The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

GEOSYNCLINE VOLCANISM OF THE MIKABU GREEN-ROCKS IN THE OKUKI AREA, WESTERN SHIKOKU, JAPAN

TAKASHI SUZUKI, NARUHIKO KASHIMA, SHIGEKI HADA, HAYAO UMEMURA

1972 Volume 67 Issue 6 Pages 177-192
Published: June 05, 1972
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.67.177

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THE LATE MESOZOIC ACIDIC IGNEOUS ROCKS OF THE NORTHERN ASHIO MOUNTAINLAND

PART 1. FIELD GEOLOGY

Keizo Yanai

1972 Volume 67 Issue 6 Pages 193-202
Published: June 05, 1972
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.67.193

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The acidic igneous rocks of the northern Ashio mountainland, which consists mainly of dacitic to rhyolitic welded tuffs, granite porphyry, granodiorite porphyry and granitic rocks, are the one of the largest volcano-plutonic complexes in Japan. The detailed field work and K-Ar dating show that the activity can be divided into the following five stages:
Sakyozan-Jizodake rhyolite (10m. y.)
Katashina, Kinugawa and Ashio rhyolites (Neogene ?)
Chuzenji acidic rocks (60-70m. y.)
Sori granitic rocks (85-90m. y.)
Matsuki granitic rocks (116m. y.)
La te Mesozoic Era
The Matsuki granitic rocks are the most melanocratic in these types varying from medium grained granodiorite to quartz diorite. The Sori granitic rocks having large porphyritic K-feldspar are leucocratic and coarse grained granodiorite. The Chuzenji acidic rocks consists of the volcano-plutonic complex which was formed with successive eruption and intrusion of the following order, rhyolitic lava, dacitic-rhyolitic welded tuffs, granodiorite porphyry, granite porphyry and granitic rocks. As a result of the eruption of pyroclastics, a great deal of welded tuff was formed in this area and major volcano-tectonic depressions was occurred on the same place.
The Katashina, Kinugawa and Ashio rhyolites are distributed in extensive area between the Ashio and Taishaku mountainlands in the southern Ashio belt. At the last stage of Tertiary period, Sakyozan-Jizodake rhyolite erupted in a southeast of Ashio.

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Rb AND Sr CONCENTRATIONS AND Sr ISOTOPIC COMPOSITIONS OF SOME MAFIC AND ULTRAMAFIC ROCKS FROM OGAWARA BODY, NAGANO PREFECTURE, CENTRAL JAPAN

Shigeru Iizumi

1972 Volume 67 Issue 6 Pages 203-208
Published: June 05, 1972
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.67.203

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Rb and Sr concentrations and Sr isotopic ratios (Sr⁸⁷/Sr⁸⁶) have been determined for some mafic and ultramafic rocks from Ogawara, Kurouchi-yama and Higashiakaishi-yama bodies. Rb and Sr concentrations of mafic rocks are nearly comparable to those of oceanic tholeiites, those of ultramafics being within the range shown by ultramafic inclusions in alkali basalts.
These rocks have Sr⁸⁷/Sr⁸⁸ ratios ranging from 0.7053 to 0.7072. No significant differences in these ratios are observed among these bodies and different rock types. These ratios are similar to those of Cenozoic continental basaltic rocks, and ultramafic inclusions in kimberlites and alkali basaltic rocks, while they lie in the lower part of the range measured in many ultramafic rocks in orogenic belts and oceanic regions.
Considering from the results of present study and previous petrographical studies, it may be the most probable interpretation that these ultramafics are derived by early fractional crystallization of some basaltic or picritic magma.

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