Lower Cisuralian brachiopod faunas from the Lhasa and South Qiangtang blocks in Tibet and their biostratigraphical and palaeobiogeographical implications

The opening time of the Bangong-Nujiang Ocean (BNO, also known as the Meso-Tethys Ocean) remains controversial. It was documented to have opened during the Permian, Early Palaeozoic or Mesozoic times. The Permian faunas and their palaeobiogeographical affinities of the Lhasa and South Qiangtang blocks bordered by the Bangong-Nujiang Suture Zone are crucial for reconstructing the tectonic evolution of both blocks and determining the opening time of the BNO. This paper describes two new lower Cisuralian brachiopod faunas from the uppermost Yunzhug Formation of the Lhasa Block and the Zhanjin Formation of the South Qiangtang Block. These two faunas exhibit a high similarity in composition, both containing species of Bandoproductus, Spirelytha and Sulciplica thailandica. Comparable brachiopod faunas are found from the glaciomarine diamictites in the blocks of the eastern Cimmerian belt including the Baoshan, Irrawaddy and Sibuma, as well as the Tethys Himalaya and the Gondwanaland. They are herein named the Bandoproductus-Spirelytha association, which exhibits apparent Gondwanan affinities. Cluster analysis and principal components analysis are employed to conduct a palaeobiogeographical reconstruction of the Asselian–Sakmarian brachiopod faunas primarily from the aforementioned tectonic units. A single palaeobiogeographical unit, the Indoralian Province, is recognised and represented by the unified brachiopod association. This suggests that most of the Cimmerian blocks, including the Lhasa and South Qiangtang blocks, were attached to the northern Gondwanan margin during the early Cisuralian. This is in contrast to our previous results revealed by multiple quantitative analyses for the Kungurian–Roadian brachiopod faunas in the Tethys, which show distinct faunal differences between the Lhasa and South Qiangtang blocks. This palaeobiogeographical discrepancy suggests that the two blocks probably had varying northward drifting tempos after they detached from the Gondwanaland, that is, the South Qiangtang Block drifted faster and incorporated into the temperate-warm region of the Northern Cimmerian Subprovince; whereas the Lhasa Block drifted relatively slowly and retained to be close to the northern Gondwanan margin and belonged to the Southern Cimmerian Subprovince during the late Cisuralian. The faunal differences provided critical implications that the BNO probably had opened in the Artinskian and reached a certain width with distinct faunal differences during the Kungurian.