G. V. Konstantinova

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif, North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic‒geochronological study of globular phyllosilicates (GPS) of the glauconite‒illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb‒Sr and K‒Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb‒Sr, 1485 ± 13 Ma; K‒Ar, 1459 ± 20 Ma) and Yusmastakh (Rb‒Sr, 1401 ± 10 Ma; K‒Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.

Rb-Sr and U-Pb Systematics of Metasedimentary Carbonate Rocks: The Paleoproterozoic Kuetsjarvi Formation of the Pechenga Greenstone Belt, Kola Peninsula

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial 87Sr/86Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial 87Sr/86Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the 207Pb/204Pb-206Pb/204Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the 208Pb/204Pb-206Pb/204Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic 208Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the 207Pb/204Pb-206Pb/204Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ13C values.

Strontium isotope composition of the lower proterozoic carbonate concretions: The Zaonega Formation, Southeast Karelia

The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite concretions contain some dolomite admixture (Mg/Ca = 0.011−0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3–2.1). The Sr content is as much as 385–505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA fraction). In individual calcite samples, discrepancy between the measured 87Sr/86Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008–0.0033. The initial 87Sr/86Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with “mild” reductive conditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite-bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87Sr from the associated siliciclastic minerals, resulting in growth of the initial 87Sr/86Sr in concretions up to 0.7108–0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. Therefore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87Sr/86Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.

Rb-Sr Geochronology of Vendian Shales, the Staraya Rechka Formation, Anabar Massif, Northern Siberia

Fine-grained clayey subfractions (SF) with particle sizes of 1–2, 0.6–1.0, 0.3–0.6, 0.2–0.3, 0.1–0.2, and <0.1 μm were extracted from shales of the Vendian Staraya Rechka Formation in the Anabar Massif and studied by XRD and Rb-Sr methods. All the clayey subfractions are represented by illite with high crystallinity indices, which are characteristic of the low-temperature diagenesis/catagenesis zone and grow with the decrease of the particle size. The Rb-Sr systematics in clayey subfractions combined with mineralogical data provide grounds for the conclusion that illite from clayey rocks of the Staraya Rechka Formation was forming during two periods: approximately 560 and 391–413 Ma ago. The first illite generation was likely formed in the course of lithostatic subsidence of the Staraya Rechka sediments and the second one, during the Devonian lithogenesis stage. It is assumed that age of the first generation (∼560 Ma) is close to that of the Staraya Rechka Formation. This inference is consistent with biostratigraphic, chemostratigraphic, and geochronological data obtained for both rocks of the Anabar Massif and Vendian sediments from other regions of Siberia.

Sm-Nd systematics of fine-grained fractions of the Lower Cambrian blue clay in northern Estonia

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1–0.4, 0.4–0.6, and 0.6–2.0 μm) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 μm) consists of mixed-layer illite-smectite. The illite crystallinity index (Ic) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790±90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Sr isotopic chemostratigraphy of Precambrian carbonate rocks in the Amderma Rise, Pai-Khoi Ridge

The Sr and C isotopic compositions of Precambrian carbonate rocks are determined for Amderma Rise, in the northeastern margin of Pai-Khoi Ridge. Based on the Sr isotopic chemostratigraphy, it is established for the first time that the Amderma Formation is referred to the Early Vendian, while the Morozovsk Formation is Late Riphean in age. This conclusion along with detailed mapping proves that the Precambrian “section” of the Amderma Rise is a series of tectonic plates combined in a nonchronostratigraphic order. Volcanic and sedimentary rocks of the Morozovsk and Sokolninsk formations make up the allochthon proper, while carbonate rocks of the Amderma Formation make up the para-autochthon. The high values of δ13С (up to +9.5‰) identified in limestones of both formations suggest a considerable distance of the Pai-Khoi paleobasin from the passive margin of the Baltic Region upon facies similarity to the Laurentia active margins.

87Sr/86Sr and δ18O chemostratigraphy and facies conditions of Pleistocene mollusks in Karelia (Gridino)

1413 Modern mass spectrometers allow us to analyze the Sr isotope composition in carbonate samples, which were not subjected to secondary recrystallization and with accuracy sufficient for distinguishing sediments even within the Quaternary period [1–3]. In addition, the values of 87Sr/86Sr in carbonate sediments com bined with the value of δ18О provide information on variations in the salinity of basins. We studied the iso tope composition of Sr, O, and C in carbonate mollusk shells collected in “old” Quaternary rocks of North Karelia for the first time. We compared the obtained isotope characteristics with the values of 87Sr/86Sr and δ18О in modern carbonate mollusks from marine and freshwater basins of North Europe. Based on these data, the existence of marine and freshwater forms in Quaternary rocks of Karelia was substantiated. The method of Sr isotope chemostratigraphy (SIS) allowed us to establish the Pleistocene age for fossil mollusks of the White Sea area. Quaternary deposits overly various Early Precam brian rocks on the territory of Karelia. Most of the Quaternary cover was formed during degradation of Scandinavian glaciation and postglacial processes of the last 9–15 thousand years. The youngest deposits have a thickness of several tens of meters and compose the modern landscape forms. These deposits forming the classic area of Late Pleistocene glaciation are well studied [4, 5]. However, the “old” Quaternary deposits occurring on the territory of Karelia and containing marine shell fauna have been studied much more poorly. They were conventionally attributed to the Mikulinskii horizon, which precedes the stages of the formation of Valdai and Holocene sediments [4, 6, 7].

Isotope composition of strontium, carbon, and oxygen in magnesian carbonates of the Onot greenstone belt

341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan marginal block of the Siberian Platform basement (Fig. 1). The OGP occurs on ancient tonalite–trondhjemite–gran odiorite gneiss containing zircons with a U–Pb age of ≈3.34 Ga [1] and is intruded by Early Proterozoic granite of the Shumikhin Complex (the U–Pb age of zircons is 1.87 Ga [2]). The belt is composed of metavolcanic rocks of the tholeiite–rhyolite series, metaterrigenous and biotite–garnet gneisses, amphib olite, quartzite, and magnesite and dolomite marbles [3, 4]. Rocks are metamorphosed under the condi tions of the amphibolite and epidote–amphibolite facies and have undergone intense ultrametamorphic and metasomatic transformations. The age of the pro tolith of metarhyolite from the OGB is 2.89 Ga (U– Pb age by zircon) [5]. The Rb–Sr age of ultrameta morphic granitoids from the Onot Complex is 2.64 Ga [6]. Thus, the supracrustal complex of the OGB is one of the oldest in Asia, and its age is consistent with that of typical Archean greenstone belts of the Earth.