AGE AND EMPLACEMENT TEMPERATURE OF A FELSIC PORPHYRY DIKE NEAR BALLARD, COUNTY GALWAY

A felsic porphyry dike intrudes along a N-S trending normal fault cross-cutting the E-W trending Coolin Thrust fault located on Mt. Ben Levy in County Mayo, Western Ireland. At this location, the Dalradian Ben Levy Formation is thrust over younger Silurian sedimentary units (Lough Mask and Kilbride Formations). The felsite porphyry is a high-SiO₂, high-K₂O rhyolite containing resorbed feldspar and quartz phenocryst ocelli, minor apatite, ilmenite, and zircon, all within a hydrothermally altered groundmass of feldspar and quartz. Trace element discrimination diagrams (Rb vs. Y+Nb and Nb vs. Y) suggest the felsic porphyry was emplaced in a post-orogenic or anorogenic tectonic environment. Ti-in-zircon crystallization temperatures range from 1050-1098°C. In contrast, temperatures calculated using Ti-in-quartz thermometry for quartz phenocrysts (520-578°C) and groundmass quartz (593-656°C) are significantly lower and may reflect post-emplacement alteration or metasomatism. LA-ICPMS analyses of zircon produced a U-Pb age of 373.9 ±4.0 Ma for the felsic porphyry. The normal faulting in the Ballard area, as well as the Coolin Thrust, are therefore constrained to be no younger than Middle Devonian in age. The felsic porphyry is similar in age and geochemistry to other late-stage felsic intrusions of the Galway Batholith to the south.