U-Pb zircon geochronology of plagiogranites from the Lough Nafooey (= Midland Valley) arc in western Ireland: constraints on the onset of the Grampian orogeny

The timing of peak Grampian metamorphism is well constrained from the detrital record of the adjacent fore-arc basin fill and geochronology of synorogenic intrusive rocks, but the onset of collision is less certain. Proximal Silurian conglomerates contain plagiogranite boulders unequivocally derived from the Lough Nafooey arc, two of which yield U-Pb secondary ionization mass spectrometry zircon ages of 489.9 +/- 3.1 Ma and 487.8 +/- 2.3 Ma. Nd isotopic evidence (epsilon(Nd(490)) c. 0) demonstrates that the plagiogranites assimilated significant amounts of old continental crust. This provides an absolute age constraint on a previously poorly constrained and inferred event, demonstrating that the arc had encountered subducting Laurentian margin sediments by 490 Ma.