Inventory and assessment of Palaeoarchaean gneiss terrains and detrital zircons in southern West Greenland

• Published in: Precambrian Research
• Main Authors: Nutman, AP, Friend, CRL, Barker, SLL, McGregor, VR
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Science Bv 2004
• Subjects: Earth Sciences; Geology; Geochronology; Greenland; Nuuk; Aasivik; Qarliit
• Online Access: https://doi.org/10.1016/j.precamres.2004.09.002; http://ecite.utas.edu.au/129468

Geochemical methods to advance knowledge on the early Earth require supplies of well-preserved >3600Ma rocks and minerals (e.g. zircon). One of the most important resources for these are small domains within high metamorphic grade Palaeoarchaean gneisses in the Archaean Craton, southern West Greenland. In the Nuuk region these gneisses occur as two terranes (totalling ∼3000km 2 ) with differences in their pre-3600Ma histories, and they are tectonically separated from each other by younger rocks. In the north is the Isukasia terrane (early metamorphic grade amphibolite facies). It is devoid of pre-3600Ma in situ partial melt and contains locally well-preserved 3690 and 3810Ma tonalites, ≥3810Ma ultramafic rocks plus the tectonically composite Isua supracrustal belt with some locally well-preserved ∼3800 and 3710Ma volcanic and sedimentary materials. To the south is the Fringehavn terrane (up to granulite facies in the Palaeoarchaean) that is dominated by 38503660Ma migmatites containing variable amounts of in situ 36603600Ma partial melt. Akilia island in this terrane contains some ∼3850Ma tonalites, mafic and sedimentary rocks the world's oldest-known sediments. Domains of lower total strain and anatexis on some of other islands near Akilia has left small amounts of well-preserved 3850Ma tonalite. There are two less known smaller bodies of Palaeoarchaean rocks north of the Nuuk region. The Qarliit Tasersuat assemblage consists of polyphase, migmatitic gneisses with lenses of mafic and siliceous rocks. Two SHRIMP zircon dates reveal 36003700Ma rocks, strongly affected by ∼3600 and 2770Ma metamorphisms. The Aasivik terrane also consists of polyphase migmatitic gneisses, and previous SHRIMP U/Pb zircon reconnaissance dating of three samples found components up to ∼3600Ma old with strong reworking in 27202550Ma events. The West Greenland Archaean Craton is a collage of Palaeo- to Neoarchaean terranes, assembled in several Archaean events. Metasediments within post-Palaeoarchaean terranes are devoid of ≥3600Ma detritus, but are dominated by zircons of the same age as major crust-forming TTG suites of their terrane. Metasediments along terrane boundaries, even those in contact with Palaeoarchaean terranes, contain very few (<5%) ≥3600Ma detrital zircons. Therefore, these sediments are not a significant resource for ancient zircons. The scarcity of ≥3600Ma detritus within these sediments supports a model that the Palaeoarchaean bodies (Fringehavn, Isukasia, Qarliit tasersuat and Aasivik) are allochthonous terranes captured within an Archaean accretionary system and comprise a tectonic assembly of juvenile crustal blocks with different age. The metasediments in contact with the Isukasia terrane are dominated by ∼3070Ma detrital zircons and were first metamorphosed (along with the adjacent Isukasia terrane) at ∼2960Ma. On the other hand, metasediments in contact with the Fringehavn terrane are dominated by ∼2831Ma detrital zircons, and thus were deposited after sediments in contact with the Isukasia terrane had already been tectonically emplaced and metamorphosed. Although the Palaeoarchaean terranes were incorporated into the Archaean terrane collage at different times, they might have been spawned from a single larger body of ancient crust broken up from ∼3500Ma onwards.