Uranium-Lead Age by Monazite of Granites of the Kropyvnytsky-Bobrynetsky Massif (Ingul Megablock of the Ukrainian Shield)

The results of uranium-lead isotope dating of monazites and optical-mineralogical study of zircon crystals from granites of the Kropyvnytsky-Bobrynetsky massif, exposed by the Bobrynets quarry, are presented. The quarry is located on the right bank of the Suhokleya River, southwest of the town of Bobrynets. The quarry exposed mainly porphyry granites, similar to granites common in the Zhyvanivsky quarry. There is a slight variability in the biotite content in the granites. The age of the granites was determined by the TIMS method using multigrain monazite samples. It was found that the age of monazites of strongly cataclased and diafluorinated granite is 2028.5±2.7 million years. The age of monazite from porphyry granite, calculated from the upper section of concordia-discordia, is 2035.1±4.6 Ma (weighted average value according to the isotopic ratio 207Pb/206Pb = 2029.7±2.0 Ma). The obtained numerical age values completely coincide with the uranium-lead isotopic dates obtained for monazite from granite other parts of the Kropyvnytskyi-Bobrynetsky massif. Optical microscopy methods revealed relict zircon nuclei of substrate rocks in the middle of zircon crystals, which suggests the formation of granites due to the transformation of older crustal material.