Home – Home – Jornals – Russian Geology and Geophysics 2021 number 5

This study presents an overview of the systematic petrography, mineralogy, and geochemistry of jadeitite and jadeite-rich rocks found as blocks in the serpentinite mélanges of the Rio San Juan Complex (RSJC) of the northern Dominican Republic. The RSJC is one of the remnants of the subduction/accretionary complex of the Great Caribbean Arc that once spanned the gap between North and South America, moved relatively eastward to its present position as the Lesser Antilles island arc, and left collisional fragments along the two continental margins. Our systematic collection of heterogeneous samples ranges from jadeitite s.str. (sensu stricto) with ≥90 vol.% jadeite to quartz-rich rocks with jadeite and lawsonite. Two suites of rock types can be recognized. In the matrix-quartz-free rock suite, albite is the principal vein-filling or interstitial phase. Quartz is present only as inclusions in the cores of some jadeite crystals. In the matrix-quartz-bearing rock suite, quartz is abundant and albite is relatively rare. The first-order question concerning jadeite-rich rocks is whether jadeite precipitated from a high-pressure aqueous fluid (“vein precipitation” or “P-type”) or whether the jadeite-rich rock formed through comprehensive metasomatic replacement of an igneous protolith (“R-type”). Some examples occur as discordant veins and are clearly P-type. For most, however, classification has been equivocal. The systematic data on the petrography and whole-rock chemistry of jadeite rocks from the RSJC presented in this paper leads to significant clarification. A major argument against R-type genesis is that the metasomatic mass transfer required to produce jadeitite and jadeite-rich rocks from any normal igneous protolith is prohibitively complex. Using whole-rock and major-element compositions, we show that many members of the matrix-quartz-bearing rock suite from the RSJC can be derived by isochemical HP/LT metamorphism of normal oceanic plagiogranites subducted together with oceanic crust. Isocon analysis shows, furthermore, that more jadeite-rich rock types and also members of the matrix-quartz-free suite can be derived from such plagiogranites primarily by straightforward desilication, a realistic scenario in a serpentine-rich environment. The quartz inclusions found in jadeite crystals of the matrix-quartz-free suite corroborate a genetic path in which the plagioclase in a plagiogranite protolith reacts to jadeite + quartz. Later desilication and the formation of albite in the Si-undersaturated rock matrix leave tell-tale quartz inclusions as relics in jadeite crystals.

The Belomorian Province (BP) of the Fennoscandian Shield is a high-grade metamorphic belt composed of Meso- to Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneisses with subordinate supracrustal complexes. The Belomorian crust is underlined by a thick mantle keel, a structural element typical of Archean cratons. Belomorian rocks were metamorphosed under conditions of mainly high-pressure amphibolite to granulite facies in the both Archean and Paleoproterozoic times. The TTG gneisses contain numerous blocks of almost completely retrogressed eclogite (eclogite-1). This paragenetic association of eclogite-1 and gneisses can be classified as an Archean eclogite-TTG gneiss mélange, a component of the Belomorian continental crust produced by subductional, accretionary, and collisional processes of the Belomorian collisional orogeny 2.9-2.66 Ga ago. The Paleoproterozoic history of the BP comprises two prominent tectonic periods: (i) early Paleoproterozoic (~2.5-2.4 Ga), related to a superplume, and (ii) late Paleoproterozoic (2.0-1.85 Ga), resulted from crustal reworking during the Lapland-Kola collisional orogeny that produced strong penetrative metamorphic and local deformational overprint. The Paleoproterozoic highest-grade metamorphic overprint is represented by patches of eclogites (eclogite-2) in Paleoproterozoic mafic dikes and eclogite-1. Field relations between eclogite-1 and eclogite-2 are described in the Gridino area of the western coast of the White Sea. So, the BP is a high-grade polymetamorphic belt formed by a superposition of the Neoarchean Belomorian and Paleoproterozoic Lapland-Kola orogenies, whose characteristic features are eclogites produced by subduction and collision.

