Lithostratigraphy, geology and geochemistry of the Tertiary volcanic rocks on Svartenhuk Halvø and adjoining areas, West Greenland

Abstract

The Palaeogene volcanic succession in the northern part of the Nuussuaq Basin in West Greenland comprises three formations: the Vaigat and Svartenhuk Formations of Paleocene age (61–58 Ma) and the Naqerloq Formation of Eocene age (57–54 Ma). In this study, we formalise and describe the volcanic stratigraphy on Svartenhuk Halvø and the areas with lavas that flowed across the basin boundary onto the adjoining basement areas in the north and east. The Vaigat Formation comprises three members. The Kakilisaat and Nerutusoq Members are of minor volume and consist of, respectively, crustally contaminated basalts and chemically enriched basalts with relatively high contents of incompatible trace elements. They are overlain by the voluminous Nunavik Member of tholeiitic picrites (MgO ≥12 wt%) and subordinate magnesian basalts. The oldest volcanic deposits are commonly foreset-bedded hyaloclastites, and the overlying subaerial lavas are mainly thin, grey, crumbling flows. Eruption sites were mainly within the basin, with depocentres in the south and hyaloclastite and lava transport directions towards the north. Thicknesses vary from up to at least 2000 m in the south to ≥380 m in the northernmost exposures close to 72°N. The Svartenhuk Formation comprises four members. The lowest, Kuugaartorfik Member, is up to 100 m thick and consists partly of quartzofeldspathic and partly volcanogenic sediments; it is restricted to northern Svartenhuk Halvø and the Innerit peninsula. The overlying volcanic Tunuarsuk, Nuuit and Skalø Members are voluminous and widespread, with a combined thickness of up to 1800 m. They consist of tholeiitic basalts with similar chemical compositions but with correlatable stratigraphic variation patterns. The Tunuarsuk Member consists of interspersed flow groups of thin, grey flows and massive, brown flows; the Nuuit Member comprises mainly massive brown flows, and the Skalø Member is dominated by light grey flows. The Svartenhuk Formation oversteps the Vaigat Formation on the basement in the north and east. In these distal areas the Tunuarsuk and Nuuit Members constitute the major volumes, and preserved thicknesses are up to 1400 m. In northern and eastern Svartenhuk Halvø and also farther to the north and east, foreset-bedded hyaloclastites indicate transport directions towards the north and possibly east from eruption sites within the basin. The Naqerloq Formation comprises one member, the Arfertuarsuk Member, consisting of flows of brown basalt with relatively enriched chemistry and a single trachyte flow. The member is only found in western Svartenhuk Halvø and on Skalø, where it conformably overlies the older lavas with up to 350 m thickness preserved after erosion. Dykes of all three formations are present. The distribution of dykes of the Naqerloq Formation suggests that this originally extended much farther east. Picrites and basalts of the Vaigat and Svartenhuk Formations are geochemically related; the picritic lavas represent erupted primitive magmas, whereas the basaltic lavas represent fractionated melts formed in deep magma chambers. The melts formed from a geochemically depleted but heterogeneous mantle; in addition melts from enriched sources were occasionally incorporated. The enriched basalts of the Naqerloq Formation arose from another mantle source. Low contents of V, Cu and Ni in some crustally contaminated lavas indicate that accumulation of these elements may be present at depth.