Svetlana Bogdanova

Frontiers in the Baltic Shield

Abstract Recent work in the Baltic Shield and its continuation in the basement of the East European Platform has created a new image of the Precambrian in the northeastern half of Europe. Towards the southeast, a continuation of the crust in the Shield can be traced to a Proterozoic system of palaeorifts in the East European Platform. These rifts apparently reproduce an earlier Precambrian crustal boundary that separates Fennoscandia, the northwestern crustal segment of the East European Craton, from its other two segments, Sarmatia and Volgo-Uralia. In the Archaean of Fennoscandia, the granite-greenstone province in Karelia is the only part that has ages in excess of 3 Ga. Interpretations of the Belomorian Belt along the White Sea and part of the Kola Peninsula as Early Archaean are therefore untenable. Rifting and break-up between 2.5 and 2.0 Ga strongly affected and partly dispersed the Archaean Domain. In the north, the resultant basins were closed by collisional orogeny between ∼ 1.95 and 1.82 Ga ago. Semi-simultaneously, the Svecofennian orogeny created the continental crust in the central Baltic Shield. The Svecofennian Orogen continues into the area southeast of the Baltic Sea where a system of beltiform structures extends to the fault boundary with central Europe, featuring westward younging from ∼ 2.0 to 1.8 Ga. In the west, the Svecofennian Orogen is truncated by the ensialic Transscandinavian Igneous Belt of ∼ 1.8-1.65 Ga age. The final stage of extensive crust formation in the Baltic Shield occurred in western Scandinavia between ∼ 1.75 and 1.55 Ga ago. Many of the granites in the interior of Fennoscandia are better explained as intracratonic manifestations of this process than as late- or post-orogenic products of the orogenies that created the immediately surrounding crust. After its formation, the continental crust of Fennoscandia underwent major reworking during the Sveconorwegian-Grenvillian and Caledonian orogenies ∼ 1.2-0.9 and 0.5-0.4 Ga ago. A previously unknown realm of crustal stacking and high-pressure Sveconorwegian metamorphism has recently been discovered in southwestern Sweden. While the formation of new and the reworking of preexisting continental crust in Fennoscandia was a consequence predominantly of processes at destructive plate margins, the geodynamics cannot be interpreted in terms of a succession of more or less uniform cyclic orogenies of similar duration. Although there were Late Archaean and Palaeoproterozoic activity maxima in the formation of new continental crust in the Baltic Shield, the definition of individual orogenies requires a new look.

Riphean rifting and major Palaeoproterozoic crustal boundaries in the basement of the East European Craton: geology and geophysics

Abstract The East European Craton, which occupies the northeastern half of the European continent, is characterized by the presence of giant, even transcratonic systems of Riphean (Meso- to Neoproterozoic) rifts (aulacogens). The largest of these Riphean rift systems tend to follow the course of Palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories. These segments, named Fennoscandia, Volgo-Uralia and Sarmatia, are marked by distinct T-MAGSAT anomalies. Genetically, the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton. In accordance with this, the Riphean rift systems vary in width, the presence or absence of Moho uplifts, and other properties like, for instance, heat flow values. However, all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting. The rift system-intersegment junction zone relationships, in consequence, similarly represent a range of variation and cannot be referred to one single model.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative eNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

EUROBRIDGE: New insight into the geodynamic evolution of the East European Craton

Abstract The Palaeoproterozoic crust and upper mantle in the region between the Ukrainian and Baltic shields of the East European Craton were built up finally during collision of the previously independent Fennoscandian and Sarmatian crustal segments at c. 1.8-1.7 Ga. EUROBRIDGE seismic profiling and geophysical modelling across the southwestern part of the Craton suggest that the Central Belarus Suture Zone is the junction between the two colliding segments. This junction is marked by strong deformation of the crust and the presence of a metamorphic core complex. At 1.80-1.74 Ga, major late to post-collisional extension and magmatism affected the part of Sarmatia adjoining the Central Belarus Zone and generated a high-velocity layer at the base of the crust. Other sutures separating terranes of different ages are found within Sarmatia and in the Polish-Lithuanian part of Fennoscandia. While Fennoscandia and Sarmatia were still a long distance apart, orogeny was dominantly accretionary. The accreted Palaeoproterozoic terranes in the Baltic-Belarus region of Fennoscandia are all younger than 2.0 Ga (2.0-1.9, 1.90-1.85 and 1.84-1.82 Ga), whereas those in Sarmatia have ages of c. 2.2-2.1 and 2.0-1.95 Ga. Lithospheric deformation and magmatism at c. 1.50-1.45 Ga, and Devonian rifting, are also defined by the EUROBRIDGE seismic and gravity models.

