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

• Published in: Journal of Petrology
• Main Authors: WHITEHOUSE, MARTIN J., KAMBER, BALZ S.
• Format: Text
• Language: English
• Published: Oxford University Press 2005
• Subjects: ARTICLES; Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/46/2/291; https://doi.org/10.1093/petrology/egh075

A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to ≥3·84 Ga, is re-investigated 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 . 3·84 Ga, 3·62 Ga and 2·71 Ga. Rare earth element abundances imply a significant role for garnet in zircon generation at 3·62 Ga and 2·71 Ga. The 3·62 Ga event is interpreted as partial melting of a c . 3·84 Ga grey gneiss precursor at granulite facies with residual garnet. Migration of this 3·62 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 2·71 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.