Emplacement conditions of a porphyritic felsite dyke and timing of motion along the Coolin Fault at Ben Levy, Co. Galway

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the E-W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellar feldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutile and zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that the dyke underwent hydrothermal alteration; however, immobile Zr vs TiO₂ for the Ben Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic crystallization temperatures were calculated as 1067±14°C using Ti-in-zircon thermometry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of <574°C based on Ti-in-quartz thermometry. An age of 373.9±4.0Ma was determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis of zircon. The Ben Levy porphyritic felsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway Batholith to the south, and may represent the northernmost extension of late-stage dyke activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between deposition of the Lough Kilbride Formation (436±428.2Ma) and intrusion of the Ben Levy Dyke at 373.9±4.0Ma.