Age of the Grampian orogeny in Scotland and Ireland

The previous conflict between stratigraphical and geochronological evidence for the age of the Grampian orogeny in Scotland and Ireland has now been largely resolved. Dalradian deposition continued on the Laurentian margin through late Proterozoic into Ordovician time. The Grampian orogeny was a brief, arc-accretion event that took place around the Arenig-Llanvirn boundary, at about 470 Ma, an extension of the Taconic orogeny of the Appalachians. Exhumation of the orogen was rapid; Rb-Sr mica ages that have been taken to indicate pre-Ordovician orogeny and slow cooling are considered to be problematical.