Buchan-type metamorphic decarbonation during the upward expansion of the South Tibetan Detachment System: A new carbon source in the Himalaya

The role of 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. The present contribution combines comprehensive field mapping with detailed petrography, pressure-temperature determination, geochemistry, and geochronology 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 orogen: Buchan-type metamorphic decarbonation of carbonate-bearing lithologies in the hanging wall of the STDS driven by juxtaposition against underlying migmatite. Specifically, calc-silicates and calc-schists within the STDS underwent Buchan-type metamorphic overprinting at P-T conditions of 630-400 degrees C and 5-3 kbar (36-48 degrees C/km). Monazite and titanite U(-Th)-Pb petrochronology indicate that metamorphism occurred between ca. 23 and 19 Ma, contemporaneous with early deformation along the STDS. Movement along the STDS continued to at least 17-16 Ma, causing deformation and resetting of titanite U-Pb systematics in some calc-silicates. Detrital zircon geochronology shows that the calc-silicates and calc-schists in the STDS have an affinity to the Tethyan Himalayan Sequence and were incorporated into the shear zone and metamorphosed during its upward expansion. Based on decarbonation reactions, protolith restoration, and decarbonation efficiency studies, the metamorphic CO2 degassing from the metamorphism of carbonate-rich rocks is estimated to be -0.6 Mt. C/yr. Recognition of the upward expansion of the STDS and associated metamorphic decarbonation provided new information key to both understanding the development of orogenscale low-angle normal-sense faulting in the Himalaya and whether the orogen was a net CO2 source or sink during middle Miocene time.