A lens of previously unknown polymictic conglomerates has been found in the axial part of the Kostomuksha greenstone belt (KGB) of the Karelian Craton. Their clastic portion consists of poorly graded, largely angular pebbles. In composition, they are comparable with rocks of the country greenstone complex: (1) amphibolites similar to the Mesoarchean KGB gabbroids and basalts, (2) magnetite–amphibole quartzites similar to the Mesoarchean magnetite–biotite–amphibole quartzites associated with banded iron formations, and (3) Neoarchean (2.75 Ga) quartz metagraywackes. The conglomerates are intensely deformed (γ >10) under conditions of a sinistral shear. The conglomerates were deposited about 2.71 Ga ago, as indicated by analysis of zircons from the matrix. Thus, a new Neoarchean stratotectonic association, the youngest in the greenstone complex, was found in KGB. It probably formed in a pull-apart basin at the final stage of the formation of the KGB from an important role of shear movements. Volcanic and sedimentary associations similar in age are known in the Khedozero–Bolshozero, Kuhmo, and Takanen greenstone belts of the Karelian Craton.

This study focuses on a boudin of Archean amphibolite that occurs in the Gridino eclogite-bearing m & eacute;lange (Belomorian Province, eastern Fennoscandian Shield) and contains two varieties of eclogite. Eclogite-1 is banded, retrogressed (omphacite is totally replaced by clinopyroxene-plagioclase symplectite), and deformed by recumbent tight folds identical to those in the TTG-amphibolite host. Zircon groups are dated at 2721 +/- 26 Ma (Zrn-I, fir-tree zoning, no or small negative Eu anomalies, flat HREE patterns, 700-730 degrees C, 14-15 kbar), ca. 2.70 Ga (Zrn-II, rims around Zrn-I, grains, 750-900 degrees C, 11-14 kbar, high-P granulite facies), and ca. 2.70 Ga (Zrn-III, rims around Zrn-II). Newly formed Paleoproterozoic zircon crystals are lacking. Retrogressed eclogite-1 in which amphibolization and epidotization totally obliterated garnet and plagioclase-clinopyroxene symplectites is intruded by 2646 +/- 46 Ma old granodiorite. Eclogite-2 is massive, contains preserved omphacite-garnet-quartz-rutile assemblages, and comprises two patches whose boundaries truncate folds in eclogite-1. A Paleoproterozoic age of eclogite-2 is evidenced by previously published 1.90 Ga ages of zircon rims around Archean zircon and newly formed crystals with omphacite inclusions. The largest patch contains tabular enclaves of three varieties of amphibolites. One variety is identical to some bands of totally retrogressed eclogite-1 changed into monomineral amphibolites in both the Neoarchean and Paleoproterozoic. The orientation and attitude of these tabular enclaves are coherent with those of the banding on limbs of recumbent tight folds in eclogite-1 within the boudin and later drag folds at its margins and form one and the same structural carcass or skeleton. These data along with recent findings of inherited 2.68 Ga old zircon with inclusions of omphacite in eclogite-2 indicate that this eclogite developed after Neoarchean eclogite-1 due to fluid infiltration along reactivated Neoarchean shear zones enveloping the boudin. These results favor the idea that the transition to the modern-style plate tectonics started not later than in the Neoarchean.

The results of geological, geochemical, and geochronological studies of granophyre rocks from the Jarva-Varaka Massif (Kola region) are presented. The 2-km section of the massif is composed of mafic and felsic norites, hypersthene diorite, pigeonite-augitic diorite, quartz diorite, and granodiorite. All these rocks contain a variable amount of granophyre (micropegmatite), from 10

