Petrogenesis of Corundum-Bearing Mafic Rock in the Horoman Peridotite Complex, Japan

Abstract

A corundum-bearing Type II mafic rock, within the Horoman in many orogenic lherzolite massifs, for example, Ronda peridotite, Japan, was petrologically examined in detail to obtain (Dickey, 1970; Obata, 1980; Garrido & Bodinier, 1999) the P–T paths of the mafic rock as well as of the host peridotite. and Beni Bousera (Kornprobst, 1969) in the Betic–Rifean Of all the mafic rocks documented from the Horoman complex, only orogenic belt, Cabo Ortegal (Girardeau et al., 1989) in the corundum-bearing mafic rock has preserved, at least partly, its northern Spain, Lanzo in the Western Alps (Bodinier, high-pressure mineralogy; all of the others have been completely 1988), and Lherz in the Pyrenees (Bodinier et al., 1987). recrystallized at low pressures. The Type II mafic rocks were initially These mafic layers vary in thickness from a few cenformed at 1·5 layers are sometimes concordant with foliation of the GPa) than the initial condition of formation. Corundum reacted peridotite and frequently exhibit folding and boudinage with clinopyroxene during exhumation of the Horoman peridotite as a result of plastic deformation. Their origins are still down to the plagioclase stability field. The field and petrographical controversial (e.g. Allegre & Turcotte, 1986; Garrido & observations of the Type II mafic rocks (± corundum) coupled Bodinier, 1999; Takazawa et al., 1999). A heterogeneous with published isotopic data suggest a complicated spiral-like P–T mantle relatively enriched in mafic components, including trajectory for the Horoman peridotite. The Type II protolith was pyroxenite–eclogite, will yield voluminous and comformed at low pressure within the peridotite at the time of initial positionally distinctive magmas upon partial melting beformation of the Horoman peridotite as a residue from primitive cause of selective fusion of the mafic components (e.g. mantle at >830 Ma. The Type II mafic rocks, as well as the Hofmann & White, 1982; Hauri, 1996; Hirschmann & surrounding peridotite, then experienced subduction to the garnet Stolper, 1996; Cordery et al., 1997; Takahashi et al., stability field. Finally, the Horoman complex ascended a second 1998). The products of selective fusion may react with time from the garnet peridotite to the plagioclase peridotite stability the more refractory wall-rock leading to refertilized, field. The Horoman peridotite is an example of multiple recycling ‘anomalous’ mantle able to act as a distinctive source in of peridotite within the mantle. later mantle upwelling (Yaxley & Green, 1998). The origins of mafic layers in peridotite complexes are, therefore, very important not only for the origin of mantle