Age profile of ophiolitic rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models

Abstract

Zircon U‐Pb ages have been determined from various ophiolitic and associated rocks across the southern part of the New England Orogen in northern New South Wales. Ophiolitic rocks of the Weraerai terrane originated during the earliest Cambrian and are the oldest rocks known from this region. The Weraerai terrane is tectonically juxtaposed against Siluro‐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 interpretation as a low Ti‐tholeiitic supra‐subduction‐zone ophiolite, analogous 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 arc rocks of the Gamilaroi terrane, the only likely nearby source. U‐Pb ages for this rock indicate arc development in the Silurian. Zircon ages from a plagiogranite within ophiolitic rocks of the Yarras Complex indicate that it is Devonian. The occurrence of Devonian supra‐subduction zone ophiolitic rocks at Yarras attests to arc rifting within the associated Birpai terrane prior to its accretion to the Gondwana margin. Zircons from felsic intrusives within a highly 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 Lachlan Fold Belt‐type rocks underlie a thin skin of New England Orogen terranes accreted to, and thrust westwards over, the eastern margin of Gondwana during the Palaeozoic.