Melting and melt segregation processes controlling granitic melt composition

Several important processes in the petrogenesis of granite are still debated due to poor understanding of complex interactions between minerals during the melting and melt segregation processes. To promote improved understanding of the mineral-melt relationships, we present a systematic petrographic and geochemical analysis for melanosome and leucosome samples from the Triassic Jindong migmatite, South China. Petrographic observations and zircon U-Pb geochronology indicate that the Jindong migmatite was formed through water-fluxed melting of the Early Paleozoic gneissic granite (437±2 Ma) during the Triassic (238±1 Ma), with the production of melt dominated by the breakdown of K-feldspar, plagioclase and quartz. The Jindong leucosomes may be divided into lenticular and net-structured types. Muscovite, plagioclase and K-feldspar in the net-structured leucosome show higher Rb and much lower Ba and Sr contents than those in the lenticular leucosome. This may be attributed to elevation of Rb and decreasing Ba and Sr abundances in melts during the segregation process, due to early fractional crystallization of K-feldspar and plagioclase. These leucosomes show negative correlation between ε_Nd(t) and P₂O₅, reflecting increasing dissolution of low ε_Nd(t) apatite during melting process. The continuous dissolution of apatite caused saturation of monazite and xenotime in melt, resulting in the growth of monazite and xenotime around apatite in the melanosome. This process resulted in a sharp decrease of Th, Y and REE with increasing P₂O₅ in the leucosome samples. This complex interplay of accessory mineral reactions in the source impact REE geochemistry and Nd isotope ratios of granites. As the granites worldwide exhibit similar compositional and isotopic patterns with the Jindong leucosomes, we suggest that both the melting and melt segregation processes strongly control the granitic melt compositions.