ISHIKAWA Kenichi College of General Education, Tohoku University; Assistant, 教養部, 助手 (20158744)
YOSHIDA Takeyoshi College of General Education, Tohoku University; Associate Professor, 教養部, 助教授 (80004505)
KANISAWA Satoshi College of General Education, Tohoku University; Professor, 教養部, 教授 (70005784)
OKAMI Kazuyoshi Faculty of Technology, Iwate University; Professor, 工学部, 教授 (60003878)
For the purpose of reexamining the tectonic subdivision for the pre-Tertiary rocks distributed in Northeast Japan, stratigraphy, geologic structure and geochemistry of the rocks were studied. The results of the investigation are as follows:
1. The Matsugadaira-Motai Metamorphic Rocks distributed in the western marginal part of the South Kitakami Belt is one of the pre-Silurian basements, not a Jurassic accretionary complex, which had been metamorphosed at ca. 500 Ma.
2. The boundaries between the South Kitakami Belt (+ Hayachine Tectonic Belt) and the neigh boring tectonic belts, which are composed mainly of the Jurassic accretionary complexes, are high-angle faults, not low-angle ones. Studies on the xenoliths in the post-Jurassic dyke rocks suggest that the Paleozoic formations in the South Kitakami and Hayachine Tectonic Belts are underlain by the metamorphic and/or pultonic basements, not the Jurassic accretionary complexes. Therefore, the South Kitakami Belt is not a Klippe rests on the Jurassic accretionary complexes.
3. The differences in the mineral and chemical compositions of the Jurassic sandstones of the Kuzumaki-Kamaishi and Akka-Tanohata Belts was reconfirmed.
4. N-MORB normalized patterns and major element chemistry for early Cretaceous granitic rocks in all zones of the Kitakami Massif have no radical variation and are characteristic for island arc and continental arc types.
From these facts the tectonic subdivision by Nakagawa et al. (1989) is adopted for pre- Tertiary sedimentary and metamorphic rocks in Northeast Japan: the Abukuma, South Kitakami , Hayachine Tectonic, Kuzumaki-Kamaishi and Akka-Tanohata Belts, from the west.