Prograde amphibolite facies to ultrahigh‐pressure transition along Nordfjord, western Norway: Implications for exhumation tectonics

Abstract

The Nordfjord area of western Norway hosts one of the best known provinces of ultrahigh‐pressure (UHP) rocks in the world, yet the nature of the transition from the UHP rocks into surrounding amphibolite‐facies crust has remained unclear. New thermobarometry of phengite‐ and kyanite‐bearing eclogites shows that this transition is unbroken by major structures: the pressure and temperature gradient increases smoothly over a horizontal distance of ∼20 km, from high‐pressure amphibolite facies (∼1.5 GPa/600–700°C) through quartz eclogite (∼2.4 GPa/600°C) into UHP eclogite (>2.7 GPa/700°C). These data support a model in which the Norwegian UHP province remained attached to lower pressure crust during exhumation. This requires that a large area of the Western Gneiss Region was metamorphosed and exhumed ∼60 km through the mantle, as a relatively coherent body.