Two early granitic units from the Ketilidian mobile belt of south Greenland yield a combined U-Pb discordia age of 1,840 ± 25 m.y. that is believed to date the peak of metamorphism; U-Pb zircon and Rb-Sr whole-rock isochron ages from four late Ketilidian granitic units indicate a widespread intrusive episode at 1,780 ± 20 m.y. B.P. Rb-Sr mineral ages agree with many recently published K-Ar mineral ages that are clustered around 1,600 m.y. B.P. The Rb-Sr and K-Ar ages evidently refect the final cooling in the mobile belt. The low initial 87Sr/86Sr ratio for the early granites (0.7022 ± 0.0010 for the two units) is probably not significantly different from that of the later granites (0.7032 ± 0.0005 for the four units).

Although field evidence suggests that the early Ketilidian granites were derived from older basement, the ages and Sr isotopic evidence do not confirm this. Different interpretations for the origin of the granites are presented, and, in each of these, the evidence points to extensive igneous differentiation that appears to be connected with large-scale remelting, crustal generation, and igneous activity in the North Atlantic and Baltic Shield regions between 1,950 and 1,700 m.y. ago.

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