Felsic metastability during continental subduction: the Norwegian Western Gneiss Complex as a case study

Abstract

Ultra-high pressure (UHP) metamorphic terranes are thought to record the Earth’s secular thermal evolution in their mineral assemblages. These mineral assemblages change from amphibolite-facies to eclogite-facies with increasing pressure during subduction. Mafic eclogite-facies rocks within UHP metamorphic terranes such as the Western Gneiss Complex (WGC), Norway, record deep burial and subsequent exhumation. Felsic orthogneisses encasing these mafic rocks are subjected to the same conditions but typically do not record UHP metamorphism. It is debated as to whether the felsic orthogneisses do not record UHP metamorphism because they remain metastable at eclogite-facies conditions. LA-ICP-MS ages of 416 ± 18 Ma and 399.8 ± 7.8 Ma, obtained from rutile grains within two mafic eclogites near Lavik, Norway, suggest that this region underwent eclogite-facies metamorphism during the Caledonian Orogeny (~430–385 Ma). Mineral equilibria forward modelling shows garnet-bearing felsic gneiss in the study area did not record pressure-temperature (P–T) conditions exceeding ~8–9 kbar, ~650-°C. By comparison, nearby high strain mafic eclogite recorded peak metamorphic conditions of ~20–22 kbar and ~650-°C. Major element zonation patterns in garnet >600 μm from high strain mafic eclogite indicate that peak metamorphism persisted for between ~10–20 Myr. A lack of high-pressure mineral phases in felsic orthogneisses imply they were metastable within the WGC for at least this time period. The anhydrous nature (H2O = 0.34 wt%) of the modelled gneiss may explain this metastability during the Caledonian Orogeny, as fluid is necessary to catalyse many metamorphic reactions. A lack of fluid would thus inhibit anhydrous felsic orthogneisses within the WGC from reacting beyond amphibolite facies. This metastability would inhibit the felsic orthogneiss from recording peak P–T conditions as hydrous mafic eclogites do. This kinetic inhibition implies anhydrous felsic orthogneisses within the WGC – and, possibly, other UHP metamorphic terranes – make poor recorders of UHP metamorphism.