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,
which show that 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, 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, it has been argued 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 in composition.
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