Basic dykes of the southern Ammassalik region, South-East Greenland: preliminary mineralogical and geochemical results

Abstract

The Ammassalik mobile belt of South-East Greenland and the Archaean gneisses to the north and south are cut by abundant variably deformed and metamorphosed Proterozoic basic dykes. Undeformed and unaltered dolerite dykes in the southernmost part of the region retain fresh igneous subophitic textures and mineral assemblages, the pyroxenes of which preserve complex chemical variations with highly variable Ca, Mg and Fe contents. The ophitic pyroxenes in neighbouring dykes of supposed Tertiary age comprise more usual, compositionally uniform pigeonite-augite pairs. The contrast between these pyroxene assemblages probably resuIts largely from differences in the oxygen fugacity of their respective parental magmas. The investigated Proterozoic dykes have chemical and mineralogical similarities with dykes of the same general age in southern West Greenland and Scotland, and they pre-date the development of the Ammassalik mobile belt, rather than being directly related to it.