Igneous stratigraphy of Archaean anorthosite at Majorqap qâva, near Fiskenæsset, South-West Greenland

Abstract

Majorqap qâva provides one of the best preserved examples of a major group of anorthosites and associated gabbroic rocks which are characterised by extremely calcic plagioclase and are widespread throughout the Archaean complex of Greenland. The anorthosite outcrop is part of a sheet of layered igneous cumulates which were deposited in a number of cycles from magma intruded into basic volcanic rocks. It consists of six major units which in upward succession are: lower gabbro (10 m), ultramafic (30 m), lower leucogabbro (60 m), middle gabbro (40 m), upper leucogabbro (60 m), and anorthosite (200 m). The lower four units are considered to represent sequences of plagioclase-pyroxene, olivine-pyroxene, plagioclase, and plagioclase-pyroxene cumulates respectively, and the upper two units are plagioclase cumulates with intercumulus hornblende and plagioclase respectively. Plagioclase is the most abundant cumulus mineral and extensively occurs as equant crystals 2 to 10 cm in diameter. The anorthosite outcrop is enclosed by gneiss which was derived by deformation from sheetlike granitic intrusions younger than the anorthosite complex. The gneiss was more duetile and partly cushioned the anorthosite complex from the ravages of later deformation and metamorphism by absorbing a greater amount of strain and recrystallisation.