LAUSR

Abstract Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian ‘super-event’ model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Faeringehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and >250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmatism at convergent plate boundaries. Each terrane is thus dominated by juvenile additions to the crust. Intra-terrane sedimentary rocks show near unimodal age distributions in contrast to those near the boundaries which are more diverse and complex. The combined geochronological, metamorphic and structural evidence of convergence of these terranes leading to collisional orogeny, this indicates that plate tectonic processes operated throughout the Archaean.