Uranium-Lead Age According to Zircon of Syenites of the Velykovyska Massif (Korsun-Novomyrhorod Pluton)

Abstract

The results of uranium-lead dating of zircons from syenites of the Velykovyska massif (Korsun-Novomyrhorod pluton) of the Ingul megablock of the USh are given. The Velykovyska massif, located in the southern part of the Korsun-Novomyrhorod anorthosite-rapakivi granite pluton, in the area of the village Velyka Vyska, composed of fayalite-hedenbergite syenites. Syenites are small isolated intrusive bodies that are conventionally united in the Velykovyska massif. Several varieties of syenites are distinguished by textural-structural features and mineral composition — fine-grained, medium-grained (predominant), mesocratic (10-15% of dark-colored minerals, occasionally 20-30%), leuco- and melanocratic (present in subordinate quantities) syenites according to alkali content belong to the normal series. The anatomy of zircon crystals from a sample (217/862) of syenite discovered by well 862 was studied. Zircon crystals from the Velykovyska syenite massif are quite diverse — from transparent yellow and lilac to slightly transparent brown, cracked. Zircon crystals in the syenites of the Velykovyska massif crystallized from magmatic melt, probably with a short break in crystallization. The age of the syenites of the Velykovyska massif was determined by the uranium-lead isotope method based on three size fractions of multi-grain weights of zircon crystals from the fayalite-hedenbergite syenite, sample (217/860) and two size fractions of multi-grain weights of zircon crystals of the sample (158/860) St. 860. The discordia intersects the concordia at two points: the upper crossing corresponds to the age of 1774 ± 18 Ma and the lower one — –129 ± 955 Ma. Taking into account the slight discordance (0.9-1.6), the weighted average value of the age based on the 207Pbr/206Pbr isotopic ratio was calculated to be 1776.2 ± 2.8 Ma, which coincides within the margin of error with the age calculated from the upper intersection of concordia with discordia and which we take the time of crystallization of zircon crystals and the age of the syenite of the Velykovyska massif.