Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to !3Á84 Ga, is reinvestigated using whole-rock major and trace-element geochemistry, and detailed cathodoluminescence image-guided secondary ion mass spectrometer analyses of zircon U-Th-Pb and rare earth elements.Complex zircon internal structure reveals three episodes of zircon growth and/or recrystallization dated to c. 3Á84 Ga, 3Á62 Ga and 2Á71 Ga.Rare earth element abundances imply a significant role for garnet in zircon generation at 3Á62 Ga and 2Á71 Ga.The 3Á62 Ga event is interpreted as partial melting of a c. 3Á84 Ga grey gneiss precursor at granulite facies with residual garnet.Migration of this 3Á62 Ga magma (or melt-crystal mush) away from the melt source places a maximum age limit on any intrusive relationship.These early Archaean relationships have been complicated further by isotopic reworking in the 2Á71 Ga event, which could have included a further episode of partial melting.This study highlights a general problem associated with dating thin gneissic veins in polyphase metamorphic terranes, where field relationships may be ambiguous and zircon inheritance can be expected.