The petrology and petrogenesis of the Tertiary anorogenic mafic lavas of Southern and Central Queensland, Australia — Possible implications for crustal thickening

Abstract

The Miocene-Oligocene volcanism of this region is part of the larger Tertiary volcanic province found throughout E. Australia. Within the S.E. Queensland region, the volcanism is strongly bimodal, and has emanated from six major centres, and many additional smaller centres. The mafic lavas (volumetrically dominant) range continuously from ne-normative through to Q-normative and are predominantly andesine-normative; Mg/Mg+∑Fe (atomic ratios range from 30–60; K₂O ranges from 0.42–2.93%, and TiO₂ from 0.81–3.6%.

Phenocryst contents are low (averaging 6.7 vol.%), and comprise olivine (Fa_18–75; Cr-spinel inclusions occur locally in Mg-rich phenocrysts), plagioclase (An_25–68), and less commonly augite, which is relatively aluminous in lavas of the Springsure volcanic centre. Very rare aluminous bronzite occurs in certain Q-normative lavas. Groundmass minerals comprise augite, olivine (Fa_33–77), feldspar (ranging from labradorite through to anorthoclase and sanidine), Fe-Ti oxides, and apatite. Within many of the Q-normative lavas, extensive development of subcalcic and pigeonitic pyroxenes occurs, and also relatively rarely orthopyroxene. Mineralogically, the ne- and ol-normative lavas, and some of the Q-normative lavas are indistinguishable, and in view of the gradations in chemistry, the term hawaiite has been extended to cover these lavas. The term tholeiitic andesite is used to describe the Q-normative lavas containing Ca-poor pyroxenes as groundmass phases.

Megacrysts of aluminous augite, aluminous bronzite, olivine, ilmenite, and spinel sporadically occur within the lavas, and their compositions clearly indicate that they are not derived from the Upper Mantle. Rare lherzolite xenoliths are also found.

The petrogenesis of these mafic lavas is approached by application of the thermodynamic equilibration technique of Carmichael et al. (1977), utilizing three “parental” mineral assemblages that could have been in equilibrium with the magmas at P and T. The models are: (a) “standard” upper mantle mineralogy; (b) an Fe-enriched upper mantle model (Wilkinson and Binns 1977); (c) “lower crust” mineralogy, based on analysed megacryst compositions. The calculations suggest that these mafic magmas were not in equilibrium with either mantle model prior to eruption, but show much closer approaches to equilibrium with the “lower crust” model. Calculated equilibration temperatures and pressures (for the “lower crust” model) range from 995°–l,391° C (average 1,192), and 7.2–16.3 kb (average 12.4). These results are interpreted in terms of a model of intrusion and magma fractionation within the crust-mantle interface region, with consequent crustal underplating and thickening beneath the Tertiary volcanic regions. Some support for the latter is provided by regional isostatic gravity anomalies and physiographic considerations.