U-Pb isotopes geochronology by monazite of granitoids of Krynychuvatsky massive (Ingul megablock of the Ukrainian Shield)

Abstract

Pegmatite (sample 13/10) exposed in the valley of the Berezivka River (Krynychuvatka village) was studied and its isotopic age was determined by monazite. Granitoids of the Krynichuvatsky massif differ from other granitoids of the Kirovohrad complex in their petrographic composition (mainly plagioclase, plagioclase-microcline), structural (aplitoid, pegmatoid inclusions) and texture (in some areas, aplitoid granites are characterized by linearly oriented grains of lamellar (lenticular) quartz, elongated feldspar crystals) and the composition of xenoliths (amphibolites, amphibole-biotite, biotite, pyroxene-biotite gneisses). To find out the time and duration of the formation of granites of the massif using the uranium-lead isotope method, we determined the age of the monazite of the apilito-pegmatoid granite (sample 12/10) — 2039.9±1.4 Ma. The age of the vein body of the pegmatite (sample 13/10) has now been determined that is widespread above the dam on the right side of the stream, the left tributary of the Berezivka River, village Krynychuvatka. Monazite occurs mainly as large (> 0.1 mm) diskshaped crystals with well-developed pinacoid facets and, in individual grains, several other facets and even ribs. The crystals are yellow, honey yellow to honey transparent and translucent. Light-yellow, water-transparent crystals with an even, smooth, shiny surface and rounded contours are present singly. Considering the fact that the main mass of granites (aplito-pegmatoid granites) exposed in the valley of the Berezivka River (Krynychuvatka village) are 2039.9±1.4 Ma, and the vein body of peg9 matite that dissects the granite was formed in 2029.1± 3.4 Ma, it can be assumed with a high probability that the Krynichuvatsky massif was formed for at least 10 million years, in the age interval of 2040—2030 Ma ago.