New Rb-Sr isotope data on two stages of the Late Paleozoic granitoid magmatism are presented. These stages widely occurred on the territory of Southern Siberia, in particular, in Transbaikalia. They occurred within the time span between the completion of the Angara-Vitim batholith formation and the beginning of large-scaled high-alkali granitoid magmatism. Three granitoid plutons of the Zazinsky suite are studied (Ulekchin, Shaluta, and Baisa), as well as two plutons made up of syenites and granites of the Lower-Selenga suite (Ust'-Khilok and Nadeino). It is established that the Zazinsky suite is most likely of Late Carboniferous age (310-290 Ma, I-Sr=0.7050-0.7060), whereas the Lower-Selenga suite is of Early Permian age (280-260 Ma, I-Sr=0.7060-0.7064). The obtained Rb-Sr isotope data, along with the earlier results of oxygen isotope study, suggest that silicic magmas in both suites were produced from different sources but with a marked involvement of mantle-derived components. Nevertheless, the silicic melts cannot be regarded as direct derivates of basalt melts.
