The Sanbagawa (Sambagawa) metamorphic belt in south-west Japan suffered a highpressure intermediate metamorphism of Cretaceous age. The regionally highest grade rocks (epidote-amphibolite facies) of the belt occur in the Besshi district in central Shikoku, and locally contain higher grade masses of coarsegrained metabasic and ultrabasic lithologies, such as the Higashiakaishi peridotite, Iratsu epidote-amphibolite, Nikubuchi peridotite and Sebadani metagabbro masses (Kunugiza et al. 1986) (Fig. 1). These masses have been regarded as large-scale tectonic blocks (maximum of 7 x 3 km 2) in a m61ange zone which formed at the initial stage of the uplifting of the Sanbagawa belt (Takasu 1984, Kunugiza et al. 1986. In this paper, quite a diverse origin and metamorphic history of these blocks will be revealed, and the tectonics associated with the development of the tectonic blocks will be discussed.
Metamorphic history of the tectonic blocks Sebadani (SB) metagabbro massThe Sebadani (SB) metagabbro mass, 0.3 x 0.2 km 2, is located in the albite-biotite zone, and consists of garnet-epidote amphibolites and relict eclogite which survived epidote amphibolization. The banding of melanocratic hornblende-rich and leucocratic epidote-rich layers records the layering of previous pyroxene and plagioclase layers of cumulate gabbro. Takasu (1984) recognized two stages of eclogite equilibrium in the SB mass; a corecore pair of 720-750°C and 12-20 kbar, and a rim-rim pair of 610-630°C and 10-17 kbar. The basic Sanbagawa schists, up to 20 m from the SB mass, underwent contact metamorphism at high pressure that produced garnet and omphacite (maximum Jd 36%) porphyroblasts from the albite-epidote amphibolite-facies assemblage. Omphacite occurs as randomly oriented poikilitic porphyroblasts including epidote and barroisite. Fe-Mg partitioning between garnet and omphacite in the basic schists gives 630-650°C, which is similar to the temperature of the rim-rim pair of the SB mass.The garnet in the pelitic schists in the contact aureole shows a peculiar composite zoning, consisting of the core and mantle with a textural and chemical discontinuity between them. The core, as well as the mantle, of the garnet shows normal zoning with outward decrease of MnO and increase of MgO. Two zones of normal zoning with discontinuity suggest two events of prograde metamorphism with a resorption stage between them. The former metamorphism corresponds to the prograde period of the Sanbagawa metamorphism, and the latter to the contact metamorphism by the Sebadani mass (Takasu 1986).The formation history of the SB mass is as follows: layered gabbro -~ high-temperature eclogite --~ emplacement into the Sanbagawa belt and contact metamorphism of the surrounding schists. The Sanbagawa schists around the SB mass have the following metamorphic history: pelitic and basic sedimentary rocks prograde metamorphism up to the epidoteamphibolite facies (Sanbagawa metamorphism) --~ resorption of garnet in the pelitic schist ~ contact metamorphism to form eclogite (Fig. 2...
The higher-grade region of the Sanbagawa metamorphic belt in Shikoku and the Kii peninsula contains metagabbro, peridotite, and serpentinite. These rocks occur exclusively in the garnet, albite-biotite, and oligoclase-biotite zones, and tend to have equilibrium mineral assemblages stable with respect to the mineral zones where they occur. Thermal histories of previous equilibrium stages can be discerned through not only the mineralogy of relics, but also the texture, megascopic structure, and bulk composition of the rocks. The protoliths were layered gabbro, cumulate and residual peridotites, and garnet clinopyroxenite. Some of these rocks were in the eclogite facies before emplacement into the Sanbagawa schists and others were in the granulite facies. They were emplaced into the metamorphic regime by solid intrusion during a later stage of the schistosity formation, that is, emplacement was syntectonic. Some peridotites were serpentinized before attaining equilibrium assemblages of the Sanbagawa metamorphism; their emplacement was during an earlier stage of the metamorphism, but not at the sedimentary stage. The source region of those exotic blocks may have been a lower crust-upper mantle region of an island arc.
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