[S-MP25] Deformed rocks, Metamorphic rocks and Tectonics

[SMP25-P15] P–T–t path of Ryoke metamorphic rocks in Kitsuki area, Kunisaki Peninsula, Oita prefecture, Japan.

*Kentaro Mutabaru¹, Toshiaki Shimura¹ (1.Yamaguchi University)

Keywords:Kitsuki area, Ryoke metamorphic belt, metamorphic P-T-t path, bathozone

The Kitsuki area of Oita prefecture belongs to the Ryoke metamorphic belt (Moriyama et al. 1983; Brown, 1998; Ishizuka et al., 2005; Isozaki et al., 2010). The Cretaceous Ryoke metamorphic rocks are less exposed in the Kitsuki area because they are widely covered by the Miocene volcanic and pyroclastic rocks. Ishizuka et al. (2005) indicated that the metamorphic grade of the Ryoke metamorphic rocks in this area corresponds to the sillimanite–K-feldspar zone in the Yanai area, Yamaguchi prefecture. This study discusses the P–T–t path of Ryoke metamorphic rocks in the Kitsuki area.
The Ryoke metamorphic rocks in Kitsuki area composed of pelitic gneiss, siliceous gneiss, and a few mafic metamorphic rocks. Ms–Bt granite and Hbl–Bt granodiorite are distributed around the Ushiyashiki and Kuranari area, respectively. Hbl–Bt granodiorite has weak foliation, and encloses pelitic gneiss as a xenolith.
The dominant mineral assemblages of pelitic gneiss in this area are the following.
Assemblage-I: And + Sil + Ms + Grt + Bt + Pl + Qz
Assemblage-II: Sil + Ms + Kfs + Grt + Bt + Pl + Qz
Assemblage-III: Sil + Kfs + Grt + Bt + Pl + Qz
Assemblage-I is recognized at the western part of this area. Assemblage-II is predominant in this area, and it contains tourmaline occurs as a minor phase. Assemblage-III is distributed in the eastern part in this area. Ti content of biotite increases from assemblages-I to III. Garnet grossular component of assemblage-I is poor than that of assemblages-II and III.
In this study area, the following two metamorphic reactions must be important. These are;
(1) And = Sil and
(2) Ms + Qz = Als + Kfs + H₂O.
Reaction curve (1) has negative slope, whereas (2) has positive slope. Because these two reaction curves are crossing each other in a P-T space, two bathozones or bathograds can be defined (e.g. Carmichael, 1978). In this study area, two bathozones (A and B) are recognized, and these two bathozones are further subdivided into three P-T fields (1, 2, and 3), as follows;

Bathozone-A (lower-P condition, ca. P < 2.2 kbar)
P–T field A-1 (lower-T condition): mineral assemblage is And + Ms
P–T field A-2 (middle-T condition): mineral assemblage is And + Kfs
P–T field A-3 (higher-T condition): mineral assemblage is Sil + Kfs
Bathozone-B (higher-P condition, ca. P > 2.2 kbar)
P–T field B-1 (lower-T condition): mineral assemblage is And + Ms
P–T field B-2 (middle-T condition): mineral assemblage is Sil + Ms
P–T field B-3 (higher-T condition): mineral assemblage is Sil + Kfs

These two prograde reactions can be observed as the following order;
And → Sil in the assemblage-I,
Ms + Qz → Sil + Kfs + H₂O in the assemblage-II.
On the other hand, these two retrograde reactions can be observed
Sil → And in the assemblage-I,
Sil (and And) + Kfs + H₂O → Ms + Qz in the assemblages-I and II.
These two retrograde reactions occurred almost at the same time. This P–T condition is the boundary between bathozones A and B, and it is about 620 ℃ and 2.2 kbar. Assemblage-I rocks passed through this point at the retrograde stage.
Metamorphic pressure condition was higher (P > 2.2 kbar) at the prograde stage, whereas it was lower (ca. 2.2 kbar) at the retrograde stage. Therefore, Ryoke metamorphic rocks in this study area was experienced a clockwise P–T–t path.

References
Brown, M. (1998) Jour. Metamorph. Geol., 16, 3–22.
Carmichael, D. M. (1978) Amer. Jour. Sci., 278, 769-797.
Ishizuka, Y. et al. (2005) 1:50,000 Geol. Map, Bungo-kitsuki. Geol. Surv. Japan., 11–18.
Isozaki, Y. et al. (2010) Jour. Tokyo Geogr. Soc., 199, 999–1053.
Moriyama, Z. et al. (1983) Fac. Educ. Oita Univ., 29–62.