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 delta O-18 values similar to those of normal mantle (+5.18 +/- 0.2 parts per thousand), but have large delta Li-7 variations, similar to+4 parts per thousand to +13 parts per thousand in harzburgites and similar to+1 parts per thousand to + 11 parts per thousand 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 (similar to 2-8 mu g/g) and negative delta Li-7 values (similar to-15 parts per thousand to -8 parts per thousand), which are beyond the ranges of normal upper mantle (<1.3 mu g/g and +4 +/- 2 parts per thousand, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high delta Li-7 but normal mantle-like delta O-18 values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-delta Li-7 clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-delta Li-7 mantle areas at deeper levels. Subsequent melting in both the high-delta Li-7 and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large delta Li-7 variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000 years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 10(6) years after the chromitite formation. (C) 2018 Elsevier Ltd. All rights reserved.