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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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- Gneisses and Granitoids of the Basement of the Nepa-Botuoba Anteclise:
Constraints for Relation of the Archean and Paleoproterozoic Crust in the
Boundary Zone between the Tungus Superterrane and Magan Terrane (South
Siberian Craton)-
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Abstract: Abstract The paper presents geochemical and geochronological data on gneisses and granitoids from three deep boreholes (Yalykskaya-4, Danilovskaya-532, Srednenepskaya-1) in the basement of the southwestern part of the Nepa-Botuoba anteclise. Based on U-Pb zircon dating, three stages of granitoid magmatism were identified: ∼2.8, 2.0 and 1.87 Ga. At ca. 2.8 Ga magmatic TTG protoliths of biotite–amphibole gneisses (Yalykskaya-4 borehole) were formed, these rocks represent the Mesoarchean crust and experienced thermal effects typical of the Tungus superterrane of the Siberian craton at the terminal Neoarchean (∼2.53 Ga). Biotite gneissic granites (∼2.0 Ga) (Danilovskaya-532 borehole), which correlate in age with the granitoids of the basement of the Magan terrane and the Akitkan orogenic belt, were derived from a metasedimentary source formed by the erosion of predominantly Paleoproterozoic juvenile crust rocks. The 1.88 Ga A-type granite (Srednenepskaya-1 borehole) corresponds to the main stage of post-collision granite magmatism within the South Siberian magmatic belt. The ca. 2.8 Ga biotite–amphibole gneisses mark the eastern boundary of the Archean crust with Paleoproterozoic juvenile crust in the south of the Tungus superterrane, which are separated by a transitional zone intruded by granites having intermediate isotopic characteristics. The isotopic composition of Paleoproterozoic gneisses and granitoids indicates that marginal southern Magan terrane in contact with the Tungus superterrane includes blocks of both Archean and Paleoproterozoic crust, thus showing similarity with the Akitkan orogenic belt and accretionary orogens. The final amalgamation of the Tungus superterrane with blocks of the eastern part of the Siberian platform basement corresponds to a milestone of 1.88 Ga.
PubDate: 2024-08-01
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- Amudzhikan Volcano-Plutonic Association of the Eastern Part of the
West-Stanovoy Superterrane (Central Asian Orogenic Belt): Age, Sources,
and Tectonic Setting-
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Abstract: Abstract Geochronological (U-Pb zircon, ID-TIMS), isotope-geochemical (Nd, Sr, Pb), and geochemical studies of rocks of the Amanan and Amudzhikan intrusive complexes and volcanic rocks of the Ukurey Formation in the eastern part of the West Stanovoy superterrane of the Central Asian Orogenic Belt were performed. The assignment of granitoids of these complexes to high-potassium C-type adakites is substantiated. It is established that the studied rocks are cogenetic and can be ascribed to a single Amudzhikan volcano-plutonic association formed in the age range of 133 ± 1–128 ± 1 Ma. The igneous complexes of this association belong to the Stanovoy volcano-plutonic belt, which extends in the sublatitudinal direction from the Pacific Ocean inward the North Asian continent for more than 1000 km, subparallel to the Mongol-Okhotsk suture zone, and assembles the tectonic structures of the Dzhugdzhur-Stanovoy and West-Stanovoy superterranes. The formation of the Stanovoy Belt is related to the closure of the Mongolo-Okhotsk Ocean and the collision between North Asian and Sino-Korean continents at ~140 Ma. The subsequent collapse of the collisional orogen, which was accompanied by large-scale lithospheric extension and delamination of the lower part of the continental lithosphere, led to upwelling of asthenospheric mantle. This caused melting of the lithospheric mantle and continental crust and, as a consequence, the formation of both mafic (shoshonitic) melts and anatectic crustal melts of the adakite type. The mixing of these melts led to the formation of the parental magmas of the Amudzhikan magmatic association. The crustal component in the source was of heterogeneous nature and finally formed as a result of the Early Cretaceous collision event. It is characterized by the upper-crustal isotopic signatures: increased Rb/Sr and U/Pb ratios and a decreased Sm/Nd ratio in the source. The mantle component is represented by enriched lithospheric mantle of the Central Asian Orogenic Belt, the formation of which is associated with subduction processes and closure of the Mongol-Okhotsk paleoocean. Metasomatic transformation of the mantle with the introduction of melts and fluids with isotopic parameters of an EMII-type source or upper crust occurred at this stage.
