TECTONIC SIGNIFICANCE OF 400 MA ZIRCON AGES FOR OPHIOLITIC ROCKS FROMTHE LACHLAN FOLD BELT, EASTERN AUSTRALIA

New SHRIMP ion microprobe data provide constraints on the age of forma tion of ophiolitic rocks from the Tumut region of the Lachlan fold bel t, southeastern Australia. Magmatic zircons from plagiogranites within both the Coolac serpentinite belt and Wambidgee serpentinite belt giv e Pb-206/U-238 ages of about 400 Ma. This Devonian age is considerably younger than those previously proposed for these ophiolitic racks (Ca mbrian-Ordovician to Early Silurian) and provides important tectonic d ata for this region of the Lachlan fold belt. We suggest that these ro cks 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 o f the Lachlan fold belt. We therefore suggest that the generation of b oth the ophiolitic rocks and granitic magmas of the Lachlan fold belt were part of the same major tectonothermal event.