[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-02] Prograde P-T evolution of eclogites from the Song Ma suture zone, northern Vietnam

*Tadashi Usuki¹, Duc Nam Nguyen¹, Ching-Hua Lo¹, Yoshiyuki Iizuka² (1.Department of Geosciences, National Taiwan University, 2.Institute of Earth Sciences, Academia Sinica)

Keywords:P-T path, eclogite, Song Ma suture zone, garnet, rutile, continental subduction

The northern margin of the Indochina and the Song Ma suture zone have a good record from the initiation of Paleotethyan subduction in the Late Carboniferous (e.g., Kamvong et al., 2014) to the termination of subduction due to collision with South China in the Triassic (e.g., Nakano et al., 2010; Zhang et al., 2013; Faure et al., 2014). Triassic high-pressure metamorphic rocks such as eclogites and high-pressure granulites have been reported from these areas (Nakano et al., 2008, 2010; Zhang et al. 2013, 2014). Triassic syn- and post-collisional granites have also been reported in the same region (e.g., Hoa et al., 2008; Hieu et al., 2017; Thanh et al., 2019; Qian et al. 2019). These indicate that eclogites and high-pressure metamorphic rocks were formed in a setting of continental collision following the subduction of the Paleotethys. Therefore, the metamorphic evolution of eclogites may give a key to understand the conditions of the final stage of subduction between the Indochina and the South China. Previous studies have estimated the P-T conditions of eclogites to be 1.7-2.6 GPa and 600-750 °C (Nakano et al., 2010; Zhang et al., 2013) using conventional geothermometers and P-T pseudosections. However, the P-T conditions estimated in previous studies are significantly different. Nakano et al. (2010) proposed P_max-T of 2.1-2.2 GPa, 600 °C and P-T_max of 1.7 GPa and 750 °C, whereas Zhang et al. (2013) proposed the peak P-T of 2.6 GPa and 700 °C. In addition, garnets in the eclogites of the Song Ma suture zone generally have pronounced prograde compositional zonings, but the relationship between the compositional zonings and the P-T path has not been examined in detail in previous studies. Therefore, in this study, we compared the garnet compositions (cores and rims) with the P-T pseudosections to estimate P-T paths. Our results indicate that garnet grew progressively from 1.7-1.8 GPa, 600°C to 2.2 GPa, 700 °C along a geothermal gradient of 10°C/km. Rutile may have started growing at ~1.5 GPa-500 °C because it includes titanite. Furthermore, the Zr content of the rutile rim in the matrix does not exceed 350 ppm, making it unlikely that the temperature increased during decompression after the peak metamorphism (~700 °C). This suggests that the P_max and T_max points of the eclogite are almost identical. Previous studies have considered that eclogites formed in a subduction zone with a low thermal gradient (8 °C/km, Zhang et al., 2013), but our results suggest that it formed in a warm subduction zone. Eclogite is thought to have formed at the time of continental collision (Nakano et al., 2010; Zhang et al., 2013), and therefore we interpret that the slowdown in the subduction rate of the slab due to the subduction of the continental crust caused the formation of the warm subduction zone.