Changes in (La/Yb) N , Eu/Eu*, t Nd (DM), and ε Nd (t) values in Upper Precambrian clay rocks on the western slope of the Southern Urals are considered. The (La/Yb) N , Eu/Eu*, and ε Nd (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 (t Nd (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 ε Nd (t) to –5.9 and decrease in t Nd (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 ε Nd (t) to –14.6, and increase in t Nd (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 ε Nd (t) to –6.8 and rejuvenation of t Nd (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.

The article attempts to solve the problem of the presence of "camouflaged" pyroclastics in sandstones and fine-grained clastic rocks of the Middle Riphean Mashak Formation of the Southern Urals based on litho- and geochemical methods and approaches. As a result of the studies, it was established that in the NKM-GM diagram, the compositional field of silt-clayey rocks of the Mashak Formation is comparable with several clusters of Jurassic slates of the Greater Caucasus, containing volcanogenic material, while the clusters of Paleogene tuffs of the Ukrainian Carpathians and the Gromashor Formation (Pai-Khoi) have a different location. On the (Na2O + K2O) - GM diagram, in the indicated field, are situated two of four clusters of metatuffoids and mixed volcanic-sedimentary rocks of the Strelna Group (Kola Peninsula), and one of two clusters of tuffs of the Redkino horizon (Vendian of the East European Platform). The same is true for a number of other examples. In the diagram (Na2O + K2O) - FM, none of the four clusters of tuffs and tuffopelites of the Ordovician of the Arga-Tas zone of the Momsky horst-anticlinorium and none of the six clusters of acidic tuffs and tuffites of Ukrainian Carpathians fit in, etc. Consequently, the localization of the compositional field of silt-clayey rocks of the Mashak Formation in the above and a number of other diagrams shows that clusters of various volcanic-terrigenous rocks, metatuffosiltstones, metatuffoids, slates, etc., containing a certain admixture of volcanic products, coincide with the diagrams in half or fewer of the cases. Thus, the analysis of the absolute contents and indicator ratios of trace elements inherent in sandstones and clayey rocks, values of petrochemical modules and the systematics of rare-earth elements (REE) does not allow us to state with certainty that magmatic processes, including subsynchronous sedimentation, were the main source of their constituent material.

The bulk chemical composition of synrift sandstones and associated mudstones is analyzed. Distribution of their fields in the discriminant paleogeodynamic diagrams SiO 2 –K 2 O/Na 2 O (Roser and Korsch, 1986) and DF1–DF2 (Verma and Armstrong-Altrin, 2013) is examined. In terms of the bulk chemical composition, the sandstones correspond mainly to graywackes, litites, arkoses, and subarkoses. The samples also include sublitites and quartz arenites, as well as a significant part of psammites (with log(Na 2 O/K 2 O less than –1.0) that are lacking in Pettijohn’s classification diagram. This fact supports our conclusion based on the results of mineralogical-petrographic studies: the sedimentary infill of riftogenic structures combines immature sandstones, clastic framework of which is derived from local provenances represented by diverse igneous and sedimentary rocks. Synrift mudstones, relative to sandstones, are composed of a more mature clastic material. Distribution of data points of mudstones in diagram F1–F2 (Roser and Korsch, 1988) suggests that they were sourced mainly from sedimentary rocks. Contents of the majority of main rock-forming oxides in synrift sandstones are almost similar to those in silty–sandy rocks of the Upper Precambrian–Phanerozoic sedimentary megacomplex in the Russian Plate, but differ from contents in the average Proterozoic and Phanerozoic cratonic sandstones, as well as the average composition in the upper continental crust. It is shown that the distribution of synrift sandstones and mudstones in diagram SiO 2 –K 2 O/Na 2 O lacks any prominent features, and their data points cluster in the fields of terrigenous rocks of passive and active continental margins. In diagram DF1–DF2, fields of the studied psammites and mudstones occur in zones of the riftogenic and collisional settings. We believe that the boundary between these zones in the diagram occupies a different position, and this suggestion needs further examination.

The article briefly describes the lithochemical features of sandstones and clayey rocks of Mashak Formation (Southern Urals), a second synrift volcano-sedimentary association in Riphean stratotype. It is shown that the sandstones of the Mashak Formation are characterised by wide variations in the bulk chemical composition (from greywacke to quartz arenite). The presence of the latter, as well as a relatively small proportion of arkoses and subarkoses, distinguishes the second synrift association from the first one (the Ai Formation, Lower Riphean). There is no overlap in between the Mashak's and Ai's psammites on the diagram log(SiO2/Al2O3)-log(Na2O/K2O) of F.J. Pettijon and co-authors. It indicates significant differences in provenances and common features of accumulation of the Mashak and Ai formations. On the basis of bulk chemical composition, the clayey rocks of the Mashak Formation are related to smectite (with kaolinite and illite admixture) clays and their chlorite-smectite-illite varieties. The clayey rocks of the Ai Formation differ by lower values of total normative alkalinity (0.27 +/- 0.06 vs. 0.42 +/- 0.07). It is established that sandstones and clayey rocks of the Mashak Formation contain a significant proportion of the "first-cycle material", i.e. they are composed mainly by products that have passed one sedimentation cycle. As opposite, there is a significant proportion of "second-cycle/lithogenic material" of the first synrift sandstones association (the Ai Formation). The high K2O/Na2O values, which are characterised for a half studied samples of the Mashak Formation, indicate that they were influenced by processes of potassium metasomatism. The distribution of data points of terrigenous rock of the Mashak Formation on diagram F1-F2 (Roser and Korsch, 1988) displays that they are composed by products of erosion of acid, average or mafic igneous and/or metamorphic rocks. The individual data points of Mashak Formation terrigenous rocks, as well as sandstone composition midpoint, are localised in the areas of riftogenic and collisional settings on the DF1-DF2 diagram (Verma and Armstrong-Altrin, 2013). It does not allow us to define their paleogeodynamic settings. In such a manner, the a critical role, traditionally, remains on general geological data and petrogeochemical characteristics of igneous rocks.

