Hydrothermal events in the Linzizong Group: Implications for Paleogene exhumation and paleoaltimetry of the southern Tibetan Plateau

Abstract

Knowledge of the thermal history of the Linzizong Group (69–47 Ma) within the Gangdese arc is critical for interpreting the geologic evolution and isotope-based paleoaltimetric results of the southern Lhasa terrane of the Tibetan Plateau. Here, we combine results from geochronologic and thermochronologic studies of this group (divided into Dianzhong, Nianbo, and Pana formations upsection) and the structurally overlying Qianggeren granite (∼52 Ma) in the Linzhou basin. Whole rock 40Ar/39Ar ages of volcanic rocks from the stratigraphically lower Dianzhong and Nianbo formations are ∼10 Myr younger than their corresponding zircon U-Pb ages, suggesting a thermal disturbance of the argon system. Zircon (U-Th)/He ages (ZHe, 63 dates) range from 54 to 24 Ma, and apatite (U-Th-Sm)/He ages (AHe, 43 dates) range from 27 to 4 Ma. Inverse modeling of the thermochronologic data from the Qianggeren granite indicates rapid cooling between 42 and 26 Ma, possibly induced by movement of the Gulu-Hamu thrust. Positive correlations between ZHe ages and effective uranium and other geologic observations provide evidence that the Dianzhong and Nianbo formations were heated 300°C at 54–50 Ma, and that the entire Linzizong Group was variably heated to 130–170°C at 42–27 Ma. These findings, together with published geochronologic and thermochronologic data, suggest that abundant 50–45 Ma zircon fission track and ZHe ages from the Gangdese arc likely reflect conductive cooling of the Gangdese arc after a ∼52 Ma magmatic flare-up episode rather than rapid regional exhumation. Areas far from Cenozoic faults and deeply incised river valleys in southern Tibet have experienced only ∼3 km of exhumation since 45 Ma, consistent with the establishment of a low-relief, plateau-like physiography by Eocene time. The (hydro)thermal events are also manifested by widespread calcite recrystallization and δ18O and Δ47 alteration in most carbonates from the Linzizong Group, which must be taken into account in past and future carbonate-based paleoaltimetric studies.