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

Morgane Ledevin, Nicholas Arndt, Mark R. Cooper, Garth Earls, Paul Lyle, Charles Aubourg, Eric Lewin

    Research output: Contribution to journalArticlepeer-review

    4 Citations (Scopus)

    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.

    Original languageEnglish
    Pages (from-to)67-79
    Number of pages13
    JournalGeological Magazine
    Volume149
    Issue number1
    DOIs
    Publication statusPublished - Jan 2012

    Keywords

    • Northern Ireland
    • Portrush Sill
    • contact metamorphism
    • magma contamination
    • thermal modelling

    ASJC Scopus subject areas

    • Geology

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