The origin of the granitic sheets and veins in the Loch Coire migmatites, Scotland

IN a previous account (Brown, ~967) of the granitic sheets and veins in the Loch Coire migmatite complex, a metasomatic origin was postulated. This has now been confirmed by new analyses. Read (I93I) described the Loch Coire complex as made of three zones; one of unmigmatized meta-sediments, one of granitic veins and one of 'injection' or migmatite. The sheets and veins of granite (sensu lato), pegmatite, and aplite, which occur in the zone of veins and in profusion in the zone of migmatite, are a distinctive feature of the complex. They are clearly defined rock bodies distinct from the more intimate admixture of granitic and host-rock material that characterizes the migmatites themselves (the injection and permeation gneisses of Read). Read (t93I) described the variety and distribution of the sheets (broadly concordant) and veins (discordant) in detail. It has been argued (Soper and Brown, I97I) that the migmatite complex was the heat source responsible for the regional metamorphic pattern in Sutherland. The granitic sheets have considerable variety but a common type is medium-grained, leucocratic, foliated, and between tens of centimetres and several metres thick, being more or less concordant. The internal structure of these sheets, and probably of the majority of foliated sheets, is of regional tectonic origin, as the internal foliation is sometimes parallel to the regional (axial plane) schistosity rather than parallel to the walls of the sheet itself and may contain a weak ESE lineation. The foliation is not of magmatic origin. The mineral assemblage is quartz-oligoclase-potassium-feldsparbiotite-late-muscovite with accessory garnet and more abundant oligoclase than potassium feldspar. Discordant veins of pegmatite are common in the peripheral parts of the complex and concordant sheets in the interior. These pegmatites have similar dimensions to the granite sheets and are distinct from the diffuse pegmatitic material common in the migmatites. Tightening of the regional ESE folds affected some pegmatites, which are ptygmatically folded or have undergone boudinage. Oligoclase-quartz-biotite pegmatites are dominant but there is wide range in mineralogy and texture, and in