Rb-Sr and U-Pb systematics of metasedimentary carbonate rocks: The Paleoproterozoic Kuetsjarvi Formation of the Pechenga Greenstone Belt, Kola Peninsula

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial ⁸⁷Sr/⁸⁶Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial ⁸⁷Sr/⁸⁶Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the ²⁰⁸Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic ²⁰⁸Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ¹³C values.     

Geochemistry and age of the complex of alkaline metasomatic rocks and carbonatites of the Gremyakha-Vyrmes massif, Kola Peninsula

This paper presents new geochemical data on the complex of alkaline metasomatic rocks and carbonatites, which hosts the rare-metal mineralization of the Gremyakha-Vyrmes massif. The contents of major and trace, including rare-earth elements were determined in the albitites, aegirinites, and carbonatites. Two types of the rare-metal ores are distinguished: niobium albitite and zirconium aegirinite ores. It was shown that the albitites and aegirinites have similar trace element distribution patterns, being most geochemically close to the foidolites. The carbonatites, albitites, and aegirinites were dated by Rb-Sr and Sm-Nd methods at 1887 ± 58 Ma, which corresponds to the formation age of the Gremyakha-Vyrmes massif. The ultrabasic rocks, foidolites, alkaline metasomatic rocks, and carbonatites were formed successively within a relatively narrow range. The geological observations and geochemical data led us to conclude that the emplacement of the fluid-saturated carbonatite solutions-melts at the final stages of the massif formation against a background of fault tectonics caused a pervasive metasomatism of the ultrabasic and alkaline rock complexes and, as a result, the formation of the alkaline albitites and aegirinites. The carbonatites could be sources of rare-metals, while foidolites served as a geochemical barrier, and their metasomatic alteration led to the formation of Zr-Nb mineralization in the albitites and aegirinites.     

Native elements in rocks of the banded iron formation,Kola Peninsula

Eleven native minerals and intermetallic alloys were identified in rocks of the banded iron formation (BIF) in the Kola Peninsula: copper, silver, gold, electrum, auricupride, cuproauride, tetraauricupride, bismuth, sulfur, tellurium, and graphite. Graphite is a common mineral of sulfide-bearing BIF and gneiss. Sulfur occurs in supergene-altered sulfide-bearing BIFs together with Fe- and Ca-sulfates. Gold of low fineness (electrum) in association with electrum, acanthite, auricupride, volynskite, hessite, cervelleite, pavonite, petzite, and bismuth is related to the areas of hydrothermally altered skarnoids with greenalite, chamosite, aegirine, and Na-Ca amphibole. Redeposited gold of high fineness associated with auricupride, hessite, silver, electrum, kostovite, cuproauride, tetraauricupride, and sperrylite occurs in low-temperature zonal hydrothermal segregations hosted in aluminous gneiss and which formed under the effect of alkalized, highly siliceous solutions at the regressive stage of BIF metamorphism.     

Spinel-group minerals in rocks of the Khibiny alkaline pluton,Kola Peninsula

Seven spinel-group minerals in various geological settings have been revealed in the rocks of the Khibiny pluton. Hercynite, gahnite, and vuorelainenite occur only in xenoliths of hornfels after volcanic and sedimentary rocks, whereas spinel and magnesiochromite occur in alkaline ultramafic rocks of dike series. Franklinite has been discovered in a low-temperature hydrothermal vein. Ubiquitous magnetite is abundant in foyaite, foidolites, alkaline ultrabasic rocks, and pegmatite and hydrothermal veins and may even be the main mineral in some foidolite varieties. The spinel-group minerals are characterized by various chemical compositions due to the fractionation of nepheline syenites resulting in formation of the Main ring of foidolites and apatite-nepheline ore. Like most other minerals found throughout the pluton, magnetite is characterized by variation in the chemical composition along the radial line from the contact with country Proterozoic volcanic rocks to the geometric center of the pluton. Toward the center, the total Ti and Mn contents in magnetite increase from 5–15 up to 40 at %.     

