The Lower Boundary of the Vendian in the Southern Urals as Evidenced by the Rb-Sr Age of Glauconites of the Bakeevo Formation

摘要

This work presents the results of the mineralogical, crystallochemical, and geochronological study of globular phyllosilicates (GPS) of glauconite-illite series from the Bakeevo Formation. This formation is the basal member of the Vendian Asha Group in the Southern Urals. The monomineral GPS fractions representing the globules varying in size and density are composed of Al-glauconite and illite. The Rb-Sr dating of the GPS of the Bakeevo Formation in combination with the modeling of cation distribution in their structure and comparison of the model data with Mossbauer and IR spectroscopy data was performed for the first time. The Rb-Sr age of the Bakeevo glauconite is 642 +/- 9 Ma. The mineralogical and geochemical characteristics in combination with model calculations show that the stratigraphic significance of this age is quite reasonable. The initial Sr-87/Sr-86 ratio in the glauconitization environment is 0.7080 +/- 0.0009 and is consistent within the experimental error with this ratio in the Vendian seawater. Thus, the lower boundary of the Vendian in the Southern Urals has an age of about 640 Ma.