Geochronology of granites of the western Korosten AMCG complex (Ukrainian Shield): implications for the emplacement history and origin of miarolitic pegmatites

The origin of large miarolitic (also known as “chamber”) pegmatites is not fully understood although they may have great economic value. The formation of cavities in magmatic bodies is related to melt degassing and gas or fluid flow through partially solidified magma. In this paper, the origin of the Volyn pegmatite field, located in the Palaeoproterozoic Korosten anorthosite–mangerite–charnockite–granite (AMCG) complex, North-Western region of the Ukrainian Shield, is discussed. Pegmatites of the field host deposits of piezoelectric quartz that is accompanied by gem-quality beryl and topaz. The Volyn pegmatite field is confined to granites located in the south-western part of the Korosten complex and extends for 22 km along the contact with the anorthosite massif within the Korosten plutonic complex. Geological data indicate hybridization of basic melts and partly crystallized granites, as well as direct impact of fluids derived from basic melts on the chamber pegmatites. The new U–Pb zircon ages obtained for granites and pegmatites of the Korosten complex confirm that the rock assemblage in the northern part of the complex crystallized between 1800 and 1780 Ma, whereas rocks in the southern part intruded mainly between 1768 and 1755 Ma. U–Pb zircon ages for granites from the south-western part of the Korosten complex indicate that granites were emplaced at 1770–1765 Ma, a few million years prior to the intrusion of the gabbro–anorthosite massif (1762–1758 Ma), while chamber pegmatites in these granites crystallized at 1760 ± 3 Ma, coevally with the basic rocks. Ultimately, the formation of the chamber pegmatites was related to the reheating of the semi-crystallized granitic intrusion and to fluids migrating from the underlying gabbro–anorthosite massif.

The Paleoarchean and Mesoarchean TTGs of the western Azov area, the Ukrainian Shield

A large anticline structure occurs in the western part of the Azov Domain of the Ukrainian Shield. It is composed of rocks of the Mesoarchean (3.2-3.0 Ga) granite-greenstone association and relics of an older basement. The anticline is divided into two parts by the Bilotserkivka structure of sub-latitudinal strike. The northern part includes the Huliaipole and Remivka blocks, and the southern part comprises the Saltycha anticline. The U-Pb age of plagiogneisses of the Lantsevo anticline of the Bilotserkivka structure is 3299 ± 11 Ma. In terms of geochemical characteristics, they correspond to TTGs. In the western part of the Bilotserkivka structure, we previously identified quartz diorites having an age of 3297 ± 22 Ma. These data show that the Bilotserkivka structure represents an ancient basement. Dislocated trondhjemites were studied in the Ivanivka area at the eastern part of the Saltycha anticline. They contain numerous relics of heavily altered amphibolites. The U-Pb age of zircons from trondhjemite is 3013 ± 15 Ma. These rocks are of the same age as TTGs of the Shevchenko Complex cutting through the sedimentary- volcanogenic rocks of the greenstone structures of the Azov Domain. They share age and geochemical characteristics with biotite and amphibole-biotite gneisses of the “Kainkulak beds” in the Zrazkove village located at the Mokra Konka river (3.1-3.0 Ga) and with biotite gneisses in the lower reaches of the Kainkulak river (2.92 Ga). Thus, gneisses of the “Kainkulak beds” actually represent the Mesoarchean TTGs of the Shevchenko Complex, transformed in the Paleoproterozoic time due to the dislocation metamorphism. The late Paleoarchean (3.3 Ga) tonalites are known in the West Azov and KMA domains; they probably also occur in the basement of the Middle Dnieper domains, where detrital zircons of this age have been reported. These data allow us to assume the existence of a large Late Paleoarchean (3.3 Ga) protocraton, in which the Mesoarchean (3.2-3.0 Ga) greenstone belts and TTGs of the eastern part of the Ukrainian Shield and the KMA Domain were formed.

