Multi-age trachytic eruptions, Western Victoria: plume-related?

Abstract

Although basaltic products dominate post-5 Ma western Victorian volcanism, a revised picture suggests basaltic activity extended back to 8 Ma and was punctuated by felsic episodes around 8, 6 and 2 Ma. The 8 Ma episode produced rhyolite (8.3 ± 0.1 Ma), zircon-bearing tuff (8.1?8.3 ± 0.5 Ma) and derived zircon-bearing sediments (8.1 ± 0.6 Ma). The 6 Ma episode formed the prominent trachytes of the Macedon-Trentham province (5.9?6.1 Ma) and provided zircon at depth for subsequent basaltic megacrysts. The 2 Ma episode is the first felsic outcrop identified in the main basaltic sequence and a trachyte plug near Creswick gives congruent K-Ar, Ar-Ar and zircon fission-track ages at 2.4 ± 0.4 Ma. The Creswick plug is a porphyritic, metaluminous potassic-sodic trachyte and each felsic episode shows petrological differences. The youngest felsic event may explain distal bentonite deposits in south-western New South Wales. Triple episodes of felsic fractionation during western Victorian basaltic activity requires a more complex genetic model for its formation. It raises the question of multiple plume-related mantle melting events.