Lawsonite-albite schists from northernmost New Caledonia

In the Boat Pass-Tiebaghi area of New Caledonia regional metamorphism of Cretaceous-Eocene sediments and volcanics is related to intrusion of large peridotite bodies along a well-defined west coast tectonic line. Three metamorphic zones have been mapped, passing eastwards from the ultramafic line: aragonite-lawsonite, calcite-lawsonite, and calcite-lawsonite-glaucophane. The relationship between structure, stratigraphy and metamorphic zones is simple; assemblages indicative of the highest ratio of pressure to temperature (aragonite-lawsonite zone) lie immediately adjacent to the intrusive peridotites and also occupy stratigraphically the highest position of all three metamorphic zones. Albite-bearing assemblages persist in all zones and spessartine and omphacite appear in the deepest zone (calcite-lawsonite-glaucophane). The high pressure mineralogy was developed by fluid overpressure generated from a normal to steep geothermal gradient within a sealed environment beneath the tectonically-emplaced peridotite plate.