The geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting

The New Caledonia ophiolite hosts one of the rare examples of crust-mantle sections built in a nascent arc environment, providing the unique opportunity to investigate the first stages of magmatism in an intraoceanic arc setting. The ophiolitic sequence dominantly consists of refractory harzburgites and dunites, overlain by km-size lenses of ultramafic and mafic cumulates (dunites/pyroxenites, wehrlites and websterites/gabbronorites in ascending order), which occur in the upper part of the sequence as decimetre to metre-thick layers. In this work, a comprehensive petrological, geochemical and Sr–Nd–Pb isotope investigation has been carried out on the gabbronorites (Secchiari et al., 2018). They are mainly formed (≃ 55 to 70 vol%) of Carich plagioclase (anorthite up to 96 mol%) and high Mg# (88–92), Al2O3-poor (1.5–2.4 wt%) clinopyroxene (8–20 vol%), often rimmed by interstitial or poikilitic orthopyroxene (6–27 vol%). Mg-rich olivine (3–15 vol%, Fo = 87–89 mol%) occurs as anhedral, resorbed crystals. Whole rock compositions display high Mg# (86– 92) and strikingly low trace element contents. They own LREE-depleted patterns, with nearly flat (0.82 ≤ DyN/YbN ≤ 1.00) and low HREE (YbN = 0.2–0.9) and positive Eu anomalies. Clinopyroxene trace element chemistry mirrors the extreme depletion of the WR. By contrast, enrichments in FME (Fluid-Mobile Elements) are observed for whole-rock and clinopyroxene. FME enrichments and the high FME/immobile elements ratios shed light on the involvement of a subductionrelated component during magma genesis. Geochemical models show that the gabbronorites crystallized from primitive, ultra-depleted melts bearing evidence of fluid contamination processes, but with significantly different geochemical signatures compared to boninitic rocks worldwide, i.e. lower LREE-MREE, and absence of Nb depletion and Zr–Hf enrichments. Nd isotopes (+ 8.2 ≤ εNd(i) ≤ + 13.1), together with radiogenic Pb isotopic ratios extending from DMM toward EM2/GLOSS-like compositions, support an origin from a DMM source variably modified by slab fluids. We propose that the gabbronorites resulted from melting a refractory mantle source, followed by crystallization of near-fractional single melt increments that were not homogenized before their emplacement in the fore-arc crust