The petrology and geochemistry of the north qoroq centre igaliko complex, south greenland

Abstract

The North Qôroq Centre is one of four major intrusive centres comprising the Igaliko Nepheline Syenite Coitplex. The centre is composed of a number of syenitic bodies all showing undersaturated character. Emplacement of the bodies was probably by ring fracture and block subsidence, combined with a degree of stoping. Petrographic, mineralogical and geochemical studies have demonstrated that the process of in situ fractionation accounts for the bulk of the variation in rock types seen in the centre. Clinopyroxene, olivine, Fe/Ti oxides and apatite were important early fractionating phases, followed by amphibole and biotite. The most important fractionating phase was, however, alkali feldspar. Its crystallization and separation resulted in peralkaline undersaturated syenites becoming more peralkaline and more undersaturated. Probe work on the majority of major mineral phases, present in the syenites, has enabled values to be placedon a number of important physical and chemical parameters. The temperature of the magma as it evolved and the values and effect of steadily varying silica activity, oxygen fugacity, water fugacity, and activity of sodium disilicate have all been considered. An alkali-rich aqueous phase probably co-existed with the more fractionated of the North Qôroq syenites. A reasonable idea of the nature and composition of this phase has been obtained and a number of features exhibited by the syenites attributed to its action. Influx of meteoric water at an early stage in the evolution of the magmas is suggested as an explanation for the common marginal pegmatites. This process could be instrumental in deciding whether magmas of trachytic composition proceed, with crystal fractionation, towards undersaturated or oversaturated residual compositions.