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The paper presents authors' original detailed data on rocks of the Archean Pon'goma-Navolok charnockite−enderbite complex in northern Karelia. The rocks practically have not been modified and are preserved within a rigid block among Paleoproterozoic zones of ductile deformations and metamorphism. The geochemistry of the rocks and their isotope−geochemical features indicate that the protolith from which the enderbite melts of the main phase of the massif were derived may have been amphibolites. The enderbite melts were derived from these amphibolites under the effect of K2O-, Na2O-, and SiO2-bearing fluids; and the enderbites were subsequently charnockitized with the involvement of fluids enriched in K2O and SiO2. Physicochemical modeling indicates that the enderbite melt was derived from the amphibolite protolith at a depth of about 45 km (P = 14.8 kbar, T = 1030−1080°C) under the effect of saline H2O−CO2 fluid. Comparison of the P−T parameters of the granulite-facies metamorphism of the metabasites and the parameters under which the enderbite melts were derived indicates that Archean granulite-facies metamorphism in the Belomorian belt in northern Karelia was of contact but not regional nature and was induced by the high-temperature field of an emplaced enderbite massif. The orthogneisses hosting the Pan'goma-Navolok massif inherit geochemical features of the unsheared, ungneissose, and unmetamorphosed enderbites. This means that enderbites analogous to those of the Pan'goma-Navolok massif may have served as the protolith of some of the orthogneisses, and that enderbites may have been spread more widely in the Archean than the currently preserved single enderbite massifs. [ABSTRACT FROM AUTHOR]

Archean gneisses cropping out on islands in the White Sea and its coast in northern Karelia host widespread bodies of Early Proterozoic metamorphosed gabbroids. Carbonate–silicate veins hosted in these metabasite bodies and constrained to their contacts with the gneisses contain Fe–Cu sulfides, whose concentrations occasionally reach economic levels. The dominant gangue minerals are plagioclase, quartz, carbonates, and chlorite. The formation stages of the veins correspond to the transition from the early quartz–plagioclase to late quartz–carbonate associations with chlorite and sulfides. The early (high-temperature) stage is discernible in the wall-rock amphibolite aureoles, which were formed at temperature of about 550–650°C (estimates by the TWQ method). This stage corresponds to the quartz–plagioclase association in the marginal zones of the veins. The transition to the late stage and the formation of veined quartz–carbonate (±biotite) associations occurred at temperatures of 540°C and lower, judging by the calcite–dolomite associations. The further development of the quartz–chlorite–carbonate and sulfide associations in the veins and wall-rock amphibolites corresponded to a temperature decrease to 350°C and below, as evaluated with the application of chlorite thermometers. The veins and wall-rock amphibolitization may have been induced by metamorphic fluids during the latest retrograde metamorphic stage in the Early Proterozoic. [ABSTRACT FROM AUTHOR]

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 mélange 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. [ABSTRACT FROM AUTHOR]

Petrographic and thermobarometric data obtained on a blastolite lens hosted in sheared rocks of the Gonzhinskii block indicate that the lens contains four mineral assemblages: relict, main, an assemblage corresponding to biotitization, and that of retrograde alterations. The relict assemblage was likely eclogitic. The main mineral assemblage corresponds to zoisite amphibolite developing after eclogite. The P−T metamorphic parameters of this assemblage are P = 13.0−15.0 kbar and T = 580−670°C. However, if the calculations are done using the iron richest outer rim of amphibole 1, the P−T parameters are P = 13.0−17.0 kbar and T = 660−780°C. The mineral assemblage corresponding to the overprinted biotitization is clearly distinct texturally. The lines of the monomineral equilibria of the biotite assemblage plotted using various combinations of the composition of the garnet, biotite, and plagioclase well converge within a compact field of P = 12.5−13.2 kbar and T = 810−830°C. The latest assemblage of retrograde minerals includes low-Al amphibole 2. Our original Sm−Nd isotope date were used to calculate an isochron for a whole-rock sample, garnet, and two amphiboles. The isochron age of the rock is 171 ± 3 Ma and is interpreted as the age of the main mineral assemblage, i.e., its crystallization at the metamorphic peak. [ABSTRACT FROM AUTHOR]

The article focuses on the proposed mechanism of the formation of the high-temperature mineral association and related autochthonous granitoid melts discovered in one of the zones of deformation in the northeastern part of the Belomorian mobile belt (BMB) in Russia. It presents a study on metasomatic rocks of the Krasnogubskii dyke field formed in the sublatitudinal zone of the tectonic melange dipping at a low angle which draped the gabbro-norite intrusion on Shavrukha Mountain. It determines four rock varieties with different chemical compositions and quantitative relationships.

On the White Sea islands and coast of North Karelia, in the Chupa sector of the Belomorian mobile belt (BMB), carbonate–silicate and quartz veins are localized in bodies of Early Proterozoic metabasites and their contacts with host gneisses. The isotopic characteristics of coarse crystal vein carbonates (δ18O and δ13C), as well as associated quartz (δ18O) and biotite (δ18O and δD), were studied. The isotopic composition of oxygen suggests biotite–quartz–carbonate equilibria during the formation of veins at temperatures of 520–450°C. The O and H isotopic characteristics of biotite correspond to the equilibrium of the fluid with metamorphic rocks. At the same time, the isotopic compositions of O and C carbonates indicate different CO2 sources in fluids. There are three groups of carbonates according to δ18О and δ13С. The first group shows "magmatic" isotopic characteristics of the CO2 source (δ18ОVSMOW = 7.1...12.1‰; δ13СVPDB = –6.3...–4.5‰). Carbonates of the second group have high δ18О and δ13С, characteristic of Precambrian sedimentary carbonates (δ18ОVSMOW = 21.9...24.7‰; δ13СVPDB = –2.2...–2‰), similar to those described earlier in the neighboring Engozero sector of the BMB. In the third group, the δ13С values are similar to the carbonates of the first group, and δ18О, to the second group (δ13СVPDB = –1.7...0.9‰, δ18ОVSMOW = 6...12‰). The O and C isotopic compositions of carbonates of the third group can be obtained from the action of a putative magmatic fluid on both the primary sedimentary source of metamorphic fluids and redeposited vein carbonates. [ABSTRACT FROM AUTHOR]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]