[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-06] Amphibole-albite rock in a metasomatic zone: a record of subduction metamorphism at the slab-mantle interface and its tectonic implications

*Dominikus Deka Dewangga¹、Wen-Han Lo¹、Chin-Ho Tsai¹、Yui Kouketsu²、Yoshiyuki Iizuka³、Hao-Yang Lee³ (1.Department of Natural Resources and Environmental Studies, National Dong Hwa University, Taiwan、2.Graduate School of Environmental Studies, Nagoya University, Japan、3.Institute of Earth Sciences, Academia Sinica, Taiwan.)

キーワード：pressure and temperature, glaucophane, Raman spectroscopy of carbonaceous material, phase equilibrium modeling, subduction interface

The peak pressure-temperature (P-T) conditions of the high-pressure (HP) rocks within melange units of the Yuli belt (eastern Taiwan) provide insights into subduction metamorphism at a fossil slab-mantle interface. These units comprise metaigneous rocks and ophiolitic serpentinites enclosed within metasedimentary schists. A metasomatic zone occurs along the boundary between pelitic schist and serpentinite; however, the metamorphic conditions of these metasomatic rocks remain poorly understood. This study aims to constrain the P-T conditions of the metasomatic zone in the Tsunkuanshan area, focusing on chlorite-albite schist (Zone I) and amphibole-albite rock (Zone II). Compositional zoning in garnet from Zone I suggests prograde metamorphic growth. Zone II records three distinct metamorphic stages: (1) magnesiokatophorite + ilmenite + titanite + albite + epidote, (2) glaucophane + barroisite + ilmenite + titanite + albite + epidote, and (3) actinolite + titanite + albite + chlorite. Phase equilibrium modeling (Perple_X) indicates that Stage 1 reached peak metamorphic conditions of 0.7–0.8 GPa and ~500 °C, followed by retrogression during Stages 2 and 3. Raman spectroscopy of carbonaceous material thermometry on the pelitic schist yielded temperatures of 480–520 °C, closely matching the peak metamorphic conditions of serpentinite (~1.5 GPa and ~550 °C). These findings suggest that metasomatism occurred at a depth of more than 25 km along the subduction interface, initiating after the peak metamorphism in serpentinite and pelitic schist, likely during the exhumation of subducted materials. Our results indicate that metasomatic rocks at the pelitic schist-serpentinite boundary preserve evidence of HP metamorphism in a subduction zone setting, providing new constraints on the evolution of slab-mantle interactions.