Lithogeochemistry of clayey rocks and volcanic tuffs in the Vendian succession of the western slope of the Middle Urals: similarities and differences

Abstract

The lithogeochemical features of gray- and green-colored, as well as red-colored clayey rocks and volcanic tuffs found among Vendian deposits of the western slope of the Middle Urals (Serebryanka Group, Koiva Formation; Sylvitsa Group, Starye pechi Formation, Chernyi Kamen Formation) are considered. The materials presented and their analysis make it possible to reasonably assume that clayey rocks of usual color and their red-colored varieties do not fundamentally differ in their characteristics. On the discriminant lithogeochemical diagrams, their data points form fields characterized by significant, if not almost complete, overlaps. Moreover, as follows from an analysis of the Strakhov and Boström module values, as well as the position of the data points on the (Y/Ho) PAAS –(Ce/Ce *) PAAS diagram, they do not contain exhalation components. The sources of the fine-grained aluminosilicoclastic material for clayey rocks of different colors were mainly acidic (magmatic?) rocks. Volcanic tuffs, on the contrary, are composed of material similar in composition to trachyandesites, andesites, and trachyandesibasalts. The data presented in the article give reason to believe that the red color of many clay rocks present among the Vendian deposits of the Middle Urals is the result of pedogenic processes, as is expected for the red-colored rocks present among the Upper Riphean and Vendian deposits of the Southern Urals. On the other hand, some of the interlayers that we took in the field for volcanic tuffs, as previously assumed, can be ordinary sedimentary rocks, the color of which was changed by secondary processes (gleying, etc.), including modern ones.