Multiple lines of evidence point to a major diversification of Eukarya in the late Tonian period, ca. 800-720 Ma ago. Regular steranes, lipid biomarkers primarily sourced from eukaryotes, are first unambiguously reported from multiple sites. Here, we present lipid biomarker stratigraphic records from two drill-cores from the Lena-Anabar Basin in northern Siberia, in addition to new biomarker data from the Walcott Member of the Chuar Group, USA. The Siberian drill-cores span the late Tonian (ca. 800-750 Ma) and late Ediacaran-early Cambrian periods (ca. 565-539 Ma), with Cryogenian strata absent. Compositional differences in sedimentary organic matter were assessed using a suite of biomarker hydrocarbon parameters, with seawater redox and phosphorus availability determined using iron speciation, trace metals, and phosphorus phase partitioning. Steranes were consistently detected in Tonian strata in the Siberian drill-cores, with sterane/hopane (S/H) values being uniformly low (< 0.05), consistent with a low-productivity, marine environment in which bacteria outcompeted eukaryotes for limited nutrients. This was confirmed by phosphorus phase partitioning data that suggest that phosphorus was a key limiter of late Tonian marine eukaryote abundance. Unlike Siberia, late Tonian samples from the Chuar Group showed consistently higher S/H values (up to 0.38) and elevated total organic carbon content (1.3-5.6 wt.%) indicative of higher algal productivity in a eutrophic marine setting. Tonian strata primarily yielded cholestane (C27 sterane), indicating red algal dominance with small contributions of cryostane and ergostane (C28 steranes), while Ediacaran strata in both Siberian drill-cores were dominated by stigmastane (C29 sterane), indicating green algal dominance, capturing this global secular shift in a single location. The distinctive C27-sterane dominance commonly found for the late Tonian is captured here for the first time from an oligotrophic marine environment that sustained oxic to weakly reducing redox conditions, supporting the idea that the Tonian rise of algae occurred widely across the oceans.

The results of Sr-chemostratigraphic study of carbonates of the Staraya Rechka and Nemakit-Daldyn formations which make up the upper part of the Precambrian cover of the Anabar Uplift in Northern Siberia are presented. A Pb–Pb age of the Staraya Rechka Formation dolostone has been obtained (549 ± 25 Ma, MSWD = 1.4) for the first time. An improved stepwise dissolution procedure was used to determine the 87Sr/86Sr, 206Pb/204Pb and 207Pb/204Pb isotopic ratios in carbonates rocks. The methodology for studying the Rb–Sr systematics included the chemical removal of about a third of the crushed sample [fraction L(Rb–Sr)1] by preliminary acid leaching in 0.2N CH3COOH and subsequent partial dissolution [fraction L(Rb–Sr)2] of the remaining part of the sample in CH3COOH of the same concentration. The Pb–Pb isotopic systematics of dolostones was studied by six to nine-step dissolution in 0.5N HBr. Chemical procedures resulted in the removal of secondary epigenetic carbonate material, which improved the quality of Sr-chemostratigraphic and U–Pb geochronological information. The initial 87Sr/86Sr ratios in the least altered carbonate material [fraction L(Rb–Sr)2] of the Staraya Rechka Formation dolostone are 0.70822‒0.70836, and in the Nemakit-Daldyn Formation limestone, 0.70854–0.70856. The Pb–Pb age of early diagenesis of dolostones of the Staraya Rechka Formation (549 ± 25 Ma) was calculated from fractions [L(U–Pb)2–L(U–Pb)n], where n for different samples varied in the range from 6 to 9. Epigenetic carbonate fractions L(U–Pb)1 are characterized by a Pb–Pb age of 360 ± 190 Ma (MSWD = 0.8). The obtained results prove that the Staraya Rechka Formation of the Anabar Uplift belong to the Late Vendian (Late Ediacaran), allowing them to be confidently correlated with the carbonate rocks of the upper part of the Yudoma Group of the Uchur-Maya region and to include the named strata into a single Yudoma Complex of Siberia.

