ABSTRACT Three Silurian basin fills, the Llandovery–Wenlock Croagh Patrick and Killary Harbour–Joyce Country successions and the Ludlow–Pridoli Louisburgh–Clare Island succession, overstep the tectonic contacts between elements of the Grampian (Taconic) accretionary history of the Caledonian-Appalachian orogeny in western Ireland. New U-Pb detrital zircon data from lower strata of these Silurian rocks provide insight into basin evolution and paleogeography. The shallow-marine Croagh Patrick succession unconformably overlies the Clew Bay Complex and the northern part of the Ordovician South Mayo Trough. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, with few younger grains. Up to 13% of the grains form a cluster at ca. 950–800 Ma, which is younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting; these may be recycled grains from Dalradian strata, derived from distal Tonian intrusions. The Killary Harbour–Joyce Country succession overlies the structural contact between the Lough Nafooey arc and the Connemara Dalradian block and records a transgressive-regressive cycle. Four samples of the Lough Mask Formation show contrasting age spectra. Two samples from east of the Maam Valley fault zone, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source, likely in north Mayo. One sample also includes 8% Silurian grains. Two samples from west of the fault overlie Dalradian basement and are dominated by Ordovician grains. Circa 450 Ma ages are younger than any preserved Ordovician rocks in the region and are inferred to represent poorly preserved arc fragments that are exposed in northeastern North America. Cambrian to late Neoproterozoic grains in association with young Ordovician ages suggest derivation from a peri-Gondwanan source in the late stages of Iapetus closure. The Louisburgh–Clare Island succession comprises terrestrial red beds. It unconformably overlies the Clew Bay Complex on Clare Island and is faulted against the Croagh Patrick succession on the mainland. The Strake Banded Formation yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician–Silurian ages. Although the basin formed during strike-slip deformation along the Laurentian margin in Ireland and Scotland, sediment provenance is consistent with local Dalradian sources and contemporaneous volcanism. Our results support ideas that Ganderian continental fragments became part of Laurentia prior to the full closure of the Iapetus Ocean.

Abstract The early Paleozoic rocks of eastern Ireland span the suture zone between the Laurentian and Ganderian continental margins of the Iapetus Ocean. The Grangegeeth Terrane comprises a Laurentian continental fragment and Ordovician volcanic arc that formed the southern margin of the late Ordovician Rathkenny Basin of Moffat Shale facies mudstone. Together, these were overstepped in Wenlock time by Laurentia-derived greywackes and became the southernmost tract of the Longford–Down Terrane accretionary prism as subduction brought them into the Laurentian-margin trench. South of the suture, Middle Ordovician failed rifting of a Ganderian volcanic arc terrane was followed by shortening during continuing subduction under the north-facing arc. In Newfoundland, a Ganderian volcanic arc migrated across the ocean in mid-Ordovician times to accrete to Laurentia. In Ireland, the late-accreted arc on the Laurentian margin was formed on a Laurentian microcontinent, and Ganderia had an active margin throughout Late Ordovician time. Silurian closure of Iapetus was between the leading edge of Ganderia and the Laurentian margin, unlike in the Canadian Appalachians, where separate Ordovician and Silurian sutures are recognized. Iapetus was narrow by Katian time but subduction-related magmatism continued into Wenlock times on both margins.