@	Alteration and calcite formation in a granitic rock in Ashio, central Japan
		K-Ar ages of feldspathization at the Yabuhara feldspar mine, Nagano Prefecture, central Japan
		Thermobarometry of the Tsukuba Metamorphic Rocks
		The low-grade Ryoke metamorphic rocks in the Wazuka district, Kyoto Prefecture, Japan
		Magnetic structures in the eastern margin of the Japan Sea | off Niigata-Aomori area |

Alteration and calcite formation in a granitic rock in Ashio, central Japan

Satoshi HAMASAKI(Mineral and Fuel Resources Department GSJ), Katsuhiro TSUKIMURA(Mineral and Fuel Resources Department GSJ), Koichiro FUJIMOTO(Geothermal Research Department GSJ), Ryuji IKEDA(National Research Institute for Earth Science and Desaster Prevention) and Kentaro OMURA(National Research Institute for Earth Science and Desaster Prevention)

1999

vol. 50 (8), P. 499-508

10 figs., 1 table, 2 appendix-tables

Keywords: granitic rock, alteration, microfractures, plagioclase, calcite, Ashio

Abstract: This paper has described the alteration minerals in host rock and microfractures of deep granitic rock, and the formation process of them. Samples were taken from the drill core, maximum depth 2002 m, in the granodiorite at Ashio, central Japan. The host granodiorite has many microfractures with width of less than 2 mm. The main alteration minerals in the host rock are illite, chlorite, calcite and fluorite. Plagioclase is altered to illite and biotite to chlorite. The microfractures are filled with quartz, illite, chlorite, laumontite or calcite. There are rare correlations between the alteration degree and the depth. The alteration degree increases toward the microfracture, so the hydrothermal solution along the fractures altered the nearby host rock. Although the measured present temperature of 2000 m deep ground water is 91ĄC, the alteration minerals formed at less than 100ĄC are not observed. Therefore the alteration minerals have not been formed by the present deep ground water. As the alteration proceeds, illite and chlorite are accompanied with much calcite. Based on the occurrence of calcite and chemical composition of primary rock-forming minerals, the alteration of An-component plagioclase and precipitation of calcite have occured intimately. It is considered that the pH and Ca2+ concentration of the hydrothermal solution caused the acceleration of both reactions. The plagioclase-water interaction and precipitation of calcite are important in the alteration process of the granodiorite in the study area

K-Ar ages of feldspathization at the Yabuhara feldspar mine, Nagano Prefecture, central Japan

TAKAGI TETSUICHI(Mineral and Resources Department GSJ), NAITO KAZUKI(Mineral and Resources Department GSJ), and SUDO SADAHISA(Mineral and Resources Department GSJ)

1999

vol. 50 (8), P. 509-513

3 figs., 1 table

Keywords: granite, feldspathization, K-Ar age, Yabuhara feldspar mine, Nagano Prefecture

Abstract: To determine the feldspathization age of the Yabuhara feldspar deposit, we have performed K-Ar age dating for an orthoclase-rich feldspar ore and a biotite fraction from fine-grained biotite granite of the Nagawa granite. The feldspar ore and biotite fraction yielded ages of 57.5 } 2.9 Ma and 61.2 } 3.1 Ma, respectively. These ages suggest that the feldspathization occurred in early Paleogene by hydrothermal fluids from the fine-grained biotite granite

Thermobarometry of the Tsukuba Metamorphic Rocks

Kazuhiro Miyazaki(Geology Department GSJ)

1999

vol. 50 (8), P. 515-525

6 Figs., 8 tables

Keywords:metamorphic rock, pressure, temperature, garnet, sillimanite, cordierite, biotite, Ryoke Metamorphic Rocks, Tsukuba Metamorphic Rocks, Tsukuba, Japan

