COLLAPSE OF ARCHEAN OROGENS AND THE GENERATION OF LATEKINEMATIC TO POSTKINEMATIC GRANITOIDS

Intrusion of late-stage granitoids into Archean granite-greenstone ter ranes may be spatially and temporally related to late-orogenic upper-c rustal extensional collapse and formation of metamorphic core complexe s. The late- to postkinematic granitoid magmas are derived through par tial melting of upper mantle and lower crustal sources, and are locall y associated with late-stage extensional structures. Thick roots of de pleted lithospheric mantle of Arcbean age (tectosphere) beneath Arcbea n cratons makes melting of the upper mantle difficult. It is proposed that the Archean tectosphere is made of imbricated slabs of buoyant Ar chean oceanic crust and lithosphere, together with trapped wedges of ' 'fertile'' mantle; this would explain several seemingly contradictory phenomena. Decompression from upper crustal extension generates signif icant amounts of basaltic melts from the trapped wedges of fertile man tle. 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 gra nitoid suite. The tectosphere in this model is less dense and colder t han surrounding asthenosphere and is consistent with eclogitic xenolit hs from subducted oceanic lithosphere found entrained in kimberlites. The direction and magnitude of shear-wave anisotropy in the upper mant le is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere.