2005), Assigning dates to thin gneissic veins in high-grade metamorphic terranes: A cautionary tale from Akilia, southwest

• Main Authors: Martin J. Whitehouse, Balz, S. Kamber
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.541.2137; http://petrology.oxfordjournals.org/content/46/2/291.full.pdf

A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to 384 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. 384 Ga, 362 Ga and 271 Ga. Rare earth element abundances imply a significant role for garnet in zircon generation at 362Ga and 271Ga. The 362Ga event is interpreted as partial melting of a c. 384Ga grey gneiss precursor at granulite facies with residual garnet. Migration of this 362 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 iso-topic reworking in the 271Ga 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