羌塘盆地晚三叠世裂谷盆地演化——来自同裂谷期那底岗日组的证据

Late Triassic rifted evolution of Qiangtang basin:Evidence from the syn-rifting Nadigangri formation

在羌塘盆地剖面地质调查基础上,综合区域最新的油气调查及地质进展,采用LA-ICP-MS测试方法,提供锆石U-Pb测年,分析羌塘盆地晚三叠世同裂谷期那底岗日组岩性组合、分布特征、喷发—沉积时限、与下伏地层接触关系及裂陷机制。结果表明:那底岗日组分为两个岩相组合类型,一类是陆上喷发系列,以基性熔岩—酸性熔岩—火山碎屑岩为主;另一类是水下沉积系列,为沉火山角砾岩—沉凝灰岩—凝灰质砂岩—粉砂岩—泥岩等,具有典型的双峰式组合特征,与其中玄武岩地球化学特征显示的、与裂陷相关的构造背景一致。那底岗日组的分布受上三叠统顶部的裂陷基底控制,与之具有良好的匹配关系;样品年龄分别为(210.5±2.1)Ma和(214.9±1.8)Ma,属于晚三叠世诺利期;那底岗日组沉积超覆于下伏地层之上。羌塘盆地在晚三叠世之前完成南北羌塘、羌塘与北部大陆的拼合,在晚三叠世诺利期—瑞替期结束前陆盆地沉积,伴随南部班公湖—怒江洋再一次打开,开启新一轮裂谷盆地演化,形成同裂谷期那底岗日组火山—沉积序列。

Based on geological section observation,the latest progress in oil and gas geological investigation and comprehensive geological survey,in this paper,we present magmatic zircons' U-Pb ages of Nadigangri formation from Shengli river and Nadigangri mountain to deal with the lithological assembleges,distribution features,eruption-sedimentary range,contact with underlying strata and Mesozoic Qiangtang basin evolution.The results show that the Nadigangri formation can be divided into two types:The lithofacies subaerial series and underwater sedimentary series,including basic lavas-felsic lavas-pyroclastic rocks and volcanic breccia-tuffite-tuffaceous sandstone-siltstone-mudstone,respectively.In addition,the volcanic rocks of Nadigangri formation have a bimodal affinity that is typical of extension-related volcanism,which is consistent with the geochemical characteristics of the basalts.Furthermore,the exposure of Nadigangri formation matched well with the rifting basement of the upper Triassic strata.The analytical data and zircon U-Pb ages in the region define a limited age range from 220.0 to 201.0 Ma.There was a depositional hiatus between the Nadigangri formation and its underlying strata,which was different between the north and the south Qiangtang.Numerous studies show that the Qiangtang basin has completed the amalgamation between the North and the south Qiangtang and the northern continent.Qiangtang basin experienced a depositional transformation from foreland basin to rift basin evolu-tion during the late Triassic along with the opening of the Bangong-Nujiang Tethys ocean.