Decompressional equilibration of the Midsund granulite from Otroy, Western Gneiss Region, Norway

The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otroy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure-temperature (P-T) pseudosection that provides an intersection of compositional isopleths of X-Mg (Mg/Mg+Fe) in garnet, albite in plagioclase and X-Na (Na/Na+Ca) in clinopyroxene in the stability field of melt+plagioclase+garnet+clinopyroxene+amphibole+ilmenite. The obtained thermodynamic model yields P-T conditions of 132-1.45 GPa and 875-970 degrees C. The relatively high P-T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP-UHP termites worldwide.