Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway: indications of incomplete transition at high pressure

Abstract

The Western Gneiss Region of Norway is a continental terrane that experienced Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under lateCaledonian amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket, Ulvesund and Kråkenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions (20-23 kbar) but show only very localized (about 5 %) eclogitization in felsic and mafic rocks, commonly related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (~95 %) metastability at pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression was limited. The degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three bodies appear to have been far less reactive than the external gneisses in this region, even though they followed a similar pressure-temperature evolution. The extent of metastable behaviour has implications for the protolith of the Western Gneiss Region, for the density evolution of high-pressure terranes and hence for the geodynamic evolution of mountain belts.

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

“…The Western Gneiss Region (WGR) is generally interpreted to represent the westward continuation of the Fennoscandian basement of Baltica (e.g., Roberts & Gee, 1985;Brueckner & van Roermund, 2004;Root et al, 2005, Spengler et al, 2009. During the Scandian phase (435−390 Ma) of the Caledonian orogeny, the subduction of Baltica beneath Laurentia produced high-pressure (HP) and ultrahigh-pressure (UHP) rocks (Griffin & Brueckner, 1980Gebauer et al, 1985;Mørk & Mearns, 1986;Andersen et al, 1991;Krabbendam et al, 2000;Terry & Robinson, 2004). The depth of subduction increases towards the northwest ( Fig.…”

“…The WGR consists mainly of granodioritic to granitic orthogneisses and is considered to represent Baltican basement (Tucker et al, 1990). The gneisses are predominantly of amphibolite-facies metamorphic grade (Bryhni & Andréasson, 1985;Krabbendam & Wain, 1997;Krabbendam et al, 2000), but in some parts granulite-facies assemblages occur Krabbendam et al, 2000). The orthogneisses are locally overlain by pelitic paragneisses, but they are very scarce .…”

“…2) is located within the UHP domain (Root et al, 2005) and the mixed HP-UHP transition zone (Wain, 1997) and preserves pre-Caledonian igneous and granulite-facies assemblages, which underwent little or no Caledonian deformation (Krabbendam et al, 2000). The main rock type of Flatraket is a micaceous quartzo-feldspathic gneiss with megacrystic K-feldspar which, in its core, equilibrated under amphibolite-facies conditions (Krabbendam et al, 2000).…”

“…Ultramafic pods have only a minor distribution in comparison with the Flatraket body. Most mafic pods are small with well equilibrated eclogite-facies assemblages (Krabbendam et al, 2000). …”

Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway

“…The deformation is strongly heterogeneous with alternating zones of intense deformation in shear zones/bands and layers and lenses of variably preserved earlier rocks and mineral assemblages as shown by the numerous lenses of eclogite and locally Pre-Caledonian metamorphic/ igneous rocks in the area (e.g. Krabbendam et al, 2000;Torsvik et al, 1997). The intense shear fabrics have amphibolite facies mineral assemblages, which post-date both eclogites (temperature 650 to 700 C; pressure 2.7 to 2.9 GPa) and later high-pressure (1.2 GPa) amphibolites, locally associated with decompression melting (Labrousse et al, 2004).…”