SM-ND ISOTOPIC EVIDENCE FOR THE PROVENANCE OF SEDIMENTS FROM THE ADELAIDE FOLD BELT AND SOUTHEASTERN AUSTRALIA WITH IMPLICATIONS FOR EPISODIC CRUSTAL ADDITION

In South Australia, Late Proterozoic and Cambrian sediments were depos ited in basins formed within Archaean to Early Mid-Proterozoic basemen t. These sequences were subsequently deformed and uplifted by the Camb ro-Ordovician Delamerian Orogeny to form the Adelaide Fold Belt. In us ing the Adelaide Fold Belt to address models for lithospheric evolutio n, it is necessary to understand the mechanisms of basin formation and whether the sequences merely reflect reworking of existing cratonic m aterial or if there were new crustal additions. This study presents ge ochemical and isotopic data on the sediments and basement rocks to com plement existing data on the Delamerian igneous rocks as a means of in vestigating these questions. The data reveal that the Archaean-Mid Pro terozoic basement rocks of the Gawler Craton have Nd depleted mantle m odel ages which average about 2.6 Ga and epsilonNd values at 800 Ma in the range -9 to -24 or at 500 Ma from -11 to -28. The Late Proterozoi c Adelaidean sediments, however, have an average model age of 1.9 Ga a nd initial epsilonNd values in the range -5 to -11. The sequence can b e divided into lowermost rift-phase sediments derived from erosion of local Gawler Craton material, and a subsequent, widely distributed, th ermal-sag-phase sedimentation whose isotopic signature is not compatib le with the basement. This implies that a more primitive or juvenile s ource was accessed via broadening of the provenance region. The Cambri an sediments have slightly older model ages (2.1 Ga) and lower epsilon Nd (-9 to -13) and can be interpreted as mixtures of basement and rewo rked Adelaidean detritus. The Early Palaeozoic sedimentary sequences o f the Lachlan Fold Belt to the east have isotopic compositions very li ke the South Australian Late Proterozoic and Cambrian sequences. They have an average model age of 1.85 Ma but much higher K2O/Na2O. This in dicates 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 t he Cambro-Ordovician mafic and felsic magmatic rocks in this orogen. O ur 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 extensio n and sedimentation.