Evolution of the paleogeodynamic settings of the formation of the Lower and Middle Riphean sedimentary sequences of the Uchur-Maya region and the Bashkir meganticlinorium

The possibility to use some widely known standard discrimination diagrams such as the K₂O/Na₂O-SiO₂/Al₂O₃, SiO₂-K₂O/Na₂O, (Fe₂O₃* + MgO)-TiO₂, F1-F2, Th-La-Sc, Sc-Th-Zr/10, and Sc/Cr-La/Y for deciphering the paleogeodynamic settings of sedimentary sequences is considered with reference to the Lower and Middle Riphean (Mesoproterozoic) deposits of the Uchur-Maya region (Far East) and the Bashkir meganticlinorium (South Urals). It was shown that only some of them can be used with a certain degree of confidence for reconstructing the settings of the platform sedimentary sequences made up of both sandstones and fine-grained terrigenous rocks.     

Lithological,Lithochemical, and Geochemical Indicators of Paleoclimate: Evidence from Riphean of the Southern Urals

Complex lithological, lithochemical, and geochemical data are commonly considered necessary for the reliable reconstruction of Late Precambrian climatic parameters. However, correlation of results deduced from different methods is becoming a more and more topical task. In this paper, we compare paleoclimatic data obtained with various lithological, lithochemical, and geochemical methods for the Riphean stratotype section on the western slope of the southern Urals. The results obtained turned out to be generally convergent, but, paleoclimatic reconstructions based on different methods are directly opposite to each other at some stratigraphic levels. Most likely, the controversy is probably caused by underestimation of recycling effects and incorrect extrapolation of conclusions related to the first-cycle sediments over the rocks that underwent multifold redeposition. Criteria for the recognition of recycled sedimentary units show that more than half of the Riphean stratotype section is composed of recycled materials. Hence, application of standard litho- and geochemical methods (CIA index and others) to these units yields virtual models only remotely resembling the reality rather than actual paleoclimatic indicators. Elaboration of new methods, which make it possible to avoid such errors, is a problem of in the near future.     

New data on the composition and age of granitoids in the northern part of the Tagil structure (Ural Mountains)

The Tagil structure representing a large fragment of the Paleozoic island arc on the eastern slope of the Urals has been sufficiently well studied in its southern part (Middle Urals). In contrast, reliable data on the age and geochemical properties of various, including granitoid, rock complexes available for its northern part are scarce. The first data on the U–Pb LA–ICP–MS age of zircons from quartz diorites of the Man’ya massif of the Petropavlovsk Complex (436 ± 3 Ma, MSWD = 1.3), tonalites of the same complex (439.4 ± 1.3 Ma, MSWD = 1.3), granites of the Yuzhno-Pomur massif of the Severorudnichnyi Complex (422.4 ± 3 Ma, MSWD = 1.5), and titanite of the same massif (423.4 ± 4.4 Ma, MSWD = 0.84) have been obtained. Based on these data combined with the geochemical properties of the host rocks, the conclusion that they were crystallized at the initial stages of the formation of comagmatic volcanic series is supported; by their composition, granitoids correspond to island arc igneous rocks.     

Distribution of some rare and trace elements in modern bottom sediments of the Caspian Sea

The features of the distribution of some rare and trace elements in modern bottom sediments of the Caspian Sea have been studied from samples collected during cruises 35, 39, and 41 of the R/V Rift and the cruise of the R/V Nikifor Shurekov in 2013. It was established that bottom sediments in different areas of the Caspian Sea vary to some degree in the contents of Zr, Hf, Th, V, Cr, Co, Ni, Cu, Sr, and Ba in comparison to suspended matter discharged by the Volga River and rivers originating in the Caucasus. As follows from the results of a comparison of the geochemical features of modern bottom sediments of the Volga River delta and different sedimentary subsystems of the Caspian Sea with the chemical composition of Middle Archean granitoids and Paleozoic and Mesozoic–Cenozoic basalts, which are regarded as geochemical images of such provenance areas as the basement of East European Platform, Urals, and Caucasus, none of these regions is considered the dominant provenance area for all sedimentary subsystems of the Caspian Sea region. Here, the revealed similarity between modern bottom sediments of the northern, central and southern Caspian Sea regions, the Volga River delta, and Post-Archean average Australian shale (PAAS) in some parameters, including REE spectra, assumes that the Volga River discharge plays a dominant role in the formation of the geochemical image of Caspian Sea subsystems. The role of clastics, including the fine-grained fraction, which is supplied to the Caspian Sea from the Caucasus region and Elburz Mountains, is insignificant already in the coastal area, which is determined both by influence of the marginal filter (MF) and large-scale cholestatic current circulation.     

Systematics of rare earth elements, Th, Hf, Sc, Co, Cr, and Ni in the vendian pelitic rocks of the Serebryanka and Sylvitsa groups from the western slope of the Central Urals: A tool for monitoring provenance composition

This paper presents the first data on the systematics of rare earth elements (REE), Th, Hf, Sc, Co, Cr, and Ni and the Nd model ages of fine-grained aluminosilicate clastic rocks of the Serebryanka and Sylvitsa groups of the Vendian from the Kvarkushsko-Kamennogorskii meganticlinorium (western slope of the Central Urals). It was found that the REE distribution patterns of shales and mudstones of the two groups are similar to those of the majority of post-Archean fine-grained terrigenous complexes. The presence of pelitic rocks with Gd_N/Yb_N > 2.0 in a number of Vendian levels suggests a contribution from an Archean component in the composition of the fine aluminosilicate clastic material. This is probably also indicated by the high degree of heavy REE depletion in some mudstone samples. The REE systematics allow us to suppose a heterogeneity of Vendian paleocatchments and variations in their composition with time. The eroded areas had the most mature composition in the beginning of Serebryanka. Starting from the second half of Serebryanka, mafic and/or ultramafic rocks started playing a significant role in the provenances. The rocks of the lower portion of the Serebryanka Group show T_Nd(DM) values of about 2.0 Ga, whereas the upper part of the section is dominated by rocks with T_Nd(DM) ? 1.77–1.73 Ga. This indicates that during the Taninskaya and Koiva time periods, fine aluminosilicate clastic material was supplied into the sedimentation region mainly from the west, from the eastern areas of the east European platform, where Archean and Early Proterozoic crystalline complexes dominated. A decrease in model ages was related to the addition of juvenile mantle material to the mature continental crust. Such processes can be illustrated by the mafic-ultramafic complexes (Dvoretskii, Shpalorezovskii, Vil’vinskii, etc.) located in the field of Vendian sedimentary sequences, which show T_Nd(DM) values from 824 to 707 Ma. It was concluded that the history of the formation of an Early Vendian rift in the western slope of the central Urals included only one rifting event (rather than three, as was previously supposed), which was supported by a variety of recent geological and isotope geochemical data.     

Riphean low-carbonaceous shales of the South Urals in the context of formation of large igneous provinces

Relationship of large magmatic/plume events, warm periods, and epochs of accumulation of variably С_org–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 С_org–rich sedimentary rocks, as well as to the local accumulation of the latters.     

A Vendian manganese-bearing basin of the Ikat terrane: Formation settings and provenance areas

The results of geochemical, mineralogical, and isotope (U–Pb and Sm–Nd) studies of metasedimentary manganese-bearing rocks from the Itantsa Formation of the Ikat terrane are presented. It is found that the carbonate-effusive-shale complex of the Itantsa Fm. formed under the continental margin environment, with volcanic activity accompanying sedimentation, in the interval of 650–540 Ma ago.