Relationships between Archaean and Palaeoproterozoic crystalline basement complexes in the southern part of the East Greenland Caledonides: an ion microprobe study

The Caledonian orogenic belt in East Greenland encompasses rock complexes with a long-lived evolution of structural and metamorphic events. Due to the strong Caledonian overprint in the region it has hitherto proved difficult to unravel the earlier history. New U-Pb zircon analyses have revealed that the crystalline basement complexes can be divided into Archaean and Palaeoproterozoic terrains. The former terrain yields U-Pb ages between approximate to 2700 and 2800 Ma, while the latter yields ages between approximate to 1900 and 2000 Ma. New Nd isotopic analyses suggest that the late Archaean terrain contains juvenile crust together with gneisses incorporating various amounts of slightly older continental crust (3000 Ma). The Archaean basement terrain is exposed in the southern part of the study area, south of 72 degrees 50'N; it is cut by Palaeoproterozoic intrusions ranging in size from centimetre-scale dykes to kilometre-size plutons. A low-strain lens in the Archaean basement preserves an approximate to 2700-Ma-old leucosome granite. North of 72 degrees 50'N the basement terrain is Palaeoproterozoic (approximate to 2000 Ma) in age, part of a major Palaeoproterozoic province that extends all the way up to 81 degreesN. In the study area (72-73 degreesN) it is not possible to differentiate between the two basement terrains on field criteria. Geochemical data suggest that the transition from Archaean to Palaeoproterozoic basement is a result of Palaeoproterozoic subduction of oceanic crust beneath the Archaean basement. No Caledonian zircons or zircon rims were found in the crystalline basement rocks suggesting that conditions for new zircon growth were not optimal during Caledonian reworking, or that the Caledonian overprint was weak in the western part of the fold belt. (C) 2002 Elsevier Science B.V. All rights reserved.