Three different series of Carboniferous-Permian volcanic rocks are reco- gnized in southwestern Yunnan of the eastern Tethyan orogenic belt, i. e. plateau tholeiitic, alkaline basaltic and MORB-like rocks. Geochemical studies support fihe understanding that the volcanic rocks are associated with the development of the Changning-Menglian rift basin do the eastern side of the Baoshan-Shan State terrane, and are unlikely to represent rocks produced in ocean floor and oceanic island envi:ronments.It is inferred that, with the thinning of the continental crust and the local emplacement of the oceanic crust, this region experienced progressive rifting. As a result of different de- grees of melting at different depths of the maatle, three different sorts of magma were formed and then evolved into three series of volcanic rocks. The closure of the lift basin was related to the westward subduction of the Lancang- jiang oc^an beneath the Baoshan-Shan State terrane,as is evidenced by the arc volcanic suite of the Lincang-Jiughon gebelt east of the Changning-Menglian belt. The corfvective mantle and the change in convection patterns are pre- sumed to have been responsible for the rifting, spreading of the initial coeanic crust and collision of terrance.