Alkali earth elements in Vendian-Cambrian clayey rocks, the East European Platform

Low Sr and Ca contents in fine-grained terrigenous rocks of the Vendian sedimentary cover in the East European Platform may be related to the following reasons: (1) the absence of authigenic carbonates in Vendian mudstones owing to desalination of epiplatformal sedimentary basins; (2) significant contribution of the chemically weathered rocks in the formation of the Vendian cover of the East European Platform.     

Rb-Sr and K-Ar Characteristics of Upper Vendian Clayey Rocks in the Russian Platform

The paper considers geochemical behavior of Rb-Sr and K-Ar systems in Upper Vendian clayey rocks of the Russian Platform. The use of additional material on size fractions of sediments recovered from boreholes drilled in the Gavrilov Yam area made it possible to confirm the previous inference on two stages of the epigenetic matter transformation (approximately 600 and 400 Ma ago). Distortions are related to transformation of sediments due to interaction in the water-rock system. The degree of interaction was more intense in the upper part of the sedimentary section relative to its lower strata. These inferences are substantiated by materials from boreholes that characterize different types of Vendian sections and different tectonic zones.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 381–402.Original Russian Text Copyright © 2005 by Vinogradov, Burzin, Bujakaite, Golovin, Gorozhanin, Veis, Murav’ev.     

Structure of the precambrian sedimentary cover and upper part of the basement in the Central Russian Aulacogen and Orsha Depression (East European Platform)

The Precambrian sedimentary section and upper part of the basement of the Central Russian Aulacogen and Orsha Depression, two largest structures located beneath the Moscow Syneclise are analyzed. It has been established that the Late Riphean Central Russian Aulacogen was initiated on the Proterozoic crust of the Transcratonic belt that separates different-aged geological blocks of the East European Platform basement. The Orsha Depression is superposed both on sedimentary complexes of the aulacogen and rocks constituting structures surrounding the Transcratonic belt. Boundaries of the sedimentary cover and basement are outlined and a new structure (Toropets-Ostashkov Trough) is defined. The Precambrian section recovered by Borehole North Molokovo is proposed to serve as a reference one for the Central Russian Aulacogen. The CMP records demonstrate seismocomplexes, which allow one to trace rock members and sequences defined by drilling. Eight seismocomplexes, combination of which varies in different structures, are defined in the Upper Riphean-Vendian part of the sedimentary section. The section of the Central Russian Aulacogen includes the following sedimentary complexes: dominant gray-colored arkoses (R₃¹), variegated arkoses (R₃²), red-colored arkoses (R₃³), and volcanosedimentary rocks (V₁²). The section of the Orsha Depression consists of dominant red-colored quartz sandstones (R₃⁴), glacial and interglacial (V₁¹), and variegated volcanogenic-terrigenous sediments. The upper seismocomplex (V₂) is composed of terrigenous and terrigenous-carbonate rocks. It represents the basal unit of the Moscow Syneclise, which marks the plate stage in development of the East European Platform. The upper part of the basement corresponds to a seismocomplex (Pr₁) represented by dynamometamorphosed rocks that form a tectonic mélange. Analysis of the lateral and vertical distribution of the defined seismocomplexes made it possible to specify the structure of the Riphean-Vendian part of the sedimentary cover and to revise their formation history in some cases.     

U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Neoproterozoic microfossils from the margin of the East European Platform and the search for a biostratigraphic model of lower Ediacaran rocks

A ca. 600 m thick siliciclastic succession in northern Russia contains abundant and diverse microfossils that document early to middle Ediacaran deposition along the northeastern margin of the East European Platform. The Vychegda Formation is poorly exposed but is well documented by a core drilled in the Timan trough region (Kel’tminskaya-1 borehole). Vychegda siliciclastics lie unconformably above Tonian to lower Cryogenian strata and below equivalents of the late Ediacaran Redkino succession that is widely distributed across the platform. The basal 10 m of the formation preserve acritarchs and fragments of problematic macrofossils known elsewhere only from pre-Sturtian successions. In contrast, the upper, nearly 400 m of the succession contains abundant and diverse large acanthomorphic acritarchs attributable to the Ediacaran Complex Acanthomorph Palynoflora (ECAP). This distinctive set of taxa is known elsewhere only from lower, but not lowermost, Ediacaran rocks. In between lies an additional assemblage of relatively simple filaments and stratigraphically long ranging sphaeromorphic acritarchs interpreted as early Ediacaran in age. Bearing in mind that knowledge of late Cryogenian (post-Strurtian/pre-Marinoan) microfossils is sparse, the Vychegda record is consistent with data from Australia and China which suggest that diverse ECAP microfossil assemblages appeared well into the Ediacaran Period. Accumulating paleontological observations underscore both the promise and challenges for the biostratigraphic characterization of the early Ediacaran Period.     

