Pressure Temperature Conditions of the Otrøy Opmhacite-Garnet Gneiss, Western Gneiss Region, Skandinavian Caledonides

Abstract: Garnet-omphacite gneisses from the island of Otrøy situated in Western Gneiss Region (WGR), Norway, Scandinavian Caledonides, were examined within this study. The WGR is one of the planets most studied ultrahigh-pressure (UHP) terranes. The studied gneisses are hosts for UHP garnet peridotites and eclogites. The presence of the high pressure mineral assemblage including e.g. omphacite and phengite together with assumed remnants of pseudomorphic transition of formerly stable coesite present in omphacite and garnet, suggest formation of the Otrøy gneisses during ultrahigh pressure metamorphism (UHPM). However, geothermobarometry based on the mineral assemblage composed of garnet + clinopyroxene + phengite yielded pressure-temperature conditions of c. 880˚C and 2.2 GPa, characteristic for just high pressure metamorphism.  Nevertheless, it can be concluded that the Otrøy gneisses were formed due to the deep burial of continental crust during the continent-continent collision. This study provides insights into the understanding of the deep subduction of continental crust and expands our knowledge about the complex metamorphic and tectonic evolution of the WGR and the Scandinavian Caledonides.