The geochronology of the basic igneous complexes

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Abstract
Synopsis: 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 × 10 10 y. for Rb 87 ). 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 (M 3 ) 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. K-Ar mica ages from the gabbro complex are extremely uniform at 461 ± 5 m.y., whereas Rb-Sr ages on some of the biotites range from 471 ± 22 m.y. to 428 ± 9 m.y. Together, these ages preclude the hypothesis of overprinting by any discrete metamorphic event younger than the main climax. Instead, they are interpreted as indicating rapid post-orogenic uplift in NE. Scotland at about 460 m.y. ago.

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