[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-P17] Rutile exsolution lamellae and residual pressure of quartz inclusions in garnets of quartz-eclogite from Mt. Gongen, Besshi region, Sambagawa metamorphic belt

*Tomohiro Takebayashi^1,2, Yui Kouketsu¹, Katsuyoshi Michibayashi¹ (1.Graduate School of Environmental Studies, Nagoya University, 2.Museum of Natural and Environmental History, Shizuoka)

Keywords:Sanbagawa, Quartz eclogite, rutile exsolusion lamellae, garnet, Subduction zone metamorphism, residual pressure

Garnet grains with needle-rutile exsolution lamellae were reported in the quartz-eclogite from the Sambagawa metamorphic belt at Gongen Mountain in the Besshi district of Shikoku (Takebayashi et al., 2023). Quartz-eclogite is divided into quartz-rich (Qz-rich), quartz-poor (Qz-poor), and mafic domains (Enami et al., 2018), and rutile exsolution lamellae are found only in the quartz-poor domains. The chemical composition of garnets containing needle-rutile was spessartine-poor and grossular (grs: 13-23 mol%, with progressive zoning in almandine¬ pyrope (Alm-Prp) (Prp: 15-41 mol %), with rutile exsolution lamellae restricted to zones with 27-34 mol% Prp. Several garnets containing needle-rutile contain quartz, and the Raman spectra of quartz show peak shifts in the A1 mode (~464 cm-1) and E mode (~205 cm-1) suggestive high pressure. The quartz inclusions near the rutile needles had a residual pressure of approximately 800 MPa. In contrast, the garnet regions without rutile needles (below 27 mol% Prp and above 34 mol% Prp) had residual pressures of ~600-700 MPa ( <800 MPa). This result suggests that rutile-bearing garnets grew near the peak conditions of metamorphism. The metamorphic peak condition of quartz-eclogite is estimated to be 2.3-2.4 GPa/675-740°C (Miyamoto et al., 2007; Tomioka et al., 2022). Assuming a metamorphic peak temperature of 740°C, the metamorphic pressure condition was estimated to be approximately 2.3 GPa based on the highest residual pressure value of quartz inclusions near the needle-rutile needle, which is similar to previous studies. The chemical compositions of garnets in the Qz-rich and Mafic-clot domains are plotted on the extended pyrope side of the Alm-Prp series of garnets in the Qz-poor domain on a triangular diagram, with some garnet compositions showing reverse zoning. These garnets possibly underwent crystal growth during the retrograde metamorphic stage. Based on the above results, we discuss four metamorphic stages recorded for garnets in the quartz-eclogite. Stage 1: Garnets began to grow as the protolith was subducted, and the temperature and pressure increased. At this stage, the temperature and pressure conditions that would allow the substitution of Ti into the garnet crystal structure have not been reached. Stage 2: Garnet growth continues owing to metamorphism during the temperature increase, and substitution of titanium into the garnet crystal structure begins when the pyrope content reaches 27 mol% or more, and the pressure reaches 800 MPa or more. Phase 3: The pressure decreased from the peak metamorphic pressure of the rock, and Ti was not incorporated into the garnet crystal structure. As the temperature continued to increase, garnet crystals continued to grow. Stage 4: Titanium saturated in the garnet via retrograde metamorphism exceeded the solvace stability region and precipitated as a needle-rutile exsolution. The distribution of rutile in the garnet, both in inclusions and in the matrix, suggests that titanium was abundant in a continuous metamorphic process. The appearance of exsolution lamellae of rutile in garnets has often been considered an indicator of their occurrence at ultrahigh pressure or ultrahigh temperature (e.g. Attoh and Nude, 2008; Glassley et al., 2014); however, recent studies have reported that they also occur at less than ultrahigh pressures (e.g., Hwwang et al., 2014; Keller and Ague., 2020). In our sample, rutile exsolution lamellae are distributed in the middle of the grain growth, which may indicate the temperature pressure at which rutile can occur. The rutile exsolution lamellae also suggest a change point for the subduction and exhumation of quartz-eclogite in the Sanbagawa metamorphic belt.