Tectonic Evolution Of The Amdo Back-arc Basin In The Central Part Of Bangong-nujiang Suture Zone

The Bangong-Nujiang suture zone is an important tectonic boundary separating the Lhasa terrane and Qiangtang terrane in central Tibet. The Amdo terrane is a large microcontinent preserved in this suture zone, wrapped by Amdo ophiolite to the north and Nagqu ophiolite to the south. Previous studies indicated the forming of the Amdo ophiolite resulted from asthenosphere upwelling associated with back-arc extension. Recently, a complete arc-back-arc basin system has also been evidenced by the identification of ca.228 Ma island arc basalts in the northern margin of the Amdo terrane. However, the duration of the bark-arc basin remains unclear. Here we present provenance analyse and geochronological study on sedimentary rocks from both the north and south sides of the Amdo ophiolite, providing better contraints on the formation and evolution of the Amdo back-arc basin.The Upper Triassic sedimentary sequences from the Amdo back-arc basin include Quehala Formation from the south side and Tumengela Formation from the north side. Our results indicate that :(1) the main rock types of Quehala Formaion are sandy conglomerate, sandstone and minor limestone, while the Tumenggela Formation is featured by argillaceous sandstone. Regional metamorphism can be observed in sedimentary rocks from the Quehala Formation, while this characteristic is absent in Tumengela Formation; We tend to interpret the metamorphism was produced by subduction process.(2) The Quehala Formation is characterized by flysch deposition and may be formed in deep water environment, however, the coal-bearing rocks of the Tumengela Formation imply the deposition environment was of shallow marine-terrigenous facies. Consequently, the paleo-water depth in the ocean basin was significantly different, showing shallow and deep water sedimentary settings in north and south respectively.(3) As shown in the Qt-F-L(or Qm-F-Lt or both of them) ternary diagram(Qm:F:Lt=0.54:0.02:0.44), the sandstones of the Quehala Formation are possibly related to recycled orogenic belt.(4) Detrial zircon age spectra of the Quehala Formation demonstrate that the age signature is similar to that of the Amdo terrane, which is likely to be the provenance region of the Quehala Formation. In contrast, the zircon ages in sedimentray rocks of the Tumengela Formation have a Qiangtang terrane affinity.(5) The resulting age spectrum of zirons from the Quehala Formation has the youngest peak at 171 Ma, indicating the sedimentary process of the Quehala Formation from Amdo may extend to Middle Jurassic.(6) Considering that the Quehala Formation experienced successive deposition in back-arc basin setting, we treat the youngest age peak(171 Ma) as the upper limit of the evolution of the Amdo back-arc basin.In combination with the reported island arc basalt, this study provides additional evidence for the existence of arc-back-arc basin in the central part of the Bangong-Nujiang suture zone between 230 Ma and 171 Ma. The significant distinctions in provenance of Late Triassic sedimentary rocks from the south and north side of the Amdo back-arc basin suggest that the rifting of this oceanic basin may occur before the Late Triassic.