B. The periodicity of granites and regional metamorphic rocks in the geologic record What do we then make of the dominant peaks in the Precambrian rock record at about 1400-1850 Ma and about 2550-3200 Ma? Are those peaks due to sampling bias, or is there another explanation? Condie et al. (2009) and Voice et al. (2011) have demonstrated that there are periodic peaks in the formation ages of granites in the Precambrian rock record. They based this on the U-Pb ages of detrital zircons that corresponded to the formation ages of the granites from which they were eroded. Fig. 7 is a plot of the frequency of 100,445 ≤ 5% discordant-filtered pooled U-Pb dated detrital zircon grains versus geologic age (after Voice et al 2011). It can be clearly seen that there are pronounced peaks at about 900-1350 Ma, at about 1650-2000 Ma, and at about 2350-2900 Ma. Nance et al. (2014) correlated these peaks with the conventionallyproposed supercontinent cycle. Within the conventional paradigm this correlation is valid because the formation of granites and regional metamorphic rocks is controlled by the global tectonics that produced the supercontinent cycle, these rocks forming in mountainbuilding collision zones and adjacent to subduction zones, and also in island arcs. While it is evident that the radiohalos frequency peaks in the Precambrian record in Fig. 5 do not exactly match those in Fig. 7, there is still a one-to-one reasonable correlation, given the sparse sampling of Precambrian granites and regional metamorphic rocks in this study. Indeed, the data reproduced in Fig. 7 represent over 100,445 U-Pb ages of detrital zircons (although eroded from an unidentified lesser number of granites and regional metamorphic rocks), whereas this study is based on some 147 granites and regional metamorphic rocks (although some data were gathered from multiple samples per rock unit). It is to be expected that further sampling of infill-aged rock units for radiohalos study would merely flesh-out the same peaks as in Fig.7. At least the comparison is also reasonable, given that it is tiny zircon inclusions in biotite grains in the granites and regional metamorphic rocks that produced the radiohalos due to the α-decay of their contained 238U. So, if these radiohalos frequency and detrital zircon U-Pb ages peaks are real, essentially coincide, and represent actual tectonic activity 0 50 0 100 150 200 250 300 350 400 450 500 550 600 5 10 15 20 25 30 35 40 45 50 55 60 65 Number of Radiohalos per Slide Millions of Years Ago (Ma) Granites Metamorphic rocks Figure 6. Plot of the number of radiohalos per slide for each rock unit in Tables 1 and 2 (y-axis) versus the conventional age in millions of years from 0-600 Ma (enlarged x-axis). The granites are marked with dots and the metamorphic rock units with red crosses. SNELLING Radiohalos through earth history 2023 ICC 554
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