Xenoliths of gneisses and the conformable,clot-like granophyres in the Marginal Border Group,Skaergaard intrusion,East Greenland

Results of this research and the earlier work of Wager and his colleagues indicate that contamination influenced the overall character of the Marginal Border Group of the Skaergaard intrusion. Precambrian gneisses were a major source of contamination and are identifiable as xenoliths in the Marginal Border. A variety of other xenoliths occur with a wide compositional range.The range of K _D values for partitioning of Fe and Mg in coexisting pyroxene pairs in xenoliths from various parts of the Marginal Border, and in hornfelses adjacent to it, is consistent with temperatures that rose steeply inward. Temperatures estimated on the basis of compositions of coexisting pyroxenes are also consistent with dehydration that exceeded the stability of amphibole (850°C). The texturally compatible association of some granophyres with gneissic xenoliths suggests that both formed during melting. These observations suggest that there was a similar range of temperature for formation of xenoliths and granophyres.The xenoliths of gneisses have bulk rock compositional features which indicate that lithophilic constituents were removed, causing an increase in basic constituents upon assimilation of the gneissic precursors. If we assume that granophyres and xenoliths of gneisses represent a consanguineous set formed by a fusion process from gneiss, enrichment and depletion factors for excluded trace elements are complementary and generally less than 10. Allowable enrichment in granophyres by crystal fractionation using the average for a large number of chilled marginal gabbro analyses as an initial composition, is also less than 10 for the same elements. The calculated low factors for enrichment of lithophile elements in granophyres by both mechanisms favor the hypothesis of partial melting of gneiss.