东昆仑大干沟火山岩SHRIMP锆石U-Pb测年及其地质意义

摘要

The Maoniushan Formation of molasse, formed in a post-orogenic environment in Dagangou area of the Eastern Kunlun Mountains, rests unconformably on pre-Devonian strata. Its formation time is critical for determining the terminal of the Caledonian orogeny and the initiation of splitting in the Late Paleozoic. The authors conducted zircon U-Pb dating for volcanic rocks of the Up⁃per Maoniushan Formation by SHRIMP. The samples (DG01 and DG02) of the Upper Maoniushan Formation yielded two average zircon U-Pb ages of 403.6±5.1Ma and 409±3.4Ma. Combined with the previous studies, the formation time of the Maoniushan For⁃mation should be Late Silurian—Early Devonian. This provides a constraint on the starting time of splitting in the Late Paleozoic and also means that the Caledonian orogeny ended before the Late Silurian—Early Devonian.%东昆仑大干沟以北地区造山后形成的牦牛山组磨拉石建造不整合覆盖于前泥盆系之上，其形成时代对限定柴达木盆地加里东造山作用的结束时间及晚古生代裂解作用的开始时间具有重要意义。应用单颗粒锆石离子探针质谱仪（SHRIMP）方法，对牦牛山组上部火山岩段的样品进行精确的锆石U-Pb测年。结果表明，牦牛山组上段火山岩样品DG01和DG02中的岩浆锆石206Pb/238U平均年龄分别为403.6±5.1Ma和409±3.4Ma。综合前人研究认为，造山后牦牛山组伸展型磨拉石的形成时代为晚志留世—早泥盆世，限定了晚古生代裂解的开始时间，也意味着早古生代加里东造山运动在晚志留世—早泥盆世之前已经结束。