Southward trench migration at ∼130–120 Ma caused accretion of the Neo-Tethyan forearc lithosphere in Tibetan ophiolites

Abstract

The preservation of ultrahigh-pressure and super-reduced phases (diamond, moissanite, etc.) in the harzburgites and chromitites of the Yarlung Zangbo ophiolites (South Tibet, China) has major implications for mantle recycling and lithosphere evolution in the tectonic system related to the closing of the Neo-Tethyan Ocean. However, important aspects of the genesis of these enigmatic ophiolites and the related geodynamic evolution are still unclear. In the Zedang ophiolite of the eastern Yarlung Zangbo Suture, detailed mineral chemical data reveal that the harzburgite domain in the east [spinel Cr# (mole Cr3+/(Cr3+ + Al3+) = 0.62–0.33] is more depleted than the lherzolite domain in the west (spinel Cr# = 0.30–0.17) and shows much lower equilibration temperatures (by ∼250–150 °C) than the lherzolites. Clinopyroxene trace-element compositions indicate that the harzburgites underwent pervasive metasomatism after melt extraction, while the lherzolites did not. New zircon U–Pb ages show that the harzburgites were intruded by dolerite dykes with chilled margins at ∼130–128 Ma, consistent with the widespread mafic magmatism at ∼130–120 Ma in the Yarlung Zangbo ophiolites. Nd–Hf isotopic data indicate that the Zedang lherzolites subcreted the pre-emplaced harzburgites concurrently with the intrusion of the dolerite dykes into the harzburgites, and that the lherzolites and dolerites both were derived from upwelling asthenosphere with minor slab input. Available zircon geochronology and Hf-isotope data show that juvenile magmatism in the adjacent Gangdese Arc was almost completely interrupted from ∼130–120 Ma.We suggest that the extension of the overlying harzburgitic lithosphere, subcretion of lherzolites, intrusion of mafic dykes, and the waning of Gangdese-Arc magmatism all reflect a southward trench migration in the Neo-Tethyan subduction system from the Gangdese Arc to the oceanic forearc lithosphere. This magmatic relocation and tectonic linkage are inferred to be the far-field effects of plate reorganization related to the Lhasa-Qiangtang collision and the breakup of Gondwanaland in the early Cretaceous. This model provides a solution for the global “ophiolite conundrum”: supra-subduction-zone type ophiolites with mid-ocean-ridge features.