Geological structure and mineral resources of Russia

The special geological issue of the Journal of the Mining Institute includes articles discussing the problems of domestic geological science and reproduction of mineral resources of the country

Composition Heterogeneity of Xenoliths of Mantle Peridotites from Alkaline Basalts of the Sverre Volcano, the Svalbard Archipelago

The article presents the results of a study of the composition of xenoliths of mantle peridotites (seven samples), collected from the Quaternary basalts of the Sverre volcano, the Svalbard archipelago. The presence of two big (more than 15 cm in diameter) xenoliths of spinel lherzolite allowed us to consider a change in their composition in the cen- tral, intermediate, and marginal parts of the samples. It is proposed to distinguish three types of xenoliths by the distribution of trace and rare earth elements. Enrich- ment of mantle peridotites with light rare earth elements, as well as high field strength (HFS)  and large-ion lithophile (LIL) elements, is presumably associated with mantlemetasomatism

Petrochemical features of tholeiites from the Shaka ridge (South Atlantic)

The article presents original data of chemical composition of tholeiitic basaltoids and andesites, dredged from the Shaka Ridge (South Atlantic) in the course of field research in spring 2016 on the scientific expedition vessel “Akademik Fedorov”. The analytical part of the work on estimating the contents of petrogenic, trace and rare-earth elements was carried out using the classical method (“wet chemistry”), X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The studied samples demonstrate elevated concentrations of large-ion lithophile elements, or LILE, (Ba, Rb, Pb) and light rare earth elements, or LREE, (La, Ce, Nd, Sm) relative to high field strength elements, or HFSE, (Nb, Ta) and heavy rare earth elements, or HREE, (Dy, Yb, Lu). The specifics of trace element geochemistry suggest a significant contribution of crustal or subduction components to the magmas of the Shaka Ridge. Discrimination diagrams of basaltoids and allied rocks with fields of different geodynamic settings indicate that they were formed in the setting of the mid-ocean ridge basalt (MORB). The reason behind the appearance of subduction and crustal marks in the rocks is possibly associated with assimilation of crustal matter by magmas or lies in their inheritance from the mantle source

Geochemistry of beryl varieties: comparative analysis and visualization of analytical data by principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE)

A study of the trace element composition of beryl varieties (469 SIMS analyses) was carried out. Red beryls are distinguished by a higher content of Ni, Sc, Mn, Fe, Ti, Cs, Rb, K, and B and lower content of Na and water. Pink beryls are characterized by a higher content of Cs, Rb, Na, Li, Cl, and water with lower content of Mg and Fe. Green beryls are defined by the increased content of Cr, V, Mg, Na, and water with reduced Cs. A feature of yellow beryls is the reduced content of Mg, Cs, Rb, K, Na, Li, and Cl. Beryls of various shades of blue and dark blue (aquamarines) are characterized by higher Fe content and lower Cs and Rb content. For white beryls, increased content of Na and Li has been established. Principal Component Analysis (PCA) for the CLR-transformed dataset showed that the first component separates green beryls from other varieties. The second component divides pink and red beryls. The stochastic neighborhood embedding method with t-distribution (t-SNE) with CLR-transformed data demonstrated the contrasting compositions of green beryls relative to other varieties. Red and pink beryls form the most compact clusters

COMMENTS ON THE ARTICLE AUTHORED BY M.V. MINTS AND K.A. DOKUKINA – THE BELOMORIAN ECLOGITE PROVINCE (EASTERN FENNOSCANDIAN SHIELD, RUSSIA): MESO-NEOARCHEAN OR LATE PALEOPROTEROZOIC?

The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga

EVALUATION OF THERMAL METAMORPHISM TEMPERATURE OF EQUILIBRATED ORDINARY CHONDRITES

The article provides an evaluation of thermal metamorphism temperatures on chondrite parent bodies using an olivine-Сr spinel geothermometer. To calculate temperatures, the results of EPMA analysis of the major and trace elements in olivine and Cr spinel of Kargapole, Orlovka, Saratov, Elenovka, Buschhof, Bjurböle and Knyahinya meteorites are used. The obtained temperatures are consistent with the onion-shell model of the planetesimal structure, in which the most metamorphosed chondrites are closer to the heating source (Buschhof, L6-734 °C), meteorites of the 5th petrologic type (p.t.) occupied an intermediate position (Orlovka, H5 – 687 °С; Elenovka, L5 – 691 °С; Knyaginya, LL5 – 700 °C), and the chondrites of the 4th p.t. were closer to the surface (Kargapole, H4 – 691 °C; Saratov, L4 – 670 °С; Bjurböle, LL4 – 682 °C). The Kargapole and Saratov meteorites are characterized by evaluation of Mg and Fe equilibrium temperatures that disagree with the onion-shell model, which can be explained by the individual history of the meteorite (for example, impact metamorphism, slow and prolonged heating)

