Junee – Narromine Volcanic Belt, Macquarie Arc, Lachlan Orogen, New South Wales: components and structure

Abstract

The Junee – Narromine Volcanic Belt of Ordovician volcanic, volcaniclastic and intrusive rocks in central New South Wales is the most westerly structural belt of the now disrupted Macquarie Arc. Although more than 200 km long (far longer than other belts of Ordovician arc volcanics), most of this belt is concealed by younger Palaeozoic to Holocene cover. Most of our knowledge of this belt thus comes from scattered outcrops and drillhole information, augmented by the interpretation of aeromagnetic and gravity data and, to a lesser extent, information from deep seismic-reflection profiling. This combined geological and geophysical approach suggests that the Junee – Narromine Volcanic Belt consists of major separate igneous complexes, each consisting of lavas and volcaniclastic sediments intruded by various mafic to felsic intrusive porphyries. Variations in age and geochemical affinities are used to suggest that the Junee – Narromine Volcanic Belt approximates most closely the magmatic core of the Macquarie Arc. The present geometry of the Junee – Narromine Volcanic Belt reflects the interplay of two major ductile – brittle fault systems: the north-northwest-trending Gilmore Fault System mainly in the south and the north-trending Tullamore Fault System, mainly in the north and centre, together with local west-northwest cross faults. These systems were generated during deformation in the Early Silurian, Early Devonian and Carboniferous.

Key words:

• geology
• geophysics
• igneous complexes
• Junee – Narromine Volcanic Belt
• Lachlan Orogen
• Macquarie Arc
• Ordovician
• structure

Acknowledgements

The geophysical datasets over the Narromine 1:250 000 map sheet were obtained by the New South Wales Department of Mineral Resources (DMR) under the Discovery 2000 Initiative. Those over the Forbes and Cootamundra map sheets (and the traverse of Figure 4) were acquired by the Australian Geological Survey Organisation (now Geoscience Australia) and the DMR as part of the National Geoscience Mapping Accord. The basic interpretations on which this paper is based were done as a contribution to those projects. The test seismic survey of Figure 10 was acquired at the suggestion of Derecke Palmer (University of New South Wales) and acquired by Geco Prakla, thanks to Grant Butler and Dennis Sweeney, and processed in their Perth office. Further processing was kindly undertaken at Geoscience Australia by Andrew Owen through the assistance of Russell Korsch. We thank Renison Goldfields Company Limited and Resolute Limited for permission to reproduce Figure 6. We thank Mike Hallett, and David Robson for discussions, Mike Hallett, Tony Crawford and Ian Percival for helpful peer reviews and Pat Lyons and Nick Direen for journal reviews. Published with permission of the Deputy Director-General, New South Wales Department of Primary Industries – Mineral Resources.

Notes

*Percival (2000) erected a new name, Northparkes Volcanic Group, which contained both constituent volcanics and sediments. We prefer to use the term Northparkes Group to refer to these rocks. Elsewhere, we use the term igneous complex to refer to spatially constrained collections of igneous rocks (e.g. lavas, volcaniclastic sediments and intrusive bodies) where rock relationships are not understood, or where recognition is based on interpretation of remotely sensed geophysical data.