班公湖-怒江缝合带上侏罗统—下白垩统沙木罗组沉积构造背景

Discussion on tectonic setting of Upper Jurassic-Lower Cretaceous Shamuluo Formation along Bangong-Nujiang suture zone

班公湖-怒江洋闭合时限的不确定制约了对青藏高原晚中生代演化及班怒成矿带成矿地球动力学背景的认识。上侏罗统—下白垩统沙木罗组及其与下伏洋壳物质的沉积不整合是解决上述问题的关键。通过整理前人的研究资料,详细地总结了沙木罗组的年代学和沉积学特征,认为上侏罗统—下白垩统沙木罗组可分为下部晚侏罗世—早白垩世晚期(牛津期—阿普特期)的浅海陆棚-斜坡相沉积和上部早白垩世末期(阿尔必期)的河口-滨海相沉积。在此基础上,提出沙木罗组下部及其沉积不整合可能是班公湖-怒江洋北向俯冲时大洋岛弧与南羌塘地体碰撞的沉积响应,而沙木罗组上部形成于班公湖-怒江洋闭合及随后拉萨-羌塘软碰撞的背景。因此,班公湖-怒江洋的中西段应该至早白垩世阿尔必期才闭合。

The uncertainty of closure of the Bangong-Nujiang ocean has restricted the understanding of the late Mesozoic evolution of the Tibeten Plateau and the Metallogenic Geodynamic background of the Bangong-Nujiang metallogenic belt.The sedimentary unconformity between the Upper Jurassic-Lower Cretaceous Shamuluo Formation and the underlying oceanic crust materials is a key to solve the above problems.In this paper,the chronological and sedimentological characteristics of the Shamuluo Formation are summarized in detail by sorting out the previous research data.Then,we propose that the Upper Jurassic-Lower Cretaceous Shamuluo Formation can be divided into the Late Jurassic-late Early Cretaceous(Oxfordsian-Aptian)shallow marine shelf-slope facies strata and the end of Early Cretaceous(Albian)estuarine-coastal facies strata.On the basis,it is suggested that the lower part and its sedimentary unconformity probably represented arc-arc and arc-continent collisions along the northern margin of the Bangong-Nujiang ocean during the Late Jurassic-Early Cretaceous,while the upper part should be formed in the background of closure of Bangong-Nujiang ocean and subsequent Lhasa-Qiangtang initial soft collision.Therefore,we believe that the middle and western segments of Bangong-Nujiang ocean was not closed until the Albian.