The Camboon Volcanics, which host major gold mineralisation at Cracow, central Queensland, are a succession of volcanic and volcaniclastic rocks and their intrusive equivalents. They are part of a Carboniferous-Permian back-arc extensional event in the New England Fold Belt that led to the development of the Bowen Basin, and they do not represent an Early Permian subduction-related arc as previously hypothesised.
The calc-alkaline Camboon Volcanics comprise a complete compositional range from basalt through to rhyolite. A new stratigraphy defined for the sequence is shown to be dominated by silicic volcanism; mafic and intermediate volcanism is volumetrically minor. The Camboon Volcanics reflect a change from the silicic-dominated volcanism of the underlying Torsdale beds with the onset of intermediate and mafic volcanism, and become bimodal towards the top of the sequence. The relative proportion of intermediate volcanic rocks is greatly increased at the monogenetic intermediate volcanic centre near Cracow township and is not representative of the Camboon Volcanics stratigraphy regionally.
A three stage model is proposed for Carboniferous-Permian back-arc extension that involves; I) initial large scale crustal melting, which is dominantly represented by the Torsdale beds, 11) accelerated extension and crustal thinning allowing access to lower crustal and mantle melts, which is represented by the Camboon Volcanics, and III) dominantly bimodal volcanism, represented by the upper part of the Camboon Volcanics and, in the northern part of the New England Fold Belt, the Rookwood Volcanics. As the Camboon Volcanics represent a transitional phase in the extensional event, their evolution also reflects the interaction of a number of magma sources and processes. Typical of basalts in back-arc extensional environments, the geochemistry of basalts in the Camboon Volcanics exhibit subduction-modified signatures, with LIL element enrichment and HFS element depletion, and transitional signatures between subduction-modified and N-MORB. Basalts with the strongest subduction signature occur at the base of the succession, while transitional basalt occurs at the top of the Camboon Volcanics. In the north of the New England Fold Belt, the Camboon Volcanics are overlain by the Rookwood Volcanics which contain basalts that dominantly show compositions similar to NMORB. Silicic rocks reflect either upper crustal melting or fractionation from intermediate melts. Intermediate rocks reflect either direct mixing of subduction modified mantle with upper crustal sources or fractional crystallisation and assimilation of lower crust.
The volume of silicic magmatism reflects extension of thick, mature, continental crust. Volumetrically the proportion of magma that can be attributed to associated subduction is minimal, especially when the Camboon Volcanics are regarded as part of a more extensive event. It is therefore suggested that extension occurred in a back-arc position and that subduction may have ceased earlier, possibly with little to no remaining subducted slab underlying the region.