Timing of late Archaean terrane assembly, crustal thickening and granite emplacement in the Nuuk region, southern West Greenland

Abstract

A breakthrough in understanding the structural and metamorphic evolution of Archaean gneiss complexes occurred with the recognition that the Nuuk region of southern West Greenland comprised separate terranes assembled in the late Archaean. From northwest to southeast these are: the Akia (3220-2970 Ma), Akulleq (3870-3600 and 2820 Ma) and Tasiusarsuaq (2920-2860 Ma) terranes. The minimum time of assembly is recorded by the first event common to all component terranes. Using SHRIMP UPb zircon geochronology the oldest events common to all terranes (including emplacement of crustally derived granites, contemporaneous metamorphism and anatexis) have been dated at 2710–2725 Ma. In the Akulleq terrane areas where in situ diatexite formed, abundant granitoid sheets were intruded and common growth of metamorphic zircon (mostly low Th U ) in most lithologies occurred. In the Akia and Tasiusarsuaq terranes there was only intrusion of a lesser number of ∼ 2720 Ma granitoid sheets, because metamorphic zircons of that age have not been found. The ∼ 2720 Ma event is interpreted as marking, or shortly following, terrane assembly, when the diverse components of the Akulleq terrane were tectonically juxtaposed with the other two. This new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of Archaean gneisses can be due to accretionary tectonics long after the individual components were first formed.