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.