Abstract
The Samondake Unit of the Mino terrane in central Japan is characterized by a large quantity of sandstone. The structure, age and tectonics of this unit have been poorly known because of its monotonous lithofacies, remarkable deformation and scarce evidence of fossils.The Samondake Unit in the Neo-Izumi area in the northern part of the Mino terrane is divided into two subunits-the Kuzawadani Subunit and the Kochidani Subunit on the basis of lithology and structure. The Kuzawadani Subunit exposed in northeastern part of the study area has been thrust over the Kochidani Subunit in the southwestern part. The Kuzawadani Subunit is mainly composed of sandstone and mudstone without chert. This subunit is gently folded and not strongly deformed.The Kochidani Subunit is composed of strongly-sheared strata and mudstone-dominant melanges including chert blocks.The deformation of rocks and the low-grade metamorphism based on illite crystallinity of the Samondake Unit are investigated in the Neo-Izumi area. The Kubler index of illite crystallinity ranges from 0.33Δ°2θ to 0.64Δ°2θ. Both illite crystallinity and deformation degree of the beds increase toward the southeastern area where the structurally lower part of the unit is exposed. This suggests that the thermal structure of the Samondake Unit is not a result of igneous intrusion but of the burial metamorphism in the subduction process. The structurally lower part showing high illite crystallinity was uplifted as a result of accretion of the seamount which made up the Funafuseyama Unit now located to the south of the Samondake Unit.
