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Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

Published online by Cambridge University Press:  10 December 2015

William McCarthy ,
R. John Reavy ,
Carl T. Stevenson  and
Michael S. Petronis
William McCarthy*
Affiliation:
Department of Earth & Environmental Sciences, University of St Andrews, St Andrews, Fife, KY16 9AL, UK. Email: wm37@st-andrews.ac.uk
R. John Reavy
Affiliation:
School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland
Carl T. Stevenson
Affiliation:
School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK
Michael S. Petronis
Affiliation:
Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, Las Vegas NM 87701, USA
*
*Corresponding author
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Abstract

The Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.

Keywords

Anisotropy of magnetic susceptibilityanticlineascent and emplacementCaledonian graniteflowfoldGalway Granite Complexmicrostructurephacolithtranspression
Type
Articles
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 106 , Issue 1 , March 2015 , pp. 11 - 28
Copyright
Copyright © The Royal Society of Edinburgh 2015 

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