High‐P metamorphism of garnet–epidote–amphibole schists from the Yuli Belt, Eastern Taiwan: Evidence related to warm subduction

Abstract

AbstractHigh‐P (HP) rocks exhumed from ancient subduction zones have been controversially argued to record much warmer geotherms than numerical modelling results. The Yuli belt in Eastern Taiwan was regarded as a mélange consisting of HP rock‐bearing tectonic blocks within metasediments (or in situ schists), witnessing the subduction of the South China Sea Plate beneath the Philippine Sea Plate and the subsequent collision during the period of 15–5 Ma. Two garnet–epidote–amphibole schist samples selected from the Tamayen block in the Yuli belt are investigated for their metamorphic evolution. The rocks comprise garnet and paragonite porphyroblasts in a foliated matrix consisting of amphibole, epidote, chlorite, quartz, albite and rutile. Large paragonite flakes crosscut clearly the main foliation, and in places develop narrow coronae of muscovite, albite or chlorite. Fine‐grained pargasite (Amp1) is the dominant variety of amphibole that defines the foliation, whereas a few coarser grains (Amp2) penetrate the foliation and both are further rimmed by a narrow light‐coloured amphibole (Amp3). Petrographic observation and phase equilibria modelling using thermocalc suggest three stages of metamorphic evolution for these rocks. The pre‐peak prograde to the peak‐stage evolution can be revealed from garnet zoning to occur in the mineral assemblages containing lawsonite and glaucophane which cannot be observed petrographically due to the post‐peak decompressional evolution. The peak conditions are constrained to be ~22 kbar/545℃ using the isopleths of the maximum pyrope and the corresponding grossular contents in P–T pseudosections. The post‐peak decompression includes three substages with different fluid behaviours: the first substage is dominated by lawsonite dehydration with the growth of paragonite; the second substage is marked by the decompressional path proceeding in fluid‐absent fields; and the third substage occurs when mineral assemblages become again saturated with water at 10–9 kbar or 12–10 kbar where glaucophane is metastably transformed into Amp1, followed by the growth of Amp2. The late‐retrograde stage is defined to be cooling with decompression on the basis of the growth of Amp3, the coronae around paragonite and the matrix albite. The peak P–T estimates for garnet–epidote–amphibole schists suggest a geotherm of ~7℃/km, much cooler than those in the previous researches in the Yuli belt, and well consistent with the results from numerical thermal modelling. We argue that HP rocks from ancient orogenic belts can robustly constrain the thermal structure of subduction zones if their peak P–T conditions as well as the post‐peak decompressional evolution are well recovered.