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[SMP28-P03] Formation of large andalusite crystals in the Ryoke belt of the Takato district
Keywords:Andalusite, formation conditions, Ryoke Belt, Takato district
Regarding the Ryoke Belt in the Shiojiri-Takato region, a sillimanite isograd has been established on the northern part of Takato Town, Ina City (Figure A). In the pelitic metamorphic rocks on the low temperature side of the isotherm, andalusite and K-feldspar are stable, while in the sillimanite zone on the high temperature side, sillimanite and K-feldspar are stable. In the area about 500 m on the high temperature side of this isotherm, large andalusite with a long diameter of 4 to 5 cm occurs in large quantities in the pelitic metamorphic rocks. This andalusite is partially replaced by sillimanite, and it would be interesting to estimate the physical conditions for the formation of large andalusite.
Physical conditions of the formation of large andalusite
It is believed that andalusite formed and grew under temperature and pressure conditions under which it was stable. Sillimanite and kyanite were not formed under stable physical conditions.
The large andalusite in question was formed in pelitic metamorphic rocks, and coexists with potassium feldspar, quartz, muscovite, and carbonaceous materials. It is believed that andalusite was produced by a chemical reaction between quartz and muscovite. In other words, it is believed that andalusite was formed in a region higher than the temperature and pressure region in which quartz and muscovite are stable.
The Ryoke metamorphism is a regional metamorphism. It is believed that the heat source was located quite deep underground, and that the metamorphic pressure was quite high. From the above considerations, it is believed that the temperature and pressure conditions under which the large andalusite formed were the region shown in blue in Figure B.
As temperature increases, so does pressure
In Figure B, in metamorphism under geothermal gradient g, the stable zone of andalusite + K-feldspar is almost completely absent. This is almost identical to the metamorphism observed in the northern margin of Takato Town. Under this geothermal gradient, large andalusite is not generated. It is presumed that large andalusite was formed under lower pressure, for example, under geothermal gradient gb. As the temperature and pressure increase, the andalusite grows and becomes larger, but at a certain stage the large andalusite becomes unstable and is partially replaced by sillimanite.
Large andalusite in the Tomigata, Ina city
As shown by the red circle in Figure C, large andalusite is also found in the southwest of Takato. The andalusite is found several kilometers south of the northern part of Takato Town, and a fault (the Osawa Fault) is believed to exist between the two areas. The area where the fault is believed to exist is covered with the Tomioka Granite, but no faults can be seen (Figure D). The Osawa Fault is thought to have been formed before the intrusion of the Tomioka Granite. Fine-grained cordierite-biotite hornfels captured in the granite are scattered on the northern part of the Tomioka Granite. The parent rock is believed to be low-temperature, fine-grained biotite schist. To the east of the fine-grained hornfels, coarse-grained gneiss containing garnet is found, which is intruded by the Tomioka Granite.
Physical conditions of the formation of large andalusite
It is believed that andalusite formed and grew under temperature and pressure conditions under which it was stable. Sillimanite and kyanite were not formed under stable physical conditions.
The large andalusite in question was formed in pelitic metamorphic rocks, and coexists with potassium feldspar, quartz, muscovite, and carbonaceous materials. It is believed that andalusite was produced by a chemical reaction between quartz and muscovite. In other words, it is believed that andalusite was formed in a region higher than the temperature and pressure region in which quartz and muscovite are stable.
The Ryoke metamorphism is a regional metamorphism. It is believed that the heat source was located quite deep underground, and that the metamorphic pressure was quite high. From the above considerations, it is believed that the temperature and pressure conditions under which the large andalusite formed were the region shown in blue in Figure B.
As temperature increases, so does pressure
In Figure B, in metamorphism under geothermal gradient g, the stable zone of andalusite + K-feldspar is almost completely absent. This is almost identical to the metamorphism observed in the northern margin of Takato Town. Under this geothermal gradient, large andalusite is not generated. It is presumed that large andalusite was formed under lower pressure, for example, under geothermal gradient gb. As the temperature and pressure increase, the andalusite grows and becomes larger, but at a certain stage the large andalusite becomes unstable and is partially replaced by sillimanite.
Large andalusite in the Tomigata, Ina city
As shown by the red circle in Figure C, large andalusite is also found in the southwest of Takato. The andalusite is found several kilometers south of the northern part of Takato Town, and a fault (the Osawa Fault) is believed to exist between the two areas. The area where the fault is believed to exist is covered with the Tomioka Granite, but no faults can be seen (Figure D). The Osawa Fault is thought to have been formed before the intrusion of the Tomioka Granite. Fine-grained cordierite-biotite hornfels captured in the granite are scattered on the northern part of the Tomioka Granite. The parent rock is believed to be low-temperature, fine-grained biotite schist. To the east of the fine-grained hornfels, coarse-grained gneiss containing garnet is found, which is intruded by the Tomioka Granite.