Metastability of granulites and processes of eclogitisation in the UHP region of western Norway

Abstract

The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

“…Amphibolite-facies metabasites occur together with mafic granulites and HP-to UHP-eclogites on km to cm scales (e.g. Austrheim and Engvik, 1997;Krabbendam et al, 2000;Wain et al, 2001). It has been shown that variability in the preserved metamorphic signature is due to heterogeneous strain and fluid ingress along the rocks' P,T evolution.…”

“…It has been shown that variability in the preserved metamorphic signature is due to heterogeneous strain and fluid ingress along the rocks' P,T evolution. Strain and fluid availability imposes kinetic control on rock and mineral reactivity, so that in absence of these factors assemblages can be metastably preserved even through episodes of extreme overstepping of equilibrium P,T conditions (Wayte et al, 1989;Austrheim, 1998;Krabbendam et al, 2000;Wain et al, 2001). …”

Heavy-mineral and garnet compositions of stream sediments and HP–UHP basement rocks from the Western Gneiss Region, SW Norway

“…The eclogites make up c. 5% of the Flatraket area and are partially retrogressed to amphibolite-facies assemblages. The granulite is also strongly affected by amphibolite-facies retrogression (Krabbendam & Wain, 1997;Wain et al, 2001). …”

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

“…Evidence from numerous localities, including the Bergen Arcs and Western Gneiss Region in Norway, and the Western Alps, suggests that granulite-facies rocks can persist metastably through a subduction-exhumation cycle so long as they remain dry and/or undeformed (Wain et al, 2001, and references therein). What is less clear is whether "wetter" continental crust behaves equally metastably under subduction conditions.…”