The Marun–Keu complex plays a significant role in our understanding of the geological evolution of the Ural orogen; however, it remains poorly understood. This study aims to provide insights into the complex’s age, protolith composition, rock formation conditions, and its position in the geological history. The zircons from the host granitic gneiss are characterized by magmatic cores with an age of 473 Ma and metamorphic rims with an age of approximately 370 Ma. We suggest that the metamorphic rims were formed during eclogite metamorphism and that the metagranitoids hosting the eclogites experienced eclogite metamorphism simultaneously with the basic and ultrabasic rocks that are common in this area. Heterogeneous zircons were also isolated from the selvage of a pegmatite vein, in which four domains are distinguished, two to three of which can be identified within single grains, as follows: (1) igneous cores with an age of approximately 470 Ma and the geochemical characteristics of zircon crystallized in basic rocks; (2) zircons recrystallized during eclogite metamorphism with geochemical characteristics intermediate between those of the magmatic cores and true eclogitic zircon; (3) pegmatitic zircon, exhibiting the most sharply differentiated REE spectra of all four domains, characterized by a prominent positive Ce anomaly and a weakly expressed negative Eu anomaly; and (4) eclogitic zircon, observed in the form of veins and rims, superimposed in relation to the other three domains. The age of the latter three domains is within the error range and is estimated to be approximately 370 Ma. This indicates that the processes of eclogite metamorphism and the formation of pegmatites occurred at approximately the same time in the studied area.

Eclogites of the Belomorian Mobile Belt (BMB) are a key object for geodynamic reconstructions not only of the BMB, but also of the entire Fennoscandian Shield. One of the main issues for BMB eclogites is the establishment of the time of their formation. The interpretation of the age of the BMB eclogites as Archean allowed some researchers to extend the geodynamic mechanisms of modern-type plate tectonics to this segment of the Archean crust. At present, the geochronological data obtained on the BMB eclogites are interpreted in different ways. Despite numerous publications on the age problem of the Belomorian eclogites (over 70 papers), a generalized overview of all points of view on the timing of eclogite metamorphism with critical analysis of underlying arguments is absent. The author's interpretation of the age of eclogite metamorphism within the BMB is based on a combination of independent isotope-geochemical dating methods: local U-Pb dating of heterogeneous zircons with magmatic cores and eclogite rims, Lu-Hf and Sm-Nd dating of rock-forming minerals of eclogite paragenesis (garnet and omphacite). All three methods independently determine the age of eclogitic metamorphism as Svecofennian, with the same value of about 1900 Ma.

Generalized overview of all points of view on the timing of eclogite metamorphism of Belomorian Mobile Belt (BMB) with critical analysis of underlying arguments was made. The author’s interpretation of the age of eclogite metamorphism within the BMB is based on a combination of independent isotope-geochemical dating methods: local U-Pb dating of heterogeneous zircons with magmatic cores and eclogite rims, Lu-Hf and Sm-Nd dating of rock-forming minerals of eclogite paragenesis (garnet and omphacite). All three methods independently determine the age of eclogitic metamorphism as Svecofennian, with the same value of about 1900 Ma.

The Belomorian Province of the Fennoscandian Shield exposes numerous Precambrian eclogites, which makes it significant for the study of early tectonic processes. The age of these eclogites has been discussed for more than 15 years and regarded as either Archean or Paleoproterozoic. In the Kuru-Vaara quarry within the northern Belomorian Province, the eclogitic assemblage is preserved in concordant mafic boudins in felsic gneisses and a partially eclogitized gabbro-norite dike cutting discordantly through the gneiss fabric. Both eclogite types preserve zircon cores with a magmatic geochemical signature that yield protolith ages of ca. 2.88 Ga for a mafic boudin and ca. 2.44 Ga for the eclogitized gabbro-norite. Ca. 1.9 Ga zircon rims and grains from the eclogites show low Th/U ratio and HREE depletion, reflecting the growth of metamorphic zircon in equilibrium with garnet. In the eclogite boudin, the Archean zircon cores yield delta O-18 = 5.1-5.9 parts per thousand typical of mantle melts; the oxygen isotope composition of garnet (delta O-18 = 4.0-5.0 parts per thousand) is in equilibrium with that of the 1.9 Ga zircon (delta O-18 = 4.5-5.4 parts per thousand). Garnet Lu-Hf geochronology coupled with U-Pb zircon geochronology constrains prograde metamorphism for the Kuru-Vaara eclogites at 1.92-1.89 Ga. Mineral inclusions of garnet, zoisite, plagioclase, kyanite, amphibole, quartz, and low-Na clinopyroxene in ca. 1.9 Ga zircon from the eclogite boudin imply epidote-amphibolite/amphibolite facies conditions for the prograde metamorphism. All data point to a Paleoproterozoic age of the eclogite facies metamorphism.

