The Neoarchean U-Th-Pb (SHRIMP-II) age of 2718 ± 7 Ma was established for the large copper–nickel Allarechka deposit located in the Kola–Norwegian region of the Fennoscandian shield. The deposit is associated with metamorphosed and deeply eroded volcano-plutonic complexes, which are the age and formational analogues of the Neoarchean greenstone belts. The compositional similarity of hyperbasites of the Allarechka ore district and host metavolcanic rocks of the Annama Formation with komatiite–tholeiitic metavolcanic rocks of the similar age of the Ura Guba–Kolmozero–Voron’ya greenstone belt suggests that they are ascribed to the komatiite–tholeiite volcanoplutonic series, the parental high-Fe komatiite melts of which were derived by partial melting of depleted upper mantle εNd(t) = +7.0 ± 0.4) at high PT parameters. The petrographic and geochemical composition of ore peridotites indicates that they are a cumulate formed by fractionation of Ol + Opx ± Pl, Mag from the primary high-Fe komatiite melt in a shallow (P ~ 4 kbar) magma chamber at a liquidus temperature of at least 1200°С. The absence of a significant crustal contamination of the hyperbasites by Mesoarchean TTG-complexes is proved by the trace and rare-earth element geochemistry, the Nd isotopic composition, as well as by the homogeneous morphology and isotopic composition of the accessory magmatic zircon. In contrast, Re-Os and sulfur isotopic systematics of sulfide ores indicate the contribution of not only mantle but likely crustal component in the evolution of sulfide melt. After separation from a primary mantle melt under subcrustal conditions, the silicate and sulfide melts evolved independently. Obtained data suggest that a local Ni, Cu, and PGE-rich mantle anomaly was formed through a plume–lithosphere interaction beneath the Kola–Norwegian Terrane (in its present-day outlines) of the Fennoscandian Shield at 2.75–1.9 Ga. This anomaly was a source of ore matter for three Early Precambrian copper–nickel ore epochs, in particular, for Neoarchean (~ 2750 Ma) deposits of the Allarechka ore field, Paleoproterozoic deposits (~ 2500 Ma) of the layered intrusions (e.g., Monchegorsk and Fedorova-Pana), and Pechenga group (~ 1900 Ma).

—In the northern Ladoga area, the age of the Sortavala Group rocks in the southeast of the Raahe–Ladoga zone of junction of the epi-Archean Fenno-Karelian Craton and the Paleoproterozoic Svecofennian province, their relationship with dome granitoids, the age of the provenances, and the time of metamorphic processes were estimated. The study was focused on the Nd isotope composition of rocks, the geochemical and isotope-geochronological parameters of zircon from the granite-gneisses of the Kirjavalakhti dome, the basal graywackes of the lower unit and the trachytes of the middle unit of the Sortavala Group, and the plagio- and diorite-porphyry dikes cutting the volcanosedimentary units of this group. The new isotope-geochemical data show a Neoarchean age of the granitoids of the Kirjavalakhti dome (2695 ± 13 Ma) and their juvenile nature (εNd(T) = +1.5). The granitoids underwent tectonometamorphic transformations (rheomorphism) in the Paleoproterozoic (Sumian) (2.50–2.45 Ga), which are recorded in the U–Th–Pb isotope system of the rims of the ancient cores of zircon crystals. The volcanosedimentary complex of the Sortavala Group formed on the heterogeneous polychronous (3.10–2.46 Ga) continental crust of the epi-Archean Fenno-Karelian Craton. With regard to the errors in determination of the age of clastic zircon, the minimum concordant U–Th–Pb ages of 1940–1990 Ma of detrital zircon from volcanomictic graywackes of the Pitkyaranta Formation can be taken as the upper age bound of terrigenous rocks, which agrees with the maximum age of the Sortavala Group rocks estimated from the U–Th–Pb (SIMS) age of 1922 ± 11 Ma of the Tervaoya diorites (Matrenichev et al., 2006). According to the proposed new tectonic model, the accumulation of the volcanosedimentary complex of the Sortavala Group, its metamorphism, erosion, and overlapping by the Ladoga Group turbidites had already occurred in the pericratonic part of the epi-Archean Fenno-Karelian Craton by the time of the Svecofennian continent–island arc collision, subduction, and formation of bimodal volcanoplutonic complexes of the young Pyhäsalmi island arcs and felsic volcanics of the Savo schist belt (1920–1890 Ma).

