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[O11-P03] Estimation of the Source Region of Zircon on the Samejima Coast of the Enshu Sea Based on Morphological Analysis
Keywords:Zircon, Ryoke belt, Granite
1. Motivation and Background
The Samejima Coast, located along the Enshu Sea, is known for its garnet sands enriched with heavy minerals, including zircon (ZrSiO4). Zircon is a common heavy mineral in igneous, sedimentary, and metamorphic rocks and is highly resistant to weathering and alteration. Due to its stability over geologic timescales, zircon serves as a reliable indicator for tracing sediment provenance.
In a previous study conducted by the school, it was estimated—based on color analysis—that the zircon present on the Samejima Coast originated from the Ryoke Belt, a major geological belt in central Japan. However, it was still unclear which specific rock body within the Ryoke Belt served as the source. This research aims to further refine the origin of these zircon grains by comparing the physical characteristics of zircon found in candidate rock bodies of the Ryoke Belt with those collected from the Samejima Coast.
2. Hypothesis
By examining the presence, shape, and color characteristics of zircon found in various Ryoke Belt rocks, it is possible to identify the specific source rock(s) that supplied zircon to the Samejima Coast.
3. Sample Collection and Target Rocks
Four types of rocks were collected from three locations along the Tenryu River, which flows through the Ryoke Belt in the Shizuoka and Nagano prefectures:
Ikuta Granite
Tenryu Gorge (Tenryu-kyo) Granite
Coarse-grained basalt
Gneiss
Zircon was observed in the Ikuta and Tenryu Gorge granites, but no zircon was found in the gneiss or basalt. Thus, the comparison was narrowed to zircon from the Ikuta and Tenryu Gorge granites. These two zircon sources are referred to collectively as “Ikuta + Tenryu Gorge zircon” in this study.
4. Methods
The distribution and formation ages of each rock body were determined using geological maps and literature.
Zircon crystals were observed using a binocular stereomicroscope.
The elongation ratio (length of the major axis divided by the minor axis) was calculated to quantify zircon shape.
A new shape classification chart was independently created to systematically categorize zircon morphology.
For color analysis, images of zircon grains were analyzed on a personal computer, and their color was broken down into RGB components.These RGB values were then plotted on a triangular diagram to compare zircon color distributions among samples.
5. Results
Both the Ikuta Granite and the Tenryu Gorge Granite are part of the Ryoke Belt, dating to the Late Cretaceous period.
Zircon grains from the Tenryu Gorge granite showed greater similarity to those from the Samejima Coast in terms of elongation ratio, shape, and color.When the zircon characteristics from both the Ikuta and Tenryu Gorge granites were combined, they matched the Samejima Coast zircon even more closely than either sample alone.The color comparison revealed that most zircons were light-colored, often appearing white or colorless, and shared similar RGB component distributions.
6. Discussion
The data suggest that the Samejima Coast zircon is not exclusively derived from either the Ikuta or the Tenryu Gorge granite alone.However, when the zircon characteristics from both granites are combined, the result shows a consistent match with the zircon observed on the Samejima Coast.Therefore, the zircon on the coast was likely supplied through sedimentary processes involving both granite bodies.These findings imply that fluvial transport from areas exposing both granites played a role in delivering zircon to the coastal sediments.
7. Conclusion
The zircon found in garnet sands on the Samejima Coast is most likely derived from both the Ikuta Granite and the Tenryu Gorge Granite of the Ryoke Belt. These findings contribute to a better understanding of the sedimentary pathways and source rock contributions in the region.
8. Future Prospects
The next phase of the research will focus on analyzing river sands from areas that do not pass over the Ikuta or Tenryu Gorge granites, to determine whether zircon is present and, if so, how its characteristics differ. This will further clarify whether other sources could also contribute zircon to the Samejima Coast sediments.
9. Acknowledgements
We thank Dr. Akira Aoshima, visiting researcher at the Fujinokuni Museum of Global Environmental History, for his valuable guidance, as well as our advisor Mr. Hiroyuki Kurematsu and the members of the Earth Science Club.
