Sanbagawa Schists Research Articles

Pressure-temperature trajectory of the Sanbagawa metamorphism deduced from garnet zoning

Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

Notes on petrography and rock-forming mineralogy (16) Sector-zoned epidote from Sanbagawa schist in central Shikoku, Japan.

Sector-zoned epidotes have been found from lower-grade (pumpellyite-actinolite facies) Sanbagawa schists of basic composition in the southern Oboke area, central Shikoku. The sector-zoned epidotes are composed of sector-zoned core and non-sector-zoned rim. The core has ten growth sectors; two {100}, two {001}, two {10_??_}, and four {110}. The strongest F-face, {100} growth surface or sector, has the highest XFe among the sectors. Intra-sectoral trend of XFe in {100} sector is opposite to those in {110} and {10_??_}, suggesting the changes in the rate of disequlibrium crystal growth of the epidote with time. The presence of sector-zoning suggests that model of interface equilibrium is not always valid in regional metamorphism.

Notes on petrography and rock-forming mineralogy (13). Detrital garnets from pelitic Sanbagawa schists in the Bessi area, central Shikoku.

Detrital garnets were found from some pelitic schists in the garnet, albite-biotite and oligoclase-biotite zones of the Sanbagawa belt in the Bessi area, central Shikoku. They are subrounded to rounded, and are always armoured by the garnet formed at the Sanbagawa metamorphism. There is a distinct chemical discontinuity at the boundary between the detrital and Sanbagawa garnets, and some of the former have composition exotic to the pelitic Sanbagawa schists.

Notes on petrography and rock-forming mineralogy (9). Coexisting 3T and 2M1 muscovites in the Sanbagawa schists.

Muscovites from the Sanbagawa schists exhibit a wide range of 2Vx, from 0° to 40°, even in a hand specimen and often in a single muscovite flake. This is due to the superposition of 3T and 2M1 polytypes of muscovite, sometimes in a scale larger than the wavelength of visible light, and sometimes less than that. Chemical environments and metamorphic grade show no preference to the types of polytypes in the range of metamorphic grade covering the glaucophane schists and epidote amphibolite facies.

Notes on petrography and rock-forming mineralogy. 7. Zonal structure of albite porphyroblast from basic Sanbagawa schists in central Shikoku.

Notes on petrography and rock-forming mineralogy. 7. Zonal structure of albite porphyroblast from basic Sanbagawa schists in central Shikoku.

THERMAL STRUCTURE OF THE SANBAGAWA METAMORPHIC BELT IN CENTRAL SHIKOKU

A detailed metamorphic zonal mapping is being in progress on the Sanbagawa metamorphic belt in central Shikoku. The mapping is based upon the distribution of index minerals, garnet and biotite in pelitic schists, and on the sliding equilibrium among silicate and oxide minerals. The distribution of mineral zones has revealed a peculiar thermal structure of the metamorphic complex that the highest-grade rocks occur in the middle of apparent stratigraphy. A large scale recumbent fold, with south vergency and extending for more than 20km, is postulated as a possible structural interpretation.It is concluded, as the most probable model we could imagine at the moment, that before the maximum temperature of metamorphism was reached, the Sanbagawa schists had been metamorphosed in more or less normal thermal regime that the temperature had increased downwards. Then a large scale recumbent fold took place, separating the higher-grade rocks from the heat source and bringing them in between the lower-grade ones. This recumbent fold was accompanied by the start of the uplift of the whole metamorphic complex, while continuing metamorphic reactions with decreasing temperature and pressure.The fact that the Sanbagawa belt is overturned suggests that a very distinctive crustal shortening took place in the present day Sanbagawa terrain in the Mesozoic time, and that the present day distribution of pre-Tertiary geologic units in the outer zone of the south-western Japan can hardly be in situ.

Pyrrhotite from the Sanbagawa pelitic schists of the Shiraga-yama area, central Shikoku, Japan

Sulfides and oxides in pelitic schists of the Sanbagawa metamorphic terrain in the Shiraga-yama area, central Shikoku, Japan, are pyrrhotite, pyrite, chalcopyrite, ilmenite, and rutile. The most abundant assemblage of opaque minerals is pyrrhotite+pyrite+chalcopyrite in the chlorite zone and a lower-grade part of the garnet zone, and pyrrhotite+chalcopyrite+ilmenite+rutile+ (pyrite) in a higher-grade part of the garnet zone and the biotite zone. In this area, in addition to monoclinic Fe7S8 pyrrhotite which is common, intermediate pyrrhotite with Fe9S10 does occur, and also Fe9Sio+pyrite assemblage is observed. This fact suggests that the Sanbagawa schists were metamorphosed above 310°C and that, although the majority of hexagonal pyrrhotite reacted with pyrite as cooling of metamorphic rocks proceeded, some pyrrhotite persisted to keep high temperature chemistry. It was also revealed by this study that pyrrhotites in the samples collected at the surface exposures are more or less oxidized but those collected from the drill cores are little oxidized.

Petrologic comparison of the franciscan and sanbagawa metamorphic terranes

Mafic schists in California carry the typical blueschist facies assemblage of glaucophane—crossite + lawsonite 4 sphene + chlorite ± aragonite or calcite ± pumpellyite. Albite, epidote and actinolite are not common in “in situ” metabasalts. In contrast, typical Sanbagawa schists contain crossite or actinolite + albite + epidote + chlorite ± calcite; the highest-grade rocks generally contain garnet and blue—green hornblende, lower grade rocks may contain pumpellyite and/or stilpnomelane, rarely lawsonite. Franciscan metaclastic rocks show three principle assemblages, all of which contain the phases quartz 4 white mica + chlorite + sphene ±lawsonite ± stilpnomelane: albite 4 calcite, albite 4 aragonite; and jadeitic pyroxene ± aragonite ± glaucophane. Quartzose Sanbagawa metaclastic rocks represent the albite ± calcite type compatibilities; lawsonite, aragonite and jadeitic pyroxene are either absent or are very rare. Many Franciscan siliceous metasediments contain, in addition to quartz, the phases riebeckite, stilpnomelane and/or spessartine. In contrast, many Sanbagawa siliceous schists carry abundant piemontite ± magnesioriebeckite in addition to spessartine. Correlation of the above mineral assemblages with experimental data suggests that blueschist-facies metamorphism took place in the Franciscan terrane at high fluid pressures, and involved temperatures of 200–300°C and lithostatic pressures of 6.5–8 kbar. Apparently Sanbagawa metamorphism reflected physical conditions transitional between the blueschist, greenschist and albite—epidote—amphibolite facies, approximating 250–400°C at 5–7 kbar lithostatic pressure.

Chlorite from Sanbagawa Schists

Chlorite from Sanbagawa Schists

On the so-called "Soda-metasomatism" of the Sanbagawa Schists in the Bessi District

On the so-called "Soda-metasomatism" of the Sanbagawa Schists in the Bessi District