Abstract: Peridotites from the southern Mariana forearc are described and the mineral compositions analyzed by EPMA. Amphibole which generally falls into Magnesiohornblende and Tremolite is common in these peridotites and the abundance of Al₂O₃ ranges from 0 wt% to 10 wt%. Composition of spinel in these peridotites is closely related to amphibole:the occurrence of amphibole rich in Al₂O₃ is always accompanied by enrichment of Al and depletion of Cr in spinel,and the occurrence of amphibole poor in Al₂O₃ by enrichment of Fe³⁺ and depletion of Al. These indicate there were two stages of hydrous metasomatism in mantle wedge under the Mariana arc:(1)Hydrous fluid (melt) enriched in Al and Ca reacted with mantle wedge under high pressure and temperature, resulting in the consumption of orthopyroxene and the occurrence of amphibole with high Al₂O₃ to lead to enrichment of Al and depletion of Cr in spinel; (2) Hydrous fluid (melt) enriched in Ca continually reacted with mantle wedge in low pressure and temperature conditions and changed the residual orthopyroxene into amphibole with low Al₂O₃ to lead to the enrichment of Fe³⁺ and depletion of Al in spinel.