Volume 26, Issue No. 2b, 2001

Silvana Martin* and Giovanni Cortiana**
 
* Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dellâInsubria, via Valleggio 11, 22100 Como, Italy (e-mail: silvana.martin@uninsubria.it).
** Dipartimento di Geologia Paleontologia e Geofisica, via Giotto 1, 35137 Padova, Italy (e-mail: pcortia@tin.it).

Keywords: blue amphiboles, blueschist, geochemistry, metaophiolite. North-western Alps.

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 gabbro-derived 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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