Garnet-biotite-muscovite granite in the Iide Mountains

Abstract

The garnet-biotite-muscovite granite in the Iide Mountains occurs as numerous small concordant bodies in sedimentary rocks of the Ashio Belt. It is medium to fine grained, leucocratic and composed mainly of quartz, plagioclase, k-feldspar, biotite, muscovite and garnet. Bulk chemical compositions of the garnet-biotite-muscovite granite are peraluminous whose Al₂O₃/(CaO+Na₂O+K₂O) (molecular ratio) are 1.05-1.19. The garnet is rich in spessartine molecule (sps.26.69-46.12%), whose rim is richer in Mn and poorer in Mg than the core indicating a reverse zoning. Mn/(Mg+Fe+Mn+Ca) (atomic ratio) of garnet and MnO/(MgO+FeOt+MnO+CaO) (molecular ratio) of whole rock are mutually related. Mg/(Mg+Fe+Mn) (atomic ratio) of biotite and MgO/(MgO+FeOt+MnO) (molecular ratio) of whole rock, anorthite content of plagioclase and CaO/(CaO+Na₂O+K₂O) (weight percent ratio) of whole rock are also correlated. The garnet crystallized from a Mn-rich peraluminous magma, because it is spessartine rich, reversely zoned and its Mn content is correlated with that of the whole rock. The biotite and plagioclase also crystallized from the peraluminous magma, as their compositions are also correlated with whole rock chemistry. The garnet bearing tonalite in the Hidaka Metamorphic Belt is poor in Or content and rich in An content and separated from the garnet-biotite-muscovite granite in the Iide Mountains by a cotectic line on the normative An-Ab-Or diagram. Therefore, it is difficult to get the garnet-biotite-muscovite granite magma of the Iide Mountains from the garnet bearing tonalite magma of the Hidaka Metamorphic Belt, which is representative of anatectic melt from pelitic granulite of high T/P type metamorphic belt. The garnet of the garnet bearing tonalite in the Hidaka Metamorphic Belt is poorer in spessartine molecule (sps.2.4-6.1%) than the garnet of the garnet-biotite-muscovite granite in the Iide Mountains. This contrast is probably caused by the difference of temperature, pressure and composition of the magma at the stage of garnet crystallization.