Interpreted 11-6 Ma age of Retrograde Metamorphism in the subduction channel of the Yuli Belt, Taiwan

Based on petrological and geochronological studies of glaucophane-bearing schist in the Cenozoic Yuli metamorphic belt, Taiwan, as one of the world's youngest high-pressure (HP) terrane, we provide insights on the metamorphic ages, the metamorphic stages/microstructural fabrics, and the exhumation mechanisms of HP rocks.

In this study we conducted field surveys of well-preserved mesoscale amphibole facies ultramafic schist outcrops along the Mayuan River, located in the Juisui area. We also carried out microstructural analysis using a microscope and SEM-EDS on oriented thin sections to distinguish different deformation stages/fabrics, along with quantitative EPMA analysis of the chemical composition of amphibole and phengite mineral. Furthermore, two different grain sizes (0.18mm and 0.25mm) of phengite were used to conduct the ⁴⁰Ar/³⁹Ar step heating dating analysis to determine the metamorphic age.

Field observation identify S₂ as the main foliation, shallowly dipping to the northwest or northeast in the Mayuan River area. The amphibole schist represents the rim of the ultramafic blocks within the matrix of the albite-quartz-mica schist. We also find a right-lateral west dipping shearing fault zone occurred between these two schists.

By adopting main Phengite fabrics as S₂, in microstructure domain, the prograde to retrograde metamorphism (subduction to exhumation) of 5 stages were identified: 1) S_2-1, defined by rotated stubby subhedral phengite; 2) S₂, defined by Phengite ± Epidote ± Calcic-amphibole; 3) S₂₊₁, defined by Phengite ± Epidote ± Calcic-amphibole ± Chlorite; 4) S₂₊₂, defined by Phengite ± Calcic-amphibole ± Titanite ± Albite; 5) and post-S₂₊₂ phengite. The amphibole revealed varied zoned compositions and evidence of both “clockwise” (high-T to low-T) and “counterclockwise” (high-T low-P to high-P low-T) metamorphism from core to rim: from Pargasite to Edenite/Mg-Hb to Actinolite, Glaucophane to Winchite to Actinolite, and Pargasite to Winchite to Actinolite. Phengite grow in every 5 stages, and the ⁴⁰Ar/³⁹Ar dating yielded plateau ages of 10.7±1.6 Ma, 10.4±0.5 Ma, 8.8±0.6 Ma, 8.9±1.9 Ma, and 11.0±0.7 Ma, indicating cooling ages after the subduction stage with an estimated closure temperature of 410-420°C. One age spectrum showed a minimum age of 6.1 Ma, possibly representing a recrystallized age during exhumation.

This study reveals the P-T-t path of the amphibole schists in the rim of ultramafic rocks in the subduction channel from subduction to exhumation. The clockwise and counterclockwise P-T path of the zoned amphibole indicated the complicated exhumation mechanism, and the retrograde metamorphism occurred in 11~6 Ma.