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

The sedimentary provenance of Palaeozoic turbidites in the southern La chlan Orogenic Belt is determinable by comparing the mean Sr isotopic ratios of the turbidites with those of potential provenance areas at t he 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 obta inable by pooling data in the geochronological literature. Sr isotopic data exist for turbidites of Early Ordovician, Late Ordovician and De vonian age located in northeastern Victoria, southeastern New South Wa les and northeastern Tasmania, respectively. All Precambrian provenanc e areas have mean Sr-87/Sr-86 ratios that are too high to be equated w ith those of the turbidites. The turbidites contain Sr with a relative ly 'juvenile' isotopic composition and the only possible equivalent se diment sources are Cambrian sedimentary rocks, such as the Kanmantoo G roup in South Australia, and igneous rocks as in western Tasmania. The palaeogeography of turbidite deposition involved a mountain belt deve loped during the Late Cambrian-Early Ordovician Delamerian Orogeny, wh ich acted as a barrier to sediment transport from the continental inte rior, and shed detritus into the turbidite basin of a passive continen tal margin.