Early Proterozoic Research Articles

Kureika–Tunguska Composite Tectono-Sedimentary Element, northern East Siberia

The Kureika–Tunguska Composite Tectono-Sedimentary Element (KT CTSE) is located in East Siberia. Geologically, it is confined to the homonymic basin in the northwestern part of the Siberian Craton. The basement of the basin is composed of Archean–Early Proterozoic metamorphic rocks. The sedimentary cover is composed of Mesoproterozoic, Neoproterosoic and Early Paleozoic predominantly carbonate rocks capped with a succession of Pennsylvanian–Permian coal-bearing sediments overlain by thick section of Early Triassic tuffs and lavas. The entire sedimentary cover, especially its upper part, is saturated with mafic intrusive bodies. The maximum depth to the top basement can exceed 10 km. The KT CTSE is characterized by a very low degree of exploration. The rather scarce prospecting work carried out on oil and gas in the CTSE has not yet yielded positive results. However, indirect data such as, for example, numerous bitumen fields and evidence of the source rocks are promising for potential discoveries including large hydrocarbon accumulations.

Bayesian Analysis for Remote Biosignature Identification on exoEarths (BARBIE). II. Using Grid-based Nested Sampling in Coronagraphy Observation Simulations for O2 and O3

We present the results for the detectability of the O2 and O3 molecular species in the atmosphere of an Earth-like planet using reflected light at visible wavelengths. By quantifying the detectability as a function of the signal-to-noise ratio (S/N), we can constrain the best methods to detect these biosignatures with next-generation telescopes designed for high-contrast coronagraphy. Using 25 bandpasses between 0.515 and 1 μm and a preconstructed grid of geometric albedo spectra, we examined the spectral sensitivity needed to detect these species for a range of molecular abundances. We first replicate a modern-Earth twin atmosphere to study the detectability of current O2 and O3 levels, and then expand to a wider range of literature-driven abundances for each molecule. We constrain the optimal 20%, 30%, and 40% bandpasses based on the effective S/N of the data, and define the requirements for the possibility of simultaneous molecular detection. We present our findings of O2 and O3 detectability as functions of the S/N, wavelength, and abundance, and discuss how to use these results for optimizing future instrument designs. We find that O2 is detectable between 0.64 and 0.83 μm with moderate-S/N data for abundances near that of modern Earth and greater, but undetectable for lower abundances consistent with a Proterozoic Earth. O3 is detectable only at very high S/N data in the case of modern-Earth abundances; however, it is detectable at low-S/N data for higher O3 abundances that can occur from efficient abiotic O3 production mechanisms.

Reflected Spectroscopy of Small Exoplanets. III. Probing the UV Band to Measure Biosignature Gases

Direct-imaging observations of terrestrial exoplanets will enable their atmospheric characterization and habitability assessment. Considering Earth, the key atmospheric signatures for the biosphere are O2 and the photochemical product O3. However, this O2–O3 biosignature is not detectable in the visible wavelengths for most of the time after the emergence of oxygenic photosynthesis life (i.e., Proterozoic Earth). Here we demonstrate spectroscopic observations in the ultraviolet wavelengths for detecting and characterizing O2 and O3 in Proterozoic-Earth-like planets, using ExoReL . For an O2 mixing ratio 2–3 orders of magnitude less than the present-day Earth and an O3 mixing ratio of 10−7 to 10−6, we find that O3 can be detected and its mixing ratio can be measured precisely (within 1 order of magnitude) in the ultraviolet (0.25–0.4 μm), in addition to visible-wavelength spectroscopy. With modest spectral resolution (R = 7) and signal-to-noise ratio (∼10) in the ultraviolet, the O3 detection is robust against other potential gases absorbing in the ultraviolet (e.g., H2S and SO2), as well as the short-wavelength cutoff between 0.2 and 0.25 μm. While the O3 detection does not rely on the near-infrared spectra, extending the wavelength coverage to the near-infrared (1–1.8 μm) would provide essential information to interpret the O3 biosignature, including the mixing ratio of H2O, the cloud pressure, and the determination of the dominant gas of the atmosphere. The ultraviolet and near-infrared capabilities should thus be evaluated as critical components for future missions aiming at imaging and characterizing terrestrial exoplanets, such as the Habitable Worlds Observatory.

