Abstract
Glaucophane-epidote eclogite occurs in the eclogitic unit of the Omi serpentinite mélange, SW Japan. The eclogite has a peak mineral assemblage of garnet+omphacite+glaucophane+epidote+quartz+rutile ±phengite. A P-T pseudosection for a bulk composition appropriate to the eclogite has been calculated using the THERMOCALC 2.75 program in the model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH) with quartz and water in excess. In the calculated phase diagram, a trivariant field of garnet+omphacite+glaucophane+zoisite occurs within the high-pressure divariant field of garnet+omphacite+glaucophane+hornblende (sensu lato)+zoisite - it is consistent with the previous P-T estimation of peak metamorphism at T=550-600ºC and P>1.8 GPa. The P-T pseudosection and average-P calculations using THERMOCALC reveal a significant pressure gap between two contrasting high-grade rocks in the eclogitic unit and the non-eclogitic unit. The glaucophane-epidote eclogite may have experienced higher pressure relative to the non-eclogitic paragonite-garnet amphibolite characterized by a trivariant field of garnet+paragonite+hornblende(sensu lato)+zoisite. Considering inferred thermal structure of present-day subduction zones, glaucophane-epidote eclogite may record subduction metamorphism of young and warm slab.
