Potassium-Argon ages for minerals from the Ross of Mull, Argyllshire, Scotland

Abstract

Sixteen K-Ar ages for samples of biotite and amphibole from the Ross of Mull and analytical data for the standards Bern 4M and W1 are presented. Ages determined for biotite and amphibole samples from the Caledonian pluton average 423 ± 4 m.y. and 416 ± 4 m.y. respectively. Field observations combined with gravity data indicate that this intrusion was guided by the Moine thrust plane. These ages therefore provide a minimum age for the Moine thrust and their concordance is interpreted in relation to the retentivity of the dated amphiboles to ⁴⁰Ar inferred from electron microprobe analyses of their composition. A biotite sample from a minette dyke cutting the pluton yielded an age of 406 ± 10 m.y., whereas two samples of amphibole from a fourchite dyke in the Moine country rock yielded early Permian ages of 276 ± 7 m.y. and 274 ± 8 m.y.