Revision of the Tanjianshan Group on the Northern Margin of the Qaidam Basin

The Tanjianshan Group distributed on the northern marg in of the Qaidam basin is an important volcanic-sedimentary for mation of the Early Paleozoic and composed of a suit of greenschist facies and strongly deformed rocks.Combined analyses of geological,palaeontological and geochronologic studies have indicated that the Tanjianshan Group was for medin the early-late Ordovician of 496～440 Ma.The revisedl it hologic and strat igraphic units for the group (from the lower part to upper part): the early-middle Or dovician volcanic-clastic formation,late Ordovician sedimentary formation,late Ordovician volcanic-subvolcanic formation.The previous C formation of the Tanjianshan Group (mauve graywakeconglomerate formation) only experienced weak phyllite-facies metamorphism and is unconformably overlain by the volcanic-subvolcanic formation.The pellite from the former gaverise a Rb-Sr isochron age of 313±1.0 Ma,which is interpreted to represent its age of metamorphism.Thus,it is thought that C formation was formed in the early Silurian and separated from the Tanjianshan Group.The new sequences of Tanjianshan Group also suggest that an open-closure cycle of intracontinental rift took place during the Ordovician in the region.