Abstract
Aso-4 pyroclastic flow is one of the largest volcanic activities in Japan and formed huge caldera of the Aso volcano at about 90,000 years b.p.. This pyroclastic flow brought about accidental clasts of granitic mylonite in the Aso-4 pyroclastic deposit in northern area of the caldera. These granitic mylonite clasts were investigated petrologically and geochronologically, and compared with the Nioki Granite in the Asaji area to the east of the volcano. The Aso-4 granitic clasts are characterized by garnet-muscovite bearing adamelite-granite, and all of them are moderately to strongly mylonitized and show porphyroclastic texture. Their SiO2 content ranges 71-74 wt.%. The range of garnet composition of the Aso-4 granitic clasts is Alm69~78, Sps14~27, Pyr0.2~3.3, Grs1.0~9.3. These mineral and chemical compositions are quite similar to those of the Nioki Granite. The K-Ar muscovite age for Aso-4 granitic clasts is 98.4±2.1 Ma, whereas Nioki Granite is 98.8±2.1 Ma. The heat of magma of the volcano did not affect the K-Ar closure system for muscovite when the pyroclastic flow took place, because heating has not fundamentally shortened the fission tracks within zircon grains in the dated clast. Accordingly, the Aso-4 granitic mylonite clasts are correlative with the Nioki Granite, which is also weakly to moderately mylonitized. The intrusive age of the Nioki Granite is also determined using CHIME monazite dating, and the result is 106±4 Ma. This suggests that subsurface ductile mylonite zone initiated around 100 Ma of the major fault extends from Asaji area to the Aso volcano. This fault coincides with the Oita-Kumamoto Tectonic Line, along which the active fault segment has been known. The occurrence of E-W extending mylonite zone is also significant evidence in considering the western extension of the Median Tectonic Line in Kyushu.
