Abstract: Abstract：In the Daba-Xiugugabu ophiolite belt of southwestern Tibet, there exists widely distributed Mesozoic chert associated with ophiolite. Geochemical parameters of major elements and rare earth elements and geochemical setting discrimination diagram of the chert associated with La' nga Co ophiolite mass suggest that the sedimentary environment of the chert was a continental margin basin, with its genesis attributed to biogenic origin and hydrothermalism. Geological section of the chert-bearing strata is composed in upward succession of quartz sandstone, debris-bearing quartz sandstone and graywacke, and chert, with the sedimentary environment being the transition from delta or shallow sea through continental margin to marginal basin. There exists abundant Radiolaria in the chert, and the radiolarian association suggests that the chert was formed in the last period of Late Jurassic. The chert-bearing strata and the ophiolite are the component parts of the small oceanic basin in northern Indian continental margin, and the oceanic basin was closed by the collision of plates which caused the collage of the strata and the ophiolite. The sedimentary environment of the chert reflects the tectonic setting of the small oceanic basin in Late Jurassic.