A. V. Samsonov

Geodynamics of the eastern margin of Sarmatia in the Paleoproterozoic

The eastern margin of Sarmatia comprises the Paleoproterozoic (2.1–2.05 Ga) rock associations of the eastern Voronezh Crystalline Massif, including the Lipetsk-Losevo volcanic-plutonic belt and the adjacent East Voronezh lithotectonic zone composed of metasedimentary rocks of the Vorontsovka Group. The isotopic and geochemical study of the available drill cores that characterize the main rock associations of the Lipetsk-Losevo belt and its nearest framework allowed us to furnish evidence for the formation of this belt in the regime of an island arc at the active margin of the Archean continent above a low-angle subduction zone. The juvenile isotopic and geochemical signatures of metaturbidites of the Vorontsovka Group indicate that only a fast growing mountain edifice with the Lipetsk-Losevo Belt in its highest part (foreland) could have been a provenance of the flysch basin. It is proposed to name this Paleoproterozoic mountain system the East Sarmatian Orogen. The hinterland of this orogen embraced the megablock of the Kursk Magnetic Anomaly as a part of the Voronezh Massif and the Azov Block of the Ukrainian Shield. It has been shown that the East Sarmatian Orogen was formed in the same way as accretionary orogens of the Cordilleran type.

Sources of Archean sanukitoids (High-Mg subalkaline granitoids) in the Karelian craton: Sm-Nd and Rb-Sr isotopic-geochemical evidence

The Sm-Nd systematics of sanukitoids with an age of 2715–2740 Ma in the Western, Eastern, and Central domains of the Karelian craton with various crustal evolutionary histories indicates that the mafic and acid rocks of the sanukitoid series were derived from two contrasting sources: enriched lithospheric mantle and lower crust. The basic sanukitoids of the Western domain were derived from the mantle enriched long before its melting [ɛNd(2715) = −0.48 ± 0.22]. The source of the acid magmas was the young juvenile crust of TTG composition [ɛNd(2715) increases to +1.2]. The mantle source of mafic sanukitoids in the Eastern domain was enriched shortly before melting [ɛNd(2740) = +1.58 ± 0.01], whereas the acid melts came from an ancient crustal source [ɛNd(2740) decreases to −3.0]. For sanukitoids in the Central domain, the time span between the enrichment of the mantle source and its melting was the shortest [ɛNd(2725) = +2.05 ± 0.15], and the contribution of the juvenile TTG crust was insignificant [ɛNd(2725) deceases to +1.7]. The variations in the isotope characteristics of the acid members of the sanukitoid series are consistent with the known age heterogeneity of the crust of the domains. The lateral isotopic-geochemical heterogeneity of the lithospheric mantle source of the sanukitoids is thought to have been related to its two-stage reworking (at 3.2 and 2.8–2.9 Ga) under the effect of TTG granitoids, which are regarded as the melting products of the subducted oceanic crust. The sanukitoids provide information on the geochemical structure of the Archean lithosphere, which is reflected in Archean crust-building processes. The Rb-Sr isotope system of the Neoarchean sanukitoids underwent transformations on the mineralogical scale and within small massifs in the course of at least two Paleoproterozoic tectono-thermal events. A trace of the event at ∼2.1 Ga is left in the Rb-Sr system of monomineralic fractions from a weakly deformed syenite of the sanukitoid series in the Central Domain. Later event (∼1.7 Ga) was recorded in the minerals of the Teloveis sanukitoid massif, which hosts a gold mesothermal deposit in the Western domain. Monomineralic fractions of muscovite and biotite from the wall-rock metasomatites and of plagioclase, microcline, and biotite from metasomatites away from the orebodies yield isochron ages of 1719 ± 60 and 1717 ± 27 Ma. This age of the metasomatic alterations of the Neoarchean sanukitoids is able to explain the broad and unsystematic variations in the Rb-Sr isotope-geochemical characteristics of these rocks. Our data on the Paleoproterozoic age of the mesothermal gold ore mineralization at the Teloveis deposit provide additional lines of evidence for the complex tectonic and metallogenic evolution of the Karelian GGT in the Early Precambrian.

