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.