Magmatic characteristics and geochronology of Ordovician igneous rocks from the Cadia – Neville region, New South Wales: implications for tectonic evolution

Abstract

The Ordovician volcanic and intrusive rocks of the Cadia – Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to ∼20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have strikingly similar geochemistry, indicating that a similar mantle source and partial melting processes were involved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in the Cadia – Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in Viti Levu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arc magmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonic setting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to pre-processing of the mantle above a subduction zone prior to extension and fragmentation of arc lithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.

Key words:

• Forest Reefs Volcanics
• geochronology
• Lachlan Orogen
• Ordovician
• shoshonite
• tectonics

Acknowledgements

This work was funded by the Australian Research Council (Strategic Partnership – Industry Research and Training) grant. We also gratefully acknowledge the support of the following sponsors: Newcrest Mining Limited, Homestake Australia, Alkane Exploration, Hargraves Resources, Goldfields Australia, Resolute Ltd, Rio Tinto and the Geological Survey of New South Wales. P. Robinson and S. Stevens from the University of Tasmania assisted with XRF and ICP-MS analyses and thin-section preparation, respectively. M. Fanning from the Research School of Earth Sciences at Australian National University is thanked for SHRIMP data, as is J. Foden from University of Adelaide for Nd isotope analyses. Peter Duerden's Honours thesis provided useful information towards this study. Phil Blevin and Anthony Harris are thanked for constructive and thoughtful reviews, as are Dick Price and Evan Leitch, who reviewed an earlier version of this manuscript.

Notes

*Appendices 1 and 2 [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).^†