Granitoid zircon forms the nucleus for minerals precipitated by carbonatite-derived metasomatic fluids at Chilwa Island,Malawi |
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| Institution: | 1. School of Geography, Geology & Environment, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK;2. Camborne School of Mines, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9FE, UK;3. Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK;4. Department of Earth and Planetary Sciences, Birkbeck College, Gower Street, London WC1E 7HX, UK;1. Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, USA;2. School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, PR China;3. Department of Earth Sciences, The University of Adelaide, Adelaide, SA 5005, Australia;4. Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea;1. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;2. Novosibirsk State University, 2, Pirogova str., Novosibirsk 630090, Russia;3. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3, Koptyug ave., Novosibirsk 630090, Russia;4. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1А Favorsky str., Irkutsk 664033, Russia;1. Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia;2. ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS)/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia;1. SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, United Kingdom;2. Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia |
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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. |
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