The mineralogy and chemistry of the anorogenic tertiary silicic volcanics of S.E. Queensland and N.E. New South Wales, Australia

Abstract

The Late Oligocene‐Early Miocene volcanism of this region is chemically strongly bimodal; the mafic lavas (volumetrically dominant) comprise basalts, hawaiites, and tholeiitic andesites, while the silicic eruptives are mainly comendites, potassic trachytes, and potassic, high‐silica rhyolites. The comendites and rhyolites have distinctive trace element abundance patterns, notably the extreme depletions of Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable enrichment of such elements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytes exhibit these characteristics to lesser degrees. The comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations (e.g., LREE, Nb, Y, and especially Zr) and Zn. The phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases: sodic plagioclase (albite‐andesine), calcic anorthoclase, sanidine, quartz, ferroaugite‐ferrohedenbergite, ferrohypersthene, fayalitic olivine, ilmenite, titanomagnetite, and rarely biotite (near annite) and Fe‐hastingsitic amphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite‐bearing rhyolites only), and allanite (amphibole and botite rhyolites only). The comendites generally contain Ca‐poor anorthoclase‐sanidine, quartz, fluorarfvedsonite, aegirine and aegirine‐augite (Zr‐bearing), aenigmatite, and ± ilmenite. Coexisting Fe‐Ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes. Where present, they indicate equilibration temperatures of 885°–980°C and fo2 between QFM and WM buffers. The magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration, which is also consistent with the general paucity of pyroclastics, the rarity of hydrous mineral phases, and the extreme Fe‐enriched ferromagnesian phenocryst compositions. The chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas, and the comendites, trachytes, and certain trachyte‐rhyolite series are considered to have evolved from a mafic parentage. The available oxygen, Sr, and Pb isotopic data, however, point to some modification of the magnas through crustal equilibration processes. The remaining high‐silica rhyolites are considered to most likely represent crustal partial melts but are again modified by extensive fractionation.