High- to ultrahigh-temperature metasomatism related to brine infiltration in the Neoarchean Limpopo Complex: Petrology and phase equilibrium modeling

• Metasomatic alteration of enderbite at high temperature (ca. 900 °C).
• Garnet + sillimanite + quartz + anti/mesoperthite in highly metasomatized rock.
• Addition of K and removal of Ca, Fe, Mg, and Na by the metasomatism.
• Brine-rich fluid infiltrated along a shear zone and triggered metasomatism.

Fluids associated with granulite-facies metamorphism are considered to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, material circulation, and high-temperature metasomatism. Here, we report new petrological data obtained from enderbite and its metasomatized equivalents from the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo Complex, South Africa, and evaluate the alteration process caused by infiltration of high-salinity aqueous fluids during granulite-facies metamorphism. At the study area on Farm Commissiedraai, enderbite (plagioclase + quartz + K-feldspar + orthopyroxene) is partly metasomatized with the formation of continuous alteration zones comprising plagioclase + quartz + K-feldspar + orthopyroxene + garnet, plagioclase + quartz + K-feldspar + garnet, and finally mesoperthite/antiperthite + quartz + garnet + sillimanite. Bulk K2O increases, and CaO, MgO, FeO, and TiO2 decreases continuously from enderbite to the most intensely metasomatized rock. The occurrence of aqueous fluid inclusions with final melting temperatures of −29.2 °C to −21.6 °C trapped as a primary phase within quartz in the highly altered sample indicates that a high-salinity brine (probably NaCl rich) with additional components such as MgCl2 and CaCl2 controlled metasomatism. We employed a phase equilibrium modeling technique to evaluate the metasomatic process in the system NCKFMASH using THERMOCALC 3.3, and discovered that addition of K to and removal of Ca and Mg from the enderbite gave rise to the metasomatized mineral assemblages. Modeling also indicated that metasomatic alteration occurred at ca. 900 °C, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the SMZ, and with temperature calculations based on ternary-feldspar geothermometry of antiperthite in the highly metasomatized sample. The brine fluid that accompanied metasomatism infiltrated along the high-temperature Petronella Shear Zone, and was probably derived from devolatilization of low-grade greenstone materials of the Kaapvaal Craton as the result of thrusting hot granulite over cool granite-greenstone belts. Our results therefore suggest that metasomatism at this locality occurred closely after the peak metamorphism having been reached at ∼1000 °C in the root zone of the Limpopo orogeny at ∼2.72 Ga.

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