Volume 46, Number 1 (2019)
Andrew Hynes Series: Tectonic Processes

Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland

Requires Subscription or Fee Letter to Editor Requires Subscription or Fee Murphy et al. Table 1-CM Requires Subscription or Fee Murphy et al. Table 2-CM Requires Subscription or Fee Murphy et al. Fig.1-revised Requires Subscription or Fee Murphy et al. Fig.2-revised Requires Subscription or Fee Murphy et al. Fig.3abc-revised Requires Subscription or Fee Murphy et al. Figs.4ab-5ab-revised x3 Requires Subscription or Fee Murphy et al. Fig.6-revised Requires Subscription or Fee Murphy et al. Fig.7-revised x2 Requires Subscription or Fee Murphy et al. Fig.8-revised x2 Requires Subscription or Fee Murphy et al. Fig.9-revised x2 Requires Subscription or Fee Murphy et al. Fig.10-revised x2 Requires Subscription or Fee Murphy et al. Fig.11-revised x2 Requires Subscription or Fee Murphy et al. Fig.12-revised x2 Requires Subscription or Fee Murphy et al. Fig.13-revised x2 Requires Subscription or Fee Murphy et al. Fig.14-revised x2 Requires Subscription or Fee Murphy et al. Fig.15-revised x2 Requires Subscription or Fee Murphy et al. Fig.16-revised x3 Requires Subscription or Fee Murphy et al. Fig.17-revised- x2 Requires Subscription or Fee Murphy et al. Fig.18-revised x3 Requires Subscription or Fee SF 1 Murphy et al. Requires Subscription or Fee SF 2a Murphy et al. Requires Subscription or Fee SF 2b Murphy et al. Requires Subscription or Fee SF 2c Murphy et al. Requires Subscription or Fee SF 2d Murphy et al. Requires Subscription or Fee SF 3a Murphy et al. Requires Subscription or Fee SF 3b Murphy et al. Requires Subscription or Fee SF 3c Murphy et al. Requires Subscription or Fee SF 3d Murphy et al. Requires Subscription or Fee SF 3e Murphy et al. Requires Subscription or Fee SF 3f Murphy et al. Requires Subscription or Fee SF 3g Murphy et al. Requires Subscription or Fee SF 3h Murphy et al. Requires Subscription or Fee SF 4 Murphy et al. Requires Subscription or Fee SF 5 Murphy et al. Requires Subscription or Fee SF 6 Murphy et al.-revised Requires Subscription or Fee SF 7 Murphy et al.-CHK Requires Subscription or Fee Murphy etal. Fig. 5 & 4 revised for galleys Requires Subscription or Fee Murphy et al. French translation
J.Brendan Murphy
Professor, Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5
R.Damian Nance
Department of Geological Sciences, Ohio University 316 Clippinger Laboratories, Athens, Ohio 45701
Logan B. Gabler
Department of Geological Sciences, Ohio University 316 Clippinger Laboratories, Athens, Ohio 45701
Alexandra Martell
Department of Earth Sciences, St. Francis Xavier University Antigonish, Nova Scotia, B2G 2W5
Douglas A. Archibald
Department of Geological Sciences and Geological Engineering Queen’s University, Kingston, Ontario, K7L 3N6
Published March 29, 2019
How to Cite
Murphy, J., Nance, R., Gabler, L. B., Martell, A., & Archibald, D. A. (2019). Age, Geochemistry and Origin of the Ardara Appinite Plutons, Northwest Donegal, Ireland. Geoscience Canada, 46(1), 31-48. https://doi.org/10.12789/geocanj.2019.46.144

Abstract

In northwest Donegal, Ireland, a large number of coeval appinitic (hornblende-plagioclase-rich) plutons and lamprophyre dykes occur around the Ardara pluton, a granitic satellite body and one of the oldest phases of the ca. 428–400 Ma composite Donegal Batholith. The appinite units form a bimodal (mafic–felsic) suite in which hornblende is the dominant mafic mineral and typically occurs as large prismatic phenocrysts within a finer grained matrix. Lamprophyre dykes are mafic in composition with a geochemistry that is very similar to that of the mafic appinite bodies. Both mafic rocks are subalkalic, with calc-alkalic and tholeiitic tendencies, and show trace element abundances indicating that the mantle source was contaminated by subduction zone fluids. 40Ar/39Ar analysis of hornblende separated from two samples of appinite yield mid-Silurian (434.2 ± 2.1 Ma and 433.7 ± 5.5 Ma) cooling ages that are interpreted to closely date the time of intrusion. Hence, according to the available age data, the appinite bodies slightly predate, or were coeval with, the earliest phases of the Donegal Batholith. Sm–Nd isotopic analyses yield a range of initial εNd values (+3.1 to –4.8 at t = 435 Ma) that, together with trace element data, indicate that the appinitic magmas were likely derived from melting of metasomatized sub-continental lithospheric mantle and/or underplated mafic crust, with only limited crustal contamination during magma ascent. The appinitic intrusions are interpreted to have been emplaced along deep-seated crustal fractures that allowed for mafic and felsic magma to mingle. The magmas are thought to be the products of collisional asthenospheric upwelling associated with the closure of Iapetus and the ensuing Caledonian orogeny, either as a result of an orogen-wide delamination event or as a consequence of more localized slab break-off.