The Serpovidny large-scale synform located in the Arctic zone of the north-eastern Fennoscandian Shield was studied with geological mapping and structural analysis, and its core with a ground detailed magnetic survey. This is a tight to isoclinal mega-sheath fold with the strongly thinned lower limb, almost undeformed upper limb, and the 'cat's eye' core. The innermost rift-related sequence of the fold core is a synformal anticline, whilst limbs are composed of Palaeoproterozoic Keivy Group (metamorphosed redeposited products of mainly deep chemical weathering) and form a syncline. The Serpovidny mega-sheath fold formed during progressive non-coaxial, high strain deformation in simple shear/general shear regimes and a northward thrusting under mid-crustal conditions. This mega-sheath fold evolved from a NNE-verging anticline originated at the base of a nappe transported north-northeastwards from the Palaeoproterozoic Imandra-Varzuga Rift-Belt during the 1.9 Ga Lapland-Kola collisional orogeny. After a thrusting of this nappe onto the Keivy Group, one anticline plunged into these rocks and was transformed into the NNE-closing Serpovidny mega-sheath fold. Its geometric parameters and/or kinematic evolution are close to those of mega-sheath folds in Cretaceous rocks in the Oman Mountains, largescale synformal anticlines in the Helvetic nappes in the Alps, and a Caledonian nappe in northwest Ireland.

Studying early Precambrian astroblems is complicated by their localization in structural and compositional complexes that underwent superimposed metamorphism and tectonic processing. Back in the 1980s, seven sites with occurrences of breccias and other rocks resembling impact structures were identified as potentially diamondiferous impact structures within the Karelian-Kola region. But at present, only two astroblems are known in the region: Janisjärvi of 725 ± 5 Ma and Suavjärvi of ~2400 Ma. In the Kola part, impact origin is assumed for two objects: the Javrozero circle structure in the Tanaelv belt and the Järva-varaka layered massif in the Monchegorsk ore area. The most promising structure to be an astrobleme with serious signs of impact origin is the Paleoproterozoic Järva-varaka massif. According to geological, petrochemical, and geochemical characteristics of rocks the Järva-varaka massif is most similar to the 1.85 Ga Sudbury structure (Canada), for which an impact origin was assumed. Shock metamorphism of the Järva-varaka massif was manifested in amorphization of zircon from the country rocks with formation of plagioclase and sillimanite glasses in inclusions, as well as planar deformations in quartz.

Первая находка псевдотахилитовой брекчии и другие признаки ударного метаморфизма в породах обрамления массива Ярва

This paper deals with a new approach to reconstructing the shape of a poorly-exposed synform that folds strongly magnetic layers. The approach is based on the solution of the 2D non-linear magnetic inverse problem in areas best suited for 2D modeling. The interpretation model consists of two parts: (i) a geometric model imitating a synform cross-section and (ii) a set of elementary layers of a fold characterized by anisotropic magnetic susceptibility. The main task is to determine the deepest point of magnetic layers and the direction and amount of their dip. It can be solved by a selection of optimal model parameters from a set of random numbers with a priory model constraints. The solution is an iterative process which defines intervals of geometrical model parameters that correspond to minimal values of the misfit between observed and modeled magnetic fields. The proposed approach is characterized by a stability of the determination of reference fold points' coordinates, which is sufficient to confidently answer a number of questions important for the geologist. These include: whether a synformal fold is upright, inclined or overturned, determining the depth of its trough line, which combined with the fold outline on the ground surface allows an evaluation of the hinge line curvature. As an example, the large Paleoproterozoic Serpovidny isoclinal synformal anticline in the Keivy terrane of the northeastern Fennoscandian Shield has been chosen. According to field observations, this synform is a giant sheath fold consisting of the poorly exposed strongly magnetic core and weakly magnetic limbs. The 3D morphology of the Serpovidny fold reconstructed using the new approach is shown to be very close to the geometry determined from field observations. Thereby, this study favors the conclusion based the sheath geometry of the Serpovidny isoclinal synformal anticline that an Alpine-type tectonics operated in the Paleoproterozoic.

A new method of the solution of the 2-D non-linear magnetic inverse problem is considered as a tool for reconstructing the shape of large synforms that contain strongly magnetic layers. It is based on an interpretation model consisting of two components. The first is a 2-D geometrical model which imitates a synform cross-section normal to the synform strike. The second is a set of elementary layers making up a fold model. A specific feature of the layers is their magnetic susceptibility values that are different in directions parallel and normal to planar shape fabrics of rocks. That is why of particular interest has been a consideration of the impact of the magnetic susceptibility anisotropy of rocks in a measured magnetic field. It has been shown that it can be significant and should be taken into account at solving the magnetic inversion.

