Korosten

Finds of aegirine syenites in the Korosten and Korsun-Novomirgorod (No. 7) plutons were first reported by Sobolev (1940). Aegirine syenites are now known in many zones of the Korosten anorthosite-granite pluton, for instance, near the villages of Guta Potievka, Mikhailovka, Stavki and Stavishche. The aegirine syenites are among the last derivatives of these plutons since they are often observed as veins in granites of the rapakivi group and in monzodiorites or quartz diorites. The aegirine syenites are, for the most part, quartz-bearing and the feldspars are represented by mesoperthite, microcline-perthite, microcline, albite and, rather rarely, oligoclase. The predominant dark-coloured mineral is aegirine-augite or aegirine, which may form 5-10% of the rock. Subordinate minerals include alkali amphibole of a riebeckite type and andradite or a titaniferous andradite; accessory minerals include zircon, titanite, fluorite, apatite and ilmenite. The amounts of aegirine syenite within the anorthosite-rapakivi granite plutons are exceedingly small, generally forming thin veins, and only near the village of Guta Potievka does the aegirine syenite cover a significant area (2.5x0.4 km). The petrogenesis of these aegirine syenites is not clear, but Krivdik and Tkachuk (1988) consider them to be fenites associated with a hypothetical carbonatite complex. They describe and illustrate the gradual mineralogical changes of the fenitization process and give analyses of rocks, pyroxenes, amphiboles and micas.