Cenozoic volcanic rocks are distributed in Songxi area of northwestern Qiangtang within northern Tibet, a marine volcanic rock association composed of andesite, dacite and rhyolite belongs to the calc-alkaline-high K calc-alkaline series. The rocks have high large-ion lithophile elements LIL and LREE, low HF SE, and relatively depleted Nb, Ta, Ti, reflecting features of terrigenous magma. SiO2 contents of basic volcanic rocks are less than 53%, suggesting that bas al tic andesite in Songxi area was not entirely produced by direct partial melting from the continental crust generation, and that the formation of basaltic andesite must have had the participation of a small amount of mantle materials. Eunegative anomalies are not obvious, implying that magma was derived from the thickening of the lower continental crust resulting from the collision and long-term squeeze between the Indian plate and the Eurasian plate, which led to the shortening and thickening of the continental crust in the northern part of Qinghai-Tibet plateau, including Qiangtang area. The northward subduction of the Lhasa block of continental lithosphere and the underplating of upwelling asthenosphere materials in the Qiangtang landmass triggered off the formation of the thickened lower crust through partial melting