Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: Evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths

The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Craton.It comprises mid-late Archaean tonalite-trondhjemite-granodiorite gneisses that were metamorphosedand deformed during the Late-Archaean and Palaeoproterozoic. A major swarm of mafic-ultramafic dykes,the Scourie Dyke Swarm, was intruded at ca. 2.4–2.3 Ga during a period of extension that can be correlatedacross the North Atlantic Craton. The majority of dykes are doleritic, with volumetrically minor picriteand olivine gabbro suites.New major and trace element geochemical data and Re-Os isotopes indicate that the Scourie DykeSwarm was not solely derived from a ‘typical’ asthenospheric mantle source region. The geochemicalsignatures of the dykes show significant negative Nb, Ta and Ti anomalies, coupled with enrichment in Th,Light Rare Earth Elements and other large ion lithophile elements. These features cannot be reproducedby simple contamination of asthenospheric sources with Lewisian granulite-facies crust. Instead they area feature of the mantle source that produced the Scourie Dykes and may have developed during Archaeansubduction episodes.Spinel lherzolite mantle xenoliths from the Isle of Lewis offer direct insight into the lithospheric mantlebelow this region. They display similar geochemical ‘enrichments’ and ‘depletions’ observed in the ScourieDykes and the magma source is thus considered to reside primarily in the sub-continental lithosphericmantle (SCLM), with some potential contribution from asthenospheric melts. Platinum Group Elementgeochemistry and trace element modelling indicate that the dolerite dykes were formed by moderate(