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

摘要

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.