Grampian and late Grenville events recorded by mineral geochronology near a basement-cover contact in north Mayo, Ireland

The Palaeoproterozoic to early Neoproterozoic Armagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Armagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U-Pb titanite analyses from Armagh Gneiss Complex gneisses have a weighted mean Pb-207/Pb-206 age of 963 +/- 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 +/- 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U-Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Armagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar-Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 +/- 4 Ma Ar-Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar-Ar hornblende and Rb-Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.