The geology of Qaersuarssuk Julianehaab district south Greenland

Abstract

An area of Precambrian basement with later dyke intrusions is described. The area is predominately of granite which has been intruded by later basic dykes. Following the intrusion of these dykes there has been reactivation of the granite. This reactivated granite locally shows intrusive features. Gneiss and a little meta-sediment also occur in the area. Some textural features in the granite are described and interpreted as the result of potash metasomatism leading to the formation of large potash feldspar porphyroblasts in a granodioritic matrix. The basic dykes (discordant amphibolites) are described in some detail and comments are made on their use for the division of the basement. The later dyke rocks consist of a variety of types including dolerites, trachy-dolerites, nepheline - micro-syenites, alkali - micro-syenite? camptonites and a single example of a spherulitic soda-rhyolite. Noteworthy among tie dyes are a group which contain numerous, large feldspar crystals (up to 50 cm in length) and feldspar aggregates (up to 1.5 m across). Some of these big feldspar dykes are composite with margins of alkali - micro-syenite. As most of the dykes with alkaline affinities are sub-parallel a division is made on their relations to movements along wrench faults. With the exception of the earliest types this gives an intrusion sequence opposite to that expected from alkaline magmas formed by successive stages of fractional crystallization. Later than all other dykes and all faulting is a dolerite dyke that is characterized by its vertical banding.