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

Abstract

The article discusses some geochemical characteristics of Upper Vendian and Lower Cambrian clay rocks that form the base of the Moscow syneclise. It is shown that during the several tens of millions of years of geological history we are considering, there was no significant change in the composition of clay rocks. Based on the Zr/Sc and Th/Sc values characteristic of argillites and argillite-like clays, the petrogenic nature of the fine-grained aluminosiliciclastics composing them is substantiated. This conclusion is also confirmed by the values of the CIA/WIP ratio. In general, according to some parameters of their composition, the Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are closer to granitoids, according to others, to basic igneous rocks. Thus, compared with the average Late Proterozoic basalts, they contain noticeably more K2O and Rb, Th, Zr, Hf, Nb, and Ta, while the average Archean granitoids have noticeably lower concentrations of TiO2, FeO, and MgO, Sc, V, Cr, Co, and Ni. The position of the data points for Upper Vendian and Lower Cambrian clay rocks on the La/Sc–Th/Co, La/Th–Th/Yb, Sc–Th/Sc and other plots confirms what has been said. Chondrite-normalized lanthanide distribution spectra in clay rocks are close to the PAAS lanthanide spectrum. It has been suggested that the transportation of suspended material to the area of sedimentation was carried out by: 1) large rivers with source areas composed of rock complexes of different composition, and 2) rivers that drained provenances composed mainly of sedimentary rocks. The average CIA values inherent in the Upper Vendian and Lower Cambrian clay rocks of the Moscow syneclise are comparable to those typical for the suspended particulate matter of modern large rivers of a humid subtropical and tropical climate, as well as rivers of dry tropical climate regions. The localization of the data points of argillites and argillite-like clays on the SiO2–(Na2O + K2O + MgO + CaO), Al2O3–(Na2O + K2O + MgO + CaO) and CIA–WIP diagrams suggests that it was the paleoclimate that determined the main characteristics of their composition. The series of α-coefficients characteristic of clay rocks of the Upper Vendian–Lower Cambrian of the Moscow syneclise is quite close to the similar series for fine-grained suspended particulate matter of modern large river systems in southern Africa.