Petrogenesis of corundum-bearing mafic rock in the Horoman peridotite complex, Japan

Abstract

A corundum-bearing Type II mafic rock, within the Horoman peridotite, Japan, was petrologically examined in detail to obtain the P-T paths of the mafic rock as well as of the host peridotite. Of all the mafic rocks documented from the Horoman complex, only the corundum-bearing mafic rock has preserved, at least partly, its high-pressure mineralogy; all of the others have been completely recrystallized at low pressures. The Type II mafic rocks were initially formed at <1.0 GPa as cumulates of olivine, plagioclase and clinopyroxene. Corundum was then formed by metamorphism and/or partial melting of the Type II protolith at higher pressures (>1.5 GPa) than the initial condition of formation. Corundum reacted with clinopyroxene during exhumation of the Horoman peridotite down to the plagioclase stability field. The field and petrographical observations of the Type II mafic rocks (± corundum) coupled with published isotopic data suggest a complicated spiral-like P-T trajectory for the Horoman peridotite. The Type II protolith was formed at low pressure within the peridotite at the time of initial formation of the Horoman peridotite as a residue from primitive mantle at 830 Ma. The Type II mafic rocks, as well as the surrounding peridotite, then experienced subduction to the garnet stability field. Finally, the Horoman complex ascended a second time from the garnet peridotite to the plagioclase peridotite stability field. The Horoman peridotite is an example of multiple recycling of peridotite within the mantle. © 2001 Oxford University Press.