A Neoproterozoic flood basalt province in southern-central Australia: geochemical and Nd isotope evidence from basin fill

Geochemical and Nd isotope studies of Neoproterozoic sedimentary successions from central and southern Australia are used to argue for the existence of a widespread flood basalt province emplaced in the Neoproterozoic, related in time to Rodinian supercontinent breakup. While major and minor element geochemistry supports a small and variable degree of input from a mafic source for at least some of the sedimentary rocks, Sm/Nd ratios and Nd isotope constraints do not allow simple physical mixing of end-member basalt and upper crustal basement terranes to achieve the relatively radiogenic epsilon(Nd) compositions of the rocks. Instead, preferential chemical mixing of the Nd composition of the end-member source regions is documented through chemical separation and analysis of different Nd reservoirs of the sedimentary material. Leaching tests undertaken to separate soluble/exchangeable Nd from that fixed in crystallographic sites of detrital phases indicate that the leachable portions of the higher initial epsilon(Nd) sedimentary samples were in isotopic equilibrium around the time of deposition in;a relatively radiogenic environment, such as that which would be produced by widespread chemical erosion of a heavily glacially scoured flood basalt province. This indication of a high Nd-143/Nd-144 diagenetic fluid or authigenic phase is in contrast to the isotopic results from separable portions of samples with an Nd isotope composition reflective of proximal basement that indicate equilibration between the two portions, detrital and diagenetic, at around the time of deposition of the sediments. Thus, although the near-shore shallow water sediments deposited in the Neoproterozoic did not, for the most part, record a physical detrital input of the flood basalt source (which, owing to its postulated weathered fine-grained nature, is seen only in the most fine-grained and slowly accumulated sedimentary units in such an environment), they do record the input of a more radiogenic component taken up by REE adsorption and/or sediment settling of REE-rich colloidal riverine material, derived from the easily chemically weathered basalt terrane. (C) 2000 Elsevier Science B.V. All rights reserved.