Granite emplacement and the retrograde P-T-fluid evolution of Neoarchean granulites from the Central Zone of the Limpopo Complex

Abstract

Petrological and fluid-inclusion data of high-grade metapelitic gneisses that occur as enclaves and in the immediate surroundings of the 2.612 Ga old Bulai granitoid intrusive are presented in this chapter. The Bulai intrusive is an important time marker in the tectono-metamorphic evolution of the Central Zone of the Limpopo Complex. The host-rock gneisses show one generation of garnet, cordierite, and sillimanite, whereas the enclave gneisses show two different generations of garnet (Grt1,2), cordierite (Crd1,2), and sillimanite (Sil1,2). The first generation defines a gneissic texture, whereas the second generation shows a random mineral orientation. Grt1 and Crd1 show a higher Mg content compared with Grt2 and Crd2. Host rock garnet and Grt1 show K-feldspar micro-veins at the contact with quartz as a result of high-temperature metasomatism. Host rock garnet, Grt1, and Grt2 are zoned and participate in two simultaneously operating reactions: sillimanite + garnet + quartz = cordierite and garnet + K-feldspar + H2O = biotite + sillimanite + quartz. The combination of petrographic, geothermobarometric, and fluid-inclusion results shows evidence of two different pressure-temperature (P-T) paths in the enclave and a single P-T path in the host rocks. The decompressional cooling P-T path in the host rock is typical of the country rocks throughout the Central Zone. The high-pressure part of the host-rock P-T path overlaps with the Grt1-Crd1-Sil1 P-T path found in the enclave rocks. The second P-T path is calculated from the Grt2-Crd2-Sil2 assemblage and is found only in the enclave rocks. The two P-T paths in the enclave rocks can be connected by a sub-isobaric heating event of ~50 °C at 5.5 kbar. This increase in temperature is followed by decompressional cooling but with a lower P-T gradient compared with that of the country rocks caused by the emplacement of the Bulai Pluton. Fluids present during granulite metamorphism include CO2 and brines. Retrograde infiltration of water in graphite-bearing country rocks under relatively reduced conditions resulted in the formation of a methane-rich fluid.