Composition and source of fluids in high-temperature graphite-bearing granitoids associated with granulites: Examples from the Southern Marginal Zone, Limpopo Complex, South Africa

P-T conditions, fluid regime and carbon isotope composition of graphite and fluid inclusions from garnet-sillimanite-bearing leucocratic tonalites, trondhjemites and granites associated with orthopyroxene-bearing granulite metapelites in the Southern Marginal Zone (SMZ) of the Limpopo Complex, South Africa, are presented in the paper. Re-integrated compositions of perthitic alkali feldspars and antiperthitic plagioclase, as well as P-T and T-X_CO2 phase equilibria modeling using PERPLE_X software indicate that the granitoids began to crystallize at temperatures of 900–940 °C and pressures of 7–9 kbar, and were equilibrated with a fluid phase with X_CO2 > 0.5–0.6 as is recorded in dense fluid inclusions in quartz. A small fraction of a saline fluid accumulated during cooling only. Average δ¹³C_PDB values for graphite (−6.52 to −8.65‰) and fluid inclusions (−2.50 to −5.58‰) from the granitoids differ substantially from the values, which have been obtained in previous studies for graphite from the surrounding SMZ metapelites. Isotope data thus indicate that fluids associated with the granitoid magmas of the SMZ originated from a source unrelated to the host metapelites. In terms of the “heavy” isotope signatures of carbon, the granitoids might carry fluids, which have been produced during devolatilization of mafic rocks (amphibolites) interlayered with hydrothermal carbonate veins in the adjacent granite-greenstone successions of the Kaapvaal craton that have been buried underneath the SMZ granulites during their exhumation. The cratonic rock could serve as a source for the trondhjemite magmas intruding the SMZ granulites. In this scenario, the studied granitoids crystallized from these magmas variously contaminated by metapelitic material. Graphite precipitated via reduction of CO₂-rich fluid as the result of dissolution of pyrrhotite-rich metapelites in the magmas.