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.

The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland

“…1). This terrane is considered part of the Ganderian margin and zircons dated from a sandstone within a volcanogenic breccia place the age of volcanism in the terrane at c. 474 Ma (McConnell et al 2015).…”

Detrital zircon characterization of early Cambrian sandstones from East Avalonia and SE Ireland: implications for terrane affinities in the peri-Gondwanan Caledonides

“…An influx of contrasting, Laurentia-derived sand, coincident with rapid flexural subsidence (King, 1994) during Silurian (Wenlock) time, marks the onset of collision with Laurentia. In contrast, in some Ganderian fragments in the northern Appalachians, the first indications of Laurentian provenance occur in Ordovician strata (Waldron, McNicoll & van Staal, 2012;MacDonald et al 2014), showing that fragments of Ganderia crossed the Iapetus Ocean earlier than in the European segment of the orogen (McConnell et al 2015).…”

Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

“…Contemporary accretion-related units in Britain and Ireland are trench-fill turbidites of the Southern Uplands terrane (Leggett et al, 1979) that are largely Laurentia-derived (Stone and Merriman, 2004;Waldron et al, 2008). This interpretation suggests that the Iapetus Ocean at the longitude of Britain and Ireland had a simpler configuration, without many of the peri-Gondwanan arcs and backarc basins that filled the northern Appalachian segment (Waldron et al, 2014a;McConnell et al, 2015)…”