PubDate: 2024-08-01
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- Three Types of Olivine Crystal Size Distribution in Dunites from the
Yoko-Dovyren Layered Massif as Signals of Their Different Crystallization
History-
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Abstract: Crystal size distributions (CSD) of olivine were obtained for 17 samples of plagiodunite and Pl‑bearing dunite from the central part of the Yoko-Dovyren massif, northern Baikal region, Russia. Three types of CSD were identified: loglinear, bimodal, and lognormal. Combining these data with the results of petrological reconstructions, which earlier revealed two main types of the Dovyren magmas (using the method of geochemical thermometry), we proposed a basic scenario of interaction between magmatic suspensions of different temperature to explain the diversity of the CSD. The intratelluric olivine transported by magmas of different temperature, which had not subjected to abrupt cooling or heating in the chamber, retained an original loglinear CSD. For some portions of the hottest magma (∼1290°C), it is assumed that the original olivine evolved into a bimodal CSD due to accelerated crystallization at faster cooling of the high-temperature injections contacting relatively cold crystal mush (∼1190°C). An interpretation of the lognormal CSD suggests that part of the olivine crystals composing the protocumulate systems efficiently interacted with the pore melt infiltrating upward during the compaction of the underlying crystal mush. This led to cycles of partial dissolution and regrowth of the olivine grains resulting in a final lognormal CSD. The infiltrating hot melt, which was undersaturated with immiscible sulfide liquid, could dissolve sulfides preexisting in the low-temperature mush. This produced dunites with lognormal CSD relatively depleted in sulfur and chalcophile elements. The lognormal CSD is considered to be a marker of crystal mush regions through which the focused infiltration of the pore melt proceeded.
PubDate: 2024-08-01
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- Titanium Partitioning between Zircon and Melt: an Experimental Study at
High Temperatures-
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Abstract: Abstract Experiments on titanium partitioning between zircon and silicate melt were conducted at temperatures 1300 and 1400°С at 1 atm total pressure. Additionally, the Ti content in zircons of a few experimental series from (Borisov and Aranovich, 2019) was measured and a critical analysis of experimental literature was carried out. It was demonstrated that at high temperatures (1200–1450°С) DTi values lie in the range from 0.02 to 0.04 regardless of pressure, melt composition, and water content. Based on obtained data, the impossibility of zircon crystallization from high temperature basic melts once more was shown. It was shown that “Ti in zircon” geothermometer cannot describe Ti content in our experimental zircons and, possibly, cannot be applied to dry high-titanium melts at 1 atm total pressure.
PubDate: 2024-08-01
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- Role of Liquid Immiscibility in the Formation of the Rare Metal Granites
of the Katugin Massif, Aldan Shield-
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Abstract: The paper discusses possible immiscibility between fluoride salt (“cryolite”) and silicate liquids into which the parental melt of the Katugin massif exsolves, and the petrological implications of this phenomenon. Results of a detailed study of the cryolite and zircon are presented. Liquid immiscibility is demonstrated to have triggered the massive crystallization of zircon and, together with the processes of subsequent evolution of the cryolite melt, contributed to the formation of the large cryolite bodies. Data on mineral-hosted inclusions were used to estimate the crystallization temperatures of fluoride salt and silicate melts and outline the pathways of their evolution during the formation of the massif. It is shown that the granites of the Katugin and West Katugin massifs were most likely derived from distinct sources, that differed mainly in fluorine content. Data on the chemical composition of three zircon generations identified in the granites of the Katugin massif are presented.
