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Petrology
Journal Prestige (SJR): 0.847  Citation Impact (citeScore): 1 Number of Followers: 6  Subscription journalISSN (Print) 0869-5911 - ISSN (Online) 1556-2085 Published by Springer-Verlag  [2468 journals]
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- Carbonation of Serpentinites of the Mid-Atlantic Ridge: 1. Geochemical
Trends and Mineral Assemblages-
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Abstract: Abyssal peridotite outcrops compose vast areas of the ocean floor in the Atlantic, Indian, and Arctic Oceans, where they are an indispensable part of the oceanic crust section formed in the slow-spreading oceanic ridges (Mid-Atlantic Ridge, Southwest Indian Ridge, and Gakkel Ridge). The final stage in the evolution of abyssal peridotites in the oceanic crust is their carbonation, which they experience on the ocean floor surface or near it. The main goal of this study was to reconstruct the geochemical trends accompanying the carbonation of abyssal peridotites using MAR ultramafic rocks as an example and to identify the main factors that determine their geochemical and mineralogical differences. The composition variations of rock-forming minerals and their characteristic assemblages indicate that the initial stages of carbonation of abyssal peridotites occurred in crustal conditions simultaneously with the serpentinization of these rocks. The final stage in the crustal evolution of the abyssal peridotites is their exhumation on the ocean floor where they were brought up along the detachment faults. On the ocean floor, the abyssal peridotites in close association with gabbro form oceanic core complexes, and the degree of their carbonation sharply increases with time of their exposure on the ocean floor. The presented data made it possible to qualitatively reconstruct the sequence of events that determined the mineralogical and geochemical features of carbonatized abyssal peridotites of the MAR.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010095
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- Grenville and Valhalla Tectonic Events at the Western Margin of the
Siberian Craton: Evidence from Rocks of the Garevka Complex, Northern
Yenisei Range, Russia-
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Abstract: Understanding the tectonic evolution of the Yenisei Range offers important clues not only for the tectonic evolution of orogenic belts at margins of ancient cratons but also for solving the problem of the incorporation of the Siberian craton into the Rodinia supercontinent. Results of mineralogical−petrological, geochemical, and isotope–geochemical studies provide an insight into the petrogenesis, geotectonic settings, thermodynamic parameters of formation, and the ages of the metamorphism and protoliths for the contrastingly compositionally different rocks of the Garevka metamorphic complex. The paper discusses the possible models for the origin of the rock complexes and the geodynamic settings in which they were formed. The western margin of the Siberian craton was determined to have been affected by two pulses of Neoproterozoic endogenic activity, which were related to the origin of the Rodinia supercontinent (930–900 and 880–845 Ma), and which correlated with Grenville and post-Grenville processes responsible for Valhalla folding. The regional geodynamic history is correlated with the coeval sequence and similar style of tectono−thermal events in the peripheries of the large Precambrian cratons Laurentia and Baltica, which is consistent with the proposed Neoproterozoic paleogeographic reconstructions of close spatiotemporal relationships between these cratons and their incorporation into Rodinia configuration.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010058
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- Crystallization Parameters, Genesis of Melts, and Sources of Magmas of the
Late Cenozoic Udokan Volcanic Plateau, Central Asia-
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Abstract: Similar to the other areas of the Late Cenozoic volcanic province of Central Asia, the Udokan volcanic plateau (UVP) was formed in the time span between the Middle Miocene and the Pleistocene. Its rocks are highly alkaline and vary from alkaline picrobasalts and basanites to alkaline trachytes. The compositional variations of the rocks were controlled by two differentiation trends, which corresponded to different generation conditions of the parental magmas. The rocks with low SiO2 contents (<45 wt %) were formed by melts of low degrees of melting, whose melts were derived under elevated pressures and temperatures. The formation of the rocks with 45–61 wt % SiO2 was associated with the differentiation of basalt melts, which were derived at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of the moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province in Central Asia, which indicates that the material of a lower mantle plume was involved in the formation of these regions.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010101
