Petrogenesis of the East Hoerba harzburgites, SW Tibet: Implications for melt stagnation in the lithospheric mantle of Neo-Tethys

In this study we present petrographic and compositional data for harzburgites from the eastern parts of the Hoerba Massif of the Yarlung-Zangbo Suture Zone (YZSZ), SW Tibet. These harzburgites are cross cut by semi-ductile mylonitic zones. The investigated rock samples contain aggregates of unstrained clinopyroxene with tapered edges. They also contain olivine with a high forsterite [Fo# = Mg/(Mg + Fe) × 100] content (89.3–91.8), orthopyroxene with a low Al2O3 content (2.0–5.6 wt%), and clinopyroxene porphyroclasts with Cr2O3 contents between 0.2 and 1.0 wt% and low lanthanide contents. Chromian spinel (Cr-spinel) in the East Hoerba harzburgites has low Cr# [100 × Cr/(Cr + Al)] and high Mg# [100 × Mg/(Mg + Fe)] values (23.7–39.2 and 62.2–76.3, respectively). These harzburgites have chondrite-normalized rare earth element (REE) patterns characterized by enrichments in the light (L-)REE and positive slopes from the middle to the heavy REE. Semi-quantitative geochemical modeling demonstrates that the bulk-rock compositions of the East Hoerba harzburgites differ from those of typical abyssal and forearc peridotites. Clinopyroxene trace element modeling suggests that the East Hoerba harzburgites initially underwent melting below a mature spreading center, followed by melting in the mantle wedge above a Cretaceous Neo-Tethyan subduction system. Enrichment of the harzburgite samples in LREE is interpreted as due to post-melting entrapment of small volumes of a LREE-rich melt in the lithospheric mantle. We suggest that the East Hoerba harzburgites are sub-oceanic peridotites that underwent melt depletion events and melt stagnation before their exhumation at the Neo-Tethyan seafloor.