From arc-continent collision to ocean closure: Lower Cretaceous Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone, central Tibet

One of most hotly debated topics concerning the Late Mesozoic evolution of Tethyan and the Tibetan Plateau is the timing of the closure of the Meso-Tethys ocean,which is represented by the Bangong-Nujiang suture zone.The Upper Jurassic-Lower Cretaceous Shamuluo Formation,which unconformably overlies the older Mugagangri Group accretionary complex,provides important information on the closure of the Meso-Tethys Ocean.This paper precisely confines the depositional age of the Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone,extending it from the Late Jurassic to the Albian.Combined with the results of previous studies,we suggest that the Shamuluo Formation in the Awengco-Baerqiong region mainly contains a bathyal Berriasian-Hauterivian subunit and a shallow-marine Albian subunit.Provenance analysis indicates that the Berriasian-Hauterivian subunit was mainly derived from the Jurassic southern Qiangtang magmatic arc,while the Albian subunit was derived from the coeval volcanic rocks and the Upper Carboniferous-Upper Permian strata in the southern Qiangtang terrane.Thus,the two subunits of the Shamuluo Formation have significant distinct sedimentary facies and provenances,indicating that they were deposited in different tectonic settings.Based on the regional geological data,we suggest that the bathyal Berriasian-Hauterivian subunit and the shallow-marine Albian subunit of the Shamuluo Formation should be interpreted as a record of the oceanic arc-continent collision and the Lhasa-Qiangtang soft-collision,respectively.Thus,the closure time of the Meso-Tethys Ocean is at least limited to the Albian.