MWEELREA IGNIMBRITES DERIVED FROM THE CONNEMARA MAGMATIC ARC, WESTERN IRELAND

The Notre Dame Arc is represented in western Ireland by the syntectonic Connemara metagabbro and orthogneiss suite (475-462Ma) and the Mweelrea ignimbrites in the upper (Llanvirn) part of the South Mayo Trough (SMT) sequence. The Connemara Dalradian block that hosts the metagabbro and orthogneiss suite is a fragment of the Laurentian margin that was emplaced to the south of the SMT during strike slip terrane amalgammation, traditionally taken as post-Llanvirn. We report a U/Pb zircon date of 466+/-6Ma for Mweelrea ignimbrite that indicates contemporaneity with the Connemara arc intrusions. Conglomerate interbedded with the ignimbrites contains granite clasts that are probably derived from the Connemara orthogneiss suite. Zircons from one granite clast have yielded an age of 486+/-16Ma. Palaeocurrents indicate that the conglomerates were derived from the south, suggesting that the Connemara terrane already lay to the south of the South Mayo Trough during arc magmatism.