Rb-Sr and U-Th-Pb systematics of alkaline rocks: the alkaline rocks from Italy

The isotopic composition of Pb and Sr and the abundances of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Cenozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre.The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different $\text{Rb}\text{Sr}$ or $\text{U(Th)}\text{Pb}$ ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived from a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences.The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. The lateral inhomogeneities in the mantle would be the result of mixing between originally mantle and crustal derived material. The mixing process itself would not have any primary connection with the Quarternary volcanic activity.     

Geochemistry of Cadmium in Mesozoic Phosphorites of the East European Platform

Cadmium is the most toxic admixture in mineral fertilizers. The Cd concentration in Mesozoic phosphorites, which are widespread in the East European Platform, has not been investigated. The present study was stimulated by the scanty and contradictory nature of the published data on this issue. We determined Cd concentration in 21 phosphorite samples from major deposits and checked the reliability of obtained results by external replicate analyses. It has been established that the Cd concentration in phosphorites varies from 5 ppm in the Late Jurassic–Early Cretaceous basin to 2 ppm in the Late Cretaceous basin. Cadmium does not enter the structure of phosphate and sulfide minerals. The Cd concentration is independent of the phosphorus abundance. However, all studied samples show a positive correlation of Cd with organic matter mainly contained in phosphates, supporting the biophilic nature of Cd. Mesozoic phosphorites of the East European Platform accumulated in epicontinental basins. They are significantly depleted in Cd relative to Mesozoic–Cenozoic phosphorites in pericontinental basins of the southern margin of the Tethys Ocean. The Cd concentration is more stable in Mesozoic phosphorites than in Mesozoic–Cenozoic deposits.     

Rb-Sr and K-Ar isotopic systems of the Upper Devonian sediments in the Pripyat Trough,Belarus

The results of Rb-Sr and K-Ar isotope studies of the Famennian upper salt and suprasalt complexes sampled from the core material of exploratory boreholes drilled at the northwestern margin of the Pripyat Trough (Belarus) are presented. The suprasalt and saliferous portions of the sequence define an age of approximately 470 Ma. Despite multiple reworking of the material, we interpret this value as preservation of the provenance “memory.” Regardless of the Sr content in sample, the strontium isotope ratios obtained for low-Rb minerals (celestine, carbonate, and halite) varies throughout the section within 0.7083–0.7095, indicating the preservation of some primary features of the matter. The lowest isotope values obtained are slightly higher than those inferred for the upper Famennian. This fact confirms the concept that the studied complex was formed in a shallow-water setting. Data on sylvites show that the present-day isotopic characteristics of potassium salts were formed significantly later.     

Middle Paleozoic impact event in the southwest of the East European Platform

A zone with high remanent magnetization and magnetic susceptibility has been identified at the boundary between Lower Devonian gray- and red-colored deposits in the basin of the Dniester River in the southwest of Ukraine (in the southwest of the East European Platform). The microparticles of native iron and nickel and iron-nickel intermetallic compounds found here might be markers of an impact event in the Lochkovian (~ 415 Ma). Electron microscope examination and probe microanalysis of the chemical composition, morphology, structures, and textures of specific microobjects showed that they are the product of melting of meteoritic substance and target rocks caused by an impact explosion. Explosive dispersion and deposition of microparticles on the Earth’s surface result in a rapid differentiation of their substance. The obtained data can be used as evidence of a reference impact event during the fall of an iron-stone meteorite, and a characteristic set of minerals and geochemical parameters can help to identify catastrophic events in the geologic history.     

Deep structure of the southeastern part of the East European Platform

The results of CMP seismic data acquisition along regional deep profiles that cross large tectonic elements in the east of the East European Platform are considered. It has been established that the Zhiguli-Pugachev Arch and the Stavropol Depression (southern part of the Melekess Basin), as well as the Volga-Kama Anteclise and Pericaspian Syneclise, conjugate along reverse-thrust faults extending to the lower crust and Moho discontinuity. The position of the southeastern reverse-thrust boundary of the South Tatar Arch has been substantially specified in plan view and illustrated by seismic sections. Based on the results obtained, it is suggested that reverse-thrust faults of different orders are widespread in petroleum provinces in the east of the East European Platform, and this suggestion should be used in geological exploration. The CMP seismic data acquisition is efficient in studying the junction zones of large tectonic elements. It also provides insights into the deep structure of the Earth’s crust and its relationship to the structure and petroleum potential of the sedimentary cover and localization of oilfields. It is expedient to reprocess and integrate earlier seismic data in order to compile tectonic (tectonodynamic) regional maps on a new methodical basis.     

