INFLUENCE OF THE WHOLE-ROCK COMPOSITION ON THE CRYSTALLIZATION OF SODIC AMPHIBOLES (PIEMONTE ZONE, WESTERN ALPS)

Silvana Martin, Giovanni Cortiana

Abstract


The recent discovery of glaucophane and crossite amphiboles in metaophiolites of the Combin Unit (Piemonte zone, north-western Alps, Italy) confirms their Alpine re-equilibration under epidote-blueschist facies conditions. Sodic amphiboles were found in two rock types: i) coarse titanite-rich metabasites derived from original Fe-Ti gabbros, and ii) epidote-rich metabasites derived from MOR basalts. Crystallisation of Fe-glaucophane and crossite in the gabbroderived titanite-rich metabasites was favoured by their high total Fe contents, whereas crystallisation of glaucophane and crossite in the basalt-derived epidote- rich metabasites was favoured by Mg depletion due to sea-floor metasomatism. The comparison of all the analysed sodic-amphibole bearing metabasites s.l. from the Combin Unit shows that the FeOtot/MgO bulk-rock ratios, independently on their intrusive or extrusive origin, ranges between 2 and 3. Therefore, it can be concluded that the growth of sodic amphiboles at peak-conditions around 0.6-0.7 GPa and 350-400 °C typical of the subduction metamorphism is favoured in metabasites by a FeOtot/MgO ratio around 2 to 3.

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DOI: https://doi.org/10.4454/ofioliti.v26i2b.166