Sm-Nd isotopic evidence for the provenance of sediments from the Adelaide Fold Belt and southeastern Australia with implications for episodic crustal addition

Abstract

In South Australia, Late Proterozoic and Cambrian sediments were deposited in basins formed within Archaean to Early Mid-Proterozoic basement. These sequences were subsequently deformed and uplifted by the Cambro-Ordovician Delamerian Orogeny to form the Adelaide Fold Belt. In using the Adelaide Fold Belt to address models for lithospheric evolution, it is necessary to understand the mechanisms of basin formation and whether the sequences merely reflect reworking of existing cratonic material or if there were new crustal additions. This study presents geochemical and isotopic data on the sediments and basement rocks to complement existing data on the Delamerian igneous rocks as a means of investigating these questions. The data reveal that the Archaean-Mid Proterozoic basement rocks of the Gawler Craton have Nd depleted mantle model ages which average about 2.6 Ga and ϵNd values at 800 Ma in the range −9 to −24 or at 500 Ma from −11 to −28. The Late Proterozoic Adelaidean sediments, however, have an average model age of 1.9 Ga and initial ϵNd values in the range −5 to −11. The sequence can be divided into lowermost rift-phase sediments derived from erosion of local Gawler Craton material, and a subsequent, widely distributed, thermal-sag-phase sedimentation whose isotopic signature is not compatible with the basement. This implies that a more primitive or juvenile source was accessed via broadening of the provenance region. The Cambrian sediments have slightly older model ages (2.1 Ga) and lower ϵNd (−9 to −13) and can be interpreted as mixtures of basement and reworked Adelaidean detritus. The Early Palaeozoic sedimentary sequences of the Lachlan Fold Belt to the east have isotopic compositions very like the South Australian Late Proterozoic and Cambrian sequences. They have an average model age of 1.85 Ma but much higher K 2O/Na 2O. This indicates that the widespread flysch of eastern Australia is the result of recycling of the Late Proterozoic and Cambrian sediments by erosion of the Adelaide Fold Belt, with some modification by the mixture of the Cambro-Ordovician mafic and felsic magmatic rocks in this orogen. Our data suggest regional, intracontinental, crustal evolution combined with recycling of older crustal material with new additions from the mantle during orogenesis and, to a lesser extent, episodes of extension and sedimentation.