Provenances of the Upper Precambrian Clay Rocks in the Southern Urals: Results of Geochemical and Sm–Nd Isotope Geochemical Investigations
А. В. Маслов, А. Б. Кузнецов, A. Yu. Kramchaninov, L. V. Shpakovich, Э. З. Гареев, V. N. Podkovyrov, С. Г. Ковалев
Abstract
Changes in (La/Yb)N, Eu/Eu*, tNd(DM), and εNd(t) values in Upper Precambrian clay rocks on the western slope of the Southern Urals are considered. The (La/Yb)N, Eu/Eu*, and εNd(t) average values in Riphean and Vendian clay rocks are 5.7–15.1, 0.58–0.74, and –14.6 to –5.1, respectively. This reflects the change in the composition of the source areas and fits well into the general outline of subglobal events established by traditional geological methods in the conjunction area of the eastern regions of the East European Platform and the modern Southern Urals. The Early and Middle Riphean sedimentary sequences (1750–1250 Ma) were formed mainly owing to the erosion products of the mature continental crust of the East European Platform (tNd(DM) = 2.8–2.4 Ga). However, the pre-Upper Riphean hiatus led, probably, to a significant change in the composition of source areas about 1 Ga. The increase in magnitude of εNd(t) to –5.9 and decrease in tNd(DM) to 2.0 Ga in clay rocks of the Biryan Subformation of the Upper Riphean Zilmerdak Formation compared to underlying deposits suggest the appearance of the juvenile crust in the erosion area. This indicates the accumulation of fine-grained sediments at the beginning of the Late Riphean under the influence of active rifting processes, which is not recorded by traditional geological methods. The significant increase in (La/Yb)N (to 13.1 on average), decrease in εNd(t) to –14.6, and increase in tNd(DM) up to 2.5 Ga in clay rocks of the Vendian Bakeevo Formation compared to the Riphean rocks are interpreted as the result of the accumulation of the Bakeevo deposits due to the supply of the products of glacial exaration of the basement mature rocks during the Marino glacial period. The significant increase in εNd(t) to –6.8 and rejuvenation of tNd(DM) to 1.8 Ga in Vendian mudstones of the Basa and Zigan formations compared to the rocks of the lower part of the Asha Group reflect the appearance of a new mantle or volcanogenic material in the catchment areas during the Middle Vendian.