Geochemistry of an ophiolitic complex from New Caledonia

The ophiolites of New Caledonia are composed of ultramafics overlain by mafic rocks, all of which were affected by low P metamorphism. The mafic rocks studied (gabbroic cumulates, and basaltic flows and dikes) from Montagne des Sources are similar to recent mid-ocean ridge rocks. They are olivine-normative with Mg/Mg+Fe²⁺ ratios ranging from 0.69 in lavas to 0.90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of Ti and V with the Mg/Fe ratio. The lavas have a flat REE pattern with a slight depletion of light REE and a La/Yb ratio <2. The dikes have three different types of REE patterns. The first type is nearly parallel to that of lavas, the second one is enriched in LREE (La/Yb4) and the third type with the lowest REE contents and a distinct LREE depletion is similar to that of cumulitic pyroxene gabbro. The variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of Langmuir et al. (1977). In agreement with structural and tectonic observations, the geochemical data suggests that the ophiolites were formed during the spreading of a mid-ocean ridge with a spreading half-rate of about 1 cm/ year.