SUBSTANTIATION OF THE VENDIAN-LOWER CAMBRIAN GEOCHRONOLOGICAL SCALE BY U-PB ZIRCON AGES

Abstract

Analysis of recently reported numerous U-Pb dates from zircons from Vendian-Lower Cambrian volcanogenic rocks demonstrated that they depend on various factors, especially on postdepositional loss of radiogenic lead. Differences also arise from using various methods (SHRIMP or IDTIMS) and analytical procedures. This results in contradictory arrays of dates for post-Tommotian Lower Cambrian, Tommotian, Nemakit-Daldynian, and, particularly, Ediacaran and Varangerian deposits. The right choice demands refinement of boundaries of main stratigraphic units in particular sections and discard of dubious or apparently erroneous dates on the basis of a critical review of paleontological, historical, and geological data. As a result, a consecutive series of dates from the roof of the Lower Cambrian to the bottom of the Vendian was compiled. These dates are much closer to those formerly adopted by a set of other methods (mainly K-Ar and Rb-Sr dating by glauconite) for determining the main boundaries in the Vendian-Lower Cambrian.