Discrepancies between neodymium, lead and strontium model ages from the Precambrian of southern East Greenland: Evidence for a Proterozoic granulite-facies event affecting Archaean gneisses

SmNd (IDM) 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 $\text{Pb}\text{Pb}$ isochrons. The data have been used to calculate $\text{Pb}\text{Pb}$ 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, $\text{Pb}\text{Pb}$ model ages are much lower than SmNd ages, locally as low as ∼ 1.80 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.