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- The Monticellite-bearing Rocks of the Krestovskaya Intrusion: Genesis
according to Melt Inclusion Study-
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Abstract: The investigation of monticellitolites and olivine–monticellite rocks from the Krestovskaya Intrusion shows that the major minerals (olivine and monticellite) have higher MgO content than the same minerals in olivinites and kugdites of the intrusion. In the studied rocks olivine contains 90–93 mol % Fo and monticellite has 41.6–42.3 mol % Fo, whereas olivine and monticellite in olivinites and kugdites contain 86–87 and 37.2–41.2 mol % Fo, respectively. Melt inclusion study in minerals of monticellite rocks demonstrates that the monticellite rocks of the Krestovskaya Intrusion were formed by mixing of volatile-rich melts of different composition: K-rich high-iron low-alumina kamafugitic melt and Na-rich high-magnesium high-alumina picritic melt. Minerals crystallized at high temperatures in the following sequence: perovskite I (1250–1230°C) → perovskite II (≥1200°C) ↔ olivine (>1200°C) → monticellite (>1150°C). Perovskite I in monticellite rocks, as well as olivine in olivinites, crystallized from K-rich high-iron (Mg# = MgO/(MgO + FeO) = 0.37), low-alumina kamafugitic melt. During crystallization of late perovskite II in the monticellite rocks, the melt became more enriched in MgO (Mg# = 0.41) and richer in Na2O and Al2O3, which is intermediate in composition between kamafugite and alkali picrite. Olivine in the monticellite rocks crystallized from melts similar in composition to melilitite, having a K-rich composition with Mg# = 0.39, whereas monticellite formed from a heterogeneous high-Mg Si-undersaturated melt, which is highly enriched with volatile components (including H2O) and salts. The crystallization of minerals was accompanied by subsequent accumulation of volatile components in mixing melts, silicate–carbonate liquid immiscibility under 1250–1190°C, and polyphase carbonate–salt immiscibility under below 1190°C. In the latter event, the exsolved carbonate melt began to split into simpler immiscible fractions: alkali–sulfate–carbonate, alkali–phosphate–carbonate, and calcio–carbonate.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010071
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- Staurolite in Metabasites: P–T–X Parameters and the Ratios of Major
Components as Criteria of Staurolite Stability-
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Abstract: Fe–Mg staurolite is a typical and widespread mineral of medium-temperature high-alumina metapelites, whereas magnesian staurolite is only relatively rarely found in metamorphosed mafic rocks (metabasites). The most significant factors controlling staurolite stability in metabasites were identified by thermodynamic modeling and analysis of the common features of the mineral-forming processes. In contrast to staurolite in low- and medium-pressure metapelites, staurolite in metabasites is stable at medium- and high-pressure metamorphism. An increase in the proportion of carbon dioxide in the water–carbon dioxide fluid shifts the staurolite-forming mineral reactions to lower temperatures and higher pressures. Al, Fe, Mg, and Ca are the major components of rocks that are critically important for the formation of magnesian staurolite in these rocks, and the contents and ratios of these components are of crucial importance for the stability of staurolite in metabasites. To understand the processes forming the mineral in metabasites, it is instrumental to subdivide metabasites into subgroups of predominantly magnesian, ferruginous–magnesian, and ferruginous protoliths. With regard to this subdivision, three petrochemical modules are proposed in the form of ratios of major components: MgO/CaO, CaO/FM, and Al2O3/FM, based on which it is possible to predict the stability of staurolite in mafic rocks at appropriate P–T parameters of metamorphism.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010034
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- Ti and Cr in High-Pressure Mica: Experimental Study and Application to the
Mantle Assemblages-
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Abstract: Experiments aimed at the synthesis of Cr- and Ti-bearing phlogopite in the silicate-carbonate systems peridotite—K2CO3 + H2O and basalt—K2CO3 + H2O at 7 GPa and 900–1200°С were carried out. It is shown that the crystallization of titanium-bearing phlogopite requires subducted crustal material at mantle depths. However, the mantle peridotite should predominate over basalt for Ti-phlogopite crystallization; otherwise, dioctahedral mica (aluminoceladonite) with (Mg + Fe)/VIAl > 1 is formed via the scheme 2VIAl = VITi4+ + VI(Mg + Fe). The competitive behavior of Ti and Cr upon incorporation into phlogopite is considered. It is shown that the presence of >1.3 wt % TiO2 introduces a limitation on the high concentrations of Cr2O3 via the scheme VI(Mg2+) + IV(Si4+) = VI(Cr3+) + IV(Al3+). This can explain the compositional patterns of phlogopite from inclusions in natural diamonds, in which the Ti content is much higher than that of Cr. The results obtained support the original idea that the composition of phlogopite may be applied to distinguish the paragenetic associations of diamond.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010113
