K. A. Savko

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

Cambrian magmatic activation of the East European Platform

822 The East European Platform (EEP), since assem bly of its constitutent landmasses in the Paleoprotero zoic, has undergone repeated magmatic activation in the Mesozoic, Neoproterozoic, and Phanerozoic. The youngest manifestations of the Neoproterozoic (Ven dian) within plate magmatism are represented by alkali volcanic rocks and their tuffs in the Belomorian rift system (570 ± 8 to 555.3 ± 0.3 Ma) [7, 9] as well as basalts and tuffs of the Volhyn–Brest flood basalt province (551 ± 4 Ma) [8]. Subsequent magmatic acti vation of the EEP occurred as late as the Devonian and resulted in the formation of basalts in the eastern part of the Voronezh crystalline massif [2], dolerite dikes (389 ± 4 to 355 ± 10 Ma) [1, 10] and alkaline rocks (380–360 Ma) [4] in the Baltic shield, and kimberlites in the White Sea region (360–340 Ma) [3, 5]. From the Upper Vendian to the Devonian time, no mag matic activity is known yet in the EEP.

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.

Paleoproterozoic evolution of the arc–back-arc system in the east Sarmatian Orogen (East European Craton): Zircon SHRIMP geochronology and geochemistry of the Losevo volcanic suite

The East European Craton has a thick sequence of volcano-sedimentary rocks preserved in the Losevo belt that developed along the junction between Sarmatian and Volgo-Uralian microcontinents. The major lithologies of the Losevo terrain (LT) are a dominant bimodal volcanic suite and a basalt–andesite–dacite–rhyolite assemblages (BADR). The LT rocks have been divided from lower to upper sequences into the Terrigene, Strelitsa and Podgornoye Formations, but the stratigraphic subdivisions have not been geochronologically tested. Here we present geochemistry and SHRIMP zircon geochronology of volcanic rocks from the LT. The volcanic suite from the Terrigene Formation has tholeiitic and calc-alkaline affinites, significant enrichment in LILE and LREE and strong depletion in HFSE with εNd(t)=+ 2.6, whereas the felsic dikes display an A-type affinity, with typical enrichment in Zr, Nb, Y, and depletion in Sr and Ti, fractionated REE patterns, and strong negative Eu anomalies with εNd(t) in the range of -0.5 to 2.6. The bimodal volcanic suite of the Strelitsa Formation is composed of tholeiites displaying minor depletion in LREE, slight enrichment of LILE, no or weak depletion of Nb resembling transition MORB with εNd(t)=+3.0 to +3.6) and rhyolites with high LREE/HREE, high Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+1.8 to +2.9) resembling slab-derived high pressure adakitic melts. The volcanic rocks of the Podgornoye Formation are bimodal with tholeiitic chemistry, lack enrichment in LILE and LREE and have a slight depletion in HFSE (εNd(t)=+3.7) together with rhyolites having high LREE/HREE, moderate Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+2.1 to +2.6) resembling slab derived relatively low-pressure adakite-like melts. The BADR assemblage has significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. Geochronological data indicate that the early LT volcanic rocks were formed during the early (Terrigene Formation) stage of intra-continental arc with a continental basement whereas the Strelitsa bimodal volcanic rocks were formed during a middle stage of back-arc extension and the Podgornoye bimodal volcanic rocks and BADR were formed during a later stage intra-oceanic arc. The identification of a 2170 to 2120 Ma back-arc basin in the East Sarmatian Orogen together with broadly coeval arcs indicate that the eastern margin of the Sarmatia was active with an arc–back-arc environment. Our new data suggest that the initial melts of the bimodal suite were adakitic derived by slab melting, followed by mantle metasomatism, whereas the basaltic magmas formed in an island arc setting. The LT and similar-aged volcanic belts in other terrains are considered to represent the initial (2.1–2.0 Ga), subduction-related growth of the Paleoproterozoic Columbia supercontinent.

Post-collisional two-stage magmatism in the East Sarmatian Orogen, East European Craton: evidence from the Olkhovsky ring complex

The Olkhovsky ring complex (ORC) is composed of a two-phase magmatic rock association with quartz diorite-quartz monzodiorite-granodiorite suite in the outer ring (OR) and trondhjemite as the inner stock (IS). Zircon U-Pb dating reveals that the rocks of the OR and IS were emplaced at 2070u2009±u20099u2005Ma and 2044u2009±u200913u2005Ma, respectively. The OR rocks are intermediate and felsic, metaluminous, low to moderate alkali. Both rocks are enriched relative to primitive mantle in Rb, Ba, Sr, K, and depleted in Nb and Ti. The OR quartz diorite and IS trondhjemite are characterized by positive values εNd(T)u2009=u20092.2 and 1.6, respectively. The ORC belongs to intermediate I-type granite with nil or weak crustal contamination, whereas the IS trondhjemite shows adakite-like features that were contaminated with components from earlier magmatic pulses. The low degree of contamination of the hypothetical melt, its basaltic-andesite composition, the elevated potassium and magnesium contents, and trace element patterns of whole rock and zircon, suggest garnet-free lower crust or metasomatized mantle wedge as the magma source for the OR, in a post-collisional setting. The mechanism for the inner stock trondhjemite was probably similar, although the magma source was characterized by the presence of garnet with a moderate degree of melting. Supplementary material: Analytical methods and tabulated analytical data are available at https://doi.org/10.6084/m9.figshare.c.3851104.

Sapphirine-Bearing Ultrahigh-Temperature Granulites of the Anabar Shield: Chemical Composition, U–Pb Zircon Ages, and P–T Conditions of Metamorphism

The results of thermobarometry yielded the P–T parameters of formation and evolution of sapphirine- bearing granulites in the Anabar shield with peak values of UHT metamorphism in the range of T = 920–1000°C at P = 9–11 kbar. Isotope–geochronological data indicate a polymetamorphic evolution of these rocks. Detrital zircon cores in the center of crystals yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later, superimposed metamorphic transformations of the detrital zircon formed rims dated to 2.4, 2.3, 2.2, and 1.83 Ga. A potential provenance source of the detrital zircons could be hypersthene plagiogneisses and metabasics of the Daldyn Group with a premetamorphic age no less than 3.32 Ga and products of their metamorphism of about 2.7 Ga old.

Bimodal intraplate magmatism of the Yenisei ridge as evidence of breakup of Rodinia and opening of the Paleoasian Ocean at the western margin of the Siberian Craton

The petrological–geochemical and isotopic–geochronological studies of contrasting rocks of the Yenisei Regional Tectonic Zone of the Yenisei Ridge allowed identification of the period of formation of riftogenic structures accompanied by intraplate magmatism. The extension processes are reflected in dike swarms of bimodal associations of anorogenic granites and intraplate mafic rocks with an intrusion age of 797–792 Ma. The formation of this belt is related to the Neoproterozoic extension along the western margin of the Siberian Craton, which occurred during breakup of the Rodinia Supercontinent and origination of the Paleoasian Ocean.