Pre-Nagssugtoqidian crustal evolution in West Greenland: geology, geochemistry and deformation of supracrustal and granitic rocks north-east of Kangaatsiaq

Abstract

The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kangaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are comprised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite paragneisses. Their association and geochemical compositions are consistent with a metamorphosed volcano-sedimentary basin (containing both tholeiitic and calc-alkali lavas) and is similar to other Archaean greenstone belts. The Kangaatsiaq granite forms a 15 × 3 km flat, subconcordant body of deformed, pink, porphyritic granite occupying the core of the supracrustal synform, and is demonstrably intrusive into the amphibolites. The granite displays a pronounced linear fabric (L or L > S). The post-granite deformation developed under lower amphibolite facies conditions (400 ± 50°C), and is characterised by a regular, NE–SW-trending subhorizontal lineation and an associated irregular foliation, whose poles define a great circle; together they are indicative of highly constrictional strain. The existence of a pre-granite event is attested by early isoclinal folds and a foliation within the amphibolites that is not present in the granite, and by the fact that the granite cuts earlier structures in the supracrustal rocks. This early event, preserved only in quartz-free lithologies, resulted in high-temperature fabrics being developed under upper amphibolite to granulite facies conditions.

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

“…The REE concentrations are slightly higher than in the regional ortho gneiss. The REE patterns in chondrite-normalised diagrams are similar to those of the regional orthogneiss except for variable negative Eu anomalies which point to retention of plagioclase in the granite source (Moyen & Watt 2006).…”

“…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).…”

“…Chemical analyses of orthogneiss obtained by Moyen & Watt (2006) and from sample 467558 (Table 1) indicate that the compositions are predominantly tonalitic and locally granodioritic. The analysed samples have strongly fractionated rare-earth element (REE) patterns relative to chondrite, with strong enrichment in light REE and chondrite-normalised La/Lu ratios commonly >100.…”

“…Field observations of gradational transitions between migmatitic veins in tonalitic orthogneiss and massive granite suggest that the granite has been derived from adjacent tonalitic-granodioritic ortho gneiss (Fig 9). The three largest occurrences are the Kangaatsiaq granite east of Kangaatsiaq (Moyen & Watt 2006) and two bodies south of Nivaap Sullua c. 20 km west of Ikamiut and south-east of Alanngorsuup Imaa (Fig. 2).…”

“…The Kangaatsiaq granite is pink, coarse-grained and K-feldspar-porphyritic, and displays a strong rodding lineation and schistosity (L to L>S tectonic fabric). It was emplaced into a series of basic to intermediate metavolcanic and pelitic rocks and subsequently folded into a complex synform a few kilometres wide (Moyen & Watt 2006, fig. 2).…”