THE PALEOARCHEAN (3.3 Ga) AND MESOARCHEAN (3.0 Ga) TTGs OF THE WESTERN AZOV AREA, THE UKRAINIAN SHIELD

A large anticline structure that includes the West Azov and Remivka blocks occurs in the western part of the Azov Domain of the Ukrainian Shield. These blocks are composed of rocks of the Mesoarchean (3.2-3.0 Ga) granite-greenstone association and relics of an older basement. The anticline is divided into two parts by the Bilotserkivka structure of sub-latitudinal strike; the northern part includes the Huliaipole and Remivka blocks, and the southern part is comprised of the Saltycha anticline. The Archean plagiogranitoids of the West Azov underwent intense dislocation metamorphism during the Paleoproterozoic. In many areas they were transformed into plagioclase gneisses that were attributed to the Paleoarchean “Kainkulak thickness” of the Azov Series. Detailed geological-structural and geochronological studies are required to define the age of these gneisses.We have chosen two areas for our studies: the Lantsevo anticline within the Bilotserkivka structure, and the Ivanivka area in the eastern part of the Saltycha anticline. The Bilotserkivka structure is composed of rocks of the Central Azov Series and highly deformed Archean formations. We have dated plagiogneisses of the Lantsevo anticline. These rocks contain large relics of metamorphic rocks of unknown age, including two-pyroxene and pyroxene crystalline schists, and pyroxenemagnetite quartzites (BIF). In terms of chemical composition, two-pyroxene crystalline schists correspond to tholeiitic basalts and basaltic komatiites. Ferruginous-siliceous rocks belong to the Algoma type typical for the Archean greenstone belts. Biotite gneisses are similar to the medium-pressure tonalite-trondhjemite-granodiorite rocks (TTGs). The U-Pb age of zircon crystallization from biotite gneisses is 3299 ± 11 Ma. At 30 km in the western part of the Bilotserkivka structure, we have previously identified quartz diorites having an age of 3297 ± 22 Ma. In terms of geochemical characteristics, they correspond to low-pressure TTGs. These data show that the Bilotserkivka structure is a block representing an ancient basement. In the Ivanivka area in the eastern part of the Saltycha anticline, the strike of the Archean rocks was reorientated from northwestern to latitudinal. The studied dislocated trondhjemites of the Ivanivka area correspond to TTGs in terms of the geochemical characteristics. They contain numerous relics of highly altered amphibolites. The U-Pb age of zircon crystallization from trondhjemite is 3013 ± 15 Ma. These rocks are of the same age as TTGs of the Shevchenko Complex cutting through the sedimentary-volcanogenic rocks of the greenstone structures of the Azov Domain. They share age and geochemical characteristics with biotite and amphibole-biotite gneisses of the “Kainkulak thickness” in Zrazkove village located at the Mokra Konka river (3.1-3.0 Ga) and with biotite gneisses in the lower reaches of the Kainkulak river (2.92 Ga). Thus, gneisses of the “Kainkulak thickness” in fact represent the Mesoarchean TTGs of the Shevchenko Complex, which were transformed in the Paleoproterozoic time due to the dislocation metamorphism. Late Paleoarchean (3.3 Ga) tonalites are known in the West Azov and the KMA domains; they probably also occur in the basement of the Middle Dnieper domains, where detrital zircons of this age have been reported. These data allow us to conclude the existence of a large Late Paleoarchean (3.3 Ga) protocraton, in which the Mesoarchean (3.2-3.0 Ga) greenstone belts and TTGs of the eastern part of the Ukrainian Shield and the KMA Domain were formed.

Gaivoron—Zavallia section of the Middle Pobuzhzhіa is the most representative part of the granulite complex of the Ukrainian Shield (structural-tectonophysical results and magnetometric studies)

In the article according to the geological and geophysical data of the well-exposed areas of development of Archean rocks of the Ukrainian Shield, two alternative approaches to establishing the structure and stratigraphy of the oldest granulite complexes are discuss. The outcrops of the enderbite-gneiss complex up to 3.6—3.8 billion years old are located along the Southern Bug River between the Gaivoron town and the Zavallia village. The first, «stratigenic-metamorphogenic» approach assumes that the main features of the composition and structure of the Lower Archean complexes are inherited from the original stratotypic strata. These strata are transformed in the conditions of quasi-isochemical metamorphism with preservation of the sequence of formation in section and the primary constitution in the form of stratification, rhythmicity, direction of change of their composition vertically and laterally. On the structural-formation map and geological section of the Gaivoron—Zavallia section, the Archean granulite complex is shown in the form of a synclinorium composed of four adjacent formations, which are equated to the world of metamorphosed volcanic-sedimentary rocks. The second, «deformation-metamorphogenic» approach, which is supported by the authors of this article, is based on the idea that the granulite complex of Pobuzhzhіa is a subvertically layered medium formed by tangential tectonic forces. The latter lead to shear deformations and displacement of matter at the atomic-molecular (with mineral transformation of rocks) and rock masses at the regional level. This creates structural and textural elements that are superimposed on the primary structure of rocks and often erase it. Field structural-tectonophysical, tectonofacial and magnetometric studies, its results are presented in the article. It was performed specifically to compare these two concepts. Magnetometric studies have shown that the enderbite-gneiss complex of the district by its magnetic characteristics belongs to the middle and lower crust of the Ukrainian Shield.

