Biotite melting in high-grade metamorphic gneisses from the Haut Allier (French Massif Central) |
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| Authors: | A Nédélec J Paquet |
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| Institution: | (1) L.A. CNRS 234 Propriétés et Structure de l'Etat Solide, Université de Lille, F-59655 Villeneuve d'Ascq, France |
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| Abstract: | In the biotite-sillimanite and biotite-sillimanite-cordierite gneisses from the Haut Allier (French Massif Central), the biotite grains are partially melted: they are embayed and replaced by an isotropic material associated with metallic oxides. The complete study of this glass by optical microscopy, Raman spectroscopy, electronic microanalysis and X-Ray diffraction was performed: the glassy state is well established (locally some very fine kaolinite crystals are present in the glass as hydrothermal reconstruction). This glass results from the incongruent melting of biotite. The alumino-silicate melt corroded the preexistent quartz and feldspar grains. Anhydrous phases crystallized from this melt: successively sillimanite-cordierite and quartz. Correlatively, the melt must have become water-saturated and a new highly hydrated fluid phase should then have coexisted with it. Potassium and silicium, together with water, may have been components of the relatively mobile hydrous phase and this one may have been responsible for some hydration reactions such as muscovitization of feldspars. The petrological implications must be emphasized: in the studied gneisses, biotite cannot be considered as a resister. In fact, biotite melts and this melting is probably an important agent of the regional anatexis. |
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