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 ≥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 an...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
2005
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/46/2/291 https://doi.org/10.1093/petrology/egh075 |
| Summary: | 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. |
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