Paleoarchean orthopyroxenites of the Bug granulite–gneiss domain at the Ukrainian shield

This report presents data on the geological structure and location of the orthopyroxenite inclusion in gneissic enderbites of the Bug granulite–gneiss domain. Three stages of orthopyroxenite formation were identified on the basis of studies of the mineral composition along with the U–Pb and Lu–Hf isotope systems of zircons.     

Metasedimentary rocks of the paleoarchean Dniester-Bug Group,Ukrainian Shield: Composition,age, and sources

Paleoarchean granulite-facies metasedimentary rocks (quartzites, garnet quartzites, garnet-pyroxene gneisses, pyroxene-magnetite and magnetite quartzites) attributed to the Dniester-Bug Group of the Ukrainian Shield were studied. On the basis of geochemical data, including REE, the primary composition of these rocks was reconstructed as association of Fe-rich sandstones and sublitharenites, Fe-shales, and BIFs. This sedimentary association is similar to the rocks of other ancient greenstone belts and ascribed to the Algama-type iron formation. The sum of Al₂O₃, CaO, Na₂O, and TiO₂, high Zr contents (>100 ppm in quartzites), and the presence of detrital zircon grains of different ages are consistent with the terrigenous nature of sedimentary rocks. The Sm/Nd, Ti/Zr, Sc/Zr, and Ni/Zr ratios indicate the predominance of granitoid rocks in the source areas. The elevated Cr contents suggest that, in addition to granitoids, the source area contained ultramafic rocks. Geochemical characteristics, such as Fe/Mn ratio, low REE contents, and variations of REE versus the sum of Ni, Co, and Cu testify that sedimentation occurred under shallow-water conditions on the continent or its slope, similarly as the formation of ancient (3.5–3.2 Ga) basalt-komatiitic series intercalated with sedimentary rocks in the Pilbara Craton. The age of supracrustal rocks of the Dniester-Bug Group was constrained within the time interval of 3.4–3.2 Ga on the basis of U-Pb zircon dating and determination of Nd isotope composition. The DM model age of quartzites varies from 3.37 to 3.5 Ga. Sedimentary rocks together with volcanic rocks represent the oldest supracrustal association of the East European Platform.     

Probable Endogenic Origin of Conglomerate-type Rocks on the Southeastern Coast of the White Sea

The southeastern coast of the White Sea and Kii Island incorporates outcrops of conglomerate-type rocks similar to boulder dikes filling tensile fractures in the early Precambrian basement of the platform. They are related to the southwestern flank of the Onega Graben representing the southeastern segment of the Onega–Kandalaksha paleorift. Genesis of these conglomerate-type rocks, previously considered sedimentary ones, is problematic. Special study of the cement of these rocks revealed that it has probably an endogenic nature. It is dominated by carbonate material replacing ultramafic volcanic glass. Carbonatization and analcime mineralization took place at the regressive stage of cement formation within a temperature range of 450–550°C with active release of H₂O- and CO₂-saturated fluids. Data on the isotopic composition (¹³C and ¹⁸O) for carbonate material of cement from the brecciated rocks testify that the carbonatization was related to input of deep-seated carbon dioxide under subsurface environment. The studies carried out allow us to suppose that these rocks were formed as a result of consolidation of solid–gaseous suspensions inside fractures in the crystalline basement. Penetration of fluidized material along them produced dike-shaped bodies. Such rocks are recently called fluidizates. The sources of solid–gaseous suspension fluxes were basic magmas with a high content of volatiles. Discharge of gases from the magmas was caused by their decompression due to the appearance of tensile zones in lithosphere during rifting.