Vol 32, No 2 (2024)

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.

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.

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.

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.