Subduction-accretion of sub-ridge mantle in the south Tibetan mélange zone

Sub-ridge mantle is widely exposed on the seafloor at slow- and ultraslow-spreading ridges, where it forms magma-poor, mantle-dominated oceanic core complexes (OCCs). Although mantle denudation processes at such ridges have been extensively documented, the emplacement mechanism of sub-ridge mantle in orogenic belts and the recognition of such magma-poor OCCs remain challenging. In this study, we present integrated field, petrological, and geochemical investigations of mantle peridotites from the Zhongba ophiolite, together with basaltic blocks from the surrounding mélange in the Yarlung–Tsangpo suture zone. The Zhongba peridotites are relatively refractory, recording < 15% anhydrous partial melting, followed by limited refertilization by discrete, weakly aggregated melts, consistent with a low and episodic magma supply. Both geological architectures and geochemical characteristics suggest a mid-ocean-ridge rather than arc-related origin for the Zhongba ophiolite, which is further supported by the sub-solidus cooling history of the mantle section. In combination with the ocean-island-basalt (OIB)-affinity of basaltic blocks within the mélange, we propose a subduction-accretion model for the emplacement of such sub-ridge OCC, due to its high topography and the “lubricant effect” of mélange. Our study underscores the recognition of fossil mantle-dominated OCCs within global OIB-hosting accretionary orogens and provides far-reaching insights into the reconstruction of the architecture and evolution of those paleo-oceans.