Peraluminous sapphirine formed during retrogression of a kyanite-bearing eclogite from Pays de Leon, Armorican Massif, France

In a quartz-free eclogite from Pays de Leon (Armorican Massif, France) kyanite has been replaced by a symplectite of plagioclase, corundum, spinel and sapphirine.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 ((Mg1.38Fe0.172+Fe0.053+Cr0.01Al2.37Al2.43Si0.57O10)-Al-VI-Si-IV) shows the highest degree of Tschermak's substitution ((Si-1Mg-1AlAlVI)-Al-IV) 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) + SiO2 --> 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 Leon 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. (C) 1998 Elsevier Science B.V.