Formation of earth's early Archaean continental crust

Subduction of oceanic crust at an unusually low-angle has been proposed as a model for the growth of continental crust older than about 2.5 Ga. At modern zones of low-angle-, or flat-subduction, magmatic additions to new crust come from partial melting of both the subducting oceanic crust (slab) and the thin wedge of mantle above the slab. Evidence for both a slab and wedge source is preserved in most late Archaean (3.0-2.5 Ga) terrains, but we find little evidence that a mantle wedge contributed to crustal growth prior to similar to3.1 Ga. This lack of evidence in part reflects a dearth of exposed crust aged between 3.0 and 3.3 Ga, but also suggests that subduction enriched mantle source regions did not develop before similar to3.3 Ga and possibly not before 3.1 Ga. In contrast to most modem terrains and some late-Archaean terrains, early Archaean (>similar to3.3 Ga) continental crust evolved through direct melting of thick mafic crust. We invoke a process of subduction that does not include the development of a mantle wedge, and call this process Archaean flat-subduction to distinguish it from modem low-angle subduction. (C) 2003 Elsevier B.V. All rights reserved.