Transformation of archaean lithospheric mantle by refertilization: evidence from exposed peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneisses at several localities in the Western Gneiss Region (WGR) of western Norway; garnet peridotites typically occur as discrete zones within larger bodies of garnet-free, chromite-bearing dunite and are commonly closely associated with pyroxenites and eclogites. The dunites of the large Almklovdalen peridotite body have extremely depleted compositions (Mg-number 92-93.6); the garnet peridotites have lower Mg-number (90.6-91.7) and higher whole-rock Ca and Al contents. Post-depletion metasomatism of both rock types is indicated by variable enrichment in the light rare earth elements, Th, Ba and Sr. The dunites can be modelled as residues after very high degrees (>60%) of melt extraction at high pressure (5-7 GPa), inconsistent with the preservation of lower degrees of melting in the garnet peridotites. The garnet peridotites are, therefore, interpreted as zones of melt percolation, which resulted in refertilization of the dunites by a silicate melt rich in Fe, Ca, Al and Na, but not Ti. Previous Re-Os dating gives Archaean model ages for the dunites, but mixed Archaean and Proterozoic ages for the garnet peridotites, suggesting that refertilization occurred in Proterozoic time. At least some Proterozoic lithosphere may represent reworked and transformed Archaean lithospheric mantle.