Relationship of large magmatic/plume events, warm periods, and epochs of accumulation of variably Сorg–rich sediments is considered by the example of Riphean low-carbonaceous shales of the western slope of the South Urals. It is shown that this relationship presumably exists for some of the indicated events (as for Early Precambrian superplume events), and does not exist for others. The formation of organic carbon-rich sediments in the Late Precambrian was likely controlled by other external and internal factors than in the Early Precambrian, relationships between which are so far poorly understood. It is also possible that the lesser scale of plume events in the Late Precambrian as compared to previous epochs could lead to the obliteration of their influence on the formation of Сorg–rich sedimentary rocks, as well as to the local accumulation of the latters.

We analyzed the distribution of wide range of trace elements in low-carbon shales of various Riphean, Vendian and Lower Paleozoic lithostratigraphic units of the South, Central, and southern part of the Northern Urals (Satka, Bolshoy Inzer, Suran, Yusha, ZigazinoKomarovo, Vels, Buton, Bezgodovo, and other suites). It was determined that they significantly differ in content of the trace elements. In the most cases, the concentration of trace elements in these shales is roughly comparable with those found in well-known geochemical units, such as the Post-Archean Australian Shale (PAAS), or is lower. Alternatively, obtained data showed significant differences between the Urals low-carbon black shales and the Vendian and Cambrian black shales from China, and other black shale formations, characterized by higher concentration of many trace elements. We consider the probable important role of the geodynamic position of black shale formation, variation in the bioproductivity, association with specific rock complexes, and extent of secondary transformations in variability of geochemical composition of the black shale sediments.

An attempt is made to compare discrimination diagrams of the first (mid-1980s) and second (early 2010s) generations compiled using data for sedimentary successions of different ages. Our results suggest that the diagrams of different generations allow more or less correct discrimination only between the platform, rift, passive margin, and island arc settings. The data for collision sediments do not form separate fields in these diagrams.

The article analyzes the adequacy of compliance on paleogeodynamic nature of Upper Precambrian sedimentary associations - Bashkir meganticlinorium, Kama-Belaya aulacogene and Uchur-Maya region, received by geological methods and by using discriminant diagrams ?. Meynard et al., B.R. Roser and R. Korsch, and S.P. Verma and ?.S. Armstrong-Altrin. It is shown that among them there are notable differences.

The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K2O/Al2O3 values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIAcorrect. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIAcorrect. and CIA values.

Results of the study of noble metal specialization of Lower and Middle Riphean terrigenous rocks in the Bashkir Anticlinorium (South Urals) are reported. The study revealed their genetic differences in the relatively unaltered, i.e., "background" terrigenous rocks in type sections of the Burzyan and Yurmatau groups and in sedimentary rocks of the same stratigraphic levels from tectonic zones subjected to local dynamothermal metamorphism of the greenschist facies and intruded by mafic rocks. It has been established that Ru serves as a geochemical marker of the impact of magmatic processes on sedimentary rocks and the redistribution of noble metals during metamorphism and local metasomatism. A generalized model is proposed for the formation of noble metal geochemical specialization of Lower and Middle Riphean terrigenous rocks in the South Urals.

The article analyzes the adequacy of compliance on paleogeodynamic nature of Upper Precambrian sedimentary associations - Bashkir meganticlinorium, Kama-Belaya aulacogene and Uchur-Maya region, received by geological methods and by using discriminant diagrams ?. Meynard et al., B.R. Roser and R. Korsch, and S.P. Verma and ?.S. Armstrong-Altrin. It is shown that among them there are notable differences.

Рассмотрены основные особенности валового химического состава синорогенных глинистых пород различных объектов (Предуральский прогиб, Олюторский террейн, Днестровский перикратон, Индо-Гангский прогиб и др.). Проанализировано расположение фигуративных точек их состава на широко известных дискриминантных палеотектонических диаграммах. Показано, что в условиях активной тектоники, присущей коллизионному этапу развития подвижных поясов, валовый химический состав этих пород отражает особенности породных ассоциаций, сформированных как на дивергентном, так и конвергентном (в том числе собственно коллизионном) этапах. Наиболее близким к их среднему составу является PAAS – интегрированный показатель состава верхней континентальной коры. Из приведенных материалов можно сделать вывод, что для суждения о палеотектонической природе тех или иных осадочных последовательностей сведения о валовых химических составах глинистых пород, отраженные на дискриминантных диаграммах (SiO2 K2O/Na2O, K2O/Na2O SiO2/Al2O3 и др.), нельзя рассматривать как минимально достаточные.

The paper is devoted to the specifics in the chemical composition of synorogenic clay rocks in various environments: the Ural foredeep, Olyutorskii terrane, Dnestr pericratonic trough, Indus-Gang trough, etc. The compositions of the rocks are plotted in conventionally utilized discriminant paleotectonic diagrams. It is demonstrated that the bulk composition of such rocks, which were formed under active tectonic conditions typical of the collisional evolutionary phase of mobile belts, reflects the nature of the rock assemblages produced during both the divergent and the convergent (including collisional) phases. Their mean composition is the closest to PAAS as an integrated parameter of the average composition of the continental crust. The materials presented in the publication suggest that data on the bulk composition of clay rocks shown in conventionally utilized discriminant diagrams, such as SiO2-K2O/Na2O, K2O/Na2O-SiO2/Al2O3, are insufficient to reproduce the tectonic environments in which these rocks were formed.