New perspectives on the order and style of granite emplacement in the Galway Batholith, western Ireland

Abstract

The late Caledonian Galway Batholith is cut by two major faults which divide it into three separate areas: the western, central and eastern blocks. The upthrown and more deeply eroded central block is bounded by these faults, in the west by the north–northeast trending Shannawona Fault and in the east by the north–northwest trending Barna Fault. We present new granite field relations from part of the central block (Inveran sector) which are fundamental in establishing the order and style of emplacement for the granites of the central block and the batholith as a whole. Unequivocal field evidence from the Inveran sector indicates upward movement of early central block granites which then became the solid roof rocks to subsequent intrusions. In the case of the Knock Granite these earlier intrusions were block stoped. We use this field evidence to review the geology of the central block in a 200 km2 area that incorporates the previously mapped Costelloe and Spiddal areas. Sharp intrusive contacts are a predominant feature of this sector of the central block and are in marked contrast to the gradational contacts recorded elsewhere in the batholith. Whereas juxtaposition of plutons in the western block occurred as the granites were partly crystallized, the central block reveals earlier, deeper level granites that were consolidated by the time they were intruded by late-stage higher level granites.