The Sarmatian crustal segment: Precambrian correlation between the Voronezh Massif and the Ukrainian Shield across the Dniepr-Donets Aulacogen

Abstract Sarmatia is the southernmost crustal segment of the East European Precambrian Craton. The Phanerozoic Dniepr-Donets Aulacogen (DDA) subdivides it into unequal halves, the Ukrainian Shield in the south and the Voronezh Massif in the north. Correlation between the Ukrainian Shield and the Voronezh Massif is reviewed, the overall structure and age-province subdivision of Sarmatia are assessed, and constraints are imposed on displacement along the DDA. It is shown that differences in the trends of the principal basement structures, highlighted by magnetic anomalies outlining Palaeoproterozoic banded iron formations, are a consequence of the heterogeneous structure of the Archaean-Palaeoproterozoic basement rather than the kinematics of the opening of the DDA. Only minor right-lateral displacement along the DDA has occurred. The main crustal units of the Ukrainian Shield, i.e. the Azov, Middle Dniepr and Ingul-Ingulets Blocks, correlate excellently with the Oskol, Sumy and Sevsk Blocks of the Voronezh Massif, which allows the definition of three coherent Oskol-Azov, Sumy-Dniepr and Sevsk-Ingulets crustal domains. Presently available radiometric datings show that these three domains all have Archaean crustal ages of 3.65-3.0, 3.2-3.1 and 3.1-2.8 Ga, respectively. The inter-domain boundaries appear largely to have existed already in the Archaean, but were subsequently masked by Palaeoproterozoic deformation and overlain by Proterozoic cover and nappe piles. Amongst the three crustal domains of central and eastern Sarmatia, the Oskol-Azov and Sevsk-Ingulets ones have been reworked substantially in the Palaeoproterozoic, while the Sumy-Dniepr Domain acted as a stable cratonic unit, although apparently rotating anti-clockwise between 2.3 and 2.1 Ga ago. The Palaeoproterozoic banded iron belts of Sarmatia can be subdivided into the cratonic-margin Krivoy Rog type and the interior, intra-domain Oskol type. The former outline the cratonic Sumy-Dniepr Domain, while the latter associate with NW-SE continental rifting that followed Archaean zones of weakness inside the Oskol-Azov Domain. The Bryansk Block in the northwestern part of the Voronezh Massif is a part of the large Bryansk-Bragin Granulite Domain which was formed between ca. 2.2 and 2.1 Ga ago, and extends into the northwestern Ukrainian Shield. In the east, the Lipetsk-Losev Volcanic Belt and the East Voronezh turbidite-schist province follow the boundary between the Sarmatian and Volgo-Uralian crustal segments. Together, these two crustal units appear to represent a Palaeoproterozoic orogenic environment, ca. 2.1-2.0 Ga in age.

Titanite-rutile thermochronometry across the boundary between the Archaean Craton in Karelia and the Belomorian Mobile Belt, eastern Baltic Shield

Abstract U–Pb isotopic dating has been carried out on titanites and rutiles from the Karelian Protocraton, the Belomorian Mobile Belt and the intervening junction zone. These are some of the principal Archaean crustal units in the Baltic Shield which have undergone regeneration to various degrees during the Palaeoproterozoic. Palaeoproterozoic resetting of U–Pb titanite ages was complete in the Belomorian Belt and almost complete in the junction zone, while it hardly affected the Karelian Protocraton. In the latter, major crustal cooling occurred at 2.71–2.69 Ga after a major igneous event at 2.74–2.72 Ga, but a tectonothermal event at 2.65–2.64 Ga was less comprehensive. In the Belomorian Belt, a northeastern marginal zone immediately underlying the collisional-thrusting suture of the Lapland-Kola orogen has somewhat higher titanite ages of ca. 1.94–1.87 Ga than the central zone where these ages range between 1.87 and 1.82 Ga. Comparison between the titanite and rutile U–Pb ages suggests a postorogenic cooling rate between 2 and 4°/Ma in these parts of the Belt. The Neoarchaean junction zone between the Karelian and Belomorian provinces was a zone of particularly intense tectonic, magmatic and hydrothermal activity during or after the Palaeoproterozoic Lapland-Kola orogeny. Dominant, newly grown titanites in that zone have ages as young as 1.78–1.75 Ga, and the age differences between the titanite and rutile U–Pb ages are substantially smaller than elsewhere.

Assembly and Breakup of Rodinia(Some Results of IGCP Project 440)

The principal results of project 440 “Assembly and Breakup of Rodinia” of the International Geological Correlation Programme (IGCP) are reviewed in this work. A map of that supercontinent compiled using geological and paleomagnetic data describes global paleogeography 900 Ma ago. The assembly of Rodinia, which comprised most of Precambrian continental blocks, lasted ca. 400 m.y. (from 1300 to 900 Ma). Its breakup presumably triggered by mantle superplume took place between 830 and 650 Ma. The correlation between tectonic events in different continental blocks is considered. Some problems concerning the Rodinia reconstruction and history, e.g., the slow growth of juvenile crust and effects of mantle-plume events during the amalgamation period and of glaciations at the breakup time, are discussed. The latter caused changes in the biosphere and climate, whereas postglacial periods stimulated progress in biota evolution.