Paleoproterozoic A- and S-granites in the eastern Voronezh Crystalline Massif: Geochronology, petrogenesis, and tectonic setting of origin

The eastern part of the Voronezh Crystalline Massif hosts coeval S- and A-granitoids. The biotite-muscovite S-granites contain elevated concentrations of Si, Al, and alkalis (with K predominance) and relatively low concentrations of Ca, Mg, Ti, Sr, and Ba, show pronounced negative Eu anomalies, and have low concentrations of Y and HREE. The biotite A-granitoids are enriched in Fe, Ti, P, HFSE, REE and have strongly fractionated REE patterns with deep Eu minima. According to their Rb/Nb and Y/Nb ratios, these rocks are classified with group A2 of postcollisional granites. The SIMS zircon crystallization age of the granitoids lies within the range of 2050–2070 Ma. Both the A- and the S-granitoids have positive ɛNd(T) values, which suggests that they should have had brief crustal prehistories and were derived from juvenile Paleoproterozoic sources. The simultaneous derivation of the A- and S-granites was caused by the melting of the lower crust in response to the emplacement of large volumes of mafic magma in an environment of postcollisional collapse and lithospheric delamination with the simultaneous metamorphism of the host rocks at high temperatures and low pressures. The S-granites are thought to be derived via the melting of acid crustal material in the middle and lower crust. The A2 granites can possibly be differentiation products of mafic magmas that were emplaced into the lower crust and were intensely contaminated with crustal material.

Geochronology and geochemistry of acid metavolcanites, Losevo Series, Voronezh crystalline massif

The Losevo structuralformation zone is a part ofthe East Sarmatian orogen [8] in the junction of theSarmatian and VolgoUralian segments of the EastEuropean Craton [9]. This structuralformation zoneis composed of rocks of the Losevo Series, which contains metavolcanites of the contrast basalt–plagiorhyolite and polymodal basalt–andesite–plagiorhyoliteassociations among the metaterrigenous units [5].Accumulation of the Losevo Series is referred to boththe Late Archean (in this case, the similarity of theLosevo structuralformation zone and greenstonebelts of the Kursk magnetic anomaly is implied [1, 3])and the Early Proterozoic (similarity between theLosevo Series and Vorontsovo Series [2, 4]). Due tothe absence of unambiguous evidence for both suggestions, the modern scheme of stratigraphy and magmatism for the Voronezh crystalline massif attributes theage of the Losevo Series to the Early Archean to LateProterozoic [7].To solve the problem regarding the age and geodynamical position of the lower (Strelitsa) stratum of theLosevo Series, which includes the contrast volcanismproducts, mineralogical–petrographic, petrochemical, geochemical, and isotopic studies of the metamorphized acid rocks from the reference section havebeen carried out.Plagiodacites, their differentiates (plagiorhyodactes and plagiorhyolites), and facial analogs (tuffs)compose the upper parts of the Strelitsa stratum section, where they interbeds with greenstone rocks(metabasites of TMORB type tholeiite series [5]).Contacts between rocks are clear and sharp; in thenearcontact zones of paleoeffusive and paleosubvolcanic bodies, metabasite inclusions (lenses of up to 2 ×4 cm in size) are presented. The predominant part ofacid rocks possesses a relic porphyritic texture.Impregnations are represented by quartz and plagioclase (the latter dominates). Granoblastic and lepidogranoblastic groundmass consists of quartz (40–60 vol %), albite–oligoclase (50–30), sericite (10–15),and chlorite and epidote (0–10). In the zones ofhigher

HT/LP metamorphic zoning in the eastern Voronezh Crystalline Massif: Age and parameters of metamorphism and its geodynamic environment

The Vorontsovskii terrane of the Eastern Sarmatian orogen underwent HT/LP metamorphism at temperatures of 430–750°C and pressures of 3–5 kbar. The TIMS monazite age of this metamorphism is 2067 ± 9 Ma and corresponds to the most probable age range (2050–2080 Ma) when large volumes of mafic and granitoid intrusions were emplaced. The time spans of the magmatic activity and metamorphic event are closely similar, which suggests that the melts could have served as sources of metamorphic heat. However, geological data on the relations between the metamorphic zones and magmatic bodies (the largest of the mafic, diorite, and granitoid intrusions are hosted in zones of low-temperature metamorphism) and the occurrence of relict metamorphic mineral assemblages and crystallization foliation in metapelite xenoliths in these intrusions suggest that the intrusions were emplaced after the metamorphism. The most probable reason for the HT/LP metamorphism was an increase in the heat flux in the course of viscous deformations and folding in the warm lithosphere of the young Paleoproterozoic Vorontsovskii terrane during collision processes.

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.

Within-Plate (Intracontinental) and Postorogenic Magmatism of the East European Craton as Reflection of the Evolution of Continental Lithosphere

