Ian G. Meighan

Combined Sm-Nd and Rb-Sr isotope systematics in the Donegal granitoids and their petrogenetic implications

The metaluminous to peraluminous granitoids of the Donegal batholith, northwest Ireland, were emplaced at c . 400 Ma into greenschist-amphibolite facies metasediments of the Dalradian Supergroup. Sm-Nd and Rb-Sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the Newry complex, Co. Down; the Donegal results reveal essentially similar initial Sr isotope ratios (0.7051–0.7068) but highly variable initial e Nd values, −1.2 to −8.3 (and −0.5 for Newry). Certain granitoids have distinctive Nd isotopic compositions characteristic of the involvement of old, LREE-enriched continental crust in some cases or young crust and/or mantle-derived magmas in others. The Nd and Sr isotopic variations can be explained by a variety of mixing hypotheses.

Sr and Nd isotope evidence for successive crustal contamination of Slieve Gullion ring-dyke magmas, Co. Armagh, Ireland

The Palaeogene Slieve Gullion Igneous Centre in southern Armagh, Northern Ireland, consists of a layered central intrusive complex surrounded by a prominent and slightly older ring-dyke that intrudes both Lower Palaeozoic sedimentary rocks and the Caledonian Newry Granodiorite pluton (452 Ma). The ring-dyke comprises two major rock types: porphyritic felsite and porphyritic granophyre. We analysed both ring-dyke lithologies, both types of country rock, and a local Palaeogene basalt dyke sample for Sr and Nd isotopes. Trace element and whole rock data for this suite suggest that there are two distinct groups of both felsite and granophyre: one Si-rich and one Si-poor, most likely representing two magmas from a zoned chamber and their mushy chamber wall equivalents (McDonnell et al. 2004). Isotope data show the low-Si rocks to be higher in radiogenic Sr than the high-Si rocks, which is inconsistent with a simple AFC-scenario of increasing sediment assimilation with higher degrees of differentiation. However, using MORB-type basalt as a starting composition, the low-Si ring-dyke rocks can be modelled through AFC with Lower Palaeozoic sedimentary rock as the contaminant. The decreasing 87 Sr/ 86 Sr trend from low-Si to high-Si dyke rocks, in turn, represents a second stage of contamination. Selective assimilation of the most fusible portions of Newry Granodiorite, which is lower in radiogenic Sr than the local sedimentary rocks, appears to be the most plausible solution. The Sr and Nd data are consistent with (a) at least a two-stage contamination history during upper crustal residence and storage, whereby fractionating magmas of basaltic and intermediate composition are contaminated by local sedimentary rocks, giving rise to rhyolite magmas that experience additional shallow contamination by Newry Granodiorite, and (b) a zoned rhyolite magma chamber where high-Si magma is stored in the upper part of the chamber where crystallization and crustal contamination are most extensive.

The origin of granite erratics in the Pleistocene Patella beach, Gower, South Wales

Rare pebbles of granite have been discovered in the raised Patella beach at Butterslade, Gower, South Wales. Their petrography, trace element geochemistry and the Rb/Sr whole rock age of 55 ± 5 Ma confirm that they are derived from the Lundy granite which is about 49 km to the southwest of Gower. Amino acid analyses of fossil gastropods in the Patella beach have provided an age of 210000 years. Various hypotheses of transportation of pebbles from Lundy and Pembrokeshire to Butterslade are considered. Erratics from Pembrokeshire were probably transported by Pleistocene ice into the area while clasts of Lundy granite were moved by progradation of beach deposits northeastwards towards Gower during glacio-eustatic marine transgressions in the Pleistocene.