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

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.

Key Words:

• accretionary wedge
• argon–argon dating
• Carboniferous
• geochronology
• New England Orogen
• provenance
• turbidites
• uranium–lead dating

Acknowledgements

This paper arose out of collaboration between Geoscience Australia and GNS Science, New Zealand on a project called ‘Trans-Tasman Tectonic Terranes (T⁴).’ We thank the Royal Society of New Zealand for partial funding through a grant from their International Science and Technology (ISAT) Linkages Fund. Preliminary results for this paper were first presented at the Gondwana 12 Conference in Mendoza, Argentina, November 2005. We thank Peter Cawood, Chris Fergusson, Natalie Kositcin, Nick Mortimer and Keith Sircombe for their constructive reviews of the manuscript, and Bianca Reese and Daniel McIlroy for drafting the figures. Hamish Campbell and Nick Mortimer assisted very materially in fieldwork associated with the project. Norman Pearson and Suzy Elhlou are thanked for their expert technical advice at the GEMOC laboratory, Macquarie University. Five zircon samples were analysed on SHRIMP B at the John De Laeter Centre of Mass Spectrometry at Curtin University, which is operated by a consortium consisting of Curtin University, the University of Western Australia and the Geological Survey of Western Australia. RJK, LPB, GLF and CF publish with permission of the Chief Executive Office, Geoscience Australia.

Additional information

Notes on contributors

W. L. Griffin

∗Appendices 2 to 5 [indicated by an asterisk (∗) in the text and listed at the end of the paper] are Supplementary Papers; copies may be obtained from the Geological Society of Australia's website (<http://www.gsa.org.au>) or from the National Library of Australia's Pandora archive (<http://nla.gov.au/nla.arc-25194>).