Origin of andalusite-kyanite-sillimanite aggregates in the Nishidohira pelitic rocks in the southernmost part of the Abukuma Plateau, Northeast Japan, and the P-T path

Abstract

Three polymorphs of Al₂SiO₅, andalusite, kyanite and sillimanite, occur in contact with each other, forming aggregates in a part of the Nishidohira pelitic metamorphics in the Hitachi area of the southernmost Abukuma Plateau. Textural relationships between the three polymorphs suggest that andalusite formed first probably as chiastolitic porphyroblasts, that kyanite and/or sillimanite grew subsequently at the expense of pre-existing andalusite, and that sillimanite continued to form even in the rock matrix thereafter. The andalusite-forming first stage may have been either a contact metamorphic event by unknown plutons or a regional metamorphic event of low-pressure type. The second kyanite and/or sillimanite-producing stage was induced by high-temperature loading, whereas the third sillimanite-forming stage possibly resulted from the contact metamorphism by the Nishidohira gabbroic intrusion exposed near Hase. Well-preserved growth zoning of garnet containing sillimanite inclusions in the andalusite-kyanite-sillimanite-bearing gneisses also indicates the high-temperature compression. The loading may be attributed to the overthrusting of the nearby Hitachi metamorphics onto the Nishidohira metamorphics. Such a P-T path of the Nishidohira metamorphics is similar to that of the Takanuki metamorphics in the central to southern part of the Abukuma Plateau. The widespread occurrence of texturally sector-zoned garnet in pelitic-psammitic rocks and the inferred short duration of high-temperature conditions are also in common to the Nishidohira metamorphics and the Takanuki metamorphics.