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite-trondhjemite-granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet +/- kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene-plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland-Kola collisional orogeny at 1.91-1.93Ga, which led to eclogite-high-pressure granulite (E-HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland-Kola collisional orogen). The Palaeoproterozoic E-HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland-Kola collision, the low Th/U 1.9-1.8Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland-Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E-HPG overprint, the areal occurrence of the 2.7-2.8Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.

This work presents the first data on the geochemistry, U-Pb (SHRIMP-II) age of zircon, U-Pb (conventional) age of titanite, Sm-Nd model age of whole rock samples, and age of reworking of metaporphyric rocks of the Serpovidnyi Ridge of the Keivy structure. These rocks are characterized by high contents of trace and rare earth elements indicative of an origin of the rocks from the enriched mantle source EM2. The negative ɛNd value (−2.4) does not exclude the contamination of mantle material. The age of two close periods is reliably identified: the crystallization of zircon (1740 ± 15 Ma) and porphyroblastesis with the formation of plagioclase and epidote glomeroblasts combined with the crystallization of titanite (1719 ± 13 Ma). The Sm-Nd isotope data indicate the Paleoproterozoic age of mafic volcanism in the Keivy terrane (2.37–1.77 Ga).

The Kola region in the northeastern Baltic Shield is characterized by diverse Paleoproterozoic collision processes. The Keivy Terrane is one of the major tectonic units in the northeastern foreland of the Paleoproterozoic Lapland-Kola Collisional Orogen, which markedly differs in a number of parameters from other tectonic units of the Kola region. The study of the Keivy Terrane allowed us to unravel one more basic difference: the large Paleoproterozoic sheath synform of the Serpovidny (Crescentic) Range localized in this terrane. Its core is occupied by volcanic and sedimentary rocks, which correlate with the fill of the Imandra-Varzuga Rift; the limbs are composed of metamorphosed mature sedimentary rocks known as Keivy paraschists of Neoarchean or Paleoproterozoic age. The lower limb of the Serpovidny Synform is strongly squeezed, whereas the upper limb consists of almost undeformed rocks. The deformed rocks underwent ductile flow under conditions of simple or general shear. In the degree of its asymmetry and main parameters, the Serpovidny Synform is similar to the plunging and recumbent anticlines in the Helvetic nappes of the Alps. It is concluded that the Paleoproterozoic core of the Serpovidny Sheath Synform, or plunging anticline, is a fragment of the almost completely eroded deep Serpovidny Nappe of the Helvetic type. During the collision related to the Lapland-Kola Orogeny (1.9–2.0 Ga), this nappe was pushed out northward from the Paleoproterozoic Imandra-Varzuga Rift, which is situated 50 km south of the Serpovidny structure, and thrust over the Keivy paraschists. The latter, together with underlying the Lebyazhka Gneiss, were folded in the process of thrusting and were involved in the structure of the Serpovidny Synform. The Keivy paraschists make up a para-autochthon or a separate nappe of the Pennine type. The Archean Lebyazhka metafelsic volcanics underlie the Keivy paraschists and overlie granitoids of the Archean basement that remained undeformed during thrusting. Most likely, they also belong to the para-autochthon; however, it cannot be ruled out that, like the Keivy paraschists, they occur as a Pennine-type nappe. The large sheath folds known in the Paleoproterozoic and Phanerozoic orogens are genetically related to deep-seated nappes or channel-flow tectonics. Paleoproterozoic and Phanerozoic orogens are similar in this respect.