Junction zone structure of the Sarmatia, Volga-Uralia, and Fennoscandia microcontinents as part of the East European Platform basement

Research subject. The structure of the pre-Paleozoic deposits and different-age Precambrian basement of the East European platform based on geological and geophysical data.Aim. To trace the evolution of the area under study and to study the geodynamics of processes in order to reconstruct the paleostructure of the Baltica continent.Materials and methods. The current state of the consolidated crust was studied using a geological interpretation of seismic profiling data (transects: “EB-1”, “Tatseys”, “Magnit”) and materials of gravity and geomagnetic surveys. The material composition of the basement rock complexes was studied based on drilling data.Results. The Baltica continent forms the basement of the modern East European Platform. After the Sveconorwegian folding about 1000 Ma, Baltica had become part of the Rodinia supercontinent and merged with the North American platform. A passive continental margin with the Riphean-Vendian sedimentary cover was formed on the eastern Baltica edge. The Petrozavodsk-Khopersky orogen, which arose in the place of the Karelian (Early Proterozoic) Petrozavodsk-Khopersky paleoocean, was reconstructed. The submeridional suture was a result of this paleoocean closure. The orogen structure, as well as that of the paleocontinent Baltica, was disturbed and partially destroyed by subsequent tectonic processes, which continue at the present time with modern rifting.Conclusions. The closure of the Petrozavodsko-Khopersky paleoocean led to the formation of the Baltica continent, which included three microcontinents – Sarmatia, Fennoscandia and Volga- Uralia. Baltica became part of the Rodinia supercontinent during the Sveconorwegian orogeny. Volga-Uralia was located on the border with the PaleoPacific and had a Late Riphean sedimentary cover of the Rodinia supercontinent. This margin underwent destruction during a collision with the Timanides. The scale and extent of the reconstructed Petrozavodsk-Khopersky orogen is comparable to Paleozoic orogenic belts, such as Cadomides, Caledonides, Variscides, or Timanides.

Relationship between radiogenic heat production in granitic rocks and emplacement age

Granites play a crucial role in the Earth's thermal regime and its evolution. Radiogenic heat production (RHP) by the decay of radioactive elements (U, Th, K) in granites is a significant parameter in estimating the thermal structure of the lithosphere. RHP variability of granites with their emplacement ages could provide insights for thermal modeling in different geological epochs. An aggregated RHP from 2877 globally-distributed granitic samples of continental crust are analyzed for this study; these sample cover the entire geological history. The average bulk RHP in all types of granitic rocks of all ages is 2.92 ± 1.86 μW/m3. The RHP tends to increase gradually with progressively younger geologic emplacement age, based on a statistical analysis of the data. However, the youngest granites do not necessarily have the highest RHP. The mean RHP in 181 representative Cenozoic Himalayan leucogranites— which are the youngest granites found on Earth, is as low as 1.84 μW/m3. This is probably related to the initial conditions of magma formation, magmatic source material, and differentiation processes in the Himalayan–Tibetan plateau. By correcting the decay factor, variations of the RHP in the emplaced granitic rocks are obtained, indicating the changing levels of heat production and different thermal regimes on Earth in various geological epochs. The highest RHP in granitic rocks emplaced in the Archean and Early Proterozoic corresponds to two global-scale collisional events during supercontinent cycles, at 2.7 and 1.9 Ga respectively. RHPs of granites can be an important indicator in the study of Earth's thermal regime and its evolution.

