Isothermal compression of an eclogite from the Western Gneiss Region (Norway): a multi-method study
Abstract
The Western Gneiss Region in Norway is constituted by a crustal nappe stack that comprises some
of the best-preserved exhumed ultra-high pressure (UHP) terranes on Earth. The UHP rocks result
from the subduction of the western edge of the Baltica craton beneath Laurentia during the
Caledonian orogeny. Mafic eclogites form lenses within granitoid orthogneisses and show the best
record of the pressure and temperature evolution. Their exhumation from the UHP conditions has
been largely studied, but the prograde evolution has been rarely quantified in the eclogites
although it constitutes an important constraint on the tectonic history of this area. This study
focused on an unaltered eclogite sample from Vågsøy in the Nordfjord region. This sample was
investigated using a large panel of methods including phase-equilibria modelling, trace-element
analyses of garnet, trace- and major-element thermo-barometry and quartz-in-garnet barometry
by Raman spectrometry. The eclogite comprises omphacite, garnet, white mica, epidote and
amphibole and accessory rutile, quartz, zircon, carbonates and kyanite. Garnet shows a grossularrich
core with inclusions of quartz, epidote, white mica and amphibole, while grossular-poor rims
are 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 °C
and 11–15 kbar, while chemical zoning in garnet suggests growth during isothermal compression
up 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-amphiboleplagioclase
symplectites. The absence of a temperature increase during compression seems
incompatible with the classic view of crystallization along a geothermal gradient in a subduction
zone and may question the tectonic significance of eclogite-facies metamorphism. Two main
tectonic scenarios are discussed to explain such an isothermal compression: (1) either the mafic
rocks were originally at deep level within the lower crust and were then buried along the
isothermal part of the subducting slab, or (2) the mafic rocks recorded significant tectonic
overpressure at constant depth and temperature conditions during the collisional stage of the
orogeny. A multi-chronometer geochronological study is currently performed and expected to
bring additional, discriminant constraints on this P–T evolution.
