Melting, carbonic fluids and water recycling in the deep crust: an example from the Limpopo Belt, South Africa

Abstract

The proposed retrograde orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt separates hydrated, amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma. Widespread rehydration of metapelitic granulites under conditions of 660–600 °C and ≥0.6 GPa, and CO2‐dominated fluid‐inclusion populations appear to suggest thorough flushing of the high‐grade crust with an externally derived carbonic fluid. However, past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium’ fluid in the evolution of the rocks. This contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes, generated by in situ fluid‐absent biotite melting along the prograde path. Model equilibrium fluid compositions suggest that reaction between this melt‐derived H2O and biogenic graphite produced CO2‐rich fluid compositions and potentially high fluid:rock ratios at the wet granite solidus. Declining temperature resulted in fluid compositions shifting to higher XH2O, with the precipitation of graphite essentially at the sites of initial fluid generation, thereby preserving original (pre‐metamorphic) isotopic heterogeneities. The hydration pattern of the Southern Marginal Zone appears to be a function of melt migration. In the hydrated zone, leucosomes generally approximate minimum melt compositions and in this zone H2O was effectively recycled between the prograde and retrograde assemblages. In contrast, leucosomes in the granulite grade portion of the terrane have lost a K2O‐ and H2O‐rich melt fraction, and although some hydration has occurred in this zone, orthopyroxene is generally preserved in metapelites. In a general context, in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid‐rock ratios at temperatures close to the wet granite solidus. This single process holds the potential for widespread retrogression of formerly high‐grade assemblages, at a variety of aH2O values, without external fluid input.