Tectono-thermal evolution of Luqu ultramafic rocks in the Xigaze ophiolites, Tibetan Plateau: Implications for oceanic lithosphere accretion at slow-to-ultraslow spreading ridges

The Luqu ultramafic rocks from the Xigaze ophiolites in the Yarlung–Tsangbo Suture Zone, southern Tibet, are interpreted as remnants of the Neo-Tethyan oceanic mantle formed at slow-to-ultraslow spreading ridges. As such, they offer valuable insights into sub-ridge mantle dynamics and evolution in this tectonic setting. This study presents a comprehensive investigation of the major- and trace-element compositions of minerals, as well as deformation microstructures, in representative harzburgites and associated pyroxenite veins. The harzburgite hosts are characterized by extreme depletion in incompatible elements and enrichment in compatible ones, consistent with high degrees (∼13–18 %) of anhydrous partial melting in a shallow (spinel-facies) sub-ridge mantle. The pyroxenite veins, with mineral compositions similar to those of the host harzburgites, likely crystallized from silica-rich melts derived from the asthenosphere, which subsequently interacted and equilibrated with the host rocks at the lithosphere–asthenosphere boundary (LAB) beneath a mid-ocean ridge. Microstructural features—including porphyroclastic texture, intracrystalline plasticity indicators (e.g., undulose extinction, subgrain boundaries, kink bands, and bent exsolution lamellae), curved olivine grain boundaries, and well-defined crystallographic-preferred orientations in olivine (A/D-type), orthopyroxene (type-AC), and clinopyroxene—indicate high-temperature (∼1000–1200 °C), low-pressure (∼3 kbar) deformation dominated by dislocation creep. This plastic deformation postdated the melt–rock interaction and was accompanied by relatively slow cooling from >1200 °C to ∼1000 °C at an estimated rate of ∼8.5 × 10⁻⁴ °C/year, driven by sub-horizontal basal shearing and conductive heat transfer at the LAB. A later, lower-temperature (<800 °C) brittle deformation stage is evidenced by widespread hydrous minerals (e.g., tremolite, anthophyllite, chlorite, talc, and serpentine) occurring along fractures and grain boundaries. This stage likely resulted from seawater infiltration along oceanic detachment faults, promoting hydration reactions and triggering rapid cooling from ∼1000 °C to <700 °C at a rate of ∼1.4 × 10⁻¹ °C/year. Based on these observations, we propose a tectono-thermal model for the evolution of the Luqu ultramafic rocks within a Neo-Tethyan slow-to-ultraslow spreading ridge environment.