Subduction-accretion process of the Neo-Tethyan Ocean: insights from the Purang mélange in the western Yarlung Zangbo Suture Zone

The Purang mélange， located within the western Yarlung Zangbo suture zone (YZSZ) in southern Tibet， provides valuable insights into the subduction-accretion process along the active southern margin of Asia during the Cretaceous-Paleocene Neo-Tethyan subduction. This mélange consists of sandstone， chert， basalt， and limestone blocks embedded in a mud matrix. We performed petrological and detrital zircon U-Pb geochronological analyses for the sandstone blocks to trace their provenances， while the whole-rock geochemical analyses for the chert and basalt to determine their sedimentary and tectonic settings. The sandstone blocks are primarily composed of quartz， with a few lithic fragments. Both of the sandstone samples exhibit detrital zircon U-Pb age ranges consistent with those of the Tethyan Himalaya. These results suggest that the sandstone blocks of the Purang mélange were derived from the passive continental margin of India. The basalt blocks exhibit three styles according to their geochemical characteristics， indicating that they originated from a subducted Neo-Tethyan slab. The chert blocks， however， display geochemical affinities of both pelagic and continental margins， indicative of a dual origin from both the subducted and overlying slabs. Based on the above observations， we propose a two-stage subduction-accretion process of the Neo-Tethyan Ocean. The basalt blocks were scraped from the subducted oceanic slab and incorporated into the subduction complexes during the Neo-Tethyan subduction stage of the Cretaceous， whereas the sandstone blocks of the passive continental margin reached the trench and were subsequently incorporated into the Purang mélange during the India-Asia collision stage of the Paleogene.