Sanbagawa Metamorphic Terrain Research Articles

関東山地三波川帯における石墨化度の測定精度の再検討と連続露頭でのシャッフルド・カード境界

Precision of the degree of graphitization (GD) is evaluated based on repeated experiments on a sample of pelitic schist, and analyses on ten pelitic schists collected from an outcrop. The experimental error is ±2 in two sigma confidential level at GD=31 and GD=26, respectively. The results suggest that if GD value difference is larger than 6, two neighboring samples have the discontinuous metamorphic grade. The systematic sampling and experiments along the direction normal to the strike, revealed the discontinuous boundaries of the metamorphic grade existing at a upper stream of the Sanbagawa-river and at an outcrop of the Arakawa-river, i.e., the boundaries between the shuffled cards Miyashiro (1994). The former boundary is serpentinitic greenschist within the pelitic schists, the latter one, the thrust plane in pelitic schists. A thick layer having continuous GD value also exists in the Sanbagawa metamorphic terrain as revealed by the experiments of the Kamabuse-yama boring core of 280m depths.

Shuffled‐cards structure and different P/T conditions in the Sanbagawa metamorphic belt, Sakuma‐Tenryu area, central Japan

AbstractThe Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d002 of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A1, A2, B1 and B2 for the Shirakura Unit; and I1, I2, II1 and II2 for the Sejiri Unit. The metamorphic grades of A1, A2, I1 and I2 are included in the chlorite zone, and that of B1, B2, II1 and II2 in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A2 and B1 zones is the same as that between I2 and II1 zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.

Manganoan winchite in a quartz schist from the Saruta-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

Manganoan winchite (up to 4.19 wt.% MnO) occurs as cores of zoned amphibole in a quartz schist from the Saruta-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan. The quartz schist consists of quartz, albite, amphibole, muscovite, garnet, chlorite, allanite, calcite, apatite, stilpnomelane, tourmaline, hematite and pyrite. The amphibole is euhedral to subhedral in form (up to 700 μm long). The core of the amphibole ranges in composition from barroisite through winchite to manganoan winchite. The amphibole is rimmed by alkali amphibole (mainly magnesioriebeckite) and winchite. Garnet in the quartz schist is rich in spessartine component (59-79%).

Strontian hollandite in a quartz schist from the Saruta-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

Strontian hollandite in a quartz schist from the Saruta-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

関東山地児玉‐長瀞町三波川変成域における層状体の構造的累積による地質構造

関東山地児玉‐長瀞町三波川変成域における層状体の構造的累積による地質構造

Ardennite in a quartz schist from the Asemi-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

Ardennite is found to occur in a piemontite-bearing quartz schist from the Asemigawa area in the Sanbagawa metamorphic terrain, central Shikoku. The ardennite-bearing quartz schist contains quartz, piemontite and garnet as major constituents with subordinate amounts of talc, chlorite, albite, calcite, hematite, braunite, rutile and apatite, together with ardennite. Ardennite is pale golden yellow and occurs as subhedral and prismatic crystals up to 1 mm in length. It is optically positive with 2VZ=57±3°. It is orthorhombic and unit cell parameters are a=5.828(2) A, b=18.540(8) A, c=8.692(3) A and V=939.1(6) A3. As2O5 and V2O5 contents of ardennite reach 10.8 and 1.3 wt.% in maximum, respectively. The empirical formula of the ardennite is (Ca0.53Mn3.67)(Mg1.02Cu0.03Fe0.203+Al4.47Ti0.01)(As0.80V0.09) Si5.25O22 (OH)6. It is slightly poor in (As + V) and rich in Si compared with the ideal formula of ardennite. Piemontite contains SrO of 0.7wt.% in maximum.

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.

Growth history of garnet in pelitic schists of the Sanbagawa metamorphic terrain in central Shikoku

During the prograde metamorphism of the Sanbagawa metamorphic terrain in central Shikoku, garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote, etc. In the quartz-albite layer, it has grown euhedrally. In the micaceous layer, its crystal morphology varies according to the different minerals in contact and their different orientations. Garnet enclosed in or sandwiched between micas is always ellipsoidal, extended parallel to the basal planes of the micas. However, when the edge of a garnet has intruded at an acute angle into the micas, euhedral garnet has grown. It is concluded that the growth of garnet at the interface with micas ceased when the {110} surface of garnet came into contact with the basal plane of the micas in the tectonic c-axis direction. Other models, such as chloritization-resorption, pressure-solution, and mechanical-erosion or grinding, do not adequately explain the origin of the flattened garnet. During retrograde metamorphism, chloritization and resorption of garnets took place in the matrix, resulting in the rough and irregular surface of garnet. Garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism.

