Refertilized Neoproterozoic sub-arc Tongbai Orogen lithospheric mantle preserved between the North China and Yangtze cratons

Abstract

Cratons form the nuclei of continents, and their margins are key locations for lithospheric mantle construction, modification and destruction. Here, we present bulk-rock major and trace element data, oxygen isotope compositions, platinum group element (PGE) concentrations, and ReOs isotopic data for the Liushuzhuang orogenic peridotites from the Paleozoic Tongbai Orogen in central China. These peridotites represent the only known mantle fragments within the Tongbai orogen, which is situated between the North China and Yangtze cratons along the former Shangdan Ocean. Building on our published petrographic and mineralogical data, which identify these rocks as phlogopite-amphibole-rich harzburgites and dunites, our new geochemical results confirm significant refertilization–as indicated by linear correlations between bulk rock MgO and Al₂O₃ and TiO₂ and, elevated (La/Yb)_N (4.2 to 14.2), enrichment in large ion lithophile elements (e.g., K, Rb, Ba, Th and U), and positive Pb anomalies. A subtle 0.24 ‰ increase of olivine δ¹⁸O (5.05 to 5.29 ‰) from the harzburgites to the dunites, translating into ∼0.17 ‰ increase (5.23 to 5.40 ‰) in reconstructed bulk compositions, indicates that the melt-rock reactions required high-δ¹⁸O slab-derived fluids. Although petrographic and geochemical evidence suggests that the dunites formed via melt-rock interaction, the most depleted dunites exhibit low Re concentrations (0.02–0.07 ppb) and unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios (0.12350–0.12464), indicating minimal disturbance of their ReOs isotope systematics. They yield Neoproterozoic T_RD model ages (0.68–0.84 Ga), representing a minimum timing of melt extraction. On the basis of the ReOs isotopic data, the Liushuzhuang peridotites are unlikely to have been from the adjacent Archean-Proterozoic cratonic mantle. Instead, they can be interpreted as fragments of sub-Tongbai arc mantle lithosphere that were assembled, depleted and refertilized during the Paleozoic. Combined with sulfide petrology and positive RuPd anomalies, we interpret that slab-derived high-fO₂, Ti-rich fluids/melts, along with S-undersaturated boninitic melts and S-saturated basaltic melts, penetrated the sub-arc Tongbai mantle. Our findings provide new geochemical and isotopic evidence for the existence of Neoproterozoic sub-arc lithospheric mantle in the Tongbai Orogen and offer rare insights into how ultra-refractory mantle domains are generated and preserved outside Archean cratons.