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.