Rare-earth element geochemistry of Archaean orthogneisses and evolution of the East Antarctic shield.

Abstract

Two distinct suites of Archaean (approx 3100 m.y.) granulite-facies orthogneisses, predominantly of intrusive origin, have been recognized in the Napier complex of Enderby Land. One, mainly of tonalitic to granodioritic composition, is strongly depleted in Y and HREE, and is characterized by positive Eu anomalies. It probably represents new continental crust derived by hydrous partial melting of a hornblende and/or garnet-bearing mafic source. A younger 'undepleted' suite, of trondhjemitic to granitic composition, has higher TiO2, P2O5, Y, Zr, Nb, REE and Ga/Al, lower CaO, Sr, Ti/Y, Ce/Y and Mg/(Mg + SIGMA Fe), and negative Eu anomalies. It originated by relatively dry, lower-P melting of felsic crustal rocks. Compositional differences between depleted tonalites and granodiorites are almost entirely confined to K, Rb, Ba, Pb, Th and U, and can be explained by interaction of either the source material or the magma (during generation or, less likely, emplacement) with an LIL-element-enriched fluid phase. Depleted granitic rocks may be representative of a deeper crustal level than undepleted granitic rocks, which, together with a variety of metasediments, predominate in adjacent lower-grade (amphibolite to low granulite-facies) late Archaean and Proterozoic terrains. The Archaean granitic rocks are dominated by two I-type suites, whereas those of post-Archaean age are largely undepleted I-type or S-type.-C.N. Div. of Petrology & Geochemistry, BMR, Constitution Ave. Parkes, Canberra, ACT 2601, Australia. Show more