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

Abstract

Mineralogical and textural criteria in high-pressure and ultrahigh-pressure metamorphic rocks in the Nordfjord-Stadlandet area of the Western Gneiss Region 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 eclogite, and quartz pseudomorphs after coesite in garnet, clinopyroxene and epidote minerals in eclogites and gneisses. Relic coesite(s) ranging from 2 to 150 μm grain size has been identified by micro-Raman spectroscopy. Preservation of coesite varies considerably, from 0 to 90 % in inclusions. Coesite, or non-annealed pseudomorphs after coesite, are only preserved as inclusions within rigid host phases that have been isolated from the rock matrix until a late stage in the exhumation history, after amphibolite-facies retrogression of the rock matrix. These textural relationships suggest cooling during exhumation. Several textural and mineral chemical features of garnets and amphiboles distinguish ultrahigh-pressure from high-pressure eclogites and gneisses. Ultrahigh-pressure eclogites and gneisses record only peak P-T conditions and subsequent retrograde metamorphism, whereas high-pressure eclogites and gneisses record a prograde evolution to eclogite-facies conditions, and subsequent retrogression, entirely restricted to the quartz stability field. These differences are evidence of a metamorphic break in the 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.