Evolution of nascent mantle wedges during subduction initiation: Li-O isotopic evidence from the Luobusa ophiolite, Tibet

The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (