The border relations of the dunite at Siorarssuit, Sukkertoppen District, West Greenland

Abstract

The dunite at Siorarssuit is conformably enclosed by granulite facies gneisses from which it is separated by a zone of bronzitite. The latter passes gradually into the dunite while it has a black-wall of phlogopitic mica against the gneiss. A rock consisting of bronzite, plagioc1ase, diopside and mica is occasionally found outside the black-wall. Joints and fractures in the dunite are occupied by rocks resembling those of the border. The bronzititic border is interpreted as a result. of reactions between gneiss and dunite at conditions of P, T corresponding to the granulite facies of regional metamorphism. The chemical changes are discussed, and it is shown that the Mg/Fe ratio of the bronzite decreases when passing from the dunite to the gneiss. The "dykes" in the dunite are regarded as results of similar reactions along the fractures in the ultrabasic mass. The reactions between ultrabasics and surroundings at different conditions of P, T are discussed, and it is emphasized that the zone of bronzitite is a high temperature equivalent of the more familiar zones of chrysotile, actinolite, talc. etc. formed at lower temperatures.