Cenozoic ultrapotassic volcanic rocks which are widely distributed in Lhasa block of southern Qinghai-Tibetan Plateau were mainly erupted between 25 Ma and 10 Ma. These ultrapotassic volcanic rocks have typical lamproite characteristics such as high SiO2, MgO, K2O and TiO2, low Al2O3 and Na2O, and high K2O/Na2O ratios. Their high abundances of incompatible trace elements like large ion lithophile elements (LILE) and light rare earth elements (LREE) also reach the extreme levels typical of lamproites. The lamproites show high radiogenic 87Sr/86Sr and low unradiogenic 143Nd/144Nd, low 206Pb/204Pb, and elevated radiogenic 207Pb/204Pb and 208Pb/204Pb ratios. Geochemical and Sr-Nd-Pb isotopic characteristics of these ultrapotassic volcanic rocks in Lhasa block suggest that they were probably derived from a phlogopite-bearing enriched mantle source related closely to metasomatism during earlier subduction events. The ultrapotassic rocks in southern Tibet may imply the uplift and extension of Tibetan plateau after the collision of Indian plate and Eurasian plate. Meanwhile, there are a number of N-S grabens (rifts) mostly formed in 23～8 Ma in Lhasa block, which might have cut the lower crust or lithosphere into the depth, suggesting the extension of southern Tibet in Miocene. Furthermore, these ultrapotassic volcanic rocks are coincident with N-S grabens (rifts) in space and time. In addition, the tectonic activities in western Lhasa block (such as Karakorum fault) in 17 Ma and in eastern Lhasa block (such as Jiali fault and Red River-Ailao Mountain fault zone) in 23～17 Ma imply that at about 20±3 Ma the tectonic stress field in southern and southern-eastern parts of Qinghai-Tibetan Plateau underwent a considerable change from the NS-trending compression to the EW-trending extension. Combined with contemporary adakitic rocks along the Yarlung Zangbo suture, the authors hold that the northward subducted Indian lithospheric mantle might have experienced break-off beneath the Lhasa block along the Yarlung Zangbo suture in early Miocene, which caused the upward and lateral migration of the asthenosphere material under the Indian lithospheric mantle along the slab window. The upward and lateral migration of the asthenosphere material led to the rapid uplift of southern Qinghai-Tibetan Plateau as well as the eastward movement of the thickened crust, which further resulted in the EW-trending extensional deformation (23～8 Ma) of the widely distributed N-S grabens (rifts) and the eruption of the ultrapotassic magma(25～10 Ma).