Tracing mantle metasomatism: Geochemical and isotopic clues from Neo-Tethyan suprasubduction zone ophiolite

Abstract Scientific diamond drilling into the Purang massif, located in the western Yarlung-Zangbo suture zone, Southwest Tibet, provides critical insights into the petrographic and geochemical evolution of Neo-Tethyan suprasubduction zone ophiolites. This study integrates structural, geochemical, and petrological data to reconstruct the formation and subsequent evolution of peridotites and associated lithologies, with implications for broader geodynamic processes. The massif predominantly consists of harzburgite, along with subordinate lherzolite, pyroxenite, and dunite (ca. 130 Ma), all of which are variably serpentinized (loss on ignition [LOI] of up to ~11 wt%). Geochemical features, including high MgO (42.68–46.24 wt%), low Al2O3 (0.7–1.18 wt%), and U-shaped rare earth element patterns, point to initial formation in a mid-ocean ridge setting with ~10%–15% partial melting, which produced Cr- and Mg-enriched residues (Cr# 0.3–0.4; Mg# 0.88–0.92). Subsequent interactions with hydrous melts in a suprasubduction zone environment, combined with distal subduction components, led to the formation of high-Cr chromitites (Cr2O3 ≤ 60%), dunite pods, and clinopyroxene-bearing harzburgite. Re-Os isotopic data (187Os/188Os: 0.12479–0.12846; γOs: −2.74 to 0.12) and pressure-temperature estimates (2.0–2.7 GPa, 745–1067 °C) suggest a polyphase genesis involving melt percolation and mantle interaction. Platinum-group element (PGE) patterns show Pt and Pd depletion and enrichment in iridium-group PGEs in dunite, which reflects fractionated melts. Additionally, Mg isotopic data (δ26Mg: −0.67‰ to −0.22‰) emphasize the role of metasomatic processes, which indicates interaction with isotopically light fluids during subduction and mantle modification. This study provides compelling evidence of the multiphase genesis of the Purang peridotites, and shows they were shaped by mid-ocean ridge extension, subduction dynamics, plume-induced lithospheric modification, and detachment faulting. These findings refine models of mantle evolution, oceanic lithosphere formation, and subduction zone dynamics, and highlight how detachment faulting, coupled with the emplacement of the Purang ophiolite, contributed to the evolution of oceanic lithosphere in the Neo-Tethyan realm.