Very Low-grade Metamorphic Evolution of Pelitic Rocks under High-pressure/Low-temperature Conditions, NW New Caledonia (SW Pacific)
Résumé
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 0Á3 GPa and 250 C in a kaolinitebearing rock in the SW, up to 1Á5 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 hightemperature-low-pressure events. Fluid inclusion study reveals an isothermal decompression in the Diahot terrane.
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