Abstract
In a quartz-free eclogite from Pays de Léon (Armorican Massif, France) kyanite has been replaced by a symplectite of plagioclase, corundum, spinel and sapphirine. The sapphirine is amongst the most aluminous yet reported in the literature. One analysis (Mg 1.38Fe 0.17 2+Fe 0.05 3+Cr 0.01Al 2.37 VIAl 2.43 IVSi 0.57O 10) shows the highest degree of Tschermak's substitution (Si −1Mg −1Al IVAl VI) yet reported for sapphirine. Omphacite has been completely transformed into clinopyroxene+plagioclase symplectite. A quantification of material transfer indicates that kyanite and omphacite breakdowns were coupled. The kyanite and omphacite pseudomorphs exchanged components during their formation but the two pseudomorphs together behaved as a more-or-less closed system. The kyanite symplectitisation is the source of the silica required for the omphacite breakdown (i.e., jadeite ss+SiO 2→albite ss). The kyanite and omphacite pseudomorphs display all the characteristics of a metasomatic system at a microscopic scale (zonation, diffusion fronts, etc.). These features are explained in terms of relative mobility of the chemical components. The kyanite and omphacite pseudomorphs give information about the P– T path of the Léon eclogites during retrograde metamorphism. The appearance of sapphirine after kyanite is attributed to a low-pressure and high-temperature evolution from eclogite-facies to granulite-facies conditions.
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