14:15 〜 14:30
[SMP21-03] Petro-fabric and seismic anisotropy of eclogites from the Sanbagawa metamorphic belt, Shokoku, SW Japan
キーワード：ペトロファブリック、エクロジャイト、地震異方性、三波川変成帯
Sanbagawa metamorphic belt, an east-west (~ 800 km long) trending high-pressure/low-temperature belt, is located to the south of the high-temperature/low-pressure Ryoke belt and both represent a well-known paired metamorphic belt of Miyashiro (1961). The Sanbagawa metamorphic belt is mainly composed of basic, quartzose, pelitic-psammitic schists and several eclogite and ultramafic bodies. An earlier study (Rehman et al. 2016) reported electron backscattered diffraction (EBSD) maps and crystal preferred orientations (CPO) of eclogites from the Sanbagawa belt and interpreted the deformation mechanism of eclogite and post-eclogite facies phases. The studied eclogites recorded multi-stage deformational history where clinopyroxene deformed under constrictive strain regime, exhibited by dominantly an L-type CPO, during peak eclogite-facies stage. Garnet, the second major constituent phase in eclogites, showed a complex fabric, with weak CPO, indicating its rigid body rotation during metamorphism. Whereas hornblende and actinolite, products of retrogression, preserve more than two types of CPO. Hornblende recorded homotactic crystal growth over clinopyroxene, produced during the early stages of retrogression whereas actinolite having slightly weak and different fabric than the one preserved in hornblende, suggests a cataclastic deformation that could have been associated with the late-stage deformation.
To understand and further elaborate the deformational history in the Sanbagawa belt, we performed analysis related to the seismic properties of eclogite aggregate (clinopyroxene + garnet + hornblende + actinolite) as well as from the individual phases. Seismic properties, estimated from the CPO data from eclogite aggregate, showed Vp of 8.34 to 8.46 km/s, Vs of 4.74 to 4.81 km/s, and an isotropy of 1.5 % for Vp and 1.1 % for Vs. The fastest direction of P-wave velocity is along the foliation direction as determined from the CPO aligned along the 001 direction. Seismic properties of clinopyroxene showed Vp of 8.38 to 8. 6 km/s, Vs of 0.2 to 2.0 km/s, with an anisotropy of 2.5 % for Vp and 2.1 % for Vs. In garnet, values of Vp ranged from 8.568 to 8.586 km/s (anisotropy : 0.2 %), Vs of 0.1 to 0.6 km/s (anisotropy: 0.6 %), suggesting insignificant effect of the deformation. Hornblende exhibited values of 6.6 to 7.1 km/s (anisotropy : 7.8 %) for Vp whereas Vs values displayed a range from 0.5 to 3.5 km/s (anisotropy : 3.7 %). In contrast, actinolite displayed values of 7.3 to 8.1 km/s (anisotropy : 10 %) for Vp and 0.5 to 5.0 km/s (anisotropy : 5.1 %) for Vs. The Vp of mantle under ambient conditions generally displays a range of 8.3 to 8.5 km/s, therefore the studied Sanbagawa eclogites deformed possibly at mantle depth. The relatively higher seismic anisotropy of hornblende compared with clinopyroxene and garnet could be attributed to a transition from blueschist facies to eclogite facies metamorphic conditions during prograde deformation when the oceanic crust was subducting whereas the seismic anisotropy of actinolite indicates relaxation during exhumation stage.
Refs:
Miyashiro, A., 1961. Evolution of metamorphic belts. Journal of Petrology 2, 277–331.
Rehman, H.U., Mainprice, D., Barou, F., Yamamoto, H., Okamoto, K., 2016. EBSD-measured crystal preferred orientation of eclogites from the Sanbagawa metamorphic belt, central Shikoku, SW Japan. European Journal of Mineralogy 28(6), 1155–1168.
To understand and further elaborate the deformational history in the Sanbagawa belt, we performed analysis related to the seismic properties of eclogite aggregate (clinopyroxene + garnet + hornblende + actinolite) as well as from the individual phases. Seismic properties, estimated from the CPO data from eclogite aggregate, showed Vp of 8.34 to 8.46 km/s, Vs of 4.74 to 4.81 km/s, and an isotropy of 1.5 % for Vp and 1.1 % for Vs. The fastest direction of P-wave velocity is along the foliation direction as determined from the CPO aligned along the 001 direction. Seismic properties of clinopyroxene showed Vp of 8.38 to 8. 6 km/s, Vs of 0.2 to 2.0 km/s, with an anisotropy of 2.5 % for Vp and 2.1 % for Vs. In garnet, values of Vp ranged from 8.568 to 8.586 km/s (anisotropy : 0.2 %), Vs of 0.1 to 0.6 km/s (anisotropy: 0.6 %), suggesting insignificant effect of the deformation. Hornblende exhibited values of 6.6 to 7.1 km/s (anisotropy : 7.8 %) for Vp whereas Vs values displayed a range from 0.5 to 3.5 km/s (anisotropy : 3.7 %). In contrast, actinolite displayed values of 7.3 to 8.1 km/s (anisotropy : 10 %) for Vp and 0.5 to 5.0 km/s (anisotropy : 5.1 %) for Vs. The Vp of mantle under ambient conditions generally displays a range of 8.3 to 8.5 km/s, therefore the studied Sanbagawa eclogites deformed possibly at mantle depth. The relatively higher seismic anisotropy of hornblende compared with clinopyroxene and garnet could be attributed to a transition from blueschist facies to eclogite facies metamorphic conditions during prograde deformation when the oceanic crust was subducting whereas the seismic anisotropy of actinolite indicates relaxation during exhumation stage.
Refs:
Miyashiro, A., 1961. Evolution of metamorphic belts. Journal of Petrology 2, 277–331.
Rehman, H.U., Mainprice, D., Barou, F., Yamamoto, H., Okamoto, K., 2016. EBSD-measured crystal preferred orientation of eclogites from the Sanbagawa metamorphic belt, central Shikoku, SW Japan. European Journal of Mineralogy 28(6), 1155–1168.