Results of isotope Sr, Ns, and O analyses of volcanic rocks from the Uda sector of the West Transbaikal Rift Zone have allowed estimation of the character of interaction of their parental mantle melts with crustal rocks. The smallest magnitude of this interaction has been found in the compositions of Late Cretaceous (83–70 Ma) volcanics, the geochemical and isotope markers of which suggest their derivation from a moderately enriched mantle compositionally resembling OIB sources. The Early Cretaceous volcanics were derived from mantle sources that included a mantle enriched by subduction. While ascending through the crust, the parental melts of the Uda Complex (130–111 Ma) were contaminated by the lower crust matter. The Zazin Complex magmas (143–135 Ma) have features suggesting their interaction with upper crustal granitoids of the Angara–Vitim Batholith.