A group of gabbroic anorthosite dykes in northwest Skye contains megacrysts of calcic plagioclase (An93–87) and olivine (FO87–78.5and plagioclase-rich xenoliths set in a matrix of low-alkali, high Ca olivine tholeiite. Field, mineralogic and petrochemical evidence indicates (1) that the dykes are filled with crystals concentrated from magmas feeding lava flows and (2) that, although the megacrysts crystallized from low-alkali, high Ca olivine tholeiite magmas at depths <4 km, the megacrysts and the matrix in each dyke are not cognate. The emplacement temperature was less than 1230 °C. Low-alkali, high Ca olivine tholeiite is an abundant Hebridean magma type, resembling unfractionated oceanic tholeiites from the Atlantic. The temporal change from alkali olivine basalt to low-alkali, high Ca olivine tholeiite in the Hebridean proto-spreading centre may mimic compositional changes during initial development of the mid-Atlantic ridge.