Intrusion of late-stage granitoids into Archean granite-greenstone terranes may be spatially and temporally related to late-orogenic upper-crustal extensional collapse and formation of metamorphic core complexes. The late- to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources, and are locally associated with late-stage extensional structures. Thick roots of depleted lithospheric mantle of Archean age (tectosphere) beneath Archean cratons makes melting of the upper mantle difficult. It is proposed that the Archean tectosphere is made of imbricated slabs of buoyant Archean oceanic crust and lithosphere, together with trapped wedges of "fertile" mantle; this would explain several seemingly contradictory phenomena. Decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle. These basaltic magmas intrude and partially melt the lower crust; becoming more silicic in composition, they migrate upward to solidify in the middle and upper crust, forming the late- to postkinematic granitoid suite. The tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites. The direction and magnitude of shear-wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere.