5:15 PM - 7:15 PM
[SMP28-P02] Metamorphic temperature and pressure conditions of pelitic rocks in the Central part of the Abukuma Belt, Northeastern Japan
Keywords:Abukuma belt, Garnet, P-T conditions, Quartz Raman spectra
The metamorphic rocks of the Abukuma Belt are represented by the Gosaisyo, Takanuki, Hitachi, Nishidohira and Tamadare pelitic rocks in the southern part of the Belt. Since Miyashiro (1958, J. Fac. Sci. Univ. Tokyo, Sec.II, 11, 219-272), numerous studies have been conducted in this field. However, petrological studies on the pelitic rocks distributed in the central part of the Abukuma Belt have not been conducted. Thus, these data are essential for unraveling the formation history of the Abukuma Belt. We report the metamorphic temperature and pressure conditions of the pelitic rocks distributed on Mt. Kamuro on the Yamagata-Akita prefectural border and in the Ozaso area of Fukushima Prefecture, located in the central part of the Abukuma Belt. Moreover, we compare the pelitic rocks in the central part of Abukuma Belt with the pelitic rocks in its southern part to clarify their geological relationships.
The pelitic rocks in the Mt. Kamuro consist of biotite gneiss and garnet-biotite gneiss. Cretaceous and Neogene granitoids are distributed near the metamorphic rock body. Similarly, the pelitic rocks in the Ozaso area also consist of biotite gneiss and garnet-biotite gneiss. Some rocks contain sillimanite (Maruyama, 2020, Bachelor thesis of the Faculty of Science, Yamagata Univ.). Cretaceous granitoids are widely distributed near metamorphic rock bodies.
The garnet in Mt. Kamuro is an euhedral crystal with a size of approximately 0.1-1.2 mm, and its compositional zonation of garnet shows a reverse zoning pattern with an increase in Sps component (XSps: 0.17-0.31) at the rim, while the core remains homogeneous. Similarly, the garnet in the Ozaso area is also an euhedral crystal with a size of approximately 0.1-0.7 mm, and its compositional zonation of garnet shows a reverse zoning pattern with an increase in Sps component (XSps: 0.24-0.31) at the rim, while the core remains homogeneous. The metamorphic temperature condition was estimated using the garnet-biotite geothermometer of Ferry & Spear (1978, CMP, 66, 113-117). As a result, it is estimated that the condition of the pelitic rocks at Mt. Kamuro were T = 631-679 °C (P = 2.1 kbar) (Mori et al., 2024, The Geological Society of Japan, T1-P-13) and those in the Ozaso area were T = 556-634 °C (P = 2.1 kbar). Metamorphic pressure condition was estimated using the Quartz Raman spectra (Enami et al., 2007, AM, 92, 1303-1315). However, it was impossible to estimate the metamorphic pressure because quartz on Mt. Kamuro has a negative residual pressure (Δω1=-1~-5 cm-1). High T/P-type metamorphic rocks sometimes have negative residual pressure, and this study yields the same result (cf. Nishiyama & Aikawa, 2011, JGUM, SMP046-05; Kouketsu et al., 2012, JGUM, SMP46-13). These results were compared with Takanuki and Nishidohira pelitic rocks of the southern part of the Abukuma Belt. Metamorphic temperature and pressure conditions of Takanuki and Nishidohira pelitic rocks are as follows: for Takanuki, T = 750-850 °C, P = 4.5-7 kbar (Hiroi et al., 1998, MG, 16, 67-81); for Nishidohira, T = 600 °C, P = 5 kbar (Hiroi & Kobayashi, GKK, 91, 220-234).
Thus, geological relationships between the central and southern parts of the Abukuma Belt revealed that the pelitic rocks in Mt. Kamuro exhibit metamorphic temperature conditions intermediate between those of the Nishidohira and Takanuki pelitic rocks, while the pelitic rocks in the Ozaso area exhibit conditions similar to those of the Nishidohira pelitic rocks. However, the estimation of metamorphic pressure conditions remains an unresolved issue. This study indicates a geological relationship between the pelitic rocks of Mt. Kamuro and the Ozaso area, located in the central part of the Abukuma Belt, and those in its southern part. It is inferred that the pelitic rocks from the southern to the central part of the Abukuma Belt were formed by the same regional metamorphic event.
The pelitic rocks in the Mt. Kamuro consist of biotite gneiss and garnet-biotite gneiss. Cretaceous and Neogene granitoids are distributed near the metamorphic rock body. Similarly, the pelitic rocks in the Ozaso area also consist of biotite gneiss and garnet-biotite gneiss. Some rocks contain sillimanite (Maruyama, 2020, Bachelor thesis of the Faculty of Science, Yamagata Univ.). Cretaceous granitoids are widely distributed near metamorphic rock bodies.
The garnet in Mt. Kamuro is an euhedral crystal with a size of approximately 0.1-1.2 mm, and its compositional zonation of garnet shows a reverse zoning pattern with an increase in Sps component (XSps: 0.17-0.31) at the rim, while the core remains homogeneous. Similarly, the garnet in the Ozaso area is also an euhedral crystal with a size of approximately 0.1-0.7 mm, and its compositional zonation of garnet shows a reverse zoning pattern with an increase in Sps component (XSps: 0.24-0.31) at the rim, while the core remains homogeneous. The metamorphic temperature condition was estimated using the garnet-biotite geothermometer of Ferry & Spear (1978, CMP, 66, 113-117). As a result, it is estimated that the condition of the pelitic rocks at Mt. Kamuro were T = 631-679 °C (P = 2.1 kbar) (Mori et al., 2024, The Geological Society of Japan, T1-P-13) and those in the Ozaso area were T = 556-634 °C (P = 2.1 kbar). Metamorphic pressure condition was estimated using the Quartz Raman spectra (Enami et al., 2007, AM, 92, 1303-1315). However, it was impossible to estimate the metamorphic pressure because quartz on Mt. Kamuro has a negative residual pressure (Δω1=-1~-5 cm-1). High T/P-type metamorphic rocks sometimes have negative residual pressure, and this study yields the same result (cf. Nishiyama & Aikawa, 2011, JGUM, SMP046-05; Kouketsu et al., 2012, JGUM, SMP46-13). These results were compared with Takanuki and Nishidohira pelitic rocks of the southern part of the Abukuma Belt. Metamorphic temperature and pressure conditions of Takanuki and Nishidohira pelitic rocks are as follows: for Takanuki, T = 750-850 °C, P = 4.5-7 kbar (Hiroi et al., 1998, MG, 16, 67-81); for Nishidohira, T = 600 °C, P = 5 kbar (Hiroi & Kobayashi, GKK, 91, 220-234).
Thus, geological relationships between the central and southern parts of the Abukuma Belt revealed that the pelitic rocks in Mt. Kamuro exhibit metamorphic temperature conditions intermediate between those of the Nishidohira and Takanuki pelitic rocks, while the pelitic rocks in the Ozaso area exhibit conditions similar to those of the Nishidohira pelitic rocks. However, the estimation of metamorphic pressure conditions remains an unresolved issue. This study indicates a geological relationship between the pelitic rocks of Mt. Kamuro and the Ozaso area, located in the central part of the Abukuma Belt, and those in its southern part. It is inferred that the pelitic rocks from the southern to the central part of the Abukuma Belt were formed by the same regional metamorphic event.