The timing of sedimentation and Buchan metamorphism in the Grampian Terrane in Scotland from 40 Ar/ 39 Ar apparent age spectra

Abstract

New 40 Ar/ 39 Ar geochronological data from the Buchan and Barrovian metamorphic sequences on the NE coast of Scotland have been obtained from in vacuo step-heating experiments conducted on white mica extracted from seven samples. The results confirm the synchronous formation of the Barrovian and Buchan metamorphic sequences, with the 40 Ar/ 39 Ar apparent age spectra obtained for samples of andalusite grade and higher recording a range of ages between 470.4 ± 1.8 and 464.6 ± 1.3 Ma. These ages are close to the accepted age for peak metamorphism during the Grampian Orogeny, c. 470 Ma, whereas 40 Ar/ 39 Ar apparent age spectra of white mica from the lower grade cordierite zone near Banff preserve pre-Grampian detrital ages. In these spectra an age of c. 560 Ma is recorded. This indicates that a tectonothermal event occurred at c. 560 Ma and may provide further insight into the timing of sedimentation. These samples also preserve evidence of ages older than 750 Ma. Supplementary material: 40 Ar/ 39 Ar step-heating data tables for all samples, data from analyses of flux monitors, determination of correction factors and mass discrimination used for data reduction and 40 Ar/ 39 Ar step-heating spectra and inverse isochron plots from each sample are available at www.geolsoc.org.uk/SUP18723 .