Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U-Pb ages mainly in the range 1100-1800 Ma but with a few grains of c. 1000 Ma, different from zircon ages (mainly 1800-2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 1000-1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900-600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c. 930 Ma are related to a 'Grenvillian' or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a 'Grenvillian' orogen in East Greenland would be of very different character than that in North America and southern Scandinavia.
- Programområde 4: Mineralske råstoffer