Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5-10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at similar to 1250 degrees C and 2-4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust-mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.