SM-ND AND U-PB ZIRCON ISOTOPIC CONSTRAINTS ON THE PROVENANCE OF SEDIMENTS FROM THE AMADEUS BASIN, CENTRAL AUSTRALIA - EVIDENCE FOR REE FRACTIONATION

The Amadeus Basin of central Australia is a Late Proterozoic to Late Palaeozoic ensialic depositional basin located between two Proterozoic basement blocks of different Nd crustal formation ages, the older Arunta Block (T(DM)Nd = 2.0 to 2.2 Ga, U-Pb zircon ages = 1.5-1.9 Ga) to the north and the younger Musgrave Block (T(DM)Nd = 1.7 to 1.9 Ga, U-Pb zircon ages = 1.0-1.7 Ga) to the south. Initial Nd isotopic compositions of the Amadeus Basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks, indicating that the sediments are dominated by essentially two source components: the older Arunta Block [Sm-147/Nd-144 almost-equal-to 0.114 and epsilon(Nd)(0) almost-equal-to -20.4] and younger Musgrave Block [Sm-147/Nd-144 almost-equal-to 0.118 and epsilon(Nd)(0) almost-equal-to -14.7] and/or their equivalents. Stratigraphically higher sediments plot more closely to the evolutionary trajectory of the Musgrave Block. This result, together with the average provenance ages, indicates the proportion of material from the Musgrave Block increases as the sediments become younger. The Sm-Nd isotopic data preclude a substantial involvement of Archaean sources. U-Pb zircon ion probe analyses of the Late Proterozoic Heavitree Quartzite and the late Cambrian Goyder Formation are consistent with the Sm-Nd isotopic constraints. Most of the zircons from the basal Heavitree Quartzite give concordant U-Pb zircon ages of 1500 to 1900 Ma, consistent with derivation from an Arunta-type source, whilst zircons from the younger Goyder sandstone give six discrete U-Pb zircon age groups of 511 +/- 20, 615 +/- 15, 960 +/- 109, 1190 +/- 54, 1633 +/- 24, and 1878 +/- 48 Ma, indicative of a dominantly Musgrave-type source with a minor contribution from the Arunta Block or reworked preexisting Amadeus Basin sediments. The two youngest U-Pb zircon ages probably indicate some contribution from Late Proterozoic-Cambrian volcanics in central Australia or a previously unrecognised younger local source in the basement blocks. The 511 +/- 20 Ma constrains the maximum depositional age of the sediment. The Amadeus Basin sediments have a surprisingly wide range of Sm-147/Nd-144 ratios (0.077-0.136), irrespective of grain size and rock types. In addition, the calculated T(DM)Nd values for samples from the same stratigraphic unit are well correlated with Sm-147/Nd-144 ratios, with samples of different stratigraphic units forming sub-parallel arrays. These phenomena are interpreted in terms of REE fractionation and preferential sorting of preexisting REE-rich phases during sedimentary recycling and deposition. This process can lead to either an increase or a decrease in Sm-147/Nd-144 ratio of a sediment relative to its source and therefore an erroneous estimate of its T(DM)Nd provenance age. A theoretical model is developed which accounts for the observed correlation between T(DM)Nd and Sm-147/Nd-144 ratios for suites of stratigraphically related sediments, and allows a reliable estimate to be made of the provenance age of individual stratigraphic units, assuming the average Sm-147/Nd-144 ratio of the source is known or can be estimated. The provenance age for each stratigraphic unit of the Amadeus Basin has been calculated using this method. The Sm-Nd isotopic systematics in the Amadeus Basin sediments suggest that (1) REE fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered; (2) the provenance of the Amadeus Basin sediments was controlled by local source regions, and cannot be used to infer large-scale continental averages. Hence, some caution must be evoked in using Sm-Nd isotopic constraints from restricted sampling for more generalised models of crustal evolution.