The Middle Ordovician submarine spilite-alkaline volcanic rock association in the Sbihuigou area in the Northern Qilian mountains was the product of island arc volcanism. Based on the studies of petrology, mineralogy, geocherr.istry of trace elements, magmatic inclusions in minerals and Sr isotope, the nature of the“ source" magma series of the spilite-alkaline volcanic rock association is determined and the equ- ence of the island arc volcanism is established in this paper. At first erupted a small amount of tholeiitic magma which was produced by the partial melting of the mantle wedge above subduction zone, then a lot of talc-alkaline magma. At the end of Middle Ordovician, the shoshonitic magma formed by a high pressure (1 GPa) fractional crystallization of calc-alkaline asalt magma in Moho-level charuber erupted, and resulted in the formation of shosho- nite lock association,Shoshonitic rocks in the Shihuigou area are stratigraphically highest and consist mainly of trachybasalt, leucite-tephrite, leucite-analcitite and leucite-trachyte porphyry. The development of the shoshonite rock association suggests that the palaeo-island arc system of the Northern Qilian mountains had reached to the rziature stage at the end of Middle Ordovician,