Abstract: Voluminous volcanic rocks of the Tiaojishan Formation outcrop in the Western Hills of western Beijing, which are unconformably overlain by the volcanics of the Donglingtai Formation. These volcanic rocks vary significantly in chemical compositions, with SiO2=51.94~77.30%，Al2O3=12.85~19.17%，Na2O=1.65 ~5.82%，K2O=0.83~4.52%，Fe2O3=0.95~9.30%，and CaO=0.13~7.08%. Silica contents show approximately linear covariations with other oxides. These volcanic rocks are characterized by highly enriched LREE and depleted HREE, and minor Eu anomalies in the chondride-normalized REE patterns. Meanwhile, they show high abundances of Sr and Ba and high Sr/Y and La/Yb. The volcanic rocks have varied and enriched Nd isotopic compositions (εNd = -11~-17), which plot intermediately between the field of the ancient continental crust and the field of the enriched subcontinental lithospheric mantle. These geochemical signatures indicate that the volcanic rocks of the Tiaojishan and Donglingtai Formations were originated from a process of hybridization between the enriched mantle-derived mafic magma and the lower continental crust.