International cooperation of S. I. Subbotin Institute Geophysics, NAS of Ukraine for 2010—2020

The geological structure of the lithosphere of the main tectonic structures has been refined for the territory of Ukraine and adjacent regions of Slovakia, Poland, Romania, Russia, as well as Bulgaria, the Antarctic Peninsula (West Antarctica) and Southeast Asia, and new data have been obtained on geophysical impacts that can affect the environment. A geodynamic scenario has been developed for the formation of large-scale folding of the Fore- Dobrudzja Trough, the South Ukrainian monocline and the Ingul block of the Ukrainian Shield, caused by tectonic events associated with the closing of the Paleotethys and Neotethys oceans in the Mesozoic. In the Pripyat-Dnieper-Donets Basin, the structure of the earth’s crust and upper mantle can reflect different intensities of rifting, from its passive stage in the Dnieper Graben to active rifting in the Pripyat Trough. An analysis of the geoelectric structure of the Earth’s crust in the Ukrainian Eastern Carpathians indicates that seismic events occur mainly in resistive solid rock domainswhich surrounded by aseismic high conductive zones consisting of at least partially melted material. The present-day mutual position of the Ukrainian shield and Fennoscandia stabilized 1720—1660 Ma. The age, distribution, orientation and composition have been studied for the LatePalaeoproterozoicdykes in the Volyn, Ingul and Azov blocks of the Ukrainian Shield. Eastern Crimea and the Sorokin Trough are fragments of a tectonic wedge formed after the Paleocene. The geothermal conditions of the Intra-Carpathian region are due to subduction during the closure of the Pannonian sea basin and the collisional interaction of the Eurasian plate with the microplates system of this region. In Bulgaria, most earthquakes occur outside high-resistive domains. The tectonic stages are reconstructed for the formation of the northern part of the Antarctic Peninsula in the Mesozoic-Cenozoic. The relationship has been established between the geomagnetic field and climate change, with it being different for the Northern and Southern Hemispheres. The results have been obtained within the framework of 25 international projects and 6 temporary international target teams of S. I. Subbotin Institute of Geophysics, NAS of Ukraine consisting of researchers from 23 countries. The results are presented in 53 publications, 38 of which are indexed in the Web of Scienct database, and 32 papers are published in 20 international journals and special publications of 10 countries with different impact factors (from 0,101 to 4,214), whose average impact factor is 3,341, and the total one is 66,815.

GEOLOGICAL INFORMATION OF MAGNETIC PROPERTIES OF GRANITOIDS OF THE PRECEMBRIAN BASEMENT OF UKRAINIAN SHIELD (ON THE EXAMPLE OF THE MELNIKOVTSY PLOT)

In Ukraine, there are all possibilities for the development of geophysical methods for solving the problems of mapping the Precambrian foundation, as there is a well-exposed Ukrainian Shield (US). The article shows the possibilities of the magnetic scanning method for geological mapping, on the example of a section composed of granitoids with xenoliths of crystalline shales. The magnetic field and magnetic susceptibility on exposures are «scanned» in detail. Two positive anomalies of the magnetic field with a similar fine internal structure have been identified. The results of the study showed that granitoids have low magnetic properties that do not allow the formation of positive anomalies of the magnetic field. It is assumed that the source of «toothed» positive anomalies of the magnetic field in the studied section of the US can be two parts of one layered geological body — with a rhythmic internal structure composed of crystalline shales. It is possible that the body was awakened by local tectonic processes and torn into parts that are significantly displaced, and even lie almost parallel in the study area.

ANALYSIS OF GRAPHITE MINERAL-ROW MATERIAL BASE OF THE UKRAINIAN SHIELD USING ECONOMIC METHODS

The analysis of the mineral base of graphite of the Ukrainian shield with the use of geological and economic eassessment methods was carried out. Directions of the rational development are outlined. The main problems of sustainable development of graphite mineral raw-material base in modern conditions are determined. Geological and technological factors of economic value of graphite deposits and occurrences are characterized. A scheme of comparative assessment estimation of graphite deposits was proposed. This scheme allows to establish the most promising objects for further exploration in the initial stages of mineral deposits prospecting. The analysis includes the main geological, economic, technological and industrial factors of economic value of graphite deposits, such as reserves / resources, graphite content in ores, depth of ores, exploration degree and complexity of the deposit and others. The following methods were used in the work: the method of rapid assessment of the minerals value for determining the potential economic value of objects and the method of scoring. At the first stage, the deposits and occurrences included in the state balance, were ranked according to geological and industrial parameters. Each of the parameters rated according to its qualitative characteristics. The number of points obtained for graphite deposits with proven development efficiency (Burtynske, Balakhivske) and the developed field (Zavalivske) were used as optimal indicators. According to them, the most promising for further exploration works graphite occurrences with the best geological and industrial indicators were selected. The commodity value of these deposits was calculated using the method of coefficients. The results of the calculation were also taken into account for determining the most promising deposits for investment. According to the obtained data, a map of potentially promising graphite deposits of the Ukrainian shield was constracted.