GEOCHEMICAL AND SM-ND ISOTOPIC STUDY OF AMPHIBOLITES IN THE SOUTHERN ARUNTA INLIER, CENTRAL AUSTRALIA - EVIDENCE FOR SUBDUCTION AT A PROTEROZOIC CONTINENTAL-MARGIN

Geochemical and Sm-Nd isotopic results are reported for mafic amphibol ites from the Alice Springs area, southern Arunta Inlier, central Aust ralia. The data, combined with those previously reported from the sout heastern margin of the Arunta Inlier, provide important constraints on the tectonic settings and crustal evolution of the region during the Palaeoproterozoic. Based on geochemical and Nd isotopic signatures and field relationships, two suites of amphibolites (Groups 1 and 2) are recognised. Group 1 rocks, which occur mainly in the south, are charac terised by fiat to LREE-depleted patterns and high epsilon(Nd)(T) valu es of +4.2 to +5.1 (T=1770 Ma), whilst Group 2 rocks, which crop out m ainly in the north, show LREE-enriched patterns and low epsilon(Nd)(T) values of - 1.0 to - 2.8. Both groups are depleted in HFS elements an d selectively enriched in LIL elements and fall broadly in the island- are basalt fields on a number of discrimination diagrams. The geochemi cal and Nd isotopic features are consistent with the magmatic precurso rs of the amphibolites being derived by 20-30% partial melting of a ma ntle wedge variably metasomatised by an enriched subduction component in an island-arc or/and back-are basin regime. Partial melting of the wedge metasomatised by LIL-enriched slab fluids is considered to be re sponsible for the generation of the Group 1 amphibolites. Partial melt ing of the wedge metasomatised by LIL- and LREE-enriched slab melt is proposed for the origin of the LREE-enriched Group 2 amphibolites. Mod el calculation suggests that about 1-2% LREE-enriched slab component e ntering the mantle wedge source region would be capable of producing t he REE and Nd isotopic signatures observed in the Group 2 rocks. Geoch emical and Nd isotopic features of the Group 2 rocks suggest they or o ther similar mafic rocks are probably the best candidate for the sourc e of the Proterozoic granites in the region. This study, combined with a number of earlier geochemical investigations in the region, indicat es that the Arunta Inlier, at least its southeastern margin, probably represented an island-arc/back-arc basin system in the Palaeoproterozo ic, possibly analogous to the Proterozoic belts in the southwestern Un ited States.