Ultra-depleted melt product preserved in the Ladong ophiolitic peridotites of the North Qilian Orogenic Belt, Northern Tibet

In Earths upper mantle, the compositions of both migrating magmas and solid residues are basically controlled by source materials and melting behavior which are related to tectonic dynamics. Thus, infrequent geochemical signatures recorded by ophiolitic lithologies may reflect a unique source or specific melting history. Here, we report whole-rock and mineral compositions of the harzburgite, wehrlite, and olivine websterite in the Ladong ophiolite complex of the North Qilian Orogen Belt (northern Tibet). The dominant spinel harzburgite has high spinel Cr# values of 65–70 and light rare earth element (LREE) depleted patterns (LaN/YbN = 0.04–0.12), resembling the residua after 20% or more of melt extraction. Wehrlite lenses within the harzburgite display extremely depleted LREE patterns (whole-rock LaN/YbN = 0.04–0.05) and low Na and Ti contents in clinopyroxene. They are interpreted as products of reaction between the host harzburgite and an ultra-depleted melt produced by asthenospheric decompression melting. Outcrops of olivine websterite have well-equilibrated triple junction textures between pyroxene grains and high-Mg olivine (Fo = 85–86; bulk rock Mg# = 93), consistent with a cumulate origin. In the olivine websterite, the depleted LREE concentrations (LaN/YbN = 0.05–0.18) in clinopyroxene suggest a less depleted nature than the wehrlite-forming melt. Thus, the Ladong ophiolite records two episodes of melt activity that produced depleted to ultra-depleted magmas, which may have been generated during subduction initiation in eastern Proto-Tethyan belt.