Redistribution of high-pressure fluids during retrograde metamorphism of eclogite-facies rocks (Voltri Massif, Italian Western Alps)

High-pressure clinopyroxenes in metasomatic mafic rocks (rodingites and metagabbros containing titanoclinohumite) and eclogites from the Voltri Massif(Ligurian Western Alps) retain primary, two-phase aqueous inclusions (H2O + NaCl +/- KCl). In the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside. The eclogites display mylonitic textures: primary fluid inclusions are hosted in early porphyroclasts of omphacite (omphacite I) and are absent in later fine-grained omphacite (omphacite II) that forms the mylonitic foliation. Omphacite I has been frequently replaced by retrograde symplectites (clinopyroxene + plagioclase +/- amphibole); in such cases the fluid inclusions are either absent, or occur in much lower number. The textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite I. The symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles, indicating that breakdown of the omphacite I to symplectites still occurred at relatively high-pressure. Post-entrapment changes have caused a large range of densities in the analyzed inclusions. Although the textural evidence constrains the entrapment of fluid inclusions to high-pressure conditions, the highest density isochores in the eclogites indicate pressure-temperature conditions (8 kbar and 550 degrees C) lower than those attained during the high-pressure stage. Such P-T estimates approach those of symplectite formation after omphacite I. Thus, the fluid inclusions preserved in omphacite were formed by low-pressure re-equilibration of a fluid presumably trapped at eclogite-facies conditions. The inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes. The compositional overlap between the analyzed inclusions and those related to the latest (greenschist) evolutionary stage, suggests further redistribution of the high-pressure fluids and close-system behaviour of these rocks during their overall exhumation path.