Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia∗

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Abstract

In easternmost Australia, the New England Orogen contains a geological record dominated by subduction-related rocks, with plate convergence during the Late Devonian to Triassic being related to a west-dipping subduction system, assuming present-day orientation, at the boundary of eastern Gondwanaland and the Panthalassan Ocean. A well-preserved Late Paleozoic accretionary wedge contains deep-marine turbidites deposited as trench fill, plus infaulted slices of oceanic crust. The turbidites are mostly first-cycle, immature, quartz-poor, volcanic-derived sedimentary rocks, some of which contain detrital hornblende, along with less-common quartz-rich sandstones to the east. In this study, detrital zircons from sandstones in various tectonic blocks of the New England Orogen are dated by the U–Pb SHRIMP and LA-ICPMS techniques and detrital hornblendes by the Ar–Ar technique to constrain the age and provenance of sedimentary rocks in the accretionary wedge. All samples, except two quartz-rich sandstones from the northern Shoalwater Formation, have maximum depositional ages of 355–316 Ma, indicating that the accretionary wedge evolved over a period of at least 40 Ma, with principal sources from a contemporaneous active continental margin volcanic arc. Quartz-rich sandstones from the easternmost part of the accretionary wedge (Shoalwater Formation and eastern Beenleigh Block) contain a greater range of individual detrital zircon ages from Late Paleozoic to Archean (several individual grains >3000 Ma). These ages indicate that, although detritus from Carboniferous volcanic arc sources was involved, quartz-rich detritus mostly derived from the continental interior dominated the depocentres. We suggest that these quartz-rich sandstones accumulated from longitudinal transport along the trench, like the modern-day Barbados Ridge accretionary wedge, along with breaching of the marginal arc by streams draining the continental interior.

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