锆石U-Pb年龄及其地质意义

LA-ICP-MS zircon U-Pb age of high-grade metamorphic rocks from the Qiangtang Basin, northern Tibet, and its geological implications

俄久卖高级变质岩位于藏北羌塘盆地中央隆起带北缘的玛依岗日地区，是目前羌塘盆地基底高级变质岩石的唯一代表。该高级变质岩由正片麻岩和副片麻岩组成，本文以正片麻岩为研究重点。正片麻岩锆石CL图像显示出明显的核一幔一边结构。根据LA—ICP—MS锆石u—Pb测年结果，锆石核部年龄范围为242~2490Ma，记录了岩浆岩源岩的年代信息：锆石幔部具有典型的岩浆振荡环带结构，年龄为207Ma±2Ma，相应的Th／u值介于0．02-0．30之间，代表正片麻岩原岩的岩浆结晶时代．该年龄与羌塘中部地区晚三叠世高压变质作用和岛弧岩浆作用在时空上相对应。锆石增生边的年龄为161～197Ma．对应的Th／u值介于0．02-0．15之间，代表片麻岩发生主变质作用的时代，可能是班公湖一怒江洋盆向北的俯冲消减作用在羌塘中部地区的响应。地球化学资料显示，正片麻岩具有类似岛弧型火山岩的地球化学特征。综合区域地质资料，俄久卖高级变质岩原岩的形成与区域上广泛存在的晚三叠世构造、岩浆及角度不整合事件相对应，可能指示羌塘盆地统一基底的形成时代为晚三叠世。这对深入认识羌塘盆地基底的时代、性质及含油气盆地资源远景评价等具有重要意义。

Located in the northern margin of the ＂Central Uplift＂ of the Qiangtang Basin, the Ejiumai high-grade metamorphic rocks composed of orthogneiss and paragneiss are only representative high-grade metamorphic rocks in the basement of the Qiangtang Basin. The orthogneiss is biotite monzonitic gneiss with magmatic features. The CL images of its zircons have the core-mantle-rim structure. LA-ICP-MS zircon U-Pb dating indicates that the ages of the core range from 242Ma to 2490Ma, which might record the information of the source. The mantle shows clearly magrnatic oscillation zoning, with age of 207Ma＋2Ma and corresponding Th/U values of 0.02-0.30. These ages imply the time of magmatic crystallization of the original rock, and are consistent with the ages of Late Triassic high-pressure metamorphism and arc-type magmatism in Qiangtang area. The rim displays ages of 161-197Ma with Th/U values between 0.02 and 0.15. These data might have recorded the peak metamorphic age, and probably indicated the response to the northward subduction of the Bangong-Nujiang Tethyan Ocean. Geochenfical compositions of the orthogneiss show the characteristics of arc-type magmlatic rocks. The protolith of the Ejiumai high-grade metamorphic rocks was related to the Late Triassic tectonic, magmatic and angular unconformity events in this area. It is thus suggested that the uniform basement of the Qiangtang Basin was formed in the Late Triassic period. The results obtained by the authors play a key role in understanding the timing and nature of the basement of the Qiangtang Basin as well as in evaluating resources of this oil-gas-bearing basin.