Abstract: Devonian volcanic rocks from Sangejing Formation in Yemaquan area in the south margin of Beishan orogenic belt form a unique bimodal volcanic rock assemblage in time and space, which is characterized by the interaction of basic volcanic rocks and acid volcanic rock.The U-Pb ages of zircons from basalts and rhyolites in the Sangejing Formation are 404.8±3.9 Ma and 402.3±1.8 Ma, respectively, which belong to the Early Devonian.The basalt is rich in aluminum and sodium, and is depleted in potassium.And it belongs to calc alkaline series.The geochemical characteristics of the volcanic rocks show that the basaltic mother magma in the study area came from the partial melting of the mantle, and a certain degree of crustal contamination occurs.The geochemical characteristics of basalts reflect that they formed in the intraplate extensional environment.The rhyolites in the study area are relatively rich in silicon and AKL, depleted in TiO₂ and MgO, and low in Al₂O₃.They are typical A-type granitoids.The rocks are the products of crystallization and differentiation of underplating basic magma in the post collision extensional environment.The determination of the bimodal volcanic rocks of the Sangejing Formation in the Early Devonian further confirmed that the Beishan orogenic belt was formed in the extension transformation period of collisional orogeny.The geodynamic background of its formation is the stage of transformation from collision to extensional tectonic environment, which indicated that from the Early Devonian the uplifted foreland of the mountain appeared local extension and subsidence to form the piedmont rift belt.It provides a new theoretical basis of the study of the Early Paleozoic structural pattern of Beishan area.