Abstract
Metamorphic biotites have been found from the pumpellyite-actinolite facies basic semischists in the northern Chichibu Belt, western Shikoku, Japan. Representative mineral assemblages of the northern Chichibu basic semischists in the study area are chlorite+phengite+calcite, riebeckite+phengite+chlorite+calcite, riebeckite+stilpnomerane+chlorite+calcite, biotite+riebeckite+chlorite+calcite and biotite+riebeckite+phengite+chlorite+calcite. Albite, titanite, hematite and quartz are common. The biotites occur in high-MgO basic semischists and are rich in phlogopite content. The mineral parageneses systematically change depending on MgO/(MgO+FeO) of bulk compositions. The bioitite-calcite-bearing basic semischists were metasomatically altered based on petrographical and geochemical data. The isocon diagram suggests that CaO and MgO were added in significant amounts and Na2O, K2O and P2O5 were removed. It seems that the increase in MgO due to metasomatism is a cause of the formation of biotite under the pumpellyite-actinolite facies.
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