Hornblende-lamprophyre dykes of basaltic parentage from the Skaergaard area, East Greenland

Summary In a complete sample of the Tertiary dykes in a small area of the Skaergaard peninsula, the majority of the forty specimens are ophitic olivine-bearing or olivine-free dolerites of a type prevalent in the East Greenland coastal dyke swarm as a whole. Some, however, are more alkaline dolerites, showing evidence of having crystallized from a magma fraction richer in volatile components and alkalis, and are later in the intrusion sequence. These oligoclase-dolerites are further seen to grade into camptonitic hornblende-lamprophyres. A genetic lineage is traced from normal dolerites, through oligoclase-dolerites, to comptonites on the basis of textures, structures and mineralogical variations in the rocks, which have almost certainly all originated from the same parental basic magma. Chemical analyses show the oligoclase-dolerites and camptonites to be poor in silica but somewhat enriched in alkalis, particularly in potash; and that there is little difference in chemical composition between oligoclase-dolerites and camptonites, the latter type being the result of crystallization at lower temperatures from a more watery magma. An explanation of the genesis of the various types in terms of fractional crystallization and filter-press action in compatible with the field, petrographic and chemical evidence.