FIELD AND GEOCHEMICAL INVESTIGATIONS OF LATE-STAGE MAGMA EMPLACEMENT IN THE GALWAY BATHOLITH, WESTERN IRELAND

We present field and geochemical studies of a suite of late-stage dacitic dikes in the Galway Batholith, Connemara, western Ireland. The batholith stitched an east-west terrane boundary related to the Southern Uplands fault and the Iapetus suture to the south. The northern side of the batholith was emplaced into a bimodal plutonic suite emplaced along a short-lived Early Ordovician magmatic arc fringing the Laurentian margin. This suite was metamorphosed and deformed during the Grampian stage of the Appalachian-Caledonian Orogeny. The southern side of the batholith was emplaced into a terrane composed of Early Ordovician greenschist-facies oceanic basalts and sediments.

A notable characteristic of the Galway Batholith is the number and variety of dikes that intrude it. The most abundant are microphyric dacitic dikes that commonly form sets trending N-NNE, perpendicular to the long axis of the batholith. The dacitic dikes discussed in this study occur west of the NNE-trending Shannawona Fault which was active during consolidation of the batholith. The dikes dip east at 80º-85º, range up to 50m wide in places, and are exposed on land for 20km length (their extent south under Galway Bay is not yet known). One of the dikes is singular in being composed of a breccia member emplaced before, and divided by, a more voluminous magmatic member. This composite structure extends along a strike length of 3km, coinciding with a high-level microgranitic host. Field relationships combined with geochemistry show a correlation between the emplacement mechanisms and dike geochemistry. Evidence will be presented that correlates change in style of intrusion with chemical variation. Major and trace element (including REE) geochemistry of this dike produces profiles matching those for the granodiorite forming the greater part of the batholith. Changes in dike chemistry along strike will be discussed in conjunction with the field relationships and a model for the geochemical evolution of the dike will be proposed in the context of emplacement style.