Shimanto Belt Research Articles

Chemical compositions of green rocks in the Shimanto Belt, Southwest Japan.

Chemical compositions of green rocks in the Shimanto Belt, Southwest Japan.

Cretaceous tectonic melange of the Shimanto Belt in Shikoku, Japan.

Cretaceous tectonic melange of the Shimanto Belt in Shikoku, Japan.

四万十帯緑色岩類の産状を中心とした成因的考察( 日本列島のオフィオライト-地団研第31回総会テーマ別討論会-)

四万十帯緑色岩類の産状を中心とした成因的考察( 日本列島のオフィオライト-地団研第31回総会テーマ別討論会-)

SHIMANTO GEOSYNCLINE AND KUROSHIO PALEOLAND

The late Mesozoic to early Neogene geosyncline in the Outer zone of Southwest Japan has been studied in detail in the Kii Peninsula by the Research Group for the Shimanto Geosyncline. The existence of the Kuroshio Paleoland to the south of the geosyncline was inferred by various sedimentologic evidences. The Shimanto belt in the Kii Peninsula is divided from north to south into three zones of Cretaceous, Eocene and Oligocene to lower Miocene. In these belts thick geosynclinal sediments were accumulated showing coarsening upward. The southward migration of the basin occurred in Cretaceous/Eocene, Eocene/Oligocene, and in early Miocene. In the present paper the reconstruction of paleogeography of the Shimanto geosyncline was attempted and the Kuroshio Paleoland was discussed in relation to the geohistory of the Philippine Sea. In spite of the detailed geologic survey in the Kii Peninsula there is no evidence of large exotic blocks nor tectonic melanges, and this does not support the plate tectonic model of the Pacific-type orogeny for the Shimanto belt.

Evaluation of Molluscan Fossils from the Mesozoic of the Shimanto Belt

Evaluation of Molluscan Fossils from the Mesozoic of the Shimanto Belt

Contact metamorphic aureole around the Kaikoma-Hoo granodiorite pluton in the northern part of Akaishi mountains, central Japan.

The contact-metamorphosed Mesozoic rocks of the Shimanto Belt at the northern part of the Akaishi Mountains are studied in detail. Two zones, i. e. Biotite and Cordierite Zones, are distinguished for pelitic rocks. The latter is further separated into two zones, Cordierite I and II Zones. Recognition fo the Cordierite I and II Zones is based on whether or not chlorite occurs in contact with white mica in the presence of cordierite. The area where chlorite not with white mica is Cordierite II Zone. The following reactions are proposed for each isograd on the basis of chemical, modal and paragenetic study of metamorphic minerals. (1) Formation of biotite at biotite isograd. celadonite molecule of white mica+chlorite→biotite+muscovite+quartz+H2O (2) Formation of cordierite at the first cordierite isograd. chlorite+Al2O3+quartz→cordierite+H2O (3) Formation of cordierite at the second cordierite isograd. celadonite molecule of white mica+chlorite→cordierite+biotite+quartz+H2O In the high grade part of Cordierite II Zone, garnet commonly occurs. This part is called “Garnet Zone” in this paper. The appearance of garnet is controlled by the difference of bulk chemical composition of host rocks, i. e. garnet occurs in metapelite rich in MnO, high in (Mn+Mg+Fe+2)/(Mn+Mg+Fe+2+Al) ratio and low in Mg/Fe+2 ratio. The following reactions are considered for the formation of garnet, although it is not clear which is more realistic. chlorite+quartz→garnet+H2O celadonite molecule of white mica+cholriet→garnet+biotite+quartz+H2O In either case chlorite rich in MnO must have participated in the reaction. The zone of garnet+potassium feldspar which has been reported by previous studies is not recognized.

Cretaceous Fossils from the Shimanto Belt of Kagoshima Prefecture

Cretaceous Fossils from the Shimanto Belt of Kagoshima Prefecture

Occurrence of the pumpellyite actinolite-bearing green rock from Shimanto Belt, Miyazaki Prefecture

Occurrence of the pumpellyite actinolite-bearing green rock from Shimanto Belt, Miyazaki Prefecture

On the Tertiary granitic rocks around Kohu basin, Yamanashi Prefecture

The Kohu plutonic body which occurs as a semicircular outcrop in Kohu basin is one of the largest Tertiary granitic bodies in Japan. Its N-S elongation is 45km. and its E-W breadth is 30km. It traverses the boundary between the Shimanto Belt and the Fossa Magna. The granitic rocks composing this plutonic body are divided into 10 types according to their occurrence, petrography, chemistry and mineralogy. These types are named: 1) Hirose, 2) Sanpo, 3) Shiodaira, 4) Yogai, 5) Tonogi, 6) Kanegawa, 7) Kurogane, 8) Ashigawa, 9) Mizugaki-Shosenkyo and 10) Huruyado. The first four types are granodiorite, carrying biotite and hornblende. The next three types are quartz-diorite, with hypersthene and c1inopyroxene (augite or salite). The Ashigawa type is a quartz-diorite without pyroxene. And the last two types are adamellite. These granitic rocks are associated with intrusions of andesite, quartz-porphyry and andesitic pyroclastic rock in the area. New chemical analyses of 32 rock samples of these granitic rocks are given in Table 4. Their chemical compositions, except that of the Asigawa type, are generally similar to the Tertiary granitic rocks in the Green Tuff Region of northeastern Japan and the Shimanto Belt. The Ashigawa type is very low in K2O and high in Na2O and resembling the Tanzawa plutonic body.