Possible non-melted remnants of subducted lithosphere: Experimental and geochemical evidence from corundum-bearing mafic rocks in the Horoman peridotite complex, Japan

We report experimental results and whole-rock trace-element characteristics of a corundum-bearing mafic rock from the Horoman peridotite complex, Japan. Coronitic textures around corundum in the sample suggest that corundum was not stable in mafic rock compositions during the late-stage P-T conditions recorded in the complex (P < 1 GPa, T < 800°C). Based on the experimental results, corundum is stable in aluminous mafic compositions at pressures of 2-3 GPa under dry conditions, suggesting that the corundum-bearing mineral assemblages developed under upper-mantle conditions, probably within the surrounding peridotite. Variations in the trace-element compositions of the corundum-bearing mafic rock and related rocks can be controlled by modal variations of plagioclase, clinopyroxene and olivine, suggesting that they formed as gabbroic rocks at low-pressure conditions, and that the corundum-bearing mafic rock was derived from a plagioclase-rich protolith. A complex P-T trajectory, involving metamorphism of the plagioclase-rich protolith at a pressure higher than that at which it was first formed, is needed to explain the origin of the corundum-bearing mafic rocks. They show no evidence for partial melting after their formation as low-pressure cumulates. The Horoman complex is an example of a large peridotite body containing possible remnants of subducted oceanic lithosphere still retaining their original geochemical signatures without chemical modification during subduction and exhumation.