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- Aegirine-Bearing Clinopyroxenes in Granulite Xenoliths from the Udachnaya
Kimberlite Pipe, Siberian Craton: Comparison of the Mössbauer and
Micropobe Data-
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Abstract: The aegirine end-member (NaFe3+Si2O6) in clinopyroxenes resulted from incorporation of Fe3+ into the mineral structure. Its presence affects the accuracy of reconstruction of the P-T conditions in the high-grade metamorphic rocks and allows the evaluation of the redox conditions of their formation. The content of this end-member in clinopyroxenes is usually estimated using crystal chemical recalculations of microprobe analyses. However, in some publications on eclogites, the comparison of microprobe-based recalculations with Mössbauer data revealed significant difference between the measured and calculated Fe3+/ΣFe ratios, which can significantly affect the results of geothermometry. This paper presents the results of the Mössbauer spectroscopy measurements of clinopyroxene fractions separated from three samples of garnet–clinopyroxene granulites from the Udachnaya kimberlite pipe. The ratios Fe3+/ΣFe = 0.22–0.26 measured in the clinopyroxenes correspond to 6–10 mol % aegirine. These estimates are in good agreement with the values obtained for the same clinopyroxenes by the recalculation of microprobe analyses using the charge balance method. Following this conclusion, we believe that crystal chemical recalculations of microprobe analyzes of clinopyroxenes from non-eclogitic rocks make it possible to correctly estimate the Fe3+ content in them. Similar recalculation of microprobe analyses of clinopyroxenes in crustal xenoliths from other localities, as well as from ferrobasalts of the continental flood basalts provinces, ferrodolerite dikes, and gabbroid xenoliths (similar in bulk chemical composition to many lower–middle crustal xenoliths) revealed significant amounts of previously unaccounted aegirine (up to 13 and 4–9 mol %, respectively), which holds the potential for deciphering redox conditions in many rocks.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010083
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- Condensate in Impact Glass Samples from the Lonar Crater, India
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Abstract: Polycomponent condensate glasses found in nature provide an insight into condensation mechanisms, which are still understood inadequately poorly. Condensate glasses found in the impactites of the Lonar crater contain nanosized inclusions of metallic Fe, Cr, Cu, Zn, Ag, In, Te, Au, Pt, and Bi, along with Fe, Cu, and Zn sulfides. This combination may be indicative either of a brief condensation window for the almost simultaneous condensation of components with so different fugacity or of a possible mechanism of cluster condensation, provided that the condensation temperatures of such clusters are close.
PubDate: 2022-12-01
DOI: 10.1134/S0869591123010046
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- Petrogenesis and Metallogeny of Intrusive Aplite Dyke from the Malanjkhand
Pluton, Central India-
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Abstract: The relationships between textural variations and structural trends of the aplite dyke enclosed in the Malanjkhand pluton were investigated in this study. The estimated zircon saturation temperature (747–835°C) and pressure of crystallization (2.5–6.1 kbar) suggested that the aplite dyke was emplaced in the lower-middle level in the continental crust. Water solubility calculations indicated that the aplite dyke originated from the silicic magma under water undersaturated conditions. Primitive mantle normalized spider diagram showed enrichment of large-ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs). The aplite dyke displayed LREE-enriched and MREE-depleted patterns, with significant positive Eu-anomaly in the REE patterns. This observation alluded the accumulation of plagioclase crystals before the crystallization of felsic magma in the reduced environment. The presence of the positive Eu-anomaly signified that the pre-existing granitic source at the lower-middle level of the crust generated aplitic magma owing to partial melting above the felsic source rock. Trace element discrimination diagrams presented evidence for possible extensional tectonic settings coupled with felsic magmatic episodes and granitic plutonic activity in a continental rift environment, thus favoring the emplacement of the aplite dyke. Th/U ratios in the aplite dyke implied that the melt fractionation in the magma chamber and the post-magmatic hydrothermal processes exerted negligible effect on the crystallization evolution of the aplitic magma. The aplite dyke pointed to a single pulse of silicic magmatism and a continuous process of injection, thus reflecting subtle variations in the physical conditions of the formation of the host Malanjkhand pluton.
