Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region,Norway: indications of incomplete transition at high pressure

The Western Gneiss Region of Norway is a continental terrane that experienc ed 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-fac ies conditions. However, three kilometre-size rock bodies (Flatraket, Ulves und and Krakenes) in Outer Nordfjord preserve Pre-Caledonian igneous and gr anulite-facies assemblages and structures. Where these assemblages are pres erved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions (20-23 kbar) but show on ly very localized (about 5%) eclogitization in felsic and mafic rocks, comm only related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (similar to 95%) metastability at pressures higher tha n other metastable domains in Norway. Late-Caledonian amphibolite-facies re trogression was limited. The degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogen ic cycle, whereby metastability is associated with a lack of deformation an d lack of fluids, either as a catalyst or as a component in hydration react ions. The three bodies appear to have been far less reactive than the exter nal gneisses in this region, even though they followed a similar pressure-t emperature evolution. The extent of metastable behaviour has implications f or the protolith of the Western Gneiss Region, for the density evolution of high-pressure terranes and hence for the geodynamic evolution of mountain belts.