Recently the timescales associated with the production of regional metamorphic belts have been determined to be on the order of a few million years. This is in contrast to the established model of crustal thickening and erosion, which requires durations on the order of 50 million years (England and Thompson, 1984). Accurately constraining the duration of regional metamorphism is important in determining the tectonic setting in which it formed. To this end the garnet-ilmenite system was investigated for its applicability to the diffusion modelling technique and was determined to be viable system. A detailed40Ar/39Ar geochronology study of the regional metamorphic belt in Scotland was also undertaken. Diffusion modelling of Ar in muscovite constrained the duration of the Buchan metamorphism in the cordierite zone to less than 1 Ma.