Ultra‐high pressure (<i>P</i> &gt; 6 GPa) garnet peridotites in Western Norway: exhumation of mantle rocks from &gt; 185 km depth

Abstract

We report here for the first time the occurrence of relics of majoritic garnet within orogenic garnet peridotites from Otrøy, Western Gneiss Region, Norway. The microstructural evidence consists of two‐pyroxene exsolution from garnet. Majoritic garnets are only stable at depths greater than 150 km. Estimates of the initial composition of the majoritic garnets imply pressures of 6–6.5 GPa indicating that the Otrøy peridotites were derived from depths &gt; 185 km. Mineral‐chemical data indicate that the present mineral compositions equilibrated at mantle conditions around 805 ± 40 °C and 3.2 ± 0.2 GPa. Estimates of the initial pressure temperature (PT) conditions and PTtime ( t ) path are consistent with a multistage, multiorogenic exhumation history with upwelling of hot asthenosphere up to ≈ 100 km in the Pre‐Cambrian followed by subsequent crustal emplacement and exhumation during the Caledonian orogeny.