Abstract:This paper presents pressure and temperature conditions for high grade rocks in the Tsukuba metamorphic rocks. Using garnet-biotite thermometer and GASP (garnet-sillimanite-plagioclase) barometer, estimated temperatures and pressures were T=627_-667_ C and P=3.2-3.5 kbar. Using garnet-cordierite thermometer and GASP barometer, estimated temperatures and pressures were T=622_-643_ C and P=2.6-3.4 kbar. Comparing these estimations with pressures and temperatures for lower grade rocks at the sillimanite isograd (Miyazaki et al., 1992), the metamorphic pressure was almost constant with increasing temperature. The rapid increase of temperature at constant pressure suggests that the Tsukuba metamorphic rocks were produced by heat of granitic magma, which intruded at ca. 12 km depth in the crust. Because the bedding plane dips of the Tsukuba metamorphic rocks are generally gentle, the isobaric plane is thought to be not highly oblique to the bedding plane

The low-grade Ryoke metamorphic rocks in the Wazuka district, Kyoto Prefecture, Japan

Keiji TAKEUCHI(Geology Department, GSJ)and Ge-Fan WANG(P.R.China)

1999

vol. 50 (8), P. 527-534

4 Figs., 1 table

Keywords:Ryoke Metamorphic Belt, Tamba Belt, Kyoto Prefecture, Kasagi, Wazuka, pelite, biotite isograd, chlorite, contact aureole

Abstract:The low-grade Ryoke metamorphic rocks which gradually changes into the Tamba Belt are distributed in the Wazuka district, Kyoto Prefecture, central Japan. The protolith is composed of mudstone, chert, small amount of sandstone, and lenses of greenstone and limestone. These strata dip steeply north or south. The sedimentary sequences from chert through siliceous mudstone to mudstone are considered to represent the oceanic plate stratigraphy. The directions of top strata and sedimentary structures of sandstone laminas suggest that closed folds with a half wave-length of several hundred meters to two kilometers developed in the Ryoke metamorphic rocks. The protolith are lithologically divided into two units. The northern unit is regarded to be the Type II Suite of the Tamba Belt, while the southern unit regarded to be the Type I Suite. The Ryoke metamorphic rocks are divided into four mineral zones for the pelitic rocks ; chlorite, transitional, chlorite-biotite, and biotite. The appearance of biotite in pelitic and psammitic rocks shows that biotite isograds can not be defined as a definite line. The contact aureoles of the Ryoke granitic rocks are much wider than previously regarded. For example, the maximum width of the Yagyu granite aureole exceeds 2km

Magnetic structures in the eastern margin of the Japan Sea

| off Niigata-Aomori area |

Rie MORIJIRI(Geophysics Dept., GSJ)

1999

vol. 50 (8), P. 535-558

18 Figs.

Keywords:Japan Sea, magnetic anomaly, structural boundary

Abstract:The distribution of apparent magnetization of basement was estimated in the eastern margin of Japan Sea, on the basis of gravity and magnetic data of the Hakurei-maru cruises by the Geological Survey of Japan. Survey line interval was about 3 km. Magnetic data were interpreted in term of lateral distribution of magnetization on the gravity basements. First, apparent magnetizations were calculated by 2.5-dimensional analysis along E-W profiles spaced at 10 km intervals. The results of 2.5-dimensional analysis divided the study area into 2 parts characterized by stronger and weaker magnetizations. The clear boundary was revealed between the north of Sado Ridge and the southern part of Okushiri Ridge, and northern side had stronger magnetization. Next, 3-dimensional apparent magnetization mapping method was applied in three areas, a) southern part of Okushiri Ridge, b) northern part of Sado Ridge, and c) southern part of Sado Ridge. Strong magnetization areas were correlated with volcanic intrusions. Moreover, they appeared along structural boundaries as seen as steep slope zones of gravity basement. However, such intrusions were often difficult to be found on the basis of gravity anomalies and single channel seismic reflection profiles. Let they be similar volcanic mounds, as those intruded around Kyuroku Island, the distribution of such volcanic intrusions should indicate active boundaries