Modification of an ancient subcontinental lithospheric mantle by continental subduction: Insight from the Maowu garnet peridotites in the Dabie UHP belt, eastern China

- The Maowu garnet dunites represent a Paleoproterozoic fragment of the SCLM.
- The orthopyroxenite veins represent metasomatic products between the dunites and silica-rich hydrous melts.
- Sulfur-saturated crustal melts significantly modified the PGE signature and destroyed the Re-Os systematics the SCLM.
- Sulfur-depleted crustal melts would not significantly modify the PGE patterns and the Re-depletion model ages of the SCLM.

Orogenic mantle-derived peridotites commonly originate from the subcontinental lithospheric mantle (SCLM) and thus provide a key target to investigate the modification of the SCLM by a subducting slab. The Maowu ultramafic rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt have formerly been debated as representing cumulates or mantle-derived peridotites. Detailed petrological and geochemical data presented in this study provide new constraints on the origin and formation of the peridotites involving melt depletion in the ancient SCLM and deep crustal metasomatism. The Maowu garnet dunites have refractory bulk compositions characterized by high Mg# (91.9-92.0) and Ni (2537-2892 ppm) values and low Al2O3 (0.26-0.76 wt.%), CaO (0.05-0.32 wt.%), TiO2 (< 0.03 wt.%), Pd/Ir (0.40-0.46) and 187Os/188Os (minimum 0.11461) values. The Paleoproterozoic model ages (TRD = 2.1 Ga, TMA = 2.3 Ga) of the most refractory dunites represent minimum estimates for the age of the initial melt extraction. The extremely depleted nature, high olivine Fo (92.7-93.9), high Cr# (82-87) of spinel, and Re-Os isotopic data suggest that the Maowu garnet dunites are the residues of ~ 40% partial melting and represent a Paleoproterozoic fragment of the SCLM beneath the southeastern margin of the North China craton. Many garnet orthopyroxenite veins crosscutting the Maowu dunites preserve abundant metasomatic textures and show variable enrichment in incompatible elements. Mineral and whole-rock chemistry indicate that these veins represent metasomatic products between the wall dunites and silica-rich hydrous melts under UHP conditions. The veins show large variations in platinum-group element (PGE) signatures and Re-Os isotopes. The garnet-poor orthopyroxenite veins are characterized by low Al2O3 (< 2 wt.%) and S (< 31 ppm) contents and have PGE patterns and 187Os/188Os ratios similar to the wall dunites, whereas the garnet-rich orthopyroxenite veins have high Al2O3 (> 6 wt.%) and S (99-306 ppm) contents and show melt-like PGE patterns and high 187Os/188Os ratios (up to 0.36910). These features, combined with the occurrence of interstitial sulfides in the garnet-rich orthopyroxenite veins, suggest that crust-derived sulfur-saturated silicate melts may have significantly modified the PGE signature and destroyed the Re-Os systematics of the SCLM. However, when the crust-derived silicate melts became sulfur-depleted, such melts would not significantly modify the PGE patterns, radiogenic Os-isotope compositions or the Re-depletion model ages of the SCLM. Consequently, deep crust-mantle interactions in continental subduction zones could induce high degrees of Os isotopic heterogeneity in the SCLM wedge.