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[SMP25-P13] Metamorphic zoning and P–T conditions of contact metamorphic rocks in the Unai area, Yamaguchi prefecture, Japan.
Keywords:Unai area, contact metamorphism, metamorphic zoning, mineral chemistry
The Unai area lies near the boundary between Ube city and Mine city, Yamaguchi prefecture, belongs to the Suo Belt. Kishi et al. (2007) reported that this area was intruded by the plutonic complex composed of Amago intrusive rocks, Kurogoro plutonic rocks, and Iwago-yama granite. These K–Ar ages are 97.9 ± 2.2 Ma, 94.0 ± 4.8 Ma, and 92.8 ± 2.0 Ma, respectively. The plutonic complex is exposed over an area of 14 × 8 km and its intrusion caused contact metamorphism to the Suo metamorphic rocks in the Unai area (Seo, 1976).
This study reports the metamorphic zoning, and P–T conditions of the contact metamorphism.
The Unai area is underlain by metamorphic rocks and granodiorites. Metamorphic rocks are composed of contact metamorphic rocks (dominant pelitic schist, and a small amount of calc-silicate schist) and the Suo metamorphic rocks (precursor).
In this area, granodiorites are distributed in the central part. Contact metamorphic rocks are distributed around the margin of granodiorite with a width of about 500 m, and most of them are pelitic schist, but calc-silicate schist also occurs as a small lens shape.
The dominant pelitic rocks had been used for zonal mapping in this area. It can be divided into six metamorphic zones. The critical mineral paragenesis are as follows.
For pelitic rocks,
Zone Ⅰ (Crd zone): Qz + Pl + Ms + Bt + Crd + (Chl)
Zone Ⅱ (And zone): Qz + Pl + Ms + Bt + And + Crd
Zone Ⅲ (Kfs zone): Qz + Pl + Bt + Ms + Kfs + Crn + And + Crd + Ilm
Zone Ⅳ (Grt zone): Qz + Pl + Bt + Kfs + Opx + Crd + And + Spl + Alm
Zone Ⅴ (Spl zone): Qz + Pl + Bt + Kfs + Opx + Crd + And + Spl
Zone Ⅵ (Sil zone): Bt + Opx + Pl + Kfs + Crd + Qz + Spl + Sil + Ilm
For calc-silicate rocks,
Zone Ⅵ (Sil zone): Pl + Cpx + Cal + Qz + Grs.
Zone Ⅰ pelitic rocks have a strong schistosity which originated precursor Suo metamorphic rocks. It is mainly composed by muscovite and biotite. This zone is characterized by the appearance of cordierite porphyroblast. Zone Ⅱ is characterized by andalusite porphyroblast. Zone Ⅲ is characterized by the occurrence of K-feldspar and euhedral corundum (up to 0.2 mm), they are coexisting with biotite. Zone Ⅳ is characterized by the appearance of orthopyroxene, which is coexisting with cordierite, biotite, and K-feldspar. Spinel (hercynite) also appear in this zone. Zone Ⅴ is almost same mineral paragenesis with that of Zone IV, but it is characterized by the disappearance of almandine. Zone Ⅵ pelitic rocks are characterized by the appearance of sillimanite (about 0.1 mm). The sillimanite is not fibrolite but is prismatic crystals. Calc-silicate rocks of Zone Ⅵ are mainly composed of calcite, clinopyroxene, and grossular.
The mineral chemistry of pelitic schist are as follows. In this abstract, we use XMg = Mg / (Fe + Mg) and XMg* = Mg / (Mg + Fe2+ + Fe3+). On spinel, Fe3+ and Fe2+ were calculated by stoichiometry. Plagioclase has An = 4.1–43.9% in composition. Biotite has XMg = 0.34–0.62. Muscovite has XMg = 0.53–0.61. Orthopyroxene has XMg = 0.42–0.43. Spinel has XMg* = 0.29–0.33, Zn = 0.002–0.007 apfu, and Cr = 0.005–0.020 apfu. Garnet has Alm55.4–58.7Prp41.0–44.3Sps0.2–0.5Grs0.1 in composition.
The mineral chemistry of calc-silicate rocks are as follows. Plagioclase has An = 91.0–96.8% in composition. Clinopyroxene has XMg = 0.68–0.79, diopside composition. Garnet has Alm10.5–16.8Prp1.3–2.2Sps0.4–1.3Grs80.1–87.5 in composition.
