High-Precision Carbonate U-Pb Dating and Clumped Isotope Constraints on the Late Cretaceous-Early Eocene Stratigraphic Age and Paleoelevation of the Northern Tibet

The uplift history of the northern Tibet is critical for understanding the growth of the plateau and regional climate evolution, but the early (Late Cretaceous-early Eocene) stratigraphic ages and paleoelevations remain debated. This study focuses on the Late Cretaceous-Cenozoic continental sedimentary sequences of the Fenghuoshan and Qaidam basins in the northern Tibet. By integrating high-precision carbonate and zircon U-Pb geochronology, mixed vapor stable isotopes, and clumped isotope paleoaltimetry, we reconstruct the early chronostratigraphic framework, paleoelevation, and drainage evolution of the northern Tibetan Plateau. Geochronological results show that in the Fenghuoshan Basin, the Fenghuoshan Group (from bottom to top: Cuojuri, Luolika, and Sangqiashan formations) yields carbonate U-Pb ages of 79–77 Ma in the lower Cuojuri Formation and volcanic tuff U-Pb ages of 65–64 Ma in its upper part. The youngest detrital zircon age of the Luolika Formation is ~54 Ma, indicating that the Fenghuoshan Group was deposited at least from the Late Cretaceous to the early Eocene (~80–54 Ma), providing a key constraint on its basal age. In the Qaidam Basin, small tubular calcite veins first discovered in the Lulehe Formation yield a U-Pb age of 50.8 ± 2.8 Ma, and the youngest zircon U-Pb age from tuffaceous sandstones at the top of the formation is 45 ± 3.2 Ma, suggesting that the Lulehe Formation was deposited at least during the early Eocene (51–45 Ma). The underlying Quanyagou Formation gives a carbonate vein U-Pb age of 87.2 ± 6.6 Ma, similar to the basal age of the Fenghuoshan Basin. For paleoelevation, after correction for sub-cloud evaporation and surface water evaporation, the mixed vapor oxygen isotope model yields a Late Cretaceous paleoelevation of 1257 m (+740/-866 m) for the Fenghuoshan Basin. The early Eocene paleoelevation of the Qaidam Basin, reconstructed by combining the oxygen isotope lapse rate and clumped isotope temperature lapse rate from paleosol calcareous nodules, is 1566 m (±981 m). Regarding the drainage reorganization in the Fenghuoshan Basin, integrating geochronology, paleoelevation, detrital zircon age distributions, and paleocurrent directions, we infer that during the Late Cretaceous–late Eocene, sediments were mainly derived from the southern Tanggula Mountains, with a unidirectional south-to-north drainage system. After the early Oligocene, tectonic uplift of the Fenghuoshan area turned it into a new sediment source region, gradually replacing the Tanggula Mountains. This change marks a reorganization of the drainage-paleoelevation pattern and also corresponds to the formation of the nascent headwaters of the modern Yangtze River system. This study demonstrates the great potential of carbonate U-Pb dating and clumped isotope paleoaltimetry in continental basins, and provides key constraints on the early uplift-drainage evolution of the northern Tibet.