Evidence for the Voluminous Silicic Volcanic Activity in the Permian–Triassic West Siberia

Permian–Triassic rifts of the West Siberian basin compose one of the largest continental rift systems in the world. They are nearly coeval to the Siberian Traps Large Igneous Province and mark the final stage of the amalgamation of the Northern Eurasia. Presence of acidic volcanic rocks along the mafic lavas is a specific feature of the West Siberian region. However, at this moment, felsic volcanics of this area are poorly studied. Here we present the first representative data on petrography, geochemistry, and U–Pb ages for silicic volcanic rocks from the drill cores of the deep boreholes in the Frolov-Krasnoleninsky region (the central part of the West Siberian basin). The largest rift structure of this area is Rogozhnikov-Nazym graben, composed of rhyolite-dacitic lavas. Lavas constitute the major part of the volcanic pile, while tuffs are subordinate (up to 15%). According to the seismic and well-logging data, the thickness of felsic volcanic rocks exceeds 0.7 km. Whole-rock chemistry of the silicic volcanic rocks of the Rogozhnikov-Nazym graben and other smaller rifts is very similar and corresponds to post-collisional extensional tectonic setting. Zircons from rhyolites and dacites yielded U–Pb (SHRIMP-II) ages ranging from 254 ± 2 to 248.2 ± 1.3 Ma (Late Permian–Early Triassic). Thus, volcanic activity in the Frolov-Krasnoleninsky region was nearly coeval to the main phase of the Siberian Traps emplacement in the Siberian platform.