Orogenic versus extensional settings for regional metamorphism: Knoydartian events in the Moine Supergroup revisited

Considerable debate exists over the tectonic regimes associated with mid-Neoproterozoic metamorphism of the Moine Supergroup, NW Scotland. Published pressure conditions imply burial to 35–40 km, a potential doubling of crustal thickness, and hence a substantial collisional event. Re-evaluation using updated thermodynamic software suggests more modest peak pressures of c. 7.5 kbar implying burial to c. 21 km. The revised P–T path has a comparatively flat clockwise evolution from early high geothermal gradient conditions. The revised P–T conditions suggest that c. 800 Ma crustal thickening within the Moine Supergroup was less significant than previously envisaged and possibly preceded by extension.