A-type magmatism in the Western Lachlan Fold Belt? A study of granites and rhyolites from the Grampians region, Western Victoria

The Victoria Valley Batholith, a granite–rhyolite complex of unusual composition, was emplaced in the Grampians–Stavely Zone of western Victoria at 396 Ma. Petrographic and geochemical features of the suite share important similarities with A-type magmas and we propose that they be classified as such. The close association between metaluminous to weakly peraluminous A-types and peralkaline granites within the Victoria Valley suite requires that, at least in some cases, these lithologies should be grouped together as part of a single A-type suite. This relationship is not necessarily a magmatic one as at least some of the peralkaline rocks in this suite appear to be the result of hydrothermal alteration. The Sr, Nd and Hf isotopic compositions of the magmas at the time of their emplacement (0.70459, 0.51220 and 0.28274) are remarkably similar to those of Cambrian mafic to intermediate lithologies calculated at 396 Ma. We suggest that the Victoria Valley suite was produced via the partial melting of basement Cambrian rocks (perhaps similar to diorites associated with the ∼ 495 Ma Bushy Creek pluton to the southeast) and that the magmas subsequently underwent variable fractional crystallisation dominated by hornblende and plagioclase. The oxidation state of the Victoria Valley magmas remains unclear, but is likely to have been higher than that observed in ferroan A-types suggesting that low oxygen fugacity is not an essential pre-requisite for A-type magmatism.