Thermal history and dinamic process of a large-scale pyroclastic-flow deposit

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Remanent magnetization and field survey of the Ata large-scale pyroclastic-flow deposit clarified relationship between lithological facies and emplacement temperatures. Palaeomagnetic measurements showed emplacement temperatures of essential fragments and matrix of the pyroclastic-flow deposits higher than 560ﾟC based on the corresponding TRM directions. Three types are identified in the directions of remanent magnetization : (1) single component with the same direction, (2) two components, low and high temperature component, obtained during progressive thermal demagnetization, and (3) components with scattered directions. These results are different from those of the Ito pyroclastic-flow deposit which shows weaker welding than the Ata, suggesting that welding processes start before emplacement.Reheating process for the Ito pyroclastic-flow deposit was investigated by measuring the annealing temperature of each mineral through track age and length of fission-track method. The anealing temperature of zircon and apatite within granitic fragments in the Ito pyroclastic-flow deposit was estimated to have been between 150ﾟC and 500ﾟC based on the measurement of track length.