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Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine

Published online by Cambridge University Press:  05 July 2018

A. C. J. M. Bambi ,
A. Costanzo ,
A. O. Gonçalves  and
J. C. Melgarejo
A. C. J. M. Bambi
Affiliation:
Departamento de Geologia, Faculdade de Cieˆncias, Universidade Agostinho Neto, Av. 4 de Fevereiro 7, 815 Luanda, Angola
A. Costanzo*
Affiliation:
Earth and Ocean Sciences, School of Natural Sciences National University of Ireland, Galway, University Road, Galway, Ireland
A. O. Gonçalves
Affiliation:
Departamento de Geologia, Faculdade de Cieˆncias, Universidade Agostinho Neto, Av. 4 de Fevereiro 7, 815 Luanda, Angola
J. C. Melgarejo
Affiliation:
Departament de Cristal·lografia, Mineralogía i Dipo` sits Minerals, Universitat de Barcelona, c/Martí i Franque`s s/n, 08028 Barcelona, Catalonia, Spain
*
*E-mail: alessandra.costanzo@nuigalway.ie
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Abstract

Three Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

Keywords

Angolacarbonatitezonation in pyrochlorefluorinehigh field strength elementsHFSEpyrochlorerare earth elementsREEmineral zonation
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 2 , April 2012 , pp. 377 - 392
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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