Two-stage exhumation of high–P rocks from the Yuli Belt, Eastern Taiwan: Insights from the metamorphic evolution in subduction channels

Metamorphic P–T trajectories of high–P rocks can well constrain the exhumation mechanism of rocks and thermal structure in a subduction zone. The Yuli Belt in eastern Taiwan is regarded as an ophiolitic mélange related to the subduction of the South China Sea Plate beneath the Philippine Sea Plate, but the metamorphic evolution of the high–P rocks therein is still controversial. Two glaucophane schists from the Tamayen block in the Yuli Belt, are chosen for detailed studies of petrography, mineral composition and phase equilibria modelling, and addressing the metamorphic evolution. The glaucophane schists show coarse-grained amphibole porphyroblasts that distribute randomly in a matrix consisting of amphibole, epidote, garnet, phengite, chlorite, albite and quartz. Amphibole porphyroblasts are characterized by core–mantle–rim zoning from sodic-, calcic- to sodic-amphiboles. Three metamorphic stages are recognized, involving prograde to the peak, post-peak decompression and subsequent cooling. The prograde to the peak stage is constrained to occur in lawsonite- and omphacite-bearing assemblages with the peak conditions of 2.1–2.3 GPa/530–560 °C on the basis of the garnet zoning and the maximum Si content in phengite. The post-peak decompression was first dominated by dehydration reactions with the expense of lawsonite and omphacite and producing glaucophane, followed by decompression in fluid-absent fields to 0.6–0.9 GPa/500–550 °C with the growth of calcic amphibole. The subsequent cooling is inferred from the regrowth of glaucophane that rims amphibole porphyroblasts. The glaucophane schists share clockwise P–T trajectories with evident isothermal decompression, and the peak conditions correspond to a low geothermal gradient of ∼250 °C/GPa, representing the metamorphic evolution within a mature subduction zone. The high-P rocks in the Yuli Belt experienced two exhumation stages: firstly, the rocks were isothermally exhumed via a matured subduction channel from ∼70 km to ∼30 km depth and incorporated into the base of the accretional wedge during 12–9 Ma; and then, further exhumation occurred through thrusting and erosion in response to the arc-continent collision started from ∼6 Ma.