The Louisiade ophiolite: a missing link in the western Pacific

Recent dredging of a 100 km long ridge along the northernmost part of the Louisiade Plateau (LP) recovered serpentinized peridotites, MORB (mid-ocean-ridge basalt) and volcaniclastic breccia–conglomerates. Clinopyroxene, Cr-spinel and bulk rocks show that the serpentinites are harzburgites to dunites, whereas hornblende phenocrysts from volcaniclastic rocks reflect hydrous, andesitic volcanism. The association of MORB, depleted mantle rocks and fingerprints of hydrous arc magmatism is typical of supra-subduction zone ocean lithosphere formed above a nascent subduction zone. Seismic and high-resolution bathymetry data reveal structures consistent with an extensive east–west elongated ophiolite estimated to have obducted onto the LP between 53 and 80 Ma. This represents a major eastwards continuation of the Papuan Ultramafic Belt and forms a crucial link with ophiolites farther south in New Caledonia, providing support for major subduction initiation events in marginal basins along the northern and eastern margins of Australia and Zealandia in the Palaeocene–Eocene.