Quantitative analysis of Early Eocene radiolarian assemblages discovered in the sedimentary mélange (accretionary prism) of the Saga area, southern Tibet provides new information to constrain the timing of Tethys closure and the initial collision of India and Eurasia. The radiolarian species of Saga includeAmphisphaera coronata (Ehrenberg), Buryella hannae Bak & Barwicz-Piskorz, Buryella clinata Foreman, Buryella tetradica Foreman, Calocycloma ampulla (Ehrenberg), Lamptonium fabaeforme constrictum Riedel and Sanfilippo, Lamptonium pennatum Foreman, Lithomespilus coronatus Squinabol and Lamptonium (?) colymbus Foreman. The adequate and reliable correlation of these radiolarians specimens indicates that the assemblage is of Early Eocene in age. The age and depositional environment of these radiolarians testify that deep ocean basins existed between India Plate and Asia Plate during the Early Eocene. The complete closure of Tethys must have taken place at least after the Early Eocene.Tibet, radiolarian, deep sea deposit, Early Eocene, TethysThe age of the Tethys closure and India-Asia Plates collision are the keys for understanding the evolution of the Himalayan orogen and the formation of the Tibetan Plateau. However, the evolution history of the Tethys and time of collision between India and Asia Plates are still controversial. Existing estimates range from 70-65 Ma [1][2][3][4] , 55-50 Ma [5][6][7][8] to 45-30 Ma [9][10][11] . These controversies are largely due to two fundamental reasons: (1) the closure processes of the Tethys and India-Asia Plates collision may have not been synchronous both spatially and temporally, that is, different regions may have occupied different tectonic positions in the same stage; (2) the ages on the closing and collision were mostly determined radiometrically, with little or no evidence from paleontology and stratigraphy. These dates thus lead to distinct views on the evolution of the Tethys. As the final disappearance of Tethys would coincide with the absolute retreat of seawater from Tibet, the transition from the latest marine to terrestrial facies would be one of the most direct proofs. Therefore, to determine the ages of Tethys closure and India-Asia Plates collision is to actually date the latest marine sediment in the region. In the past decade, much attention has been paid to studying the latest marine strata in the Yarlung Zangbo region. Somewhat curiously, the results are diverse from some other conclusions [1,2,[12][13][14][15] .During our recent geological investigation in Sage area, southern Tibet, we discovered Early Eocene radiolarian assemblages in the sedimentary mélange of the
