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

An alkali-rich set of dykes near Freestone, eastern Victoria (Australia) ha s an age and geochemical signature similar to alkali basalts in western Vic toria, which are found interbedded with low-Ti tholeiites of the Ferrar mag matic province. Both occurrences of alkali basalts are shown to have EM 1 m antle-type trace element and Nd isotopic signatures, while the Sr isotopic signature in the Freestone basalts is likely to have been affected by secon dary processes. The Jurassic alkali basalts are also similar to the Cenozoi c alkali basalts of the Newer Volcanic Province in West Victoria. This sugg ests 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. Alt ernatively, both the Jurassic and Cenozoic alkali basalts could have had a plume source, in which case the geochemical similarity between these two pe riods of magmatism is presumably coincidental. (C) 1999 Elsevier Science Li mited. Ail rights reserved.