Zircons from the Wambidgee Serpentinite Belt, southern Lachlan Orogen: evidence for oceanic crust at the Cambrian–Ordovician boundary

Ophiolitic rocks of the Lachlan Orogen are important because they can provide key information used to refine existing tectonic models for the orogen. This study focuses on the serpentinite belts within the greater Tumut region of the Lachlan Orogen in southeastern Australia. U–Pb, Lu–Hf, O isotopic and trace-element data collected on zircons from a plagiogranite of the Wambidgee Serpentinite Belt are consistent with their crystallisation from mantle-derived melts. Their juvenile Hf-isotope compositions, with εHf ranging from +7 to +14, and δ¹⁸O values ranging from 4.35 to 5.07‰, are close to typical mantle values and clearly within the range defined for zircons from plagiogranites of ophiolitic complexes worldwide. When combined with zircon data available for the Coolac Serpentinite and recent petrogenetic modelling for the Coolac Serpentinite Belt, a new U–Pb age of 486 ± 3 Ma of the Wambidgee zircons constrains the age of formation of oceanic lithosphere in the greater Tumut region to Cambro-Ordovician.KEY POINTS An age of 486 ± 3 Ma is reported for zircons from plagiogranite of the Wambidgee Serpentinite Belt in the Tumut region, southern Lachlan Orogen. We tentatively conclude that all serpentinite belts of the Tumut region may be of the same age, at the Cambrian–Ordovician boundary. This Cambro-Ordovician age provides the best estimate for oceanic crust formation in the greater Tumut region.