Korgeredaba

Korgeredaba extends over 36 km2 and truncates Upper Proterozoic marbles, Lower Palaeozoic gabbro-diorite and leucocratic granites of Devonian age. It was intruded in three phases. The first, and principal, phase is represented by hastingsite foyaites which occupy an area of 29 km2 and have in the northern part a distinct zoned structure. The zoning has been produced by interaction of the foyaite with older gabbro-diorites. The latter have been intensively altered which resulted in the formation of oligoclase essexite and pulaskite fenites. The zoned fenite aureole varies in width from 1.5 km in the northern part to several hundreds of metres in the western and eastern parts. Adjacent to the fenites at the periphery of the intrusive body there is a distinct outer contact zone of pulaskites which are variably fine- to coarse-grained and heterogeneously textured. The pulaskites have distinct boundaries with the fenites, which are of similar composition, but there is a gradual transition to the hastingsite foyaites. The second intrusive phase is represented by aegirine-arfvesonite foyaites and abundant peralkaline pegmatites containing endialyte, astrophyllite, rinkolite and other rare earth minerals. The third intrusive phase comprises dyke and stock-like bodies of peralkaline syenites and granosyenites. At the contacts the hastingsite foyaites lose nepheline and become enriched in anorthoclase and quartz. Postmagmatic processes caused the development of albite-rich rocks with aegirine, which are referred to as mariupolites, and the crystallization of various rare earth minerals including eudialyte, catapleite, astrophyllite and rinkolite.