TIMING OF LATE ARCHEAN TERRANE ASSEMBLY, CRUSTAL THICKENING AND GRANITE EMPLACEMENT IN THE NUUK REGION, SOUTHERN WEST GREENLAND

A breakthrough in understanding the structural and metamorphic evoluti on of Archaean gneiss complexes occurred with the recognition that the Nuuk region of southern West Greenland comprised separate terranes as sembled in the late Archaean. From northwest to southeast these are: t he Akia (3220-2970 Ma), Akulleq (3870-3600 and 2820 Ma) and Tasiusarsu aq (2920-2860 Ma) terranes. The minimum time of assembly is recorded b y the first event common to ail component terranes. Using SHRIMP U-Pb zircon geochronology the oldest events common to all terranes (includi ng emplacement of crustally derived granites, contemporaneous metamorp hism and anatexis) have been dated at 2710-2725 Ma. In the Akulleq ter rane areas where in situ diatexite formed, abundant granitoid sheets w ere intruded and common growth of metamorphic zircon (mostly low Th/U) in most lithologies occurred. In the Akia and Tasiusarsuaq terranes t here was only intrusion of a lesser number of similar to 2720 Ma grani toid sheets, because metamorphic zircons of that-age have not been fou nd. The similar to 2720 Ma event is interpreted as marking, or shortly following, terrane assembly, when the diverse components of the Akull eq terrane were tectonically juxtaposed with the other two. This new d ocumentation of metamorphic and associated igneous events within an ac creting cratonic region is an illustration that the stabilisation of e xtensive areas of Archaean gneisses can be due to accretionary tectoni cs long after the individual components were first formed.