THE HOROMAN PERIDOTITE COMPLEX, JAPAN: THE UPPER MANTLE VARIATIONS IN ONE PLACE

Abstract

The spectrum of ultramafic and mafic rocks in the Horoman peridotite complex provides a near-perfect natural laboratory to study the process of melt generation, mixing of crustal component, melt-rock interaction, metasomatism and deformation in the mantle and/or during exhumation. It physically preserves the so-called “source rock” (lherzolite), the residue after partial melting (harzburgite), and the crystallized melts (ultramafic and mafic rocks) all in one place, allowing earth scientists to directly observe the consequences of a fundamental planetary process. Based on a synthesis of extensive research findings and our field observations, the Horoman Peridotite Complex is thought to have formed as follows: The incipient petrogenetic stage at approximately 1 Ga involved polybaric partial melting of a MORB-source mantle during its ascent across the garnet–spinel facies boundary, generating a heterogeneous residual suite (MHL) that was accompanied by the crystallization of gabbroic protoliths for Type II mafic layers with cumulus ultramafic rocks (SDW). Following their initial formation in the shallow oceanic lithosphere, the MHL and SDW suites including the Type II mafic layer underwent profound tectonic burial into the garnet peridotite stability field. The BDH suite, which is a product of high-Mg andesitic magmatism in an arc setting, was incorporated as exotic blocks. Subsequently, the integrated lithological framework was intruded by melts for the formation of Type I (and Type III) mafic layers, which crystallized as garnet pyroxenites. This entire assemblage then underwent tectonic ascent—potentially as a mantle diapir—transitioning from the garnet- to the plagioclase-peridotite stability field. During this exhumation, melt–rock interactions at approximately 50 Ma facilitated the formation of mega-olivine dunite channels, while localized partial melting of the MHL suite yielded characteristic plagioclase-rich veins. Multi-stage metasomatic events driven by the infiltration of slab-derived fluids impacted all lithological units and facilitated the localized crystallization of hydrous phases, specifically phlogopite and pargasite, at ca. 23 Ma. While late-stage processes have overprinted much of the complex, the MHL plagioclase peridotite uniquely preserve isotopic signatures of ancient depletion dating back to 1 Ga. These results provide a comprehensive model for the tectonic transport of mantle lithologies from mid-ocean ridge environments to subduction zones.