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ABSTRACT

Three Cainozoic intraplate volcanic suites in the Bingara to Inverell area, northeastern New South Wales, have been discriminated on the basis of differing geophysical responses and contrasting K–Ar ages. Major isotopic and chemical characteristics can also be used to distinguish the three suites. These newly defined suites are the Middle Eocene–Early Oligocene Maybole Volcanic Suite; the Late Oligocene–Early Miocene Delungra Volcanic Suite; and the Middle Miocene Langari Hill Volcanic Suite. Four basaltic volcanic units within the Delungra Volcanic Suite have also been distinguished: Mount Russell Volcanics; Derra Derra Volcanics; Inverell Volcanics; and Bingara Volcanics. The Maybole Volcanic Suite is dominated by mafic volcanic rocks of alkaline affinity. These rocks include hawaiite, transitional basalt, basanite and rare phonolite (not included in this study). Volcanogenic and non-volcanogenic sedimentary units are minor but significant components, hosting world-class concentrations of sapphires in the Inverell–Glen Innes region. The Maybole Volcanic Suite occupies the eastern portion of the study area, forming ridges that outline the radial drainage pattern of the deeply eroded Eocene–Oligocene Maybole shield volcano. The Delungra Volcanic Suite is geochemically diverse and consists of alkaline members (Inverell and Bingara Volcanics) and tholeiitic members (Mount Russell and Derra Derra Volcanics). These are dominated by mafic lava flows with minor interflow sedimentary horizons. The Delungra Volcanic Suite forms broad elevated plains and prominent plugs in the central and western portions of the study area. Diamond occurrences in the Bingara district are spatially associated with the Bingara and Derra Derra Volcanics. The Langari Hill Volcanic Suite consists of a mafic tholeiitic lava flow that is spatially restricted to a prominent east–west ridge east of Inverell overlying the Maybole Volcanic Suite. The Langari Hill Volcanic Suite is significantly younger than the Maybole and Delungra Volcanic suites and represents the youngest recognised volcanic episode in the Bingara–Inverell area.

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