Petrogenesis of the Tertiary Anorogenic Volcanic Series of Southern Queensland, Australia, in the Light of Trace Element Geochemistry and O, Sr and Pb Isotopes

Abstract

The magmas of the Tertiary volcanic province of S. Queensland are chemically bimodal, and occur in numerous volcanic centres, at least three representing original shield volcanoes. The mafic lavas are dominantly hawaiites and tholeiitic andesites, whereas the silicic magmas comprise mainly trachytes, rhyolites, and comendites.The silicic rocks exhibit variable trace element abundance patterns. There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn, and P, through the trachytes to the rhyolites and comendites while the behaviour of Zr, Nb, LREE, Y and Zn is very variable. Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour, becoming most generally concentrated in the comendites and rhyolites. These trace element patterns are modelled by application of the Rayleigh distillation model, using partition coefficients based on analysed phenocrysts from the S. Queensland silicic lavas. Trace mineral phases, namely zircon, chevkinite, and allanite, are shown to be important in the probable control of LREE, Zr, and Th abundances, while Nb and Zn are probably controlled during fractionation by magnetite. Trace element data for the hawaiites and tholeiitic andesites also indicate extensive although variable levels of fractional crystallization of these magmas.The Sr and O isotopic compositions of the mafic lavas, trachytes, comendites and rhyolites are as follows: initial 87Sr/86Sr ratios; 0.70357-0.70456, 0.70432-0.70589, 0.70495-0.70917, and 0.70708-0.70863 respectively. δ18O range between 5.6-7.0 (mafic lavas), 4.9-8.7 (trachytes), 5.0-7.6 (comendites) and 8.1-10.4 per mil (rhyolites). Pb isotopic compositions are variable, showing a variation of 6.7 per cent for 206Pb/204Pb ratios through the range of volcanic compositions. The rhyolites exhibit a much greater divergence in their O, Sr, and Pb isotopic compositions compared with those of associated mafic lavas, than is found in the trachytes and comendites. Within the silicic volcanics, positive correlations exist between δ18O and initial Sr ratios, and between Pb isotopic compositions and initial Sr ratios (with one group of trachytes providing a noteworthy exception). These correlations are not so clearly defined for the mafic lavas, although these do exhibit positive correlations between differentiation index, δ18O, and initial Sr isotope ratios.The development of the silicic magmas, excepting two groups, is interpreted in terms of a model in which assimilation and fractional crystallization occur concurrently, involving a basalt or hawaiite magma component and a crustal component (modelled on the analysed Carboniferous basement greywackes outeropping in the region); the data indicate, however, that differentiation continued in isotopically closed systems (i. e. isolated from the wallrocks). The highly depleted Sr and Ba abundances of the rhyolites and comendites suggest that contamination did not occur after differentiation had ceased. The rhyolites have the highest isotopic input of the crustal components and are interpreted as crustal anatectic melts, produced locally within the crust in response to basalt/hawaiite magma intrusion, whereas most of the trachytes and comendites are interpreted as primarily the differentiated products from original mafic parental magmas, with variable assimilation of crustal wallrock components. The isotopic data suggest that only the Minerva Hills trachytic lavas, and a Glass House comendite, have not been significantly modified by wallrock assimilation processes. The erpted mafic magmas were also evidently modified by isotopic crustal wallrock interactions, which independent petrological data suggest has occurred at intermediate to lower crustal depths.