The detailed mineralogical-geochemical study of eclogites and their retrograde products (amphibolites in a rim of eclogite boudin) from Vichennaya Luda Island (Keret Archipelago, White Sea) revealed systematic variations in major-, trace and rare-earth element composition of rock-forming minerals, which should be taken into account in geochronological and thermobarometric studies. Garnets from garnet–amphibole interlayers in the eclogites demonstrate a prograde zoning. In addition, garnet rims differ from their cores in a “hump-like” REE pattern owing to the elevated Sm, Eu, Gd and Dy contents and a negative slope of HREE pattern, as well as in an increase of Sm/Nd and a decrease of Lu/Hf ratio. It has been established that the eclogites contain clinopyroxene with depleted (to chondritic level) REE content, a positive Eu anomaly, and lowered Ti, V, Cr, Y, Zr, and Hf contents. Based on these geochemical features, the Cpx can be ascribed to relict that have preserved during peak eclogite metamorphism. Amphiboles in amphibolite rim of boudin differ sharply from amphiboles in eclogite in the lowered contents of LREE and some HREE. In addition to almost a two-fold decrease of Ti content in the eclogite–amphibolite sequence, amphiboles demonstrate a significant decrease of V, Sr, Y, Nb, and Hf contents. All biotites have sinusoidal REE pattern, which is typical of minerals formed through fluid-induced disequilibrium processes. Biotite from eclogites has higher Ti content and elevated contents of REE, Nb, V, Cr, Ba, and Hf as compared to biotites from the amphibolization rim. The eclogites and amphibolites developed after them are similar in major, trace, and rare-earth element composition.

The gold mineralization in the Central Aldan ore district is genetically related to potassic calc-alkaline and alkaline magmatism dated at 115–150 Ma. The objective of this study is to establish the age of hydrothermal processes that accompanied the formation of Au-Te mineralization at the Samolazovsky Deposit. Based on the isotope-geochemical study of zircons from quartz-feldspar metasomatic rocks of the deposit, the granitoids and charnokites of the Nimnyr Complex (1 900–1 960 Ma) at the contact with the Yukhta monzonite-syenite massif (∼127 Ma) were studied. Zircon U-Pb dating was performed on a SHRIMP-II ion microprobe, and rare-earth and trace elements analyses of zircon were carried out in the same craters by secondary-ion mass spectrometry on a Cameca IMS-4f ion microprobe. It is revealed that the studied zircons have heterogeneous structures: dark core and lighter rim, which differ greatly in isotope-geochemical parameters. Zircon rims are cut by a network of fractures, extending into the central part of zircon grains. The rims yield a subconcordant age of 1 937±24 Ma, with an average total REE content of 550 ppm, which corresponds to the formation age of the Nimnyr Complex. All zircon cores yield a discordant age of 83±11 Ma and are characterized by a higher total REE content (∼1 812 ppm), as well as higher contents of U and non-formula elements (Ca, Sr, and Y) with respect to rims, due to the effect of fluid on zircons. Despite the limited number of zircon grains, the additional geochronological study of zircons from syenites of the ore-bearing Ryabinovy Massif has revealed the presence of two distinct age clusters: ∼125–138 and 76–83 Ma. The older ages of zircons from syenites are typical for the Central Aldan ore district. Until now, there is still no explanation for an age range (76–83 Ma) of single zircon grains from ore-bearing syenites of the Ryabinovy Massif. The obtained data suggest that the processes of activation (the effect of fluid) within the Central Aldan ore district continued until Late Mesozoic. With regards to the equivocal geotectonic position of the Mesozoic potassic magmatism in the study area and its high metallogenic potential, there exists an absolute necessity to determine the geochronological age of the rock formations. Therefore this study presents the Late Cretaceous geochronological data for the first time which can constrain the time-frame for the formation of gold-bearing magmatic and metasomatic rocks of the Aldan ore district.

Аннотация.В работе приводятся результаты исследования химического и минерального составов эклогитов и их протолитов комплекса Берген Аркс.Полевые работы проводились на острове Холсной на юго-западе Норвегии.Эклогиты приурочены к тектонически ослабленным зонам

