The age and tectonic significance of the Warraweena Volcanics and related rocks, southern Thomson Orogen

Magmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417 ± 3.5 and 414 ± 4.0 Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNdt values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411 ± 5.5 Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNdt values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty. The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian. Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100 km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system.