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

^{1 1}Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland (e-mail: stephen.daly@ucd.ie)
^{2 2}Present address: British Antarctic Survey, c/o NERC Isotopes Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh 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 Annagh Gneiss Complex gneisses have a weighted mean ²⁰⁷Pb/²⁰⁶Pb 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 Annagh 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.

Key Words: Grenvillian Orogeny • Caledonides • Grampian Orogeny • Dalradian • Ireland

This article has been cited by other articles:

				S. C. Sherlock, K. A. Jones, and R.G. Park Grenville-age pseudotachylite in the Lewisian: laserprobe 40Ar/39Ar ages from the Gairloch region of Scotland (UK) Journal of the Geological Society, 2008; 165: 73 - 83. [Abstract] [Full Text] [PDF]

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