Deep infiltration of surface water during deformation? Evidence from a low- δ18O shear zone at Koegel Fontein, Namaqualand, South Africa

Abstract

The 144–133Ma Koegel Fontein Igneous Complex is notable for the high proportion of magmatic rocks with very low δ18O values (<6‰), including some that crystallized from low-δ18O magmas (< 6‰). Previous work has suggested that this is a consequence of partial melting of country rock with very low δ18O values. Here we demonstrate that low-δ18O country rocks (as low as —2‰) are present and are associated with a N-S trending ~545 Ma Pan-African high-temperature ductile shear zone. Wall rocks adjacent to the shear zone have normal δ18O values. We interpret this to mean that the fluid-rock interaction that lowered the δ18O values of the country rock was enabled by deformation associated with the shear zone (or an earlier brittle precursor) at some point in the history of this structure. Recrystallization and annealing after deformation and variable (>10‰) δ18O values on a cm scale in the shear zone are consistent with peak temperature and accompanying deformation post-dating, at least in part, fluid infiltration. The low permeability and lack of brittle overprinting of the shear zone are inconsistent with fluid flow post-dating deformation.Modelling of the inferred oxygen isotope exchange demonstrates that fluid-rock interaction must have occurred at either high temperatures (for infiltrating water with a δ18O value of −10‰, W/R is ~1 at 700 °C) or with very low δ18O values for the infiltrating fluids. If the infiltrating fluid had δ18O values < −20‰, the temperature of interaction makes little difference to the required water/rock ratio (W/R) calculated from mass-balance models. Meteoric water might have had δ18O values as low as —30‰ during the proposed ~547 Ma global glaciation, during the period when the shear zone is thought to have been active. Kinetic considerations require temperatures of >250 °C for exchange to occur in a reasonable time-period, even if the δ18O values were abnormally low. Previous studies have suggested fluid infiltration into mid-crustal fault systems and shear zones may play an important role in the rheological evolution of these structures. This study provides direct evidence for such infiltration and suggests that the identification of similar settings may help to establish the frequency and significance of these processes.