(Ultra)high-pressure metamorphic rocks provide valuable insights into properties of slab-derived fluids. Here, we report CH4-rich fluid inclusions in garnet of a metapelite from the Zermatt-Saas ophiolite, western Alps. Two types of metapelite, a CH4-bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P-T-fO2 conditions for preservation of CH4 in high-pressure metapelite. In the CH4-bearing pelitic schist, CH4-rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtⅡ). GrtI records P-T conditions of ~2.85 GPa and ~555 °C, whereas GrtⅡ records a prograde P-T path from ~1.75 GPa at 510 °C to ~2.0 GPa at 530 °C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ~1.9 GPa at 510 °C to ~2.12 GPa at 545 °C. CH4-rich fluid formation may primarily rise from graphite reduction at high-pressure reduced conditions (ΔFMQ -3.5 to -4, 2.85 GPa, ~550 °C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ~ 0, 2.12 GPa, 545 °C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depth. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non-oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab-derived COH fluids and offers new insights into the deep carbon cycle.

The exhumation mechanism of the low-temperature/high-pressure (LT/HP) rocks, is critical for understanding the formation of the central Qiangtang metamorphic belt (CQMB), Tibetan Plateau, but it is still hotly debated. Here, we report field, petrological, phase-equilibria and petro-physical modelling data on the newly discovered lawsonite-bearing eclogites, epidote eclogite and lawsonite-bearing blueschists from the Lanling area in the CQMB. The mineral characteristics and phase equilibria modeling reveal that the LT/HP rocks record peak P-T conditions from peak pressure (Pmax) of 22.5–23.5 kbar at 460–480 °C to peak temperature (Tmax) of 530–550 °C at 20–22.5 kbar. Combined with previous documented geochronological data, a clockwise P-T-t path for these LT/HP rocks is obtained, which is characterized by pronounced heating decompression (~223–221 Ma), subsequent isothermal decompression (~221–219 Ma), and final cooling decompression (~219–212 Ma). Modeled densities and net buoyancies (defined as the density difference between Preliminary Reference Earth Model and LT/HP rocks) show that all LT/HP samples are buoyant at Pmax, but gradually become denser during heating decompression and evolve to neutrally or negatively buoyant around Tmax. Later mixing with lower-density garnet-phengite schists at Tmax, help the density of the exhuming LT/HP unit reduce to lower than that of the surrounding mantle again during continued isothermal decompression. We concluded that exhumation of eclogites and blueschists is short-lived (~10 Ma) and multi-stage buoyancy-driven characterized by early self-exhumation via diapiric rise and post-Tmax carried-exhumation along subduction channel.