Structural evolution and granite chronology of the central Molong Zone, Eastern Lachlan Fold Belt, Australia

Abstract

The details of the deformation of the Molong Zone after its development are poorly constrained because of the paucity of dates on significant structural features, the difficulty of mapping faults that are poorly exposed, and the limited understanding of the timing and kinematics on key faults. Bedding and foliation in the Ordovician metavolcanics and greywackes in the northern Molong Zone record pre-granite Benambran deformation and up to three post-granite foliation-forming events (Bowning – Bindian, Tabberabberan and Kanimblan). There appears to have been multiple cycles of foliation development along meridional trends. The granites were intruded along pre-existing faults and overall this area shows post-granite southward and anticlockwise movement during dominant east – west shortening. Fault movement occurred during sedimentation, prior to, during and after granite intrusion. Many major faults have a complex history involving a significant component of dextral strike-slip movement sometimes after normal, dip-slip histories. The granodiorites and granite intruded at different structural levels and were progressively uplifted during generally east-directed thrusting between the bounding Wongalong – Werribee and Copperhannia – Godolphin Fault Systems. Zircons from the Carcoar Granodiorite were dated using single crystal and SHRIMP dating methods and recorded a crystallisation age of 425.6 ± 2.3 Ma (mean) and 434.4 ± 5.5 Ma, respectively. Conventional U – Pb on three zircon grains from the Barry Granodiorite gave a crystallisation age of 425.8 ± 1.9 Ma. U – Pb in zircon SHRIMP age for the Sunset Hills Granite yielded a crystallisation age of 430.1 ± 3.7 Ma. The Ar – Ar hornblende radiometric ages for the granodiorites suggest that there is a 2 – 28 million years interval between granite crystallisation and hornblende closure. This is unlikely given these are high-level granites and suggests that intercalibration problems between the U – Pb and Ar – Ar radiometric methods may in part explain the apparent extended period between crystallisation and closure of hornblende.