Middle Jurassic radiolarians and chert geochemistry, Dajiweng ophiolite, SW Tibet: Implications for Neotethyan Ocean evolution

Abstract

Well-preserved Middle Jurassic (upper Bajocian to lower Bathonian) radiolarian assemblages from bedded cherts constrain the age of cherts associated with the Dajiweng ophiolite in the northern sub-belt of the western section of the Yarlung Tsangpo suture zone (YTSZ). Sixty-one species belonging to 39 genera are recognized and illustrated. They represent the most diverse Middle Jurassic radiolarian fauna yet documented in this ophiolite and support correlation with UA zones 4–5 of Baumgartner et al. (1995). They are thus, the oldest known radiolarian-bearing strata in the northern sub-belt of the western YTSZ. The fauna exhibits a close degree of similarity to the other coeval assemblages previously reported from middle and eastern segments of the YTSZ, reveal widespread development of Middle Jurassic or even earlier oceanic crust within the Neotethyan Ocean. The geochemistry of these bedded cherts, in particular ratios of Al/(Al + Fe + Mn), Ce/Ce*, and LaN/YbN indicate they accumulated in a transitional environment between a continental margin and an oceanic basin.