The significance of the research is determined by the lack of information on the composition of rocks of the gabbro-diorite intrusion and their role in continental crust formation and the age of manifestation of basic- intermediate magmatism on this area. Aim. The studies are aimed at the investigation of geological structure and description of the rock material composition and to determine the time frame of the development of the main magmatism within the Central Transbaikal region. Methods. The study employs complex structural-geological researches including explorative-surveying and geological-prospecting works carried out earlier. The analysis of the material composition of magmatic rocks is performed by means of modern petrological-geochemical and isotope-geochronological methods using the composition of rare and rare-earth elements, microprobe minerals identification as well as isotope (U-Pb) data. Results and conclusions. The intrusion consists of basic and intermediate rocks with a predominance of gabbroids formed as a result of fractional crystallization of the parent melt corresponding to high-alumina basalt. The Late Paleozoic gabbro-diorite rocks originated from the spinel-bearing lherzolitic mantle, which melted under «hydration conditions». Results for U–Pb analysis of magmatic zircon from a gabbro sample of the Khorul massif yielded and age of 337 ±3 Ma. It is assumed that the formation of intrusions is associated with the processes of intraplate magmatism at the post-orogenic stage with the participation of plume magmatism.