Associations between zircon and Fe–Ti oxides in Hiltaba event magmatic rocks, South Australia: atomic- or pluton-scale processes?

The Hiltaba Suite intrusive rocks and penecontemporaneous Gawler Range Volcanics (GRV) comprisethe 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia.Zircon is principally associated with Fe–Ti oxides and clusters of touching crystals in these rocks,including in the Roxby Downs Granite (RDG), host of the Olympic Dam iron oxide–copper–golddeposit, and in other intrusive rocks that comprise the Olympic Province. There has been no explicitevaluation and explanation of potential origins published for concentrations of zircon withFe–Ti oxides (herein zircon-rich clusters) found in these and similar rocks of western North Americaand elsewhere. Here we use U–Pb geochronology, mineral morphologies and compositions, andinsights from surface chemistry and liquid-bound particle interaction studies to investigate zirconrichclusters and provide a model for their formation. U–Pb geochronology does not reveal anyconcordant zircon populations older than ca 1590Ma, so it is unlikely that there are significantxenocrystic zircon grains or that the zircons include significant inherited cores. The lack of premagmaticzircon, consistent intra-grain and inter-grain zircon compositional trends, the predominanceof oscillatory zoned zircon with morphologies indicating growth from hot, evolved silicatemelts, and the lack of evidence for zircon recrystallisation, indicates that zircon crystallised in thehost GRV and RDG magmas. Variable zircon compositions within individual clusters does not supportepitaxial nucleation of zircon on Fe–Ti oxides, but it is likely that some zircon grains grewfrom seed crystals formed by exsolution of Zr from Fe–Ti oxides. Aggregation of isolated, liquidboundcrystals is energetically favourable, and the grainsize discrepancy between larger crystals(Fe–Ti oxides, pyroxenes) and smaller accessory minerals (zircon, apatite) maximises the disparity inparticle velocities and hence enhances the opportunities for collisions and adhesion between thesecrystals. We propose that zircon adheres to Fe–Ti oxides with greater ease and/or with greaterbond strengths, than to other phases present in the parental magmas. It is possible that this associationis related to interactions between zircon and Fe–Ti oxide surface sites with opposingcharges, presuming the distance between phase surfaces is sufficiently small. The occurrence ofsmall zircon grains within Fe–Ti oxides and both euhedral zircon and zircon with asymmetricgrowth zonation in contact with Fe–Ti oxides indicates that several processes are responsible forthe high concentrations of zircon crystals in some Fe–Ti oxide clusters.1. Zircon is principally associated with Fe–Ti oxides in 1.59 Ga Gawler Range Volcanics (GRV) andRoxby Downs Granite (RDG)2. U–Pb geochronology does not reveal any concordant zircon populations older thanca 1590Ma3. Zircon compositions and morphologies indicate that zircon crystallised in the host RDG andGRV magmas and suggest recharge, reheating and mixing occurred in these magmaticsystems4. Seed crystals, aggregation and surface chemical affinities contributed to the strong associationof zircon and Fe–Ti oxide