Jurassic alkali-rich volcanism in Victoria (Australia): Lithospheric versus asthenospheric source

An alkali-rich set of dykes near Freestone, eastern Victoria (Australia) has an age and geochemical signature similar to alkali basalts in western Victoria, which are found interbedded with low-Ti tholeiites of the Ferrar magmatic province. Both occurrences of alkali basalts are shown to have EM1 mantle-type trace element and Nd isotopic signatures, while the Sr isotopic signature in the Freestone basalts is likely to have been affected by secondary processes. The Jurassic alkali basalts are also similar to the Cenozoic alkali basalts of the Newer Volcanic Province in West Victoria. This suggests that the source for both the Jurassic and Cenozoic alkali basalts may have resided in the subcontinental lithospheric mantle. This has also been suggested as the source for the low-Ti tholeiites, which are likely to have come from a shallower part of the lithosphere than the alkali basalts. Alternatively, both the Jurassic and Cenozoic alkali basalts could have had a plume source, in which case the geochemical similarity between these two periods of magmatism is presumably coincidental.