Laurentian Provenance of Archean Mantle Fragments in the Proterozoic Baltic Crust of the Norwegian Caledonides

The Proterozoic gneisses of the Western Gneiss Region (WGR) of western Norway experienced HP-UHP metamorphism during the 435-390 Ma Caledonian (Scandian) orogeny, and locally enclose numerous large bodies of Archean peridotite. Competing models for the emplacement of these peridotites into the gneisses involve either (1) upthrusting of subcontinental mantle into overlying gneisses or (2) 'sinking emplacement' of peridotites from the Laurentian mantle wedge into the upper surface of the subducting Baltica plate. The first model implies the existence of Archean lower crust below the outcropping gneisses. To evaluate these models we have carried out a regional survey of the U-Pb age, Hf isotope composition and trace-element compositions of detrital zircon grains collected from drainages in the northern half of the WGR. The zircon data indicate that the gneisses that make up the crust of the WGR were originally generated during the 1·7-1·5 Ga Gothian orogeny. The Hf isotope signatures of these zircons indicate a juvenile (i.e. mantle) origin; there is no evidence, from either inherited zircons or the Hf isotope data, that Archean crustal materials were involved in the genesis of these gneisses. The Sveconorwegian orogeny (1·3 to < 1·0 Ga) that overprinted the Gothian gneisses involved both juvenile magmatic additions to the crust and remelting of the Gothian basement; these Sveconorwegian-age magmas also show no evidence of Archean contributions. A population of zircons collected from a drainage area containing large mantle-derived peridotite bodies includes anhedral to rounded grains with distinctive trace-element patterns consistent with derivation from depleted rocks; these are inferred to be derived from the peridotites and/or their enclosed eclogites and pyroxenites. These zircons give Archean Hf model ages, but 207Pb/206Pb ages ranging from Archean to Caledonian, suggesting that the younger ages reflect resetting during later thermal events. The Archean zircon ages are consistent with Archean Re-Os model ages previously obtained on the peridotites. In the absence of any evidence for Archean crust (and hence Archean mantle) beneath southern Baltica, we infer that the peridotite massifs represent fragments of the subcontinental lithosphere beneath Laurentia, and were introduced tectonically into the gneisses during the Caledonian subduction of Baltica beneath Laurentia

Archean mantle hidden in Proterozoic crust in SW Norway

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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.26 page(s