RIS ID
82597
Abstract
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