Coexisting chromian omphacite and diopside in tremolite schist from the Chugoku Mountains, SW Japan: the effect of Cr on the omphacite-diopside immiscibility gap

Chromian clinopyroxenes with exsolution textures were found in high-pressure (HP) tremolite schist from the Osayama serpentinite melange in the Chugoku Mountains, Southwestern Japan. Chromian omphacite [jadeite [(Jd).sub.23.3-41.4] diopside [(Di).sub.46.7-.60.7] kosmochlor [(Ko).sub.4.5-19.6] aegirine [(Ae).sub. Chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite, chlorite, and pumpellyite. The wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and Si-rich chromian phengite indicate T < 300-400 [degrees]C and P > 0.8 GPa. This P-T estimate is consistent with parageneses of minerals in the host serpentinite. The variation of Cr content in chromian silicates reflects the extent of Cr [left and right arrow] A1 substitution, and may be related to a chemical heterogeneity of the Cr-bearing fluid.