A whole-rock Rb-Sr isochron for the Donegal Granite gives an age of 470±1 m.y., which places it between the "Older Granites" (pre-500 m.y.) and the "Newer Granites" (400± m.y.) of Scotland and Ireland. The initial Sr⁸⁷/Sr⁸⁶ ratio of the Donegal Granite, together with the field evidence, suggests that some contamination by crustal material has occurred.

Two groups of Dalradian rocks of near stratigraphic equivalence--sillimanite-grade Connemara Schist from western Ireland and metasedimentary rocks of lower green-schist grade from Islay, Scotland--yield isochron ages of 490±30 and 540±20 m.y., respectively. It is postulated that the latter is very nearly the age of the main Dalradian metamorphism, which is shown from stratigraphic evidence to have occurred probably during the Middle or Late Cambrian. The Connemara Schist, because of higher metamorphic temperature, took longer to cool to a temperature at which homogenization of strontium and rubidium isotopes ceased in the whole rock on the scale of a hand specimen. Previous work has shown that individual minerals in the Connemara Schist remained as open chemical systems ntil about 450 m.y. ago, and that low-grade Dalradian slate became closed to argon diffusion at 505±6 m.y. ago.