Origin of Permian exotic limestone blocks in the Yarlung Zangbo Suture Zone, Southern Tibet, China: With biostratigraphic, sedimentary and regional geological constraints

Abstract

The Yarlung Zangbo Suture Zone in Southern Tibet represents the collision zone between the Indian and Eurasian continents. It is, to a lesser extent, also the boundary between the Himalayan region and the Lhasa Block. A fascinating aspect of it is the appearance of Permian limestone blocks in Late Triassic (and sometimes Late Jurassic–Cretaceous) siliciclastic sediments south of the ophiolite belt. They are meter- to kilometer-scale, randomly scattered blocks in a clastic matrix. Large blocks often form ridges of mountains or occur as projecting rocks; small blocks are often seen half buried or floating within matrix. They are therefore often noted as exotic blocks.Fossils from the limestone blocks have ages mainly in the range of Guadalupian–Lopingian. The faunas have mixed Gondwanan and Cathaysian features. Petrographic study of the limestone blocks shows that their deposition processes were often influenced by terrigenous clasts and suggests that they were deposited in a generally proximal environment. The age range, lithofacies and fossil content of these limestone blocks in the Yarlung Zangbo Suture Zone are highly compatible with those of the Permian successions north and south of the zone in the framework of northern Greater India. They were originally deposited in peri-Gondwana environment, like the Permian limestone successions of the Lhasa Block and the Himalayan region.The exotic Permian limestone blocks are dismembered units of Permian carbonate successions broken during the rifting process in the Late Triassic. Their entering in the Late Triassic clastic sediments is resulted from either block faulting or olistostrome. Final emplacement of different components, including the limestone blocks, in the suture zone were achieved in the Paleogene accompanied by strong deformation.