Fast tectonometamorphism and exhumation in the type area of the Barrovian and Buchan zones

The beginning of the Grampian episode of tectonometamorphism in Scotland is dated by collision and obduction of the Ballantrae Ophiolite Complex at 478 +/- 8 Ma. For the first time we have used zircon and garnet to radiometrically date the peak of the Caledonian Buchan and Barrovian tectonometamorphism in their type areas as contemporaneous at 467 +/- 2.5 Ma. Detrital garnet from the neighboring Southern Uplands flysch terrane has the same radiometric age and occurs in 465 +/- 2.5 Ma molasse in the Midland Valley. We show that the entire Grampian episode lasted 15 +/- 2.5 m.y. and that it took fewer than 7.6 m.y. to expose high-grade rocks following the peak of metamorphism. Those who study Precambrian orogeny should note the brevity of this Scottish tectonometamorphic episode and the speed of exhumation.