Aspects of the mineralogy and chemistry of the intermediate‐silicic Cainozoic volcanic rocks of eastern Australia. Part 1: Introduction and geochemistry

Abstract

Comparative data are presented on the major and trace element chemistry of ‘evolved’ volcanic samples from the major volcanic centres from central Queensland to Victoria. Compositional types represented include peralkaline and non-peralkaline quartz trachytes and phonolites, trachyrhyolites, trachybasalts, trachyandesites, comendites, and rhyolites. Within the southern Queensland region, the volcanic compositions are strongly bimodal due to the extensive development of rhyolites, and the normal dominance of mafic lavas (basalts, hawaiites and tholeiitic andesites). Within the remainder of E Australian region, the compositions are skewed with progressively decreasing frequencies towards the most siliceous compositions. Undersaturated compositions are most frequent in the NSW and Victorian regions. Chemically, the various trachytic, phonolitic, and peralkaline types exhibit a sequence of progressive depletions of Mg, Ca, P, Mn, Cr, Ni, V, Sr, and Ba, which become extreme in the comendites and most siliceous trachytes; in addition, variable enrichment patterns are found for Rb, Th, Nb, Ce, Zr, and Zn. REE patterns exhibit Eu/Eu* anomalies (0.97-0.05) which tend to correlate with decreasing La/Lu ratios. The rhyolites are dominantly high-silica potassic types, with strongly depleted Ca, Mg, P, Mn, Sr, Ba, V, Cr, and Ni abundances, and strong Eu/Eu* anomalies (0.12-0.002); they exhibit certain mineralogical, trace element, and isotopic differences from the trachytic magma types, and are not simply the result of extended trachyte fractionation. The general patterns of trace and minor element behaviour, which are consistent through all the sampled E Australian centres, are modelled in terms of Rayleigh fractionation; it is shown that this can explain the observed trace element patterns, including the Eu/Eu* anomalies; f values of 0.3-0.2 are sufficient even for the most Sr, Ba, and Eu depleted samples.