Experiments and thermodynamic modelling on the blueschists in the Longmu Co-Shuanghu Suture Zone, North Tibet: Estimation of the metamorphic conditions and implications for garnet stability in the subduction zone

The pressure-temperature (P-T) path of blueschist is usually applied to understanding the evolution of the subducted oceanic crust. Garnet plays a key role in calculating the metamorphic conditions, but sometimes it is absent in the blueschists. Nevertheless, the high-P experimental simulation may serve as a useful tool to constrain the P-T conditions. In the middle of the Longmu Co-Shuanghu Suture, North Tibet, blueschist blocks embedded in the marble were discovered, where garnet is abundant in the core but absent in the marble-contacting margin. These two kinds of blueschist contain similar mineral assemblage (glaucophane, epidote, and phengite) and show comparable basaltic character. Pseudosection modelling was applied to the garnet-bearing blueschist that calculated the peak metamorphic condition of ca. 1.9 GPa and 530 degrees C. As for the garnet-absent blueschist, experiments (1.2-2.3 GPa and 500-800 degrees C) were performed on the designed mixture of 93 wt% basalt and 7 wt% calcite. The results show that the total Fe content in the glaucophane and barroisite is positively correlated with the pressure but not with temperature, which could be regarded as a geobarometer. Accordingly, the peak condition was constrained at 2.3 GPa and 500-600 degrees C. Therefore, the equivalent metamorphic conditions of the blueschists indicate that the Palaeo-Tethys oceanic crust subducted into at least 70 km. Furthermore, both the experiments and the T-X (CaCO3) pseudosection, constructed based on basaltic lithology, support that the absence of garnet in the blueschist is predominantly caused by the mixing of calcite. In summary, this study provides a valid attempt in exploring metamorphic conditions by experiment simulation, and evaluates the profound influence of the interaction between the basaltic crust and overlying carbonate-bearing sediments in the subduction zone.