Abstract
In Sin, Ar-40/Ar-39 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(1)) 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(2)) 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(1) and eight Bt(2) crystals. The two biotite generations yielded two statistically distinct age distributions with Bt(1) maxima at c. 433 Ma and Bt(2) maxima at c. 417 Ma. A small number of analyses (11% Bt(1) and 26% Bt(2)) 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(1) crystals are younger (than the Bt(2) population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt(2) analyses are predominantly older (than the Bt(2) population) and are interpreted as indicating localized excess Ar-40. Such dis cordant analyses indicate intracrystalline variation reflecting both excess Ar-40 (Bt(2) population) and Ar-40 loss (Bt(1) population), and were therefore discarded when determining geologically meaningful ages.
Original language | English |
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Pages (from-to) | 337-345 |
Number of pages | 9 |
Journal | Journal of the Geological Society |
Volume | 163 |
Issue number | 2 |
Publication status | Published - Mar 2006 |
Keywords
- OLD RED SANDSTONE
- SINISTRAL TRANSPRESSION
- NORTHWESTERN IRELAND
- ARGON
- GEOCHRONOLOGY
- TEMPERATURE
- EVOLUTION
- DIFFUSION
- SCOTLAND
- DONEGAL