Neoarchaean tectonothermal evolution of the SE Nuuk region, southern West Greenland

Abstract

The Nuuk region is dominated by complexly deformed Archaean TIG orthogneisses with subordinate supracrustal lithologies that have been variably metamorphosed at upper amphibolite to granulite facies. The region is interpreted as a mid- to lower crustal exposure of a Meso- to Neoarchaean terrane complex, collectively affected by high grade deformation and metamorphism between 2720 and 2650Ma. New studies of the previously little-understood Kapisillit area, SE Nuuk region, suggest significant exposures of the recently-proposed Kapisillik terrane, a crustal block characterised by a distinct magmatic and metamorphic history between ca. 3100 Ma and 2650 Ma. A fuller understanding of the tectonic units within the Kapisillit area, including the extent and internal age-event relations of the Kapisillik terrane, thus has important implications for our understanding of the tectonothermal evolution of the Nuuk region.

Three field areas were selected in west, east and north Kapisillit (Norsanna, Tummeralik and Aputitooq Mountain respectively). Norsanna is situated at the boundary between the ca. 2825Ma Tre Br~dre and 3800-3300 Ma Freringehavn terranes whereas Tummeralik lies entirely within the proposed Kapisillik terrane. Aputitooq Mountain is situated within 2km of the purported boundary zone between the Freringehavn and Kapsillik terranes. The tectonothermal evolutions of the three areas were investigated using field mapping, metamorphic petrology of pelitic assemblages and UlPb zircon geochronology of key orthogneisses and structurally constrained leucosomes. P-T -t paths for each of these areas were constructed and considered in a regional context through integration with complementary geochronological, structural and metamorphic information obtained in parallel studies conducted both in the Kapisillit area and more broadly in the Nuuk region as a whole.

Metamorphic, structural and geochronological data suggest that N orsanna and Tummeralik share a polydeformational history during the Neoarchaean, between ::52817Ma and 2659Ma. Early isoclinal folding (D}) under kyanite zone (Ml) conditions gave way to peak gamet-sillimanite-biotite-plagioclase-quartz assemblages (M2). Subsequent fold limb attenuation and decompression (D2) at high temperatures (M3) occurred by ca. 2720Ma, constrained by high grade, syn-D2 leucosome. Pelites from Aputitooq Mountain follow a similar P-T path, but dating by other workers has yielded ca. 2650Ma ages from upper amphibolite facies leucosome and metamorphic rims on detrital zircon. Although an isotopic disturbance at 2650 Ma is recorded in a leucosome at Tummeralik, this is interpreted as a partial reset age instigated by a Proterozoic greenschist facies overprint, rather than a high grade metamorphic event.

The study suggests that Norsanna and Tummeralik (the Tre Br~dreFreringehavn and Kapsillik terranes respectively) were juxtaposed by ca. 2720Ma and that the high-grade, ca. 2650Ma event in the Kapsillik terrane becomes less significant moving south towards and across Ameralik Fjord. The ca. 2720 Ma tectonothermal event is recorded in most terranes/crustal units of the Nuuk region and therefore is still considered to represent final amalgamation. Subsequent metamorphism and deformation at ca. 2650 Ma is attributed to post-amalgamation reactivation, shearing and refolding focused along the previously established boundaries between juxtaposed blocks.