A discussion of possible heat sources during nappe stacking: the origin of Barrovian metamorphism within the Caledonian thrust sheets of NW Scotland

Geochronological data indicate that the Caledonian thrust sheets of NW Scotland were assembled in 18 Ma, yet regional Barrovian metamorphism to amphibolite facies and local melting occurred syn-thrusting. From theoretical studies, it is clear that there is insufficient time to generate the high metamorphic temperatures during the thrusting event. We suggest that a plausible explanation for the heat source is that it resulted from a 20–25 Ma period of crustal heating in a back-arc setting between the Grampian and Scandian orogenic events. Most of the orogenic heat therefore came from the pre-existing back-arc rather than the orogenic process itself.