Abstract

The Færingehavn–Tre Brødre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Færingehavn terrane, largely composed of early-Archaean Amîtsoq gneiss cut by younger Archaean granitic gneiss defined here as the Sätut gneiss; (2) Tre Brødre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nûk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nûk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Færingehavn and Tre Brødre terranes which at c. 2800 Ma experienced a lower grade of metamorphism.

Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages.

This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amîtsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nûk gneiss precursors described from Godthåbsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthåbsfjord region.

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