西藏班公湖地区竟柱山组时代及其构造意义

上白垩统竟柱山组呈近EW向分布于班公湖–怒江缝合带内,该组以陆相磨拉石建造为特征,角度不整合在蛇绿岩及老的海相地层之上,从早到晚由河流相向湖泊相演化。本文以班公湖–怒江缝合带西段的班公湖地区出露的竟柱山组为主要研究对象,对其岩性特征、沉积环境及形成时代进行了分析,认为竟柱山组为班公湖–怒江特提斯洋全面闭合后的陆相山间盆地沉积,是洋陆转换全面完成之后的陆相沉积。本文首次对班公湖地区竟柱山组进行了ESR年代学、磁性地层学研究,得出了研究区竟柱山组底部砾岩的ESR年龄为92.0±9.0 Ma,古地磁测年显示该组的底界年龄约为96 Ma。班公湖地区在96 Ma左右全面完成了由洋到陆的转换,进入了陆内环境。

Age and Tectonic Significance of Jingzhushan Formation in Bangong Lake Area,Tibet

The Upper Cretaceous Jingzhushan Formation distributes along the east-west Bangong Lake-Nujiang suture. It consists mainly of continental molasse formation overling unconformably the ophiolites and marine strata. The sedimentary features changed from early fluvial to later lacustrine facies. This paper focus on the Upper Cretaceous Jingzhushan Formation in the Bangong Lake-Nujiang suture zone, and the petrology, sedimentary environment and age of the sequences were analyzed. It is suggested that the Jingzhushan Formation was deposited in a continental intermontane sedimentary basin that formed subsequent to the closure of the Bangong Lake-Nujiang Tethys Sea and evolved into a continental sedimentary basin when the ocean-continent transition finished. It is the first time to research the ESR chronology and magnetostratigraphy on the Jingzhushan Formation in the Bangong lake area. The ESR results indicate that the age of the lower part of the Jingzhushan Formation is about 92.0±9.0 Ma, and the magnetostratigraphical data show that the bottom age is about 96 Ma. It’s concluded that in the Bangong Lake area the ocean-continent transition ended and evolved into intracontinental environment in 96 Ma.