Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland |
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Authors: | WHITEHOUSE, MARTIN J. KAMBER, BALZ S. |
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Affiliation: | 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 |
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Abstract: | A granodiorite from Akilia, southwest Greenland, previouslysuggested to date putative life-bearing rocks to 3·84Ga, is re-investigated using whole-rock major and trace-elementgeochemistry, and detailed cathodoluminescence image-guidedsecondary ion mass spectrometer analyses of zircon UThPband rare earth elements. Complex zircon internal structure revealsthree episodes of zircon growth and/or recrystallization datedto c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rareearth element abundances imply a significant role for garnetin zircon generation at 3·62 Ga and 2·71 Ga. The3·62 Ga event is interpreted as partial melting of ac. 3·84 Ga grey gneiss precursor at granulite facieswith residual garnet. Migration of this 3·62 Ga magma(or meltcrystal mush) away from the melt source placesa maximum age limit on any intrusive relationship. These earlyArchaean relationships have been complicated further by isotopicreworking in the 2·71 Ga event, which could have includeda further episode of partial melting. This study highlightsa general problem associated with dating thin gneissic veinsin polyphase metamorphic terranes, where field relationshipsmay be ambiguous and zircon inheritance can be expected. KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon |
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Keywords: | : Archaean geochronology Greenland secondary ion mass spectrometry zircon |
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