Grampian orogenesis and the development of blueschist-facies metamorphism in western Ireland

Rb-Sr and [sup.40]Ar/[sup.39]Ar step-heating and in situ [sup.40]Ar/[sup.39]Ar laserprobe dating of fabric-forming micas provide new constraints on the timing of Grampian orogenesis and the associated development of blueschist-facies metamorphism at the Laurentian margin in NW Ireland. Early ([MP.sub.1]) blueschist-facies assemblages were developed in metabasites of the Dalradian Supergroup deposited near the edge of the incipient Laurentian margin, contemporaneous with Barrovian metamorphism in the Dalradian closer to the Laurentian foreland. The regional [D.sup.2] event is associated with the formation of orogen-scale fold nappes and is constrained in the Dalradian by [S.sub.2] muscovite ages of c. 460 Ma, which are probably recording crystallization. Importantly, the Clew Bay Complex, previously considered as an exotic terrane (correlated with the Highland Border Complex of Scotland) and the Dalradian are in structural continuity. Muscovite from the [S.sub.2] nappe fabric in the Clew Bay Complex also yields identical c. 460 Ma ages. During [D.sub.3], dextral shearing tilted the recumbent [D.sub.2] nappes into a vertical, downward-facing orientation adjacent to the Laurentian margin. [D.sub.3] is constrained by [S.sub.3] muscovite ages of c. 448 Ma. Synchronous deformation of the Dalradian Supergroup and the Clew Bay Complex in the mid-Ordovician Grampian orogeny casts doubt on both the validity of Silurian microfossil dates obtained from the Clew Bay Complex and the exotic status of both this 'terrane' and the correlative Highland Border Complex of Scotland. Keywords: Dalradian Supergroup, Clew Bay Complex, Ar/Ar, Rb- Sr, structural geology.