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

New 40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar 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.

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