Characteristics of fault rocks in Koujaku granite using the weathering index W value

Iwamori, Akiyuki*; Ogita, Yasuhiro  ; Shimada, Koji   ; Tateishi, Ryo*; Takagi, Hideo*; Ota, Toru*; Kanno, Mizuho*; Wada, Shinya*; Ono, Akihiro*; Otsuka, Yoshiharu*

We investigated the W value, which is an index showing the progress of weathering, for the fault rocks in the Kojak granite distributed in the eastern Wakasa area. The W value represents the contribution due to weathering calculated from the chemical composition, and along with the M value (contribution of the mafic component) and F value (contribution of the felsic component), a triangular diagram of M + F + W = 100% showing trends from protolith to fault rocks (cataclasite or fault gouge) can be drown. We also investigated on the fault at the geological boundary between the Kojaku granite and the Mino-Tamba metabasalt, and on the difference in characteristics from the fault rock in the Kojaku granite. Granite protolith has an F value of 94.2% and W value of 4.9%, and the fault rock sample has an M value of about 3% regardless of whether it is an active fault or an inactive fault. As weathering progresses, the F value decreases and the W value increases. Metabasalt has an M value of 88.2% and a W value of 6.6%, and the cataclasite has an almost constant F value. As weathering progresses, the M value decreases and the W value increases. Some of the F value increases with the increase of the W value, which is consistent with the contamination of granite-origin quartz fragments found in the basaltic fault gouge. As a result of examination, it was confirmed that NaO and CaO have a great influence on the increase and decrease of the W value.