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To study the formation of S-type ultra-potassic granitic melts under the conditions of the continental crust, we conducted experiments on partial melting of plagioclase-free garnet–two-mica metapelite (with accessory apatite and ilmenite) at 6, 10, and 15 kbar and 700–900°C. Rock melting is driven by a series of peritectic melting reactios as a function of pressure and starts at temperatures of 750–800, ~800, and ~850°C at 6, 10, and 15 kbar, respectively, reflecting the positive dP/dT slope of the solidus. These P–T-conditions of melting, compositions of mineral assemblages and granitic melts formed during melting of plagioclase-free metapelites are similar to those formed during melting of plagioclase-bearing two-mica assemblages. The high K2O content (7–8 wt %) at a K2O/Na2O ratio of >8–10 is the main feature of the composition of the melt, which forms upon melting of plagioclase-free metapelite. These features are similar to the compositions of S-type ultra-potassic rhyolites, which form in extension tectonic settings. [ABSTRACT FROM AUTHOR]

One of the mechanisms explaining relationships between CO2-rich fluids and granitoid magmas at high-temperature crustal metamorphism is the melting of protoliths that originally contained carbonate minerals. In order to study the coupled processes of dehydration/decarbonation and melting, experiments were conducted with carbonate–biotite gneiss from an Archean greenstone belt at pressures of 6, 10, and 15 kbar in the temperature range of 800–950°C, and phase relations in this rock were modeled using the pseudo-section method. The experiments and modeling revealed a subvertical positive dP/dT slope of the solidus of the rock. In comparison to the calculated solidus temperatures, the experiments showed higher melting temperatures (~800°C at 6 kbar and ~850°C at 10 and 15 kbar). The products of the experiments at pressures of 6 and 10 kbar and temperatures >850°C were found out to contain assemblages of clinopyroxene, orthopyroxene, and ilmenite. The products of the experiments at 15 kbar did not contain either orthopyroxene or ilmenite, but calcium garnet and rutile were stable. The first portions of the near-solidus melts at 6 and 10 kbar were poor in SiO2 (44–50 wt %) and were formed because carbonate phases were involved in the melting reactions. With a temperature increase, the melts acquired a granite composition that was close to the composition of melts formed during the melting of the carbonate-free plagioclase + biotite + quartz assemblage. An aqueous–carbonic fluid containing Ca–Mg–Fe carbonate components coexisted with the melts. The phase assemblages and compositions of the granite melts obtained in the experiments are consistent with the modeling results. Comparison of the experimental results with published data on the partial melting of the carbonate-free plagioclase + biotite + quartz assemblages led us to the preliminary conclusion that Ca–Mg–Fe carbonates are able to decrease the melting temperature. The experiments have demonstrated that granite magmas can be derived together with aqueous–carbonic fluids from a carbonate-bearing protolith during high-grade metamorphism in the middle and lower crust. The occurrence of clinopyroxene or two-pyroxene assemblages in granitoids can be considered as a mineralogical indicator of this process. [ABSTRACT FROM AUTHOR]