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

Abstract

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