笠岡福山地方における基盤岩類の地質構造と圧砕花崗岩類の産状

The Geologic Structure of Basement Rocks and the Mode of Occurrence of Mylonitic Granites in the Region of Kasaoka-Fukuyama, Chûgoku Province, Southwest Japan

HIDE Kei

廣島大學地學研究報告

Geological report of the Hiroshima University

In the present paper, the outline of geology, the geologic structure of the basement rocks, and the mode of occurrence of the mylonitic granites, observed in the region concerned, have been described with special reference to the "Yakuno Intrusive Rocks."
The basement rocks are divisible into the Paleozoic sedimentaries, the green-rocks and the my-lonitic granites. The first of the three consists mainly of black slate intercalated with sandstone, conglomerate, rhyolitic tuff, and lava which are found occurring locally as thin beds, and the second consists mainly of metadiabase, hornblende-metagabbro and partly of albite-epidote-horn-blende-schist. These basement rocks might have, in most part, been sheared prior to deposition of the Inakura Inkstone group of the Lower Cretaceous and thermally metamorphosed by intrusion of the Hiroshima granites of the Upper Cretaceous.
With only few data for bedding planes •and without those for axial planes and for fold-axes concerning the basement rocks, the author has, on the basis of the pattern of their distribution and the trend of the boundary surfaces between the green-rocks and the Paleozoic formation, interpreted the geologic structure of the green-rocks as sheet-like forms spreading horizontally as a whole. Their main mass reveals an asymmetrical open-fold with an axis traceable at least 20 km in the direction E-W, the axial plane dipping to the north, and the wave-length estimated at about 5 km. This structure is believed to have essentially been formed before sedimentation of the Inakura Inkstone group.
The mylonitic granites are found occurring as small masses with the diameter ranging from a few to 100 m not only in the inner parts, as well as along the margins, of the main mass of green-rocks but also in the Paleozoic formation. The green-rocks and the mylonitic granites seem to have given no thermal effects on their wall rocks.
Taking the lithologic characters and the tectonic situation into account, the green-rocks and the mylonitic rocks distributing in the region under consideration are to be included in the "Yakuno Intrusive Rocks" or "Yakuno Complex," the distribution of which has hitherto been traced from the Yakuno region to the central part of the Okayama Prefecture.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676