New SHRIMP ion microprobe data provide constraints on the age of formation of ophiolitic rocks from the Tumut region of the Lachlan fold belt, southeastern Australia. Magmatic zircons from plagiogranites within both the Coolac serpentinite belt and Wambidgee serpentinite belt give 206Pb/238U ages of about 400 Ma. This Devonian age is considerably younger than those previously proposed for these ophiolitic rocks (Cambrian-Ordovician to Early Silurian) and provides important tectonic data for this region of the Lachlan fold belt. We suggest that these rocks formed within a narrow marginal basin akin to those of Cretaceous age within the Chilean Andes of South America. The dates presented are within the 420–390 Ma range assigned to most of the granitic bodies of the Lachlan fold belt. We therefore suggest that the generation of both the ophiolitic rocks and granitic magmas of the Lachlan fold belt were part of the same major tectonothermal event.