Age of Gold Mineralizations of Dongping and Xiaoyingpan Gold Deposits in North China Craton: Constraints from the Zircon U–Pb Dating and Fission-Track Analysis

Northwest Hebei province is one of the gold-producing areas in China. Based on a geochronological analysis of the Zhangjiakou-Xuanhua area, zircon U–Pb with an age of 2487–142 Ma and zircon fission-track (ZFT) with a cooling age of 155–66 Ma were obtained. Zircon U–Pb dating of the Xiaoyingpan deposit revealed two ages of 2487 ± 92 Ma and 1745 ± 89 Ma, representing the Archaean and Early Proterozoic metamorphism of the Sanggan Group. The ZFT cooling age of 155 ± 10 Ma represents the deposit’s lower limit metallogenic age. The Shuiquangou (SQG) alkaline complex emplaced at 388.9 ± 3.0 Ma and experienced magmatic activities until the Late Devonian (ca 377 Ma), considering the response to the collision between the Siberian plate and the North China Craton. The emplacement age of Shangshuiquan (SSQ) granite is 142.3 ± 1.1 Ma and is coeval with the thinning of the North China Craton during the Late Jurassic–Early Cretaceous. The intrusion of SSQ might partially reheat the SQG complex, resulting in the Yanshanian gold mineralization in Dongping. The SQG complex and SSQ granite underwent a relatively consistent rapid cooling process in the Cretaceous based on the ZFT ages.

Age and Provenance of the Daur Series Sedimentary Rocks (Riphean), Argun Continental Massif: Results of U–Th–Pb and Lu–Hf Isotope Studies of Detrital Zircons

The first results of U–Th–Pb and Lu–Hf isotope studies of detrital zircons from sedimentary rocks of the Daur Series (Argun continental massif of the Central Asian Fold Belt) are presented. It is established that the youngest detrital zircons from sandstones of the Urulungui and Dyrbylkei formations of the Daur Series have the ages of 899 and 771 Ma, respectively. These data determine the maximum depositional age of the Daur Series and agree with the current ideas about the Late Riphean age of this series. The majority of Late Riphean detrital zircons from sedimentary rocks of the Daur Series are characterized by Early Precambrian Lu–Hf model ages. In addition, the Archean and Early Proterozoic detrital zircons are present in the studied rocks. This suggests either the presence of ancient (Early Precambrian) recycled material in the sedimentary rocks of the Daur Series or the fact that the studied part of the Argun massif is an “exotic block,” whose origin requires additional research.

Age of Detrital Zircons and Sources of Terrigenous Deposits of the Olokit Zone (Northern Baikal Region)

This work presents the results of U–Th–Pb (LA–ICP–MS) geochronological study of detrital zircons from terrigenous deposits of the Olokit zone of the Baikal–Patom fold–thrust belt of the Siberian Craton. It was established that the age of deposits of the Olokit Formation is in the range of 0.86–0.72 Ga. Archean and Early Proterozoic rocks of the southern part of the Siberian Craton and the Neoproterozoic complexes of the Baikal–Muya belt were the sources of detrital zircons of the terrigenous rocks of the Olokit zone. There is no evidence of rocks that could have been the sources of Mesoproterozoic (1.08–1.46 Ga) and Late Paleoproterozoic (1.65 Ga) detrital zircons for the deposits of the Ondok paleo-uplift in the Olokit zone, in the Baikal mountain region, and in the Siberian Craton. The data obtained and the results of paleomagnetic reconstructions indicate that Siberia and Laurentia in the Meso- and Early Neoproterozoic were in a fixed position relative to each other; an unknown continental block composed of Mesoproterozoic rocks could have occupied the space between them.

