西藏仲巴中-晚三叠世放射虫化石特征及其地质意义

摘要

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.%在1∶25万霍尔巴幅区域地质调查中,在雅鲁藏布江结合带穷果群上部及修康群中发现了较多的硅质板岩,其内含有较丰富的放射虫化石,共有48属35种,可识别为Cenosphaera-Phormocyrtis, Capnuchosphaera-Angulobracchia和Parahsuum-Citriduma 3个组合,时代为中—晚三叠世。厘定修康群的形成时代为晚三叠世,穷果群的形成时代为中三叠世。这一发现为确定蛇绿岩的时代提供了重要的古生物依据,表明该区蛇绿岩的形成时代应早于中三叠世。结合区域地质特征,修康群应形成于大陆斜坡-深海沉积环境,位于南带特提斯洋盆区的边部地带。