PubDate: 2022-12-01
DOI: 10.1134/S086959112301006X
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- The Kulikovs: A Family of Geologists. Vyacheslav Stepanovich Kulikov,
Viktoria Vladimirovna Kulikova, and Yana Vyacheslavovna Bychkova
(Kulikova)-
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PubDate: 2022-12-01
DOI: 10.1134/S0869591122060108
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- The Great Dyke of the Kola Peninsula as a Marker of an Archean
Cratonization in the Northern Fennoscandian Shield-
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Abstract: The results of geochronological and petrological studies of the largest mafic dyke in the northern part of the Fennoscandian Shield, called the Great Dyke of the Kola Peninsula (GDK), are presented. According to U-Pb D-TIMS baddeleyite dating, the GDK crystallization age is 2680 ± 6 Ma. The age of host granites is 2.75–2.72 Ga (U-Pb, zircon, SHRIMP-II). The dyke has a simple internal structure with no signs of multistage melt injection. It comprises equigranular and plagioclase-porphyritic dolerites and gabbro that are amphibolitized to varying degrees. All rocks are low-Mg (Mg# less than 0.37) with low concentrations of Cr and Ni, and were derived through differentiation of more primitive melts. The analysis of geochemical and Sr-Nd isotopic data suggests that GDK melts could be formed by mixing of two types of mantle melts: depleted asthenospheric melt and enriched melt formed via melting of a lithospheric mantle. The weakly fractionated HREE patterns indicate that primary GDK melts originated at shallow (<60 km) depths outside the garnet stability field. The generation and injection of melts of the Neoarchean GDK occurred immediately after large-scale granitic magmatism and main crustal growth event in the Murmansk Craton and marked the cratonization of the continental lithosphere in the northeastern part of the Fennoscandian Shield.
PubDate: 2022-12-01
DOI: 10.1134/S086959112206008X
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- The Effect of CO2 Reduction in Low-Water Melts at Low Hydrogen Fugacity:
Experiment at 500 MPa and Thermodynamic Model-
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Abstract: Formation of graphite was observed in experiments on synthesis of dry carbon-bearing albite glasses in platinum capsules in an Internally Heated Pressure Vessel at 500 MPa and Т = 1200–1250°С. A thermodynamic model is proposed that explains the achievement of low oxygen fugacity near QFM-2 in the melt at low fugacity of hydrogen formed due to the decomposition of trace amounts of water in a compression medium (Ar gas). The unexpectedly low fugacity of oxygen is explained by the shift of equilibrium between the gases dissolved in the melt CO2 + H2 = H2O + CO to the right due to the low activity of molecular water at a low total content of H2O ~ 0.1–0.5 wt %. The high local СО concentrations in the melt lead to the platinum dissolution in form of carbonyl, corrosion of capsule walls, and redeposition of the metal at the contact with melt. With increase of water concentration in the melt (>1 wt %), the effect of reduction disappears.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060078
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- Tantalite Solubility in Granitoid Melts and Evaluation of the Ta and Nb
Diffusion Coefficients-
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Abstract: The paper presents experimental data on tantalite solubility in water-saturated granitoid melts with various alumina and alkaline elements concentrations at T = 650–850°C and P = 100 MPa. The maximum Ta concentration (effective solubility) in melt is shown to be always higher than the Nb concentration. As the melt composition is changed from alkaline to Al2O3-enriched, the Ta and Nb concentrations decrease by one to two orders of magnitude, and the Nb/Ta ratio simultaneously decreases (from ~0.8–0.7 to ~0.4–0.1) because the Nb concentration decreases notably more rapidly than that of Ta. This effect is enhanced at decreasing temperature. The effective Ta solubility in melt is demonstrated to be practically independent of the composition of the dissolving mineral of the columbite-tantalite series. The Ta, Nb, Mn, and Fe diffusion coefficients in granitoid melts are estimated. The Ta and Nb diffusion coefficients at T = 750°C and P = 100 MPa are ~10–10 cm2/s, and those of Fe and Mn are ~10–8.5 cm2/s. With an increase in temperature from 740 to 980°C, all of the diffusion coefficients increase by approximately 1.5 orders of magnitude. The configurations of the diffusion profiles of Ta concentration in melts change differently depending on change in the composition of the melt, temperature, or pressure.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060030
