Late Triassic sedimentary environments and detrital zircon provenance analysis in the Amdo area of the Tibetan Plateau: Implications for the evolution of the Meso-Tethys Ocean

Aspects of the early Mesozoic evolution of the Meso-Tethys Ocean remain uncertain, and detrital zircon provenance analyses may be useful for constraining the development of this region. In this paper, we describe Late Triassic sedimentary facies, petrography, geochemistry, and detrital zircon UPb geochronology for the Tumengela Formation (South Qiangtang Basin) and the Quehala Formation (Amdo back-arc basin) in the Tibetan Plateau. In Late Triassic times, the South Qiangtang Basin was characterized by marine deltaic environments adjacent to an evaporative platform, whereas the Amdo back-arc basin was characterized by a shallow marine shelf fringed by tidally-influenced deltas; in both basins, marine palaeoenvironments first deepened and then shallowed. By analyzing the characteristics of detrital zircon UPb age spectra for each microcontinent on the northern margin of Gondwana, and combining them with previously published data, we infer that the North and South Qiangtang Terranes amalgamated in Late Triassic times, and were separated from the Lhasa Terrane by the wide Meso-Tethys Ocean. During the Late Triassic, the Meso-Tethys Ocean began to be subducted, and the southern edge of the South Qiangtang Terrane developed as an active continental margin. The Quehala Formation accumulated in the back-arc basin of the Amdo microcontinent during subduction of the Meso-Tethys Ocean, and the uppermost part of Member 2 of the Quehala Formation is dated as no older than Early Jurassic.