The variations of these mineral paragenesis and the variations in mineral chemical compositions reflect the metamorphic grade. The metamorphic pressure is about 0.8 kbar, and the metamorphic temperatures are about 500 °C for Zone Ⅰ, 500–550 °C for Zone Ⅱ, and 550–600 ℃ for Zone Ⅲ, 600–750 °C for Zone Ⅳ and Zone Ⅴ, and about 750 °C or higher for Zone Ⅵ. The highest metamorphic condition reached to the pyroxene hornfels facies.
References
Kishi et al. (2007) Jour. Geol. Soc. Japan., 113, 479-491.
Seo (1976) Professor G. Kojima Memorial Volume, 276-285.
This study reports the metamorphic zoning, and P–T conditions of the contact metamorphism.
The Unai area is underlain by metamorphic rocks and granodiorites. Metamorphic rocks are composed of contact metamorphic rocks (dominant pelitic schist, and a small amount of calc-silicate schist) and the Suo metamorphic rocks (precursor).
In this area, granodiorites are distributed in the central part. Contact metamorphic rocks are distributed around the margin of granodiorite with a width of about 500 m, and most of them are pelitic schist, but calc-silicate schist also occurs as a small lens shape.
The dominant pelitic rocks had been used for zonal mapping in this area. It can be divided into six metamorphic zones. The critical mineral paragenesis are as follows.
For pelitic rocks,
Zone Ⅰ (Crd zone): Qz + Pl + Ms + Bt + Crd + (Chl)
Zone Ⅱ (And zone): Qz + Pl + Ms + Bt + And + Crd
Zone Ⅲ (Kfs zone): Qz + Pl + Bt + Ms + Kfs + Crn + And + Crd + Ilm
Zone Ⅳ (Grt zone): Qz + Pl + Bt + Kfs + Opx + Crd + And + Spl + Alm
Zone Ⅴ (Spl zone): Qz + Pl + Bt + Kfs + Opx + Crd + And + Spl
Zone Ⅵ (Sil zone): Bt + Opx + Pl + Kfs + Crd + Qz + Spl + Sil + Ilm
For calc-silicate rocks,
Zone Ⅵ (Sil zone): Pl + Cpx + Cal + Qz + Grs.
Zone Ⅰ pelitic rocks have a strong schistosity which originated precursor Suo metamorphic rocks. It is mainly composed by muscovite and biotite. This zone is characterized by the appearance of cordierite porphyroblast. Zone Ⅱ is characterized by andalusite porphyroblast. Zone Ⅲ is characterized by the occurrence of K-feldspar and euhedral corundum (up to 0.2 mm), they are coexisting with biotite. Zone Ⅳ is characterized by the appearance of orthopyroxene, which is coexisting with cordierite, biotite, and K-feldspar. Spinel (hercynite) also appear in this zone. Zone Ⅴ is almost same mineral paragenesis with that of Zone IV, but it is characterized by the disappearance of almandine. Zone Ⅵ pelitic rocks are characterized by the appearance of sillimanite (about 0.1 mm). The sillimanite is not fibrolite but is prismatic crystals. Calc-silicate rocks of Zone Ⅵ are mainly composed of calcite, clinopyroxene, and grossular.
The mineral chemistry of pelitic schist are as follows. In this abstract, we use XMg = Mg / (Fe + Mg) and XMg* = Mg / (Mg + Fe2+ + Fe3+). On spinel, Fe3+ and Fe2+ were calculated by stoichiometry. Plagioclase has An = 4.1–43.9% in composition. Biotite has XMg = 0.34–0.62. Muscovite has XMg = 0.53–0.61. Orthopyroxene has XMg = 0.42–0.43. Spinel has XMg* = 0.29–0.33, Zn = 0.002–0.007 apfu, and Cr = 0.005–0.020 apfu. Garnet has Alm55.4–58.7Prp41.0–44.3Sps0.2–0.5Grs0.1 in composition.
The mineral chemistry of calc-silicate rocks are as follows. Plagioclase has An = 91.0–96.8% in composition. Clinopyroxene has XMg = 0.68–0.79, diopside composition. Garnet has Alm10.5–16.8Prp1.3–2.2Sps0.4–1.3Grs80.1–87.5 in composition.
The variations of these mineral paragenesis and the variations in mineral chemical compositions reflect the metamorphic grade. The metamorphic pressure is about 0.8 kbar, and the metamorphic temperatures are about 500 °C for Zone Ⅰ, 500–550 °C for Zone Ⅱ, and 550–600 ℃ for Zone Ⅲ, 600–750 °C for Zone Ⅳ and Zone Ⅴ, and about 750 °C or higher for Zone Ⅵ. The highest metamorphic condition reached to the pyroxene hornfels facies.
References
Kishi et al. (2007) Jour. Geol. Soc. Japan., 113, 479-491.
Seo (1976) Professor G. Kojima Memorial Volume, 276-285.