Phase Equilibria in the Process of Anatexis in High-grade Metapelites

Anatexis is a widespread phenomenon in high-grade metamorphic rocks, which has drawn considerable attention of geologists. Due to the advancements in the study of phase equilibrium since the beginning of the 1990s, the formation and fractionation of melts and relevant retrograde reactions can be quantitatively studied using the software THERMOCALC and the pseudosection method. There are three main melting regimes in metasedimentary rocks: melting at water-saturated solidus, muscovite dehydration melting, and biotite dehydration melting reactions. Calculation of phase equilibria in KFMASH and NCKFMASH systems indicates that the phase relations of ferromagnesian minerals are controlled by the KFMASH system. But the H2O-saturated solidus in the KFMASH system is 50–60°C, higher than the actual minimum solidus in metapelites. Thus, the phase relations at temperature above the solidus of metapelite can be better simulated in the NCKFMASH and more complicated systems. Recent researches indicate that the preservation of granulite facies requires the melt loss. The formation of migmatite includes partial melting, partial segregation and extraction of melt, and partial back and retrograde reactions.