Omphacite-diopside vein in an omphacitite block from the Osayama serpentinite melange, Sangun-Renge metamorphic belt, southwestern Japan

Omphacite (Jd_46.1-52.0Ae_o-8.4Aug_48.o-51.2) and diopside (Jd_4.3-6.3Ae_0-0.4AUg_93.6-95.6) coexist in a vein cutting an omphacitite block in a serpentinite melange of the Oeyama ophiolite, central Chugoku Mountains. The compositional gap between omphacite and diopside is significantly wider than for other omphacite-diopside pairs reported in the literature. The intergrowth texture of the omphacite-diopside vein suggests that the clinopyroxene pair was contemporaneously crystallized in the fracture from a Ca-bearing, alkali-rich fluid in a sub-solvus condition. Such a fluid may have been introduced from the surrounding serpentinized clinopyroxene-bearing harzburgite. The stability of omphacite and Al-rich pumpellyite in the matrix and the omphacite-diopside vein indicate that the earlier matrix recrystallization and later fracture filling may have both taken place under high-P-T condition during the melange-forming stage.