Late Caledonian granitoids and timing of deformation in the Iapetus suture zone of eastern Ireland

Late Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture ‘line’ in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.