PETROGENESIS OF LATE-STAGE MAGMATISM AT HOLD WITH HOPE, EAST GREENLAND

The Myggbukta caldera complex and a swarm of basic dykes constitute th e latest Tertiary magmatism in the Hold with Hope region, East Greenla nd. The Sr and Nd isotope ratios of these rocks show coherent variatio ns which extend to high Sr-87/Sr-86 and low Nd-143/Nd-144 values and r equire a contribution from continental lithosphere. Broad correlations with major element differentiation indices suggest that the continent al component was incorporated during magmatic differentiation thereby favouring a crustal contamination process. Trace element concentration s are strongly correlated with isotopic compositions but display range s for many incompatible elements which extend beyond likely crustal co ntaminant compositions. This is readily modelled by AFC processes in w hich the dominant cause of trace element enrichment is the concentrati on effect of fractional crystallisation rather than the composition of the contaminant. The simplest such models still require unrealistical ly high degrees of fractional crystallisation to explain the ten-fold enrichment of some trace elements, This can be overcome if the primary magmas entering the crust already had highly variable trace element c ompositions. Such variability is readily achieved if melts from differ ent parts of the melting column escape without thorough homogenization . An AFC model which incorporates variability in parental magma compos ition is then able to simulate the range of compositions observed at H old with Hope. This carries the implication that the Variations observ ed are more readily attributed to changes in uncontaminated parental m agma than to variations in the composition or amount of contaminant.