Figure 5. ²⁰⁷Pb/²⁰⁶Pb ion microprobe ages of zircons from tonalitic gneiss intruding amphibolite + banded iron-formation on Akilia island, West Greenland. What we interpret as the magmatic zircon population in these rocks (top right) has highest Th/U values and tightest spectrum of ages at 3850 ± 10 Ma (data in green from Nutman et al., 1997; data in red from Mojzsis and Harrison, 1999). All other ages are for lower Th/U metamorphic zircons (arrow) that correlate with well-known metamorphic events in West Greenland at 3650 Ma and 2700 Ma (Baadsgaard et al., 1976; Griffin et al., 1980).