Low δ18O, ultrahigh-P garnet-bearing mafic and ultramafic rocks from Dabie Shan, China

The Maowu mafic–ultramafic complex from the Dabie ultrahigh-pressure metamorphic belt of China is enclosed in coesite-bearing gneiss. The complex mainly consists of layered garnet ortho- and clinopyroxenites in addition to minor harzburgite, eclogite and rare omphacitite. The rocks contain pyrope-rich garnet, enstatite, diopside and forsterite and minor clinohumite, magnesite, chromite, rutile, monazite and apatite with rods of monazite. Thermobarometry and coesite inclusions in omphacite constrain peak metamorphism at about 700±50°C and 40–60 kbar. The δ18O (SMOW) values of garnet, olivine, orthopyroxene, clinopyroxene and omphacite from seven samples were determined by in situ analysis with an Ultraviolet Laser Microprobe. Except for omphacite, the δ18O data define a narrow range between +2.9 and +4.4 per mil; omphacite has the lowest δ18O values (+1.5±0.3). Furthermore, fractionation of 18O and 16O between coexisting mineral pairs (e.g., ΔGrt-Cpx, ΔOl-Cpx, and ΔGrt-Opx) is less than ±0.5 indicating high-temperature O-isotope equilibrium. High REE contents and extreme LREE-enrichment [(La/Yb)N up to 17 400] for whole rock, as well as low δ18O data of minerals suggest the ultramafic–mafic rocks were metasomatized by a LREE-enriched fluid and reacted with meteoric water at a shallow crustal level prior to ultrahigh-P metamorphism at upper mantle depths.