An Overview of the Geochemical and Isotopic Characteristics of the Eastern Australian CainozoicVolcanic Provinces

Abstract

A broad zone of intra-plate volcanism occurs for some 3000 km along eastern Australia. Mafic lavas dominate, and include the following types (with frequency % occurrence, based on 1757 analyses): leucitites (2·1), melilitites, nephelinites, and analcimites (5·4), basanites (12·7), alkali basalts (7·0), nehawaiites and hawaiites (44·4), transitional basalts and Ol-tholeiites (17·4), and Q-tholeiites (11·0). These lavas are erupted through a wide variety of crustal-tectonic environments, from Proterozoic to Mesozoic. Marked differences in chemistry exist between the lavas erupted from central volcano provinces (in which most ‘evolved’ lava types occur) and lava-field provinces, the former exhibiting greater isotopic variability and evidence for more extensive crystal fractionation (AFC). More evolved lava types include mugearites, benmoreites, icelandites, peralkaline and non-peralkaline trachytes and phonolites, comendites, low-silica and high-silica rhyolites. Marked regional differences exist with respect to distribution of various lava types; northern Queensland and Tasmania, for example, apparently have very few strongly evolved lavas, the latter region also containing a disproportionately high percentage of nephelinites. Trace element geochemistry of the mafic lavas is very variable, but typically continental; the lavas are enriched in incompatible elements, but enrichment varies greatly, being extreme in the leucitites, melilitites, and nephelinites, and slight (relative to MORB) in certain Q-tholeiites. It is shown that the patterns of the more extreme incompatible element enrichments are consistent with recent work on extraction of small melt fractions. Nevertheless, marked source inhomogeneities are indicated by the data, believed to be lithospheric; arc-modified lithosphere is suggested as a source for at least some lava field tholeiites. It is clear, however, that the majority of lavas have been modified by some degree of low-medium pressure crystal fractionation processes (olivine ± augite ± plagioclase ± Fe-Ti oxides). The critical role of fractional crystallization is even more apparent in the chemistry of the intermediate and silicic lavas, which exhibit dual patterns of progressive and ultimately extreme element enrichment (e.g., Pb, Th) and depletion (e.g., Mg, V, Ni, Cr, Sr, Ba, Eu). These patterns are readily modelled by Rayleigh fractionation, but require elevated KD values, appropriate to silicic magmas; continually varying KDs are also indicated by some data sets. The mafic lavas exhibit a wide, but continuous variation of isotopic compositions, there being marked regional differences, but with the leucitite exception, no particular compositional ranges characterize particular compositional types. Correlations are observed between Sr, Nd, and to a less extent Pb isotopic compositions with, for example, Bh/Th, Ba/Nb, and mg-ratios. Much of the observed geochemical-isotope data, excepting the most undersaturated lavas, can be modelled in terms of AFC processes, utilizing upper and lower crustal models. The isotopic data of the alkaline Tasmanian lavas are distinctive and are interpreted as asthenospheric; these compositions, and those of rare magnesian alkaline lavas from elsewhere in the region, suggest a mixed mantle source containing a component approaching the ‘St. Helena-type’. The leucitites have a marked DUPAL isotopic signature, and it is noted that these occur above an interpreted Proterozoic rift system, suggesting a lithospheric source. Isotopic and geochemical data for the trachytes and low-silica rhyolites are consistent with AFC processes, with variable assimilation, modelled in terms of upper crustal components. The high-silica rhyolites are isotopically distinctive, and are interpreted as local upper crustal melts, but modified by subsequent crystal-liquid fractionation.