10. References
Igarashi et al. (2023) – Estimation of the origin of zircon in the Samejima Coast of Enshu Sea
Yamamoto, T. (1959) – Zircon in volcanic rocks
Ryoke Research Group (1972) – Interrelationship of granitoids in the Chubu District
The Samejima Coast, located along the Enshu Sea, is known for its garnet sands enriched with heavy minerals, including zircon (ZrSiO4). Zircon is a common heavy mineral in igneous, sedimentary, and metamorphic rocks and is highly resistant to weathering and alteration. Due to its stability over geologic timescales, zircon serves as a reliable indicator for tracing sediment provenance.
In a previous study conducted by the school, it was estimated—based on color analysis—that the zircon present on the Samejima Coast originated from the Ryoke Belt, a major geological belt in central Japan. However, it was still unclear which specific rock body within the Ryoke Belt served as the source. This research aims to further refine the origin of these zircon grains by comparing the physical characteristics of zircon found in candidate rock bodies of the Ryoke Belt with those collected from the Samejima Coast.
2. Hypothesis
By examining the presence, shape, and color characteristics of zircon found in various Ryoke Belt rocks, it is possible to identify the specific source rock(s) that supplied zircon to the Samejima Coast.
3. Sample Collection and Target Rocks
Four types of rocks were collected from three locations along the Tenryu River, which flows through the Ryoke Belt in the Shizuoka and Nagano prefectures:
Ikuta Granite
Tenryu Gorge (Tenryu-kyo) Granite
Coarse-grained basalt
Gneiss
Zircon was observed in the Ikuta and Tenryu Gorge granites, but no zircon was found in the gneiss or basalt. Thus, the comparison was narrowed to zircon from the Ikuta and Tenryu Gorge granites. These two zircon sources are referred to collectively as “Ikuta + Tenryu Gorge zircon” in this study.
4. Methods
The distribution and formation ages of each rock body were determined using geological maps and literature.
Zircon crystals were observed using a binocular stereomicroscope.
The elongation ratio (length of the major axis divided by the minor axis) was calculated to quantify zircon shape.
A new shape classification chart was independently created to systematically categorize zircon morphology.
For color analysis, images of zircon grains were analyzed on a personal computer, and their color was broken down into RGB components.These RGB values were then plotted on a triangular diagram to compare zircon color distributions among samples.
5. Results
Both the Ikuta Granite and the Tenryu Gorge Granite are part of the Ryoke Belt, dating to the Late Cretaceous period.
Zircon grains from the Tenryu Gorge granite showed greater similarity to those from the Samejima Coast in terms of elongation ratio, shape, and color.When the zircon characteristics from both the Ikuta and Tenryu Gorge granites were combined, they matched the Samejima Coast zircon even more closely than either sample alone.The color comparison revealed that most zircons were light-colored, often appearing white or colorless, and shared similar RGB component distributions.
6. Discussion
The data suggest that the Samejima Coast zircon is not exclusively derived from either the Ikuta or the Tenryu Gorge granite alone.However, when the zircon characteristics from both granites are combined, the result shows a consistent match with the zircon observed on the Samejima Coast.Therefore, the zircon on the coast was likely supplied through sedimentary processes involving both granite bodies.These findings imply that fluvial transport from areas exposing both granites played a role in delivering zircon to the coastal sediments.
7. Conclusion
The zircon found in garnet sands on the Samejima Coast is most likely derived from both the Ikuta Granite and the Tenryu Gorge Granite of the Ryoke Belt. These findings contribute to a better understanding of the sedimentary pathways and source rock contributions in the region.
8. Future Prospects
The next phase of the research will focus on analyzing river sands from areas that do not pass over the Ikuta or Tenryu Gorge granites, to determine whether zircon is present and, if so, how its characteristics differ. This will further clarify whether other sources could also contribute zircon to the Samejima Coast sediments.
9. Acknowledgements
We thank Dr. Akira Aoshima, visiting researcher at the Fujinokuni Museum of Global Environmental History, for his valuable guidance, as well as our advisor Mr. Hiroyuki Kurematsu and the members of the Earth Science Club.
10. References
Igarashi et al. (2023) – Estimation of the origin of zircon in the Samejima Coast of Enshu Sea
Yamamoto, T. (1959) – Zircon in volcanic rocks
Ryoke Research Group (1972) – Interrelationship of granitoids in the Chubu District