PROTRACTED END-CALEDONIAN GRANITE MAGMATISM IN IRELAND: FIELD EVIDENCE FROM THE GALWAY BATHOLITH

The Galway Granite is one of several early Devonian (~ 400 Ma) batholiths in Ireland. It is exposed over an area of 600 km2 on the northern side of Galway Bay in the West of Ireland. Ice-polished shoreline platforms provide superb evidence for the field relations among differing granite lithologies. The southern part of the batholith extends under early Carboniferous sedimentary strata on the floor of the Bay. The Granite intruded into a ~ 470 Ma-old island arc metagabbro-granite gneiss belt to the north, and an oceanic metabasalt-chert suite to the south. The WNW-ESE long axis of the batholith trends oblique to the pre-emplacement Skird Rocks Fault, considered to be a splay from the continuation of the Southern Uplands Fault of Scotland across Ireland. The batholith is cut by two major syn-consolidation faults dividing it into three blocks: western, central and eastern. There is strong field evidence that the central block was uplifted and unroofed during granite emplacement. Evidence includes the late intrusion of leucogranites exhibiting chilled margins against the earlier granite that had crystallised at a deeper level. The uniform lithology of these leucogranites contrasts strongly with the earlier calcalkaline megacrystic granodiorite and its zones of hybridised and mingled granite-hydrous diorite. One of these zones runs along the axis of the batholith, and shows evidence for a WNW-directed component of flowage. Post-dating all the plutons are swarms of microgranite and hypabyssal latite-rhyolite dikes that mostly trend NNE-SSW, perpendicular to the batholith long-axis. Dikes invariably show chilled margins against the roof rocks of the batholith, including the late leucogranites. Incipient pyroclastic texture in dikes in proximity to breccia pipes suggests that they were fissure feeders for ignimbritic volcanoes. The preserved dike swarms are almost entirely restricted to the western and eastern blocks of the batholith, contrasting with their rarity in the uplifted central block.