Prograde temperature-time evolution in the Barrovian type-locality constrained by Sm/Nd garnet ages from Glen Clova, Scotland

The timing of garnet growth during metamorphism associated with the Grampian Orogeny in the sillimanite zone of the Barrovian type-locality in Glen Clova, Scotland, was determined by Sm/Nd geochronology. Two high precision garnet-whole-rock ages were achieved by employing HF partial dissolution of garnet separates to optimize purity. Multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data: an early stage, at 472.9 +/- 2.9 Ma, during D-2 deformation under garnet zone conditions (c. 500-550degrees), and a later stage, at 464.8 +/- 2.7 Ma, during or slightly after D-3 deformation mostly under sillimanite zone conditions (peak temperature of c. 660degreesC), but possibly including some growth during kyanite zone conditions. When combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of Barrow's zones were achieved roughly contemporaneously. The difference between garnet zone and sillimanite zone peak temperature attainment is 2.8 +/- 3.7 Ma. The near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over-thickened crust. We suggest that local igneous intrusions, with synmetamorphic ages, provided that additional heat.