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

Abstract

The Proterozoic gneisses of theWestern Gneiss Region (WGR) of western Norway experienced HP^UHP metamorphism during the 435^390Ma 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 theWGR.The zircon data indicate that the gneisses that make up the crust of theWGR 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 to51·0 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 Pb/Pb 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.