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

• Published in: Geological Magazine
• Main Authors: Ledevin, M., Arndt, N., Cooper, M.R., Earls, G., Lyle, P., Aubourg, Charles, Lewin, E.
• Other Authors: Université Joseph Fourier - Grenoble 1 (UJF), Geological Survey of Northern Ireland, Colby House of Ireland, Laboratoire des Fluides Complexes et leurs Réservoirs (LFCR), TOTAL FINA ELF-Université de Pau et des Pays de l'Adour (UPPA)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2012
• Subjects: contact metamorphism; magma contamination; Northern Ireland; Portrush Sill; thermal modelling; [SDU.STU.GM]Sciences of the Universe [physics]/Earth Sciences/Geomorphology; North Atlantic
• Online Access: https://hal.archives-ouvertes.fr/hal-00684828; https://doi.org/10.1017/S0016756811000537

International audience 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.