The paper discusses the melt sources and formation parameters of the Khokhol-Repyevka granitoid batholith that compose the Don terrane of the Volga–Don orogen in the East European craton. The batholith consists of three granitoid types: Pavlovsk granitoids (quartz monzonites–granites, mostly without pyroxenes), Potudan granitoids (quartz monzogabbro–granodiorites containing pyroxene), and hybrid ones (quartz monzodiorites, monzonites, and quartz monzonites). These three types of rocks occur together and have a similar age of 2050–2080 Ma, similar geochemical characteristics (high contents of Ba, Sr, and highly fractionated REE patterns with GdN/YbN = 2–11), but differ in petrographic and isotopic geochemical parameters. The initial isotope characteristics of the sources of the Pavlovsk-type rocks are εNd(t) = +0.2 to ‒3.7 and Sri = 0.70335, those of the Potudan type are εNd(t) = –1.7 to –3.8, Sri = 0.70381–0.70910, and the hybrid rocks have εNd(t) = –8.8, Sri = 0.70596. In addition to granitoids, the batholith was found out to host two types of leucogranite dikes. One of them is characterized by εNd(t) = –3.8 and fractionated HREE patterns (GdN/YbN = 2.1–3.8) and could be formed as a result of the deep differentiation of Pavlovsk-type magma. The other type has εNd(t) = –7.8 and less fractionated HREE patterns (GdN/YbN = 1.1–1.6), which likely resulted from the melting of a crustal source at shallow depths. The Rb–Sr isotope-geochemical characteristics of rocks of the Pavlovsk and Potudan types indicate that their melts were derived from different sources. Therefore the melts of the Khokhol–Repyevka batholith were derived from at least three sources: (1) lower (or buried oceanic) crust of predominantly mafic composition and/or enriched mantle, which was metasomatized in the Proterozoic, whose involvement is reflected in the composition of the Pavlovsk granitoids; (2) an enriched mantle source, which was likely subcontinental lithospheric mantle (SCLM) that had been metasomatized during an earlier stage of the geological development of the region, specific of the Potudan-type monzonitoids; and (3) Archean crust consisting mostly of TTG gneisses and metasediments, which underwent melting and participated in the formation of some of the leucogranite dikes and hybrid rocks. The results of thermodynamic modeling indicate that the mixing of two melts contrasting in composition (Potudan-type mafic and Pavlovsk-type intermediate–felsic) could form only some of the hybrid rocks. The others could be formed by mafic melt contaminated with anatectic melts derived from the Archean crust of the Kursk block.

The work reports on the results of the mineralogical and petrographic study of the Borodino meteorite (H5). For the first time, meteorite minerals were described and their chemical compositions were given. The following were found in the Borodino meteorite: olivine (Fa 18.16 ± 1.15), Ca-poor pyroxene – (clino)enstatite (En 81.37 ± 1.73, Wo 1.18 ± 0.31), Ca-rich pyroxene – augite (En 57.23 ± 1.57, Wo 39.38 ± 2.68), diopside (En 51, Wo 45), pigeonite (En 69, Wo 6), plagioclases – oligoclase (An 12.16 ± 1.24, Or 5.68 ± 2.12), andesine (An 48.23 ± 1.84, Or 1.23 ± 0.12), anorthoclase (An 0, Or 36) and sanidine (An 0, Or 40.00 ± 1.1), and poorly crystallized glasses of the feldspathic composition, merrillite and chromespinelide. The obtained data made it possible to estimate a degree of terrestrial weathering of the meteorite as W0 and the stage of impact metamorphism (S1-2), which suggests good preservation of the meteorite material. The composition of olivine and Cr-spinel, determined by the Electron Probe Microanalysis, was used to estimate the peak temperature of thermal metamorphism at 720°C, which falls within the temperature range (670–740°C) typical of chondrites of a petrological type 5. The presence of Ca-rich pyroxenes, large grains of Ca–Na–Mg phosphates and chromite-pigeonite aggregates in the meteorite matrix indicate the prolonged heating of the material.

