Tuff ages are derived from the weighted mean206Pb/238U dates of the analyzed
zircons after excluding visibly older analyses interpreted as
xenocrystic or detrital (Fig. 4). No young zircon analysis has been
excluded. Calculated age uncertainties are reported in the ±X /Y /Z Ma format, where X is the internal
95% confidence interval uncertainty in the absence of all external
errors, Y incorporates the tracer calibration errors and Zincludes Y as well as the U decay constant uncertainties of
Jaffey et al. (1971). Age results are summarized in Figure 4. Yneeds to be taken into account when U-Pb ID-TIMS dates are compared to
those produced by in situ techniques such as SIMS. Similarly, the
accuracy of the SIMS ages, derived from the reproducibility of mineral
standard measurements, need to be considered in such comparisons (see
Discussion).
4 Results
4.1 U-Pb Geochronology
Five single zircon grains were analysed from each sample. Excluding a
slightly older analysis suspected of being detrital or xenocrystic in
each case the remaining four analyses produced statistically coherent
clusters of 206Pb/238U dates from
which weighted mean dates can be calculated (Fig. 4). These are 163.541
± 0.052/0.090/0.20 Ma (MSWD = 0.66) for the crystal lithic lapilli tuff
24-1-18-1 and 163.555 ± 0.071/0.10/0.20 Ma (MSWD = 1.5) for the
ignimbritic lapilli tuff 22-1-18-5. The two dates are statistically
indistinguishable within uncertainty and are Middle Jurassic (Callovian)
based on the latest calibration of the Jurassic time scale (Hesselbo et
l., 2020), which places the Middle-Late Jurassic boundary at 161.5 ± 1.0
Ma.