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[MIS23-P03] The origin of large chert blocks in the western part of the Matsumoto Basin, Nagano Prefecture, central Japan
Keywords:slope failure, Complex in mino belt, debris flow
The Large chert blocks with several meters in diameter are distributed around the Azusa River in the western part of the Matsumoto basin, Nagano Prefecture, central Japan. Among these, the chert block with 7 meters in diameter, which is located in Yamato, Azusagawa area, is called Hiuchiiwa and another chert block with 8 meters in diameter in Hata is called Mitsuiwa. The origin of these large chert blocks is still obscure with the possibilities of an apex of the buried mountain ridge and a transported rolling stone (Harayama et al., 2009).
In this study, we examined the microstructure and secondary minerals in these chert blocks to clarify the origin of the blocks.
The microstructure under microscope using thin sections shows that both large blocks are weakly metamorphosed with formation of secondary mica and opaque minerals. As a result of EDS(energy dispersive X-ray spectroscopy) analysis, it is characteristic that the Hiuchiiwa contains Mn-oxide and k-feldspar in the vein, whereas the Mitsuiwa includes Mn- and Mn-Fe oxides in the vein. Besides, the Hiuchiiwa includes radiolarian pseudomorphs with wider diameter compared with those in the Mitsuiwa. Thus, it is considered that the Hiuchiiwa was affected by intense thermal alternation rather than the Mitsuiwa.
Besides, the rolling chert gravels, which were collected from the upstream area of Azusa River, include biotite, calcite, and Ti-oxide minerals as the products of higher temperature alternation. These minerals imply that these rolling chert gravels were suffered from strong thermal influence.
Considering the geology of the surrounding area of these large chert blocks, most bed rocks distributed in the western part of Matsumoto Basin were affected by thermal metamorphism by Nagawa granite body located in the upper stream of the Azusa River. However, the two large chert blocks showed a different grade of thermal alternation, which indicates the origin of these blocks was quite different thermal conditions. Also, strongly alternated Hiuchiiwa is located farther from the Nagawa granite body, though the weakly alternated Mitsuiwa was located close to the granite body. Thus, it is possible that both are allochthonous blocks or that either is the exposure of the bedrock. In both cases, it is considered that the large slope failure and large debris flow events were occurred to carry the large chert block from the source area.
In this study, we examined the microstructure and secondary minerals in these chert blocks to clarify the origin of the blocks.
The microstructure under microscope using thin sections shows that both large blocks are weakly metamorphosed with formation of secondary mica and opaque minerals. As a result of EDS(energy dispersive X-ray spectroscopy) analysis, it is characteristic that the Hiuchiiwa contains Mn-oxide and k-feldspar in the vein, whereas the Mitsuiwa includes Mn- and Mn-Fe oxides in the vein. Besides, the Hiuchiiwa includes radiolarian pseudomorphs with wider diameter compared with those in the Mitsuiwa. Thus, it is considered that the Hiuchiiwa was affected by intense thermal alternation rather than the Mitsuiwa.
Besides, the rolling chert gravels, which were collected from the upstream area of Azusa River, include biotite, calcite, and Ti-oxide minerals as the products of higher temperature alternation. These minerals imply that these rolling chert gravels were suffered from strong thermal influence.
Considering the geology of the surrounding area of these large chert blocks, most bed rocks distributed in the western part of Matsumoto Basin were affected by thermal metamorphism by Nagawa granite body located in the upper stream of the Azusa River. However, the two large chert blocks showed a different grade of thermal alternation, which indicates the origin of these blocks was quite different thermal conditions. Also, strongly alternated Hiuchiiwa is located farther from the Nagawa granite body, though the weakly alternated Mitsuiwa was located close to the granite body. Thus, it is possible that both are allochthonous blocks or that either is the exposure of the bedrock. In both cases, it is considered that the large slope failure and large debris flow events were occurred to carry the large chert block from the source area.