A revised intrusion sequence for the Donegal batholith: Evidence from its aureole in Lettermacaward

The Thorr pluton has until now been regarded as the oldest pluton of the (c. 400Ma) Donegal batholith, and the Ardara pluton the next oldest. However, cross-cutting relationships are not observed. The region around Lettermacaward, NW Donegal, lies within the structural aureole of the Ardara pluton and within the thermal aureole of the Thorr pluton. This study provides new petrological and microstructural evidence to support a revision of this emplacement sequence, showing that in Lettermacaward, Thorr-related fibrolite is superposed on regional Ardara-related D₄ crenulations. The Ardara pluton therefore precedes the Thorr pluton. This revision has significant implications for models of the Donegal batholith that rely on the order of emplacement of these two early plutons.