Late Mesozoic low-P/high-T metamorphism preceding emplacement of Cretaceous granitic rocks in the Gosaisho-Takanuki district, Abukuma metamorphic belt

Abstract The central part of Abukuma metamorphic belt consists of two geologic units, the Gosaisho Group and the Takanuki Group. Although the deformation styles differ between the Gosaisho and the Takanuki Groups, their rock facies show a gradual transition. In both Groups early regional low-pressure (over 3 kb) metamorphism has been overprinted by contact metamorphism. Evidence for the P/T condition of the regional metamorphism is recorded in cores of armored minerals. Metamorphic zones have been defined on mineral rim assemblages of meta-mafite, meta-pelite and meta-calc-siliceous schist and on the degree of graphitization of meta-pelite. The mineral-core chemistry of plagioclase, Ca-amphibole and garnet changes with increasing metamorphic grade, and indicates that the regional metamorphism of the Gosaisho Group took place in a high pressure region of the andalusite stability field. The Takanuki metamorphic rocks are structurally overlain by the Gosaisho Group and have undergone regional metamorphism whose conditions have passed near the triple point of Al-silicates and kyanite has crystallized. The contact aureoles in both groups are developed around middle Cretaceous granitic intrusions. Rims of plagioclase, Ca-amphibole and garnet overgrew on the mineral-cores during the contact metamorphism. The regional metamorphism began after the sedimentation of Jurassic chert and was succeeded by the contact metamorphism in the middle Cretaceous.