A chronology of foreland deformation: ultra-violet laser 40Ar/39Ar dating of syn/late-orogenic intrusions from the Variscides of southwest Ireland

Abstract

The Upper Palaeozoic sedimentary rocks of the Munster and South Munster Basins, southern Ireland, lie within the Rhenohercynian Zone of the European Variscan orogeny. This foreland region accommodated shortening during the Asturian phase of Variscan deformation at the end of the Carboniferous by the development of kilometre-scale and lower order folding, high angle reverse faulting and regional fabric development. At the southwestern extremity of the belt lies the Black Ball/White Ball Heads area of the Beara Peninsula where high-level igneous intrusions locally exhibit a close relationship with both the ductile and late brittle phases of Variscan deformation. 40Ar/39Ar ultra-violet laser analysis of phlogopite crystals from these intrusions (principally trachytic dykes) has yielded ages that, when combined with structural field relationships, help to constrain the timing of Variscan deformation in southern Ireland. These ages include 314.44±1.00 Ma for a penetratively deformed lamprophric pipe on Black Ball Head, 301.98±1.47–298.08±0.61 Ma for dyke material associated with later stage brittle deformation and a date of 296.88±0.60 Ma for an undeformed post-orogenic dyke on White Ball Head