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

Citation

Ferguson, MRM and Ehrig, K and Meffre, S and Cherry, AR, Associations between zircon and Fe-Ti oxides in Hiltaba event magmatic rocks, South Australia: atomic- or pluton-scale processes?, Australian Journal of Earth Sciences, 67, (2) pp. 201-220. ISSN 0812-0099 (2020) [Refereed Article]

Copyright Statement

Copyright 2019 Geological Society of Australia

DOI: doi:10.1080/08120099.2019.1653990

Abstract

The Hiltaba Suite intrusive rocks and penecontemporaneous Gawler Range Volcanics (GRV) comprise the 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. Zircon is principally associated with Fe傍i oxides and clusters of touching crystals in these rocks, including in the Roxby Downs Granite (RDG), host of the Olympic Dam iron oxide膨opper鉾old deposit, and in other intrusive rocks that comprise the Olympic Province. There has been no explicit evaluation and explanation of potential origins published for concentrations of zircon with Fe傍i oxides (herein zircon-rich clusters) found in these and similar rocks of western North America and elsewhere. Here we use U鳳b geochronology, mineral morphologies and compositions, and insights from surface chemistry and liquid-bound particle interaction studies to investigate zirconrich clusters and provide a model for their formation. U鳳b geochronology does not reveal any concordant zircon populations older than ca 1590Ma, so it is unlikely that there are significant xenocrystic zircon grains or that the zircons include significant inherited cores. The lack of premagmatic zircon, consistent intra-grain and inter-grain zircon compositional trends, the predominance of oscillatory zoned zircon with morphologies indicating growth from hot, evolved silicate melts, and the lack of evidence for zircon recrystallisation, indicates that zircon crystallised in the host GRV and RDG magmas. Variable zircon compositions within individual clusters does not support epitaxial nucleation of zircon on Fe傍i oxides, but it is likely that some zircon grains grew from seed crystals formed by exsolution of Zr from Fe傍i oxides. Aggregation of isolated, liquidbound crystals is energetically favourable, and the grainsize discrepancy between larger crystals (Fe傍i oxides, pyroxenes) and smaller accessory minerals (zircon, apatite) maximises the disparity in particle velocities and hence enhances the opportunities for collisions and adhesion between these crystals. We propose that zircon adheres to Fe傍i oxides with greater ease and/or with greater bond strengths, than to other phases present in the parental magmas. It is possible that this association is related to interactions between zircon and Fe傍i oxide surface sites with opposing charges, presuming the distance between phase surfaces is sufficiently small. The occurrence of small zircon grains within Fe傍i oxides and both euhedral zircon and zircon with asymmetric growth zonation in contact with Fe傍i oxides indicates that several processes are responsible for the high concentrations of zircon crystals in some Fe傍i oxide clusters.

1. Zircon is principally associated with Fe傍i oxides in 1.59 Ga Gawler Range Volcanics (GRV) and Roxby Downs Granite (RDG)

2. U鳳b geochronology does not reveal any concordant zircon populations older than ca 1590Ma

3. Zircon compositions and morphologies indicate that zircon crystallised in the host RDG and GRV magmas and suggest recharge, reheating and mixing occurred in these magmatic systems

4. Seed crystals, aggregation and surface chemical affinities contributed to the strong association of zircon and Fe傍i oxides

Item Details

Item Type:Refereed Article
Keywords:Gawler Range Volcanics, Roxby Downs Granite, crystal clusters,
Research Division:Earth Sciences
Research Group:Geology
Research Field:Igneous and Metamorphic Petrology
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Earth Sciences
UTAS Author:Ferguson, MRM (Mr Matthew Ferguson)
UTAS Author:Meffre, S (Dr Sebastien Meffre)
UTAS Author:Cherry, AR (Mr Alex Cherry)
ID Code:136844
Year Published:2020
Funding Support:Australian Research Council (LP130100438)
Deposited By:CODES ARC
Deposited On:2020-01-20
Last Modified:2020-04-02
Downloads:0

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