The first of two communications is devoted to the study of lithogeochemical features of the pilot collection of Upper Precambrian sandstone and siltstone samples taken from four boreholes: Bogushevskaya 1, Bykhovskaya, Lepel 1, and Kormyanskaya (Belarus). This article analyzes the general features of their bulk chemical composition and shows the possibilities and limitations for further reconstructions. It has been established that Riphean and Vendian rocks included in the pilot collection, visually identified as sandstones, are actually quartz, feldspar–quartz, and arkosic varieties with different cement types. In terms of geochemical characteristics, the Vendian “siltstones” correspond to the coarse- and fine-grained siltstones and, to a greater extent, mudstones with a predominance of illite, as well as various admixtures of berthierine, kaolinite, and smectite. Based on the comparison of enrichment factor (EF) of the trace element, these rocks are marked by several dissimilarities related to both variations in the source rock composition and sedimentary environment. Data points of the samples on the Zr/Sc–Th/Sc diagram indicate that all of the studied Riphean and Vendian rocks are dominated by the first sedimentation cycle material, suggesting that the lithogeochemical characteristics of the pilot collection rocks quite correctly reflect similar features of the source rock complexes. Therefore, they can be used to reconstruct the paleogeodynamic and paleoclimatic factors that controlled the accumulation of Riphean and Vendian sedimentary sequences in Belarus.

This paper completes the examination of results of the study of lithogeochemical characteristics of a pilot collection of the Riphean–Vendian sandstones, siltstones, and claystones (last one based on the interpretation of results) in Belarus. Data on the age of detrital zircon published in recent years suggest that Upper Precambrian rocks of this region were sourced from the Osnitsk–Mikashevichi and the Trans-Scandinavian igneous belts, the Volyn–Brest large igneous province, rapakivi granites, as well as various rock associations of Sarmatia, Danopolonian orogen and Svecofennides. Based on the lithogeochemical characteristics, the distribution of data points of the studied clastic rocks on discriminant diagrams provides insight into the possible provenance, paleogeodynamic, paleogeographic, and paleoclimatic settings and suggests several conclusions. The considered Riphean–Vendian stratons in Belarus are composed mainly of erosion products of within-plate granitoids, as well as various island-arc and syncollisional felsic igneous rocks. The share of erosion products of mafic rocks among them generally does not exceed 30

The geochemical characteristics (REE, trace elements) and Sr and Nd isotopic composition of apatite from corundum-bearing metasomatites of the Khitoostrov occurrence (Belomorian Mobile Belt), associated plagioclasites, and host rocks represented by garnet amphibolites and kyanite–garnet–biotite gneisses of the Chupa sequence have been studied. Apatites from the corundum-bearing metasomatites and kyanite–garnet–biotite gneisses are enriched in medium REE and have a negative Eu anomaly (Eu/Eu* 0.20–0.35). Apatite from the corundum-bearing rocks differs from apatite from the gneisses of the Chupa sequence in the increased content of Sr, LREE, decreased content of HREE, as well as a lower 87Sr/86Sr(t) ratio and an increased ɛNd(T) value: 0.70 865–0.70 896 and –9.3 ± 0.2 compared to 0.72 533 and –8.1, respectively. Apatite from the garnet amphibolites is enriched in MREE, lacks Eu-anomaly (Eu/Eu* 0.98), and has a low ɛNd(T) = –9.3 and the lowest 87Sr/86Sr(t) ratio of 0.70 560. The Sm-Nd age estimate of apatite is 1.80 ± 0.15 Ga, which coincides with the Svecofennian metamorphism in the Belomorian Mobile Belt. The geochemical features of the apatite and 87Sr/86Sr(t) ratios indicate that the metasomatic alteration of the gneisses was caused by the lower crustal fluid and was accompanied by the influx of LREE and the removal of HREE. The slightly lower Eu anomaly and higher Ce vs Th and REE vs La/Sm relations reflect the fact that apatite from the corundum-bearing metasomatic rocks was formed in a more oxidizing environment than apatite from host rocks. Neither the corundum-bearing metasomatites and plagioclasites, nor the host rocks revealed any Sr-isotopic and REE-geochemical traces of interaction with surface (meteoric) waters.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23070243

