Lawsonite–Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

The Diahot terrane of NE New Caledonia contains an interbedded sequence of Cretaceous to Eocene metasediments, felsic and mafic metavolcanics that experienced c. 40 Ma high-P/T metamorphism. Metabasaltic assemblages define two prograde events (M1 and M2) and a tectonically disrupted crustal profile that extends from lawsonite–blueschist conditions in the SW to paragonite–eclogite conditions in the NE. Weakly deformed metabasalts from lowest-grade parts of the Diahot terrane contain M1 omphacite, chlorite, lawsonite and glaucophane-bearing assemblages that partially pseudomorph igneous plagioclase and augite, and reflect P = 0·7–1·0 GPa and T = 350–400°C. M1 assemblages are enveloped by a steeply SW-dipping S2 foliation that becomes progressively more intense towards the NE over a distance of c. 15 km. S2 assemblages are divided into four zones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine; (3) clinozoisite–hornblende–almandine; (4) almandine–omphacite. S2 assemblages reflect a P–T gradient that spans the exposed 15 km of the Diahot terrane from P = 0·8–1·0 GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7 GPa and T = 550–600°C (Zone 4). The systematic mineralogical changes reflect parts of a P–T array between 1·0 and 1·7 GPa that was extensively disrupted by tectonic thinning during exhumation.