Geology and Mineralogical-Geochemical Characteristics of the Volcanic Rocks in the Tunguska Syneclise, Permo-Triassic Siberian Traps Province

The Siberian Traps have been a reference object for geologists for many years in understanding the origin of Large Igneous Provinces on Earth. Data from high-magnesium rocks in its northern region have traditionally been used to create genetic models. However, the central and southern parts, consisting of the tholeiitic basaltic rocks (platform basalts, or typical traps), remain significantly unexplored, despite their important role in volcanic evolution within the province. This article presents new structural, mineralogical, and geochemical data from the tuff-lava sequences in the central regions of the Tunguska syneclise, i.e., along the Lower Tunguska River Valley and near Vanavara settlement in the Podkamennaya Tunguska Valley. The studied sections include pyroclastic rocks of the Korvunchansky Formation and overlaying basalts and tuffs of the Nidymsky and Kochechumsky Formations (Early Triassic, 251 Ma). Major and trace element analyses of 64 samples show that basalts from these formations are similar in composition, with MgO contents ranging from 4.2% to 7.8%. Trace element patterns exhibit negative Ta-Nb anomalies and positive Pb anomalies, which are typical of the Siberian platform basalts. Tuffs, on the other hand, differ from the basalts by having higher SiO2 and K2O contents and steeper rare earth element (REE) patterns. Isotopic analysis (Sr-Nd) confirms the difference between basalts and tuffs and suggests their origin from two distinct magma sources. The Nidymsky basalts have positive ƐNd values (+2.21 to +3.10), similar to those of the Morongovsky and Mokulaevsky basalts in the Norilsk area. Tuffs, on the other hand, exhibit negative ƐNd (−1.33 to −4.5) and elevated 87Sr/86Sr (>0.706), in comparison with the basalts (0.704–0.706). Geological observations reveal that pyroclastic rocks play a significant role at the initial stage of volcanic activity. The main result of this study is that tuffs from the Korvunchansky and Khakanchansky Formations (in the south and north of the province, respectively) are identical in their geochemical characteristics. This similarity suggests the existence of a single pyroclastic cover, closed in composition to Lower crust, over the Siberian platform. These pyroclastic rocks mark the beginning of trap magmatism in the Siberian province, rather than picrites, as suggested by the plume model.