Isotopic geochronology of the volcanic rocks from the Yeba Formation in the Gangdise zone,Xizang

The Yeba Formation in the Dagze region,Xizang is built up of slightly metamorphosed basalt,dacite and acidic tuff intercalated with purplish red sandstone.The dacite gives a SHRIMP U-Pb zircon age of 181.7±5.2 Ma,which may represent the age of the Yeba Formation.The examination of geochronology and rock associations suggests the subdivision of the Yeba Formation into two parts:upper and lower.The volcanic rocks in the Dagze region may be traced back to the Early Jurassic.The acidic volcanic rocks intercalated with clastic rocks and carbonate rocks from the Yeba Formation intermediate between Gongbo'gyamda,Sangri and Maizhokunggar are Middle and Late Jurassic in age.The volcanic rocks in the Yeba Formation may be considered as the products originated from southward subduction and consumption of the Tethyan Ocean in the Bangong-Nujiang suture zone.The SHRIMP U-Pb zircon age of 131.9±5.5 Ma for the metamorphic rocks represents that of the metamorphism event during the Early Cretaceous,which may largely be the expressions of superimposition and reworking of early geological records caused by intense magmatism in the Gangdise zone during the Yanshanian.