No 4 (2024)

The first of two publications is devoted to the lithogeochemical features study of the pilot collection of the Upper Precambrian sandstones and siltstones samples, selected from four boreholes: Bogushevskaya 1, Bykhovskaya, Lepel 1 and Korminskaya (Belarus). This article analyzes the general features of their bulk chemical composition and shows the possibilities and limitations for further reconstructions. It has been established that the Riphean and the Vendian rocks included in the pilot collection, visually identified as sandstones, are actually quartz, feldspathic-quartz and arkose varieties with different cement types. The Vendian “siltstones” in their geochemical characteristics correspond to coarse- and fine-grained siltstones and, to a greater extent, mudstones with a predominance of illite, as well as various admixtures of berthierine, kaolinite and smectite. Considering the trace elements enrichment factors of these rocks a number of dissimilarities were identified, caused both by the source rocks composition and sedimentary environments changes. The data point position of the studied samples on the Zr/Sc–Th/Sc diagram suggests that the composition of all the Riphean and the Vendian rocks is dominated by first sedimentation cycle material. This suggests that the lithogeochemical characteristics of the rocks of the pilot collection quite correctly reflect similar features of the source rock complexes and can be used to reconstruct the paleogeodynamic and paleoclimatic factors that controlled the accumulation of the Riphean and the Vendian sedimentary sequences of Belarus.

The article considers the material composition of sandy rocks from the Upper Triassic deposits of the Mongugai formation of South-Western Primorye and presents the results of its paleogeodynamic interpretation. It has been established that, according to their mineralogical and geochemical parameters, sandstones are petrogenic, correspond to graywackes and, only partially, to lithite arenites, are characterized by a significant degree of geochemical maturity of the clastic material, and their formation occurred due to the erosion of significantly weathered parent rocks of the source areas. Generalization and paleogeodynamic interpretation of the data obtained indicate that in the Late Triassic in the southwestern part of Primorye, sedimentation occurred in basins associated with the environment of the active continental margin and, probably, complicated by strike-slip dislocations along transform faults. The dominant source of clastic matter was continental land: cratons and projections of crystalline basement framed by rift zones or along transform faults. Acidic granite-metamorphic complexes were eroded with the participation of sedimentary rocks enriched with ancient components. A secondary source was a deeply dissected continental margin magmatic arc, which supplied the basin with an additional amount of sialic material, as well as a small amount of the basic-medium volcanoclastic. U-Pb geochronological studies of detrital zircons from sandy rocks of the formation made it possible to establish the age and possible location of magmatic complexes, due to the destruction of which the deposits were formed.

The results of studies of a basaltic andesite sample complicated by a mineralized crack and voids, with a crack and gas voids filled with secondary mineralization dredged on the Esmeralda underwater volcano, are presented. A detailed comparative study of the mineral composition of the substance lining the crack, the space around the crack, and the part of basaltic andesite unaffected by secondary changes made it possible for the first time for the underwater Esmeralda volcano to establish the presence of an association of minerals that is not characteristic of unaltered volcanic rocks. In the intracrack space and adjacent zones of basaltic andesite, wide ranges of plagioclase composition were determined, isomorphism in the Fe-Ca-pyroxene series was studied, REE oxides, hydroxides and fluorohydroxides were studied, and variability in the composition of minerals in the magnetite-hematite series was shown. It is assumed that tectonic movements led to the emergence of permeable zones in the previously formed basaltic andesites through which new portions of the melt leaked. In a limited space, high fluid gas saturation, temperature and pressure made it possible to extract metal compounds from the melt and host rocks.