Testing the model of late Archean terrane accretion in southern West Greenland: a comparison of the timing of geological events across the Qarliit nunaat fault, Buksefjorden region

Abstract

The prevailing tectonic model for the Archean gneiss complex of southern West Greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between ∼2.80 and 2.72 Ga. This model was tested by using detailed mapping and U–Pb geochronology to compare the timing of geological events across the Qarliit nunaat fault, the boundary between terranes in the Buksefjorden region. The fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite (or retrograde amphibolite) facies gneisses of the Tasiusarsuaq terrane (TT) from prograde amphibolite facies gneisses of the Akulleq terrane (AT), and it separates tonalitic gneisses of different age (3.87–3.60 and 2.80 Ga in the AT versus 2.92–2.86 Ga in the TT). Dating of similar homogeneous gneisses in each terrane yielded ages of 2843±3 Ma in the TT (tonalite) and 2824±2 Ma in the AT (granodiorite, Ikkattoq gneiss). Granulite facies metamorphism in the TT is thought to have coincided with intrusion of a 2805±2 Ma orthopyroxene pegmatite, and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779±2 and 2660±10 Ma, respectively. Metamorphic minerals in the AT are younger: zircon and titanite from amphibolitic gneiss are 2717±3 and 2510±10 Ma, respectively, and monazite from pelitic schist is 2732±5 and 2708±4 Ma. A concordant pegmatite layer that underwent a substantial amount of shearing along the Qarliit nunaat fault was dated at 2725±2. Other pegmatites that intruded relatively late in the structural history crystallized at 2710±2 in the TT and 2541±2 Ma in the AT. These results support the prevailing tectonic model by showing that (i) similar homogeneous gneisses in each terrane are different age; (ii) ∼2805 Ma granulite facies metamorphism in the TT predated 2740–2700 Ma amphibolite facies metamorphism in the AT; and (iii) shearing along the terrane boundary occurred after ∼2725 Ma.