Middle Carboniferous-Early Triassic eclogite-blueschist blocks within a serpentinite mélange at Port Macquarie, eastern Australia: implications for the evolution of Gondwana's eastern margin

Allen P. Nutman*, Solomon Buckman, Hiroshi Hidaka, Tomoyuki Kamiichi, Elena Belousova, Jonathan Aitchison

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)


The New England Orogen of easternmost Australia is dominated by suites of Palaeozoic to earliest Mesozoic rocks that formed in supra-subduction zone settings at Gondwana's eastern margin. On the northern New South Wales coast at Rocky Beach, Port Macquarie, a serpentinite mélange carries rare tectonic blocks of low-grade, high-pressure, metamorphic rocks derived from sedimentary and igneous protoliths. Dominant assemblages are glaucophane. +. phengite. ±. garnet. ±. lawsonite. ±. calcite. ±. albite blueschists and lawsonite-bearing retrogressed garnet. +. omphacite eclogites. In some blocks with sedimentary protoliths, eclogite forms folded layers within the blueschists, which is interpreted as Mn/(Mn. +. Fe) compositional control on the development of blueschist versus eclogite assemblages. Review of previous studies indicates pressure-temperature conditions of 0.7-0.5. GPa and ≤. 450. °C. Three samples of high-pressure metasedimentary rocks contain Archaean to 251. ±. 6. Ma (Permo-Triassic) zircons, with the majority of the grains being Middle Devonian to Middle Carboniferous in age (380-340. Ma). Regardless of age, all grains show pitting and variable rounding of their exteriors. This morphology is attributed to abrasion in sedimentary systems, suggesting that they are all detrital grains. New in situ metamorphic zircon growth did not develop because of the low temperature (≤. 450. °C) of metamorphism. The Permo-Triassic, Devonian and Carboniferous zircons show strong heavy rare earth element enrichment and negative europium anomalies, indicating that they grew in low pressure igneous systems, not in a garnet-rich plagioclase-absent high pressure metamorphic environment. Therefore the youngest of these detrital zircons provides the maximum age of the metamorphism. A titanite. +. rutile porphyroblast within an eclogite has a U-Pb age of 332. ±. 140. Ma (poor precision due to very low U abundances of mostly <. 1. p.p.m.) and provides an imprecise direct age for metamorphism. In the south of the Port Macquarie area, the Lorne Basin ≥. 220. Ma Triassic sedimentary and volcanic rocks unconformably overlie serpentinite mélange, and provide the minimum age of the high-pressure metamorphism. Our preferred interpretation is that the 251. Ma zircons are detrital and thus the Port Macquarie high-pressure metamorphism is constrained to the end of the Permian-Early Triassic. Emplacement of the serpentinite mélange carrying the Rocky Beach high-pressure rocks might have been due to docking of a Permian oceanic island arc (represented by the Gympie terrane in southern Queensland?) and an Andean-style arc at the eastern Australian margin (expressed in the New England Orogen by 260-230. Ma north-south orientated magmatic belts). Alternatively, if the 251. Ma grains are regarded as having grown in thin pegmatites, then the dominant Devonian-Carboniferous detrital population still indicates a maximum age for the high pressure metamorphism of ca. 340. Ma. A ≤. 340. Ma age of metamorphism would still be much younger than the previously suggested ca. 470. Ma (Ordovician) age, which was based on Ar-Ar dating of phengites.

Original languageEnglish
Pages (from-to)1038-1050
Number of pages13
JournalGondwana Research
Issue number3-4
Publication statusPublished - Nov 2013


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