Ladinian radiolarian fauna, siliceous rock from the Xianshuihe Belt, West Sichuan and their tectonic significance

摘要

Ladinian radiolarian fauna, including Muelleritortis, Baumgartneria, Oertlispongus, Paroertlispongus, Pseudoertlispongus, etc., was discovered from the siliceous rock of the Runiange Formation in the Xianshuihe belt, West Sichuan Province. Geochemical test on five samples from the siliceous rock indicates that SiO2 content varies in 71.16%—90.06% and Si/Al ratio, in 49—71, which shows that the siliceous rock contains more terrigenous mud sediments. The siliceous rock is characterized by the large ratios of Al_2O_3/(Al_2O_3+Fe_2O_3) (0.63—0.81) and Ti/V ( >26), the low ratio of V/Y (< 2.8), and low vanadium content (< 23 μg/g), which are similar to the geochemical characteristics of continental margin siliceous rock. The Ce/Ce~* ratios of the four samples vary in 1.02—1.47 and the La_N/Ce_N ratio, in 0.75—1.07, which imply that the siliceous rock was deposited in the continental margin basin. But only one sample is similar to the oceanic siliceous rock in REE. Turbidite-siliceous rock bearing radiolarian-basalt assemblage and the geochemical characteristics of the siliceous rock indicate that the Xianshuihe belt is in the strong rift stage in the Ladinian age.