New isotopic data are presented in this paper that make it possible to determine the stratigraphic position of the Talja Formation on the regional stratigraphic scale of the Early Precambrian. The U‒Th‒Pb age of 1926 ± 7 Ma obtained for the first time for metarhyodacites of the Talja Formation indicates that this formation is coeval to the Kaskama Formation, which, together with the similar isotope–geochemical composition of the type varieties of the Kaskama and Talja formations (komatiitic basalts, aluminous metabasalts, and metarhyodacites), significantly expands the understanding of the extent of the area composed of the Paleoproterozoic Kalevian Superhorizon within the Kola–Norwegian region of the Fennoscandian shield.

This study presents Sr-isotope data to determine the stage affiliation of different parts of the Miocene Sertunaiskaya and Kurasiyskaya formations in Sakhalin, as well as the upper part of the Togeshita Formation in Hokkaido. Additionally, it confirms a diachronous boundary between the Sertunaiskaya and Kurasiyskaya formations. The data received will be useful for globally correlating the Miocene formations of the SakhalinHokkaido area. The study indicates that the Japan and Kuril back-arc basins in the Western Pacific underwent their final opening stages during the Middle Miocene (18.1 - 17.7 and 15.9 - 14.2 Ma, respectively), as determined by the SIS method. This process was likely influenced by both global and local factors. One of the local influences on the formation of back-arc basins in the Western Pacific appears to be the influx of cool boreal waters, as indicated by the delta 18 O signals. The Miocene data from the Sakhalin-Hokkaido area, including evidence from Sr, O, and C isotopes, combined with corresponding paleontological, stratigraphic, palaeomagnetic, and palaeobiogeographic data from the Paleogene and Neogene, provide a four-stage chronological sequence of events related to the opening of the Western Pacific ' s back-arc seas. This paper presents original information on the Miocene C-isotope excursion in the Western Pacific, which are correlated with the Monterey Event. Recent data on this topic may have enhanced our comprehension of the conditions required for the formation of hydrocarbons, which are found in substantial amounts in the Miocene of northern Sakhalin and Hokkaido.

FeSiO3 is a pyroxene mineral - ferrosilite. It remains unclear whether FeSiO3 can be synthesized at atmospheric pressure, as most mineral-focused research leans toward the improbability of such a phenomenon. However, in our previous studies of synthetic glasses melted under atmospheric pressure, the FeSiO3 phase was observed. In this work, FeSiO3 crystallization was studied in more detail in the low-alkali synthetic glass of the Na2O-B2O3- SiO2-Fe2O3-FeO system by XRD, DTA, SEM and Mo center dot ssbauer spectroscopy depending on the duration of heat treatment (700 degrees C, 2-130 h). The glass was synthesized using conventional melting at atmospheric pressure at 1500 degrees C in air. Additional heat treatment caused the formation of magnetite (79-0416), iron metasilicate (FeSiO3, 76-1638), and cristobalite. Increasing the duration of heat treatment led to the growth of FeSiO3 spherical inclusions to more than 2000 nm, and the oxidation of magnetite to hematite. The crystallization of iron metasilicate with an anorthite (triclinic) structural type at atmospheric pressure in synthetic borosilicate glass during thermal treatment at 700 degrees C was established. The possible reasons for the formation of the FeSiO3 phase are discussed.

