Late-magmatic mineral assemblages with siderite and zirconian pyroxene and amphibole in the anorogenic Mt Gibraltar microsyenite, New South Wales, Australia, and their petrological implications

The Mt Gibraltar intrusion near Mittagong and Bowral in New South Wales, Australia (lat. 34°27'54″S, long. 150°25'44″) is a small intrusive body of hypersolvus microsyenite emplaced into the Triassic Hawkesbury Sandstone of the Sydney Basin in Jurassic time, possibly related to extensional faulting. The rock itself consists of intermediate alkali feldspar with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to ≈Hd ₃₄Aeg ₆₅ in composition. It is crosscut by an irregular system of late-magmatic veins consisting of homogeneous alkali feldspar (≈Ab ₅₀Or ₅₀), clinopyroxene evolving from sodic hedenbergite to zirconium-rich aegirine, arfvedsonite and siderite. During postmagmatic evolution of the veins, microcrystalline or amorphous silica precipitated together with calcite filling miarolitic cavities. The late-magmatic mineral assemblage of the veins indicate crystallisation (at assumed 700bar pressure) at T=650-670°C, log f _O2=-22. This corresponds to conditions very close to the magnetite-wustite curve. Zirconium-bearing pyroxene has a restricted stability field in the system SiO ₂-ZrO ₂-FeO-FeO _1.5-NaO _0.5-HO _0.5, at moderately elevated peralkalinity and intermediate silica activity. Under such conditions, pyroxene and amphibole will act as effective sinks for Zr, preventing crystallisation of magmatic zircon or more exotic Zr silicates. The Mt Gibraltar microsyenite is therefore a rare example of an igneous rock in which zirconium is camouflaged in pyroxene and amphibole rather than forming its own minerals.