Discovery of Biotite-Bearing Schists Blocks in the Garnet Zone of the Sambagawa Belt of the Asemi District: an Evidence of Tectonic Erosion of Hanging Wall Rocks by Subducting Sediments

Biotite-bearing schists have been discovered in the garnet zone (Fuyunose nappe) of the Sambagawa belt of the Asemi district, central Shikoku, which is covered by the biotite zone (Saruta nappe II and Saruta nappe I). The biotite-bearing schists (subunit II schists of the Fuyunose nappe) are pelitic schists, siliceous schists and basic schists and have plagioclase porphyroblasts, which crystallized during the prograde phase of metamorphism, like the case of the Saruta nappe (I +II) schists. They occur as lenses in the biotite-free schists (subunit I schists of the Fuyunose nappe) which have plagioclase porphyroblasts of the rerograde phase. Amphibole, which crystallized in hematite-bearing basic schists of the subunit I of the Fuyunose nappe during the peak metamorphism, is glaucophane. Biotite of the subunit II schists is commonly found only in plagioclase porphyroblasts, and the inclusion biotite in hematite-bearing siliceous schists of the subunit II occurs together with barroisite, katophorite and taramite. Barroisite of the subunit II schists, which crystallized together with biotite, have distinctly lower values of NaB content than that of the prograde phase of the Saruta nappe (I +II) schists (biotite zone schists) and than that of the retrograde phase of the subunit ( I +II ) schists, showing that the subunit II schists were derived from shallow tectonic positions of subduction zone. The subunit II schists had already been intermingled with the subunit I schists when the peak metamorphism of the latter had begun. It has been concluded in this paper that the origin of the subunit II schists is ascribed to the tectonic erosion and subduction of the hanging wall rocks [probably low pressure parts of the Saruta nappe ( I +II ) schists] of the subduction zone during the subduction of the original sediments for the subunit I schists,which induced great decrease of temperature along the subduction channel.