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Vol 32, No 2 (2024)
Articles
Paleoproterozoic Dacite Dykes of the Vorontsovka Terrane, Volga-Don Orogen: Geochemistry, Age and Petrogenesis
Abstract
The metamorphosed dacitic porphyry dykes were established for the first time in the western part of the Vorontsovska 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. They are ferrous, metaluminous rocks of calc-alkali series and belong to I-type granitoids. Sodium specialization, low concentrations of Mg, Cr, Ni, incoherent elements with sufficient REE fractioning and absence of Eu*-anomalies, high Sr/Y ratio and especially (Gd/Yb)n values (>10), and also radiogenic Nd isotopic composition suppose the juvenile mafic source for the dacitic melts. According to petrogenetic estimations, such conditions could be caused by partial melting of depleted N-MORB type basites at the equilibrium with the eclogitic restite. Supposed mechanism for the dacitic magmas forming is the partial melting of the basites from the lower horizons of crust with highly enlarged thicknes (>60 km) due to preceding collision processes.
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Model of the formation of monzogabbrodiorite-syenite-granitoid intrusions by the example of the Akzhailau massif (East Kazakhstan)
Abstract
This paper presents a model of the formation of a multiphase granitoid Akzhailau massif, formed within a Caledonian block of the Earth’s crust in Hercynian time. This work is based on the results of studies of petrogenic and rare elements 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 ideas about the multicomplexity and polychronicity of this intrusive: 1) the formation of moderate alkaline A2-type leuсogranites (308–301 Ma); 2) intrusion of monzodiorites into the base of leucogranites (~295 Ma) increasing of partial melting degree of substrates with the formation of syenites and moderate alkaline granites of I-type (294–292 Ma); 3) the intrusion of dikes and small bodies of alkaline ferroekermanite A1-type leucogranites in the west and north of massif (~289 Ma). The Akzhailau massif was formed in the interval of about 15 million years in the middle-upper crust during the interaction of subalkaline basitic magmas of plume nature with metamorphosed crustal substrates of the orogenic structure.
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Genesis of Dunite from the Guli puton According to Olivine-Hosted Melt Inclusion Data
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 of dunite according to literature data. It suggests the influx of picrito-meimechite melts and the mixing of latter with melanephelinite magma were carried out 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 near the undepleted mantle, at different depths and varying degrees of partial melting.
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Metamorphic mineral reactions and mineral paragenesis in the rocks of the Meyeri tectonic zone (the south-eastern part of the Fennoscandian shield, Russia)
Abstract
Mineral reactions were studied in metamorphic rocks from the Meyeri tectonic zone, and the P–T path of the development of this structure was calculated. According to the P–T path, the Proterozoic granulite complex of the Svecofennian Belt was thrust onto low-temperature rocks of the Archean Karelian Craton’s margin. Relict staurolite and other minerals preserved as inclusions in the garnet porphyroblasts made it possible to identify P–T parameters of the pre-peak stage of metamorphism using the compositions of the relict minerals. The temperature on the prograde trend of metamorphism was 500–600°C at a pressure of about 5 kbar. The peak metamorphic conditions of the Meyeri tectonic zone are estimated at T > 700°C and P ~ 7 kbar. The post-peak stage began with a decompressional P–T path at the aforementioned temperatures, with a change from granulite hypersthene-containing paragenesis to lower-temperature amphibole-containing ones. The subsequent metamorphic retrogression was characterized by the development of numerous hydrous minerals as a result of the activation of fluids in the shear zone. The P–T path of the tectonic zone is clockwise and reflects the exhumation of the Svecofennian granulite complex during the orogenic events.
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The First Discovery of Archean Dolerite Dikes in the Western Part of the Aldan Shield
Abstract
In the western part of the Aldan terrane, in the middle reaches of the Tokko river, dolerite dikes have been studied. These dolerite dikes form a swarm of submeridional trend about 1 km wide. In the thickest dike, dolerites have well-preserved primary textural and structural features and mineral composition: plagioclase + pigeonite + augite + titanomagnetite. Dolerites from the chilled margins and inner parts of the dike are homogeneous in composition, correspond to low-Mg tholeiites, have low contents of Ti and other HFSE, with weak enrichments in light REE spectra and small negative Nb anomalies. Sm-Nd isotopic studies of magmatic dolerite minerals from the central part of the dike in isochron coordinates yielded a good linear correlation corresponding to an age of 2510 ± 64 Ma, which probably records the time of crystallization of the basaltic melt. The metadolerites in the shallow dike retain plagioclase-porphyritic structures, but the pyroxenes in them are completely replaced by amphibole and chlorite. Metadolerites are contrasted by low contents of MgO, Cr and Ni and higher contents of TiO2, Fe2O3, P2O5, Nb and all REEs. The differences in the composition of the dikes may be related to the long-term (about 65%) crystallization differentiation of the initial melt and the flow of residual melt from the shallow intermediate magmatic chamber along the opening cracks. Such conditions probably existed in tectonically stable intraplate settings. The age of the studied dolerites of the dike swarm is comparable to that of the anorogenic granites of the Nelyuki Complex (~2.4–2.5 Ga), which are widespread in the western part of Aldan granulite-gneiss Terrane. The data obtained complement the characterization of the intraplate anorogenic magmatism that occurred in the western part of the Aldan Shield in the Late Archean and marked the final consolidation of a large block of Archean crust in the Chara-Olekma granite-greenstone area.
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Experimental modeling of the interaction of fluorine-containing granite melt and calcite marble
Abstract
At 750°C and a pressure of 1 kbar, an experiment was carried out simulating the contact-reaction interaction of calcite and a deeply differentiated fluorine-containing granite melt. The water content in the system did not exceed 10% of the dry charge mass. The possibility of interaction of magmatic melt with calcite is shown. The experimental products contain a zoned column composed of liquid phases along with crystalline minerals. In the apocarbonate part, the newly formed phases are represented by cuspidine, quartz, wollastonite, grossular and the non-crystalline carbonate-fluoride phase LCF. Phase parageneses 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 the carbonate part, and calcium in a small amount is transferred in the opposite direction.
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Separation of salts NaCl and CaCl2 in aqueous-carbon dioxide deep fluids
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.
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