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[SMP27-08] The metamorphic conditions and the origin of mafic schists of the Sanbagawa belt in the Shibukawa area, central Japan
Keywords:Sanbagawa belt, Raman spectroscopy, lower-grade, metamorphic condition, protolith
We applied Raman spectroscopy to Sanbagawa crystalline schists in the Shibukawa area, central Japan in order to constrain peak metamorphic conditions. The Shibukawa area is located near the boundary between Aichi and Shizuoka prefectures and consists of the Sanbagawa crystalline schists (chlorite zone) and the Mikabu greenstones in addition with ultramafic rocks and hornblendites (Makimoto et al., 2004). Some characteristic metamorphic minerals, such as jadeite, lawsonite, and sodic amphibole have been reported in the crystalline schists (Seki et al., 1959; Isogai, 1977), but no quantitative metamorphic conditions have been studied before.
Estimated metamorphic peak temperatures by Raman carbonaceous material (CM) geothermometry are in the range of 307–395℃ and increase to the northwest towards the Median Tectonic Line. By applying Raman geobarometry combined with Raman CM geothermometry, we obtained the peak metamorphic condition of 0.76–0.93 GPa and 360–390℃. These results are consistent with the occurrence of metamorphic minerals, and equivalent to the peak pressure condition of the garnet zone in the central Shikoku.
Some mafic schists retain traces of protoliths, such as gabbroic texture and relicts. Major chemical compositions of clinopyroxene relicts indicate that some protoliths could be alkali basalts. The amphibole relicts are rich in chlorine and titanium, suggesting that they had been derived from the ocean-floor metamorphism. These features are similar to those of the hornblendite bodies associated with the Mikabu greenstones in this area. Therefore, it may suggest that a part of the protolith of the mafic schists could be originated from hornblendite bodies as the hanging wall materials while the protolith of the Sanbagawa crystalline schists were subducted.
Isogai (1977) Hiroshima University Press, 237-245.
Makimoto et al. (2004) Geological survey of Japan, 7p.
Seki et al. (1959) Jour. Geol. Soc. Japan 65, 618-623.
Estimated metamorphic peak temperatures by Raman carbonaceous material (CM) geothermometry are in the range of 307–395℃ and increase to the northwest towards the Median Tectonic Line. By applying Raman geobarometry combined with Raman CM geothermometry, we obtained the peak metamorphic condition of 0.76–0.93 GPa and 360–390℃. These results are consistent with the occurrence of metamorphic minerals, and equivalent to the peak pressure condition of the garnet zone in the central Shikoku.
Some mafic schists retain traces of protoliths, such as gabbroic texture and relicts. Major chemical compositions of clinopyroxene relicts indicate that some protoliths could be alkali basalts. The amphibole relicts are rich in chlorine and titanium, suggesting that they had been derived from the ocean-floor metamorphism. These features are similar to those of the hornblendite bodies associated with the Mikabu greenstones in this area. Therefore, it may suggest that a part of the protolith of the mafic schists could be originated from hornblendite bodies as the hanging wall materials while the protolith of the Sanbagawa crystalline schists were subducted.
Isogai (1977) Hiroshima University Press, 237-245.
Makimoto et al. (2004) Geological survey of Japan, 7p.
Seki et al. (1959) Jour. Geol. Soc. Japan 65, 618-623.