Uranium-Lead Geochronology of Two-Feldspar Granites of the Inhul Megablock (Ukrainian Shield) by Monazite

Abstract

In the area of the Novooleksandrivka village, the valley of the Bokovenka river crosses a powerful strip of metamorphic rocks of the Inhulo-Inhulets series with numerous small granitoid bodies of the Kirovohrad complex. In the right bank of the river valley north of Novooleksandrivka there are almost continuous rock outcrops of porphyry-like, mostly coarse-grained garnet-biotite granites, which are cut by veins of aplite-pegmatoid and pegmatoid granites. Uranium-lead isotope systems of accessory monazites from porphyry-like granite, layered body of uneven-grained granite and from veined body of aplite-pegmatoid granite have been studied. The age of the first two, more coarse-grained granite varieties, is 2043.2 ± 2.6 and 2041 ± 2.3 million years, respectively. Significantly younger are the monocytes from the vein of aplite-pegmatoid granite - 2030 ± 0.3 million years. In granites in this sequence, in addition to structural and textural characteristics (in general, decrease in grain size), there is a decrease in the amount of SiO2 (from 73.14 to 70.93%) connected with a significant increase in K2O (from 3.96 to 7.58%), (their inverse correlation coefficient is 0.98), a significant decrease in the CaO content from 2.04 to 0.97%, and a slight decrease in the MgO content from 1.14% to 0.82%. These changes are probably caused by the crystallizational differentiation of the original granite melt.