External UHP garnet websterites and orthopyroxene eclogites in the Western Gneiss Region, Norwegian Caledonides: metamorphism or metasomatism?

The spectacular “pegmatitic” orthopyroxene eclogites of the Western Gneiss Region, Norway, have attracted much attention and their origins have been hotly debated. They were termed “external” to distinguish them from similar, mantle-derived orogenic peridotites. They form part of a texturally diverse suite of opx-bearing eclogites (OpE) and garnet websterites (GtW). They are often interlayered with opx-free eclogites and olivine-bearing garnet pyroxenites (“peridotites”) (e.g. Jamtveit, 1987) belonging to the “Fe-Ti” suite of orogenic peridotites with protoliths in Proterozoic layered intrusions (Carswell et al., 1983). It is evident that at least some OpE and GtW had a metasomatic origin: Vein systems are found in several of these bodies, such as Kolmannskog, Myrbaerneset (Otrøy) and Remøysund (Sørøyane). Veins are cored by garnetite and have symmetrical wall-rock zones with outward variation from clinopyroxenite to ultra-coarse-grained phlogopite garnet websterite, grading into a host of olivine-garnet websterite. Garnetite is commonly enriched in apatite, monazite and zircon. Felsic sheets that may represent the metasomatising agent are found coring the garnetite veins or cutting across the olivine websterite. Adjacent to the host gneiss or internal felsic veins the ultramafic rocks grade into phlogopite glimmerites. At Svartberget (Vrijmoed et al., 2013) ultra-coarse, decussate garnet websterites host microdiamond showing that the veins formed at ultrahigh pressure. Here, mass-balance calculations show a metasomatic agent with an andesitic (Vrijmoed, 2013) or trachyandesitic (Quas-Cohen, 2014) composition similar to experimental melts in granite-peridotite systems (e.g. Rapp et al., 1999). Hence the agent responsible for generating OpE and GtW may have been similar to granitic leucosomes in the host gneiss that reacted with the marginal peridotite before penetrating along fractures into the core of the body. Further south at Arsheimneset a variant has HFSE-enriched phengite-quartz glimmerite rimmed by garnet and omphacitite. Overall, the evidence points to the generation of ultra-coarse GtW and OpE by infiltration of a felsic, supercritical melt enriched in LIL and HFSE. The classic examples at Grytting and nearby Arsheimneset, which lack exposed olivine-rich precursors, are deduced to have had a similar origin. New U-Pb accessory mineral ages from vein systems indicate vein formation in the Arsehimneset body at 414.0+5.6Ma (zircon) and in the Svartberget body at 410.6±2.6Ma (zircon) and 410.6±2.6Ma (monazite). Some opx eclogites (e.g Nybø) lack vein systems and show normal metamorphic tectonite fabrics, so may represent simple metamorphic transformation of igneous protoliths, or possibly recrystallisation of ultra-coarse metasomatic rocks. The Norwegian opx eclogites and garnet websterites show many similarities to metasomatised mantle xenoliths and offer a fruitful field of investigation in which field-scale studies can be made of mantle processes through these analogous systems.