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

Published online by Cambridge University Press:  19 September 2011

MORGANE LEDEVIN ,
NICHOLAS ARNDT ,
MARK R. COOPER ,
GARTH EARLS ,
PAUL LYLE ,
CHARLES AUBOURG  and
ERIC LEWIN
MORGANE LEDEVIN*
Affiliation:
Institut des Sciences de la Terre (ISTerre), Université de Grenoble, Grenoble 38401, France
NICHOLAS ARNDT
Affiliation:
Institut des Sciences de la Terre (ISTerre), Université de Grenoble, Grenoble 38401, France
MARK R. COOPER
Affiliation:
Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast, Northern Ireland, BT9 5BF, UK
GARTH EARLS
Affiliation:
Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast, Northern Ireland, BT9 5BF, UK
PAUL LYLE
Affiliation:
Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast, Northern Ireland, BT9 5BF, UK
CHARLES AUBOURG
Affiliation:
Université Cergy-Pontoise, 5 mail Gay-Lussac Neuville-sur-Oise, F-95031 Cergy-Pontoise
ERIC LEWIN
Affiliation:
Institut des Sciences de la Terre (ISTerre), Université de Grenoble, Grenoble 38401, France
*
†Author for correspondence: morgane.ledevin@ujf-grenoble.fr
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Abstract

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.

Keywords

Portrush Sillcontact metamorphismmagma contaminationthermal modellingNorthern Ireland
Type
Original Articles
Information
Geological Magazine , Volume 149 , Issue 1 , January 2012 , pp. 67 - 79
Copyright
Copyright © Cambridge University Press 2011

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