Palygorskite Mineralization in the Induan Sediments of the Moscow Syneclise as an Effect of Regional and Global Paleogeographic Changes at the Permian–Triassic Boundary

Detailed study of the Permian–Triassic continental deposits of the Moscow syneclise has shown the widespread occurrence of palygorskite mineralization in the Early Triassic (Induan) in the southeastern (basin of the Vetluga River), in the northeastern (basin of the Sukhona–Yug–Severnaya Dvina rivers), and in the western parts (Gavrilov-Yam borehole, NPO Nedra, city of Yaroslavl). Palygorskite is found in almost all types of Induan sediments where, together with smectites, it is one of the main clay minerals; its highest concentrations are recorded in paleosols. The morphology of aggregates and the nature of their relationships with other sedimentary components indicate an authigenic origin of palygorskite. The presence of high-magnesian minerals (serpentine, Mg-chlorite, talc–chlorite), which compose the fragments of meta-ultrabasites, is associated with the palygorskite mineralization. The values of the Cr/Ni ratio (1.3–1.5) in the Induan rocks suggest active erosion of the ultrabasic Ural complexes at that time. In the Permian–Triassic transition, the ratio of the alkali and alkaline-earth elements to aluminum (Σbases/Al) increases and the CALMAG weathering index decreases in the continental sediments, which is consistent with the global climatic trend of warming and aridization at the Permian–Triassic boundary. The widespread occurrence of palygorskite in the Early Triassic is caused by increased delivery of chemically unstable high-magnesian detrital material of ultrabasic rocks and products of their early metamorphism from the Urals, as well as its specific treatment (low mobility and a high degree of accumulation of leached components) by soil pore waters in the continental environments with an arid climate. Since palygorskite is absent in the Upper Permian sediments of the Moscow syneclise, it can be used for stratigraphic purposes, viz., to distinguish the Upper Permian and Lower Triassic continental rocks (and the cycles they form) within the boundary interval.