The paper discusses the possible conditions and involvement of sources in genesis of the Khokhol-Repyevka batholith granitoids, that build up the Don terrane in the Volga-Don orogen of the East European Craton. In the batholith, three types of granitoids are distinguished – pavlovsk (quartz monzodiorite–granites, mainly pyroxene-free), potudan (quartz monzogabbro–granodiorites containing pyroxene) and hybrid (quartz monzodiorites, monzonites, quartz monzonites). These three types of rocks are spaсely co-located and have a similar age of formation 2050–2080 Ma, similar geochemical characteristics (high contents of Ba, Sr, highly fractionated REE patterns (GdN/YbN= 2–11)), however, they differ in petrographic and isotopic geochemical parameters. Primary isotope characteristics of sources for rocks of the pavlovsk type εNd(t) = +0.2…–3.7, Sri= 0.70335, for potudan εNd(t) = – 1.7 ... –3.8, Sri= 0.70381–0.70910, for hybrid εNd(t) = – 8.8, Sri= 0.70596. Apart from granitoids, two types of leucogranite dikes were found in the batholith. The first type is characterized by εNd(t) = –3.8 and fractionated HREE patterns (GdN/YbN= 2.1–3.8) and could have formed as a result of deep differentiation of pavlovsk-type magma. The second type is with εNd(t) = –7.8 and less fractionated HREE patterns (GdN/YbN = 1.1–1.6), which presumably appeared as a result of melting of a crustal source at shallow depths. Rb-Sr isotope-geochemical characteristics of rocks of the pavlovsk and potudan types indicate their formation from different sources. In total, at least three sources took part in the formation of the Khokhol-Repyevka batholith: 1) lower (or buried oceanic) crust, predominantly of mafic composition and/or enriched mantle, metasomatized in the Proterozoic, the participation of which is reflected in the composition of the Pavlovsk granitoids; 2) an enriched mantle source, probably represented by subcontinental lithospheric mantle (SCLM), possibly metasomatized during the previous stage of geological development of the region, specific for Potudan-type monzonitoids; 3) Archean crust, consisting mainly of TTG gneisses and metasediments, which underwent melting and participated in the formation of part of the leucogranite dikes and hybrid rocks. The results of thermodynamic modeling indicate that the mixing of two melts contrasting in composition – mafic (potudan-type) and intermediate-felsic (pavlovsk-type) can lead to the formation of only part of the composition of hybrid rocks. The formation of the rest was influenced by the contamination of mafic melt by anatectic melts from the Archean crust of the Kursk block.

The Rb–Sr age of glauconite and the U–Pb LA-ICP-MS age of detrital zircons from sandstones of the Riphean Okos Formation and the Vendian Ust-Pinega Formation were determined in the Keltmen-1 parametric borehole, drilled in the Vychegda trough of the Mezen syneclise in the northern part of European Russia. The model Rb–Sr age of glauconite (870–820 Ma) and the U–Pb age of the youngest detrital zircon (1005 ± 14 Ma) limit the time of accumulation of the Okos Formation to the first half of the Late Riphean, which is in good agreement with the biostratigraphic data available. The U–Pb age of the youngest detrital zircon from the lower part of the Ust-Pinega Formation suggests that accumulation of Late Vendian sandstones into the Mezen Basin began about 575 Ma. Thus, the duration of the stratigraphic hiatus between the Riphean and Vendian is estimated to be about 250–300 Ma. In the Late Riphean and Late Vendian, the Mezen basin was filled with terrigenous material of Archean–Late Proterozoic age (from 3.25 to 1.02 Ga), the sources of which were rocks of the Baltic Shield. In the lower part of the Ust-Pinega Formation, a population of detrital zircon of Vendian age (730–575 Ma) was discovered, a possible source of which could have been rocks of the Proto-Ural–Timan Orogen.

