Origin of high-Cr chromite deposits in nascent mantle wedges: Petrological and geochemical constraints from the Neo-Tethyan Luobusa ophiolite, Tibet

Abstract

The petrography and compositions of chromitites and peridotites from the Tibetan Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere, are used to study the origin of high-Cr chromite deposits and relevant magmatic evolution in nascent mantle wedges during subduction initiation. The mantle sequence of the ophiolite has a two-layered structure with cpx-poor and cpx-rich harzburgites dominating the upper and lower layers, respectively, and locally contains dunite and chromitite bodies in the upper layer. The cpx-rich and cpx-poor harzburgites contain clinopyroxene having finger-like protrusions and chromite showing both irregular and euhedral shapes, and display spoon-shaped and U-shaped REE patterns, respectively. The dunites and chromitites followed a crystallization sequence Ol-Chr-Cpx and present spoon-shaped to U-shaped REE patterns. Clinopyroxene grains in the cpx-rich harzburgites have major and REE compositions slightly more depleted than those in abyssal peridotites, whereas clinopyroxene grains in the cpx-poor harzburgites and dunites have boninitic affinity. Chromite grains in the chromitites and cpx-rich harzburgites have the highest and lowest Cr#s [100 × Cr/(Cr + Al)] (75–80 vs. 20–40), respectively, whereas those in the cpx-poor harzburgites and dunites have intermediate Cr#s. The dataset above indicates that the Luobusa harzburgitic mantle wedge was once modified by slab-derived fluids and later refertilized by melts with compositions intermediate between MORB and boninitic lavas. Under high geothermal conditions induced by slab rollback and asthenospheric upwelling, subsequent partial melting in the refertilized H2O-bearing mantle formed high-Ca boninitic to arc picritic melts, which reacted with the overlying cpx-rich harzburgites and generated high-Cr chromitites, dunites and cpx-poor harzburgites under conditions of diminishing melt/rock ratios. Chromitites and peridotites in mantle sequences of forearc ophiolites keep important information for reconstructing the geodynamic evolution of subduction initiation.