Ordovician marine alkaline volcanic rocks from the west part of the north Qili an M ountains belong to the shoshonitic series varying from basic to intermediatea cid and consis t ing of potassic trachybasalt, shoshonite, latite, trachyte and benmoreite. Chemi cally, they have both the FeO*MgO evolution trend of the calcalkaline rock series and a low FeO* content, high Fe2O3/FeO and K2O/Na2O ratios, with varin g Al2O3 contents. Studies on REE and trace element geochemistry show that they occurred in an isl and arc tectonic environment, which symbolizes the matured arc volcanism in the trench  a rcbasin system of the north Qilian Mountains in the Ordovician.