The Sr chemostratigraphic characteristics of marine sediments of the Lower Permian in the Omolon massif of Northeast Asia have been obtained for the first time based on a study of well-preserved brachiopod shells. The 87Sr/86Sr ratio in brachiopods from the Lower Permian regional horizons (regional stages) is within the following ranges: 0.70786–0.70794, Orochian; 0.70744–0.70786, Ogonerian; 0.70745–0.70748, Koargychanian; and 0.70742–0.70735, Khalalian. The obtained Sr-isotope data demonstrate good agreement with the standard variation curve of the 87Sr/86Sr ratio in the Permian Ocean. New Sr-isotope data can be used to correlate the units of the Regional Stratigraphic Scale of Northeast Russia directly with the stages of the International Permian Stratigraphic Scale. Based on recent data, the Asselian age of the Orochian regional stage of the lower part of the Permian section in the Omolon massif has been proved, and the regional correlation of the Upper Sakmarian interval established previously from biostratigraphic data has been refined.

We provide the first strontium isotope data on molluscs from the lowermost Bathonian (Besnosovi Zone) and lower Aptian (Tenuicostatus and Volgensis zones) of the Saratov Volga region and some new O- and C-isotope results on early Aptian fossils from this region. Despite minor diagenetic changes in the earliest Bathonian bivalve shells from the Besnosovi Zone, they show 87Sr/86Sr values very close to that (0.70706) of a well-preserved belemnite rostrum from the same zone. New data on the Sr-isotopic composition of the Jurassic and Early Cretaceous (early Aptian) molluscs from the Saratov Volga region, combined with information on the stable isotope content of these molluscs, suggest that the isolation and partial freshening of the Middle Russian Sea occurred both in the late Oxfordian-Kimmeridgian, as has recently been reported, and in the early Aptian (during the Volgensis, to a lesser extent the Tenuicostatus chrons). This information on the lower Aptian is consistent with the available stable isotope, palynological and palaeoecological data. The pronounced early Aptian 87Sr/86Sr value deviation detected in many well-preserved fossils mainly from the Volgensis Zone seems to have contributed to the active input of radiogenic strontium into the basin during a series of repetitive events associated with some episodic isolations and partial freshening of the Middle Russian Sea, starting at the Tenuicostatus Chron and intensifying at the onset of the Volgensis Chron associated with the Oceanic Anoxic Event 1a (OAE 1a). New isotope data, collected from certain molluscs residing in the lower Aptian Tenuicostatus Zone of the Saratov Volga region and living in normal seawater salinity conditions, when combined with literature evidence sourced from other locations such as Ulyanovsk Volga region, Central Pacific and Germany suggest the presence of early Aptian climatic optimum. Further molluscs from the lower Aptian Volgensis Zone of the study area offer insight into the habitat conditions of Mesozoic ammonoids.

The U–Th–Pb age (SIMS) of magmatic (T = 730–744°C) zircon from intermediate and felsic metavolcanics (1923–1926 Ma) of the supracrustal complex in the Kaskama block of the Inari terrane (northwestern part of the Kola–Norwegian region of the Fennoscandian Shield) were obtained for the first time. This makes it possible to attribute these metavolcanics to the Paleoproterozoic Kalevian suprahorizon. The source of primary metarhyodacite and metabasalt in the Kaskama block was the Paleoproterozoic continental lithosphere not younger than 2390–2384 Ma with an essential contribution of juvenile material (εNd(Т) = +1.2…+2.8). The ages of the spatially related tonalites of the Kuroaivi massif (1936 ± 7 Ma, εNd (T) = +0.1) and of the granitoid massifs of the Southern Pechenga zone (1950–1940 Ma) are older, within the errors of the U–Th–Pb age determinations, than the volcanosedimentary deposits of the Kaskama unit and the Vepsian of the South Pechenga zone. This stage of granite formation separates two geodynamic stages in the Kola–Norwegian region of the Fennoscandian Shield: the early continental rifting in the Yatulian–Ludikovian time and a stage similar to the suprasubductional formation setting of the continental lithosphere of the Inari terrane (1926–1850 Ma).

