Middle-Late Triassic radiolarian fossils from the Zhongba region, Xizang and their geological implications

Abundant radiolarian fossils were discovered in the siliceous slates from the upper part of the Qiongguo Group and Xiukang Group in the Yarlung Zangbo suture zone in southern Xizang during the regional geological survey of the 1:250 000 Huoerba Sheet. These radiolarian fossils are composed of 35 species and 48 genera, including Cenosphaera-Phormocyrtis, Capnuchosphaera-Angulobracchia, and Parahsuum-Citriduma assemblages which are identified to be Middle to Late Triassic in age, and indicate the Late Triassic age for the Xiukang Group and the Middle Triassic age for the Qiongguo Group. The findings in this study may provide important paleontological evidence for the age dating of the ophiolites. The ages of the ophiolites in the study area should be earlier than the Middle Triassic. The Xiukang Group should be formed in the continental slope-abyssal sedimentary environments around the margins of the Tethyan ocean basin in southern Xizang.