Involvement of fluids in the metamorphic processes within different zones of the Southern Marginal Zone of the Limpopo complex, South Africa: An oxygen isotope perspective

• K-feldspar-rich gneiss may have formed due to melting of tonalite gneiss (TG).
• Melting of the TG can be produced by externally derived saline H2O–CO2 fluid.
• Leucogranite within the SMZ indicate genetic relation to the tonalitic gneisses.
• O-isotope composition of metapelites was modified during fluid infiltration.
• Leucosome-like band can represent injection of felsic melt into the metapelite.

We present oxygen isotope data obtained by laser fluorination for three outcrops from the South Marginal Zone (SMZ) of the Limpopo granulite complex, South Africa. These outcrops are located within the granulite zone (Bandelierkop quarry), on the retrograde (anthophyllite-in) isograd (Manamead locality), and within the zone of rehydrated granulite south of the retrograde isograd (Klipbank locality). Studied samples also include felsic (granitoid) rocks that occur as intrusive bodies, veins and veinlets in the host tonalite gneisses or the metapelite. The tonalite gneiss (TG) and the K-feldspar-rich segregations (Kfs-G) at Klipbank have marginally different δ18О (whole rock, WR) values (7.9 and 7.3% correspondingly). The Kfs-G may have formed by partial melting of the TG in the presence of external saline H2O–CO2 fluid. Metapelites at Manamead locality have δ18O values (9.5–10.3‰) lower than those at Bandelierkop quarry (10.0–11.4‰). The two-mineral oxygen isotope temperature estimates for metapelite minerals from Manamead cluster around two values: 620 ± 30 and 730 ± 40 °C. Oxygen isotope temperature estimates for the metapelites from Bandelierkop quarry define a narrow temperature range of 700–740 °С. The oxygen isotope temperatures are in reasonable agreement with the results of “classical” geothermobarometry. The δ18О values of the whole rocks and individual constituent minerals of metapelites strongly negatively correlate with the rocks’ volatile content (LOI values). This suggests that a fluid phase was involved in the hydration of metapelites in the vicinity of the regional anthophylite-in isograd of the Southern Marginal Zone. The fluid may have been released during crystallization of leucogranite melt. Oxygen isotope and geochemical features of the leucogranite indicate genetic relation to the tonalitic Baviaanskloof gneisses, but do not support the formation of leucogranite via in situ partial melting of metapelites.