Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain best record of pressure temperature evolution this ultrahigh-pressure (UHP) terrane. Their exhumation from UHP conditions has been extensively studied, but their prograde rarely quantified although it represents a key constraint for tectonic history area. This study focused on well-preserved phengite-bearing eclogite sample Nordfjord region. The was investigated using phase-equilibrium modelling, trace-element analyses garnet, trace- major-element thermobarometry quartz-in-garnet barometry by Raman spectroscopy. Inclusions garnet core point to crystallization amphibolite facies at 510–600°C 11–16 kbar, whereas chemical zoning suggests growth during isothermal compression up peak 28 kbar 600°C, followed near-isobaric heating 660–680°C. Near-isothermal decompression 10–14 is recorded fine-grained clinopyroxene–amphibole–plagioclase symplectites. absence increase seems incompatible with classic view along geothermal gradient subduction zone may question significance metamorphism. Two end-member scenarios are proposed explain such an compression: Either (1) rocks were originally depth lower crust consecutively buried portion subducting slab or (2) 14 overpressure constant collisional stage orogeny.