Geochemistry of Upper Vendian and Lower Cambrian Clay Rocks in the Moscow Syneclise: Some Traditional and Modern Approaches

The article discusses some geochemical characteristics of Upper Vendian and Lower Cambrian (hereafter, Upper Vendian–Lower Cambrian) clay rocks that make up the base of the Moscow syneclise. It is shown that there was no significant change in the clay rock composition during several tens of million years of the geological history under consideration. Based on the typical Zr/Sc and Th/Sc values, the petrogenic nature of the fine-grained aluminosiliciclastics of mudstones and mudstone-type clays is substantiated. This conclusion is also confirmed by the CIA/WIP values. In general, according to some parameters of their composition, Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are closer to granitoids; according to other parameters, to basic igneous rocks. Thus, compared with the average Late Proterozoic basalts, the clay rocks are markedly enriched in K₂O and Rb, Th, Zr, Hf, Nb, and Ta, whereas the average Archean granitoids have notably lower concentrations of TiO₂, FeO, MgO, Sc, V, Cr, Co, and Ni. Positions of data points of the Upper Vendian–Lower Cambrian clay rocks on the La/Sc–Th/Co, La/Th–Th/Yb, Sc–Th/Sc, and other diagrams confirm the above statement. The chondrite-normalized lanthanide distribution in clay rocks is close to the PAAS lanthanide spectrum. It is suggested that the suspended material was transported to the sedimentation area by: (1) large rivers with source areas composed of rock complexes of different composition; (2) rivers that drained provenances composed mainly of sedimentary rocks. The average CIA values inherent in the Upper Vendian–Lower Cambrian clay rocks of the Moscow syneclise are comparable to those typical for the suspended particulate matter (SPM) in modern large rivers of the humid subtropical and tropical climate, as well as in rivers of dry tropical climate regions. Localization of data points of mudstones and mudstone-type clays on the SiO₂–(Na₂O + K₂O + MgO + CaO), Al₂O₃–(Na₂O + K₂O + MgO + CaO), and CIA–WIP diagrams suggests that the main characteristics of their composition was governed by the paleoclimate. The series of α-coefficients characteristic of the Upper Vendian–Lower Cambrian clay rocks in the Moscow syneclise is quite similar to those for the fine-grained SPM of modern large river systems in southern Africa.