Genesis of mantle peridotites and high-Cr chromitites from the Jiangco ophiolite, northern Tibetan Plateau: Insights from mineralogy, geochronology, and geochemistry

• The Jiangco ophiolite records multi-stage oceanic basin evolution. • Mantle peridotites reveal complex melting and metasomatism. • The genesis of chromitite is coupled with long-term evolution of oceanic lithosphere. The origin of ophiolitic chromitites has long been debated. In the central Bangong-Nujiang suture zone, multiple episodes of ophiolites developed during the ocean-continent transition, seafloor spreading, and subsequent subduction-closure processes, providing an ideal setting to investigate long-term mantle evolution and chromitite formation. This study presents integrated geochronological, mineralogical, and geochemical constraints on chromitite genesis within the Jiangco ophiolite of the Bangong-Nujiang suture zone, northern Tibetan Plateau. The chromitite-bearing ultramafic complex is dominated by dunite with subordinate harzburgite and is crosscut by multiple generations of troctolite, pyroxenite, and gabbroic dikes, recording a polyphase magmatic history. Zircon U-Pb ages from chromitites define several age populations (∼547 Ma, ∼251 Ma, and ∼ 184 Ma), with younger zircons clustering at ca. 170–150 Ma. Troctolite and gabbroic dikes from the mantle–crust transition zone yield crystallization ages of 184.3 ± 1.7 Ma and 161.2 ± 0.8 Ma, respectively, indicating episodic mantle-derived magmatism during ocean-basin evolution and subsequent subduction. Two types of mantle peridotites are distinguished. Type I peridotites (harzburgite and dunite) are highly refractory and forearc-like, recording high degrees of partial melting and metasomatism by subduction-related melts/fluids, whereas Type II peridotites are relatively enriched in Al and Ca and show transitional characteristics from abyssal to subduction-modified mantle peridotite. Chromitites occur exclusively within dunite and contain uniformly high-Cr chromian spinel (Cr# = 64.8–72.3; very low TiO 2 ), together with IPGE-enriched platinum-group element patterns typical of high-Cr chromitite. These observations suggest that chromitite formation in the Jiangco ophiolite was closely linked to focused melt-rock interaction between boninitic melts and a refractory mantle during Jurassic subduction of the Bangong-Nujiang Ocean. The Jiangco peridotites and chromitites thus record a key stage in the long-term evolution of oceanic lithosphere and provide insight into chromitite genesis in complex subduction-influenced ophiolite systems.