Preferential retrogression of high-P metasediments and the preservation of blueschist to eclogite facies metabasite during exhumation, Diahot terrane, NE New Caledonia

High-P metabasites of the Diahot terrane, NE New Caledonia occur as spatially restricted (cm to km-scale) lenses, boudins and layers in psammitic to pelitic metasediments. Although interlayered, the two rock types preserve distinct, tectonically disrupted metamorphic profiles in the transition from lawsonite blueschist in the SW to low-T eclogite in the NE. As the metabasites experienced comparatively low strain, igneous textures, and, more rarely, igneous minerals are preserved. Metamorphic assemblages in the metabasites range from lawsonite–omphacite–glaucophane-bearing assemblages (Zone 1; P=7–10 kbar, T=350–400 °C) in the SW, through clinozoisite–almandine-bearing assemblages (Zone 2; P=14–16 kbar, T=450–550 °C) and finally to clinozoisite–almandine–hornblende–omphacite-bearing assemblages (Zones 3 and 4; P=16–18 kbar, T=550–600 °C) in the NE. Metasedimentary assemblages range from spessartine–lawsonite–albite–chlorite (Zone 1), through almandine–clinozoisite–albite–chlorite (Zone 2) and almandine–epidote–omphacite–albite-bearing (Zone 3) lithologies. Mineral inclusion trails in coarse-grained albite and garnet in Zones 2 and 3 metasediments record prograde mineral assemblages that match the P–T conditions recorded by the metabasites (P=14–18 kbar, T=450–600 °C). The metasediments were intensely deformed at peak conditions (P=12–16 kbar, T=550–600 °C) and during retrogression, and developed coarse-grained albite, phengite and chlorite-bearing assemblages. Differences in strain history (competency contrasts), the timing of major dehydration reactions and variation in whole rock composition all played vital roles in developing the distinctive field relationships involving blueschist to eclogite facies metabasites enclosed within strongly retrogressed high-T blueschist to greenschist facies metasedimentary lithologies exposed in NE New Caledonia.