A composite breccia and magmatic dacite dike in the Galway Granite Batholith

The Galway Batholith (∼410-380Ma) in western Ireland hosts several dike sets in its western and eastern sectors. The more pervasive of these sets trend N-NNE, perpendicular to the long axis of the batholith, expressing stress relaxation during the cooling and consolidation of the batholith. The dikes of the Ard Mhóir-Gabhla-An Gharmain set of the western block are synto post-plutonic intrusions composed predominantly of quartz-plagioclase microphyric dacite. One of these dikes is singular in exhibiting three breccia units - coarse, fine and incipient tuffisite breccias - that are intermittently developed adjacent to, or outside and parallel to, the dike margins. The breccias were emplaced immediately before entry of magma into the fissure. The thickest unit is a coarse breccia composed mainly of cobbles of a pre-existing breccia. The cobbles are cemented with dacite that preceded the main dacite intrusion. This coarse breccia was emplaced in association with splitting of the walls of the fissure and also with south-directed offshoots from the magmatic dike. A paroxysmal emplacement of the dike is inferred.