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Volume 80, Issue 3
Displaying 1-4 of 4 articles from this issue
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Shunichi Suzuki, Yukito Oda, Tadahiko Abe1985 Volume 80 Issue 3 Pages 83-93
Published: March 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSVitrinite reflectivity measurements were made on core samples from eight boreholes which penetrated the Poronai Formation and the Ishikari Group of Paleogene age. The Ishikari Group, the productive coal measures, is divided into the Noborikawa, Horokabetsu, Yubari and Wakanabe Formations in ascending order, and is unconformably overlain by the Poronai Formation. The estimated maximum depth of burial is about 3300m at the basal part of the Poronai Formation.
Vitrinite reflectivity changes gradually with stratigraphic position in the Poronai Formation, and increases rapidly in the Ishikari Group. The values of vitrinite reflectivity average 0.7% at the basal part of the Poronai Formation; 0.87% at the top of the Yubari Formation about 100m below the unconformity ; 0.94% at the bottom of the Yubari Formation about 250m below the unconformity. The paleotemperature estimated from vitrinite reflectivity of the basal part of the Poronai Formation is 130°C with a geothermal gradient of 3.5°C/100m, which is higher than the present gradient of 2.9°C/100m. The measured values of vitrinite reflectivity in the Ishikari Group are higher than values expected from the geothermal gradient for the Poronai Formation. These facts suggest that thermal effect of deep-seated igneous activity was superimposed on the normal burial coalification.View full abstractDownload PDF (792K) -
Atsushi Sakayori1985 Volume 80 Issue 3 Pages 94-103
Published: March 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSGeological survey on Minami Zao volcano has been carried out in order to constract the evolutional history of the volcano and for further petrological investigation.
Minami Zao volcano is situated on the Pacific side of Nasu volcnic zone in Miyagi Prefecture, northeastern Japan, consists of six cones, and occupies approximately 100km2. The evolutional history can be divided into the following three stages: Stage I. Nonomoriyama, Manokamidake, and Eboshidake cones were built on the eastward inclining basement in this order, and Sugigamine cone was formed on the northwestern elevated basement probably at the same time of the formation of the Nonomoriyama cone. Andesite lava flows are dominante, and andesitic pyroclastic flow deposits are found in the successions of the Manokamidake cone. Stage II. Following volcanic activity began to the west of the Manokamidake cone resulting in formation of Byobu cone. First, basic lava had been erupted and was followed by andesitic lavas and pyroclastics. Stage III. To the south of the Byobu cone, volcanic eruptions began and formed Fubo cone consisting of basic lava and pyroclastics.
After the activity or during the late to last activity, two large calderas formed by erosion on the eastern side of the ridge between the Byobu and Fubo cones, and the eruption centers of the Manokamidake, Eboshidake, Byobu and Fubo cones became vague.
The topography of the basement leads to the volume evaluation of only 9.4 km3 for the volcanic body. This can be attributed to the elevated basement under the volcano.View full abstractDownload PDF (955K) -
Tatsuro Horiuchi1985 Volume 80 Issue 3 Pages 104-112
Published: March 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSGabbroic rocks occur in Kajishima, a small islet in the Seto Inland Sea about 20 kilometers north of Niihama city. Layered structures are well-developed in the gabbroic body and cumulate textures are recognized under microscope. The gabbroic rocks are divided into the following five rock types: (1) Opx allivalite, (2) Noritic gabbro-A, (3) Noritic gabbro-B, (4) 01 gabbro, (5) Hyperite. Fo content of olivine ranges from 58 to 65, and the Mg/(Mg+Fe2+) value of parental magma is estimated to be from 0.31 to 0.38 by using the iron-magnesium distribution coefficient between olivine and silicate melt. Plagioclase is An83-96 in composition. The high An content suggests a high vapour pressure. A small amount of diorite is also found in lenticular form in hyperite. Diorite contains quartz, and plagioclase is more sodic than that in hyperite. Diorite is considered to be the later stage product of the Kajishima magma. Parental magma of the Kajishima gabbroic body is considered to be a tholeiitic one that has high Fe/Mg value and high vapour pressure.View full abstractDownload PDF (1555K) -
Mitsuo Hashimoto1985 Volume 80 Issue 3 Pages 113-117
Published: March 05, 1985
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Japanese metamorphic or tectonic belts where glaucophanitic rocks occur are classified into the following two types of contrasting geologic features.
First type represented by the Sambagawa belt
1. Large areal extent
2. Traceable and analysable geologic structure
3. Low proportion in volume of ultrabasic rocks to metamorphic rocks
4. Single facies series of metamorphism
5. Uniform radiometric ages
Second type exemplified by the Kamuikotan tectonic belt
1. Relatively small areal extent, especially narrowness in width
2. Highly complicated, untraceable and unanalysable geologic structure
3. High proportion in volume of ultrabasic rocks to metamorphic rocks
4. Multiple facies series of metamorphism
5. Two or more clusters of radiometric ages
Most of the Japanese glaucophanitic terranes belong to the second type, but only the Sambagawa belt is the representative of the first type. Although the Sambagawa belt constitutes a typical paired metamorphic belts in association with the Ryoke low P/T belt, most of the second type terranes do not always be paired.View full abstractDownload PDF (373K)
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