Eclogite and blueschist in the southern New England Fold Belt: P–T–t conditions and long-lived subduction on the Gondwanan eastern margin

Abstract

Eclogite and blueschist in the Tasminides represent clear evidence for subduction-related metamorphism on the Gondwanan eastern margin during the Palaeozoic. These eclogites and blueschists are located in the serpentinite-bearing Peel Manning Fault System in the Southern New England Fold Belt (SNEFB) of eastern Australia. U–Pb zircon and Lu–Hf and Sm–Nd multimineral geochronology give ages of ca. 490 Ma for lawsonite-bearing eclogite and ca. 470 Ma for garnet-bearing blueschist at Port Macquarie in the SNEFB, in agreement with Cambro-Ordovician dates for eclogite metamorphism across the eastern Gondwanan margin. In combination with this, Ar–Ar data dates exhumation and cooling in the subduction channel at ca. 460 Ma, suggesting that high-pressure metamorphism at Port Macquarie was continuously active for upwards of 40 Ma. This is supported by mineral equilibria forward modeling, which demonstrates that 24–27 kbar eclogite from Port Macquarie and Pigna Barney in the SNEFB experienced high-pressure but low-temperature retrograde paths, consistent with their residence in the subduction channel. Geochemical and isotopic results suggest that MORB as well as oceanic arc-related material was subducted and metamorphosed in a westwards dipping subduction system on the Australian cratonic margin during the ca. 515–490 Ma Delamerian Orogeny, and subsequently entrapped in the subduction channel during rollback. This rollback resulted in the development of a large backarc system on the upper plate in which the protoliths to the Lachlan Orogen accumulated, as well as on-going blueschist-facies metamorphism in the subduction channel. Ultimately, rollback led to translocation of subduction products and their exhumation over 2000 km eastwards to their current position in the New England orogen. In contrast to this, further south in Tasmania and Antarctica, the subduction of continental material led to rapid burial and exhumation of eclogite, representing differing styles of Cambro-Ordovician high-pressure metamorphism on the eastern Gondwanan margin.