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Crystallization trends of pyroxenes from agpaitic phonolites (Cantal, France)

Published online by Cambridge University Press:  05 July 2018

R. Brousse
Affiliation:
Laboratoire de Pétrographie-Volcanologie, Université Paris XI, 91405 Orsay, France
J. Ph. Rançon
Affiliation:
Bureau de Recherches Géologiques et Miniéres, Départment Géothermie, BP 6009, 45060 Orléans Cédex, France

Abstract

Ca- and Na-rich pyroxene compositions of four agpaitic phonolites from the Cantal Volcano (France) have been studied. The pyroxenes from each sample show a range of compositions between X2+ pyroxencs (with X = Ca, Mg, Fe2+) and aegirine. The main substitution, X2+ by (Fe3+ + Na+), occurs when Na-Mg = 0.5, by which time Si is already equal to 2.0. The crystallization of Fe-Na-rich pyroxenes seems to be related to the increasing peralkalinity of the environment, in which the initially high temperature was suddenly reduced. Other factors, such as the structure of the liquid, are also considered.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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