Melt-forming and melt-loss reactions controlling granitic melt composition

Partial melting is a primary process for the continental differentiation. Composition of granitic melts are dominated by the melting process and possibly modified by later interaction with residual minerals in the source. Systematic petrographic study and geochemical analyses were conducted on the Triassic Jindong migmatites (238±1 Ma) from the Indosinian orogen, South China. The Jindong migmatites were formed through water-fluxed melting process with reaction of 0.035 Bt + 0.40 Qz + 0.365 Kfs + 0.20 Pl = 1 melt. The leucosomes formed by the melting process are all characterized by high SiO₂, K₂O and Na₂O and low Fe₂O₃, TiO₂ and MgO. The retrograde reaction during the melt segregation caused fractionation between Rb, Sr and Ba, which released Rb into melts via breakdown of biotite and extracted Ba and Sr from melts by subsequent intergrowth of muscovite and plagioclase, resembling the dehydration melting process. Th, Y and REE contents are controlled by the incongruent melting of apatite with precipitation of monazite and xenotime. Dissolution of low ε_Nd(t) apatite caused negative correlation between ε_Nd(t) and P₂O₅ of leucosomes. Whole-rock REE content of granite may not be used to infer behaviors of major mineral phases during melting of source. Missing “garnet effect” in the upper crust is not a solid evidence for the continental differentiation at shallow depth. Both “melt-forming” and “melt-loss” reactions are responsible for granitic melt composition, which may be common processes involved in the petrogenesis of granite because similar chemical evolution trends are also observed in the Himalayan leucogranite.