Dunites and serpentinites in the Sanbagawa metamorphic belt, central Shikoku and Kii Peninsula, Japan.

Abstract

Large dunite-serpentinite masses of the Sanbagawa metamorphic belt occur in the axial zone of large scale recumbent fold. The texture and chemistry of olivines from the Higashi-Akaishi and Shiraga bodies (central Shikoku) and the Ryumon body (Kii peninsula) demonstrate the two types of dunite to be present in this belt. Dunites of the Higashi-Akaishi body had experienced two stages of metamorphism, resulting in slight compositional change of their olivines, but the dunites remained dunite, whereas those of the Shiraga and Ryumon bodies consist of olivines newly formed by the metamorphism of serpentinite. The newly formed olivines contain the minerals of serpentinite, such as magnetite, antigorite, chlorite, brucite and magnesite, as inclusions, and some of them have a positive correlation between XFe and NiO content being the reverse to the compositional trend formed by igneous crystallization. The presence of olivine+antigorite in the Shiraga and Ryumon bodies is in harmony with the thermal structure of the Sanbagawa terrain. This harmony and the widespread occurrence of newly formed olivine indicate that this type of dunite was formed by the metamorphism of serpentinite during the Sanbagawa metamorphism. From the view point of thermal history, not all the peridotites in the Sanbagawa metamorphic belt are simple dismembered masses of the Higashi-Akaishi-type peridotite.