From the southern Gangdese Yeba arc to the Bangong-Nujiang Ocean: Provenance of the Upper Jurassic-Lower Cretaceous Lagongtang Formation (northern Lhasa, Tibet)

The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic-Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong–Nujiang suture, no debris from Bangong–Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.