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Les phonolites agpaïtiques et miaskitiques du Cantal septentrional (Auvergne, France)

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L’étude de seize massifs phonolitiques dénombrés dans le nord du Cantal permet de mettre en évidence deux types de phonolites: agpaïtique et miaskitique.

Leurs earactéristiques minéralogiques et géochimiques sont discutées et il est envisage l’évolution petrogenétique des magmas peralealins dont elles tirent origine.

La lignée des phonolites miaskitiques parait prolonger celle des téphrites (ordanchites) et de ces miaskites apparaissent les phonolites agpaïtiques qui constituent le terme hyperalcalin de la serie.

En terme final, l’origine de ces diverses roches alealines est abordée.

Abstract

The phonolitic units of Northern Cantal are studied in detail, in a mineralogical, petrological and geochemical point of view. This study distinguishes two essentially different types of phonolites: agpaitic and miaskitic phonolites.

The miaskitic phonolites have a normal trend of crystallisation and the following mineralogical association: anorthoclase, noseane (phenocrysts) and sodalite (microcrysts), salite, magnetite, kearsutite, sphene and zircon. In these rocks, the rate in alkalies does not exceed that in Aluminium.

The agpaitic phonolites have an inversed trend of crystallisation and the following mineralogical association: anorthose, black sodalines, analcime, aegyrine, mosandrite. Let us note the lack in noseane, iron oxydes, sphene and zircon. In these rocks, the rate in alkalies exceeds the rate in trivalent ions (Al+Fe+Ti).

After a detailed description of both these poles, a liliation is traced from the tephrites to the miaskitic phonolites, with many transition-types. The agpaitic phonolites seem to derive from the miaskitic phonolites through a relative sodium enrichment; again with intermediates.

The miaskitic phonolites are considered as representing the liquid residual of fractional crystallisation of an alkali-olivine basalt. All intermediates are found in Cantal: alkali-olivine basalt, analcime basanite, tephrite, miaskitic phonolite.

The agpaitic phonolites are considered as deriving from the latter, through a process of gaseous transfer (Na associated with Cl and SO4), as showed by the conditions of high (O2, H2O, Cl, SO4) pressure.

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  1. Laboratoire de Petrographic, Faculte des Sciences, Orsav - 91, France

    J. Varet

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Varet, J. Les phonolites agpaïtiques et miaskitiques du Cantal septentrional (Auvergne, France). Bull Volcanol 33, 621–656 (1969). https://doi.org/10.1007/BF02596529

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