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

Geochemical methods to advance knowledge on the early Earth require supplies of well-preserved >3600 Ma rocks and minerals (e.g. zircon). One of the most important resources for these are small domains within high metamorphic grade Palatoarchaean gneisses in the Archaean Craton, southern West Greenland. In the Nuuk region these gneisses occur as two terranes (totalling similar to3000 km(2)) with differences in their pre-3600 Ma 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-3600 Ma in situ partial melt and contains locally well-preserved 3690 and 3810 Ma tonalites, greater than or equal to3810 Ma ultramafic rocks plus the tectonically composite Isua supracrustal belt with some locally well-preserved similar to3800 and 37 10 Ma volcanic and sedimentary materials. To the south is the Faeringehavn terrane (up to granulite facies in the Palaeoarchaean) that is dominated by 3850-3660 Ma migmatites containing, variable amounts of in situ 3660-3600 Ma partial melt. Akilia island in this terrane contains some similar to3850 Ma tonalities. 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 3850 Ma tonalite. There are two less known smaller bodies of Palaeoarchaean rocks north of the Nuuk region. The Qarliit Tasersuai assemblage consists of polyphase, migmatitic gneisses with lenses of mafic and siliceous rocks. Two SHRIMP zircon dates rev.-A 3600-3700 Ma rocks, strongly affected by similar to3600 and 2770 Ma 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 similar to3600 Ma old with strong reworking in 2720-2550 Ma 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 greater than or equal to 3600 Ma 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%) greater than or equal to3600 Ma detrital zircons. Therefore, these sediments are not a significant resource for ancient zircons. The scarcity of greater than or equal to 3600 Ma detritus within these sediments supports a model that the Palaeoarchaean bodies (Faeringehavn, 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 similar to3070 Ma detrital zircons and were first metamorphosed (along with the adjacent Isukasia terrane) at similar to2960 Ma. On the other hand, metasediments in contact with the Faeringehavn terrane are dominated by similar to2831 Ma 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 similar to3500 Ma onwards. (C) 2004 Elsevier B.V. All rights reserved.