Dehydration-melting of MORB-composition amphibolites at 7--22.5 kbar and 775--1050°C: Constraints on phase relations, metamorphic facies transitions and origin of Archean TTG magmas

Long duration dehydration-melting experiments were conducted on two amphibolites with compositions similar to mid-ocean-ridge basalts. The experiments were done using a piston cylinder apparatus in the pressure-temperature range 775-1050°C and 7-22.5 kbar. The amphibolites underwent dehydration melting at temperatures above 800°C through hornblende breakdown reactions: Hornblende + Plagioclase + Quartz = Clinopyroxene +/- Orthopyroxene + Melt and Hornblende + Plagioclase + Quartz = Garnet + Clinopyroxene + Melt. The onset of melting in both amphibolites is at temperatures above the water-saturated solidus of basaltic rocks. The residual assemblage produced by these melting reactions corresponds to a low-P (7-10 kbar) orthopyroxene-bearing granulite and a high-P (>10 kbar) orthopyroxene-free garnet granulite. Tight experimental brackets and phase reversal experiments indicate that the low-P amphibolite-granulite transition occurs at temperatures >850°C whereas the high-P garnet-granulite assemblage forms at temperatures >825°C. At pressures above 17.5 kbar, plagioclase disappears from the assemblage and the residue becomes eclogitic. The transformation from garnet granulite to eclogite is characterized by decreases in the abundance of hornblende and plagioclase and an increase in the garnet/clinopyroxene ratio. Between 10-22.5 kbar the proportion of garnet in the residue is linearly correlated with pressure and the amount of garnet controls the density of the residue.;The melts produced in the experiments are felsic (SiO2 58-73 wt.%; Al2O3 ∼15-23 wt.%; K2O 1-5 wt.%, Na2O/K2O >1) and similar in composition to the Tonalite-Trondhjemite-Granodiorite (TTG) rocks that dominate many Archean cratons. Trace-element modelling using La and Yb as representative rare-earth-elements indicate that characteristic early Archean TTG signatures (<1 ppm Yb, La/Yb >15) requires the presence of ∼20 wt.% garnet in the melt residue, which is only achieved at pressures above 15 kbar (>48 km depth). This depth constraint is inconsistent with early crust evolution models that posit melting at the base of oceanic plateaus or oceanic crust to produce TTGs. Models involving melting of subducted oceanic crust are more consistent with the depth constraint. However, operation of subduction process in the Archean is considered problematic. A new model of subduction initiation by lateral compositional contrasts between converging oceanic plateau and normal thickness oceanic lithosphere is developed to explain the origin of early Archean TTG and by inference that of the first continental nuclei.