Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican Massif, France): a potential geobarometer

Abstract The garnet blueschists from the Ile de Groix (Armorican Massif, France) contain millimetre‐ to centimetre‐sized pseudomorphs consisting of an aggregate of chlorite, epidote and paragonite. The pseudomorphed phase developed at a late stage of the deformation history, because it overgrows a glaucophane–epidote–titanite foliation. Garnet growth occurred earlier than the beginning of the ductile deformation, and thus garnet is also included in the pseudomorphs. Microprobe analyses show that garnet is strongly zoned, with decreasing spessartine and increasing almandine and pyrope contents from core to rim. Grossular content is higher in garnet cores (about 35 mole%) compared to garnet rims (about 30 mole%). Blue amphibole has glaucophane compositions with a low Fe³⁺ content and become more magnesian when inclusions in garnet (X_Mg = 0.62–0.65) are compared with matrix grains (X_Mg = 0.67–0.70). Matrix epidote has a pistacite content of about 50 mole%. On the basis of their shape and the nature of the breakdown products, the pseudomorphs are attributed to lawsonite. A numerical model (using Thermocalc ) has been developed in order to understand the reactions controlling both the growth and the breakdown of lawsonite. Lawsonite growth could have taken place through the continuous hydration reaction Chl + Ep + Pg + Qtz + Vap = Gln + Lws, followed by the fluid‐absent reaction Chl + Ep + Pg = Grt + Gln + Lws. Peak P–T conditions are estimated at about 18–20 kbar, 450 °C. This indicates that lawsonite growth took place at increasing P and T, hence can be used as a geobarometer in the buffering assemblage garnet–glaucophane–epidote. The final part of the history is recorded by lawsonite breakdown, after cessation of the ductile deformation, and recording the earliest stages of the exhumation.