The Nangqen Basin is one of the Tertiary pull-apart basins situated at the east of the Qiangtang block. Similar to the adjacent Dingqing and Bashu Basins, there occurred a series of potassic volcanic and sub-volcanic rocks ranging from basic, intermediate to intermediate-acid. Based on a study of petrology, mineralogy and geochemistry including REE, trace elements, isotopic elements and chronology, we suggested that the Cenozoic potassic volcanic rocks in the Nangqen basin were formed in a post-collisional intra-plate tectonic setting. The geochemical variability of the basic, intermediate and intermediate acid rocks may be mainly attributable to the different partial melting degrees of the mantle sources formed at about 50～80 km depth, instead of a relation of magmatic differentiation or evolution. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixture of components mainly derived from the crust. The partial melting of the sources of the volcanic rocks in some pull-apart basins in the Hengduan Mts. area may be related to the underplating of the small-scattered magma upwelling derived from the asthenosphere. The tectonic movements of the large-scale strike-slip faults provided the channel-ways for the melts ascending.