Fahlbands of the Keret archipelago, White Sea: the composition of rocks and minerals, ore mineralization

This paper presents a complex mineralogical and geochemical characteristic (based on SEM-EDS, ICP-MS analysis) of the fahlband rocks of the Kiv-Guba-Kartesh occurrence within the White Sea mobile belt (WSMB). The term “fahlband” first appeared in the silver mines of Kongsberg in the 17th century. Now fahlbands are interlayers or lenses with sulfide impregnation, located in the host, usually metamorphic rock. The level of sulfide content in the rock  exceed the typical accessory values, but at the same time be insufficient for massive ores. Fahlbands are weathered in a different way than the host rocks, so they are easily distinguished in outcrops due to their rusty-brown color. The studied rocks are amphibolites, differing from each other in garnet content and silicification degree. Ore mineralization is represented mainly by pyrrhotite and pyrite, and pyrrhotite grains are often replaced along the periphery by iron oxides and hydroxides, followed by pyrite overgrowth. At the same time, the rock contains practically unaltered pyrrhotite grains of irregular shape with fine exsolution structures composed of pentlandite, and individual pyrite grains with an increased Ni content (up to 5.4 wt.%). A relatively common mineral is chalcopyrite, which forms small grains, often trapped by pyrrhotite. We have also found single submicron sobolevskite and hedleyite grains. The REE composition of the fahlband rocks suggests that they are related to Archean metabasalts of the Seryakskaya and Loukhsko-Pisemskaya structures of the WSMB, rather than with metagabbroids and metaultrabasites common in the study area

Composition of spherules and lower mantle minerals, isotopic and geochemical characteristics of zircon from volcaniclastic facies of the Mriya lamproite pipe

The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D"

Age and metamorphic conditions of the granulites from Capral-Jegessky synclinoria, Anabar shield

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

КОММЕНТАРИИ К СТАТЬЕ М.В. МИНЦА И К.А. ДОКУКИНОЙ «СУБДУКЦИОННЫЕ ЭКЛОГИТЫ БЕЛОМОРСКОЙ ЭКЛОГИТОВОЙ ПРОВИНЦИИ (ВОСТОК ФЕННОСКАНДИНАВСКОГО ЩИТА, РОССИЯ): МЕЗОАРХЕЙ, НЕОАРХЕЙ ИЛИ ПОЗДНИЙ ПАЛЕОПРОТЕРОЗОЙ?»

The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.Работа представляет собой критические комментарии к статье М.В. Минца и К.А. Докукиной «Субдукционные эклогиты Беломорской эклогитовой провинции (восток Фенноскандинавского щита, Россия): мезоархей, неоархей или поздний палеопротерозой?» (Geodynamics & Tectonophysics. 2020. 11 (1), 151–200). Беломорская провинция Фенноскандинавского щита – ключевой пункт в исследованиях ранней тектоники, поскольку с ней связаны многочисленные находки докембрийских эклогитов. Проблема возраста эклогитового метаморфизма габброидов Беломорского подвижного пояса неожиданно оказалась крайне актуальной не только для геологии данного региона, но и для докембрийской геологии в целом, поскольку определения рядом авторов возраста эклогитов как архейского (2.7–2.8 млрд лет) делают Беломорский подвижный пояс единственным в мире примером архейского эклогитового метаморфизма и, следовательно, единственным продатированным свидетельством в пользу плейт-тектонической модели эволюции земной коры на самом раннем этапе ее становления.В статье последовательно проведен критический разбор аргументов, выдвигаемых сторонниками архейского возраста эклогитов Беломорского подвижного пояса. Специальный акцент в работе сделан на изотопно-геохронологических и геохимических особенностях состава циркона из эклогитов, а также на особенностях фазового и химического состава и характера распределения минеральных включений. Авторская трактовка возраста эклогитового метаморфизма, приведшего к образованию эклогитов Беломорского подвижного пояса, основанная на комплексе независимых изотопно-геохимических методов датирования – локальном U-Pb методе по гетерогенным цирконам с магматическими ядрами и эклогитовыми каймами, Lu-Hf и Sm-Nd методах по минералам эклогитового парагенезиса – гранату и омфациту, принципиально другая, чем в рассматриваемой статье: все три метода независимо друг от друга определяют эклогитовый метаморфизм как палеопротерозойский, с одним и тем же значением возраста – около 1.9 млрд лет. По нашим данным, эклогиты Беломорского подвижного пояса являются одними из самых древних высокобарических пород с достоверно установленным возрастом метаморфизма около 1.9 млрд лет при различном возрасте магматического протолита в интервале от 2.2 до 2.9 млрд лет