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

Elizabeth A. Johnson, Christopher Sutherland, M. Amelia V. Logan, Scott D. Samson, Martin Feely

Research output: Contribution to journalArticle

5 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)1-13
Number of pages13
JournalIrish Journal of Earth Sciences
Volume29
DOIs
StatePublished - Sep 29 2011

ASJC Scopus subject areas

  • Earth and Planetary Sciences(all)

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