Abstract: I-type granites,aplites and pegmatites outcropping in Mesozoic metamorphic belt of the southeastern coast of Fujian Province contain commonly magmatic garnets and primary muscovites.The garnets are characterized by high Mn,Fe and low Mg,Ca,up to 40～50％ spessartite component.The btotites in the granites also have high MnO and A1203 contents-up to 0.84～1.25％ and 16.04～18.03％ respectively.Primary muscovites exhibit high Fe and low A1 and Na.T-P calculations indicate that these Al-rich minerals crystallized at 750～600％ and 0.3～0.2 GPa.Both petrographic features and the geochemical modeling also show that they are the late-stage crystallization phases.Earlier crystallization of felsic minerals than biotite and garnet is related with much lower crystallization pressure.Phase diagram analyses demonstrate that week peraluminity and high Mn／(Mg+Fe) ratio (>0.060)are two key factors for the formation of garnet in the granitoids.Granitic magma with Mn／(Mg+Fe)>0.06O may produce bt+grt or bt+ rt+ins,and that with Mn／(Mg+Fe)<0.055 only forms bt or bt+ins.Grt+ins assemblage is the ultimate products of strongly evolved magma.Garnet and primary muscovite from these I-type granites have similar composition to those from S-type granites, therefore they cannot be simply used for classifying granite types.