中国科学院南京地质古生物研究所机构知识库

Radiolarian-bearing cherts are widely exposed in the volcanic-sedimentary sequences associated with the Purang ophiolitic melange in the western segment of the Yarlung Tsangpo suture zone. Highly diverse and well preserved Lower Cretaceous (Valanginian-Hauterivian) radiolarian assemblages that correlate to the Cecrops septemporatus Zone occur in bedded cherts in the Peiersanggangmu section of the southern Purang ophiolitic melange. Fifty-five species belonging to 38 genera, including characteristic species: Archaeodictyomitra apiarium (Rust), Cecrops septemporatus (Parona), Hemicryptocapsa capita Tan, Pseudodictyomitra carpatica (Lozyniak), and Svinitzium pseudopuga Dumitrica, have been recognized.Geochemistry of the cherts reveals high SiO2 contents (average: 92.23 wt%), Al/(Al + Fe + Mn) ratios, and low La-N/Ce-N ratios. Post Archean Australian Shale (PAAS) normalized rare earth element (REE) abundance patterns exhibit moderately positive Ce anomalies (mean Ce/Ce* =1.35) with relative enrichment in light rare earth elements (LREEs). These features are indicative of deposition in an oceanic basin close to a continental margin environment. Lithological characteristics and geochemical results suggest that the paleogeographical location for these radiolarian-bearing bedded cherts closely matches northern regions of the Indian continent, where existed enhanced upwelling activity and surface mixing resulting from monsoon activities.