Orogen-scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention

Abstract

Granulite facies metapelitic gneisses collected over a $200\times 120$ km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K-feldspar–ilmenite  $\pm$ plagioclase  $\pm$ magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite- or sillimanite-free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of $5.5\pm 1$ kbar and $790\pm 30$°C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re-equilibration due to minor melt retention after peak metamorphism. Re-equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H₂O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum $P$– $T$ conditions in the Kakamas Domain reflect the upper- $T$ stability of biotite, and also corresponds to the intersection of the well-known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the $P$– $T$ history of exhumed terranes.