Discovery of Middle Jurassic trench deposits in the Bangong-Nujiang suture zone: Implications for the timing of Lhasa-Qiangtang initial collision

The Mesozoic trench deposits in the Bangong-Nujiang suture zone in central Tibet can provide critical information for reconstructing the paleotectonic evolution of the Bangong-Nujiang Ocean and constraining the timing of the Lhasa-Qiangtang initial collision. In this paper, the Gamulong Formation, which is well exposed in the Dong Co area, western Bangong-Nujiang suture, central Tibet, was studied. The sedimentological analysis shows that the Gamulong Formation was deposited in a submarine fan environment. The youngest detrital zircon age and fossils from limestone pebbles within the Gamulong Formation combine to determine a Middle Jurassic age. The pebbles in the conglomerate are dominated by limestones and sandstones, which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components. Detrital zircon U-Pb ages from the sandstones yield age populations of 164-178 Ma, 200-500 Ma, 700-1000 Ma, 1700-2100 Ma and ~2500 Ma. The corresponding zircon εHf(t) values are distributed between -23.5 and +11.1. These zircon features are similar to those from the Qiangtang terrane. Accordingly, the provenance results imply that the Gamulong Formation was recycled from the accretionary complex (Mugagangri Group mélange) in the Bangong-Nujiang suture zone and the forearc basin deposits (Lower-Middle Sewa/Shaqiaomu/Jiebuqu formations) in the southern Qiangtang subterrane. The Gamulong Formation is characterized by sedimentary recycling in the trench, likely in response to a tectonic event (possibly the continental collision). Together with the previous studies, this work on the occurrence of the Gamulong Formation indicates that the Lhasa-Qiangtang initial collision could have occurred in Middle Jurassic time (~166 Ma)