RÉSUMÉ
Dans le nord-ouest du Donegal, en Irlande, un grand nombre de plutons appinitiques (riches en hornblendes ou en plagioclases) et de dykes de lamprophyres contemporains se retrouvent autour du pluton d’Ardara, un corps satellite granitique et l’une des phases les plus anciennes du batholite composite de Donegal, âgé d’environ 428–400 Ma. Les unités de l’appinite forment une suite bimodale (mafique–felsique) dans laquelle la hornblende est le minéral mafique dominant et se présente généralement sous forme de grands phénocristaux prismatiques au sein d’une matrice à grains plus fins. Les dykes de lamprophyres ont une composition mafique dont la géochimie est très similaire à celle des corps d’appinite mafique. Les deux roches mafiques sont subalcaliques, avec des tendances calcoalcalines et tholéiitiques, et elles montrent des teneurs en éléments traces indiquant que la source du manteau a été contaminée par des fluides de zone de subduction. L'analyse 40Ar/39Ar des hornblendes provenant de deux échantillons d'appinite donne des âges de refroidissement du Silurien moyen (434,2 ± 2,1 Ma et 433,7 ± 5,5 Ma) qui sont interprétés comme étant proches de la date de l’intrusion. Par conséquent, selon les données d’âge disponibles, les corps d’appinite sont légèrement antérieurs ou contemporains des toutes premières phases du batholite de Donegal. Les analyses isotopiques Sm–Nd aboutissent à une gamme de valeurs εNd initiales (+3,1 à -4,8 à t = 435 Ma) qui, associées aux données des éléments traces, indiquent que les magmas appinitiques sont probablement dérivés de la fusion d'un manteau lithosphérique souscontinental métasomatisé et / ou d’une croûte mafique sousplaquée, avec une contamination crustale limitée lors de l'ascension du magma. Les intrusions appinitiques sont interprétées comme s'étant mises en place le long de fractures profondes de la croûte qui ont permis au magma mafique et au magma felsique de se mélanger. On pense que les magmas sont les produits de la remontée (upwelling) asthénosphérique collisionnelle associée à la fermeture de l’océan Iapetus et à l'orogenèse calédonienne qui s'ensuit, soit à la suite d'un délaminage à l'échelle de l'orogène, soit à la suite d'une rupture plus localisée de la plaque.
Requires Subscription or Fee Letter to Editor Requires Subscription or Fee Murphy et al. Table 1-CM Requires Subscription or Fee Murphy et al. Table 2-CM Requires Subscription or Fee Murphy et al. Fig.1-revised Requires Subscription or Fee Murphy et al. Fig.2-revised Requires Subscription or Fee Murphy et al. Fig.3abc-revised Requires Subscription or Fee Murphy et al. Figs.4ab-5ab-revised x3 Requires Subscription or Fee Murphy et al. Fig.6-revised Requires Subscription or Fee Murphy et al. Fig.7-revised x2 Requires Subscription or Fee Murphy et al. Fig.8-revised x2 Requires Subscription or Fee Murphy et al. Fig.9-revised x2 Requires Subscription or Fee Murphy et al. Fig.10-revised x2 Requires Subscription or Fee Murphy et al. Fig.11-revised x2 Requires Subscription or Fee Murphy et al. Fig.12-revised x2 Requires Subscription or Fee Murphy et al. Fig.13-revised x2 Requires Subscription or Fee Murphy et al. Fig.14-revised x2 Requires Subscription or Fee Murphy et al. Fig.15-revised x2 Requires Subscription or Fee Murphy et al. Fig.16-revised x3 Requires Subscription or Fee Murphy et al. Fig.17-revised- x2 Requires Subscription or Fee Murphy et al. Fig.18-revised x3 Requires Subscription or Fee SF 1 Murphy et al. Requires Subscription or Fee SF 2a Murphy et al. Requires Subscription or Fee SF 2b Murphy et al. Requires Subscription or Fee SF 2c Murphy et al. Requires Subscription or Fee SF 2d Murphy et al. Requires Subscription or Fee SF 3a Murphy et al. Requires Subscription or Fee SF 3b Murphy et al. Requires Subscription or Fee SF 3c Murphy et al. Requires Subscription or Fee SF 3d Murphy et al. Requires Subscription or Fee SF 3e Murphy et al. Requires Subscription or Fee SF 3f Murphy et al. Requires Subscription or Fee SF 3g Murphy et al. Requires Subscription or Fee SF 3h Murphy et al. Requires Subscription or Fee SF 4 Murphy et al. Requires Subscription or Fee SF 5 Murphy et al. Requires Subscription or Fee SF 6 Murphy et al.-revised Requires Subscription or Fee SF 7 Murphy et al.-CHK Requires Subscription or Fee Murphy etal. Fig. 5 & 4 revised for galleys Requires Subscription or Fee Murphy et al. French translation