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

In situ ⁴⁰Ar/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 aquired 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 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 discordant analyses indicate intracystalline variation reflecting both excess ⁴⁰Ar (Bt₂ population) and ⁴⁰Ar loss (Bt₁ population), and were therefore discarded when determining geologically meaningful ages.