New U–Pb baddeleyite and zircon ages for the Scourie dyke swarm: A long-lived large igneous province with implications for the Paleoproterozoic evolution of NW Scotland

Abstract

We report new U–Pb baddeleyite and zircon ages for doleritic and noritic dykes from the NW–SE trending Scourie dyke swarm located in the Precambrian Lewisian Gneiss Complex of northwest Scotland. These new ages indicate that the majority of the dykes were intruded over a ∼40Ma period between ∼2418 and 2375Ma and we redefine the term ‘Scourie dykes’ to refer to this earliest period of dyke intrusion, thus avoiding confusion with the nearly 400Ma of known mafic magmatism in the area.Recent tectonic interpretations of the Lewisian Gneiss Complex subdivide the area into several small terranes, which amalgamated during the ca. ∼1.9–1.7Ga Laxfordian metamorphic event. However, the majority of Scourie dykes intruded the Lewisian Gneiss Complex shortly after a ∼2.5Ga (Inverian) amphibolite grade metamorphic event, previously interpreted to be related to the collision of the Rhiconich and Assynt terranes. The timing and placement of the Scourie dykes indicates that they could have formed during a period of post-collisional extension. Scourie dykes located in the Tarbert terrane on the Isle of Lewis in the Outer Hebrides and the Assynt terrane of the Scottish mainland indicate that the two areas, along with the Rhiconich terrane, shared a common ancestry prior to dyke emplacement, rather than amalgamating during the Laxfordian orogeny.The new geochronology, along with paleomagnetism, and geochemistry of the Scourie dykes allow the Lewisian Gneiss Complex to be placed in a broad paleogeographic context. Various Neoarchean supercontinents have been proposed based on broad similarities in the timing of major cratonic events. New data on the Scourie dykes, combined with the regional geology of the NW Scottish Highlands, indicate that the Lewisian Gneiss Complex, and potentially the North Atlantic craton, were separate crustal entities from the proposed supercontinent Nunavutia during the Neoarchean and Paleoproterozoic.