Laser ablation <sup>40</sup>Ar/<sup>39</sup>Ar dating of metamorphic fabrics in the Caledonides of north Ireland

<p><em>In situ</em> <sup>40</sup>Ar/<sup>39</sup>Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt<sub>1</sub>) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt<sub>2</sub>) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt<sub>1</sub> and eight Bt<sub>2</sub> crystals. The two biotite generations yielded two statistically distinct age distributions with Bt<sub>1</sub> maxima at <em>c</em>. 433 Ma and Bt<sub>2</sub> maxima at <em>c</em>. 417 Ma. A small number of analyses (11% Bt<sub>1</sub> and 26% Bt<sub>2</sub>) from within crystals yielded ‘main population’ ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt<sub>1</sub> crystals are younger (than the Bt<sub>2</sub> population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt<sub>2</sub> analyses are predominantly older (than the Bt<sub>2</sub> population) and are interpreted as indicating localized excess <sup>40</sup>Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess <sup>40</sup>Ar (Bt<sub>2</sub> population) and <sup>40</sup>Ar loss (Bt<sub>1</sub> population), and were therefore discarded when determining geologically meaningful ages. </p>