Retrogressive development of glaucophane in some eclogites from “Massif Armoricain” (East of Nantes,France)

A petrological and mineralogical study, using an electron microprobe, of a blue-amphibole eclogite occurring near Nantes (Massif Armoricain, France) has enabled us to characterize this amphibole as glaucophane resulting from a secondary reaction in the rock. This sodic amphibole was formed at the expense of primary eclogite paragenesis including omphacite, garnet and quartz, according to a sliding reaction which it was possible to study quantitatively: 3.24 omphacite+0.90 SiO₂+0.76 garnet+1.08 H₂O =1 glaucophane+0.55 grossular (S.S. in the garnet) +0.04 paragonite.This reaction is accompanied by a variation in the distribution of iron and magnesium between the amphibole, the garnet and the omphacite.The appearance of the glaucophane can be explained as the beginning of a retromorphic evolution from the stable physical conditions of the primary eclogite paragenesis (650±100° C; minimum pressure 15 Kb).