The oscillatory intergrowth of feldspars in titanian andradite, Little Dromedary, NSW, Australia

Veins bearing titanian andradite were found in a metasomatically altered clinopyroxenite cumulate of an alkali magmatic suite (Little Dromedary, NSW, Australia). The titanian andradite is up to 2.5 cm in size, mainly subhedral and displays a large concentric, oscillatory intergrowth with layers consisting of plagioclase and alkali feldspar. Detailed field work and petrography showed that there is a gradual change in paragenesis from the cumulitic clinopyroxenite with interstitial plagioclase to an apatite- and scapolite-bearing plagioclase-titanian andradite-pyroxene rock, which hosts the titanian andradite veins. Clinopyroxene composition changes from a magmatic Ti-bearing subsilicic ferrian aluminian diopside to a metamorphic Ti-free, Al-poor diopside. Plagioclase changes composition from oligoclase (Ab₈₁An₁₄Or₅) in the pyroxenite to bytownite (Ab₁₄An₈₆) in the garnet-pyroxene rock and to oligoclase (Ab₇₆An₂₁Or₃) in the vein. Orthoclase occurs exclusively in the vein and contains 12% albite component. The titanian andradite-bearing veins and the change in paragenesis, bulk rock and mineral composition provide evidence for the reaction of a late magmatic, oxidizing hydrothermal fluid with the magmatic pyroxenite. The temperature of alteration is constrained by the composition of the coexisting plagioclase and alkali feldspar in titanian andradites to ∼ 420 °C. Crystallization of the titanian andradite indicates that Ti was transported on a metre scale from the magmatic clinopyroxene to the vein by a fluid. Ti solubility may have been enhanced by ligands such as F^-, Cl^-, PO₄^3-, SO₄^2-, and CO₃^2-, which are all present in the fluid as documented by their incorporation in metamorphic apatite, scapolite, and carbonate coexisting with titanian andradite. KCl and NaCl salt inclusions in feldspar within the titanian andradite were found with transmission electron microscopy (TEM) and indicate that the fluid was partly supersaturated in these components. The alternating intergrowth of titanian andradite and feldspars mirrors an oscillating evolution of the fluid. TEM images of the garnet-feldspar interface suggest that garnet has been partly dissolved prior to or during feldspar precipitation, supporting the hypothesis of significantly changing fluid composition during vein formation. The concentric intergrowth is interpreted as the result of both the changing chemistry of the fluid and the different growth behaviour of garnet and feldspar.