Sapphirine-bearing, orthopyroxene-sillimanite ± garnet granulites of the Palni Hill Ranges record evidence of ultrahigh-temperature metamorphism during which primary mineral assemblages were partially overprinted by a succession of reaction coronas and symplectites that together suggest decompression. Model system petrogenetic grids allow constraints to be placed on the evolution of P–T conditions as recorded by reaction relationships of successively developed mineral assemblages; thermobarometry on garnet-bearing assemblages allows minimum constraints to be placed on P–T conditions for the primary mineral assemblages. Primary mineral assemblages suggest P–T conditions of c. 12 kbar and c. 900-1000 "C. Secondary mineral assemblages in reaction textures show a two-stage decompression to probable P–T conditions of c. 5 kbar and c. 850 °C. The inferred P–T path is clockwise.

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