Plate Tectonics and the Archean Earth

If we accept that a critical condition for plate tectonics is the creation and maintenance of a global network of narrow boundaries separating multiple plates, then to argue for plate tectonics during the Archean requires more than a local record of subduction. A case is made for plate tectonics back to the early Paleoproterozoic, when a cycle of breakup and collision led to formation of the supercontinent Columbia, and bimodal metamorphism is registered globally. Before this, less preserved crust and survivorship bias become greater concerns, and the geological record may yield only a lower limit on the emergence of plate tectonics. Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. This transition is recorded by changes in geochemical proxies and interpreted based on numerical modeling. Improved understanding of the secular evolution of temperature and water in the mantle is a key target for future research. ▪ Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. ▪ Plate tectonics can be demonstrated on Earth since the early Paleoproterozoic (since c. 2.2 Ga), but before the Proterozoic Earth's tectonic mode remains ambiguous. ▪ The Mesoarchean to early Paleoproterozoic (3.2–2.3 Ga) represents a period of transition from an early tectonic mode (stagnant or sluggish lid) to plate tectonics. ▪ The development of a global network of narrow boundaries separating multiple plates could have been kick-started by plume-induced subduction.

Geochemical Environments of Precambrian Sedimentation

Distribution of carbonate sediments of different composition in the Precambrian section is reviewed. It is shown that calcium carbonates prevailed in the Archean–initial Early Proterozoic, and iron carbonates dominated in the banded iron formations. Dolomites and magnesites were more widespread in the Proterozoic. Such distribution suggests an acid reductive geochemical environment in the Archean–Early Proterozoic, which determined the corresponding character of continental weathering: appearance of dissolved forms of Fe and silica that were transported to ancient oceans and deposited, resulting in the formation of ferruginous–quartzites or jaspilites (banded iron formations). Large-scale and almost explosive development of photosynthesizing cyanobacteria at the Archean–Proterozoic boundary stipulated, first, the utilization of carbon dioxide and, correspondingly, the change of acid environment into alkaline, leading to the formation of magnesian carbonates; and, second, to the appearance of free oxygen.

Age, HF-isotope systemantics of detritial zircons and the source of conglomerates of the mt. Southern Demerdzhy, Mountainous Crimea

The U–Pb dating and Hf isotope systematics of detrital zircons from a sandstone interbed in the section of the upper conglomerate sequence of the Mt. South Demerdzhi were carried out.
 The dominant populations of detrital zircons in the studied sample characterize episodes of magmatic activity within the source of the Upper Jurassic conglomerates. Magmatism was manifested in the Vendian-Cambrian, Carbon-Triassic and Late Jurassic. The åHf values of detrital zircons of these ages indicate the insignificant role of the ancient (Archean–Early Proterozoic) continental crust in the protolith of magmatic chambers. The similarity of the detrital zircons age distribution from the Middle Jurassic and Upper Jurassic conglomerate strata suggests that they are molasses of the Cimmerian orogen. The absence of products of Middle Jurassic magmatism in molasses of the Cimmerian orogen, which we fixed, limits position of the Cimmerian orogen in the southern part of the Scythian plate. It is shown that the primary source of the Precambrian detrital zircons were mobilized within the Cimmerian orogen the crustal fragments of the Peri-Gondwanan origin, rather than the basement complexes of the East European Platform, similar to the complexes of the Ukrainian shield.
 The reconstruction of the main stages of the accumulation of the coarse-grained strata of the Mountaineous Crimea in the context of the tectonic evolution of the southern margin of Laurasia during the Mesozoic is presented.

Age, Hf-Isotope Systematic of Detrital Zircons and the Sources of Conglomerates of the Southern Demerdzhi Mountain, Mounatinous Crimea

