【摘要】 柴达木盆地北缘牦牛山组为一套由砂砾岩组成的陆相紫红色粗碎屑沉积岩,其时代一直存在争议。本文对两个剖面牦牛山组的沉积特征、古水流、砾石成分进行了研究,并对砂质充填物中锆石进行了LA-ICP-MS测年分析。结果表明,牦牛山组发育冲击扇沉积,古流向主要为自北西至南东和自南东至北西两个方向。两个剖面砾石成分差异显著,城墙沟剖面砾石成分以碳酸盐岩为主,而旺尕秀剖面砾石成分则较为复杂。砂质充填物中碎屑锆石U-Pb年龄可分为3个组:360～560Ma、890～1050Ma和2200～2500Ma。它们代表本区3次构造事件,包括早古生代柴达木盆地北缘由洋-陆至陆-陆的碰撞过程,新元古代Grenville造山事件及新太古代陆壳的增长。其中最小的碎屑锆石U-Pb年龄为365±3Ma,结合区域地质背景与古生物等资料,表明牦牛山组的时代为晚泥盆世。本区早古生代第三期和第四期花岗岩类的锆石U-Pb年龄与本文最年轻的一组锆石U-Pb年龄在误差范围内一致,表明这两期岩体发生了快速抬升剥蚀。结合前人研究,柴达木盆地北缘在牦牛山组沉积期处于后碰撞阶段。更多还原

【Abstract】 The Maoniushan Formation in the northern margin of the Qaidam Basin consists mainly of purple red terrestrial sand and conglomerate, but its depositional age has long been controversial. This study has carried out an investigation for the depositional characteristics, paleocurrent direction and composition of gravels and LA-ICP-MS dating. The results show that the Maoniushan Formation is characterized by alluvial deposit, with the main paleocurrent directions from northwest to southeast and from southeast to northwest. The composition of gravels is quite distinct in two profiles, with the Chengqianggou profile dominated by carbonate and the Wanggaxiu profile being quite complicated. The youngest U-Pb ages of detrital zircons in sandy fillings of conglomerate can be divided into three populations: 360～560 Ma, 890～1050 Ma and 220～2500 Ma. They represent three tectonic events, including ocean-continent subduction to continent-continent collision of the North Qaidam in the Paleozoic, Grenville collisional orogeny in the Neoproterozoic and continent growth in the Late Archean. The youngest U-Pb age of detrital zircon is 365±3 Ma. Combined with regional background and paleontological data, we suggest that the depositional age of the Maoniushan Formation is Late Devonian. The U-Pb ages of the third and forth granitoids are consistent with the youngest U-Pb age of our detrital zircons, indicating the two episodes of granites have undergone rapid exhumation. Combined with previous studies, we suggest that the North Qaidam had evolved into the post-collision stage during the deposition of the Maoniushan Formation.更多还原