Highly refractory ultramafic complex from Hong'an, western Dabie orogen, central China: An ancient document for subduction initiation?

Ultramafic rocks outcropped within orogenic belt or collision margin always attract numerous attention. Most natural objects are commonly compositionally modified, making up a special and complicated type which worth advanced study onto their origins, records of fluid/melt-rock interaction and tectonic evolution. Our research reported a set of highly refractory ultramafic rocks emplaced within Mesoproterozoic schists in Hong'an, Western Dabie orogen, central China. A new discussion was established about these mantle rocks which may indicate ancient ocean-arc transformation , implying an unfiled ophiolite or at least in part .

The ultramafic massif consists of serpentinized peridotite and minor amphibolite and ecologite. Peridotites were highly hydrated with two main lithologies of harzburgite and dunite recognized. Low SiO₂ and CaO, Al₂O₃ contents with highly depleted, "U" shaped REE pattern imply a inherited highly refractory signiture of typical mantle wedge peridotite without refertilization. Olivine relicts in dunites show consistently high Fo content (96~97). Chromites derive mostly from dunites, occurring mainly as disseminated discrete grains with zoning texture under microscope. Two types of chromites cores are recognized with medium-Cr# and high-Cr# respectively, both low TiO₂ (<0.058wt%) content suggests a forearc-related but distinct origins: 1) medium-Cr# chromite core (Al₂O₃=15.7~17.5wt%, Cr#=0.67~0.70, Mg#=0.59~0.62) retaining the abyssal-like spinel phase; 2) high-Cr# chromite core (Cr#0.81~0.99, Mg#=0.38~0.55) with boninitic affinity which suggests an arc condition. Meanwhile lizardite, brucite and dolomite of early serpentinization show affinity to fluids derived from oceanic slab. We propose the ultramafic rocks in the research represent combined products associated to initial subduction process with a primary transformation from ocean to arc setting.