History of burial, carbonate diagenesis, and exhumation of the Eocene Xianqian Basin in western central Tibet

Lacustrine carbonates in intermontane basins have the potential to archive paleoelevation and paleoclimate information of orogenic belts. Carbonate oxygen and clumped isotopes are commonly used to reconstruct paleo-meteoric water isotopic compositions and depositional temperatures; however, post-depositional processes, including water-rock interaction and solid-state reordering, can introduce uncertainties that complicate such interpretations. To evaluate the fidelity of lacustrine carbonate records, we combine clumped isotope thermometry with apatite thermochronology to constrain the thermal history and carbonate diagenesis in the Xianqian Basin, central Tibet, where >2 km of strata accumulated between 50 and 43 Ma. Low-temperature thermochronometric data indicate that apatite U-Th/He ages were fully reset, whereas apatite fission-tracks were only partially annealed, suggesting burial temperatures between 60 and 120 °C. These burial temperatures are insufficient to raise plausible near-surface carbonate clumped isotope temperatures to the observed 47–77 °C through solid-state bond reordering alone. Together with petrographic, mineralogical, and geochemical evidence, we interpret the elevated clumped isotope temperatures as the result of dolomitization during the shallow-burial stage. Consequently, the calculated oxygen isotope compositions reflect those of diagenetic fluids (δ 18 O w = +1‰ to +6‰), but not those of surface freshwater required for paleoelevation reconstruction. This study underscores (1) the value of integrating low-temperature thermochronology with clumped isotope thermometry to resolve carbonate diagenetic histories; and (2) the need to re-evaluate previous conclusions that the Qiangtang terrane had reached near-present elevations during the Paleogene.