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

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.