M. Krabbendam et al., Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region,Norway: indications of incomplete transition at high pressure, GEOL MAG, 137(3), 2000, pp. 235-255
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.