[S-IT20] New perspectives on accretion and exhumation processes during oblique plate convergence

[SIT20-P03] New evidence of high-pressure metamorphism in the Wanjung area, Yuli belt, eastern Taiwan: new P-T constraints and implications

*Wen-Han Lo¹、Enggar Wisnu Cahyaning Ratri¹、Chin-Ho Tsai¹、Yui Kouketsu²、Yoshiyuki Iizuka³ (1.Department of Natural Resources and Environmental Studies, National Dong Hwa University, Taiwan、2.Graduate School of Earth and Planetary Sciences, Nagoya University, Japan、3.Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan)

キーワード：subduction, exhumation, omphacite, glaucophane, pseudosection modelling, elastic barometry

The Yuli belt is a high-pressure (HP) metamorphic terrane located at the convergent boundary between the Eurasia plate and the Philippine Sea plate. It is mainly composed of metapelites with minor serpentinites and metaigneous rocks. In the northern part of the belt, HP metamorphism is by far only reported on the omphacite-bearing metagabbro in serpentinite surrounded by greenschist in the Wanjung area. This rock association has been interpreted as a mafic-ultramafic block tectonically thrust onto the eastern flank of the Yuli belt. Whether this Wanjung tectonic block has undergone HP metamorphism and how it was incorporated into the Yuli belt are still unclear. Here we report the first finding of glaucophane preserved in some garnet-bearing layers intercalated with greenschist. The layers include mafic-intermediate gneiss, amphibole-albite rock, and barroisite schist. Amphibole compositional zoning in these layers, from core to rim, is commonly glaucophane, barroisite, and actinolite, implying a clockwise pressure-temperature (P-T) path. Peak P-T conditions of 14–17 kbar and ~500 ºC for these layers are constrained by quartz-in-garnet Raman barometry, garnet-hornblende thermometry, and pseudosection modelling (Perple_X). The new P-T estimates reflect subduction metamorphism at depths of ~50 km and a geotherm of ~10 ºC/km. We speculate that this mafic-ultramafic block represents a tectonic slice exhumed from a deep slab-mantle interface in a fossil subduction zone, which is probably related to the Miocene subduction of the South China Sea oceanic crust beneath the Philippine Sea plate. The present position of this HP tectonic block is situated at the Yuli belt's eastern margin, implying that this margin is an important structural boundary.