PubDate: 2024-08-01
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- Properties of Fluids during Metasomatic Alteration of Metamorphic Rocks
under P–T Conditions of the Middle Crust: An Example from the Bolshie
Keivy Region, Belomorian–Lapland Orogen, Fennoscandian Shield-
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Abstract: Properties of fluids under P–T conditions of the middle crust were studied with reference to the metasomatic alteration of metamorphic rocks (amphibolite facies) of the Bolshie Keivy nappe of the Keivy terrane of the Belomorian–Lapland collision orogen of the Fennoscandian shield. Properties of the fluids were studied in five selected types of rocks: metamorphic schists and gneisses with graphite, metasomatic quartz rocks with a high content of graphite, kyanite–quartz veins with wall-rock metasomatites, and metasomatic quartz-bearing kyanite rocks and anchimonomineral quartz veins. NaCl, CaCl2, CO2, N2, CH4, heavier hydrocarbons, and graphite were identified in the fluid inclusions using microthermometry and Raman spectroscopy. Using the method of multiequilibrium thermobarometry for mineral associations and the density of CO2 inclusions, a retrograde P–T path was calculated, which reflects the P–T exhumation history of the rocks. An explanation was proposed for the presence of water inclusions with NaCl of low salinity among inclusions of high salinity with NaCl and CaCl2. Comparison of data on the H2O activity (inferred from mineral equilibria) and salt content (data on fluid inclusions) with those of a model fluid (thermodynamic model of the H2O–NaCl–CaCl2–CO2 system) showed a good agreement between natural and model data. Natural and model data were synthesized to analyze variations in the phase state and chemical composition, fluid properties, including H2O activity, density, and salinity along the retrograde P–T trend.
PubDate: 2024-08-01
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- Petrogenesis of the Neoproterozoic Peraluminous Orogenic Granite and
Tertiary Phonolites from Jabal Fezzan in Southern Libya-
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Abstract: The majority of Neoproterozoic rocks exposed in southern Libya, are comprised of intrusive coarse-grained porphyritic, two-mica, and high K-calc alkaline granite. The Jabal Fezzan Granite (JFG) is located in southwestern Libya. In this study, trace elements and whole-rock geochemistry have been used to understand the origin and the process of petrogenesis of the studied granites. The JFG is high-Si, Rb, Y, Nb, and (ASI values greater than 1.1). Mineralogically, it is characterized by the presence of minor muscovite and biotite and a lack of hornblende, exhibiting features of S-type granites, and having a character that belongs to an alkali-calcic series. According to geochemical value, light REE-enriched, characterized by moderate enrichments in LREE (La/Sm), HREE, and weak negative Eu-anomalies. The geochemical modeling of the (JFG) reveals that the JFG derived from the melting of the crust and underwent high fractional crystallization (~50%) of plagioclase and K-feldspar at (H-P) conditions (750–980°C/1–4 GPa). The Jabal Fezzan (JFG) formed during Pan-African orogenic events during the destabilization of the interior Saharan metacraton due to compression stress and transpressive movements along pre-existing weakness and reactivation of shear zones inherited from Paleoproterozoic evolution. The Neoproterozoic basement forms the northernmost margin of the intracratonic Muruzq Basin, as evidenced by (greenschist facies) and intruded granitic rocks derived at the syn-collision stage (630–540 Ma).
