open access

https://hdl.handle.net/2115/36725

The Tokoro Belt, a Tectonic Unit of the Central Axial Zone of Hokkaido

英語

1984-02

Hokkaido University

Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy

21(1)

21-75

The Tokoro Belt, a tectonic unit occupying the eastern side of the Central Axial Zone of Hokkaido, is made up of Jurassic greenstones and Cretaceous clastic sedimentary rocks. The former is called Nikoro Group and the latter Saroma and Yubetsu Groups. The Nikoro Group is separately distributed forming two masses called the Eastern and the Western masses. The geologic constitutions of the two masses have been studied and the following distinct contrasts between them were revealed: The Western mass is characterized by the presence of abundant hyaloclastites and pillow lavas which are alkalibasalt or trachyte. Limestones associated with chert are rather common in the mass. Strata-bound Mn-oxide ores and massive sulfide ore deposits both in a small scale are found here. On the other hand, the Eastern mass is characterized by the preponderant occurrence of pillow lavas of tholeiitic basalt. It is noteworthy that manganiferous hematite deposits predominate in the Eastern mass, especially in the northern part. Hyaloclastites and limestones are poorer here. REE abundances of pillow lavas and related rocks from the two masses were examined. The results obtained show that the pillow lavas from the Eastern mass is the solid type whereas those from the Western mass the liquid type. Gabbro and troctolite believed to be of tectonic blocks are found in the northern Tokoro Belt though the quantity is scant. The above-stated facts suggest that the Nikoro Group of the Tokoro Belt is composed of two masses those are different in origin, both of which are presumed to have been derived from an oceanic crust having ophiolite sequences. The Eastern mass was possibly produced in an abyssal sea floor, while the Western mass represents seamounts (Niida et al., 1983) of Jurassic time.

紀要論文

Version of Record

0018-3474