U-Pb ISOTOPIC GEOCHRONOLOGY OF THE MONAZITE GNEISSES OF THE «ZELENOLEVADIVSKA SUITE» OF THE MIDDLE BUG REGION

Abstract

On the right bank of the South Bug river valley, opposite the village of Zelena Levada crystalline rocks are observed in separate scattered rock outcrops, more than 1 km long along the river bed. A bundle of hypersthene-biotite and biotite plagiogneisses is exposed here, which in some areas has undergone calishpatization, manifested in the form of unevenly placed porphyroblasts of potassium feldspar, thin veins of quartz-feldspar composition and migmatization. Gneisses are dissected by single veined and lenticular bodies of granites. The gneiss pack is assigned to the Zelenolevada stratum of the Dniester- Bug series. The section is considered to be a stratotype for the Zelenolevada Formation. So far, the following values of isotopic ages have been obtained for monazites from microclinized plagiogneiss and from a lenticular body of granite exposed in this area — 2031.5 ± 6.7 million years and 2030.3 ± 1.7 million years, respectively. This publication presents the dating results of monazites from calishpatized plagiogneiss. microclination is manifested in the formation of thin veins composed of quartz+microcline+plagioclase+ biotite and porphyroblasts of microcline. Optical studies of polished sections of zircon crystals revealed that the crystals of this mineral have a complex structure, they are formed by heterogeneous, often with rounded contours, light pink (brownish pink) cores and shells of milky white zircon. The presence of heterogeneous, including cores with rounded (rolled) cores indicates the primary-sedimentary (volcanogenic-sedimentary) nature of plagiogneiss. According to the results of uranium-lead isotope dating of multigrain samples of monazite, the age of monazite (weighted aver- age value based on the ²⁰⁷Pb/²⁰⁶Pb ratio) is 2031.5 ± 2.7 million years.

Keywords

• plagiogneiss
• monazite
• uranium-lead isotope system
• middle bug region