Geochemistry and tectonic setting of some Neoproterozoic and Early Cambrian volcanics in western New South Wales

Geological and geochemical investigation of rocks previously mapped as the Mt Wright Volcanics in western New South Wales shows that two very different magma series are present in this region. An earlier transitional alkaline basalt-trachybasalt-trachyandesite-trachyte-alkali rhyolite suite, dated by SHRIMP U-Pb on zircons at 586 ± 7 Ma, is overlain and intruded by a calc-alkaline basalt-andesite-dacite suite. The latter is faulted against the Early Cambrian Cymbric Vale Formation, consisting mainly of vitric tuffs and pelitic sedimentary rocks for which a zircon SHRIMP U-Pb age of 525 ± 8 Ma has been previously reported. The calc-alkaline suite hosts intercalated Early Cambrian archaeocyathan limestones that contain the same faunal assemblage as basal Cymbric Vale Formation limestones, suggesting that these two units may be closely associated temporally. The transitional alkaline suite at Mt Wright is compositionally very close to the basalt-trachybasalt-trachyte suite of the Nundora-Packsaddle-Mt Arrowsmith region, extending northwest of Mt Wright for more than 150km. Our study confirms the Neoproterozoic age for the Kara beds to which these alkaline volcanic sequences have previously been assigned. We propose a continental rift setting for the eruption of these lavas and note their pronounced compositional similarities to the basaltic to alkali rhyolite lava piles in the Afar Rift, bordering the Red Sea. The calc-alkaline suite is more difficult to assign to a particular tectonic setting of eruption. Despite a broad compositional similarity to medium-K calc-alkaline lavas in mature island arc or active continental margin settings (e.g. Japan, Mexico), the thin pile of aphyric and sparsely plagioclase-phyric coherent lavas of the Mt Wright calc-alkaline suite are unlike typical lava breccia-pyroclastic-dominated arc sequences. By analogy with well-known calc-alkaline lava sequences in the Basin and Range extensional setting in western USA, we suggest that the calc-alkaline suite rocks at Mt Wright formed by partial melting of subcontinental lithospheric mantle in an immature continental rift around 525 Ma. This extensional episode may record an early stage of development of the Kanmantoo Trough. Later magmatism associated with more advanced extension generated the tholeiitic basalts and dykes, some approaching MORB compositions, known from the Kanmantoo sections of eastern South Australia and western Victoria.