The U–Pb dating and Hf isotope systematics of detrital zircons from a sandstone interbed in the section of the upper conglomerate sequence of the Mt. Southern Demerdzhi were carried out. The dominant populations of detrital zircons in the studied sample characterize episodes of magmatic activity within the source of the Upper Jurassic conglomerates. Magmatism was manifested in the Vendian-Cambrian, Carbon-Triassic, and Late Jurassic. The eHf values of detrital zircons of these ages indicate the insignificant role of the ancient (Archean–Early Proterozoic) continental crust in the protolith of magmatic chambers. The similarity of the detrital zircons age distribution from the Middle Jurassic and Upper Jurassic conglomerate strata suggests that they are molasses of the Cimmerian orogen. The absence of products of Middle Jurassic magmatism in molasses of the Cimmerian orogen, which we fixed, limits position of the Cimmerian orogen in the southern part of the Scythian plate. It is shown that the primary source of the Precambrian detrital zircons were mobilized within the Cimmerian orogen the crustal fragments of the Peri-Gondwanan origin, rather than the basement complexes of the East European Platform, similar to the complexes of the Ukrainian shield. The reconstruction of the main stages of the accumulation of the coarse-grained strata of the Mountaineous Crimea in the context of the tectonic evolution of the southern margin of Laurasia during the Mesozoic is presented.

Rare Elements—Markers of the Formation Setting of Manganese and Iron Ores in the Kalahari and Postmasburg Manganese Fields (South Africa): Communication 2. Postmasburg Iron and Manganese Field

Deposits in the Postmasburg field, South Africa, confined to rocks of the Transvaal Supergroup, occupy a significant place in world resources of the manganese and iron ores. The ore deposits represent karst residual accumulations. Several elements (B, Cr, Ni, Zn, Ge, As, Se, Mo, Ag, Cd, Sb, Te, W, Pb, REE) in them make up an assemblage providing insight into the geochemistry of the processes of ore formation. Among these elements, Mo, As, Ag, and REE are most representative. Molybdenum is marked by the chemosorptional incorporative nature of accumulation in these ores, sometimes with the formation of epictic overgrowths of the ferrimolybdate-type minerals. Arsenic, which is leached from the substrate rocks and accumulated in karst ferromanganese and iron ores, reflects total impact of the dominating iron oxide minerals upon its mobility. The behavior of Ag is controlled by the processes of supergene alteration of Archean–Early Proterozoic carbonate rocks and ores of the banded iron formation (BIF). Comparison of the distribution of rare earth elements (REE) in the karst iron, ferromanganese, and manganese ores, as well as banded iron ores, demonstrates that these elements display similar values of the cerium (C/Ce*) and europium (Eu/Eu*) anomalies, but differ in terms of the HREE and LREE fractionation (typical values: Ce/Ce* = 0.7‒1.0; Eu/Eu* = 0.8‒1.1). Banded iron and associated manganese ores were accumulated in the marginal anoxic-dysoxic marine basin, which was constrained by the continental land. Anoxic and dysoxic conditions were created by an intense hydrothermal activity.

Mineral and geochemical composition of the Onega Ice Lake sediments

An analysis of the mineral-geochemical composition and structure of the Holocene–Pleistocene bottom sediments was performed on Lake Polevskoye, a small lake in the northern Lake Onega area in Russian Karelia, which is considered representative to describe the Late Weichselian Onega Ice Lake sediments. The analysis was accomplished using modern analytical methods, including scanning electron microscopy and ICP-MS, which allowed us to interpret their genesis in a new light. It is assumed that the distribution of rare earth elements (REE) in the bottom sediments of the recent Lake Onega and in the sediments of Onega Ice Lake will improve our understanding of the history of sedimentogenesis. It is apparent in the distribution of REE, their composition and data on the geochemical and mineral composition of the Holocene bottom sediments of Lake Onega and Lake Polevskoye (varved clays) that in their composition of terrigenous matter the material introduced from the north-western part of the catchment area is mainly composed of Archaean and Early Proterozoic crystalline complexes. However, the values of several indicator ratios of elements in the lower part of the of varved clays with shungite interlayers indicate the presence of mixing of clastic material from two sources of different geochemical origin: the north-western part of the catchment area (source of shungite rocks) and the south-eastern part of the catchment area (Phanerozoic sedimentary rocks of the Russian Platform).

