Production of the classic Barrovian metamorphic facies series in Scotland was associated with a period mountain building referred to as Grampian orogenesis. Recent geochronological work from the Grampian terrane, Scotland has the Grampian orogenic episode beginning at c . 478 Ma and lasting for less than 15 Myr. Such brief orogenesis is at odds with the generally accepted model for Barrovian metamorphic heating by thermal relaxation following lithospheric thickening and suggests an alternative, smaller scale heat source is responsible for metamorphism.

We have studied garnets from the garnet to sillimanite zones of the Barrovian sequence to quantify diffusion related modifications to major element zoning in garnet with increasing metamorphic grade. Forward modelling of the diffusion process suggests regional metamorphism involved crustal heating over a few Myr. This places further restrictions on the size of heat sources responsible for production of the type Barrovian sequence.