Carbonate metasomatism in a cold subduction zone: Evidence from pyroxenite-derived mafic rocks of the Banpo backarc basin, southeastern Tibetan plateau

Identifying magmatic rocks related to carbonate metasomatism in cold subduction zones is critical for deciphering carbon cycling dynamics in these settings. This study presents an integrated geochemical investigation of mafic rocks from the Banpo region along the southern margin of the Changning−Menglian orogenic belt, southeastern Tibetan plateau, which is widely recognized as a classic cold subduction zone. The zircon U-Pb age of the Banpo mafic rocks is 295.6 ± 2.8 Ma, indicating that they formed in the Early Permian. They were not significantly affected by crustal contamination, but they were subjected to variable degrees of fractional crystallization of olivine, pyroxene, plagioclase, and ilmenite. Rare earth element (REE) inversion modeling indicates that most samples exhibit 0.5%−20% partial melting of spinel pyroxenite, while a minority are derived from 10% to 25% melting of garnet pyroxenite. The studied rocks exhibit pervasive carbonate metasomatism, evidenced by specific geochemical markers: clinopyroxene with elevated Mg# (>74), high Ca/Al (>5), enriched (La/Yb)N (3.8−12.4), and low Ti/Eu (850−1850). This interpretation is further supported by whole-rock Zr/Hf ratios (24.3−40.27), zircon and feldspar geochemistry, and integrated isotopic signatures. Monte Carlo simulations reveal a low carbonate-carbon flux of only 0.02 Mt/yr in the study area, significantly below global averages. We suggest that primary carbon originates from the fluid-mediated dissolution of carbonates. This decarbonation mechanism dominates in cold subduction zones, where high fluid fluxes—generated by localized dehydration (e.g., breakdown of chlorite or serpentine) at intermediate depths—combine with low temperatures to enhance carbonate solubility. These findings provide the first evidence for carbonate metasomatism in the Changning−Menglian orogenic belt and support an intra-oceanic arc−backarc basin evolution for the Paleo-Tethys Ocean.