The Late Permian Foraminifers In The Tsochen And Xainza Area,Lhasa Block,Tibet And Their Paleobiogeographical Implications

The Qinghai-Tibet Plateau,consisting of multiple blocks,has a complicated geological history.The different geographical locations of these blocks resulted in different fauna during the geological history because of the presence of pronounced thermal gradients.The Permian faunas and their paleobiogeographic affinities have been proved to be the reliable data to recognize the paleogeographic position for many Tibetan blocks.The Upper Permian strata of Lhasa Block have been previously considered as terrestrial strata with plants fossils.Consequently,it was believed that the Lhasa Block had uplifted above sea-level during the Late Permian.However,the Lopingian marine strata have been reported in Lhasa Block in recent years,which challenged the traditional viewpoint.However,the taxonomy and palaeobiogeography of these Late Permian marine faunas remain unclear.This study selects two sections to focus on the taxonomy and the palaeobiogeography of Late Permian foraminifers in the Lhasa Block.They are the Mujiucuo section at the Xainza area and the Aduogabu 2 section at the Tsochen area.Totally,62 species in 33 genera are recognized in the Mujiucuo section,and 26 species in 18 genera in the Aduogabu 2 section.The Mujiucuo foraminiferal fauna mainly consists of Reichelina,Codonofusiella,Dunbarula,Colaniella,Pachyphloia,Globivalvulina,Agathammina,Hemigordius,Midiella and Multidiscus,indicating a Wuchiapingian age.The Aduogabu fauna is mainly composed of Colaniella,Reichelina,Sphaerulina,Neodiscus,,Langella and Geinitzina,indicating a Changhsingian age.The most typical feature of this fauna is the absence of Palaeofusulina.More importantly,no Palaeofusulina fauna has been reported in the whole Lhasa Block and such is the case for those Gondwanan blocks.But the Palaeofusulina species has been reported in the west of South Qiangtang Block.It suggests that the South Qiangtang Block would have drifted northward to a low latitude region during the Changhsingian,but the Lhasa Block may still locate at a relatively southern position.This indicates that theBangong-Nujiang Ocean has a considerable width at that time,which is consistent with the view that the Bangong-Nujiang Ocean opened before the Middle-Late Permian.On the other hand,the Permian sedimentary sequence and faunas of the Lhasa Block exhibits a great difference to the South Qiangtang Block and the Tethys Himalaya region.Therefore,it is considered that the Lhasa Block was an independent block between the Bangong-Nujiang Ocean and the Neotethys Ocean during the Changhsingian.