Metamorphic record of accretionary processes during the Neoarchaean: The Nuuk region, southern West Greenland

The Nuuk region of southern West Greenland consists of several distinct terranes, including, from NW to SE, the Færingehavn, Tre Brødre, and Tasiusarsuaq terranes. Extensive high-pressure metamorphism and a clockwise P-T evolution of the Færingehavn terrane at ca. 2720-2710. Ma has been interpreted to be a result of crustal thickening and thrusting of the Tasiusarsuaq terrane on top of the Tre Brødre and Færingehavn terranes. Pseudosection modelling constrains the P-T path for the Færingehavn terrane to be characterised by initial burial, followed by heating at depth to peak conditions of ~700. °C and 10. kbar and subsequent isothermal decompression to conditions of 700. °C and 6. kbar. These data are consistent with the results of previous studies, pointing to a relatively cool apparent geothermal gradient of <20. °C/km during prograde metamorphism. The tectonically overlying Tasiusarsuaq and Tre Brødre terranes record a contrasting metamorphic history. Prior to final collision the Tasiusarsuaq terrane experienced granulite facies metamorphism along a distinctly hotter apparent geothermal gradient of ~35. °C/km, followed by prolonged isobaric cooling during NW-vergent thrusting to conditions of ~700. °C and 6.5-7. kbar. These retrograde conditions are similar to the peak conditions of 620-660. °C and 6. kbar in the Tre Brødre terrane, which have been dated at 2751. ±. 4. Ma. The contemporaneous existence of different thermal regimes and contrasting P-T paths, coupled to the strong structural evidence for regional-scale tectonic thickening indicate that these terranes of the Nuuk region are a Neoarchaean paired metamorphic belt. Here we propose a new tectonic model for the Nuuk region that involves the southwards subduction of the Færingehavn terrane underneath the Tre Brødre and Tasiusarsuaq terranes. In our model, the Tre Brødre terrane is not regarded as a separate tectonic entity, but rather as the leading edge of the upper plate, prior to, and during terrane amalgamation in the Neoarchaean. The prolonged period of convergence recorded in the Nuuk region does not seem to have resulted in deep subduction of crustal rocks, perhaps reflecting that Neoarchaean convergence rates were much slower than today or that subduction was intermittent and inefficient.