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700012
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- Rocks of the Ary-Bulak Ongonite Massif: Relationship between Geochemical
Features, Mineral-Phase Assemblages, and Formation Processes-
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Abstract: Abstract The paper reports the study of geochemistry, mineral-phase assemblages of rocks of the Ary-Bulak ongonite massif, compositions of major, minor and accessory minerals (quartz, feldspars, topaz, zinnwaldite, prosopite, rare Ca–Al-fluorides, W-ixiolite, columbite, zircon, cassiterite, and fluocerite), fluoride–calcium (F–Ca) phase, and fluorite formed from it. The rock-forming minerals of porphyritic ongonites are quartz, albite and sanidine, and minor minerals are topaz and zinnwaldite. The ongonitic matrix is composed of a quartz–sanidine–albite assemblage with micron-sized needle-shaped topaz crystals. In transitional porphyritic rocks and in the endocontact aphyric zone, the interstices between matrix minerals are filled with a F–Ca phase formed from a F–Ca (fluoritic) stoichiometric melt. Fluoride–silicate liquid immiscibility in ongonitic magma and fluid-magmatic processes led to the redistribution of REE, Y, and many trace elements between melts, fluids, minerals and a contrasting change in mineral-phase assemblages in the rocks. This is associated with the appearance of M-type (T1 La–Nd, T4 Er–Lu) and W-type (T3 Gd–Ho) tetrad effects in the chondrite-normalized REE patterns of rocks. Degassing of magmatic fluids through the endocontact aphyric zone was accompanied by the crystallization of Sr-bearing prosopite and hydrous Ca–Al-fluorides. Aphyric rocks, compared to porphyritic ongonites and porphyritic transitional rocks, are enriched in H2O, Sr, Ba, Rb, Sn, W, Ta, Be, Zr, Hf, Sb, As, Sc, but contain less Li, Pb, Zn, Y and REE. During the effect of magmatic fluids on rocks enriched in Ca and F, especially in the endocontact aphyric zone, albite was partially or completely replaced by the F–Ca phase and kaolinite, and the F–Ca phase recrystallized into aggregates of micron-sized grains of stoichiometric fluorite without trace elements. Rb-Cs mica also crystallized in the rim of zinnwaldite laths, the zones of which maximally enriched in rubidium with the cation relation Rb > K > Cs may be a new mineral. The geochemistry of the rocks, the features of their mineral-phase assemblages, the compositional evolution of the minerals and the F–Ca phase are a consequence of the formation of the Ary-Bulak massif from ongonitic magma during a fluid-magmatic process complicated by fluoride–silicate liquid immiscibility with the participation of fluoritic and other fluoride melts, as well as magmatic fluids of P–Q and the first types.
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700061
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- Age and Origin of the Subalkaline Magmatic Series of the
Khibiny–Lovozero Complex-
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Abstract: The paper presents data on the miaskite syenites of the Lovozero massif, pulaskites of the Khibiny massif, rocks of the larvikite–lardalite series of the Kurga massif, and subalkaline volcanics, which are preserved as remnants in the roof of the massifs. The studied rocks are characterized by a low agpaitic coefficient of <1, the absence of minerals typical of peralkaline rocks (eudialyte, aenigmatite, etc.), and the presence of zircon. The morphological features and chemical composition of zircon from miaskite of the Lovozero massif syenite indicate that the mineral is of magmatic nature. The crystallization age of the miaskites was dated on zircon at 373 ± 5 Мa. The isotope-geochemical characteristics of rocks of the subalkaline series indicate that the miaskites of the Lovozero massif are of mantle origin, show no indications of their crustal contamination, and were produced during the evolution of ankaramite melt. The pulaskites of the Khibiny massif were formed according to an analogous scenario, except their assimilation with crustal material, whose proportion did not exceed, according to model calculations, 10%.
