Juvenile island arc accretion
Many Archean granite-greenstone terranes are interpreted as juvenile island arc sequences that grew above subduction zones and later amalgamated during collisional orogenesis to form new continental crust. The island arc model for the origin of the continental crust is supported by geochemical studies that show the crust has a bulk composition similar to arcs. Island arcs are extremely complex systems that may exhibit episodes of distinctly different tectonics, including accretion of ophiolite fragments, oceanic plateaux, and intra-arc extension with formation and preservation of back arc and intra-arc basins. Many juvenile arcs evolve into mature island arcs in which the magmatic front has migrated through its own accretionary wedge, and many evolve into continental margin arcs after they collide with other crustal fragments or continental nuclei.
Although accretion of immature oceanic arcs appears to have been a major mechanism of crustal growth in Archean orogens, some people argue that oceanic arc accretion alone is insufficient to account for the rapid crustal growth in Precambrian shields. Furthermore, most oceanic arcs are characterized by mafic composition, whereas the continental crust is andesitic.
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