40Ar/39Ar dating has identified a succession of middle Miocene (14–13 Ma) basaltic lavas in East Greenland that overlie Eocene flood basalts that were erupted during continental breakup ca. 56–55 Ma. The long postbreakup magmatic history (∼40 m.y.) of the East Greenland margin precludes a simple relationship between this later igneous activity and the track of the Iceland hotspot. Chemical and isotopic data suggest that the postbreakup magmas were produced from mantle that had been metasomatized by light rare earth element–enriched, H2O- and CO2-bearing melts originating from the Iceland plume. Episodic melting of recently metasomatized shallow mantle beneath Greenland and the North Atlantic can explain both the composition and the long-lived nature of postbreakup magmatism along the East Greenland margin, as well as lavas on Jan Mayen Island that have enriched, Icelandic-type isotopic signatures.