Mantle pyroxenites in a young subduction system: the New Caledonia record

Mantle pyroxenites in supra-subduction zone settings are mainly testified by veins and layers in refractory peridotites, where they represent a key witness of melt migration in the forearc/subarc mantle. The New Caledonia ophiolite is dominated by highly depleted harzburgites, locally overlain by mafic-ultramafic cumulates representing a protoarc crust generated by boninitic to depleted tholeiitic melts. Pyroxenitic rocks intruding the harzburgites occur in a 5-km wide high-temperature mantle shear zone (Bogota Peninsula Shear Zone, BPSZ), classically referred to as an ancient oceanic transform fault. The pyroxenites (~5-15 cm-thick) may be undeformed or display ductile shearing or boudinage when parallel to foliation within the high-strain domain of the BPSZ. The samples of this study mostly consist of amphibole-bearing websterites. Minor olivine-cpx-bearing orthopyroxenites are also present. Textures range from cumulitic to porphyroclastic or granoblastic-polygonal. Accessory phases include highly calcic plagioclase (An82-86), Cr-rich spinel, sulfides and, occasionally, apatite. Pyroxenes display high Mg# (Mg# Opx= 91-92; Mg# Cpx= 84-93) and low Al2O3 contents (0.97-1.92 wt% and 1-2.42 wt% for orthopyroxene and clinopyroxene, respectively). Amphibole is high Mg# (78-85) hornblende. Bulk rocks are characterized by variable Mg# (82-91) coupled to REE concentrations ranging between 0.1-10 times chondritic values, with the orthopyroxenites displaying the lowest absolute contents. Clinopyroxene REE patterns mirror bulk rocks at higher absolute values. Putative melts in equilibrium with clinopyroxene of the orthopyroxenites display high Mg# (~80) and boninitic affinity. By contrast, the websterite-forming melts show enriched compositions (REE abundances up to 100 x ch.), variable LREE-HREE fractionation (LaN/LuN=3-19) and flat to fractionated HREE (GdN/LuN=1-2), sharing some analogies with pre-obduction adakite-like dikes locally intruding the harzburgites. Our results, coupled to the compositional variability of the melts in the New Caledonia proto-arc sequence, reinforce the notion that geochemical heterogeneity may be a fingerprint of early magma production during subduction inception.