Search

Geological-structural and petrological data acquired by the authors on the most widely spread metamorphic associations of the Belomorian Mobile Belt in northern Karelia, Russia, indicate that the rocks belong to five age groups (0–IV), which were produced by metamorphism of protoliths of different composition and genesis. The Paleoproterozoic metamorphic associations were formed by three successive metamorphic episodes, which corresponded to discrete evolutionary episodes of the Lapland–Karelian Orogen. The first episode of Paleoproterozoic metamorphism was high-pressure transformations that produced steep zones of ductile flow of submeridional and northeastern trend. The second episode was high-temperature metamorphism at intermediate and high pressure, which was triggered by the development of gently sloped ductile flow zones and tectonic melange of roughly latitudinal trend. The third metamorphic episode involved amphibolization of the rocks during high-temperature decompression cooling and was likely related to the development of randomly oriented fractures when the rocks of the Belomorian Mobile Belt were brought to middle and upper crustal levels. Each of the episodes was characterized by different P–T parameters of metamorphism, which are recorded in the compositions of the rock-forming minerals. The former two episodes corresponded to the prograde metamorphic evolution, and the third one was a retrograde evolution. The beginning of the first episode was responsible for the origin of amphibolites at Т = 560–640°C and Р = 6.7–9.5 kbar, with the metamorphic peak producing eclogites at Т = 600–690°C and Р = 11.3–12.9 kbar and eclogite-like rocks at higher temperatures of Т = 650–780°C and Р = 10.3–13.0 kbar. The peak of the second metamorphic episode corresponded to the origin of abundant plagiomigmatite leucosomes, which were formed within a broad temperature range (640–840°C) and a narrow pressure range (9.0–11.9 kbar). The widespread amphibolization of the metabasites of all types occurred only during the third metamorphic episode, whose P–T parameters broadly varied: 590–740°C and Р = 7.5–10.8 kbar. All dike varieties of the complex of eclogitized garnet metagabbro were formed at the metamorphic peak at the same P–T parameters as the host metamorphic rocks in the corresponding structures: metagabbro I was formed in steep ductile flow zones, and metagabbro II was produced in the gently sloped zones.

The paper studies redistribution of alloying elements (Al, V, Mo) in the pseudo-alpha-titanium structure under graded stretching until destruction of the sample. Local sections of the titanium structure and of the fracture surface have been scanned by electron microscopy and X-ray microanalysis. The change in the chemical and phase composition of the alloy has been compared with the initial state stimulated by deformation impact. The structural-phase state of the material under high plastic strains is metastable, fixed by the system of deformation defects of the crystal lattice.

The low-angle dip schistosity zones of the Belomorian mobile belt of northern Karelia are zones of plastic flow of thrust origin. They were formed from 1.85 to 1.90 Ga: 1879 ± 21 Ma according to 40Ar/39Ar for amphibole from amphibolites and 1857 ± 13 Ma according to the Sm–Nd isochron in amphibolites. The P–T parameters of rock metamorphism in low-angle dip schistosity zones correspond to the boundary of amphibolite and granulite facies of metamorphism: T = 640–765°C, rarely rising to 826°C; P = 8.0–11.7 kbar. The hypothesis of the two-stage Paleoproterozoic metamorphism of rocks of the Belomorian mobile belt was introduced.

Soils of low-mountain landscapes in the northwest of Karelia have been studied. The soil cover of the studied area is mainly represented by Al–Fe-humus soils (Podzols); thin soils (Leptosols) are widespread. Characteristic morphological features of all the studied soils are relatively shallow profiles, high stone content, and underlying by hard bedrock with fine earth material in crevices between large boulders. The studied soils have the high carbon and low nitrogen content, which points to unfavorable conditions of organic matter transformation. The content of most macro- and microelements is not high, which is typical for soils of the region; the content of copper and zinc exceeds the regional background two–three times. Regularities of the vertical zonality in the properties of soils of mountain ecosystems manifest themselves in decreasing thickness of the soil profile at higher altitude above sea level in parallel to decreasing thickness of the layer of loose rocks, while the stone content increases. In soils of the forest-tundra zone, the organomineral horizon with the high organic matter content is formed immediately under the forest litter. The litter horizon is the soils of this zone is characterized by increased concentrations of calcium, magnesium, phosphorus, and zinc.

The Belomorian Mobile Belt (BMB) in northern Karelia mostly consists of gently sloping shear zones, whose gneisses and migmatized amphibolites and blastomylonites are typically thinly banded, with their banding consistently dipping north- and northeastward. These gently sloping shear zones were not affected by folding after they were produced and are not cut by Paleoproterozoic metabasite dikes. Intrusive metabasites in the gently sloping shear zones make up relatively small (usually