Abstract
The Permian Shiroumadake Formation is distributed in the Shiroumadake area of the northeast Hida Gaien belt. The Shiroumadake Formation is redefined as a coherent succession of shallow-marine sediments which are composed mainly of felsic pyroclastic rocks deposited in a volcanic arc with extrusion of basalt and intrusion of gabbro, and divided into three members. A bimodal volcanism which is characterized by felsic tuff, felsic tuff breccia and subaqueous eruption of basalt is recognized in lower member of the Shiroumadake Formation. The volcanism was suspended, and shale and sandstone were deposited in middle member. The bimodal volcanism was reactivated during the deposition of upper member, and felsic tuff breccia with limestone breccias and blocks was accumulated due to slumping or collapse of slope deposits.
The Shiroumadake Formation was thrust by serpentinite melange containing amphibolite blocks with west vergence. After the thrusting, NW-SE and NE-SW trending and steep-dipping faults were formed. The faulting resulted in an emplacement of the Shiroumadake rocks as tectonic blocks within the serpentinite melange.
