Chemical variation within three alkaline complexes in South Greenland

Abstract

Comparison is made between three hypersolvus alkaline complexes of Precambrian age occuring in the same igneous province. Two represent the cores of deeply eroded central volcanoes; the third comprises a suite of trachytic and rhyolitic dykes. In each complex a similar suite of trachytic and comenditic magmas was intruded and the gross mineralogy and chemistry are closely comparable. However, such differences as exist are explicable in terms of variation in cooling rates, controlling the degree to which the rocks represent cumulates and the extent to which excess alkalis in liquid residua were lost to the atmosphere and wall-rocks. In the Kûngnât Fjeld complex where cooling was slowest, alkali loss is inferred to have been greatest and the bulk of the constituent syenites are mildly peraluminous in consequence. In the dyke complex cooling was rapid and alkali loss less severe; the majority of the trachyte/rhyolite dyke rocks are mildly peralkalic. The central ring-complex of Tugtutôq represents an intermediate situation in relation to Kûngnât Fjeld and the dyke complex.