NewP-Tconstraints on the Tamayen glaucophane-bearing rocks, eastern Taiwan: Perple_X modelling results and geodynamic implications

New pseudosection modeling was applied to better constrain the P-T conditions and evolution of glaucophane-bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane-bearing high-pressure rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type-I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type-II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type-III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type-I), barroisite → Mg-katophorite/taramite → Fe-glaucophane (Type-II), glaucophane → winchite (Type-III). Using petrographic data, mineral compositions, and Perple_X modeling (pseudosections and superimposed isopleths), peak P-T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type-I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type-II (thermal peak), and 11 ± 1 kbar and 530 ± 30 °C for Type-III. The calculations yield higher pressures and temperatures than previously thought; the difference is about 1-6 kbar and 50-200 °C. The three rock types record similar P-T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near-isothermal conditions (Type-I and Type-III), with the probable exception of Type-II where decompression followed colder geotherms. The P-T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11-14 ˚C km-1. This article is protected by copyright. All rights reserved.