Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt, central Shikoku, Japan

Metamorphic olivine veins in serpentinites are considered to be a metamorphism dehydration product. However, the previous descriptions of these veins have been from paleo-collision belts such as those in the Alps, Norwegian Caledonides, and Spain. Here the unique metamorphic olivine veins in a mass of serpentinite in the high-pressure/low-temperature (high P/T) Sanbagawa belt was reported. The Sanbagawa belt is a product of the subduction of oceanic plates during the Cretaceous. Elongated olivine-talc veins were observed along shear bands in serpentine mylonite. The olivine's chemical composition is nearly pure forsterite; magnetite is distributed along the boundary of the olivine vein. This clearly suggests that the olivine was formed by the antigorite terminal reaction at a pressure less than 1.5GPa. The reaction was caused by local heating in the mylonite as there is little evidence of the continuous antigorite breakdown reaction in the serpentinite. Metamorphic olivines are possibly formed in the intraslab earthquake zone of a relatively warm subduction zone such as southwest Japan after the antigorite terminal reaction at P less than 1.5GPa.