Abstract: The Eastern Mongolian Volcanic Area (EMVA) is part of the Late Mesozoic–Early Cenozoic volcanic and plutonic belt in Northeastern Asia. The EMVA evolved in three stages, with volcanic rocks of different composition produced during each of the stages and with the parental melts of the rocks derived from different sources and formed by different mechanisms. The rocks of the Early Cretaceous stage (135–100 Ma), which form the volcanic flow complex of the EMVA, are predominantly differentiated alkali basaltoids. Data on isotopic features of these rocks, particularly their Pb isotope composition, allowed us to identify the nature of their sources: peridotites of the Continental Metasomatized Lithospheric Mantle (CMLM) and lower continental crustal eclogitic rocks. The alkali basaltoids of the extrusive complex of the Uldza-gol volcanic field were formed during the next evolution stage of the EMVA at 104–90 Ma. According to their geochemical and isotope features, the melts of these rocks were derived from the same sources as those of the volcanics of the previous Early Cretaceous stage, except only that eclogite material played a more significant role in forming of the Uldza-gol basaltoid melts. During the concluding stage of the EMVA evolution in the Late Cretaceous–Early Cenozoic (87–51 Ma), OIB-like rocks of the basanite–trachybasalt association were formed in the Central Gobi in the southwestern flank of the EMVA. Asthenospheric and recycled pyroxenite components, together with not so much CMLM peridotites, were involved in forming of these rocks. The various sources of the EMVA volcanic rocks reflect two mechanisms of their formation. In the Early to Late Cretaceous, magmatism was triggered by the ascent of the asthenospheric mantle and delamination of the lithospheric mantle, whereas the Early Cenozoic magmatism was induced by the activity of a mantle plume.