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700024
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- Associations and Formation Conditions of a Body of Melilite Leucite
Clinopyroxenite (Purtovino, Vologda Oblast, Russia): an
Alkaline–Ultrabasic Paralava-
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Abstract: Abstract A novel petrogenetic scheme is discussed for the formation of a melilite leucite clinopyroxenite body from an alkaline–ultrabasic paralava in the Purtovino area. Its protolith was likely a mixture of Upper Permian sedimentary rocks (aleurolite, marl, among others). Degassing, evaporation, and thermal (contact) metamorphism have significantly influenced the petrogenesis to produce a wide diversity of species present in mineral associations. The crystallization of paralava in a shallow setting was accompanied by an intense degassing and vesiculation of the melt, causing locally high porosity in the rock. An elevated degree of oxidation of the initial melt and progressive rise of fO2 were likely related to the H2 loss during the vesiculation and dissociation of H2O. Consequently, ferrian magnesiochromite (Mchr) and chromian spinel (Fe3+-enriched) were the early phases to crystallize; they were followed by members of the magnesioferrite–magnetite series. In situ melting of quartz-bearing and carbonate–clay rocks led to the development of domains of peralkaline felsic glass that surround partially resorbed quartz grains. Numerous grains of wollastonite and rare larnite formed during contact pyrometamorphism. The alkalis increased progressively during crystallization, with a notable enrichment in Na (up to 0.30 apfu) in the åkermanite–gehlenite series. The formation of leucite following melilite is indicated. Euhedral grains of Cpx display concentric cryptic zonation, with a zone of extreme Mg enrichment due to a local deficit in Fe2+. As consequences of the continuing rise in fO2, esseneite crystallized in the rim of zoned clinopyroxene. Two schemes of coupled substitution account for the composition of Cpx grains analyzed in various textural relationships: Mg2+ + Si4+ → (Fe3+ + Al3+) and (Ti4+ + Al3+) + (Na + K)+ → 2Mg2+ + Si4+. The pre-existing grains of olivine (associated with Mchr) were likely replaced completely by sepiolite–palygorskite associated with brownmillerite and its probable Fe3+-dominant counterpart, srebrodolskite. The investigated layer of alkaline microclinopyroxenite is unique in the Russian Plate, and a search is thus required to recognize other pyrogenic products. Also, further research is required to evaluate the contents and volumes of coal (or other sources of hydrocarbons) that could cause spontaneous and long-lasting combustion to form the considerable volume of paralava recognized in the Purtovino area.
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700073
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- Malyi Zadoi Peridotite−Gabbronorite Massif: Computational Modeling of
Crystallization and Evaluation of Indicator Geochemical Parameters of the
Parental Melt-
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Abstract: Model calculations were used to estimate the compositions of melts during fractional crystallization corresponding to the formation of the Malyi Zadoi massif, which is located in the Irkut block of the Sharyzhalgai uplift in the southwest of the Siberian craton. It is shown that the gabbronorites of the massif are comagmatic to the plagioperidotites and olivine gabbronorites. The estimates obtained for the composition of the model melts are used to characterize the composition of the mantle source of the parental melt. The geochemical characteristics led us to suggest that the parental melt of the Malyi Zadoi massif was formed by melting an enriched source, a conclusion consistent with isotope data that indicate that the mantle Sm/Nd ratio decreased in the Archean. The probable source of the parental melt could consist of depleted lithospheric mantle material metasomatized by felsic melts coming from rocks of a subducting oceanic plate.
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700036
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- Fenner Trend and the Role of Fractional Crystallization and Ferrobasaltic
Magma Immiscibility in Granophyre Petrogenesis: the Case of the
Mesoproterozoic Valaam Sill in the Ladoga Graben, Karelia-
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Abstract: Abstract This paper presents the results of petrographical, mineralogical, geochemical, and isotope-geochemical studies of granophyres and host ferrogabbros, quartz ferromonzogabbros, quartz monzodiorites, and quartz monzonites in the Mesoproterozoic Valaam sill in the Ladoga Graben on the Karelian Craton. The sill is poorly layered: the ferrogabbros compose the lower part of the sill, the middle part consists of quartz gabbro-monzonites and quartz monzonites, while graphic leucogranites (granophyres) form a dense network of veins mainly in the upper part of the sill. Geochemical features of ferrogabbro, iron-rich composition of olivine and pyroxene, and low Ca composition of plagioclase indicate the evolution along the Fenner trend. The granophyres have petro- and geochemical characteristics of anorogenic alkaline granites, with negative Eu/Eu* = 0.15–0.49 and REE distribution patterns similar to those of granophyres of layered intrusions. All rocks of the sill have close Sr isotopic composition (87Sr/86Sr)T = 0.7043–0.7066, and εNd values ranging from −9.6 to −11.2. Model calculations show that fractional crystallization can lead the initial ferrogabbro melt into the immiscibility field. Ilmenite–magnetite–silicate microstructures have been identified in the ferrogabbro and ferromontzogabbro from the sill; similar microstructures in layered intrusions are considered as evidence for the immiscibility of Fe-rich and Si-rich liquids (Holness et al., 2011; Dong et al., 2013). The segregation of the high-silica melt may have occurred in an intermediate crustal chamber at around 350 MPa and 960oC; magma was supplied as crystalline mush at the sill emplacement level at around 70 MPa and acidic melt migrated through it. This melt underwent fractional crystallization and interacted with host minerals. At the level of sill emplacement, it crystallized under supercooling into granophyre aggregates. The example of the Valaam sill shows that when the Fenner fractionation reaches the final composition–-ferrogabbro, its further evolution with a conjugate decrease in SiO2 and Fe contents can be related to the incomplete separation and mixing of Fe-rich melts and immiscibly split felsic melt. Such a mechanism can be implemented for the formation of the mafic part of AMCG-type massifs.
