Two-layered oceanic lithospheric mantle in a Tibetan ophiolite produced by episodic subduction of Tethyan slabs

The origin and evolution of the Yarlung Zangbo ophiolites (South Tibet, China) is the key to the tectonics of the Neo-Tethyan Ocean between Greater India and Asia and the underlying upper mantle dynamics. This study presents a detailed investigation of the Zedang ultramafic body (comprising a harzburgitic and a lherzolitic domain) in the eastern Yarlung Zangbo Suture. Major-element compositions and Ti, Y, and HREE concentrations in peridotites and their minerals indicate that the harzburgites experienced higher degrees of melting than the lherzolites (∼13–19% versus ∼7–12%). The overall enrichment of LREE, Zr, and Sr in harzburgites and their clinopyroxenes suggest that the harzburgites were pervasively metasomatized (cryptically) by silicate melts. The harzburgites also record local strong metasomatism close to melt channels. Nd isotopes indicate that both metasomatic agents were derived from forearc basaltic magmas that intruded the harzburgites at ∼130–120 Ma. The lherzolites did not experience such metasomatism. Thermometry shows that the harzburgites experienced a thorough, lower-temperature reequilibration process in lithosphere, while the lherzolites rapidly accreted from the asthenosphere and preserved high equilibration temperatures (up to ∼1320°C). Comparable enrichment in fluid-mobile elements and radiogenic Sr-isotope compositions in both harzburgitic and lherzolitic pyroxenes reflect slab-fluid infiltration into both mantle domains. All the evidence and the presence of subduction-related chromitites in the harzburgites suggest that the Zedang harzburgites formed in a possibly Jurassic mature subduction system, while the lherzolites accreted later in an early Cretaceous forearc during subduction initiation. The two-layered lithospheric mantle reflects the episodic subduction of the Tethyan slabs.