Mineralogical and textural criteria in high-pressure and urtrahigh-pressure
metamorphic rocks in the Nordfjord-Stadlandet area of the Western Gneiss R
egion of Norway demonstrate in situ metamorphism of both high-pressure and
ultrahigh-pressure eclogites within their enclosing gneisses. In ultrahigh-
pressure rocks, coesite occurs in garnet, clinopyroxene and kyanite in eclo
gite, and quartz pseudomorphs after coesite in garnet, clinopyroxene and ep
idote minerals in eclogites and gneisses. Relic coesite(s) ranging from 2 t
o 150 mu m grain size has been identified by micro-Raman spectroscopy. Pres
ervation of coesite varies considerably, from 0 to 90 % in inclusions. Coes
ite, or non-annealed pseudomorphs after coesite, are only preserved as incl
usions within rigid host phases that have been isolated from the rock matri
x until a late stage in the exhumation history, after amphibolite-facies re
trogression of the rock matrix. These textural relationships suggest coolin
g during exhumation. Several textural and mineral chemical features of garn
ets and amphiboles distinguish ultrahigh-pressure from high-pressure eclogi
tes and gneisses. Ultrahigh-pressure eclogites and gneisses record only pea
k P-T conditions and subsequent retrograde metamorphism, whereas high-press
ure eclogites and gneisses record a prograde evolution to eclogite-facies c
onditions, and subsequent retrogression, entirely restricted to the quartz
stability field. These differences are evidence of a metamorphic break in t
he Nordfjord area. It appears not to be related to kinetic factors, and so
supports a proposed tectonic boundary to an ultrahigh-pressure unit in the
Western Gneiss Region. These features are consistent with exhumation of UHP
rocks in the Nordfjord-Stadlandet area by a subduction-type model.