A Late Jurassic To Early Cretaceous Radiolarian Fauna From Changku,Gyangze,Southern Tibet

The Yarlung-Tsangpo suture zone is located at the boundary between the India and Eurasia Plates,marking the closed position of the Neo-Tethyan Ocean.The ophiolites and related marine strata within this suture zone together recorded the evolution of the Neo-Tethyan basin and the collision between India and Eurasia continents.Radiolarians are important records of deep-marine sedimentation within the suture zone,and often considered to be critical to constraining the timing of major events.The study of radiolarian biostratigraphy is significant for interpretating the tectonic nature of associated oceanic terranes,and has been widely applied to investigations of the Neo-tethys.Radiolarian-bearing cherts are extensively distributed on both sides of the ophiolite belt in southern Tibet.Abundant Jurassic and Cretaceous radiolarians have been discovered and described from these cherts in previous radiolarian works.However,the radioarian biostratigraphic sequence in southern Tibet has not been systematically established.Abundant well-preserved and identifiable radiolarian fossils have been discovered from a set of brecciated cherts near Changku,Gyangze,southern Tibet.A total of 59 species belonging to 40 genera have been identified from 8 radiolarian-bearing samples collected from the brecciated cherts.The radiolarian fossils extracted from the brecciated cherts were studied systematically,and the radiolarian biostratigraphic framework was further improved.According to the species distribution characteristics and detailed biostratigraphic comparison,five radiolarian assemblages have been recognized as follows: Zhamoidellum ovum-Pseudoxitus gifuensi assemblage(late Kimmeridgian to early Tithonian),Archaeodictyomitra minoensis-Archaeodictyomitra mitra assemblage(early to early late Tithonian),Syringocapsa amphorella-Archaeodictyomitra lacrimula assemblage(early to early late Berriasian),Dicerosaturnalis dicranacanthos-Thanarla pulchra assemblage(late Berriasian to late Vanlanginian)and Hemicryptocapsa capita-Dictyomitra pseudoscalaris assemblage(late Valanginian to earliest Hauterivian).The stratigraphic range of the important taxa in each assemblage were further verified,and then the age of radiolarian cherts is more precisely defined.The discovery of relatively well-preserved radiolarian fossils,extracted from the brecciated cherts,belong to UAZ 11-18 of Mesozoic radiolarian zonation in the Tethyan realm.The protoliths of radiolarian breccia cherts may represent pelagic sedimentation of Late Jurassic to Early Cretaceous.The temporal and spatial distribution characteristics of radiolarians have certain differences in both latitude and water depth.Unlike most previous studies dealing with the palaeogeography of radiolarians,this study not only explored the possible factors affecting the distribution of Mesozoic radiolarians within Tethys,but also interprets the paleo-water depth reflected by the key species and the nassellarians versus spumellarians ratios.