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- Eocene Calc-Alkaline Volcanic Rocks from Central Iran (Southeast of Khur,
Isfahan Province); an Evidence of Neotethys Syn-Subduction Magmatism-
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Abstract: Eocene volcanic rocks with basaltic-trachyandesite and trachybasalt composition which cross-cut the Cretaceous sedimentary rocks, are exposed in the northwestern part of the Central-East Iranian Microcontient (CEIM) (SE of Khur, Isfahan Province, Iran). The rock-forming minerals of these volcanic rocks are olivine (chrysolite and hyalosiderite, Mg# = 0.69–0.71), clinopyroxene (augite with Mg# = 0.74–0.84), orthopyroxene (enstatite with Mg# = 0.61–0.62) and plagioclase (andesine and labradorite with An48.3-65.1). Phenocrysts set in a fine-grained matrix of the same minerals plus sanidine (Or59.1Ab36.6An4.3) with minor amounts of opaque minerals (magnetite and ilmenite). Secondary minerals are chlorite and calcite. The main textures of these volcanic rocks are porphyritic, microlitic porphyritic, poikilitic, and glomeroporphyritic. The Eocene volcanic rocks of the Khur area are characterized by SiO2 content of 51.8 to 54.9 wt %, Al2O3 amounts of 14.35 to 16.47 wt %, and TiO2 values of 0.88 to 0.92 wt %. They exhibit strong enrichment in light rare earth elements (LREE) relative to heavy REE (HREE) (La/Lu ratio up to 102.35), enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSE), and present negative anomaly in Eu (Eu/Eu* = 0.72–0.87). Chemical characteristics and homogeneity of these volcanic rocks reveal their calc-alkaline nature and suggest that they were derived from a same parental magma and underwent a similar melt extraction. Major and trace elements geochemical features of the analyzed samples indicate that the parental magma was possibly derived from relatively low degrees of partial melting of a mantle wedge spinel lherzolite which was previously enriched by fluids/melts released from the Neo-Tethyan subducted slab.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060042
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- Comparative Characteristics of the Layering of Mafic–Ultramafic
Intrusions of the Oulanka Group, Northern Karelia-
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Abstract: The Oulanka group is a compact group of three peridotite–gabbronorite intrusions that is convenient for testing various petrogenetic concepts. The three intrusions are similar in age and occur not far from one another but differ in the composition of their original magmas, are characterized by different sets of cumulus mineral assemblages, and are different in inner structure and rhythmic layering. We applied cluster analysis of the contents of major elements to reproduce the cumulus mineral assemblages of the isochemically altered rocks of the Tsipringa and Lukkulaisvaara massifs. Although the parental magmas of the Kivakka and Tsipringa massifs were of different composition and their crystallization sequences were also different, the vertical sections of these massifs can be clearly subdivided into zones according to their cumulus mineral assemblages, with the limited development of rhythmic interbedding (with individual rhythms ranging from a few to a few dozen meters in thickness). Conversely, the Lukkulaisvaara intrusion does not possess any clearly distinguishable cumulus zones, and large-scale rhythmic layering is traceable throughout the entire thickness of the massif (with rhythms ranging from a few dozen to a few hundred meters in thickness). The different character of the rhythmic layering of the three intrusions may provide an insight into the different scenarios of magma convection in the chambers.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060091
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- Re-Os Isotope and HSE Abundance Systematics of the 2.9 Ga Komatiites and
Basalts from the Sumozero-Kenozero Greenstone Belt, SE Fennoscandian
Shield: Implications for the Mixing Rates of the Mantle-