The paper reports new data on the composition and age of the Neoarchean calc-alkaline volcanic rocks of the Uraguba–Kolmozero–Voron’ya greenstone belt (UKV GB). Petrological-geochemical modeling indicates a polygenetic origin of primary melts of the basalt–andesite–dacite association and non-subduction geodynamic mechanisms for the crustal growth in the largest greenstone belt of the Kola–Norwegian Block of the Fennoscandian shield.

Numerical modeling of the generation and evolution of parental melts of the komatiite–tholeiite association of the Uraguba structure was carried out using previously obtained geochemical and isotope data. It was established that komatiite, komatiite and tholeiite basalts depleted in LREE and having ε Nd (Т = 2.79) = +2.9…+3.2 were generated by equilibrium partial melting ( F > 15%) of a depleted source (garnet-bearing Ol 0.63 + Opx 0.22 + Cpx 0.06 + Grt 0.09 mantle peridotite) at 4–8 GPa, while the genesis of primary melts of LREE-enriched komatiites (La N /Sm N ~ 1.2–1.6) with ε Nd (Т = 2.79) = +2.5…+2.2 was related to the equilibrium partial melting ( F > 20%) of an “enriched mantle peridotite” (EM– Ol 0.60 + Opx 0.20 + Cpx 0.08 + Grt 0.12 ) at pressure of 2.5–4 GPa. Coexistence in space and time of two types of melting products of mantle peridotites formed at different depths is explained by melting of different parts of adiabatically ascending mantle plume.

The paper reports newly obtained geological and isotopic-geochemical data on the volcano-sedimentary complex of the Uraguba Neoarchean greenstone structure in the Kola–Norwegian province of the Fennoscandian Shield. New U–Th–Pb geochronologic data (SIMS) on the metadacite (2790 ± 9 Ma) from a rock unit of interbedding metadacite, komatiite tuff, and lava breccia and on veins of plagioclase–microcline granite (2697 ± 10 and 2696 ± 9 Ma) that cuts the komatiite constrain the time span when supracrustal complex of the Uraguba structure was produced and underwent tectono-metamorphic transformations to approximately 100 Ma. The metavolcanic rocks of the komatiite–tholeiite association of the Uraguba structure belong to two distinct isotopic-geochemical types, which are spatially separated from one another and were produced by melting different mantle sources. Geological and isotopic-geochemical data indicate that the Uraguba structure is analogous to such unique tectonic structures on cratons as the Neoarchean Belingwe and Bulawayo belts in the Zimbabwe Shield, Kalgoorlie Belt in the Eastern Goldfilds province at the Yilgarn Craton, Kuhmo–Tipasjarvi Belt in the Karelian epi-Archean craton, and the Warawoona Paleoarchean Belt in the Pilbara Craton.

New data are reported on U-Pb (SHRIMP-II) age (2662 ± 7 Ma), isotope (Sm-Nd) and geochemical compositions of the anorthosites of the Patchemvarek Massif and “ovoidal” anorthosite sills of the Neoarchean Kolmozero-Voron’ya greenstone belt. Mesoarchean (2938 ± 8 Ma) zircons found in the Patchemvarek anorthosite have low Th/U ratio, are overgrown by a thin rim, and may be interpreted as xenogenic crystals assimilated by primary melts of the gabbro-anorthosite massifs from host Mesoarchean tonalites during crystallization in a magmatic chamber. The “ovoidal” anorthosite sills are dated at 2730–2740 Ma on the basis of U-Pb local zircon isotope analysis. The sills of the “ovoidal” anorthosites in the Kolmozero-Voron’ya GSB represent the older (2730–2740 Ma) rock group, which differs from the Patchemvarek anorthosites in strongly depleted Nd isotope composition and some geochemical features. In terms of age and Sm-Nd isotope characteristics, the “ovoidal” anorthosites are close to the komatiites of the lower volcanogenic sequence (εNd(Т) + 3.0–3.2), and metaandesites (2778 ± 5.4 Ma, U-Pb TIMS, εNdТ + 3.5) and metatholeiites of the upper volcanogenic sequence (εNd(Т) + 3.5–3.7) of the supracrustal complex of the Kolmozero-Voron’ya GSB.

