Radiometric evidence for Middle Devonian inversion of the Hill End Trough,northeast Lachlan Fold Belt

The publication of a new geological time-scale by the Australian Geological Survey Organisation and radiometric dates from the Hill End goldfield have prompted the re-examination of the timing of deformation of the Hill End T rough to determine whether it occurred in Middle Devonian or Early Carbonif erous time. Palaeontological evidence from the western trough margin and th e Capertee High dates the end of deposition in the trough as late Emsian or early Elfelian (385-382 Ma). After a mid-Devonian hiatus of at least 15 mi llion years, paralic sedimentation commenced on the Molong and Capertee Hig hs in late Frasnian or early Famennian time (367-363 Mo). No Upper Devonian sedimentary formations occur in the Hill End Trough. Structural relationsh ips indicate that the oldest mineral veins at Hill End preceded cleavage fo rmation in the deformed trough sedimentary rocks. Early Vein muscovites hav e Middle Devonian Ar-41/Ar-39 dates of 380-370 Ma. Regional metamorphic bio tites from Hill End have well constrained Ar-40/Ar-39 closing ages of 360-3 58 Ma (mid-Famennian). The metamorphic (thermal) maximum which outlasted pe netrative deformation, is estimated here by modelling to have been about 37 0 Ma (latest Givetian). This clearly places the earlier main deformation in the Middle Devonian. Deformation probably began by terminating trough depo sition in latest Emsian to early Elfelian time and ended in early Givetian time at about 375 Ma ago, Published pressure and temperature data from the Hill End goldfield suggest that deformation thickened the 6 km sediment col umn to around 11 km. The thermal model suggests there was post-deformation erosion of about 4 km and little if any further erosion occurred during Lat e Devonian to Early Carboniferous time. The shortening accompanying the inv ersion of the northern Hilt End Trough may have been taken up in the region to the south, both east and west of the Copperhannia Thrust, and east of t he southern termination of the Capertee High. The general conclusions are n ot affected by a new Devonian time-scale discussed in Appendix 2.