Geochemistry of Precambrian gneisses: relevance for the evolution of the East Antarctic Shield

Archaean granulite-facies orthogneisses of the Napier Complex in Enderby Land, metamorphosed 3070 Maago, comprise two chemically distinct suites. The more abundant, mainly of tonalitic to granodioritic composition, shows strong Y depletion, explicable by hydrous partial melting of a garnet-bearing source (garnet amphibolite or possibly eclogite); it apparently represents new continental crust. Other gneisses (predominantly of trondhjemitic to granitic (s.s) composition) do not show Y depletion, and have higher TiO₂, Zr, Nb, La, Ce and Ga/Al, and lower CaO, Sr and Mg/(Mg + total Fe); they probably originated by relatively dry melting of predominantly felsic crystal rocks. Both suites show evidence for loss of Rb (relative to K), Th, and U during metamorphism. Late Archaean (?2800 Ma) amphibolite-facies gneisses of MacRobertson Land are of ‘undepleted’ type and may be representative of a higher crustal level than those of Enderby land. Late Proterozoic (1000 Ma) granulite-facies gneisses of Enderby Land (Rayner Complex) are to a large extent remetamorphosed Napier Complex rocks of igneous derivation; in contrast, gneisses of similar age in MacRobertson Land include a much higher proportion derived, either directly or by partial melting, from sedimentary protoliths.