Coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro, Western Alps, North Italy

Abstract

The objective of this work is to study the most significant static and syn-tectonic transformations which accompanied the eclogite metamorphism (450 ± 50°C, 12–15 kbar) of an ophiolite metagabbro from the Western Alps. In partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or (incomplete) pseudomorphic reactions. The disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate-limiting factors which prevented a large-scale eclogite equilibration. The undeformed volumes are bounded by syn-eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric/foliation develops. The enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation. The bulk-rock chemistry also played an important role in determining the kind of eclogite assemblage; in particular only sheared gabbros rich in Fe can produce bimineralic eclogites.