Omphacite-bearing orthogneisses in the Western Gneiss Complex, Sunnfjord, Norway: Densification of subducted felsic crust

Geodynamic models for the behaviour of continental margins during collision and subduction require realistic information on rock density and rheology as they evolve during the subduction-eduction cycle. In the Western Gneiss Complex (WGC), a giant mid-late Silurian continental HP-UHP terrain in the southern Scandinavian Caledonides, the predominant exposed lithology is granitoid orthogneiss. Preservation of peak Scandian HP or UHP mineral parageneses is rare due to pervasive amphibolite-facies overprinting and partial melting. Where this exhumation-related ductile deformation is weak there is widespread evidence for the formation of phengite in orthogneisses [1,2] but the extent to which plagioclase transformed to denser jadeitic pyroxene is not yet well known.

In the southern part of the WGC P-T conditions recorded in eclogites are in the quartz-stable, HP range with temperatures significantly lower (550^o-700^oC) than the better-known UHP rocks north of Nordfjord. In the Dalsfjord area in Sunnfjord, omphacite-bearing dioritic to granodioritic orthogneisses have been found to outcrop continuously over an area ~10km² and possibly considerably larger [3,4]. They can be shown to have evolved from a Mesoproterozoic two-pyroxene, garnet-free, charnockitic precursor by transformation of feldspar to domains of czo+ky+qz±pg±phe and mafic domains to omp+grt+rt (altered to symplectites of amp+plag±biotite). Peak P-T conditions are estimated to be 2.5GPa at ~675^oC. The gneisses are L>S tectonites with a strong omphacite aggregate linear shape fabric. Syn-tectonic veinlets consist of coarse quartz, omphacite, phengite and rutile.

Transformation of charnockite to an eclogite-facies mineral assemblage involves a significant increase in density. For a Dalsfjord tonalitic omphacite gneiss at ambient HP conditions, density has been estimated at ~3.2g.cm³ [5]. The change from massive charnockite to a strongly-lineated omphacite gneiss also suggests a change in rheology. If similar rocks were originally widespread in the WGC, these changes are likely to have influenced mechanical behaviour of the crust during Scandian subduction and eduction. Such transformations require a pervasive influx of aqueous fluid, but no local source of fluid is evident. Hence fluid flux of several kilometers is implied, possibly from metasedimentary cover rocks or previously hydrated, overlying subduction-zone mantle. Preservation of dense, peak eclogite-facies mineral assemblages is likely to have been controlled by a combination of bulk composition, availability of an external source of water and continued potential for reactivity during exhumation. Preservation is not favoured where spontaneous dehydration reactions can take place during decompression, such as in K-rich granitoids and pelites dominated by phengite[6]. However, the Dalsford orthogneisses are richer in Ca, favouring abundant anhydrous omphacite that requires an external source of water to promote its decomposition.

1 Engvik, A.K., Austrheim, H., Andersen, T.B. (2000) Journal of the Geological Society, London, 157, 121-134.
2 Hacker, B.R., Andersen, T.B. et al. (2010) Tectonophysics 480, 149–171.
3 Cuthbert, S.J. (1985) Unpublished PhD Thesis, University of Sheffield.
4 Skår, Ø. (1997) Norwegian Geological Survey Report no. 97.077.
5 Hacker, B.R. & Abers (2004) Geochemistry, Geophysics, Geosystems, 5, doi:10.1029/2003GC000614
6 Proyer, A. (2003) Lithos, 70, 183–194