Abstract
Sm-Nd ( t DM) model ages for gneisses from the coastal region between 66° and 68°N in southern East Greenland range from 3.02 to 2.79 Ga and indicate that these basement rocks were formed in a major late Archaean episode of sialic crust formation between 3.0 and 2.8 Ga ago. Very low concentrations of U have resulted in unradiogenic Pb-isotopic compositions, so that most samples do not yield chronologically useful Pb/Pb isochrons. The data have been used to calculate Pb/Pb model ages, and in the northern and southern parts of the study area these are in good agreement with the Sm-Nd model ages. In the central part of the area, however, Pb/Pb model ages are much lower than Sm-Nd ages, locally as low a.80 Ga. This discrepancy is attributed to loss of U long after the crust formation event, and probably related to a phase of Proterozoic granulite-facies metamorphism that has not previously been recognised. Rb-Sr isotope data support this interpretation.
Published Version
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