Abstract: The Currywongaun and Doughruagh masses of northern Connemara represent a section through a syntectonic, partially layered intrusion formed of ultrabasic cumulates all having plagioclase of around An94. Differentiation and contamination of the magma from which these cumulates were precipitated ultimately produced an acid residuum which was responsible for the hydrous metasomatism of the earlier formed ultrabasic rocks resulting in the extensive formation of quartz-amphibole-anorthite gneisses in the petrogenetically ‘upper’ parts of the intrusion. Coexisting Ca-rich and Ca-poor amphiboles in the gneisses are interpreted as ‘frozen’ reaction intermediates that were in local equilibrium. The bulk-rock chemical data presented confirm that the compositions of the quartz-rich gneisses can be attributed almost entirely to the addition of Si to the ultrabasic rocks, coupled with the removal of Mg and Ca. The data also suggest that Zr, which reaches around 1000 ppm in some of the gneisses, may have been ‘mobile’ under these conditions of hydrothermal metasomatism.