NUMERICAL AGES OF VOLCANIC-ROCKS AND THE EARLIEST FAUNAL ZONE WITHIN THE LATE PRECAMBRIAN OF EAST POLAND

U-Pb age determinations by ion microprobe on zircons from two tuff samples within the Neoproterozoic-early Cambrian successions in Poland are presented. One sample, from the Kaplonosy borehole is within, or conformably below, rocks that contain the Sabellidites-Vendotaenia fossil assemblage of the Upper Vendian. The second sample is from the Ksiaz Wielki IG-1 borehole, from rocks that were referred alternatively to the early Cambrian or to the Vendian on lithostratigraphic evidence. The Kaplonosy zircons are euhedral and free of visible zircon cores, both optically and as back-scattered electron images, but they exhibit a range in Pb-206/U-238 ages that exceeds analytical error. The combined data-set can be resolved into three age-components in different proportions, which overlap in apparent age due to measurement errors. There is a well-defined principal age component at 551 +/- 4 Ma (95% limits) and two minor detrital or inherited components at 588 +/- 8 Ma and 635 +/- 10 Ma. The age of the Kaplonosy tuff is interpreted as equal to that of the youngest and principal component, 551 Ma. This age allows a maximum time difference of 17 +/- 4Ma between the top of the Slawatycze Formation and Lower Cambrian strata of the Heliosphaeridium dissimilare-Skiagia ciliosa acritarch Zone, the latter being time-equivalent to Middle Tommotian strata in northeastern Siberia, recently dated as 534.6 +/- 0.5 Ma. The tuff from the Ksiaz Wielki IG-1 borehole has a similar spectrum of zircon ages, but also contains several detrital grains that have concordant and separate early Proterozoic and Archaean ages. The age of the major component is 549 +/- 3 Ma (72% of the total), and there are two older components at 578 +/- 9 Ma (13%) and 619 +/- 8 Ma (15%). The deposition of the tuff from the Ksiaz Wielki Formation therefore occurred at 549 Ma, indicating that the Ksiaz Wielki Formation is not early Cambrian but Upper Vendian, unless all zircons in the tuff are detrital.