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

[SIT20-04] A petrological perspective on exhumation of high-pressure metamorphic rocks in the Yuli belt, eastern Taiwan

★Invited Papers

*Chin-Ho Tsai¹ (1.Department of Natural Resources and Environmental Studies, National Dong Hwa University)

キーワード：orogenic evolution, subduction metamorphism, serpentinite, metamafic rocks, collision, extrusion

Taiwan's unique tectonic framework makes it one of the best places in the world to study orogenic evolution involving subduction, accretion, collision, exhumation, and uplift. In particular, the Yuli belt is the only geologic unit containing high-pressure (HP) metamorphic rocks; it bears key information for deciphering the geodynamic history of the Taiwan orogen. One outstanding question about HP metamorphic rocks worldwide is how they were exhumed back to the surface. Several hypotheses and theoretical models had been proposed over the past three decades, but a global consensus does not seem to exist yet. This presentation aims to contribute some thoughts on the exhumation of Taiwan's HP rocks on the basis of recent findings from field and petrological observations. Glaucophane-/omphacite-bearing HP metaigneous rocks and adjacent garnet pelitic schists are spatially associated with antigorite serpentinites and their peak metamorphic conditions are similar, indicating that the rock associations represent exhumed slices from an intermediate slab-mantle interface in a subduction channel. Serpentinite and buoyancy might have played a vital role in the early-stage exhumation. Garnet compositions of the HP rocks (including pelitic schist) mostly record prograde zoning patterns and the highest-grade (>15 kbar, ~550 °C) metamafic rocks show little or no retrograde overprint, implying rapid decompression. The two sub-units with HP metamorphic rocks are both sandwiched between slightly lower-grade units (with ~100 °C less)—a configuration likely resulted from compressional extrusion. The distribution of two major HP blocks along the eastern edge of the Yuli belt suggests that these blocks were transported and brought up by tectonic forcing, probably collision-related. In summary, exhumation of the Yuli belt's HP metamorphic rocks is a geologically fast but complex journey that might have been driven by buoyancy and serpentinite diapirism, and facilitated by collision-related tectonic extrusion.