No Exploits back-arc basin in the Iapetus suture zone of Ireland

Abstract

A controversial aspect of the closure history of the Iapetus Ocean concerns the existence of the Exploits basin in Ireland and Britain. The Exploits–Tetagouche back-arc basin of the Canadian Appalachians opened during the Middle Ordovician as the Popelogan–Victoria volcanic arc migrated northwards from the Ganderian margin. The Bellewstown Terrane, within the Iapetus suture zone of Ireland, lies between the Ganderian–Avalonian Leinster Terrane and the Laurentian Grangegeeth Terrane. An early Ordovician ‘Celtic’ shelly fauna is hosted in volcanogenic breccia that we demonstrate is stratigraphically overlain by shales with no volcanic horizons. The lower 24 m of shale are unfossiliferous, but shale above this stratigraphic level has yielded a new graptolite fauna consistent with the upper part of the artus Biozone (Darriwilian, c . 464 Ma). U–Pb zircon dating of the volcanic horizon yields an age of c . 474 Ma. The shale, therefore, appears to represent c . 10 myr of deposition and a significant gap in volcanism. This, together with a Sandbian Anglo-Welsh fauna, suggests that the Bellewstown Terrane remained in a peri-Gondwanan position throughout the Ordovician. Hence, Exploits back-arc basin opening and Ganderian arc migration did not occur in the Irish sector of Iapetus. Supplementary materials: The table of U–Pb zircon data, CL images of zircon grains, concordia plot of all U–Pb zircon analyses, and additional drawings and notes on the graptolite fauna are available at www.geolsoc.org.uk/SUP18858.