Corundum-group minerals in rocks of the Khibiny alkaline pluton,Kola Peninsula

Five minerals of the corundum group have been identified in the Khibiny pluton with certainty. Corundum proper and karelianite occur only in hornfels after volcanic and sedimentary rocks. Xenoliths of hornfels mark the ring faults that bound foidalite within the field of foyaite. Hematite occurs in hydrothermally altered nepheline syenite and crosscutting hydrothermal veins related to the ring faults. Minerals of the ilmenite-pyrophanite series are present in all rocks of the pluton, including veins. Accessory ilmenite in foyaite varies from the manganese variety and pyrophanite in the inner and outer parts of the pluton to manganese-free ilmenite in zone of the Main Ring Fault. In xenoliths of volcanic rocks and alkaline ultramafic rocks, ilmenite is enriched in magnesium. The zoning in distribution of the above-mentioned minerals and the character of variation in their compositions from margins of the pluton to its center are consistent with the petrochemical zoning formed as a result of foyaite alteration of near ring faults.     

The Turiy Massif, Kola Peninsula, Russia: Isotopic and Geochemical Evidence for Multi-source Evolution

The Turiy Massif, lying within the Kandalaksha Graben, and onthe southern coast of the Kola Peninsula, contains carbonatites,phoscorites, melilitolites, ijolites and pyroxenites withinone central and four surrounding satellite complexes. Sr–Ndisotopic data from the central complex phoscorites and carbonatites,and the nearby Terskii Coast kimberlites, combined with otherrecently published data on the Devonian Kola Alkaline Province,allow us to redefine the position of the Kola Carbonatite Line(KCL) of Kramm (European Journal of Mineralogy 5, 985–989,1993). We propose that the revised-KCL mantle sources includea lower-mantle plume, and a second enriched source, which alsocontributed to the Terskii Coast and Archangelsk kimberlites.The Turiy Massif silicate rocks and northern complex carbonatiteshave more enriched isotopic signatures than the distinct, anddepleted signatures of the central complex phoscorites and carbonatites,particularly with respect to     

New Promising Gold-Ore Objects in the Strelna Greenstone Belt,Kola Peninsula

Data on gold ore objects in the Strelna Greenstone Belt in the southeastern Kola Peninsula are presented in the paper. The studied Vorgovy and Sergozero ore occurrences are localized in the zone of tectonic contact of the Neoarchean complexes making up the greenstone belt and the volcanic–sedimentary sequences of the Paleoproterozoic Imandra–Varzuga Zone. The Vorgovy gold occurrence is related to stockwork of carbonate–quartz veins and veinlets hosted in a biotite gneiss transformed into chlorite–sericite–quartz metasomatic rock with pyrrhotite–arsenopyrite dissemination. The Sergozero occurrence is localized in amphibolites corresponding to komatiitic and tholeiitic basalts hosted in biotite gneiss (metapelite). Mineralization is confined to the zone of tectonized contact between komatiitic and tholeiitic basalts, where it is controlled by a strip of metasomatic biotite–calcite rock with gersdorffite–arsenopyrite dissemination. The native gold grains medium to high in fineness are up to 0.1 mm in size and mainly localized at the contact of arsenopyrite and gersdorffite with gangue minerals. Gold mineralization is of superimposed character, and, as indicated by isotopic geochronology, was formed at the retrograde stage of the Svecofennian regional metamorphism. Comparison of ore occurrences localized in the Strelna Greenstone Belt with gold deposits in greenstone belts of the western Fennoscandian Shield and the Superior Province in Canada allows us to suggest a high perspective of the entire Strelna Belt for gold.     

