Multistage and diverse melt-mantle interaction in dunite-harzburgite channel systems beneath oceanic slow-ultraslow spreading centers: Evidence from the Xigaze ophiolite (Tibet)

Abstract

High-porosity, melt-focusing channel systems (comprising reactive dunite and its surrounding harzburgite) reflect an essential type of melt extraction within the mantle under mid-ocean ridges, proposed mainly by investigations of ophiolites produced in fast-spreading centers. However, relevant melt-migration processes from mantle wall rocks into dunitic channels under slow-ultraslow spreading centers and their effects are less well understood. We present systematic petrographic observations and whole-rock/mineral compositional analyses of a dunite-harzburgite channel system (∼250 m in width) within the Dazhuka mantle section (∼3–4 km thick) of the Xigaze ophiolite (South Tibet), produced in a Neo-Tethyan slow-ultraslow spreading center. The harzburgites, closely surrounding a dunite-rich zone, show large variations in pyroxene/olivine ratios (0.1–0.6), whole-rock MgO (41.2–45.9 wt%), Al2O3 (0.23–1.87 wt%) and CaO (0.38–2.80 wt%), as well as spinel Cr# (molar Cr3+/(Cr3++Al3+), 0.20–0.73) and pyroxene major and trace elements, almost covering the whole ranges of the Xigaze ophiolitic mantle. The harzburgites closer to the dunite-rich zone are gradually more depleted than those further ones, and finally replaced by the most depleted dunites. Modeling of decompressional melting from a depleted-MORB-mantle (DMM) source shows that the harzburgitic compositions correspond to melt depletion degrees of ∼6.9–19.2%, requiring a ∼30-km solely decompressional distance that is much greater than the sampling size of the channel system. It suggests that the wall-rock harzburgites can be produced by the reaction between high-fluxing, silica-undersaturated melts and the least-depleted harzburgites during the formation of reactive dunite-harzburgite channels. In addition, clinopyroxenes in dunites and some harzburgites show metasomatic enrichments, reflected in higher Na2O (up to 0.32 wt%), TiO2 (up to 0.17 wt%) and concentrations of Zr, Hf, light rare earth elements (LREE) and fluid-mobile elements relative to those in other harzburgites. These distinct signatures suggest the later localized metasomatism by batches of basaltic melts (some are volatile-rich) flowing through the harzburgitic wall rocks into the dunite-rich zone. We therefore propose that the dunite-harzburgite channel system archives at least the formation of reactive channels in deep upwelling asthenosphere during the high-melt-flux stage, and the subsequent metasomatic enrichments by basaltic melts reusing the channels in the shallow asthenosphere. The multistage and diverse melt-mantle interaction styles may characterize the melt extraction processes under oceanic slow-ultraslow spreading centers.