Petrology and Isotope Geochemistry of Tertiary Lavas from the Northern Flank of the Tweed Volcano, Southeastern Queensland

Abstract

The generalized stratigraphic sequence (20-21.8 m.y.) of the northern flank of the Tweed Volcano is: Beechmont Basalt (base)-Rhyolite (composed of two distinct units, the Springbrook and Binna Burra rhyolites)-Hobwee Basalt. In addition, comendite occurs as a postrhyolite intrusive phase. Chemically and mineralogically, the 'basalts' are tholeiitic andesites, which are conveniently divided into olivine-normative and quartz-normative types. Phenocryst mineralogy is olivine and labradorite (microphenocrystic) in the olivine-normative lavas, and plagioclase plus rare augite in the quartz-normative lavas.Rhyolites (which constitute some 7 vol. per cent of the Tweed Shield volume) are of the potassic two-feldspar type; these are characterized by highly fractionated trace element patterns, which are most extreme in the Binna Burra rhyolites. The latter, for example, have low K/Rb (<100) and La/Yb, highly depleted Eu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb, Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine, ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz, sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite, and very rare resorbed grains of extremely ulvospinel-rich titanomagnetite (Binna Burra rhyolite). Phenocryst equilibration temperatures are estimated to be in the range {reversed tilde equals}900-1050 °C for the Springbrook rhyolite and {reversed tilde equals}800-950 °C for the Binna Burra rhyolite, at oxygen fugacities in proximity to the WM buffer. The comendites are characterised by sanidine, quartz, fluor-arfvedsonite, minor acmite, and ilmenite.Pb isotopic compositions indicate at least two distinct groups of mafic lavas; certain olivine-normative tholeiitic andesites with compositions less radiogenic than modern oceanridge basalts (possibly indicative of lower crustal contamination), and a second more radiogenic group including the remaining isotopically analysed tholeiitic andesites. Sr isotopes reveal small differences between the Beechmont and Hobwee Groups. Pb and Sr isotopic compositions of the three rhyolitic groups are distinct, and all more radiogenic than the mafic lavas.It is concluded that the tholeiitic andesites represent the fractionation products from an olivine-normative tholeiitic basalt, and calculations suggest that olivine-plagioclase-aluminous clinopyroxene-Fe-Ti oxides were the likely fractionating mineral phases. The potassic rhyolites are interpreted also in terms of fractionation from a basaltic parent, although the geochemistry of the Binna Burra rhyolite has been further modified by continued fractionation at the quartz-feldspar minimum. Chemical and mineralogical data, however, suggest some modification of the rhyolitic magmas by crustal equilibration (possibly lower crust). Although the comendite is isotopically distinct from the exposed rhyolites, various least squares mixing calculations suggest that the comendite may have developed by continued late stage quartz-feldspar ternary minimum fractionation of rhyolitic magma.