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23050094

For the first time, the U–Pb and Rb–Sr age of granodiorites of the Birandzhinskii massif, with which the gold mineralization of the Kutyn deposit in Khabarovsk krai is spatially connected, has been determined. Isotope–geochronological data show good convergence: the concordant U–Pb age of zircon is 90.7 ± 1.7 million years (SIMS), and the Rb–Sr age of the rock shaft and minerals is 92.7 ± 0.4 million years. Low εNd( t ) values (about –0.8) and high primary 87 Sr/ 86 Sr ratios (0.7051–0.7053) relative to the parameters of the depleted mantle suggest that granodiorites were formed with the participation of the continental crust. The formation of the Birandzhinskii massif coincides with the second stage of the development of the Khingan–Okhotsk volcano-plutonic belt. The Rb–Sr age of gold-bearing quartz–carbonate–sericite metasomatites of the Kutyn deposit is 79.3 ± 0.5 million years. The obtained isotope–geochronological data indicate a time gap (about 10–12 million years) between the crystallization of granodiorites and the formation of metasomatites, which suggests the allometasomatic nature of gold mineralization.

Middle Jurassic sedimentary strata of Siberia remain poorly studied chemostratigraphically. We contribute to the knowledge with pioneering C, O, and Sr isotopic data for carbonate material of belemnites from the Yuryung-Tumus Peninsula and the lower reaches of the Lena River, as a basis for comprehensive description of the north Siberian Bajocian and lower Bathonian. The obtained chemostratigraphic constraints, with new 87Sr/86Sr ratios and previous delta 13C and delta 18O estimates, also include data for the lower Bathonian in the Sokur section (Central Russia). Despite the limited amount of material, chemostratigraphy, along with the available biostratigraphic data, allows reliable correlation of the Boreal sections with the primary standard of Northwest Europe, which is impossible for these strata with any of the two methods alone. The delta 13C, delta 18O, and 87Sr/86Sr patterns correlate with the records of eustatic, climatic, tectonic, and paleogeographic events. The new delta 18O data and the inferred paleotemperatures for the latest early Bajocian and the Bajocian/Bathonian boundary reveal two excursions of notable seawater warming near the Siberian Arctic coast, which were synchronous with episodes of global sealevel rise. It was presumably during the eustatic events that the N-S Komi Strait (first naming) opened twice in the territory of the Russian Plate. The strait connected the Boreal and Tethyan seas and thus changed the oceanic circulation patterns. Specifically, it opened a gateway for a warm current from the south to northern Siberia responsible for the high seawater temperatures recorded in the delta 18O patterns of belemnites.

Neoarchaean greenstone belts of the Zimbabwe craton host microbialite-bearing limestone successions that range in age from -2.82 to 2.68 Ga. The best-preserved successions, according to Raman geothermometry, are situated in the Bulawayo and Belingwe greenstone belts where they have been subjected to lower greenschist facies metamorphism with a peak temperature of -350 degrees C. Elsewhere, such as in the Masvingo belt, peak temperatures reached -540 degrees C, giving rise to marbles. The carbonate rocks consist mainly of calcite and, to a lesser extent, dolomite and contain variably admixtures of siliciclastic detritus. Samples with the least amount of siliciclastic detritus have shale-normalized rare earth element and Y distribution patterns that indicate open marine conditions, with positive La, Eu, and Gd anomalies, superchondritic Y/Ho ratios, and depleted light rare earth elements relative to the heavy rare earth elements. The & delta;13CVPDB values are -2.0 to 2.1 %o, and & delta;18OVPDB values cluster in the range of -20 to -10 %o. Strontium isotope ratios vary in concert with siliciclastic detritus. The lowest initial 87Sr/86Sr ratio of 0.70155 is close to the depleted mantle value at the time of deposition, suggesting minimal contribution from evolved felsic crust to the Neoarchean ocean. Only samples from the Masvingo belt show radiogenic Sr isotope ratios (0.70202-0.70340) indicative of an epicratonic setting with exposed felsic basement. For the rest, open marine conditions prevailed in a proto-cratonic setting characterized by mantlederived greenstone volcanism. Carbonate sedimentation on the Zimbabwe craton thus predate the fundamental rise in the marine 87Sr/86Sr Sr isotope signal in the late Neoarchaean.