The P–T gradient in a Late Eocene low-T high-P metamorphic belt in northern New Caledonia increases from SW to NE. Metapelites in the pumpellyite–prehnite and blueschist zones contain lawsonite, Mg-carpholite, Fe-stilpnomelane and Fe-glaucophane. Thermodynamic calculations indicate a progression of metamorphic conditions from less than 0·3 GPa and 250°C in a kaolinite-bearing rock in the SW, up to 1·5 GPa and 410°C in a lawsonite–glaucophane-bearing sample in the NE of the Diahot terrane. Through a multi-method investigation of phyllosilicates, organic matter and fluid inclusions, we demonstrate that the evolution of organic matter and illite crystallinity depends strongly on the evolution of the P–T path with time. In addition, we show that the illite–muscovite b cell dimension provides a robust estimate of maximum pressure reached in low-temperature domains with polyphase metamorphic histories, despite subsequent high-temperature–low-pressure events. Fluid inclusion study reveals an isothermal decompression in the Diahot terrane.