UHP eclogite from western Dabie records evidence of polycyclic burial during continental subduction

Understanding the behavior of continental crust during subduction is important for investigating dynamic processes at convergent plate margins. Although simulations have predicted continental crust may experience multiple burial-partial exhumation cycles during subduction, petrological evidence of these cycles is scarce. In this study at Sidaohe, western Dabie, we combine microstructural observations and mineral chemistry with phase equilibrium modeling, Amp-Pl thermobarometry and Zr-in-rutile thermometry to constrain the P-T evolution for three eclogite samples. All samples have a similar mineral assemblage of garnet + omphacite + symplectite (amphibole + plagioclase +/- clinopyroxene) + quartz, with accessory rutile/ilmenite. Element mapping and analytical traverses across large garnets from two samples show obviously systematic variations in Ca and, less strongly, Mg, Fe, and X-Mg [Mg/(Mg+Fe2+)]. Based on phase equilibrium modeling and calculated isopleths for grossular, pyrope and X-Mg in garnet, we show that P first increased from 23.0 to 28.5 kbar, then decreased to 24.0 kbar, before increasing again to a maximum of 30.5 kbar (+/- 1.0 kbar, 2 sigma error) concomitant with a small increase in T from 580 to 605 degrees C (+/- 20 degrees C, 2 sigma error) at the late prograde stage. These data are interpreted to indicate multiple burial cycles and partial exhumation of eclogite during ongoing continental subduction. After the P-max stage, T first increased to a maximum of 664-644 degrees C at 25.0-20.0 kbar, then decreased to 581-561 degrees C (+/- 30 degrees C, 2 sigma error) at 15.0-10.0 kbar based on results of Zr-in-rutile thermometry. Further decompression and cooling occurred across P-T fields of 590-567 degrees C at 12.0-10.0 kbar and 520-504 degrees C (+/- 40 degrees C, 2 sigma error) at 8.0 kbar. Fine-grained symplectite (clinopyroxene + plagioclase +/- amphibole) in the matrix is interpreted to have formed after omphacite due to dehydroxylation of nominally anhydrous minerals during decompression from the P-max stage. By contrast, formation of coarse-grained symplectite (amphibole + plagioclase) and a veinlet of rutile + quartz that crosscuts one sample may be related to influx of externally sourced H2O. This study shows that: (1) evidence of cyclic burial and partial exhumation may be retained in low-T eclogite during continental subduction, and (2) fluid contributing to widespread retrogression of eclogite during exhumation may be internally and/or externally sourced.