On the peak metamorphic conditions of the Yuli belt, eastern Taiwan: new constraints using garnet isopleths

Two metamélange units in the Yuli belt contain high-pressure (HP) metamorphic rocks attesting to subduction zone metamorphism. These metamélange units are characterized by blocks of serpentinites, metaigneous, and metavolcaniclastic rocks in a matrix of garnet pelitic schist. Some of the blocks contain HP minerals (glaucophane, omphacite), but their peak metamorphic conditions are still poorly constrained. This presentation reports a compilation of garnet textures and compositions from these blocks and adjacent pelitic schists. Phase equilibrium modeling (Perple_X) with garnet isopleths is applied to estimate the P-T conditions quantitatively. We recognized two types of garnet compositional nature in these HP blocks. Garnet in the glaucophane schists (GlnS) displays a Mn-rich core and Fe-rich rim. By contrast, garnet in the garnet-paragonite-epidote amphibolites (GPEA) is nearly homogeneous and relatively Mg-rich. Garnet of the adjacent pelitic schists (PS) shows a similar zoning to that of the GlnS. The peak P-T conditions of these HP rocks constrained by garnet isopleths reveal a P-T range in 10–18 kbar and 500–580 ºC. The inferred P-T paths for the GlnS and PS are clockwise, whereas the one for the GPEA seems counter-clockwise. Our new P-T constraints suggest that these HP blocks and the metapelitic matrix likely formed in a paleo-subduction interface. The discrepancy in P-T data and paths among different rock types may reflect a kind of tectonic mixing.

Keywords: subduction zone, subduction interface, tectonic mélange, tectonic mixing, phase equilibrium modeling, garnet chemical zoning