Geochemistry of mafic extrusive lavas in the Bayankhongor Ophiolite, Mongolia

This study presents new geochemical data on pillow lavas from the Bayankhongor Ophiolite in western Mongolia, revealing compositions that range from sub-alkaline to alkaline basalts. While most of these basalts are tholeiitic, some show transitional geochemical variations. Except for one sample resembling normal mid-ocean ridge basalt, all others are enriched in light rare earth elements and closely resemble enriched mid-ocean ridge basalt and ocean island basalt. The presence of high-field strength elements such as Th, Ta, Nb, Zr, Hf, and Ti indicates a mantle-derived origin. Tectonic discrimination diagrams demonstrate a transition between enriched mid-ocean ridge basalt and within-plate basalt, characteristic of mid-ocean ridges. The samples are categorized into three groups based on the LREE variation: low- normal mid-ocean ridge basalt, high- enriched mid-ocean ridge basalt, and ocean island basalt, suggesting advanced partial melting of the mantle and a mixing of mid-ocean ridge basalt with ocean island basalt. The Bayankhongor Ophiolite is widely recognized as a subduction-unrelated ophiolite and represents one of the largest Neoproterozoic oceanic rift basins in the Central Asian Orogenic Belt. Consequently, subduction-unrelated tectonic models are often favored for the Bayankhongor Ophiolite, which somewhat aligns with our findings. However, recent studies highlight the importance of subduction-related models and timing. Our model aims to integrate both aspects.