PubDate: 2024-06-01
DOI: 10.1134/S086959112470005X
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- Metamorphosed Ultramafic and Mafic Lithoclasts and Detrital Minerals from
Sandstones of Clastic Ophiolitic Deposits of the Rassokha Terrane: A
Setting of Formation of the Chersky Range Ophiolites-
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Abstract: Ophiolite-derived clastic rocks of the Rassokha terrane in the Chersky Range of the Verkhoyansk−Kolyma folded area were studied to obtain representative characteristics of the eroded source metamorphosed ultramafic and mafic rocks, to gain an insight into the possible geodynamic setting in which the protoliths of these rocks were formed, and to identify the possible source of the eroded material. The composition of lithoclasts and detrital minerals of the serpentinite and listwanite sandstones suggests that their source was composed of serpentinite, chloritite, listwanite, and dolomite rocks and that this source was proximal. Prior to the source erosion, the ultramafic and mafic rocks were metamorphosed and recrystallized, listwanite was formed, and the ultramafic rocks were tectonically disintegrated and combined with units of carbonate rocks (dolomite). Ultramafic rocks from lithoclasts experienced allochemical metamorphic retrogression during at least the latest stage of their serpentinization in a nonoceanic setting, where also the listwanite was formed. The Late Neoproterozoic ophiolites of the collisional belt of the Chersky Range were the most probable source for the protoliths of the clastic material. The protoliths of the ophiolite rock were probably formed in a backarc setting. Considered together with the published ages, our data indicate that relics of suprasubduction oceanic lithosphere of the Neoproterozoic basin occurred in the Chersky Range.
PubDate: 2024-06-01
DOI: 10.1134/S0869591124700048
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- Genesis of Dunite from the Guli Puton according to Olivine-Hosted Melt
Inclusion Data-
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Abstract: Abstract Olivine from the dunite of the Guli pluton crystallized from olivine–melanephelinite magma at temperatures above 1260°C according to the melt inclusion study. The melts were enriched with volatile components (S, CO2, F, H2O, slightly Cl) and contained high amount of incompatible elements. In addition, olivine hosts sporadic inclusions of picrite-basalt composition, which are close to picrite–meimechite melts preserved in chromite from dunite according to literature data. This suggests the influx of picrite–meimechite melts and their mixing with melanephelinite magma during the formation of dunites in the magma chamber. Based on the indicator ratios of incompatible elements, these melts and melanephelinite magma had different sources, which were located in undepleted mantle at different depths and derived through different degree of partial melting.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020061
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- Experimental Modeling of Interaction between Fluorine-Containing Granite
Melt and Calcite Marble-
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Abstract: At 750°C and a pressure of 1 kbar, an experiment was carried out simulating the contact-reaction interaction of calcite and a highly evolved fluorine-containing granite melt. The water content in the system did not exceed 10% of the dry charge mass. The possibility of interaction between magmatic melt and calcite is shown. The experimental products contain a zoned column composed of liquid phases and crystalline minerals. In the apocarbonate part, the newly formed phases are cuspidine, quartz, wollastonite, grossular, and a non-crystalline carbonate–fluoride phase LCF. The mineral assemblages in the zones of the apocarbonate part of the column vary depending on the ratio of CO2 and HF activities. In the silicate part, aluminosilicate glass, alkali feldspar, and plagioclase of variable composition were found. Silicon and fluorine are intensively transferred from the silicate to carbonate part, and a small amount of calcium is transferred in the opposite direction.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020024
