Polybaric petrogenesis of mafic layers in the Horoman peridotite complex, Japan

Two major types of granulite layers occur within the Horoman peridolite, an 8 km x 10 km x 3 km orogenic therzolite exposed in the high-T and low-P Hi daka metamorphic belt of Hokkaido, Japan. The mineral assemblages and textu res of these layers reflect subsolidus reactions occurring during uplift fr om the upper mantle to the crust. Nevertheless, their whole-rock compositio ns can be used to infer the primary mineralogy of these layers, and a genet ic relationship to melts geochemically similar to mid-ocean ridge basalts ( MORB). The interlayer compositional variation of Type I layers (Al-Ti augit e type mafic granulites) shows that the centers formed as garnet clinopyrox enites in equilibrium with an incompatible element depleted melt that cryst allized to form the margins. In contrast, the Type II layers (Cr-diopside t ype mafic granulites) formed at relatively shallow depths and are much olde r, similar to 830 Ma, than the Type I garnet pyroxenites, which formed at s imilar to 80 Ma. The temporal sequence supports the hypothesis that the Hor oman peridolite represents shallow MORB-related oceanic mantle that had sub sided to deeper depths before crustal emplacement.