藏北阿木岗群、查桑群和鲁谷组放射虫的发现及有关问题讨论

通常人们一直认为阿木岗(戈木日)群是所谓“羌塘地块”的前寒武纪或前泥盆纪基底,曾发现古生物化石的“中泥盆统”查桑群和“下二叠统”鲁谷组等是其上的晚古生代盖层沉积。本次工作在阿木岗群和鲁谷组中分别发现中-晚三叠世放射虫化石组合;在查桑群中发现二叠纪中-晚期放射虫化石组合。结合野外实际考察和前人研究成果,本文就有关问题作了讨论。

DISCOVERY OF RADIOARIANS IN THE AMUGANG AND CHASANG GROUPS AND LUGU FORMATION IN NORTHERN TIBET AND SOME RELATED GEOLOGICAL PROBLEMS

The Chabug-Chasang region is the central part of the so-called Qiangtang Block, which is remote from the human society. The Amugang Group there is believed to be the Precambrian or pre-Devonian basement of the Qiangtang block, and the Chasang Group, Lugu Formation, etc. , to be the sedimentary cover on this block. The Chasang Group and Lugu Formation were assigned to the Middle Devonian and Lower Permian respectively based on the paleontologic data got before. Out radiolarian fossils were found in all the three stratigraphic units. Archaeospon-goprunum sp. , Triassocampe sp. , Yeharaia (?) sp. , Livarella (?) sp. Pseudostylosphaera sp. , and Plafkerium sp. were found in the Amugang Group. The age of this radiolarian assemblage is Middle-Upper Triassic. Abaillella (?) sp. , Follicuculus sp. , F. (?) sp. F. cf. scholasticus Ormiston et Babcock, Latentifistula (?) sp. , Latentifistulidae gen. et sp. indet. , Lacisus (?) sp. , Copicyntra sp. and Ruzhencevispongus (?) sp. were found in the Chasang Group. The age of this fossil assemblage is Middle-Upper Permian. Plafkerium (?) sp. , Sarta (?) sp. , Canop-tum sp. , C. (?) sp. , Triassocampe sp. , T. (?) sp. , Archaeospongoprunum sp. , Tripocyclia (?) sp. and Nasselaria gen. et sp. indet. were found in the Lugu Formation. This is a Middle-Upper Triassic radiolarian assemblage.On the basis of this new discovery and regional geological data, we believe that the so-called Amugang Group is virtually an accretionary complex formed by subduction of the pre-Jurassic (or pre-Middle Jurassic) Paleotethys oceanic crust beneath the south margin of the Tarim Plate, the Chasang and Lugu Formation represent exotic blocks of oceanic islands in the complex; while the Upper Permian Rejuechaka Formation, which is characterized by the occurrence of the Cathaysia flora and coal-bearing terrestrial deposits, is deposits on the accretionary wedge.