Rb-Sr whole-rock isochrons for the Upper Zone of the Insch mass and aureole schists of the Haddo House mass indicate crystallisation of the basic sheet of NE. Scotland at 486± 17 m.y. ago (t½ = 4.85 × 1010 y. for Rb87). It is argued that intrusion occurred into hot country rocks during, or very soon after, the migmatite overprint phase of the climax of regional metamorphism (M3) in the Dalradian Series, so that this is also dated at about 486± 17 m.y. Small bodies of gneissose rocks in the Arnage mass give a whole-rock isochron age of 479± 12 m.y. and are interpreted as intrusions slightly younger than the Newer Gabbros. Post-metamorphic granites at Insch and Haddo House are dated at 459± 8 m.y.