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Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

Published online by Cambridge University Press:  03 November 2011

Gawen R. T. Jenkin ,
Paul Mohr ,
John G. Mitchell  and
Anthony E. Fallick
Gawen R. T. Jenkin
Affiliation:
Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, Rankine Avenue, Scottish Enterprise Technology Park East Kilbride, Glasgow G75 0QF, UK
Paul Mohr
Affiliation:
Department of Geology, University College Galway, Galway, Ireland
John G. Mitchell
Affiliation:
Department of Physics, The University, Newcastle-upon-Tyne NE1 7RU, UK
Anthony E. Fallick
Affiliation:
Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, Rankine Avenue, Scottish Enterprise Technology Park East Kilbride, Glasgow G75 0QF, UK
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Abstract

The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht are investigated using stable isotope, water content and K–Ar data for whole rocks and mineral separates. Using an isochron approach the Logmór dyke in the north is re-dated to 308±4 Ma; previously determined older whole-rock ages reflect excess 40Ar. The ∼ 305 Ma age previously proposed for the Teach Dóite suite in the south is reinforced by a 305 Ma age on a pyroxene separate, although the severe resetting of most samples is emphasised by other pyroxene and plagioclase ages of ∼210 Ma. Pyroxene δ18O values for these Upper Carboniferous dykes are mostly 5·5 to 6·1%, indicating negligible crustal contamination. Logmór whole-rock samples have water contents of 1·7–2·1 wt.%, δ5D= 59 to –47‰ and δ18O = 9·4 to 9·6‰; plagioclase shows little mineralogical alteration but its δ18O is 9·7‰. Hydrothermal alteration involving a local formation or metamorphic water took place at high fluid/rock ratios and high temperature during cooling after intrusion, most probably in a thermally-driven convection system. Teach Dóite dykes have water contents of 2·0–4·2 wt.%. δD= –58 to –38‰ and δ18O = 3·6 to 9·2‰, and were mostly altered in two stages; hydration upon intrusion to ∼ 2 wt.% water by contemporaneous meteoric water at low fluid/rock ratios was followed by extensive chemical and isotopic alteration at ∼210 Ma (Upper Triassic) by surface waters. This latter event could also have caused the extensive alteration observed in the host rocks.

Keywords

Connemaradolerite dykeGalway Granitestable isotopesK–Ar datesexcess 40AralterationCarboniferousTriassicPaleocenepyroxenechloritesericiteconvection
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 88 , Issue 4 , 1997 , pp. 225 - 243
Copyright
Copyright © Royal Society of Edinburgh 1997

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