DEVELOPMENT OF THE EARLY PALEOZOIC PACIFIC MARGIN OF GONDWANA FROM DETRITAL-ZIRCON AGES ACROSS THE DELAMERIAN OROGEN

Detrital-zircon age spectra have been determined for sedimentary rocks from the Delamerian orogen, southern Australia. In Neoproterozoic sed imentary rocks, patterns progressively change from Mesoproterozoic- to Neoproterozoic-dominated detritus and there are few zircons that are close to the depositional age. The base of the Cambrian Kanmantoo Grou p marks an abrupt change in provenance to detrital patterns dominated by Ross and Delamerian (600-500 Ma) and Grenvillean ages (1200-1000 Ma ), These patterns are strikingly similar to those obtained from Lachla n fold belt sedimentary rocks, indicating that the sedimentation recor ded in the Kanmantoo Group marks a change from deposition of sediments derived from the Australian cratons to those representative of the ea rly Paleozoic Gondwana mudpile. If sedimentary rocks with zircon-prove nance characteristics such as those of the Kamnantoo rocks extend unde r elements of the Lachlan fold belt, they mould provide suitable proto liths for the S-type granites of southeastern Australia.