Kimberlites and lamproites of the East European Platform: Petrology and geochemistry

Several episodes of kimberlite magmatism occurred in the East European Province (EEP) during a long (about 1.5 Gyr) time period, from the Late Paleoproterozoic (ca. 1.8 Ga) in the Archean Ukrainian and Baltic shields to the Middle Paleozoic (ca. 0.36 Ga) mainly in the Arkhangelsk, Timan, and adjacent regions. Based on the analysis of data on 16 kimberlite occurrences and four lamproite occurrences within the EEP, five time stages can be distinguished; one of them, the Middle Paleozoic stage (Middle Ordovician and Devonian), is the most productive epoch for diamond in the northern hemisphere (EEP, Siberian Craton, and part of the China Craton). The analysis of petrological and geochemical characteristics of kimberlites (lamproites were studied less thoroughly) revealed variations in rock composition and their correlation with a number of factors, including the spatial confinement to the northern or southern Archean blocks of the craton, time of formation of the source of kimberlite melts, contents of volatiles and autoliths, etc. Three petrogeochemical types of kimberlites were distinguished: high-, medium-, and low-Ti (TiO₂ > 3 wt %, 1–3 wt %, and <1 wt %, respectively). There are two time intervals of the formation of kimberlite and lamproite sources in the EEP, corresponding to T_Nd(DM) values of about 2 Ga (up to 2.9 Ga in the Por’ya Guba occurrence) and 1 Ga. The latter interval includes two groups of occurrences with model source ages of about 1 Ga (low-and medium-Ti kimberlites of the Zolotitsa and Verkhotina occurrences) and about 0.8 Ga (high-Ti kimberlites of the Kepino and a number of other occurrences); i.e., there seems to be an evolutionary trend in the composition of kimberlites. Concentric zoning patterns were recognized. The role of the crust in kimberlite sources is discussed; it is assumed that buried remnants of the oceanic lithosphere (megaliths) may underlie whole continents. A unique feature of the composition of low-Ti kimberlites, for instance, kimberlites of the Zolotitsa occurrence (to a smaller extent, medium-Ti kimberlites of the V. Grib pipe) is the distinct depletion of highly charged elements and pronounced negative anomalies of Ti, Zr, Th, U, Nb, and Ta in trace-element distribution patterns, which indicates a contribution of crustal material to the source of these kimberlites. It was shown that autoliths exert a significant influence on the differentiation of kimberlite material, resulting in the enrichment of rocks in the whole spectrum of incompatible elements. It was argued that geochemical criteria can be used together with traditional criteria (including those based on indicator minerals) for the assessment of diamond potential in EEP occurrences. We hope that such a combined approach will yield important outcomes in the future.     

REE Geochemistry of Precambrian Metamorphic Rocks in Wutaishan Region

The metasedimentary-volcanic series of the Wutai and Hutuo groups experienced regional metamorphism and thus turned into moderate-to low-grade metamorphic rocks.REE abundances and REE distribution patterns in the Shizui and Taihuai Subgroup metasedimentary-volcanic rocks are typical of the Archean,whereas the Gaofan Subgroup and the Hutuo Group show post-Archean REE geochemical char-acteristics.Five types of REE distribution pattern are distinguished:(1)rightward inclined smooth curves with little REE anomaly(Eu/Eu*=0.73-0.95) and heavy REE depletion (e.g.the Late Archean metasedimentary rocks);(2)rightward inclined V-shaped curves with sharp Eu anoma-ly (Eu/Eu*=0.48-0.76) and slightly higher ∑REE (e.g.the post-Archean metasedimentary rocks);(3) rightward inclined steep curves with negative Eu anomaly(Eu/Eu*=0.73-0.76) and the lowest ∑REE (e.g.the post-Archean dolomites);(4)rightward inclined,nearly smooth curves with both positive Eu anomaly and unremarkable positive Eu anomaly(Eu/Eu*=0.95-1.25)(e.g.the meta-basic volcanic rocks);and (5) rightward inclined curves with Eu anomaly(Eu/Eu*=1.09-1.19)and heavy REE depletion(e.g.the meta-acid volcanic rocks).Strata of the two groups are considered to have been formed in an island-arc belt-an instable continental petrogenetic environment.     

Tectonic differentiation of Precambrian basement formations in East European Platform

Principal unconformities and terminal folding, on the time scale from Belomorian through Caledonian, are correlated tentatively for constituents of the East European Platform (Baltic Shield, Ukrainian Massif, Eastern Margin, and their subordinates), Canadian Shield and Greenland, parts of Great Britain, and northern margin of the African Platform. — IGR Staff.     

Excess europium content in precambrian sedimentary rocks and continental evolution

We propose that the europium excess in Precambrian sedimentary rocks, relative to those of younger age is derived from volcanic rocks of ancient island arcs, which were the source materials for the sediments. Precambrian sedimentary rocks and present-day volcanic rocks of island arcs have similar REE patterns, total REE abundances and excess Eu, relative to the North American shale composite. The present upper crustal REE pattern, as exemplified by that of sediments, is depleted in Eu, relative to chondrites. This depletion is considered to be a consequence of development of a grandioritic upper crust by partial melting in the lower crust, which selectively retains europium.     

Seismogenic Ancient Structures of the Center and North of the East European Platform

Doklady Earth Sciences - The location of earthquake epicenters in the central and northern parts of the East European Platform in 2009–2016 has been analyzed. The earthquake epicenters have...