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- Paleoproterozoic Dacite Dikes of the Vorontsovka Terrane, Volga–Don
Orogen: Geochemistry, Age, and Petrogenesis-
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Abstract: Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020073
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- Separation of Salts NaCl and CaCl2 in Aqueous-Carbon Dioxide Deep Fluids
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Abstract: The possibility of changing the ratio of the concentrations of NaCl and CaCl2 salts in fluid phases formed as a result of heterogenization of the H2O–CO2–NaCl–CaCl2 fluid with a decrease in P-T parameters has been studied. A well-known experimental fact regarding the ternary systems H2O–CO2–NaCl and H2O–CO2–CaCl2 is the greater tendency of the H2O–CO2–CaCl2 system to separate into coexisting predominantly aqueous-salt and aqueous-carbon dioxide phases compared to the similar system H2O–CO2–NaCl. This experimental fact can be interpreted as a greater affinity of NaCl for CO2 compared to CaCl2. Using a recently developed numerical thermodynamic model of the H2O–CO2–NaCl–CaCl2 quaternary fluid system, it was possible to identify geologically significant consequences of this difference in the interaction of NaCl and CaCl2 with CO2. Multistage heterogenization of the H2O–CO2–NaCl–CaCl2 fluid with a significant decrease in P-T parameters ultimately leads to the formation of aqueous-carbon dioxide fluid phase f2, the salt component of which is significantly enriched in NaCl and depleted in CaCl2 compared to the initial fluid. The fluid phase f1 formed at each stage of heterogenization has a predominantly water-salt composition with the ratio of the mole fractions of NaCl and CaCl2 salts, differing little from that in the initial fluid. However, the total mole fraction of salt in the f1 phase, as a rule, significantly exceeds that in the original fluid. The density of phase f1 significantly exceeds the density of phase f2. During the process of multistage heterogenization of the fluid phase f1, there is no formation of a fluid with a significant enrichment of CaCl2 compared to the initial ratio of the mole fractions of NaCl and CaCl2. At the same time, successive multiple separation of the f2 phase leads to the enrichment of its salt component in NaCl. Under favorable conditions, this process can lead to the formation of a fluid with almost pure NaCl salt. Changes in the salt composition of the fluid H2O–CO2–NaCl–CaCl2 are considered in application to the evolution of fluid composition along the regressive branch of the P-T trend of HP metamorphism and syngranulite metasomatism in the Lapland granulite belt.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020036
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- Model of the Formation of Monzogabbrodiorite–Syenite–Granitoid
Intrusions by the Example of the Akzhailau Massif (Eastern Kazakhstan)-
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Abstract: Abstract This paper presents a model of the formation of a multiphase Akzhailau granitoid massif formed within a Caledonian block of the Earth’s crust in the Hercynian time. This work is based on the results of major and trace element composition, geochronological, mineralogical and isotope-geochemical studies. Three stages of the formation of the Akzhailau massif are distinguished, which differ significantly from the previously accepted concepts about the multicomplex and polychronous origin of this intrusion: (1) the formation of moderately alkaline A2-type leuсogranites (308–301 Ma); (2) intrusion of monzodiorites into the base of leucogranites (~295 Ma), increasing degree of partial melting of protoliths with the formation of syenites and moderately alkaline granites of I-type (294–292 Ma); (3) intrusion of dikes and small bodies of alkaline ferroeckermannite A1-type leucogranites in the west and north of massif (~289 Ma). The Akzhailau massif was formed within about 15 Myr in the middle–upper crust through the interaction of plume-related subalkaline basitic magmas with metamorphosed crustal protolith of the orogenic structure.
