PROVENANCE OF PALEOZOIC TURBIDITES IN THE LACHLAN OROGENIC BELT - STRONTIUM ISOTOPIC EVIDENCE

The sedimentary provenance of Palaeozoic turbidites in the southern Lachlan Orogenic Belt is determinable by comparing the mean Sr isotopic ratios of the turbidites with those of potential provenance areas at the time of sedimentation. The possible provenances encompass rocks of Precambrian to Cambrian age extending from central South Australia to western Tasmania and estimates of their isotopic compositions are obtainable by pooling data in the geochronological literature. Sr isotopic data exist for turbidites of Early Ordovician, Late Ordovician and Devonian age located in northeastern Victoria, southeastern New South Wales and northeastern Tasmania, respectively. All Precambrian provenance areas have mean Sr-87/Sr-86 ratios that are too high to be equated with those of the turbidites. The turbidites contain Sr with a relatively 'juvenile' isotopic composition and the only possible equivalent sediment sources are Cambrian sedimentary rocks, such as the Kanmantoo Group in South Australia, and igneous rocks as in western Tasmania. The palaeogeography of turbidite deposition involved a mountain belt developed during the Late Cambrian-Early Ordovician Delamerian Orogeny, which acted as a barrier to sediment transport from the continental interior, and shed detritus into the turbidite basin of a passive continental margin.