Abstract:The Donglingtai volcanic rocks exposed in Yanhecheng area, Western Hills of Beijing are composed of basaltic trachyandesite in the lowermost segment (1st member of Donglingtai Formation), rhyolitic pyroclastics in the lower portion (2nd member), and trachyte, rhyolite in the upper portion (3rd and 4th member). According to geochemistry of these rocks, the lowermost mafic rocks belong to alkaline series, and exhibit Coombs trend in total alkali vs. silica (TAS) diagram; however, the intermediate and acid rocks belong to highpotassic calcalkaline series. As a whole, the Donglingtai volcanic rocks exhibit straddle trend in TAS diagram, which evolves from alkalirich mafic rocks to silicasaturated intermediate and acid rocks. The mafic rocks are derived from fractional crystallization of primitive basaltic magma, which is enrichment of Ba, Sr, LREE, and K, under intermediate pressure; meanwhile, these mafic rocks had been assimilated by lower crust materials during petrogenetic process. The trachytes/dacites in upper Donglingtai Formation are divided into three categories: the aluminousrich potassic trachyte (KAT), and the aluminousrich sodic trachyte or dacite (NAT), and the aluminouspoor trachyte or dacite (PAT). The petrogenetic analysis shows that, KATs are produced by melting interaction between basaltic magma and middle crust material; and the fluidbearing melting of solidified basaltic magma generates NATs; however, PATs are originated from mafic rocks through fractional crystallization accompanying wallrock assimilation, i.e. AFC process. The acid volcanic rocks in the 2nd member of Donglngtai Formation are generated by lowdegree melting of lowpotassic rock of lower crust under relative high oxygen fugacity condition. Whereas the lowsilica endmembers of acid rocks in upper Donglingtai Formation are the products of melting interaction between basaltic magma and middle crust material, and the highsilica endmembers of acid rocks is derived from lowsilica endmembers by fractional crystallization. It is inferred that the largescale early—middle Early Cretaceous magmatism in Yanshan belt was originated from melting interaction between mantlederived magma and crustal rocks. Donglingtai volcanism is an exhibition of the crustal heating by intraplated basaltic magma in Yanshan region during middle Early Cretaceous epoch. The increased temperature and partial melting within the lower portion of crust triggered lateral flow of crust; eventually, the Early Cretaceous Yanshan changed from a high relief mountainous chain into a gentlyundulated plateau.