Isothermal compression of an eclogite from the Western GneissRegion (Norway): a multi-method study

International audienceThe Western Gneiss Region in Norway is constituted by a crustal nappe stack that comprises someof the best-preserved exhumed ultra-high pressure (UHP) terranes on Earth. The UHP rocks resultfrom the subduction of the western edge of the Baltica craton beneath Laurentia during theCaledonian orogeny. Mafic eclogites form lenses within granitoid orthogneisses and show the bestrecord of the pressure and temperature evolution. Their exhumation from the UHP conditions hasbeen largely studied, but the prograde evolution has been rarely quantified in the eclogitesalthough it constitutes an important constraint on the tectonic history of this area. This studyfocused on an unaltered eclogite sample from Vågsøy in the Nordfjord region. This sample wasinvestigated using a large panel of methods including phase-equilibria modelling, trace-elementanalyses of garnet, trace- and major-element thermo-barometry and quartz-in-garnet barometryby Raman spectrometry. The eclogite comprises omphacite, garnet, white mica, epidote andamphibole and accessory rutile, quartz, zircon, carbonates and kyanite. Garnet shows a grossularrichcore with inclusions of quartz, epidote, white mica and amphibole, while grossular-poor rimsare enriched in pyrope and middle rare-earth elements and include omphacite and rutile.Inclusions in garnet core point to crystallisation conditions in the amphibolite facies at 550–600 °Cand 11–15 kbar, while chemical zoning in garnet suggests growth during isothermal compressionup to the peak pressure of 28 kbar at 600 °C, followed by near-isobaric heating to 640–680 °C.Isothermal decompression to 8–13 kbar is recorded in fine-grained clinopyroxene-amphiboleplagioclasesymplectites. The absence of a temperature increase during compression seemsincompatible with the classic view of crystallization along a geothermal gradient in a subductionzone and may question the tectonic significance of eclogite-facies metamorphism. Two maintectonic scenarios are discussed to explain such an isothermal compression: (1) either the maficrocks were originally at deep level within the lower crust and were then buried along theisothermal part of the subducting slab, or (2) the mafic rocks recorded significant tectonicoverpressure at constant depth and temperature conditions during the collisional stage of theorogeny. A multi-chronometer geochronological study is currently performed and expected tobring additional, discriminant constraints on this P–T evolution