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[SCG53-P04] Traces of Thermal Metamorphism of the Kanmon Group around Maeda, Shimonoseki City
Keywords:Kanmon Group, thermal metamorphism, metamorphic rock, mica
Introduction
The Kanmon Group is distributed over an area extending from northern Kyushu through the Kanmon Strait to the Chugoku District [1]. The Group is divided into the lower Wakino Subgroup and the upper Shimonoseki Subgroup, each of which is characterized as a sedimentary rock-dominated or volcanic rock-dominated formation, and traces of regional metamorphism, contact metamorphism, and hydrothermal alteration have been confirmed [2]. The boundary of the subgroup is thought to exist around Shimonoseki City but is obscured by the surrounding igneous complex and urbanization. Volcanic rocks are a major constituent material of the Shomonoseki Subgroup and are thought to have caused contact metamorphism on the Wakino Subgroup. Batholiths of granitic rocks are also located near the region and also cause thermal metamorphism. Maeda area is expected to be the contact area of Wakino and Shimonoseki sub-groups, but there is no information about it. Therefore, we are investigating this area to clarify mineralogical signatures of contact metamorphism.
Method
Samples were collected in the Maeda area, Shimonoseki, Yamaguchi Prefecture. We made thin sections for further analysis. The sections were examined by optical microscope and Raman spectroscopy at Okayama University of Science.
Result
We identified two main types of rocks collected from the Maeda area, igneous rocks and sedimentary rocks. The rocks are in contact with each other. We found two kinds of igneous rocks. One shows white color, the other is blue color. The white colored rock shows porphyritic texture consisting of microcrystalline lithic matrix with phenocryst (100 to 800 µm in size). The phenocrysts of the rock are quartz, plagioclase, biotite, pyroxene and opaque minerals. The matrix is fine grained quartz and plagioclase.
Blue colored igneous rock also shows porphyritic texture with phenocrysts ranging from 50 to 1000 µm in size embedded in a fine-grained matrix of plagioclase and quartz.
The sedimentary rocks show dark gray in color and dominated by angular minerals (~ 50 µm in size). It composed of plagioclase, clay minerals, quartz, and opaque minerals. We also found Muscovite in the rock.
Discussion and Summary
Based on the mineral species (clay minerals) found in the adjacent rocks, the temperature as a thermal source is assumed to be low and the aspect as an effect of hydrothermal alteration is relatively strong. A number of plagioclase and clay minerals (such as muscovite) were observed throughout the sample, suggesting at least low-grade effects of regional metamorphism, contact metamorphism, and diagenesis. However, further analysis is required, focusing on mineral quantification and chemical composition.
Reference
[1] Imaoka T. et al. (1993) J. Min. Retr. Econ. Geol. Japan, 88, 265-271. [2] Utada M. et al. (2005) Jour. Geol. Soc. Japan, 4, 206-216. [3] Uyeda Y. (1956) J. Geol. Soc. 63, 26-34. [4] Nakae S. et al. (1998) Geol. Surv. Japan, 126.
The Kanmon Group is distributed over an area extending from northern Kyushu through the Kanmon Strait to the Chugoku District [1]. The Group is divided into the lower Wakino Subgroup and the upper Shimonoseki Subgroup, each of which is characterized as a sedimentary rock-dominated or volcanic rock-dominated formation, and traces of regional metamorphism, contact metamorphism, and hydrothermal alteration have been confirmed [2]. The boundary of the subgroup is thought to exist around Shimonoseki City but is obscured by the surrounding igneous complex and urbanization. Volcanic rocks are a major constituent material of the Shomonoseki Subgroup and are thought to have caused contact metamorphism on the Wakino Subgroup. Batholiths of granitic rocks are also located near the region and also cause thermal metamorphism. Maeda area is expected to be the contact area of Wakino and Shimonoseki sub-groups, but there is no information about it. Therefore, we are investigating this area to clarify mineralogical signatures of contact metamorphism.
Method
Samples were collected in the Maeda area, Shimonoseki, Yamaguchi Prefecture. We made thin sections for further analysis. The sections were examined by optical microscope and Raman spectroscopy at Okayama University of Science.
Result
We identified two main types of rocks collected from the Maeda area, igneous rocks and sedimentary rocks. The rocks are in contact with each other. We found two kinds of igneous rocks. One shows white color, the other is blue color. The white colored rock shows porphyritic texture consisting of microcrystalline lithic matrix with phenocryst (100 to 800 µm in size). The phenocrysts of the rock are quartz, plagioclase, biotite, pyroxene and opaque minerals. The matrix is fine grained quartz and plagioclase.
Blue colored igneous rock also shows porphyritic texture with phenocrysts ranging from 50 to 1000 µm in size embedded in a fine-grained matrix of plagioclase and quartz.
The sedimentary rocks show dark gray in color and dominated by angular minerals (~ 50 µm in size). It composed of plagioclase, clay minerals, quartz, and opaque minerals. We also found Muscovite in the rock.
Discussion and Summary
Based on the mineral species (clay minerals) found in the adjacent rocks, the temperature as a thermal source is assumed to be low and the aspect as an effect of hydrothermal alteration is relatively strong. A number of plagioclase and clay minerals (such as muscovite) were observed throughout the sample, suggesting at least low-grade effects of regional metamorphism, contact metamorphism, and diagenesis. However, further analysis is required, focusing on mineral quantification and chemical composition.
Reference
[1] Imaoka T. et al. (1993) J. Min. Retr. Econ. Geol. Japan, 88, 265-271. [2] Utada M. et al. (2005) Jour. Geol. Soc. Japan, 4, 206-216. [3] Uyeda Y. (1956) J. Geol. Soc. 63, 26-34. [4] Nakae S. et al. (1998) Geol. Surv. Japan, 126.