ND ISOTOPE STUDY OF GRANITES FROM THE ARUNTA-INLIER, CENTRAL AUSTRALIA - CONSTRAINTS ON GEOLOGICAL MODELS AND LIMITATION OF THE METHOD

Three chemically distinct types of granite (''high Sr/low Y'' calc-alkaline, Palaeoproterozoic ''normal'' and ''enriched'') are present in the Palaeoproterozoic Arunta Inlier of central Australia. Twenty-one representative samples were selected for Nd isotopic analysis to complement geological and geochemical data. Most samples analysed have depleted-mantle Nd isotope model ages (T-DM, following McCulloch, 1987) of 2.3 to 2.1 Ga, similar to most of Palaeoproterozoic granites of northern Australia (McCulloch, 1987). These ages do not show a younging trend from the Northern to Southern provinces, although some post similar to 1750 Ma granites from the Central and Southern provinces, with typical felsic crust Sm-147/Nd-144 ratios (0.09-0.11), have younger model ages (less than 2.0 Ga). Post 1700 Ma, ''normal-type'' granites from the Southern Province have chemical compositions similar to older granites (1880-1760 Ma) of the same suite, even though they have younger model ages (1960-1930 Ma). This implies an increase in a younger component within southern source regions through melting of new underplate and/or additional mantle input. The syn- or post-tectonic granites (1720-1140 Ma) of the ''enriched-type'' (high heat producing granites rich in Th, U and K) have similar T-DM model ages to older granites of the ''normal-type'' from the same areas. The ''enriched-type'' granite from the Teapot Granite Complex of the Southern Province are characterised by low Cs, Sr and high Rb/Cs (30-50), Th/U (7-20) ratios and high Y contents. These chemical characteristics are consistent with magma being formed through anatexis of ''normal-type'' granite sources. In contrast, an 1820 Ma, S-type granite from the Harverson suite in the Northern Province has an older T-DM model age (2.32 Ga) than the T-DM 2.18 Ga age of the nearby Aileron Metamorphics, suggesting that the source rock of the granite may contain a significant Archaean component. The Nd isotope data, when integrated with geological and chemical information, are compatible with the recycling of Archaean crustal material. Such a process might have taken place very early in the history of the Arunta Inlier through subduction along a continental margin made up of a stretched, dismembered, thin Archaean basement. Magmatic underplating may have taken place shortly before 1850 Ma and continued episodically during subsequent tectono-thermal events. Crustal melts of mixed source rocks, formed during these events, could contain different amounts of an Archaean component.