Journal of Petrology Advance Access originally published online on November 19, 2004
Journal of Petrology 2005 46(2):291-318; doi:10.1093/petrology/egh075
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Journal of Petrology vol. 46 issue 2 © Oxford University Press 2004; all rights reserved
Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland
1 SWEDISH MUSEUM OF NATURAL HISTORY, BOX 50007, SE-104 05 STOCKHOLM, SWEDEN
2 ADVANCED CENTRE FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE), UNIVERSITY OF QUEENSLAND, ST LUCIA, QLD 4072, AUSTRALIA
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.
KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon
* Corresponding author. Telephone: +46 8 519 551 69. Fax: +46 8 519 540 31. E-mail: martin.whitehouse{at}nrm.se
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