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Intrusion history of the Portrush Sill, County Antrim, Northern Ireland : evidence for rapid emplacement and high-temperature contact metamorphism

Ledevin, Morgane; Arndt, Nicholas; Cooper, Mark R.; Earls, Garth; Lyle, Paul; Aubourg, Charles; Lewin, Eric. 2012 Intrusion history of the Portrush Sill, County Antrim, Northern Ireland : evidence for rapid emplacement and high-temperature contact metamorphism. Geological Magazine, 149 (1). 67-79. 10.1017/​S0016756811000537

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Abstract/Summary

The gabbroic Portrush Sill in Northern Ireland, part of the North Atlantic Igneous Province, intruded Lower Jurassic mudstones and siltstones about 55 Ma ago. We used petrologic observations and geochemical analyses to study how the sill interacted with the sedimentary rocks. Field relationships show that an Upper Sill and numerous associated Minor Intrusions were emplaced in the sedimentary host rocks before intrusion of the Main Sill, some 10 m above its upper contact. Geochemical analyses reveal two magma contamination processes: Nb and Ta anomalies, coupled with incompatible element enrichment, record contamination by deep crustal rocks, whereas Li, Pb and Ba anomalies reveal a superficial contamination through fluid circulation at the contact between magmatic and sedimentary rocks. Analysis of mineral assemblages and geochemical data from the contact aureole demonstrate uniform metamorphic conditions between the two main intrusions and an absence of a thermal gradient. The identification of pyrrhotite by magnetization analyses and of orthopyroxene by microprobe analyses indicates very high temperatures, up to 660°C. Thermal modelling explains these temperatures as the coupled effects of the Main Sill and the earlier intruded Upper Sill and Minor Intrusions. Even though the chemical composition of the Main Sill suggests another type of parental liquid, all three units were emplaced in a very short time, certainly less than five years.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1017/​S0016756811000537
Programmes: BGS Programmes 2010 > Geological Survey of Northern Ireland
ISSN: 0016-7568
Date made live: 03 Jan 2012 14:18
URI: http://nora.nerc.ac.uk/id/eprint/16214

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