Permian-Triassic rifts of the West Siberian basin: evidence of voluminous felsic volcanic activity

Permian-Triassic rifts of the West Siberian basin compose one of the largest continental rift systems in the world. The Koltogor-Urengoy and Khudosey rifts of meridional strike are the main structures in the eastern part of the basin and are filled mainly by basaltic lavas with clastic sediments. However, in the central part of the West Siberian plate felsic lavas are widespread along with mafic volcanics. Here we present the detailed data on composition of lavas, whole-rock geochemistry, geophysical features and U-Pb ages from the Frolov-Krasnoleninsky region in the central part of the West Siberian basin.

Within the studied region, Permian-Triassic rifts of NW and NE strike are predominant. The main structure is Rogozhnikov-Nazym graben of NW strike, composed of rhyolite-dacitic lavas.  According to the seismic data, this volcanic area comprises multiple local eruptive centers (1-5 km in diameter). Lavas constitute the major part of the volcanic pile, while tuffs are subordinate (up to 15%). Deep boreholes did not reach the base of volcanic sequence, but its thickness exceed 0.5 km.

The main geochemical features of the Rogozhnikov-Nazym volcanics are: 1) acidic composition and increased alkali content; 2) signs of supra-subduction setting: Ta-Nb and Pb anomalies; 3) high ratios of all incompatible trace elements. According to these features, volcanic rocks of the Rogozhnikov-Nazym graben were formed in the setting of post-collisional extension. Furthermore, coeval felsic lavas are widespread in smaller structures of the Frolov-Krasnoleninskiy region and demonstrate similar geochemical characteristics.

We obtained 9 U-Pb (SHRIMP) ages from felsic lavas of the Rogozhnikov-Nazym graben and other rift structures. All samples yielded ages in the range from 254±2 to 248.2±1.3 Ma (Late Permian – Early Triassic). Thus, volcanic activity in the Frolov-Krasnoleninsky region was nearly synchronous to the main phase of Siberian Traps magmatism in the Siberian platform.

Volcanic rocks of the Frolov-Krasnoleninsky region constitute rifts of NW strike (mainly felsic lavas, including the Rogozhnikov-Nazym graben) and NE strike (mainly mafic lavas, geochemically similar to the Siberian Traps basalts). We suggest that orientation of rifts inherits two conjugate strike-slip fault systems, which mark the W-E compression during the preceding collisional event in the Early-Middle Permian, and the mechanism of extension is similar to pull-apart model. The contrasting composition of volcanics can be caused by different-depth zones of magma generation.

The Permian-Triassic volcanics are overlain by continental coal-bearing coarse-grained volcanoclastic sediments of the Chelyabinsk Group (Middle Triassic – Early Jurassic). These deposits fill the local depressions in the paleotopography. The Middle Jurassic clastic Tyumen Formation overlays both volcanic rocks and Chelyabinsk Group, covers almost the entire territory of the Frolov-Krasnoleninsky region and marks the initiation of post-rift subsidence in the West Siberian basin.