• Pre-Oxfordian (> 163 Ma) Ophiolite Obduction in Central Tibet

      Ma, Anlin; Hu, Xiumian; Kapp, Paul; BouDagher-Fadel, Marcelle; Lai, Wen; Univ Arizona, Dept Geosci (AMER GEOPHYSICAL UNION, 2020-05)
      The timing of Bangong-Nujiang suture ophiolite obduction between the Lhasa and Qiangtang terranes in central Tibet is important for understanding the closure history of the Meso-Tethys but remains poorly constrained. We investigated subaerial to shallow marine strata of the Dongqiao Formation that sit unconformably on Bangong-Nujiang suture ophiolites that crystallized in a supra-subduction zone setting at 189-181 Ma. Based on foraminiferal and coral studies, the depositional age of the Dongqiao Formation is constrained to be Oxfordian and Kimmeridgian (Late Jurassic). Provenance analyses including detrital modes, geochemistry of detrital chromian spinels, and U-Pb age populations of detrital zircons suggest the Dongqiao Formation was sourced from uplifted Bangong-Nujiang suture ophiolites and sedimentary and metamorphic rocks of Lhasa terrane affinity to the south. We conclude that Bangong-Nujiang suture ophiolites were obducted soon after crystallization (prior to Oxfordian time; >163 Ma) onto the Lhasa terrane or a microcontinent of Lhasa terrane affinity. Plain Language Summary Ophiolite obduction often occurs when a passive continental margin enters an oceanic subduction zone and thus may mark when an arc-continent or continent-continent collision begins. In central Tibet, the Dongqiao ophiolite represents a relict of Meso-Tethys oceanic lithosphere. Initial research during the 1980s provided rough constraints on the timing, polarity, and mechanism of Dongqiao ophiolite obduction. However, there has been little advancement in our knowledge of the obduction history since. Here we report results of sedimentologic, stratigraphic, and provenance studies on the Dongqiao Formation overlying the Dongqiao ophiolite. Our foraminiferal and coral biostratigraphic data show that subaerial to shallow marine strata of the Dongqiao Formation were deposited between 163 and 152 million years ago. We demonstrate that the clastic rocks in the Dongqiao Formation received detritus from both the underlying Dongqiao ophiolite and Lhasa terrane affinity continental crust. We propose that a continental margin of Lhasa terrane affinity entered a north-dipping oceanic trench, and the Dongqiao ophiolite was obducted southwards on to it, no later than 163 million years ago.