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

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 m lange 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 lawsonitebearing 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 pressuretemperature conditions of 0.7-0.5 GPa and <= 450 degrees 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 degrees 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 m lange, 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 m lange 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. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.