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

[SIT20-P02] Subduction metamorphism recorded in metasomatic rocks from the Yuli belt, eastern Taiwan: metamorphic conditions and tectonic implications

*Dominikus Deka Dewangga¹, Chin-Ho Tsai¹, Yoshiyuki Iizuka², Yui Kouketsu³ (1.Department of Natural Resources and Environmental Studies, National Dong Hwa University, Taiwan, 2.Institute of Earth Sciences, Academia Sinica, Taiwan, 3.Graduate School of Earth and Planetary Science, Nagoya University, Japan)

Keywords:high-pressure, glaucophane, RSCM, pseudosection modeling

The Yuli belt attests to tectonic evolution involving subduction metamorphism because a few mélange units contain high-pressure (HP) blocks of metaigneous rocks and serpentinites enclosed in metasedimentary rocks. Metasomatic rocks commonly occur at the contact between the metasedimentary rocks and serpentinites. However, the metamorphic conditions of the metasomatic rocks remain poorly constrained. Meter-scale metasomatic rocks in the Tsunkuanshan area display five zones, from pelitic schist (PS) to serpentinite (SP), including (I) chlorite-albite schist, (II) amphibole-albite rock, (III) albite-chlorite schist, (IV) epidote-chlorite schist, and (V) chlorite-talc schist. Minor garnet in zone I and amphibole in zone II show chemical zoning. We estimate metamorphic conditions of the metasomatic rocks on the basis of the mineral assemblages and compositions with the temperature constrained by Raman spectroscopy of carbonaceous material (RSCM) thermometry and pseudosection modeling (Perple_X). The peak metamorphic conditions of metasomatic rocks are ~1.2 GPa and ~500 °C. This result indicates that relict blueschist-facies metamorphism is preserved in the zone I and II, whereas the zone III-V might have been overprinted by retrograde greenschist-facies metamorphism. The RSCM thermometry on the PS yielded a temperature range of 480-520°C. Pseudosection modeling of the SP resulted in a possible peak metamorphic temperature around 500-550 °C. These pressure and temperature estimations imply subduction metamorphism and are similar to the peak metamorphic conditions of Yuli belt’s HP blocks (1.4-1.7 GPa and ~550 °C). We interpret that metasomatic rocks between PS and SP were formed during subduction metamorphism followed by exhumation processes along the subduction interface.