Evolution of Neoarchaean supracrustal belts at the northern margin of the North Atlantic Craton, West Greenland

Abstract

The Archaean North Atlantic Craton of West Greenland collided at c. 1.9 Ga with a lesser-known Archaean craton to the north, to form the Nagssugtoqidian orogen. The Palaeoproterozoic metamorphic grade and strain intensity decrease northward through the orogen, allowing investigation of the reworked Archaean components in its northern part. Two Archaean supracrustal belts in this region – the Ikamiut and Kangilinaaq belts – are investigated here using field mapping, aeromagnetic data, zircon geochronology, and geochemistry. Both belts comprise quartzo-feldspathic and pelitic metasedimentary rocks, amphibolite, and minor calc-silicate rocks, anorthosite and ultramafic rocks. PbPb and U-Pb dating of detrital zircons and host orthogneisses suggest deposition at c. 2800 Ma (Kangilinaaq belt) and after 2740 Ma (Ikamiut belt); both belts have zircons with Neoarchaean metamorphic rims. Metasedimentary rocks and orthogneisses at Ikamiut share similar steep REE signatures with strong LREE enrichment, consistent with local derivation of the sediment and deposition directly onto or proximal to the regional orthogneiss precursors. Zircon age data from Kangilinaaq indicate both local and distal sources for the sediment there. Geochemical data for Kangilinaaq amphibolites indicate bimodal, mixed felsic–mafic source rocks with island-arc basaltic affinities, consistent with a shelf or arc setting. Both belts experienced a similar tectono-metamorphic history involving Neoarchaean amphibolite facies peak metamorphism at c. 2740–2700 Ma, possibly due to continued emplacement of tonalitic and granodioritic magmas. Nagssugtoqidian lower amphibolite facies metamorphism at c. 1850 Ma was associated with development of the large-scale F2 folds and shear zones that control the present outcrop pattern. The observed differences in the sources of the Kangilinaaq and Ikamiut belts and their shared post-Archaean history suggest they were formed in different Neoarchaean environments proximal to and on a continental plate, and were amalgamated in a convergent margin setting shortly after their deposition.

Descriptive text to the Geological map of Greenland, 1:100 000, Kangaatisiaq 68 V.1 Syd and Ikamiut 68 V.1 Nord

“…As mentioned above the Kangaatsiaq-Ikamiut map area is underlain by the Aasiaat domain, most of which consists of Neoarchaean orthogneiss with interspersed supracrustal belts of generally undated Archaean amphibolite, mica schist (± sillimanite, garnet), and garnetbearing quartzo-feldspathic paragneiss (Hollis et al 2006;Moyen & Watt 2006;Garde et al 2007a;St-Onge et al 2009;Garde & Hollis 2010). Palaeoproterozoic metavolcanic and metasedimentary rocks are restricted to the Naternaq supracrustal belt in the eastern part of the Kangaatsiaq map area, the islands of Isuamiut-Qaqqarsuatsiaq and Equutiit Killiat about 10 km northeast of Aasiaat, and presumably also the island group of Hunde Ejlande (Garde & Hollis 2010).…”

“…Medium-grained, biotite-and locally hornblende-bearing, tonalitic to granodioritic orthogneiss is by far the most voluminous rock association in the map area. Such rocks from Ikamiut have been described in some detail by Hollis et al (2006), who also published age data (see below). The metamorphic grade is upper amphibolite facies throughout the central and northern Aasiaat domain; the biotite in the orthogneiss mostly occurs as evenly distributed flakes 2-3 mm long, suggesting that the rocks have not been through granulite facies metamorphism and subsequent retrogression.…”

“…around the narrow Amitsoq fjord, especially where the latter rocks are not garnetiferous. Hollis et al (2006) carried out zircon U-Pb ion microprobe and laser ablation ICP-MS age determinations of two samples of amphibolite facies orthogneiss from Ikerasassuaq /Langesund about 20 km east of Aasiaat and obtained ages of 2831 ± 23 Ma and 2741 ± 53 Ma, respectively. The former age was interpreted as the emplacement age of the magmatic precursor.…”

“…2). The granite west of Ikamiut, which is located within the hinge zone of a large antiform, was described by Hollis et al (2006). It is porphyritic and holds a weak tectonic fabric, much weaker than found in the surrounding supracrustal rocks and ortho gneisses.…”

“…The partial melting event in this area has not been dated. However, age determination of metamorphic rims on zircon grains from an adjacent metasedimentary rock collected on the south coast of Nivaap Paa suggests that the area underwent high-grade metamorphism at c. 2740-2700 Ma (Hollis et al 2006). It is therefore considered most likely that the partial melting of the biotite-garnet-sillimanite schist was Archaean and broadly contemporaneous with partial melting in the southern part of the Aasiaat domain (and thus unrelated to Nagssugtoqidian thermal reworking, see later).…”