Decompressional equilibration of the Midsund granulite from Otrøy, 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 Otrøy,a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsundgranulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmeniteand titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT systemresulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths ofXMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt +plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effectof equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HTat lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previouslythought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR andother HP–UHP terranes worldwide.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

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