Supporting Information: Buchan-type Metamorphic Decarbonation during the Upward Expansion of the South Tibetan Detachment System

Abstract:The role of continent collisional belts in the global carbon budget remains controversial. Collisional orogens have traditionally been considered a net carbon sink, but recent studies have highlighted significant CO₂ fluxes. This study carried out comprehensive field mapping, petrography, pressure-temperature determination, geochemistry, and geochronological data along three transects of the South Tibetan Detachment System (STDS) in the Mount Everest and Nyalam regions of the central Himalaya. The results outline a previously unrecognized carbon source in the Himalaya: Buchan-type metamorphic decarbonation of carbonate-bearing lithologies in the migrating hanging-wall of the STDS driven by the juxtaposition against hot migmatite/magma in the footwall of the structure. Specifically, calc-silicates and schists incorporated into the active STDS underwent Buchan-type metamorphic overprinting at P-T conditions of 630–400°C and 5–3 kbar (36–48°C/km) compared to Barrovian-type metamorphism (28°C/km) in the footwall. Monazite and titanite U(-Th)-Pb petrochronology indicate that metamorphism within the STDS occurred between ca. 23 and 19 Ma, which is contemporaneous with deformation along the STDS evidenced by the ages of mylonitized leucogranites. Activity along the STDS sustained to 17–16 Ma, causing resetting of titanite U-Pb ages in some calc-silicates. Detrital zircon geochronology shows that the Yellow Band and North Col Formation in the STDS have an affinity to the Tethyan Himalayan Sequence and were involved in the shear zone during its upward expansion into hanging-wall rocks. Based on decarbonation reactions, protolith restoration, and decarbonation efficiency studies, the metamorphic CO₂ degassing from the metamorphism of calc-silicate rocks is quantified to be ~0.8 Mt C/yr during 23–19 Ma. The quantification of upward expansion of the STDS, the resulting juxtaposition of underlying hot migmatites/magma with cold hanging wall-rocks, and the proposed metamorphic decarbonation phenomenon are crucial to understanding the development process of orogen-scale low-angle normal-sense faulting and the resulting carbon sources during Himalayan orogenesis.