Geologic traverse of the Sambagawa metamorphic belt in central Shikoku, SW Japan:

Carbonaceous material in pelitic schists of the Sanbagawa metamorphic belt in central Shikoku, Japan

c-Axis fabrics and microstructures in quartz schist from the Sambagawa metamorphic belt, central Shikoku, Japan

In the Asemi River region of the Sambagawa metamorphic belt, central Shikoku (Fig.1), dozens of quartz schist layers are intercalated, in which well-defined quartz c-axis fabrics penetratively developed. In this pictorial, we show the close relationship between microstructures and c-axis fabrics in the deformed quartz, which indicates that both the microstructures reflect the deformation geometries caused by ductile flow in the Sambagawa metamorphic belt.The geological structure in the region is characterized by E-Wtrending and Ndipping foliation (Sb), and horizontal mineral lineation (Lm). ThesamplesforFigs.2, 3, 4 and 5 are collected respectively from the metamorphic zones as shown in Fig.1. In all the figures, X, Y and Z directions denote the directions parallel to Iineation, parallel to foliation and normal to lineation, and normal to foliation, respectively, and thesedirections can be approximated as the principal directions of strain ellipsoid(X>Y>Z).

Cooling history of the Sanbagawa metamorphic belt inferred from fission track zircon ages

四国中央部三波川変成帯の変成分帯

Metamorphic zonation and metamorphism of pelitic schists in the higher grade region of the Sambagawa metamorphic belt in central Shikoku, Japan

The Sanbagawa basic schists in the Shiragayama area, central Shikoku contain hematite, ilmenite, rutile, sphene and some magnetite. Mineral assemblages of Fe-Ti oxide and silicate minerals in hematite-bearing ones are significantly different from those in hematite-free variety. Excluding albite, quartz, epidote, chlorite and phengite, the mineral assemblages of hematite-bearing ones are hematite+riebeckitic actinolite+sphene or hematite+crossite+sphene in the lower grade zone and hematite+ilmenite+rutile+hornblende±magnetite in the higher grade zone. In the other variety, the mineral assemblages are actinolite+sphene±rutile in the lower grade zone, and ilmenite+hornblende in the higher grade zone. Combining the mode of occurrence and chemistry of Fe-Ti oxides and sphene as well as the Mn-Fe2+ partitioning among magnetite, hematite and ilmenite, stability and paragenesis were determined. Magnetite, ilmenite and rutile in the hematite-bearing basic schists occur in the garnet and biotite zones, whereas, in the hematite-free ones, rutile is restricted to the garnet zone at the prograde stage, and ilmenite occurs in the biotite zone. Sphene is widespread in all the zones, but its occurrence is restricted to the lower grade zone at the prograde stage of metamorphism; its occurrence in the higher grade zone postdates major mineralization. The stable oxide mineral assemblages are magnetite+hematite, hematite+rutile, magnetite+hematite+ilmenite, magnetite+hematite+rutile, hematite+ilmenite+rutile and magnetite+hematite+ilmenite+rutile. Ilmenite and hematite contain significant amounts of MnO; the maximum MnO content of ilmenite and hematite are 24.1 and 2.1 wt. per cent, respectively. Therefore, the paragenesis of Fe-Ti oxide minerals can be determined only in the FeO-Fe203-Ti02-MnO system. The stability of Fe-Ti oxide minerals and sphene is controlled by the bulk-rock chemistry as well as pressure, temperature and oxygen fugacity.

Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Yact > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.

The higher grade metamorphic zonation of the Sambagawa (= Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg‐Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re‐equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.

In the Asemi-gawa area of intermediate high-pressure and low-temperature Sanbagawa metamorphic belt in central Shikoku, Japan, piemontite-quartz schists are common in which cation exchange between piemontite and garnet has been studied. Piemontite in contact with garnet usually contains two zones in which core is enriched in Mn 3+ and surrounded by a rim rich in Fe 3+ . Garnet is Ca-Febearing spessartine and is slightly heterogeneous in which core rich in Mn is surrounded by a narrow rim poor in Mn and rich in Ca. The chemical composition of piemontite changes in contact with garnet in matrix or whenever it is included in the garnet porphyroblast. In both types of contacts, piemontite tends to be richer in Mn 3+ and poorer in Fe 3+ while garnet tends to be richer in Ca and Fe 3+ and poorer in Mn 2+ . P-T curves were calculated using piemontite-garnet equilibria and show that the chemical variation from core to rim in piemontite-ga rnet pair is related to increasing temperature whilst cation exchange in boundary of piemontite-garnet can be caused by decreasing temperature. Log f (O2)-T calculations show that cores of piemontite-ga rnet pair have been formed under more oxidizing condition than their rims. Calculations also show cation exchange between piemontite and garnet has been occurred under more oxidizing condition than the piemontite-garnet rim.

Thermal Structure of the Sanbagawa Metamorphic Belt in Central Shikoku

Antigorite CPO formation mechanism in the shallow wedge mantle: Case studies from the Besshi and Shiraga bodies of the Sanbagawa metamorphic belt in central Shikoku, Japan

Phase relations of pumpellyite-actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan

Sodic, calcic and subcalcic amphiboles are expressed by the combination of tremolite, edenite, tschermakite, glaucophane, magesioriebeckite and Fe-tremolite. Because such complex compositions of amphiboles provide enough chemical variables to constrain equilibrium conditions of the metabasic system, amphibole composition makes it possible for Gibbs' method to be applied to this system. We made a framework for estimating quantitative P–T paths from compositions of a zoned amphibole in metabasite consisting of amphibole, chlorite, epidote, plagioclase, quartz and water in the system Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–SiO2–H2O. For deriving total derivatives simply, mole fractions of cations in each site were introduced as the variables of amphibole composition. In obtaining non-ideality terms of amphibole activity, we used the regular solution models with 15 Margules parameters derived from published data. Application of the differential thermodynamics to a natural basic schist from the Sanbagawa metamorphic belt in central Shikoku provides both prograde and retrograde paths. The ranges of calculated P–T conditions are in good agreement with the previous study, and calculated compositions of coexisting minerals are consistent with the values from analyses of minerals in the sample. Activity models for amphiboles, especially non-ideality of tremolite–tschermakite (tr–ts) and tremolite–edenite (tr–ed) joins, provide significant effects on the calculated P–T paths. The initial P–T conditions given at the amphibole rim affect the calculated P–T paths slightly, but the shapes and lengths of the paths almost do not change. If compositional changes in each step of the amphibole zoning are small enough, Gibbs' method is able to transform the change of amphibole chemistry into changes of P–T conditions that are independent of the histories of the compositional changes of amphibole. This implies that all amphibole chemistry can be translated into the specific P–T condition, considering the relative compositional change from the reference amphibole, as long as the same mineral assemblage is preserved.

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.