Archaean terranes, Palaeoproterozoic reworking and accretion in the Ukrainian Shield, East European Craton

Abstract The Ukrainian Shield is a large coherent region of exposed Archaean and Proterozoic crust in the southwestern, Sarmatian part of the East European Craton. It is traditionally divided into blocks, or domains, separated by major suture zones. The Azov Domain in the east and the Podolian Domain in the SW are Archaean complexes that have been highly reworked in the Palaeoproterozoic; in contrast, the Archaean (3.2-3.0 Ga) granite-greenstone terrane dominated Middle Dniepr Domain, in the central part of the Shield, is virtually untouched by Proterozoic processes. Palaeoproterozoic rocks dominate the Kirovograd domain in the central Shield. We review previous and recent geochronological results and demonstrate that the Volyn Domain and adjacent parts of the Ros-Tikich Domain in the NW are largely juvenile, c. 2.2-2.0 Ga segments of Palaeoproterozoic crust accreted to the Palaeo- to Mesoarchaean crust in the Podolian Domain. The Podolian Domain includes 3.65 Ga granitoids, with traces of 3.75 Ga material. It has been reworked, at 2.8 Ga and c. 2.0 Ga. Its temporal evolution is thus similar to that of the Azov Domain in the eastern part of the Shield. However, in view of the complex terrane pattern of Sarmatia, this does not necessarily mean that the Podolian and Azov domains were parts of the same continent in the Archaean.

The “Saamian” of the Belomorian Mobile Belt: new geochronological constraints

Abstract Most gneisses and amphibolites of the Belomorian Belt in the White Sea region have previously been considered to represent the oldest, “Saamian” crustal component in the Baltic Shield which has been referred to the Early Archaean. New UPb zircon ages of some of the most typical “Saamian” rocks from the northern segment of the Belomorian Belt are reported. These are: (1) ∼ 2.65-2.60 Ga for the main (“Rebolian”) event of folding, high-P metamorphism and migmatization; (2) ∼ 2.8 Ga for the protolith of an orthogneiss in a granulite-facies relic area within the region of Rebolian folding on the eastern shore of Lake Kovdozero; (3) ∼ 2.74 Ga for tonalitic orthogneisses that dominate the Belomorian gneiss terrain around Kandalaksha Bay of the White Sea; (4) ∼ 2.4 Ga for a gabbro-granite intrusion on the Tolstik Peninsula of the White Sea. This intrusion has previously been considered to be one of the type-rock units of the Early Archaean. These and other age determinations previously only reported in Russian constrain the age of the assumed “Saamian” in the Belomorian Belt. In conjunction with new field observations, they place most of the dated rock units within the same age group as the Late Archaean, Lopian rocks. As a result of the age determinations, the concept of a single array of Late Archaean geodynamic events in the Baltic Shield is advanced. The Belomorian Belt is interpreted as an edge of the Late Archaean granite-greenstone terrain in Karelia which had been involved in strong structural reworking during an event of tectonic stacking and thrusting ∼ 2.65-2.60 Ga ago. Part of the Belomorian Belt was subsequently also affected by Palaeoproterozoic deformation. The new results throw doubt on the previously assumed Late Archaean, Lopian age of some metavolcanic rock units occurring within the Belomorian Belt but overlying the presently dated orthogneisses. These metavolcanics which comprise, for instance, the so-called Kolvitsa Belt and its continuation towards the northwest, are assigned tentatively to the Palaeoproterozoic. They may be age equivalents of Lapponian-Sumian, ∼ 2.5-2.4 Ga old rocks underlying the Lapland Granulite Belt.

Palaeoproterozoic UPb zircon ages from Belorussia: new geodynamic implications for the East European Craton

Abstract The crust in the northwest of the East European Craton has traditionally been regarded as part of an Archaean nucleus in eastern Europe. Recent correlation with the Baltic Shield in Scandinavia, however, suggests the need of reappraisal. To study this problem, zircons from three boreholes in the basement of the Russian Platform in Belorussia (Belarus) have been dated by the U-Pb and Pb-Pb methods. The ages indicated by two U-Pb discordias are 1982±26 Ma for a metadacite from the Central Belorussian Belt of volcanic-arc igneous rocks, and 1800±7 Ma for mafic granulite from the Baltic-Belorussian Granulite Belt largely built up of mafic igneous rocks. For another Central Belorussian metadacite, a Pb-Pb age of 1975±10 Ma was obtained. In previous “stratigraphies” of Belorussia, all three dated rocks have been considered to be important components of an Archaean, even Early Archaean crust. Analyses of abraded crystal cores of complex zircons in one sample and separated core and rim zircons in another do not suggest substantially older ages of the core materials. Together with previous ages of granitoids and charnockites, which had, however, been explained as a result of late magmatism and metamorphism, and therefore not characterizing the age of the crust, our data indicate the dominance of Palaeoproterozoic rocks in the basement of the northwestern part of the East European Craton that adjoins the Tornquist-Line boundary between central and northeastern Europe. In conjunction with the results of a Finnish-Estonian study of similarly allegedly Archaean, but in actual fact Proterozoic rocks in southern Estonia, our results suggest the existence of large Palaeoproterozoic crustal belts extending all the way from the Finnish Gulf to eastern Poland. This implies a substantial revision of established concepts on the Precambrian geology in the northeastern half of the European continent.