17:15 〜 19:15
[SMP28-P13] 紀伊半島西部三波川帯に産する泥質片岩の石英包有物が保持する残留圧力と歪み
キーワード：石英、残留圧力、歪み、ラマン地質圧力計、三波川帯
In central and eastern Shikoku, eclogite facies rocks (i.e., eclogite units) are sporadically exposed within lower-grade schists associated with the main Sanbagawa metamorphism. The spatial distributions of eclogite units in the Sanbagawa belt have not yet been adequately constrained, except within Shikoku Island. Although there have been some petrological studies of the Sanbagawa schists in the western Kii Peninsula, the detailed metamorphic evolution remains undetermined owing to the influence of pervasive retrograde metamorphism. To quantitatively evaluate the metamorphic conditions of this region, we focused on the residual pressures retained in the quartz inclusions of the garnets within the pelitic schists. Conventional quartz-in-garnet (Quig) Raman barometry (Kouketsu et al., 2014 AM; Reynard and Zhong et al., 2023 SE) and the software packages based on strain analysis (stRAinMAN: Angel et al., 2019 ZKCM; EntraPT: Mazzucchelli et al., 2021 AM) were utilized to estimate the prograde metamorphic conditions of pelitic schists.
In the western Kii Peninsula of this belt, the geological structure is divided into the Iimori and Tomobuchi units. Previous studies suggested that the metamorphic conditions of the Iimori unit are equivalent to those of garnet and albite-biotite zones. Raman spectra of quartz inclusions in the garnets were obtained from pelitic schists in the Iimori units (Funato, Takefusabashi, Imoyama, and Funaokayama areas). To apply Quig Raman barometry, two parameters, Δω1 (cm−1) and P206 (GPa), were used in this study. Strains acting on quartz inclusions entrapped in garnets were calculated using the peak shifts from standard quartz and the Grüneisen tensor of quartz under deviatoric stress. The strain components in the X- and Y-plane directions (ε1+ε2) and Z (c-axis) direction (ε3) of quartz were estimated. In the Funato and Takefusabashi areas, quartz inclusions in pelitic schists exhibit Δω1 and P206 value ranges of 4.2–11.0 cm−1 and 0.24–0.68 GPa, and those of 4.0–10.0 cm−1 and 0.22–0.61 GPa, respectively. The Δω1 values of quartz inclusions in these rocks show high Δω1 >10.0 cm−1, indicating eclogite facies conditions. In the Imoyama and Funaokayama areas, most quartz inclusions show relatively low Δω1 and P206 value ranges of 3.9–8.1 cm−1 and 0.20–0.50 GPa, and those of 4.0–7.6 cm−1 and 0.21–0.42 GPa. The P–T conditions of the Funato and Takefusabashi areas show P = 1.6–2.0 GPa and P = 1.5–1.8 GPa assuming T = 400–600°C. These values are comparable to those of the Sanbagawa eclogite units in Shikoku. In contrast, the Imoyama and Funaokayama areas show P = 1.2–1.6 GPa and P = 1.1–1.5 GPa assuming T = 400–600°C, indicating pressure differences even in the Iimori unit. The strain components of quartz inclusions in the samples showed ε1+ε2 ranging from −0.021 to −0.009 and ε3 from −0.002 to 0.004 (Funato), ε1+ε2 from −0.020 to −0.008 and ε3 from −0.003 to 0.003 (Takefusabashi), ε1+ε2 from −0.016 to −0.008 and ε3 from −0.002 to 0.002 (Imoyama), ε1+ε2 from −0.017 to −0.009 and ε3 from −0.002 to 0.004 (Funaokayama), respectively. The presence of quartz with high residual pressures provides valuable information for the characterization of the eclogite units of the western Kii Peninsula. In all samples, the strains of the quartz inclusions tended to be retained away from hydrostatic conditions; however, the deviatoric stress on these inclusions showed relatively small values. The converted P–T values from the quartz strains are compatible with those obtained from Quig Raman barometry. Some researchers have questioned the Quig barometry method, which directly converts the residual pressure to metamorphic pressure. However, our results suggest that the influence of deviatoric stress on quartz inclusions has little effect on pressure estimation in the case of the Sanbagawa schists.
In the western Kii Peninsula of this belt, the geological structure is divided into the Iimori and Tomobuchi units. Previous studies suggested that the metamorphic conditions of the Iimori unit are equivalent to those of garnet and albite-biotite zones. Raman spectra of quartz inclusions in the garnets were obtained from pelitic schists in the Iimori units (Funato, Takefusabashi, Imoyama, and Funaokayama areas). To apply Quig Raman barometry, two parameters, Δω1 (cm−1) and P206 (GPa), were used in this study. Strains acting on quartz inclusions entrapped in garnets were calculated using the peak shifts from standard quartz and the Grüneisen tensor of quartz under deviatoric stress. The strain components in the X- and Y-plane directions (ε1+ε2) and Z (c-axis) direction (ε3) of quartz were estimated. In the Funato and Takefusabashi areas, quartz inclusions in pelitic schists exhibit Δω1 and P206 value ranges of 4.2–11.0 cm−1 and 0.24–0.68 GPa, and those of 4.0–10.0 cm−1 and 0.22–0.61 GPa, respectively. The Δω1 values of quartz inclusions in these rocks show high Δω1 >10.0 cm−1, indicating eclogite facies conditions. In the Imoyama and Funaokayama areas, most quartz inclusions show relatively low Δω1 and P206 value ranges of 3.9–8.1 cm−1 and 0.20–0.50 GPa, and those of 4.0–7.6 cm−1 and 0.21–0.42 GPa. The P–T conditions of the Funato and Takefusabashi areas show P = 1.6–2.0 GPa and P = 1.5–1.8 GPa assuming T = 400–600°C. These values are comparable to those of the Sanbagawa eclogite units in Shikoku. In contrast, the Imoyama and Funaokayama areas show P = 1.2–1.6 GPa and P = 1.1–1.5 GPa assuming T = 400–600°C, indicating pressure differences even in the Iimori unit. The strain components of quartz inclusions in the samples showed ε1+ε2 ranging from −0.021 to −0.009 and ε3 from −0.002 to 0.004 (Funato), ε1+ε2 from −0.020 to −0.008 and ε3 from −0.003 to 0.003 (Takefusabashi), ε1+ε2 from −0.016 to −0.008 and ε3 from −0.002 to 0.002 (Imoyama), ε1+ε2 from −0.017 to −0.009 and ε3 from −0.002 to 0.004 (Funaokayama), respectively. The presence of quartz with high residual pressures provides valuable information for the characterization of the eclogite units of the western Kii Peninsula. In all samples, the strains of the quartz inclusions tended to be retained away from hydrostatic conditions; however, the deviatoric stress on these inclusions showed relatively small values. The converted P–T values from the quartz strains are compatible with those obtained from Quig Raman barometry. Some researchers have questioned the Quig barometry method, which directly converts the residual pressure to metamorphic pressure. However, our results suggest that the influence of deviatoric stress on quartz inclusions has little effect on pressure estimation in the case of the Sanbagawa schists.