Abstract

Using the sensitive high resolution ion microprobe (SHRIMP), U-Pb ages have been determined for zircons separated from rhyolitic obsidian from the Tardree (Sandy Braes) Rhyolite Centre and granites of the Mourne Mountains and Slieve Gullion central igneous complexes of the British Tertiary Volcanic Province. These ages (Tardree, 58.4 ±0.7 Ma; Mourne, 56.4 ±1.4 Ma and 55.3 ± 0.8 Ma and Slieve Gullion, 56.5 ±1.3 Ma) are in close agreement (within error) with results from independent dating methods (Rb-Sr, K-Ar and ⁴⁰Ar/³⁹Ar) and are interpreted as crystallization ages. The euhedral, clear, bipyramidal nature of all separated zircons is indicative of a magmatic origin and, combined with the paucity of zircon grains with any history of inheritance, has implications for granite petrogenesis. When considered with recently published high-precision ⁴⁰Ar/³⁹Ar ages for sanidine separates from the Hebrides, these data help to establish a timeframe for Tertiary magmatism in the NW British sector of the North Atlantic Tertiary Volcanic Province and show that central complex magmatism in NE Ireland was confined to the later stages of activity.