U-PB DETRITAL ZIRCON EVIDENCE FOR ACTIVE MARGIN TECTONISM AFTER THE GRAMPIAN OROGENY IN WESTERN IRELAND

The Grampian Orogeny occurred when an oceanic volcanic arc collided with Laurentia in the Early Ordovician (ca. 470 Ma). Subsequently, the mountains experienced extensional collapse and subduction was initiated along the Laurentian margin with the opposing polarity, converting the old passive margin into an active one. The Silurian stratigraphy of western Ireland is divided into a number of apparently independent sequences separated by major strike-slip faults. We dated detrital zircons from five sandstone formations using U-Pb methods in order to understand the tectonic setting in which they were deposited and to test whether the sequences are really divided. The Glen Formation (deposited ca. 420 Ma) comprises proximal fluvial sandstones that incorporated material eroded from Grampian-related Dalradian metamorphic rocks themselves dominated by erosion of sources in the Grenville orogen (ca. 1 Ga) and Baltica (1.6-1.8 Ga), as well as post-orogenic volcanic rocks, indicating re-establishment of subduction magmatism after Grampian arc collision. Sequences in South Mayo show that erosion from the Dalradian progressively reduced between 430 and 425 Ma. Erosion from magmatic rocks of Grampian or slightly younger ages becomes dominant as water depths increased. Either the Dalradian metamorphic sources subsided and/or the basin was transported by strike-slip faulting along the Laurentian margin after the end of the Grampian Orogeny. The Lettergesh (ca. 426 Ma) and Loch Mask (ca. 430 Ma) Formations show abundant younger grains postdating the Grampian Orogeny, again consistent with the re-establishment of arc magmatism. The rocks south of the South Mayo Trough contrast with those on the north side, suggesting basins that were separated from one another potentially by significant topography or bathymetry perhaps similar to the modern-day Ryukyu Arc of the west Pacific.