We provide new isotope–geochronological evidence for the synchronous occurrence of Late Paleozoic basic and granitoid magmatism in western Transbaikalia; this is a strong argument for the contribution of mantle magmas to granitoid petrogenesis. The Late Paleozoic basic rocks originated from the phlogopite–garnet-bearing lherzolitic mantle, which melted under “hydration conditions.” The specific features of Late Paleozoic magmatism in western Transbaikalia were determined by the combination of the activity of a low-energy mantle plume with the final stage of the Hercynian orogeny in space and time. At the early stage of magmatism, during the formation of the Barguzin granites, the plume had only a thermal influence on the crustal rocks heated as a result of Hercynian fold–thrust deformations. The mixing of mantle basic and crustal salic magmas at different levels marked the transition from crustal to mixed (mantle–crustal) granites, which include all post-Barguzin complexes (probably, except for alkali granites). In the geologic evolution of Transbaikalia, the Late Paleozoic magmatism was postorogenic, but it was initiated and influenced by the mantle plume.