Late Magmatism of the Galway Granite Batholith: I. Dacite Dikes

Abstract:The Galway Granite is singular among the Irish latest Caledonian (-400Ma) batholiths for the number and variety of syn- and 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.