S2: Water Rock Interaction (Special Session)

[S2P-06] C-isotope composition of graphite associated with green V-bearing grossular and origin of COH fluid from Menipa, Sør Rondane Mountains, East Antarctica

*Tetsuo KAWAKAMI¹, Tasuro Adachi³, Satish-Kumar M.², Fumiko Higashino¹, Masaoki Uno⁴ (1. Kyoto Univ. Sci., 2. Niigata Univ. Sci., 3. Kyushu Univ. Soc., 4. Tohoku Univ. Env.)

Keywords:Continental collision zone, COH fluid, metamorphism, graphite, C-isotope composition

The Sør Rondane Mountains (SRM) is divided into the NE terrane and the SW terrane. The granulite-facies gneisses in the NE terrane records clockwise P-T-t path whereas those in the SW terrane records counter-clockwise P-T-t path. Kawakami et al. (2022) used a Sil-Bt-Grt gneiss and graphitic Sil-Bt gneiss containing green V-bearing grossular from Menipa in the SW terrane to construct a P-T-t path. They concluded that Menipa experienced a clockwise P-T-t path and ca. 600-560 Ma was the timing of prograde metamorphism and the peak metamorphic conditions of ~1.0 GPa, ~800 ^oC were attained at ca. 560 Ma. The green grossular is surrounded by symplectite that consists of An+V-Cpx+V-Grt+V-Ttn+V-Zoi+Qz+Ap+V-Ti-Fe oxide+Pyh+Py+Cal, and the symplectite formation was dated to be ca. 550 Ma by U-Pb dating of titanite in the symplectite. The graphitic Sil-Bt gneiss rarely contains green grossular of ~10 cm in diameter, which is associated with a crust of quartz aggregate. The quartz includes fluid inclusions containing CO₂+CH₄. This suggest that COH fluid played a role in coarsening the green grossular. Some symplectites replacing the rim of green grossular are filling their pressure shadows, suggesting that the symplectite formation was contemporaneous with or postdated the formation of the matrix foliation defined by the arrangement of Sil+Bt. The outermost part of the symplectite is coarse-grained than the inner part and accompanies graphite which shows δ¹³C=-24.74 ±0.01, which is a typical biogenic value. The symplectite is also developed along the cracks in the green grossular. The extension of the crack turns into healed cracks recognized as a linear, higher-V grossular narrow band along which calcite inclusions are found. Therefore, COH fluid infiltration took place during the symplectite formation. Graphite is also found in the symplectite developed along the crack, indicating that some graphite crystals recrystallized from the COH fluid. Since precipitation of V requires a reducing environment, the graphite was probably already present in the protolith of the graphitic Sil-Bt gneiss. Therefore, the COH fluid is likely a reaction product of graphite in the gneiss and aqueous fluid.