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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]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]

Abstract: The paper summarizes data on the petrology of granitoid intrusions in the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo granulite complex, South Africa, and discusses the thermal and fluid effects of these intrusions on the granulites. The intrusions were emplaced in SMZ at 2680-2640 Ma, when the granulite complex was overthrust on the rocks of the adjacent Kaapvaal Craton. The mineral assemblages of the granitoids reflect temperatures above 900°C for the magmas that crystallized under pressures of 6-9 kbar. The granitoid magmas assimilated host rocks and were thereby enriched in MgO, FeO, and Al2O3, which was favorable for the crystallization of garnet, spinel, sillimanite, and corundum in the granitoids. Fluid inclusions in the granitoids and estimates of the fluid composition based on mineral equilibria indicate that the CO2/H2O ratio of the fluids broadly varied. Along with H2O-CO2 fluid, the magmas carried H2O-salt fluids, which penetrated the host rocks and triggered various metasomatic reactions in them. The thermal effects of the intrusions on the host granulites resulted in the development of partial melting zones that host orthopyroxene with >7 wt % Al2O3. Depending on the fluid regime and temperature, the orthopyroxene is found in equilibrium with either garnet and potassic feldspar or with biotite. The ages of the partial melting zones are comparable with those of the intrusions. [ABSTRACT FROM AUTHOR]