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Intrusive history of the Slieve Gullion ring dyke, Ireland: implications for the internal structure of silicic sub-caldera magma chambers

Published online by Cambridge University Press:  05 July 2018

S. McDonnell ,
V. R. Troll ,
C. H. Emeleus ,
I. G. Meighan ,
D. Brock  and
R. J. Gould
S. McDonnell
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
V. R. Troll*
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
C. H. Emeleus
Affiliation:
Department of Earth Sciences, University of Durham, DH1 3LE, UK
I. G. Meighan
Affiliation:
Institute of Lifelong Learning, Queen's University Belfast, Belfast BT7 1NN, UK
D. Brock
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
R. J. Gould
Affiliation:
Department of Geology, University of Dublin, Trinity College, Dublin 2, Ireland
*
*E-mail: [email protected]
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Abstract

The Palaeogene Slieve Gullion Igneous Complex comprises a layered central intrusion surrounded by a slightly older ring dyke. The ring dyke contains two major intrusive rock types. About 70% of the ring dyke is occupied by porphyritic granophyre and 30% by porphyritic felsite. Locally complex relationships between the two lithologies are observed. Major and trace element compositions suggest that there are two distinct chemical groups within each lithology: a Si-rich felsite, concentrated in a ~1 m wide zone at the outer margins of the dyke which grades into a less Si-rich felsite towards the interior. Similarly, a Si-rich granophyre, concentrated in the centre of the intrusion grades outwards into a Si-poor granophyre facies.

These rock relationships and geochemical variations suggest that a complex magma chamber hosted a stratified granitic magma body and various wall/floor magma facies. Low density, high-Si felsite magma from the top of the chamber was tapped first, followed by less Si-rich felsite magma as evacuation proceeded. The granophyres probably originate from the chamber walls/floor, representing more mushy equivalents of the felsite magma. Little granophyre magma was tapped during the early stages of the evacuation sequence. As evacuation continued, probably aided by trap-door caldera collapse, the ‘granophyre magmas’ intruded the already emplaced and slightly cooled felsite, forming the complexly zoned structure of the Slieve Gullion ring intrusion.

Keywords

Slieve Gullionring dykesmagma chamber dynamicscaldera collapse
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 5 , October 2004 , pp. 725 - 738
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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