Tectonic switches and the exhumation of deep-crustal granulites during Neoarchean terrane accretion in the area around Grædefjord, SW Greenland

Abstract

Terrane accretion in the Nuuk region in the North Atlantic Craton in Greenland shares many characteristics with modern accretionary systems, including compelling evidence for regional-scale crustal thickening as well as the presence of terranes with distinct PT histories. The Nuuk region is therefore interpreted to contain a paired metamorphic belt, formed during the convergence and collision of the Færingehavn terrane with the structurally overlying Tre Brødre and Tasiusarsuaq terranes between ca. 2720 and 2700Ma. The Tasiusarsuaq terrane exposes a deeply eroded section through the upper plate of the orogen, dominated by granulite and amphibolite facies TTG gneiss and mafic supracrustal rocks. In its centre, nappes of deep crustal granulites (800–950°C, 9–12kbar) were exhumed to mid crustal levels at conditions of 660–810°C and 7–8kbar. Nappe emplacement occurred during NW-vergent thrusting and the steepening of fabrics between ca. 2760 and 2720Ma, coeval with the underthrusting of the Færingehavn beneath the Tasiusarsuaq terrane. Pervasive ductile fabrics and the presence of abundant leucosomes and syntectonic pegmatites suggest that mid-crustal nappe emplacement was melt-assisted. The final collisional event between 2720 and 2700Ma was associated with a switch from convergence-related thrusting and non-coaxial shearing to co-axial shortening, leading to the development of the subvertical, linear Grædefjord gneiss belt that records the vertical extrusion of material. The Grædefjord gneiss belt is the most prominent of these late-stage ca. 2700Ma extrusion zones that was previously interpreted to be located at or near a boundary between two tectonic blocks.