A comparative analysis of within-plate (intracontinental) and orogenic magmatic series formed during various evolution stages of the East European Craton (EEC) was performed using geological-petrological, geochemical, and isotopic data. The example of Baltic shield indicates that the compositions and tectonic settings of mantle melts in the Early Precambrian (Archean and Early Paleoproterozoic) significantly differed from those in the Phanerozoic. The Early Precambrian magmas were dominated by high-Mg low-Ti melts of the komatiite-basaltic and boninite-like series; this tectonomagmatic activity was determined by the ascent of mantle superplumes of the first generation, which originated in the depleted mantle. In the interval of 2.3–2.0 Ga, high-Mg mantle melts gradually gave place to the Fe-Ti picrites and basalts that are typical of within-plate Phanerozoic magmatism; at ∼2 Ga, plume tectonics of the Early Precambrian gave way to plate tectonics. This is considered to be linked to the activity of mantle superplumes of the second generation (thermochemical), which originated from the liquid metallic core/mantle interface. Owing to the presence of fluid components, these superplumes reached much higher levels, where spreading of their head portions led to the active interaction with overlaying thinned rigid lithosphere. Sm-Nd isotopic studies showed that orogenic Neoarchean and Middle Paleoproterozoic magmatism of the Baltic shield was connected to the melting of the lithospheric mantle and crust; the melting of crustal sources gave rise to felsic members of the considered complexes. The systematic geochemical variations observed in these rocks with time presumably reflect a general trend toward an increase of the thickness of the continental crust serving as the basement for orogens. Beginning at ∼2 Ga, the Meso, Neoproterozoic, and Phanerozoic including, no systematic variations were observed in the isotopic-geochemical characteristics of within-plate magmatism. All considered age sections demonstrate that isotopic-geochemical characteristics of parental mantle melts were strongly modified by crustal contamination. Mesoproterozoic magmatism of EEC was unique in the development of giant anorthosite-rapakivi granite complexes. Kimberlites and lamproites were repeatedly formed within EEC in the time interval from 1.8 to 0.36 Ga; their maximal development was noted in the Late Devonian. It was shown that only kimberlites derived from weakly enriched mantle are diamondiferous in the Arkhangelsk province; in the classic diamond provinces (Africa and Yakutia), diamondiferous kimberlites were derived from both depleted and enriched mantle.

An Archaean Tonalite–Trondhjemite–Granodiorite Association of the Kursk Block (Voronezh Massif): Composition, Age, and Correlation with the Ukrainian Shield

Framing of the Archaean greenstone belts of the Kursk Block (KB) of the East Sarmatia preserves rocks of the TTG association: those do not form massifs with distinct boundaries, but occur as fields gradually transiting into gneisses and migmatites. According to Sm–Nd isotope–geochemical data, the TTG are characterized by positive values of εNd(2960) = +0.3…+1.6 and protolith model ages of ТNd(DM) = 3100–3200 Ma. Magmatic protoliths of the Kursk Block TTG were formed about 2960 Ma by melting from a juvenile basite source. These age estimates are significantly younger than heterochronous (3.19, 3.13 and 3.07 Ga) TTGs of the Middle Dnieper granite–greenstone terrane. On the other hand, the similarity of εNd(T) implies a single source of their protoliths. Consequently, the KB TTGs, apparently, are a result of transformation of an older sial crust preserved within the Middle Dnieper Block.

The Sarmatia megablock as a fragment of the Vaalbara supercontinent: Correlation of geological events at the Archean‒Paleoproterozoic transition

The results of correlation between geological events in the period of 2.8‒2.0 Ga provide grounds to assume that the Sarmatia lithospheric megablock definable in the southern part of the East European Craton belonged to the ancient Vaalbara supercontinent consisting of the Pilbara and Kaapvaal cratons. In the period of 2.8‒2.6 Ga, all of them represented fragments of the continental crust consolidated at approximately 2.8 Ga and subjected to continental rifting, which was accompanied by intense basite volcanism. In the period of 2.50‒2.45 Ga, these three cratons were characterized by similar tectonic settings and accumulation of banded iron formations. Precisely these banded iron formations of the largest Transvaal, Hamersley, Kursk, and Kremenchug‒Krivoi Rog iron ore basins accumulated in the period of 2.50‒2.45 Ga in a single oceanic basin serve as a basis for adequate paleotectonic reconstructions of the Vaalbara supercontinent. In the period of 2.45‒2.20 Ga, all three cratons were subjected to a long-lasting break in sedimentation followed by activation of continental rifting with terrigenous sediment deposition, which terminated with basite volcanism ca. 2.2 Ga. These events gave start to the Vaalbara breakup, which represented a multistage process with alternating divergence and convergence phases of supercontinent fragments until the Kaapvaal and Zimbabwe, Pilbara and Yilgarn, and Sarmatia and Volgo-Uralia cratons, respectively, became eventually united.

The 2405 Ma doleritic dykes in the Karelian Craton: A fragment of a Paleoproterozoic large igneous province

New data on the age and composition of doleritic dykes of the Karelian Craton on the Fennoscandian Shield are reported. Based on the results of U–Pb dating of baddeleyite, a new age episode (2404 ± 5 Ma) in the formation of basic rocks on the Karelian Craton is established. Comparison of the composition of the studied dolerite with that of dykes of the same age from other Archean cratons worldwide shows their essential similarity and allows us to suggest their formation within a single large igneous province. The data obtained support the current models of supercontinental reconstructions for the period of 2400 Ma.