The paper presents the results of the isotope, geochemical and thermobarometric study of plagio-crystalline schist containing in the Upper Anabar series of the Anabar Shield. Granulite complexes of the paleoplatforms are the most important issue in addressing the fundamental problem of the Earth's crust origin and its composition. The early stages of crust formation which correspond to the deeply metamorphosed rocks of the platform basements, available for study within the shields, are of particular interest. The study of the age and metamorphic conditions of granulites by the case of the Upper Ananbar series allows specifying the stages the Anabar Shield's ancient crust formation. Isotope-geochemical (U-Pb geochronology for zircon and Sm-Nd for garnet-amphibole-WR) and thermoba-rometric (Theriak-Domino) studies of plagio-crystalline schist allowed to identify two Paleoproterozoic metamorphism stages within the territory of the Anabar Shield with an age of about 1997 and 1919 million years. The peak conditions of granulite metamorphism are determined as 775±35 С and 7.5±0.7 kbar, the parameters of the regressive stage are 700 C and 7 kbar. The sequence of the rocks metamorphic transformations can be assumed: high-thermal metamorphism of the granulite facies (T ≤ 810 C) and subsequent sub-isobaric (about 7 kbar) cooling to 700 C with a water activity increase and formation of Grt-Amp paragenesis corresponding to the transition from the granulite to amphibolite facies. Data on the REE and other trace elements distribution in zircon and rock-forming minerals obtained by the ion microprobe analysis contribute significantly to the isotope-geochemical data interpretation.

Several types of metabasites of different age were identified in the southern part of Pezhostrov Island: eclogites with a magmatic protolith age of about 2200 Ma and 2500 Ma old metagabbroanorthosites that retained no eclogitic assemblage. It is shown that the Paleoproterozoic eclogites dominate volumetrically over Archean eclogites in the Belomorian eclogitic province. Eclogites with the youngest Jatulian protolith age (no older than 2200 Ma) occur with the same frequency as those with the Archean protolith age. A new find of eclogites with a Paleoproterozoic age of magmatic protolith and generalization of accumulated geochronological data confirm the recognition of an extended zone of high-pressure metamorphism with an age around 1900 Ma in the Belomorian mobile belt.

The paper reports the first data obtained by state-of-the-art analytical techniques on the composition of minerals and the metamorphic age and metamorphic parameters of eclogite-like rocks from Sidorov and Ileiki islands, Keret Archipelago, White Sea. The U-Pb zircon magmatic age of the metabasites lies within the range of 2400–2480 Ma. The rocks were subjected to eclogite metamorphism at 1870–1890 Ma, with this age estimate consistent with analogous estimates for eclogite metamorphism elsewhere within BMB. Simultaneously garnetite zones were produced. The metabasites were eclogitized at 12 kbar and 700°C, i.e., near the boundary between the eclogite and amphibolite facies of relatively high pressure, because of a local pressure increase during rock cooling in the presence of fluid. The retrograde metamorphic episode proceeded under pressures from 12 to 6.5 kbar at temperatures from 700 to 600°C. The contact amphibolization of the metabasites at a temperature close to 620°C and pressures of 2–2.5 kbar (low-pressure amphibolite facies) occurred at 1870 Ma and is pronounced in the form of an amphibolite rim around a boudin of eclogitized basite and in significant changes in the trace-element and REE composition of the eclogite zircon.

Впервые проведено исследование геохимии титанита из эклогитов и сопряженных с ними пород Беломорского подвижного пояса (БПП) методом вторично-ионной масс-спектрометрии, которое позволило установить особенности состава титанита из гранатсодержащего и безгранатового парагенезисов. Для титанита из гранатсодержащих парагенезисов характерно: спектр REE выпуклой формы и пониженное содержание HREE относительно LREE, также величина GdN/YbN отношения в среднем составляет около 16.5. Титанит из метаультрабазита наследует специфические особенности состава вмещающей породы, что необходимо учитывать при сравнении с таковым из метагаббро. Результаты U-Pb датирования (TIMS) титанита подтверждают правомерность выделения раннего и позднего этапов свекофеннского метаморфизма в изученных районах БПП: раннего метаморфизма с возрастом достижения пиковых эклогитовых параметров около 1900 млн лет и ретроградной стадии при параметрах амфиболитовой фации в районе 1870–1880 млн лет и позднего метаморфизма аллохимической направленности, сопровождаемого пегматитообразованием, с возрастом около 1840 млн лет.

The first geochemical study of titanite from eclogites and associated rocks of the Belomorian Mobile Belt (BMB) by secondary ion mass spectrometry made it possible to establish the compositional features of this mineral in the garnet-bearing and garnet-free assemblages. Titanite from garnet-bearing assemblages is characterized by upward convex REE pattern and lowered HREE content relative to LREE, as well as the average Gd N /Yb N ratio around 16.5. Titanite from metaultrabasic rock inherits the specific features of the host rock, which should be taken into account when comparing with titanite from metagabbro. Results of U-Pb (TIMS) dating of titanite confirms the identification of the early and late stages of the Svecofennian metamorphism in the studied areas of BMB: early metamorphism with the peak eclogite facies conditions at around 1900 Ma, retrograde amphibolite facies metamorphism at 1870–1880 Ma, and late allochemical metamorphism accompanied by the pegmatite formation with an age of 1840 Ma.