Over the past 4.5 billion years (Ga) the Earth has undergone a secular change in tectonic regimes. Archaean tectonics was a markedly different process than observed today, involving a hotter planet with a thinner lithosphere, thicker oceanic crust, and smaller proto-continents. The transition to a more modern type of plate tectonics required the initiation of subduction of oceanic lithosphere. The onset of subduction in the Meso- to Neoarchaean (ca. 3.2 to 2.5Ga) is shown by geochemical signatures indicative of crustal recycling and petrologic evidence of tectonically driven thickening of the continental crust. This transition is recorded in the rock record of the North Atlantic Craton, which preserves a history of crust formation through to terrane accretion and the collision of crustal blocks. Here we present petrologic, chronologic, chemical and geophysical evidence from the Majorqaq Belt, a ~100km wide east-west striking zone, defined by a high gravity anomaly, voluminous metasedimentary units, olivine-rich ultramafic enclaves, and anomalously high K 2O, Ni, and MgO stream-sediment chemistry. We use a 207Pb/ 206Pb zircon LA-ICP-MS age of 2.557±0.005Ga (2σ) from an upper-amphibolite facies metapelitic migmatite to propose Neoarchaean suturing of the Maniitsoq crustal block to the North Atlantic Craton. Ti-in-zircon temperatures and zircon-garnet REE distribution coefficients constrain zircon crystallisation to 815-729°C, which is at, or near the solidus during leucosome crystallisation. Our data reveal a clockwise P-T path with a high-pressure assemblage of garnet-kyanite-plagioclase-rutile-biotite-quartz which grew at an apparent thermal gradient of ~660°C/GPa during subduction-driven compression of surface-derived sediments to a minimum-peak-pressure of ~1.15GPa at ~760°C followed by the growth of gedrite-cordierite-sillimanite-ilmenite at minimum-peak-temperature conditions of ~810°C at ~0.94GPa. We propose the evolution of the Majorqaq belt formed from the collision of the Maniitsoq block with the proto-North Atlantic Craton following closure of an oceanic basin with south-dipping subduction, providing a potential source of volatiles for the ca. 2.55 Ga QÔrqut granite complex situated ~ 150 km further southwards. These conditions necessitate orogenesis and indicate that plate tectonics was active by ca. 2.56Ga during the final assembly of the North Atlantic Craton.
- Programområde 4: Mineralske råstoffer