The generation of trondhjemites and tonalites on a massive scale during the Archean (3.8-2.5 Ga ago) marked the transition from a simatic to a sialic crust, and represents the magmatic contribution of cratonization. Petrogenetic models for the origin of these rocks based on their highly fractionated, HREE-depleted rare earth patterns suggest a mafic crustal source, either through a process of partial melting of amphibolite, garnet-amphibolite, or eclogite, in which hornblende and/or garnet are essential residual phases, or by hornblende-controlled fractionation of hydrous basltic magma. A series of vapor-absent (i.e., P(fluid) < P(total)) melting experiments on four natural basaltic compositions were conducted at 8, 16, 22 and 32 kbar in order to assess the validity of models for the origin of Archean granitoids which assume a mafic crustal source. Melt compositions produced by 10-40% melting are tonalitic-trondhjemitic at all pressures investigated; residual assemblages are amphibole + plag +/- opx +/- Fe-Ti oxide at 8 kbar, garnet + cpx +/- amphibole +/- plag +/- opx +/- Fe-Ti oxide at 16 kbar, and garnet + cpx +/- rutile at 22 and 32 kbar. REE pattens for most of the trondhjemitic-tonalitic partial melts, calculated on the basis of estimated modal proportions of melt and residual phases, are highly fractionated (La/Yb is 30-50), heavy rare earth-depleted (Yb(N) is 1-10) when garnet is present to some extent in the residue; these REE patterns are similar to those of trondhjemitic and tonalitic gneisses from several Archean "grey gneiss" and granite-green-stone terrains. A consideration of estimated Archean geotherms with respect to the experimental P-T conditions indicates that a temporally diminishing Archean geotherm might have progressively swept through a P-T regime in which trondhjemitic-tonalitic melts could have been generated initially from a water-saturated (P(f) = P(t)) to undersaturated (P(f) < P(t)) amphibolite source by partial melting at 5-8 kbar. Subsequent relaxation of the geotherm through the mid- to late-Archean would have produced similar melts by vapor-absent melting of garnet-amphibolite at 16 kbar and eclogite at 22-32 kbar: However, the degree of melting required to produce melts of trondhjemitic-tonalitic composition increases with pressure, 10-15% melting being appropriate at 8 kbar in a amphibole-dominated residue, but 25-35% melting being required at 22-32 kbar, where garnet dominates the residue. Supposing the tendency for melt segregation and/or magma mobilization mechanisms to be more effective at higher degrees of melting, an origin by partial melting of eclogite seems to be the most likely source for massive plutonic trondhjemite-tonalite contributions to the juvenile continents. Such a source is consistent with the generation of trondhjemite-tonalite protocontinental cores in any number of plausible Archean tectono-thermal scenarios, though not necessarily in a conventional subduction-zone setting.
