Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides: petrogenesis, preservation and implications for scandian collision dynamics

Geodynamic models for continental collision require information on rock density and rheology as they evolve during the subduction-eduction cycle. The Western Gneiss Complex (WGC) in the Scandinavian Caledonides represents a continental margin that underwent transient subduction during the Scandian collision. Preservation of peak eclogite facies metamorphic parageneses in the predominant granitoid gneisses is rare due to amphibolite-facies overprinting and partial melting.
In the Dalsfjord area of the southern WGR the gneisses escaped the migmatisation prevalent in the higher-T, UHP, northern WGC. Grey bt+pl+qz+ep±grt±Ca-amp orthogneisses enclose areas of green, omphacite-bearing granitoid gneiss. These evolved from a Mesoproterozoic charnockitic precursor to omp+grt+czo+ky+qz+rt±pg±phe (omp extensively altered to amp+pl symplectite). They are L>S tectonites with a strong omphacite-aggregate shape fabric. Peak P-T was ~650^oC at 2.3GPa.
Density of the omphacite gneiss at peak P&T was about 3.2 g.cm^-3 based upon the estimated mineral mode and isochemical phase diagram sections. This would have been neutrally or slightly positively buoyant relative to anhydrous lithospheric mantle but negatively buoyant relative to a serpentinised mantle wedge. Phase diagram analysis shows that the HP assemblage of the grey gneisses would have been rich in phengite and poor in omphacite. Phengite spontaneously decomposes on decompression, while omphacite requires addition of an aqueous fluid. Hence the grey gneiss remained reactive so the HP paragenesis was lost, but the reactivity of the omphacite gneiss was constrained by the availability of a source of water so the HP paragenesis was preserved.
The evolution of density and rheology during the subduction cycle depended on an initial ingress of water followed by its differential consumption and release by different bulk compositions. The tectonite fabric indicates that transformation from charnockite resulted in a major change in rheology. However, the localised survival of pristine protolith shows that the transformation was inefficient.