≥ 3850 Ma BIF and mafic inclusions in the early Archaean Itsaq Gneiss Complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites

• Published in: Precambrian Research
• Main Authors: Nutman, Allen, McGregor, Victor, Shiraishi, Kazuyuki, Friend, Clark, Bennett, Victoria, Kinny, Peter D
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier
• Subjects: Keywords: iron; zirconium; Archean; dating method; geochronology; gneiss; migmatite; protolith; accuracy; article; chronology; evolution; granite; Greenland; injection; measurement; rock; sediment; Greenland Akilia; Migmatites; Palaeoarchaean; U/Pb zircon dating; Qilanngaarsuit
• Online Access: http://hdl.handle.net/1885/72026; https://doi.org/10.1016/S0301-9268(02)00045-1; https://openresearch-repository.anu.edu.au/bitstream/1885/72026/5/MigratedxPub3012_RSD_2002.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/72026/7/01_Nutman_%E2%89%A5_3850_Ma_BIF_and_mafic_2002.pdf.jpg

The southern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on Akilia and adjacent islands consists of polyphase dioritic-tonalitic-granitic injection components with inclusions of metavolcanic amphibolites, chemical sediments such as banded iron formation (BIF), gabbros and ultramafic rocks. Incipient in situ partial melting and strong deformation during several Archaean tectonothermal events strongly modified these injection components, so that they are now mostly banded, schlieric migmatites with neosome produced during several events. The margins of many inclusions have been loci for either segregation of neosome and/or higher strainobliterating the relationship between the inclusions and the older components of the migmatites. An added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon, which would provide precise, direct dates. Instead, minimum ages of inclusions are obtained by dating invasive components of the granitic (sensu lato) migmatites. Previous age determinations of the inclusions were centred on a ∼ 200 m long body of amphibolites, BIF and ultramafic rocks on the southwestern corner of Akilia. Geochronology of this locality has been controversial, with our proposed age of ≥ 3850 Ma making the inclusion the world's oldest-dated sedimentary and mafic rocks. We continue the debate on the age of the inclusion on southwest of Akilia, and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of Akilia. This (1) addresses the contentious ages of these rocks, (2) examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies, and (3) addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths. On Qilanngaarsuit (island), a gneiss sheet cutting a metaperidotite inclusion has an age of ∼ 3850 Ma. On an islet ∼ 2 km west of Akilia, tonalite and ...