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

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 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 exhumation-related ductile deformation is weak there is widespread evidence for the formation of phengite in orthogneisses (Engvik et al., 2000; Hacker et al., 2010) but the extent to which plagioclase transformed to denser jadeitic pyroxene (to be expected under the pressures calculated from metabasic eclogites in the WGC) is not yet well known. In the southern part of the WGC P-T conditions recorded in eclogites are in the quartzstable, 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 of the Gjölanger Unit have been found to outcrop continuously over an area ~6km2 and possibly considerably larger (Cuthbert 1985, Skår, 1997). 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 2.3GPa at ~650^oC. The gneisses are L>S tectonites with a strong omphacite aggregate linear shape fabric. Syn-tectonic veinlets consist of coarse quartz, omphacite, phengite, rutile and carbonate.
Transformation of anhydrous charnockite to a hydrous eclogite-facies mineral
assemblage involves a significant increase in density. 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, both such 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 transport of fluid over several kilometers is implied, possibly from metasedimentary cover rocks or previously hydrated, overlying subduction-zone mantle.

Cuthbert, S.J. (1985) Petrology and tectonic setting of relatively low-temperature eclogites and related rocks in the Dalsfjord area, Sunnfjord, West Norway. Unpublished PhD Thesis, University of Sheffield.
Engvik, A. K., Austrheim, H., Andersen, T.B. (2000) Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid‐limited polymetamorphism, Western Gneiss Region, Norway. Journal of the Geological Society of London 157:121-134.
Hacker, B.R., Andersen, T.B., Johnston, S., Kylander-Clark, A.R.C., Peterman, E.M., Walsh, E.O., Young, D. (2010) High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway. Tectonophysics 480:149–171.
Skår, Ø. (1997) Protoliths of the gneisses and mafic rocks in the Hellevik-Flekke area in Sunnfjord, Western Norway. Norwegian Geological Survey Report no. 97.077.