Hydrous mafic-ultramafic intrusives at the roots of a proto-arc: implications for crust building and mantle source heterogeneity in young forearc regions

The New Caledonia ophiolite represents a rare example of proto-arc section originated during subduction infancy. The sequence is dominated by refractory harzburgites overlain by ultramafic (dunites and wehrlites) and mafic (gabbronorites) lithologies. In this contribution, we report the first occurrence of amphibole-bearing intrusives in the New Caledonia forearc sequence. This study deals with a petrological and geochemical investigation of a pyroxenite intrusion cut by mafic–ultramafic dikes. The intrusion consists of medium grain websterites, composed of orthopyroxene (30–75 vol %), clinopyroxene (20–50 vol %) and amphibole (2–30 vol %), which occurs as interstitial or poikilitic phase. Whole rocks display moderate Mg# (71–82) and concave downward REE patterns, bearing depleted to flat LREE (LaN/NdN = 0.5–1) and flat HREE segments (DyN/LuN = 0.8–1.1). Bulk rocks mirror clinopyroxene at higher absolute values. Fluid mobile element (FME) enrichments, coupled to Zr–Hf depletion, are observed for both clinopyroxene and bulk rock. Mineral major element variations and textural relationships indicate that the investigated lithotypes derived from hydrous magmas, which underwent extensive fractional crystallisation and post-cumulus processes. Geochemical modelling shows that the parental melts in equilibrium with the pyroxenites share remarkable similarities with the New Caledonia CE-boninite. However, they significantly differ from the equilibrium melts previously reported for the other intrusive rocks of the sequence. As a whole, our new results highlight a greater compositional variability for the liquids ascending into the Moho transition zone and lower crust. This may be also related to the involvement of a highly heterogeneous mantle source during subduction initiation.