Dyke Rocks of the Korsun-Novomyrhorod Anorthosite-Rapakivigranite Pluton

Abstract

In spite of significant intensity and composition variability of dyke magmatism in the Ingul mega-block, in the Korsun- Novomyrhorod anorthosite-rapakivi granite pluton (KNP) dyke magmatism is restricted. To present day, dykes of basic composition have not been described in this pluton. Previously mentioned gabbro-diabases dykes or diabase porphyrites should be classified as medium rocks as well as they are enriched by SiO2, Na2O and K2O. The authors succeeded studied the dyke from the Nosachiv massif, which can be considered the only known dyke of basic composition in KNP. By chemical composition, this rock is similar to the Ti-enriched gabroids of KNP, but differs in the presence of high-Ti magnetite (not typical mineral in plutonic rocks of KNP), and in terms of SiO2 and Na2O + K2O it should be classified as subalkaline. Most of the dikes known in the KNP are mainly monzonites, monzosienites, less widespread syenites, granite dikes are quite rare too. As our studies have shown, in the previously described porphyry dykes diabase structure is not exihibited. Furthermore pyroxene content is the first percent, and a significant (or most) of them are probably xenogeneic. According to chemical composition such dykes should be classified as medium in composition, because the SiO2 content in them is in the range of 54-60 % and elevated content of K2O, often prevails over Na2O, by low MgO and higher TiO2 they are similar to gabroid of KNP. In these rocks, normative orthoclase and quartz are calculated in significant quantities. Therefore, we offer to consider these rocks as dyke analogues of monzonites (quartz-containing or quartz) and regard as monzonite porphyry. The significant distribution of monzonites and syenites and their dyke analogues denote clear pronounced monzonite-syenite trend of evolution in KNP.