Abstract

J. A. winchester & M. D. Max write: In postulating a Newfoundland origin for granitic clasts in the Southern Uplands of Scotland, which yielded dates of 1265 ± 130 and 1230 ± 120 Ma, Elders (1987) has proposed a distant source when other much closer potential sources exist. Even though no extensive areas of Precambrian basement of this age range are currently exposed in the British Isles, rocks of comparable age crop out in a small area of NW Ireland. In NW Co. Mayo (which Elders dismissed as a potential source area, quoting the 1070–30 Ma age of granitic pink gneiss emplacement) pre-Grenville plagioclase–quartz–biotite migmatitic gneisses of granodioriti composition in the Annagh Division .have yielded U-Pb zircon ages of 1297 + 164/−101 and 1224 + 192/−64 Ma (Aftalion & Max 1987). These ages are comparable with those obtained by Elders for clasts from Corsewall Point and Glen Afton. Indeed, they correspond more closely to the age of the clasts than the ages of 1081 ± 165 and 1132 ± 154 Ma obtained from the Newfoundland plutons which Elders suggested as potential source rocks. Within the Annagh Division, granitoid migmatitic segregations are well-developed but frequently display only a weak foliation, a feature also observed by Elders in many clasts in the Southern Uplands.

In addition, geophysical evidence [Max et al. 19831 suggests that much of the continental shelf to the northwest of Ireland probably consists of Proterozoic rocks similar to the Annagh Division. Because areas of this unsampled

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