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

Alice Wain, David Waters, Andrew Jephcoat and Helmut Olijynk
The high-pressure to ultrahigh-pressure eclogite transition in the Western Gneiss region, Norway
European Journal of Mineralogy (June 2000) 12 (3): 667-687


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 mu m 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.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 12
Serial Issue: 3
Title: The high-pressure to ultrahigh-pressure eclogite transition in the Western Gneiss region, Norway
Affiliation: University of Oxford, Department of Earth Sciences, Oxford, United Kingdom
Pages: 667-687
Published: 200006
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 40
Accession Number: 2000-049409
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
N61°00'00" - N61°00'00", E07°00'00" - E07°00'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200016
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