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Abstract: Rhenium-Os isotope and highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) abundance systematics of Archean komatiites can be used to estimate the stirring rates of the mantle for the HSE and the timing of homogenization of late accreted materials within the mantle. In this study, we report Re-Os isotope and HSE abundance data for ~2.9 Ga komatiites and basalts from the Sumozero-Kenozero greenstone belt in the SE Fennoscandian Shield. The lavas are characterized by excellent preservation of the primary textural, chemical, and Re-Os isotope characteristics. The Re-Os isotopic data for spinifex-textured and cumulate komatiite and massive basalt samples from the lowermost sequences define a precise 10-point isochron (MSWD = 2.6) with an age of 2904 ± 18 Ma and an initial 187Os/188Os = 0.10758 ± 18 (γ187Os(2904) = +0.45 ± 0.17). This is the first direct age determination for the Sumozero-Kenozero lower komatiite-basalt sequences. Our modeling indicates that the mantle source of the komatiites and basalts evolved with a time-integrated 187Re/188Os = 0.418 ± 6. This ratio is well within the uncertainty of the bulk chondritic average 187Re/188Os = 0.410 ± 51 (2SD), also consistent with the chondritic evolution of the majority of komatiite mantle sources observed globally. The mantle source of the Sumozero-Kenozero komatiites has been calculated to contain the total HSE abundances of 58 ± 7% of those in the estimates for modern Bulk Silicate Earth (BSE). This estimate is in the middle of the range for other late Archean and Proterozoic komatiite systems. Using the estimated HSE abundances in the sources of komatiite systems as a function of their ages and ISOPLOT regression analysis, we calculated the average time in the past by which late accreted materials have been completely homogenized within the mantle to be 2.48 ± 0.23 Ga. These data require that the residence times of the late accreted planetesimals within the mantle, before complete homogenization, were on average 1.92 ± 0.23 Ga. This estimate represents a constraint on the average mixing rates of the mantle in terms of the HSE abundances in the Hadean and the Archean.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060054
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- Eastern Margin of the Neoarchean Tunguska Superterrane: Data from
Boreholes in the Central Part of the Siberian Platform-
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Abstract: The paper presents data on granites and gneisses recovered by Kulindinskaya-1 hole drilled in the central part of the Siberian Craton. The biotite granites retain a porphyritic texture, correspond to I-type according to their compositional features, are enriched in LREE and moderately depleted in HREE, and have negative Eu, Sr, and Nb and positive Zr anomalies. The U−Pb zircon age of the granites is Neoarchean (2525 ± 10 Ma), with single cores of zircon grains dated at about 2.6 Ga, which likely suggests a crustal source of the granitic magmas. The model age TNd(DM) = 2.77 Ga of the granite shows that the crust from which the initial melts were derived had been formed shortly before the melting episode. In terms of age and all characteristics, the granites are close to those of the Yurubchen massif, which was drilled through in the western part of the Tunguska superterrane. The biotite gneiss was apparently derived from sedimentary rocks and was heavily reworked when the granites were emplaced. The enrichment of the gneiss in Cr and Ni is probably inherited from the sedimentary protolith, whereas the REE, HFSE, and LILE concentrations and distribution in the gneiss are similar to those of the granite. The concordant (D < 1%) U−Pb zircon ages (according to LA-ICP-MS data) broadly vary from 3284 to 2620 Ma, with two major peaks at 2717 and 2678 Ma. The model age of the gneiss TNd(DM) = 2.91 Ga confirms a contribution of the ancient crustal component to the sedimentary protolith of the rock. The minimum age of the detrital zircon, 2.62 Ga, determines the maximum age limit for sedimentation, and the minimum age limit is set by the age of the granite intrusions at 2.53 Ga. According to our data, the Archean gneisses and granites recovered by the Kulindinskaya-1 drillhole probably compose the eastern part of the Neoarchean Tunguska superterrane. Ereminskaya-101 drillhole, which was drilled 20 km northeast of Kulindinskaya-1, recovered gneisses with model ages TNd(DM) from 2.30 to 2.37 Ga, which belong to the adjacent Taimyr−Baikal suture zone with widespread Paleoproterozoic rocks. The contrasting crustal history of the adjacent complexes provides grounds to suggest that they were tectonically combined, which is an additional reason to consider the Taimyr−Baikal suture zone as a Paleoproterozoic collisional orogen.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122050058