PubDate: 2024-04-01
DOI: 10.1134/S086959112402005X
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- Mineralogical Characteristics and Magmatic Evolution Significance of
Garnets in the Late Jurassic Granites in Xingcheng, Eastern North China
Craton-
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Abstract: A series of Mesozoic granites related to the subduction of the ancient Pacific plate are widely developed in the eastern part of the North China Craton. These granites contain garnet as a minor phase. Garnet records important information such as magma composition, temperature and pressure, and is an important object in the study of petrological evolution of granites. We take the late Jurassic garnet–bearing monzogranite in Xingcheng, Liaoning as the research object, and the magmatic evolution process in the study area is discussed by the in–situ element geochemical variation and the Raman effect of magmatic garnet girdle. It is found that the garnet has a zonation structure in which the contents of MnO and CaO increase and then decrease, while the contents of FeO and MgO decrease and then increase from the core to the edge. Through the simulation of the crystallization sequence of garnet and biotite, it is found that the garnet has “M” type “spessartine bell–shaped profile”, Mn/(Ca + Mg + Fe) value and Mn/Fe value, which reflects that the crystallization pattern of minerals in the magma is dominated by biotite crystallization to garnet crystallization. It is considered that forward and reverse girdle can exist in the same garnet; the separation and crystallization of minerals in the late magmatic evolution is the main reason for the change in the compositional profile of the garnet. This is consistent with the late evolution of granitic magmas in the northeastern of the North China Craton after the late Jurassic.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020085
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- Serpentine Mineral Association, Texture and Composition as Keys to
Serpentine Origin in Kimberlites-
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Abstract: Syn-emplacement serpentine is one of the most abundant late minerals in kimberlites; its multiple generations can be distinguished by various textural positions and parageneses. Composition of the primary kimberlite melt cannot be accurately determined if we do not recognize distinct origins for several textural varieties of serpentine. This study aims to find compositional indicators of the serpentine origin by characterizing millimetre-sized serpentine domains in hypabyssal kimberlites. Serpentine forms as segregations in the groundmass or when serpentine replaces olivine or metasomatized silicate xenoliths. The latter textural variety of serpentine has not been recognized previously; it develops in Si-rich basement xenoliths ranging from basalt to granite. This serpentine is associated with abundant diopside, pectolite, phlogopite and chlorite and less prominent amphibole, hydrogarnet, wollastonite, xonotlite and other rare Ca hydrosilicates. We report petrography and textures of reacted silicate xenoliths in Renard 65, Orapa AK15, BK1, Gahcho Kué 5034 and Jericho kimberlites and provide a global summary of the phase compositions in the xenoliths. This study discovered that NiO content < 0.05 wt %, Al2O3 content > 1.3 wt % and MnO > 0.3 wt % in serpentine are clear signs of formation after felsic xenoliths. Serpentine/chlorite replacing olivine always have 1.5–4 wt % more FeO than serpentine after silicate xenoliths. The compositional contrast results from the immobile behaviour of conserved Al, Ni and Mn. The proposed criteria were tested on a pyroclastic kimberlite with an enigmatic origin of round serpentinized clasts overgrown by fibrous clinopyroxene and identified the precursor of these clasts as felsic. We also determined mineralogical characteristics of serpentine parageneses that can be used for reconstruction of the initial xenolith lithology. Serpentine coexists with the more abundant calcic hydrosilicates (hydrogarnet, xonotlite, amphiboles) in reacted mafic xenoliths. There, serpentine and chlorite crystal structures show less ideal stoichiometry indicative of a higher volume of nanometre-scale interstratification with smectites. Serpentine-rich assemblages in reacted xenoliths formed metasomatically at T < 600°C due to skarn-like mass transfer with the host kimberlite that controlled the gain of Ca and Mg and desilication. These metasomatic assemblages are remarkably identical to rodingites. Serpentine production appeared to be limited by the availability of Si in and around silicate xenoliths, but by the H2O availability in pseudomorphed olivine/monticellite.
PubDate: 2024-04-01
DOI: 10.1134/S0869591124020048
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