Кольский регион на северо-востоке Балтийского щита является областью проявления разнообразных коллизионных процессов палеопротерозоя. Одной из главных тектонических единиц северо-восточного форланда палеопротерозойского Лапландско-Кольского коллизионного орогена является Кейвский террейн. По ряду параметров он заметно отличается от тектонических структур Кольского региона. Проведенные нами исследования Кейвского террейна выявили еще одно его принципиальное отличие: на северо-западе этого террейна установлена и охарактеризована крупная палеопротерозойская Серповидная колчановидная синформа. Ее ядро сложено вулканогенными и осадочными рифтогенными породами, коррелируемыми с образованиями палеопротерозойского рифта Имандра-Варзуга, а крылья – метаморфизованными зрелыми осадочными толщами, известными как кейвские парасланцы дискуссионного неоархей–палеопротерозойского возраста. Нижнее крыло Серповидной структуры сильно раздавлено, а верхнее сложено почти недеформированными породами. Деформированные породы испытали пластическое течение в условиях либо простого, либо общего сдвига. По степени ее асимметричности, а также по главным параметрам Серповидная структура подобна ныряющим и лежачим антиклинальным складкам в гельветских покровах Альп. Сделан вывод, что палеопротерозойское ядро Серповидной колчановидной синформы или ныряющей антиклинали является фрагментом почти полностью эродированного глубинного тектонического покрова гельветского типа, названного Серповидным. Во время лапландско-кольской коллизионной орогении 1.9–2.0 млрд. лет назад этот покров был выдвинут к северу из палеопротерозойского рифта Имандра-Варзуга, расположенного в 50 км к югу от Серповидной структуры, и надвинут на кейвские парасланцы. Последние вместе с подстилающими их лебяжинскими гнейсами смяты в надвиговые складки, причем кейвские парасланцы участвуют в строении Серповидной колчановидной синформы. Кейвские парасланцы слагают параавтохтон или самостоятельный Кейвский тектонический покров пеннинского типа. Архейские лебяжинские кислые метавулканиты подстилают кейвские парасланцы и перекрывают недеформированные во время надвигообразования гранитоиды архейского фундамента. Они, скорее всего, принадлежат параавтохтону, но не исключена вероятность того, что они, подобно кейвским парасланцам, слагают тектонический покров пеннинского типа. Крупные колчановидные складки, выявленные в коллизионных орогенах палеопротерозоя и фанерозоя, генетически связаны как с глубинными тектоническими покровами, так и с тектоникой канального течения. Палеопротерозойские и фанерозойские коллизионные орогены в этом отношении имеют принципиальное сходство.

The Keivy Terrane in the northeastern Baltic Shield appreciably differs from the adjacent tectonic blocks. In the northwestern part of this terrane (the Serpovidny Range), an outlier of Paleoproterozoic supracrustal rocks called the Serpovidny structure is surrounded by Archean (?) Keivy high-alumina paraschists. As follows from structural and magnetic data, the Paleoproterozoic rocks are deformed into a tight sheath fold 8 × 2 km in size at the surface and 5 km in length along the sheath axis. Faults parallel to the boundaries of the layers and locally cutting them off at an acute angle are involved in folding as well. The outer boundaries of the Serpovidny structure are tectonic. This structure is complementary to a larger tectonic lens composed of the Keivy mica schists. It is concluded that all of the supracrustal rocks of the Serpovidny Range are in fact tectonic sheets and lenses deformed into sheath folds. The literature data show that kilometer-scale sheath folds occur throughout the Keivy paraschist belt and most likely were formed owing to thrusting of the Murmansk Craton onto the Keivy Terrane in the south-southwestern direction. Foliation and lineation related to thrusting have been established in the Archean silicic metavolcanics and peralkaline granites occupying the most part of the terrane. In contrast, the granitoids and gabbroanorthosites of the Archean basement, which form a block 90 × 20 km in the southwestern Keivy Terrane, were not affected by Paleoproterozoic deformation. In other words, a detached assembly of tectonic sheets composed of the upper and middle crustal rocks that underwent deformation at the initial stage of the Paleoproterozoic Lapland-Kola Orogeny and the Archean basement, which is free of this deformation, are distinguished. The depth of detachment is estimated at 20–25 km. The detachment of the upper and middle crust in the Keivy Terrane and its position in the structure of the Baltic Shield are consistent with a spatiotemporal succession that resulted in the formation of a Paleoproterozoic supercontinent and the Baltic Shield as its fragment. This succession began with the amalgamation and deformation of the Archean terranes in the northeast of the Baltic Shield during the Lapland-Kola Orogeny, the Keivy Terrane showing a record of the earliest reworking (1.97–1.93 Ga). The succession completed in the southern and southwestern parts of the shield (1.80 Ga) after the Svecofennian Orogeny, expressed in the accretion of island-arc terranes composed of Paleoproterozoic juvenile crust to the continent.