Establishment of a 3.83-Ga magmatic age for the Akilia tonalite (southern West Greenland)

A recurring challenge to ion microprobe U-Pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains. This has proved particularly problematic in studying the polyphase metamorphic rocks that dominate Early Archean gneissic terranes. For example, differing interpretations exist for the origin of complex zircon populations investigated by U-Pb ion microprobe zircon geochronology from a granitoid (orthogneiss) body, previously used to establish a minimum age for a supracrustal enclave on Akilia (island) in southern West Greenland. We describe a method whereby the geochemistry of the Akilia orthogneiss, coupled with a U-Th-Pb vs. age depth profile in a zircon from the same rock, permits direct assessment of zircon inheritance. Results reveal evidence for three phases of concentric zircon growth at 3.83. Ga, ~3.6 Ga and 2.7-2.5 Ga; zircon growth at both ~3.6 Ga and 2.7-2.5 Ga is consistent with precipitation from a metamorphic fluid. Depth profile U-Th-Pb data demonstrate that only the >3.8-Ga zircon core could have crystallized from the host rock. We conclude that the magmatic age of this rock is 3.83 ± 0.01 Ga, not ~3.65 Ga as has been previously proposed. The U-Th-Pb zircon depth profile technique has wider applications to resolving other geochronological debates in high-grade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution/re-precipitation features.