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

• Main Authors: Holmberg Johanna, Bukała Michał, Jeanneret Pauline, Klonowska Iwona, Majka Jarosław
• Format: Article
• Language: English
• Published: Earth Science Institute, Slovak Academy of Sciences, Slovakia 2019-12-01
• Series: Geologica Carpathica
• Subjects: scandinavian caledonides; thermodynamic modelling; granulite facies metamorphism; decompression
• Online Access: http://www.degruyter.com/view/j/geoca.2019.70.issue-6/geoca-2019-0027/geoca-2019-0027.xml?format=INT

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 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 XMg (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 of 1.32–1.45 GPa and 875–970 °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 terranes worldwide.