Early Cretaceous radiolarians and chert geochemistry from western Yarlung Tsangpo suture zone in Jiangyema section, Purang county, SW Tibet

Abstract

Moderately to well-preserved radiolarian faunas belonging to 51 genera and 81 species have been identified in a continuous bedded chert succession in the Jiangyema section in the west segment of the Yarlung Tsangpo suture zone (YTSZ), Purang county, SW Tibet. Based on the occurrence of some characteristic species, especially the evolutionary appearance of age-diagnostic species two radiolarian zones and two subzones are recognized and respectively named as the Cecrops septemporatus zone, Aurisaturnalis carinatus zone, Aurisaturnalis carinatus carinatus subzone and Aurisaturnalis carinatus perforatus subzone. These can be correlated with coeval Hauterivian to upper Barremian radiolarian biozones in both the western Tethys and Japan. They provide a better biostratigraphical framework and reliable age constraints for parts of the Neotethyan Ocean floor that were subducted beneath supra-subduction zone (SSZ) ophiolites in the western part of the YTSZ.Geochemical data indicate that SiO2 contents of bedded cherts range from 79.84 to 94.65% with an average of 89.39%. Comparison of geochemical characteristics on discriminant graphs suggests that the bedded cherts were deposited in a deep oceanic basin near a continental margin. Our results and integration of available radiolarian studies from bedded cherts along the YTSZ suggest that the Neo-Tethys was a deep ocean between the Indian and Eurasian continents in which pelagic sedimentation was on-going until at least the late Barremian.