Early Proterozoic isotopic ages in the East Greenland Caledonian fold belt

Rb-Sr whole rock isochron ages of 1950 m.y. and 1725 m.y. obtained on rock units of the Hagar migmatite sheet indicate that it is an early Proterozoic gneiss complex and not part of a mobile Caledonian infrastructure. New isotopic ages and recent fieldwork give no support to the concept of mobile Caledonian migmatitic masses rising into the suprastructure as envisaged in the classic stockwerke model for the East Greenland Caledonian fold belt. However, K-Ar mineral ages ranging from 415–462 m.y. indicate a significant Caledonian thermal event. Relationships between the Hagar sheet and bordering metasediments are discussed, including evidence for a middle Proterozoic group of sediments metamorphosed before deposition of the late Precambrian Petermann Series and Eleonore Bay Group.