The Kap Washington Group volcanic rocks occur on the north coast of Greenland and were extruded at or just before the Cretaceous–Tertiary boundary. The suite is strongly bimodal with basaltic lavas, comendites and comenditic ash-flow tuffs predominating. The basic lavas are of transitional basalt type but have characteristically alkaline minor-element contents. The riebeckite-bearing comendites have strong light REE enrichment and large negative Eu anomalies as commonly found in continental peralkaline suites. As well as the comendites there are other rhyolite lavas and minor intrusions of distinctly different chemistry which are thought to have originated by crustal refusion. The peralkaline nature of the siliceous rocks is in keeping with the extensional tectonic environment suggested by an intense alkali dolerite dyke swarm which immediately preceded the extrusive activity. Association with the development of the Makarov Basin in Cretaceous times is suggested.

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