The First Evidence for Mesozoic Magmatism on the Western Slope of the Southern Urals

Numerous Riphean mafic dykes cross-cut Precambrian clastics and carbonates on the western slope of the Southern Urals. An andesite dyke with an isotope age of 71 ± 1 Ma (U–Pb SHRIMP II on zircons) encountered among metamorphic rocks of the Taratashskii complex (Archean–Early Proterozoic) at the northern terminus of the Bashkirian meganticlinorium along with two gabbro bodies with zircons of similar ages near the town of Bakal provides the first evidence of Mesozoic magmatism in this region.

Keivy Paraschists (Archean–Early Proterozoic): Nanobacteria and Life

Nanobacteria, buried in situ, were discovered in the Early Precambrian paraschists (Keivy, Kola Peninsula). It is suggested that occurrence of nanobacteria indicates that a biological factor played a role in the formation of enclosing rocks.

Petrology and age of Precambrian Aksu blueschist, NW China

The first occurrence of blueschist in the Neoproterozoic signifies a transition in thermal regime from a warmer Archean–Proterozoic Earth to modern colder subduction characterized by blueschist facies or ultrahigh pressure metamorphism. To evaluate the thermal regime of the transition, we apply phase equilibrium modeling to the Aksu blueschist in northwestern China to constrain the pressure (P) and temperature (T) conditions of the peak mineral assemblages and to calculate the thermal regime recorded by the blueschist. Phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–O system indicates that the peak mineral assemblage (glaucophane/riebeckite + epidote + phengite + albite + quartz + chlorite) was developed at P–T conditions of 6.8–8.7 kbar and 320–410 °C for a stilpnomelane-bearing epidote blueschist (AK04) and of 6.5–8.0 kbar and 310–380 °C for an epidote blueschist (AK06). These P–T data define an apparent thermal gradient of ∼470 °C/GPa for the peak stage which is typical of the high-pressure intermediate type metamorphic facies series. In addition, we present new LA–ICP–MS U–Pb ages from detrital zircons from two blueschists (AK04 and AK06) and SHRIMP U–Pb ages from zircon from a cross-cutting post-metamorphic mafic dyke (AK134) to constrain the age of the metamorphism. The youngest concordant detrital zircon ages give a maximum depositional age for the blueschist protoliths of 806 ± 10 (AK04) and 803 ± 10 Ma (AK06), respectively, and the SHRIMP U–Pb ages of zircons from the mafic dyke yields a weighted mean age of 769 ± 19 Ma interpreted to date crystallization. These results constrain the age of Aksu blueschist facies metamorphism to between 769 ± 19 and c. 805 Ma.

Problems of geodynamics of the Ukrainian shield in Precambrian

The main achievements of Ukrainian geologists and geophysicists in the studies of processes of geodynamic development of the Ukrainian Shield in the Archean—Early Proterozoic are considered as well as the main problems and tasks for their solving. For further effective studies of Precambrian geodynamic processes it is proposed to increase the extent of examinations of the deep structure of the Shield by laying international geo-traverse of deep seismic sounding, which crosses the typomorphic geodynamic zoning of the Shield in latitudinal direction. The extent of radiogeochronologic and deep petrologic studies of the Shield and their integration with paleomagnetic and tectonophysical data is also important.

Caledonian formation of gold-bearing sulfide depositions in Early Proterozoic gabbroids in the northern Ladoga region

We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ∼450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = 238U/204Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores.

The zircon-age distribution in metamorphic rocks of the Taratash block, Southern Urals (an initial provenance signal)

A concept for the interpretation of the initial provenance signal in rocks of the Taratash block (in the Southern Urals) using the zircon isotope dating of the Archean and Early Proterozoic igneous and metamorphic rocks was substantiated and carried out. Based on 132 zircon-age datings with a discordance of as much as 10%, a probability-density diagram was compiled first to compare these age data with those of detrital zircons from Lower Riphean sandstones of the Ai Formation and, secondly, with the probability density of zircon ages in metamorphic rocks of the Aleksandrovsk block, which is located to the east. The similarity of the distributions was verified using the Kolmogorov–Smirnov test.