Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: Crustal growth in the Archean with modern analogues

The ca. 700 km long, Archean craton of West Greenland consists of six Meso–Neoarchean (ca. 3000–2720 Ma) shear zone — bounded crustal blocks that display similar cross-sections; from south to north Ivittuut, Kvanefjord, Bjørnesund, Sermilik, Fiskefjord, Maniitsoq. Each block has a southerly upper and a northerly lower zone, thus each faces upwards to the south. Upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies, whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies. Upper and lower zones consist predominantly of tonalite–trondhjemite–granodiorite (TTG) orthogneisses; geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins. The gneisses contain km-thick metavolcanic amphibolite layers typically bordered by km-thick layers containing anorthosite and leucogabbro. Most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic, andesitic rocks. The two most-prominent metavolcanic belts in the Fiskefjord block at Qussuk (andesitic–volcaniclastic rocks; Garde, A.A., 2007. A mid-Archaean island arc complex in the eastern Akia terrane, Godthåbsfjord, southern West Greenland. Journal of the Geological Society (London) 164, 565–579.) and Ivisaartoq (mafic–ultramafic rocks and anorthosite–leucogabbro from upper and lower parts of a supra-subduction zone system; Polat, A., Frei, R., Appel, P.W.U., Dilek, Y., Fryer, B., Ordóñez-Calderón, J.C., Yang, Z., 2008. The origin and compositions of Mesoarchean oceanic crust: evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland. Lithos 100, 293–321.) have island arc geochemical signatures. The 2 km-thick Fiskenæsset complex (Bjørnesund block) comprises chromite-layered anorthosites, leucogabbros and gabbros, and local pillow-bearing roof pendants from overlying metavolcanic amphibolite. The style of deformation changes downwards within crustal blocks; upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding, and lower zones by kilometre-scale double-triple fold interference patterns. Everywhere TTG protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones, often so extensively that only anorthositic or amphibolitic lenses are preserved. The Meso-Neoarchean crust was thickened by a combination of thrusting, isoclinal folding and continued TTG injection. Dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes. These crustal blocks provide an exceptional example of how continents evolved in the Meso–Neoarchean. Comparable Archean examples in Kapuskasing and Pikwitonei (Canada) and modern analogues in Fiordland (New Zealand), Kohistan (Himalayas), Southern California batholith, Peruvian Andes, and Hidaka (Japan) demonstrate that processes of continental growth from island arc to continental arc magmatism (and by implication to continental collision) were broadly similar throughout most of Earth history.