The Cenozoic highK alkaline volcanic rocks are widely distributed in th e Nangqi an Basin，Qinghai province，east of the Tibetan Plateau．Most of the Volcani c rocks in the whole area are trachyandesite—trachydacite，with 43%～66% of SiO2，8%～10% of K2O+Na2O，belonging to the potassic—ultrapotassic series ．Based on a host of analyses，the authors concluded that the volcanic roc ks in the Nangqian Basin were derived from the same source，which might expe rience intense mixing and metasomatism before its formation．They are the product of crustmantle interaction，so closely related to the bulk silicate e arth components．