The Western Gneiss Region of Norway is a continental terrane that experienced
Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show
either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under late-Caledonian
amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket,
Ulvesund and Kråkenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies
assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected
by Caledonian deformation. The three bodies experienced high-pressure conditions
(20–23 kbar) but show only very localized (about 5%) eclogitization in felsic and mafic rocks, commonly
related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (∼ 95%) metastability at
pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression
was limited. The degree of reaction is related to the protolith composition and the interaction
of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of
deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three
bodies appear to have been far less reactive than the external gneisses in this region, even though they
followed a similar pressure–temperature evolution. The extent of metastable behaviour has implications
for the protolith of the Western Gneiss Region, for the density evolution of high-pressure
terranes and hence for the geodynamic evolution of mountain belts.