Complex petrological, geochemical, and isotope studies of igneous rocks sampled from the core of parametric Maizasskaya BH-1 showed a predominance of dolerite sills, which formed earlier (~263 ± 4 Ma) than most of basalts in the basement of the West Siberian sedimentary basin and in the Siberian Platform traps (248–251 Ma). Their formation took place during the crystallization of basaltic melt in intrusive chambers existing between layers of Silurian sedimentary rocks. The petrochemical, geochemical, mineralogical, and thermobarogeochemical data show that the sills resulted from the activity of complex magmatic systems different from typical oceanic and plateau-basalt melts and related, most likely, to the formation of rift structures under the influence of mantle plume. Study of melt inclusions provided data on the conditions of generation of primary melts from mantle substratum (≤1570 ºC, depths to 105–120 km) and crystallization parameters of dolerites—1130–1155 ºC, 1.5–2 kbar. The results obtained show that the studied basalt complexes in West Siberia are genetically related to the mantle plume activity, which led to the breakup of ancient crust and rifting. Formation of oceanic crust took place in the largest rifts; the ascending magma penetrated into the enclosing ancient strata to form sills.