AGE PROFILE OF OPHIOLITIC ROCKS ACROSS THE LATE PALEOZOIC NEW-ENGLANDOROGEN, NEW-SOUTH-WALES - IMPLICATIONS FOR TECTONIC MODELS

Zircon U-Pb ages have been determined from various ophiolitic and asso ciated rocks across the southern part of the New England Orogen in nor thern New South Wales. Ophiolitic rocks of the Weraerai terrane origin ated during the earliest Cambrian and are the oldest rocks known from this region. The Weraerai terrane is tectonically juxtaposed against S iluro-Devonian Gamilaroi and Djungati terranes but no unambiguous pre- Permian links exist between these three terranes. Age and geochemical characteristics of the Weraerai terrane are consistent with its interp retation as a low Ti-tholeiitic supra-subduction-zone ophiolite, analo gous to those within the Lachlan Fold Belt located to the west of the New England Orogen. A felsic volcanic rock which occurs as a tectonic block in melange along the Peel Fault zone is geochemically identical to intra-oceanic island are rocks of the Gamilaroi terrane, the only l ikely nearby source. U-Pb ages for this rock indicate are development in the Silurian. Zircon ages from a plagiogranite within ophiolitic ro cks of the Yarras Complex indicate that it is Devonian. The occurrence of Devonian supra-subduction zone ophiolitic rocks at Yarras attests to are rifting within the associated Birpai terrane prior to its accre tion to the Gondwana margin. Zircons from felsic intrusives within a h ighly disrupted 'ophiolitic' assemblage at Port Macquarie appear to be entirely detrital with no magmatic component recognized. The youngest zircons are Triassic with other age components present at 300-400 Ma (associated with magmatic activity in southeastern Australia) as well as 500-600, 1000-1200 and up to Archaean ages. The latter are typical of inheritance in Ordovician Lachlan Fold Belt turbidites and granites . Port Macquarie zircon ages are consistent with a scenario in which L achlan Fold Belt-type rocks underlie a thin skin of New England Orogen terranes accreted to, and thrust westwards over, the eastern margin o f Gondwana during the Palaeozoic.