东昆仑早古生代花岗岩锆石U-Pb年龄及其地质意义

应用激光烧蚀多接收器电感耦合等离子体质谱(LA-MC-ICPMS)方法,对东昆仑五龙沟地区花岗岩样品进行了锆石 U-Pb 定年研究。结果表明,黄龙沟中部的糜棱岩化二长花岗岩(B20-14)中岩浆锆石206Pb/238U年龄平均值为(417.7±2.0) Ma,黄龙沟上部的糜棱岩化二长花岗岩(B21-2)中岩浆锆石206Pb/238U年龄平均值为(419.7±2.3) Ma,深水潭粗中粒二长花岗岩(B20-8)中岩浆锆石206Pb/238U 年龄平均值为(419.9±2.0) Ma,红旗沟中部的粗中粒二长花岗岩(B25-9)岩浆锆石206Pb/238U年龄平均值为(419.0±2.0) Ma。它们代表了五龙沟地区所研究花岗岩的形成年龄,其形成时代均为晚志留世,记录了东昆仑早古生代的岩浆活动。本文获得的东昆仑五龙沟地区的晚志留世花岗岩可能与东昆仑早古生代洋盆闭合后的碰撞造山作用有关。花岗岩中1861 Ma、1666 Ma古元古代继承锆石的发现,表明东昆仑造山带的基底物质为古元古代。

Zircon U-Pb Dating of Early Paleozoic Granites from the East Kunlun Mountains and Its Geological Significance

Zircon U-Pb dating was conducted for four granite bodies in Wulonggou area by LA-MC-ICPMS. The mylonitized monzogranite (sample B20-14) from the middle of Huanglonggou yielded an average zircon U-Pb age of (417.7±2.0) Ma, the mylonitized monzogranite (sample B21-2) from the upper part of Huanglonggou yielded an average zircon U-Pb age of (419.7±2.3) Ma, the middle-coarse grained monzogranite (sample B20-8) from Shenshuitan yielded an average zircon U-Pb age of (419.9±2.0) Ma, and the middle-coarse grained monzogranite (sample B25-9) from the middle of Hongqigou yielded an average zircon U-Pb age of (419.0±2.0) Ma. These ages should represent the formation epoch of the four granite bodies in Wulonggou area i.e., Late Silurian, and thus record magmatic activities of the Early Paleozoic period. It is held that granites of the late Silurian period in Wulonggou area might have been concerned with the collisional orogeny after the closure of the Early Paleozoic Ocean. The discovery of 1861 Ma and 1666 Ma zircons of the Palaeoproterozoic epoch in the granite shows that the metamorphic basement materials of the East Kulun orogenic belt should be of Palaeoproterozoic period.