This paper is concerned with study of tephriphonolites, rare rocks that have been first found in the pre-Jurassic basement of West Siberia. Based on geological observations of the bedding of different types of volcanics and their geochemical peculiarities, the conclusion about the synchronous occurrence of subalkaline and alkaline volcanism is drawn. This is confirmed by the coexistence of compositionally contrasting volcanics in the borehole section. The hypothesis is put forward that these rocks alternate in the section, as is the case in the Tunguska syneclise of the Siberian Platform. The new data on Sr and Nd isotopy suggest the participation of crustal material in the formation of alkaline rocks. The rift nature of the regional intraplate volcanism is confirmed.