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- А History of Coronitic Metagabbronorites in the Belomorian Province,
Fennoscandian Shield: U-Pb (CA-ID-TIMS) Dating of Zircon–Baddeleyite
Aggregates-
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Abstract: The estimation of crystallization and metamorphic reworking ages of mafic rocks in the polycyclic Precambrian areas is a difficult problem. Magmatic baddeleyite can be partially or completely replaced by polycrystalline zircon within a wide range of temperature and pressures, from greenschist to granulite facies. Evaluation of the age of each phase of the zircon–baddeleyite aggregates can provide information on both the age of the magmatic crystallization and metamorphism. U-Th-Pb (SHRIMP-II) and U-Pb (ID-TIMS) geochronological studies were carried out for single baddeleyite grains and zircon–baddeleyite aggregates from gabbronorites (“drusites”) of the Ambarnsky massif (Belomorian Province, Fennoscandian Shield). The petrological studies indicate the simultaneous growth of coronas at the olivine–plagioclase boundary and zircon rims around baddeleyite. U-Pb (ID-TIMS) dating of single baddeleyite grains yielded 2411 ± 6 Ma crystallization age of gabbronorites of the Ambarnsky massif. U-Pb (ID-TIMS) dating coupled with the discrete chemical abrasion give an age of 1911 ± 35 Ma for metamorphic zircon rims. The obtained results indicate that coronitic textures in the gabbronorites were formed 500 million years later than the magmatic crystallization of rocks as a result of the granulite-facies metamorphism that was probably related to the Lapland-Kola orogeny.
PubDate: 2022-12-01
DOI: 10.1134/S0869591122060066
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- Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of
Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia):
Intracontinental Rifting as a Precursor of Continental-Margin Subduction-
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Abstract: Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende ([B]Na = 0.07–0.16, IVAl = 0.79–1.69, [A](Na + K + 2Ca) = 0.14–0.64, [C](Al+ Ti + Fe3+) = 0.58–1.29, Fe2+/(Fe2+ + Mg) = 0.18–0.46 at Fe3+/(Fe3++Al) = 0.18–0.77), low-to-medium-Ca plagioclase (An24–36), and epidote–clinozoisite (0.08 < \({{X}_{{{\text{F}}{{{\text{e}}}^{{{\text{3 + }}}}}}}}\) < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak P-T conditions up to 570–630°С and 7–9 kbar ascribed to the high-T epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (εNd(550) from +5.1 to –9.1) and Sr ((87Sr/86Sr)550 = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar P-T conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).
PubDate: 2022-10-01
DOI: 10.1134/S0869591122040051
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- Pauzhetka Caldera (South Kamchatka): Еxploring Temporal Evolution and
Origin of Voluminous Silicic Magmatism-
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Abstract: The Pauzhetka Caldera (27 × 18 km) was formed in the South Kamchatka during the Golygin Ignimbrite eruption (420–440 ka), the largest known eruption in the region in the past 1 Myr. The eruption was preceded by the 3 Ma-old mafic and intermediate volcanism. After the caldera-forming eruption, a variety of products, from basalt to rhyolite, were ejected within the caldera. For understanding the origin of voluminous silicic magmatism in thin mafic South Kamchatka crust, we used geochemical and isotope data. Our research has characterized the major and trace element composition of Golygin ignimbrite, intra-caldera hydrothermally altered deposits, pre-caldera (Mt. Orlinoe Krylo, Mt. Klyuchevskaya) and post-caldera (Kambalny Ridge, Chernye Skaly) eruptive centers. The Sr–Nd isotope composition of the Golygin ignimbrite and some eruptive post-caldera products was investigated. The isotope variations indicate that parental magmas for all rocks of the Pauzhetka area were obtained from a weakly evolved source derived through fluid-assisted melting of a subducted slab. Geochemical data support that the formation of most magmas of the Pauzhetka caldera was mainly controlled by fractional crystallization in the lower to middle crust. MELTS-modelling agrees with geochemical data. The fractional crystallization of Kambalny basalt with 2 wt % H2O at 6 kbar provides the best fit to the observed composition of the Golygin dacite.
PubDate: 2022-10-01
DOI: 10.1134/S0869591122050022
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