The paper presents the results of the geochemical study of the accessory chrome spinels from mesoarchean (3.0–2.8 Ga) AUDK-type komatiites from the greenstone structures of the Fennoscandian Shield. In the komatiites coexistence of several generations of crome spinels was established, including relicts of the primary magmatic chromite-alumochromite (Cr2O3 – 43.31–51.61 wt %, Al2O3 – 7.66–13.64 wt %, #Cr – 70–79, #Mg – 6–11, #Fe <10), which is the equilibrium mineral phase in the komatiitic melts, crystallizing in the temperature range 1340–1370°C after olivine. Identified geochemical zoning in primary chromite is the result of the equilibrium crystallization of komatiitic melt with a gradual decrease of temperature, which leads to the formation of alumochromite-subferrialumochromite solid solutions. Later generation chrome spinels are presented by chrommagnetite-magnetite, crystallized in the final metamorphic stages of rock recrystallization in the temperature range 380–410°C (determined by magnetite-ilmenite thermometer), which corresponds to the temperature regime of metamorphic transformations of komatiitic complexes. It is shown that metamorphic transformations of rocks at pressure ≥5 kbar and temperatures ≥450–500°C lead to a complete recrystallization of the primary accessory phases, such as spinels.

Проведен анализ геологического положения, геохимического состава и изотопной систематки Nd тоналит-трондьемит-гранодиоритовых (ТТГ) серий древних кратонов. Показано, что формирование ТТГ происходило в течение 1600 млн. лет (от 4.2 до 2.6 млрд. лет назад) в пределах древнейших ядер континентов, причем для многих ТТГ отсутствует временная связь с зеленокаменными поясами. Это следует из эволюции древних кратонов, таких, как Слейв (Канадский щит), Водлозерский (Балтийский щит), кратоны Пилбара и Йилгарн (Австралийский щит), в которых ранние зеленокаменные ассоциации появляются позже древнейших ТТГ. На примере Балтийского щита установлено, что образование первичных расплавов мезоархейских ТТГ присходило на меньших глубинах (Р 150 млн. лет) интервал времени между возрастом протолита и временем его плавления с образованием расплавов состава ТТГ. Исходя из расчетов скорости охлаждения литосферных плит, эти данные свидетельствуют о малой вероятности образования большинства архейских ТТГ в субдукционно-аккреционных конвергентных геодинамических обстановках. Приведены изотопные и геохимические ограничения для реконструкции состава протолитов архейских ТТГ. Петрологическое моделирование условий формирования и изотопный состав Nd метабазальтов зеленокаменных поясов свидетельствуют, что эти метабазальты не могли быть источником ТТГ. Наиболее вероятным изотопно-геохимическим аналогом протолита ТТГ являлись архейские амфиболиты (ENd-мафиты), отличающиеся от метабазальтов зеленокаменных поясов более низкими значениями Sm/Nd отношений и повышенными содержаниями ряда литофильных элементов. Совокупность приведенных в работе данных предполагает возможность генерации первичных расплавов ТТГ при плавлении амфиболитов и гранулитов такого состава в нижней коре.

This study presents new data on the geochemical and Sm-Nd isotope compositions, as well as the U-Pb age and geodynamic nature, of the Neoarchean basalt-andesite-dacite (BAD) association from the Kolmozero-Voron’ya greenstone belt. As it was first demonstrated by the example of the Neoarchean greenstone belt, the formation of BAD associations within a single Neoarchean greenstone structure may be explained by the long-lasting evolution of separate mantle or crustal sources not related to subduction processes.

The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic-Cenozoic structures, respectively. In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa. The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These processes occurred under various thermodynamic conditions (T, P, and \(f_{O_2 } \)) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts.