The Cretaceous suturing process of the eastern Bangong–Nujiang Ocean, Basu area

Abstract

The Bangong–Nujiang Ocean played an important role in the formation of the Tibetan Plateau prior to the Cenozoic India–Eurasia collision. However, there are still uncertainties about the subduction polarity and timing of the Lhasa–Qiangtang collision. We conducted sandstone petrologic and detrital zircon U–Pb–Hf isotopic analyses on the Cretaceous Wada mélange, trench-fill strata and Duoni Formation in the Basu area in the eastern Bangong–Nujiang Suture Zone. The Wada mélange (∼114 Ma) exhibits block-within-matrix features and a detrital zircon U-Pb age spectrum characterized by multiple peaks at 114–180 Ma, 200–300 Ma, and 1800–2000 Ma. The trench-fill strata consist of coherent chert, sandstone, and mudstone, with a detrital zircon age spectrum dominated by a single peak at ∼120 Ma. The εHf(t) values of the 110–300 Ma detrital zircon grains in the Wada mélange and trench-fill strata range from −20 to +10 and are consistent with those of the South Qiangtang Terrane. The εHf(t) values of the ∼120 Ma detrital zircons are all negative, and these grains were likely derived from a remnant Cretaceous arc in the South Qiangtang Terrane to the north. These data suggest that the accretionary wedge was derived from the South Qiangtang Terrane during the northward subduction of the Bangong–Nujiang Ocean. The Duoni Formation (∼113 Ma) represents peripheral foreland basin deposits with the Lhasa Terrane as the basement. Provenance analysis indicates that these deposits received clastic material mainly from the South Qiangtang Terrane and to a lesser degree from the North Lhasa Terrane. Our results suggest that the Lhasa–Qiangtang collision occurred in the Early Cretaceous (∼113 Ma) in the eastern segment of the Bangong–Nujiang Suture Zone.