Abstract: The Wusu volcanics,west of the Ailaoshan Suture Zone,are located in the Simao Massif.They are composed of basalt,pillow lava,dacite,rhyolite and tuff,about 800m thick,occurring in the early Carboniferous sequence.As compared to N-MORB,the Wusu basalt has a higher content of TiO₂ and REE,and higher Ti/V and Zr/Y ratios characteristic of E-MORB.The basalts are LREE-enriched,with La_N/Yb_N=1.9-2.4,Yb_N=11-23.The rhyolites are higher in REE than the basalts,but with La_N/Yb_N=2.0-2.8.This shows that both of them may have the same origin,i.e. the bi-modal type.Such composition of the Wusu basalt indicates an extension environment rather than an island arc environment.The higher Th/Ta ratios (1.4-4.0) may imply a hibridization with the continental crust during its rise to the surface.