EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE AND TIMING OF MOTION ALONG THE COOLIN FAULT AT BEN LEVY, CO. GALWAY

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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 Ti02 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 thermo metry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of < 574CC 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. Introduction Coolin Fault. The Coolin Fault lies within a thr ust belt associated with the Caledonian Orogeny A felsic porphyritic dyke outcrops along the foot(Phillips 2001). Based upon field relationships in this wall scarp of a NNE-trending normal fault, location, motion along the Coolin Fault must have which transects Ben Levy near Ballard, Co. Galway occurred after deposition of the Kilbride Formation. (Fig. 1). The dyke has a strike of ~040°, dips steeply The Kilbride is constrained to the Telychian Stage to the W, and is about 2-3 metres wide in locations (436^428.2Ma) of the Llandovery Series (Holland where contacts can be clearly seen on both sides 1985; Cocks 1989) by the presence of the brachiopod of the porphyritic felsite (Locations A and C). The Eocoelia curtisi curtisi within the Annelid Grits porphyritic felsite extends through the Dalradian member in the lower Kilbride (Harper et al. 1995). Ben Levy Formation and into the Lower Silurian In this study, the timing of motion along the Lough Mask and Kilbride Formations on the north Coolin Fault is constrained using U-Pb age dat side of Ben Levy, cross-cutting the E-W-trending ing. Cathodoluminescence (CL) imaging, major and Irish Journal of Earth Sciences 29 (2011), 1-13 © 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.1 This content downloaded from 157.55.39.217 on Mon, 18 Apr 2016 08:03:01 UTC All use subject to http://about.jstor.org/terms 2 Irish Journal of Earth Sciences (2011 ) Ordovician-Silurian Sediments and Volcanics Carboniferous Oughterard Granite p Connemara Metamorphic Complex Galway Batholith