Les variations chimiques des clinopyroxènes calciques, témoins de la complexité des processus pétrogénétiques à l'intérieur d'un réservoir magmatique (Exemple des ponces des Champs Phlégréens Campanie, Italie)

Abstract

Diopside and salite occur together in latitic and trachytic pumices in Phlegrean Fields. Optical and chemical studies by electron microprobe (Camebax) on clinopyroxenes, and optical thermometry on the two phases inclusions (glass + gaz) (which can be used to get a minimal crystallization of the host mineral) present in these crystals show that : — diopside crystallize from a trachybasaltic melt 1,155 °C ± 10 °C, — cognate diopsidic pyroxene exist either as phenocrysts in a latitic and trachytic pumices, or as core of salite in latitic pumice, — inversed zoning and oscillatory zoning (with diopsidic and salitic compositions) occur in these salites, — minimal crystallization temperatures measured on the melt inclusions are : 1,020 °C ± 10 °C, 1,040 °C ± 10 °C for more ferriferous salite in trachyte. These variations can be correlated with compositional variations of the phenocrysts. — Melt inclusions of some apatite included in the pyroxene of trachyte point out the same temperature 1,020 °C ± 10 °C. The results suggest the coexistence of different cogenetic magmas at different differentiation degrees. The observed compositional change from trachybasaltic and trachytic melts is consistent with a fractional crystallization process with removal of diopside and olivine in the first step then salite, plagioclase, magnetite and a more and more important role of the sanidine in the most evolved liquids. These hypotheses are in agreement with the magma chamber model proposed by Armienti et al. (in press).