Geochemical studies of gneisses from the Rosslare Complex have indicated that the grey gneisses comprise a suite of sediments consisting mainly of greywackes before metamorphism. These were associated with tholeiitic gabbros, which are now represented by the Greenore diorite, and early granodiorites during migmatization. After this early high amphibolite-facies metamorphic event, the St Helen's gabbro was intruded. Subsequently, tholeiitic older basic dykes were also emplaced. A low amphibolite-facies metamorphic event, accompanied by localized alkali metasomatism, modified the chemistry and mineralogy of these basic intrusive rocks. Finally, tholeiitic younger basic dykes, which have subsequently been altered or mildly metamorphosed, were emplaced before the complete crystallization of the Ordovician Saltees granite.