The Ohsa-yama ultramafic body, which consists of several types of serpentinized peridotites, underwent contact metamorphism caused by a Cretaceous granite intrusion ; this metamorphism resulted in the formation of contact aureole, 1.5-2.0 km wide, around the granite intrusion, and produced progressive mineral changes in metaserpentinites toward the contact between the Ohsa-yama body and the granite intrusion. On the basis of analysis of mineral paragenetic relations, the Ohsa-yama ultramafic body can be divided into three zones with progressive changes in mineral assemblages as follows :
Zone Ⅰ : serpentine ± chlorite ± brucite
Zone Ⅱ : olivine + talc ± tremolite ± chlorite
Zone Ⅲ : olivine + orthopyroxene ± tremolite ± spinel
Zone Ⅰ corresponds to the parts unaffected by the thermal event, and Zones Ⅱ and Ⅲ correspond to the thermally metamorphosed parts of the Ohsa-yama body. The results obtained in this study are generally consistent with those of the previous studies on metamorphic peridotites from the Sangun and Muzuru zones.