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    Granitoid zircon forms the nucleus for minerals precipitated by carbonatite-derived metasomatic fluids at Chilwa Island, Malawi

    Dowman, E. and Wall, F. and Jeffries, T. and Treloar, P. and Carter, Andrew and Rankin, A. (2017) Granitoid zircon forms the nucleus for minerals precipitated by carbonatite-derived metasomatic fluids at Chilwa Island, Malawi. Gondwana Research 51 , 64 - 77. ISSN 1342-937X.

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    Abstract

    Mineralogical assemblages are fundamental to the interpretation of geological processes. Zircon is an integral petrographic component of the mineral assemblages present in fenites (rocks formed by alkaline metasomatism) associated with the 136 Ma-aged Chilwa Island carbonatite complex,Malawi. Zircon exhibits contrasting characteristics and properties across the fenite aureole that surrounds the carbonatite stock. It shows intense grain dissolution and subsequent replacement by pyrochlore in the more intensely metasomatised ‘high-grade’ fenite of the innermost part of the aureole. By contrast, relict zircon crystals form the nucleus for the development of apatite-ilmenite-REE mineral assemblages in less altered zones. These changes in zircon properties are considered to be evidence of the diverse nature of fluids that metasomatised the Chilwa Islands fenite aureole. Although zircon is a principal component of the fenite mineral assemblages, when dated by LA-ICP-MS techniques it was found to predate the other minerals present in the mineral assemblages and thus, the age of carbonatite intrusion, by over 380 Ma. Instead of co-crystallising with the assemblage, zircon is therefore interpreted as providing a focus around which the minerals in the fenite assemblage formed. This implies that caution is needed both in the interpretation of Zr mobility in metasomatic assemblages, and also in attributing a zircon age to the assemblage as a whole in such sequences.

    Metadata

    Item Type: Article
    Keyword(s) / Subject(s): Zircon, Geochronology, Fenite, Metasomatism, Carbonatite
    School: School of Science > Earth and Planetary Sciences
    Depositing User: Andy Carter
    Date Deposited: 15 Aug 2017 10:24
    Last Modified: 06 Jul 2020 14:41
    URI: https://eprints.bbk.ac.uk/id/eprint/19353

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