Leaching rare-earth and radioactive elements from alkaline rocks of the Lovozero Massif,Kola Peninsula

The paper presents data on the leaching efficiency of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and radioactive (Th and U) elements by various reagents from alkaline rocks of the Lovozero Massif. Element concentrations were analyzed by ICP-MS and instrumental neutral activation (INAA). A new complex technique is suggested for analyzing elements on the solid phase of polymer hydrogels. This technique makes it possible to enhance the sensitivity and selectivity of INAA when these elements are simultaneously contained in rocks. Data are presented on the selective leaching of trace elements and the application of environmentally safe reagents.     

Eclogites from the Belomorian Mobile Belt (Kola Peninsula): geology and petrology

The paper focuses on the metamorphic geology of the oldest crustal eclogites discovered in the Late Archean tonalite-trondhjemite-granodiorite (TTG) complex of the Belomorian Mobile Belt on the Kola Peninsula. Eclogite bodies are, most likely, widespread. We studied one of the key objects, the Kuru-Vaara quarry, where several tens of retrogressed eclogite blocks randomly embedded in the TTG gneisses were stripped at the benches. Based on the field observations, two visually different types of eclogites have been recognized: “southern”, strongly retrogressed coarse-grained, and “northern”, well-preserved fine-grained. The southern eclogite blocks bear evidence of their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence of melting. Despite the significant mineralogic difference, both types of eclogites can be assigned to amphibole eclogite facies. The applied jadeite solubility geobarometers yielded the minimum pressures of ～12 kbar for the northern eclogites and ～14–14.5 kbar for the southern ones. The used geothermometers yielded ～700°C and ～750°C, respectively. But the presence of quartz lamellae in Na-clinopyroxenes in both types of eclogites and their bulk compositions corresponding to high-Mg basalts suggest that the Kuru-Vaara eclogites might have reached the field of ultrahigh-pressure metamorphism. Analysis of the tentative P-T paths of metamorphic evolution for both types of eclogites showed that their burial–exhumation cycle might have taken as short as a few million years. The set of presented data suggests that the formation of the Kuru-Vaara eclogites was related to the subduction of the Archean oceanic crust, which should have differed in composition and structure from the modern oceanic crust.     

The formation conditions of labuntsovite-group minerals in the Kovdor massif,Kola Peninsula

In the Kovdor massif, labuntsovite-group minerals occur in dolomite carbonatite veins (labuntsovite-Mg), in a natrolite-calcite vein (lemmleinite-Ba and labuntsovite-Fe), and in calcite pockets and veinlets cutting fenites (late labuntsovite-Mg). They are closely intergrown with paragenetic carbonates, and this makes it possible to estimate their crystallization temperature from the fluid inclusions entrapped in dolomite (≥265°C) and calcite (175–225°C). The earlier labuntsovite-Mg was formed under relatively acidic conditions, whereas later labuntsovite-calcite mineralization was derived from alkaline solutions.     

The geochemistry and isotopic age of eclogites from the Belomorian Belt (Kola Peninsula): evidence for subducted Archean oceanic crust

We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–repre-cipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ～2.86–2.87, ～2.82–2.80, and ～2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.     

Globular ferropicritic rocks at Pechenga, Kola Peninsula (Russia): Liquid immiscibility versus alteration

The uppermost volcanic unit of the early Proterozoic Pechenga Group comprises ferropicritic volcanic and hypabyssal rocks which possess well-developed, light-weathering globular (up to 10 cm in size) or layer-like structures mesoscopicly similar to those found in some Archean ocellar komatiites. These structures are superficially reminiscent of features attributed to liquid immiscibility including spheroidal aggregates in all stages of coalescence. However, several lines of field, microscopical, mineralogical and chemical evidence argue against the immiscibility hypothesis. These include identical modal proportions, grain sizes, morphologies and orientations of crystalline phases in the globules and matrix, as strikingly demonstrated, for example, by the spinifex-textured varieties of the globular rocks. A solid-state hydrothermal/metamorphic diffusional process that affected the originally glassy mesostasis is concluded to be the only feasable mechanism for the formation of the globular structures in the Pechenga ferropicrites.