青藏高原羌塘中部中—上奥陶统达瓦山组的建立及意义

首次在藏北羌塘中部达瓦山地区识别出一套复理石沉积,厚度大于3000m,未见顶底,岩石组合以变质砂岩、杂砂岩和千枚岩为主,夹双峰式火山岩,火山岩岩性为流纹岩和玄武岩。用LA-ICP-MS同位素测定技术对流纹岩夹层中的锆石进行了U-Pb同位素测定,获得锆石的年龄为454.0±2.0Ma和458.3±1.8Ma,时代为中晚奥陶世达瑞威尔—桑比期,代表了达瓦山组的主体形成时代。在剖面测制和区域对比的基础上建立了中—上奥陶统达瓦山组,代表冈瓦纳大陆北缘中—晚奥陶世裂谷型深水-半深水复理石沉积。中—上奥陶统达瓦山组的发现和建立完善了该区中晚奥陶世的地层系统,进一步确定该区中—晚奥陶世洋盆的存在,对冈瓦纳大陆北缘中—晚奥陶世岩相古地理研究具有重要意义。

Establishment of Middle-Upper Ordovician Dawashan Formation in central Qiangtang, Tibetan Plateau, and its significance

In this paper, a set of flysch sediments were firstly discovered in central Qiangtang, northern Tibet. Their thickness is more than 3000 m, and the top and bottom were not observed. Rock association consists of metamorphic sandstone, greywacke and phyllite, intercalated with bimodel volcanic rocks. The main lithology of the volcanic rocks includes rhyolite and basalt. The results of zircon LA-ICP-MS U-Pb dating indicate that the rhyolite rocks formed at 454Ma and 458Ma, indicating Middle-Late Ordovician Darriwilian-Sandbian age, which represents the formation age of Dawanshan Formation. The Middle-Upper Ordovician Dawashan Formation was established based on the section measurement and regional correlation, which shows that the abyssal-bathyal flysch was formed in the rift of northern Gondwana continent margin in the Middle-Late Ordovician. The establishment of Dawashan Formation contributes to the improvement of Ordovician stratigraphic system and also confirms the existence of Middle-Late Ordovician Ocean in this area. The results obtained by the authors are of great importance for the study of the Middle-Late Ordovician lithofacies paleogeograghy on the northern margin of Gondwana continent.