Polyorogenic Nature of the Southern Caledonian Fold Belt in East Greenland: An Isotopic Age Study

The southern part of the Caledonian fold belt in East Greenland comprises a complex of pre-Caledonian rock units as demonstrated by numerous age determinations and interpretation of field work. U-Pb isotopic age determinations on zircons, Rb-Sr analyses of whole-rock and mineral samples and one K-Ar mineral age are presented. They date some of the key rock units fundamental to the unravelling of the development of this part of the fold belt, and support the polyorogenic nature of the Caledonian fold belt. Three main groups and two orogenic events with related migmatizations are observed: (1) Archean basement: early intrusive granites and basic dikes imply a minimum age of 2500 m.y. for the basement; (2) Proterozoic intrusions: bodies and sheets of augen granites were emplaced about 1000 m.y. before present. The intrusions coincide with the first (i.e. Proterozoic) orogenic event as well as the first migmatization recognized in the area; (3) Caledonian intrusives: a massive hypersthene monzonite sheet representing an early phase of the second (i.e. Caledonian) orogeny, was emplaced about 475 m.y. ago. Younger granitic stocks as well as sheets of granite associated with the second migmatization were formed in the period between 425 and 400 m.y. ago.