Polybaric Petrogenesis of Mafic Layers in the Horoman Peridotite Complex, Japan

Abstract

Two major types of mafic granulite layers occur within the Horoman host peridotite (e.g. Nicolas & Jackson, 1982). [In this paper we use ‘mafic layers’ to describe pyroxenites to peridotite, an 8 km× 10 km× 3 km orogenic lherzolite exposed ‘gabbroic’ rocks (mafic granulites) whose composition in the high-T and low-P Hidaka metamorphic belt of Hokkaido, and mineral assemblage differs substantially from the Japan. The mineral assemblages and textures of these layers reflect host peridotites.] Hypotheses for the origin of these subsolidus reactions occurring during uplift from the upper mantle layers in orogenic peridotites include: (1) solidification to the crust. Nevertheless, their whole-rock compositions can be used of melt (Niida, 1984; Shiotani & Niida, 1997); (2) to infer the primary mineralogy of these layers, and a genetic crystal cumulates in melt conduits or sills (Loubet & relationship to melts geochemically similar to mid-ocean ridge basalts Allegre, 1982; Bodinier et al., 1987; Suen & Frey, (MORB). The intralayer compositional variation of Type I layers 1987; Bodinier, 1988; Pearson et al., 1993; Kumar et (Al–Ti augite type mafic granulites) shows that the centers formed al., 1996); (3) recycling of ancient subducted oceanic as garnet clinopyroxenites in equilibrium with an incompatible crust that was dispersed and thinned during mantle element depleted melt that crystallized to form the margins. In convection (Polve & Allegre, 1980; Allegre & Turcotte, contrast, the Type II layers (Cr-diopside type mafic granulites) 1986). Subsequent to their formation these layers may formed at relatively shallow depths and are much older, ~830 Ma, have experienced partial melting (Loubet & Allegre, than the Type I garnet pyroxenites, which formed at ~80 Ma. 1982) and interacted with metasomatic fluids (Garrido The temporal sequence supports the hypothesis that the Horoman & Bodinier, 1999). Ultimately, some mafic layers may peridotite represents shallow MORB-related oceanic mantle that be important source components for basalts (e.g. had subsided to deeper mantle depths before crustal emplacement. Hirschmann & Stolper, 1996). In addition to layers within massive peridotites, pyroxenites also occur as discrete xenoliths and as portions of composite pyroxenite–peridotite xenoliths