U-Pb zircon geochronology of silicic tuffs and chronostratigraphy of the earliest Old Red Sandstone in the Munster Basin, SW Ireland

Abstract

Abstract Newly acquired U-Pb magmatic zircon dates from silicic tuffs within the Old Red Sandstone (ORS) magnafacies of the Munster Basin (SW Ireland) are intercalibrated with newly discovered (late Givetian) and reappraised (mid-Frasnian) miospore assemblages to provide the first biostratigraphically constrained numerical ages in the Irish Devonian succession. The weighted mean 207 Pb/ 206 Pb isotopic age determined for the Keel Tuff Bed (385.0±2.9 Ma) is indistinguishable from that of the previously investigated Enagh Tuff Bed (384.9±0.7 Ma). In conjunction with very similar rare earth element (REE) signatures, this confirms their correlation, placing a minimum age of 384.9±0.4 Ma on the newly discovered Reenagaveen microflora, which is assigned to the late Givetian TCo Oppel zone. The equivalence of the Keel and Enagh Tuffs constrains a vertebrate fauna containing Bothriolepis and the Valentia Island tetrapod ichnofauna to pre-date this event. Isotopic dating of thickly bedded subaerial tuffs from the Lough Guitane Volcanic Complex, a major accumulation of rhyolites and silicic volcaniclastic rocks, reveals ages of 384.5 ± 1.0 Ma (Killeen Volcanic Centre), indistinguishable from the Keel-Enagh Tuff Bed, and 378.5±0.2 Ma from the Horses Glen Volcanic Centre, previously considered to be the oldest of the complex. The Horses Glen Centre post-dates the Moll’s Gap Quarry microflora, the only current biostratigraphical control on the age of the early ORS in the east of the basin depocentre, thus indicating a minimum age for the (mid-Frasnian) IV Oppel zone, the revised biostratigraphic age of this assemblage. These controls on the early ORS (1) suggest that Munster Basin initiation occurred before late Givetian time and (2) give time-averaged (compacted) accumulation rates of c. 0.17–0.25 and 0.18 mm a −1 for eastern and western Iveragh, respectively. The minimum basin duration time was c. 23 Ma to the end of the Devonian period. The implications of these data for the depocentre stratigraphy, volcanic events, proposed ORS cyclicities and the geohistory of the Munster Basin are examined.