5:15 PM - 7:15 PM
[HQR05-P11] Major element glass composition of large-scale pyroclastic flow deposits over the past three million years in the Kagoshima Graben, south Kyushu, SW Japan
Keywords:tephra, major element composition, large-scale pyroclastic flow eruptions, southern Kyushu
Widespread tephras are a crucial chronological indicator, and detailed tephrostratigraphic frameworks are being constructed and integrated for the stratigraphy and distribution of Late Pleistocene tephra in Japan. On the other hand, the establishment of a tephrostratigraphic framework for Early to Middle Pleistocene tephras has not yet been completed because of the difficulties involved in constructing the regional stratigraphy of the source volcanic area. The southern Kyushu caldera region composed of the several large calderas, located in southwest of the Japanese Islands, is one of the most active volcanic regions in Japan and an important source area for widespread or voluminous tephras. This region is located in the Kagoshima Graben, which is recognized as a volcano-tectonic depression more than 100 km long in the north-south direction and 20 to 30 km wide, spanning the southern part of Kyushu in the Shimanto Supergroup. In this rift, there are large calderas, such as Kikai, Ata, Aira, Kakuto, and Kobayashi caldera from the south, which have provided huge ignimbrites by large-scale caldera-forming eruptions throughout the Quaternary period. Controversies remain in the proximal tephrostratigraphic study of the Early–Middle Pleistocene in southern Kyushu. Some of the proximal pyroclastic flow deposits (PFDs) are known to contain petrographically similar PFDs and similar distal ash fall deposits, which leads to difficulties and errors in widespread correlation. In addition, some PFDs of unknown volume and local distribution have not been incorporated into the regional stratigraphy due to erroneous correlations based solely on lithology, and exposure limitations caused by landscape modification from caldera forming eruptions.
In order to clarify the stratigraphy and distribution of PDFs in the Southern Kyushu caldera region, petrographic characteristics, including mineral compositions, were determined for more than 20 PDFs, with a focus on the major element compositions of their volcanic glass shards. The samples were collected from PFDs erupted from calderas lies in the Kagoshima Graben during the Holocene, Pleistocene, and Late Pliocene, and originated from large-scale pyroclastic flow eruptions ranked at VEI= of 6–7 that were associated with caldera formation. In addition to those, major element data were obtained from PDFs that were incorporated into the regional stratigraphy but whose volume, distribution, and stratigraphic position were unknown. Major element compositions of the volcanic glass shards were determined by energy dispersive X-ray spectrometers (JEOL JSM-6930 and EDAX GENESIS APEX2) owned by Tokyo Metropolitan University at an acceleration voltage of 15 kV and current of 0.6 nA.
Tephras with similar petrographic characteristics need to be studied in more detail by clarifying their trace element compositions. Based on the petrographic characteristics, we plan to construct a tephrostratigraphy of this provenance, including undefined PFDs, and to reexamine the widespread correlations. This work was supported by JSPS KAKENHI Grant Number JP19K13438.
In order to clarify the stratigraphy and distribution of PDFs in the Southern Kyushu caldera region, petrographic characteristics, including mineral compositions, were determined for more than 20 PDFs, with a focus on the major element compositions of their volcanic glass shards. The samples were collected from PFDs erupted from calderas lies in the Kagoshima Graben during the Holocene, Pleistocene, and Late Pliocene, and originated from large-scale pyroclastic flow eruptions ranked at VEI= of 6–7 that were associated with caldera formation. In addition to those, major element data were obtained from PDFs that were incorporated into the regional stratigraphy but whose volume, distribution, and stratigraphic position were unknown. Major element compositions of the volcanic glass shards were determined by energy dispersive X-ray spectrometers (JEOL JSM-6930 and EDAX GENESIS APEX2) owned by Tokyo Metropolitan University at an acceleration voltage of 15 kV and current of 0.6 nA.
Tephras with similar petrographic characteristics need to be studied in more detail by clarifying their trace element compositions. Based on the petrographic characteristics, we plan to construct a tephrostratigraphy of this provenance, including undefined PFDs, and to reexamine the widespread correlations. This work was supported by JSPS KAKENHI Grant Number JP19K13438.