Abstract: Volcanic rocks of Fangshankou Formation are widely distributed in Hangwula area on the northern side of Engger Us ophiolitic belt in the Alxa area.These rocks are very essential for interpreting tectonic evolution of the southern part of Central Asian orogenic belt.Early Permian rocks in this stratum mainly consist of grayish green andesite and andesitic breccia lava, partly with agglomerate.The andesite has SiO₂ content of 54.23%~60.81%, Al₂O₃ content of 13.67%~15.96%, MgO content of 4.26%~5.76%, and Mg^# of 42~50 with the content of Na₂O(2.21%~3.48%)less than that of K₂O(2.51%~3.49%).These rocks also have high TFe₂O₃(7.98%~11.24%)and low CaO(2.52%~3.36%)as well as TiO₂(0.75%~1.32%).They should be peraluminous calc-alkaline rocks.Andesite samples are enriched in light rare earth elements(LREE)and depleted in heavy rare earth elements(HREE).The differentiation between LREE and HREE is not obvious with(La/Yb)_N equal to 3.73~9.86.Samples show negative Eu anomalies with δEu being 0.52~0.75.The chondrite-normalized REE patterns are similar to E-MORB.The samples of andesite are enriched in such elements as Rb and K and depleted in such elements as Nb, Ta and Ti.What's more, they have ε_Nd(t)=-4.3~-14.2 and ε_Hf(t)=-19.8~+1.4.These characteristics suggest that andesites were sourced from mixing of the mantle and the crust and formed in intraplate rift.Considering a lot of geological materials related to this area, the authors conclude that the branch of the PAO(Paleo-Asian Ocean)represented by Engger Us ophiolitic belt had been closed before Late Carboniferous, and that, during Late Carboniferous to Early Permian, magmatic activities in the study area occurred in an intraplate extensional setting.