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...

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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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Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the highgrade blueschists at Qingshuigou. White mica 40 Ar/ 39 Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1-453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The 40 Ar/ 39 Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high-grade metamorphism during the same subduction event. From this chronological evidence and the presence of well-developed Silurian remnant-sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.

Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km ¥ 1 km) in a high-grade portion of the Sambagawa schists. The Tonaru mass experienced high-P/low-T prograde metamorphism from the epidote-blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P-T conditions are T = 300-450∞C and P = 0.7-1.1 GPa for the epidote-blueschist facies, and the peak P-T conditions for the eclogite facies are T = 700-730∞C and P ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase-biotite zone were reached. The high-P/low-T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep dP/dT geothermal gradients despite their diverse peak P-T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep dP/dT geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite-bearing tectonic blocks. Therefore, the eclogite-bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later-subducted Sambagawa high-grade schists during their exhumation.