The high-pressure to ultrahigh-pressure eclogite transition in the WesternGneiss Region, Norway

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.