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U-Pb data for zircons from post-tectonic monzonite, syenite and norite, and country rock gneiss and migmatite from the Kap Farvel-Prins Christian Sund area of South Greenland indicate two distinct intrusive episodes at ∼1740 m.y. and ∼1755 m.y. The norites have the same age as the older granitic rocks. Similar intrusions further north along the southeast coast have the same age (∼1755 m.y.) and geochemical character as those of the Kap Farvel-Prins Christian Sund area. Our U-Pb ages are about 100 m.y. older than previously determined K-Ar ages. The youngest peak of regional Ketilidian metamorphism in this area is placed at about 1800 m.y. based on a linear array of seven magnetic and size fractions from a syntectonic granitic part of a migmatite. This age is distinctly different from the post-tectonic intrusions and, along with other data, precludes the possibility of in situ formation of the intrusions by remelting of the country rocks. In one hypersthene migmatite sample collected near a post-tectonic intrusion, clear overgrowths and separate clear grains with low uranium concentrations were identified. The clear grains of zircon have the same age as the intrusion, indicative of new zircon growth during the granulite facies recrystallization of the gneiss. In contrast, rounded red zircons from an early subconcordant granitic sheet have clear uranium-rich overgrowths which probably formed during regional metamorphism. If the deep-red zircons observed in one of the post-tectonic intrusions were derived from the surrounding metamorphic rocks, they have had their U-Pb systems reset to the time of intrusion. In accord with observations (Krogh, 1971) on non-magnetic zircon fractions from volcanics, the Greenland zircons contain very low amounts of common lead ranging from |
