Rb-Sr ages of early Archaean supracrustal rocks and Amîtsoq gneisses at Isua

• Published in: Nature (London) 1977-11, Vol.270 (5632), p.43-45
• Main Authors: MOORBATH, S, ALLAART, J. H, BRIDGWATER, D, MCGREGOR, V. R
• Format: Article
• Language: English
• Subjects: Humanities and Social Sciences; letter; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/270043a0

PREVIOUS field descriptions 1–4 and isotopic age determinations 5,6 left unresolved several important points about the relationships between the Isua supracrustal rocks and the surrounding Amîtsoq gneisses. In scattered areas of comparatively low deformation, field relationships show unequivocally that gneissose granitic veins intrude the supracrustal sequence. Geochemical studies 7 show that these veins are closely comparable to early tonalitic components in the polyphase Amîtsoq gneisses surrounding the supracrustal belt. Here we show that the veins are isotopically indistinguishable from the main outcrops of Amîtsoq gneisses. They do not represent a period of granitic activity significantly younger than the Amîtsoq gneisses, such as the ∼ 2,900 Myr-old Nûk gneisses of the Godthaab area. Thus the veins may be classified as Amîtsoq gneisses. Present field and isotopic evidence combined suggest that at least part of the ∼ 3,700 Myr-old Amîtsoq gneisses are younger than the Isua supracrustal belt. We have found no field or isotopic evidence that any part of the Amîtsoq gneisses so far mapped pre-date the deposition of the supracrustals.