Early uplift and exhumation of the Tanggula granitoid pluton since the Late Cretaceous: Implications for the stepwise topographic growth model in the eastern Qiangtang terrane

Abstract

Reconstructing the uplift process of the eastern Qiangtang terrane is crucial for understanding the growth model of the central Tibetan Plateau. However, due to the limited amount of data available, it is not well constrained. The Tanggula granitoid pluton is an outstanding geological feature in the eastern Qiangtang terrane, and thus could provide crucial constraints on its uplift history. We applied multiple thermochronologic systems over a broad temperature range, including apatite U-Pb, biotite and K-feldspar 40Ar/39Ar, apatite and zircon fission-track, and zircon (U-Th)/He, to study samples from the Tanggula granitoid pluton. The results exhibit the expected relative age order of these thermochronologic systems, with 242−238 Ma apatite U-Pb ages, 218−204 Ma biotite 40Ar/39Ar ages, 197−191 Ma K-feldspar 40Ar/39Ar ages, 94−81 Ma zircon fission-track ages, 70−58 Ma zircon (U-Th)/He ages, and 61−39 Ma apatite fission-track ages. Using these thermochronologic ages and thermal history modeling results, we reconstructed a comprehensive thermal history for the pluton, from which three rapid cooling phases were revealed. The earliest rapid cooling phase (220−180 Ma; ∼5.25 °C/m.y.) closely followed the emplacement of the Tanggula granitoid pluton, and thus is primarily an expression of natural cooling triggered by conduction with the surrounding rocks. In contrast, the rapid cooling during 100−60 Ma and since 20 Ma can be interpreted to represent intense exhumation, with corresponding exhumation of 5.0−6.0 km and 2.3−2.8 km, as well as an average exhumation rate of 0.125−0.150 mm/yr and 0.115−0.140 mm/yr, respectively. According to the thermal history, the earliest uplift in the Tanggula region could have been initiated as early as the Late Cretaceous. Using the published data, we determined that the onset of rapid uplift and exhumation in the entire eastern Qiangtang terrane had a northeastward, stepwise propagation process. The region within or around Anduo first experienced rapid uplift and exhumation that initiated during the late Early Cretaceous (ca. 130 Ma), the Tanggula region to the northeast experienced rapid uplift and exhumation that initiated during the Late Cretaceous (ca. 100 Ma), and the region farther to the northeast in Tuotuohe and Yushu−Nangqian experienced rapid uplift and exhumation that initiated in the late Paleocene (ca. 60 Ma). The northeastward stepwise uplift and exhumation in the eastern Qiangtang terrane was likely caused by the combined Lhasa−Qiangtang and India−Asia continental collisions.