Ultra-high pressure (P > 6 GPa) garnet peridotites in Western Norway: exhumation of mantle rocks from > 185 km depth

We report here for the first time the occurrence of relies of majoritic gar net within orogenic garnet peridotites from Otroy, Western Gneiss Region, N orway. The microstructural evidence consists of two-pyroxene exsolution fro m garnet. Majoritic garnets are only stable at depths greater than 150 km. Estimates of the initial composition of the majoritic garnets imply pressur es of 6-6.5 GPa indicating that the Otroy peridotites were derived from dep ths > 185 km. Mineral-chemical data indicate that the present mineral compositions equili brated at mantle conditions around 805 +/- 40 degrees 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 approximate to 100 km in the Pre -Cambrian followed by subsequent crustal emplacement and exhumation during the Caledonian orogeny.