Geochemistry of Xenoliths in Rapakivi Granites of the Korsun-Novomyrhorod Pluton (Ukrainian Shield)

Abstract

Rapakivi of the Korsun-Novomyrhorod pluton (KNP), being the latest Precambrian granites of the central part of the Ukrainian shield, contain older acidic and intermediate rocks as xenoliths, the geochemical features of which reflect the Early Proterozoic geodynamic situation. Rapakivi granites, their differentiates and three types of xenoliths in them, represented by plagiogneiss, pink granite and charnockitoid (mangerite), were selected in the northwestern part of the KNP for geochemical studies. Analysis findings were compared with the geochemical characteristics of similar rocks of igneous and metamorphic Precambrian complexes that surround the KNP and could serve as a source of xenoliths. Based on distribution patterns of rare earth elements, lithophile elements content and their ratios relations in the rocks, taking into account their petrographic composition, it was established that plagiogneiss belongs to the upper part of the Inhul-Inhulets series, pink granite belongs to the Kirovohrad complex, and charnokitoid belongs to the intermediate rocks of the Tashlyk complex. In the latter, acid charnockitoids (enderbites) are probably younger than intermediate ones and were formed in another geodynamic setting. Pink granite, like the granites of the Kirovohrad complex, shows collisional geochemical features, in contrast to the rapakivi granites containing these xenoliths, which have predominantly within-plate geochemical characteristics. Thus, in the form of xenoliths in rapakivi those rocks were preserved that had been generated before and during the collision that occurred about 2 billion years ago and constructed a crust thickening at the site of the Inhul megablock, under subsequent extension of which about 1.75 billion years ago the KNP was intruded. Acid charnockitoids of the Tashlyk complex and rocks of the Novoukrainian complex, which are absent in xenoliths, probably were formed after the collision.