In situ40Ar/39Ar 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 (Bt1) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt2) 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 Bt1 and eight Bt2 crystals. The two biotite generations yielded two statistically distinct age distributions with Bt1 maxima at c. 433 Ma and Bt2 maxima at c. 417 Ma. A small number of analyses (11% Bt1 and 26% Bt2) 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 Bt1 crystals are younger (than the Bt2 population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt2 analyses are predominantly older (than the Bt2 population) and are interpreted as indicating localized excess 40Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess 40Ar (Bt2 population) and 40Ar loss (Bt1 population), and were therefore discarded when determining geologically meaningful ages.

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