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Archean greenstone belts, elongate to triangular tracts of metavolcanic and metasedimentary rocks, 1-1000 km in scale and preserved among intrusive granitoids and more deeply eroded domal gneiss domains, represent the most important and informative part of the ancient rock record, from the oldest preserved supracrustal sequences at ca. 3.85 Ga to the numerous and diverse sequences of the Neoarchean.
Occurring on every craton around the world, they generally represent the onset of one or more major phases of crustal growth in any particular area Yet, only rarely do they contain the very oldest record in any fragment of crust, which tends to come from nearby gneiss complexes or inherited zircons.
Greenstone belts form part of a (punctuated) continuum of similar volcano-sedimentary belts through time and space-a continuum that clearly extends into the Proterozoic and, very likely, into a variety of modern tectonic settings. This consensus view is best illustrated by numerous greenstone belts and granite-greenstone terrains having been mapped historically as Archean, with younger ages only becoming apparent after the application of modern geochronology.
Likely modern analogs are volcano-sedimentary sequences forming in convergent margin settings, arcs and back-arcs, but also those forming in magmatically active extensional settings within or peripheral to modern continents, in particular volcanic rifted margins. Oceanic plateaus, especially those with a complex, cyclic, history and some degree of internal melting and differentiation, almost certainly represent another endmember of the greenstone belt spectrum.
Evolved arcs incorporating older continental slivers (e.g., New Zealand), back-arc basins partly underlain by remnant continental crust (e.g., Japan Sea), and volcanic rifted margins underlain by variably attenuated continental crust (e.g., the Norwegian margin) may be overrepresented in the Archean greenstone record due to the inherent crustal-scale buoyancy of such settings. In contrast, immature oceanic arcs and small undifferentiated oceanic plateaus, unmodified by arc processes, are likely under-represented due to their low preservation potential during subduction of ambient lithosphere. This partly explains the paucity of Archean ophiolites, even among the numerous "oceanic" greenstone belts that are isotopically juvenile and lack any clear association with older crust.
Nearly all greenstone sequences show the imprint of complex polyphase deformation histories associated with basin closure, structural telescoping, and final collision, followed by transpressional modification. During the hot end-phases of these orogenic episodes, density inversions represented by thickened, internally complex, greenstone piles overlying less dense but radiogenic granitoid crust invariably caused a terminal phase of buoyancy-driven amplification of vertical structures, a process long recognized by eminent Nordic scientists such as Eskola and Ramberg.
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