THE URANIUM-LEAD MONAZITE ISOTOPIC AGE OF GRANITOIDS OF THE KROPYVNYTSKYI MASSIF, ON THE EXAMPLE OF THE SUBOTTSI VILLAGE OPEN PIT (INHUL DOMAIN OF THE UKRAINIAN SHIELD)

The object of the study is biotite granite exposed in the open pit of the Znamianka dimension stone deposit in the village of Subottsi, which is located in the marginal northeastern part of the Kropyvnytskyi massif. The aim of the study is to investigate the anatomy of accessory zircon and monazite crystals, the chemical composition of the latter, and to define the U-Pb monazite isotopic age. Previously, the U-Pb zircon age of these granites was determined at 2062 ± 11 Ma, while the inherited core of one of the crystals yielded a concordant age of 2787 ± 24 Ma. The results of our study show that zircon crystals often contain inherited cores derived from the substrate rocks. The mineral composition of inclusions in monazite has been studied. It has been found that one of the monazite fractions is characterized by significant direct discordance, significantly increased uranium content, and a much higher 206Pb/208Pb ratio than the other fractions. In our opinion, this is due to the capture of uranium by the crystals of this fraction shortly after their crystallization. Such scenario is realistic, given the presence of epigenetic inclusions in some monazite crystals. Without taking this fraction into account, the weighted average age is 2024.8 ± 4.7 Ma. Thus, the results of uranium-lead dating of zircons differ markedly from the results of monazite dating. This may be due to different crystallization times of zircon and monazite from the granite melt. Another, more likely reason for this difference in numerical values of isotopic ages is the presence of well-diagnosed relict cores inside many zircon crystals, which are at least Neoarchean in age. It is quite likely that zircon crystals with no relict nuclei were captured by ancient radiogenic zircon during crystallization, which causes some (about 40 Ma) difference in isotopic dating.