U-Pb Age, by Zircon, of the Pre-Klesiv Breed Association of the Osnytsky Block

Abstract

The Osnytsky block is an integral part of the Osnytsky-Mikashevytsky volcanic-plutonic belt, represented by early proterozoic igneous rocks of different compositions. This is a new type of igneous belts for the early precambrian – a paleotypical analogue of intracontinental volcanic-plutonic belts by the phanerozoic, which was formed in the conditions of mature continental crust in the initial stage of radical structural rearrangement of the cratonized gneiss base of the Eastern European platform. Volcanogenic formations have undergone regional metamorphism at a level not higher than the epidote-amphibolite facies, classified in the klesiv series, which combines effusives of basic, acidic and medium composition and occupies a higher stratigraphic position compared to the metamorphites of the teteriv series. Intrusive rocks (gabbro-diorite-granodiorite granite association) – to the osnitsky complex. Rocks of the osnitsky complex are widespread, they account for 80-85% of the area of the Osnitsky block. The pre-klesiv association, represented mainly by gneisses of the teteriv series and plagiogranitoids of the sheremetivsky complex, has a limited distribution. The LA-ICP-MS U-Pb method was applied to date zircon from plagiogranite and plagiomigmatite of the sheremetivsky complex, and from gneiss and amphibolite. The plagiogranite yielded an age of 2110 ± 8, and plagiomigmatite of 2165 ± 22 Ma. Hence, granitoids of the sheremetivsky complex in the Osnitsk block were formed between 2.16 and 2.11 Ga. The age of the metamorphism of gneisses was defined as 2198 ± 29 Ma, whereas zircon from amphibolite crystallized at 1981 ± 36 Ma. Correspondingly, gneiss can be attributed to the teteriv series, whereas amphibolite can belong to either osnitsky complex, or nartsyzivsky complex.