Some Features of Evolution of Bulk Chemical Composition of Clay Rocks of the Upper Precambrian Sedimentary Groups: Example of South Urals

The article presents the results of study of some general features of the evolution of bulk chemical composition of clay rocks of the Upper Precambrian sedimentary groups of South Urals, which indicate the composition of source rocks of their alumosilicoclastic material (ln(Ni/Co) and ln(Th/Sc) values) and paleogeodynamic settings of formation (discriminant functions DF1 and DF2). Our conclusions are compared with ideas on the formation of these groups, which are based on general geological data and analysis of the formational nature of volcanic rocks known in some levels of the Riphean stratotype. It is considered in the last few decades that the Yurmatau and Karatau or Burzyan and Yurmatau groups have rift-related depression/platform origin. This suggests that their sedimentary complexes formed similarly and should be reflected in lithogeochemical characteristics of the latter. It is shown that the architecture of the Upper Precambrian sedimentary groups of the South Urals is largely diverse in terms of the “lithochemistry of fine-grained clastic rocks–paleogeodynamics”. The trends of the variation in bulk chemical composition of clay rocks in each groups in the DF1–DF2 diagram are not comparable in most cases with the model “sedimentary rocks of rift-related settings → the same of platform settings” trend. In the ln(Ni/Co)–ln(Th/Sc) diagram, the clay rocks of the sedimentary groups are characterized by two types of variation in bulk chemical composition. Each group is specific in these parameters. Thus, the features of bulk chemical composition of clay rocks, which are caused by the composition of the provenance rocks, exhibit no similar variation trends from bottom to top along the sedimentary group. It is evident that all the aforesaid indicates a significant role of paleogeodynamics and the composition of the provenances, as well as numerous (?) local factors in the formation of sedimentary filling of the Early, Middle, and Late Riphean, as well as Vendian, sedimentary basins of the South Urals.