Search

Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)

The Galway Granite is singular among the Irish latest Caledonian (-400Ma) batholiths for the number and variety of synand post-consolidation sheet intrusions. The most numerous and persistent are microphyric dacitic dikes, typically arranged in sets trending N-NNE, perpendicular to the long axis of the batholith. The sets cut the eastern and western blocks of the batholith, in contrast to their absence from the uplifted and eroded central block. Dacite geochemistry is similar to that of the megacrystic granodiorite that comprises the greater part of the batholith. However, the longer dike sets manifest mineralogical and chemical gradients along strike, from rhyodacite at the batholith axis to andesitic dacite outside the batholith. Hybridisation of granitoid magma with a more mafic magma is considered to have produced the dike magmas. Diking was induced by east-west stress relaxation of the batholith, coeval with a final plutonic episode involving high-level intrusion of alkali leucogranite sills. Regional ENE-WNW transcurrent faulting became active before the diking had concluded.