The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction-collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm-Nd isochron from the diamondiferous calc-silicate and garnet-pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2-1.1 Ga was obtained by U-Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate and garnet-pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggests that they were metamorphosed under granulite facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet-kyanite-mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm-Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation. The close ɛ_Nd(1100) values of the basement rocks and garnet-kyanite-mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

The paper presents new data on the Rauer Islands, one of the unique objects of the East Antarctic Shield. The interest in this area is triggered by its complex geologic structure, including both Archean and Proterozoic fragments of the Earth’s crust, and by its multiphase formation. A detailed scheme of the geologic structure of the area is proposed, new petrologic complexes are revealed, and the stages of tectonomagmatic activity at ~1400-1320 Ma and 1150 Ma are reliably dated. This serves as a factual basis for comparison of the study area with other regions of East Antarctica. Based on the geological and isotope data obtained, the Meso-Neoproterozoic Filla Terrane in the area of the Rauer Islands is recognized. It is composed of metamorphic and primarily intrusive rocks, whose protoliths formed in the time interval 1400-950 Ma. Three periods of tectonothermal activity have been established in the Filla Terrane: Mid-Mesoproterozoic (1400-1320 Ma), Meso-Neoproterozoic (1150-886 Ma), and early Cambrian (536-504 Ma). The first period is the formation time of Mesoproterozoic crust, and it is time-correlated with the tectonogenesis phase in the adjacent Rayner province. The second period corresponds to the later phase of tectonothermal activity in the Rayner province. In the Filla Terrane, this period can be divided into two intervals, 1150-1100 Ma and 1010-886 Ma. The former interval is treated as intense crustal growth in the course of granitoid and mantle magmatism. The latter interval is a period of tectonothermal processes accompanied by intense deformations, high-temperature metamorphism, and intrusion of porphyritic granitoids. Apparently, the gap between the first and the second intervals is the time of deposition of the sedimentary protolith of paragneisses, which, together with the surrounding rocks, underwent high-temperature metamorphism and deformations at 950-914 Ma. The synchronous evolution of the Archean block and the Filla Terrane began at least within 1100-1000 Ma. The youngest, early Cambrian, period of tectonic activity coincides with the development of local low-temperature mylonite zones and the intrusion of synkinematic pegmatite veins. Thus, the tectonothermal evolution of the Filla Terrane includes almost the same main phases of crustal growth and transformation as the Rayner province. This indicates that the Filla Terrane is a fragment of the Rayner province, which accreted to the Archean terrane at least in the late Mesoproterozoic.

The structure of the initial and predicted oil resources of the West Siberian petroleum province is quantitatively assessed. The assessment is based on the law of mass distribution of hydrocarbon accumulations, i.e., the truncated Pareto distribution and simulation modeling of the general set of oil fields. This approach makes it possible to estimate the amount of oil and the total oil resources concentrated in intervals of any size, in particular, in intervals of small and fine fields, in order to determine the economic efficiency of their development. The considered estimates do not apply to unconventional resources, such as the shale oil of the Bazhenov Formation.

We present the preliminary results of a study of the Bystrinskoe earthquake, which occurred in the southern Baikal region on 21 September 2020 and was accompanied by shaking with an intensity of VI-VII on the MSK-64 scale in the epicentral area and with an intensity of V in large cities of southern East Siberia (Irkutsk, Angarsk, Usolye-Sibirskoe, Zakamensk, etc.). A preliminary characteristic of the seismic event is given on the basis of a comprehensive analysis of seismological, structural-tectonic, strain, emanation, and hydrogeochemical data obtained during the monitoring of hazardous geologic processes in the Baikal natural territory. We have estimated the seismologic parameters of the Bystrinskoe earthquake, characterized the accompanying phenomena, and identified the effects that are of interest as probable precursors of future strong earthquakes in the Baikal region. The data obtained suggest that the earthquake occurred in the zone of the Main Sayan Fault as a result of strike-slip movement along the W-NW fault. The earthquake focus was apparently located at a shallow depth, as evidenced by the duration of the shocks, macroseismic manifestations, and the strong rumble heard at different directions from the epicenter.