Coexisting sodic augite and omphacite in a Sanbagawa metamorphic rock, Japan

Coexisting sodic augite and omphacite were found in a zoisite amphibolite from the Iratsu epidote amphibolite mass in the Sanbagawa metamorphic terrain of central Shikoku, Japan. The occurrences of the sodic augite-omphacite pairs are classified into four types by texture: independent, composite, intergrowth and exsolution types. Sodic augite and omphacite of the independent and composite types (pair A) have XNa (=Na/(Na + Ca)) = 0.15 and 0.35, respectively, and were stable in the epidote amphibolite facies during the Sanbagawa progressive metamorphism. On the other hand, XNa values of sodic augite and omphacite of the intergrowth and exsolution types (pair B) are 0.10 and 0.44, respectively. The Na-poor augite and Na-rich omphacite of the pair B were formed by re-equilibration of the pair A at lower temperature. The pair A of the Iratsu sample suggests that a compositional gap lies between sodic augite and C2/c omphacite under epidote amphibolite facies conditions, and is in marked contrast to the coexistence of sodic augite and P2/n omphacite reported from some low-grade, high-pressure metamorphic terrains. A possible phase diagram to explain the chemistry and mode of occurrence of the coexisting sodic pyroxenes is proposed.

Isotopic studies of marbles in the Sanbagawa metamorphic terrain, central, Shikoku, Japan.

Carbon, oxygen and strontium isotopic studies were carried out on marbles occurring in crystalline schists and epidote amphibolites of the Sanbagawa metamorphic terrain, central Shikoku, Japan, in order to estimate metamorphic temperatures and to elucidate their origin. Carbon isotopic fractionation between calcite and graphite shows the metamorphic temperature of 460°C at the transitional part between the garnet and albite-biotite zones. Marbles are isotopically classified into two groups. (1) some marbles in epidote amphibolite masses show characteristically negative δ13C values and low 87Sr/86Sr ratios. These marbles are interpreted as have been derived from magmatic or deep-seated carbonates. (2) marbles collected from the crystalline schists and from the marginal part of epidote amphibolite masses, have high 87Sr/86Sr ratios and δ13C values similar to those of typical sedimentary carbonates. They were probably derived from sedimentary carbonates and/or carbonates re-equilibrated with metamorphic fluid segregated from crystalline schists during the Sanbagawa metamorphism.

Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic belt

The oligoclase‐biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa= 0.10–0.28)‐bearing assemblages in pelitic schists, and represents the highest‐grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH2O = Psolid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high‐pressure type of metamorphic belt and with those in a generalized‘upper staurolite zone’as an example of a medium‐pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:

Oligoclase-biotite zone of the Sanbagawa metamorphic terrain in the Bessi district, central Shikoku, Japan

Oligoclase-biotite zone of the Sanbagawa metamorphic terrain in the Bessi district, central Shikoku, Japan

Notes on petrography and rock-forming mineralogy (11). Green biotite in Sanbagawa basic schists in the Kanto Mountains, Japan.

In the northeastern part of the Kanto Mountains, central Japan, in basic schists, green biotite widely occurs in a lower-grade part where garnet is common in pelitic and psammitic schists, while brown biotite is present in a higher-grade part where brown biotite commonly occurs in them. Comparing with brown biotite from higher-grade part of the Sanbagawa metamorphic terrain in this area and central Shioku, green biotite in lower-grade part of this area contains lower Al2O3 and higher Fe2O3, and slightly lower TiO2. On the basis of the paragenetic relation of the green biotite-bearing assemblages, the occurrence of green biotite is explained by high Fe2O3, (FeO+MgO), and low Al2O3 contents of the host rocks. Brown biotite in basic schists in a higher-grade part is formed by the following reaction; green biotite+ (Fe, Mg-rich) chlorite+epidote+albite+quartz=brown bitite+hornblende+(Al-rich) chlorite+magnetite+H2O. This reaction enlarges the stability field of biotite, and in a higher-grade part, brown biotite-bearing assemblages widely occurs in the common basic schists.

三波川帯,八幡浜地域のざくろ石一単斜輝石角閃岩類

Garnet-clinopyroxene amphibolites from the Kawamai tectonic mass emplaced in the Sanbagawa metamorphic terrain of the Yawatahama district, southwestern Shikoku have been studied chemically. The amphibolites are characterized by the assemblage brown hornblende-garnet-clinopyroxene-calcic plagioclase indicating, at least, the amphibolite facies.Their bulk rock compositions are always undersaturated with silica. The ferromagnesian minerals are saute with appreciable amounts of acmite and Tschermak's components, Ca-rich almandine and ferroan pargasite. From chemical comparison of clinopyroxene and of hornblende the formation temperature seems to be higher in the Kawamai amphibolites than in the Iratsu amphibolites. The partition coefficient of Fe and Mg between garnet and clinopyroxene suggests that the Kawamai amphibolites equilibrated at a temperature higher than or similar to that of the Higashi-Akaishi-yama garnet clinopyroxenites.It can be concluded that the Kawamai mass suffered the Sanbagawa metamorphism corresponding to the amphibolite facies of which Ps-T conditions were similar to those of the Higashi-Akaishi-yama mass, but prevailing PH2O must have been different between the two. Retrogressive reactions have been caused by the subsequent uplift of the Kawamai mass during the Sanbagawa metamorphism.

Surface microtopography of metamorphic white micas

Gold decoration and platinum-carbon-shadowing techniques of electron microscopy have been applied to study the surface microtopographs of noncleavage (001) faces of porphyroblastic white micas and sericite crystals collected from the chlorite, biotite, and garnet zones in the Shiragayama area in the Sanbagawa metamorphic terrain, Shikoku, Japan.

Unusual pyralspite-ugrandite garnets from the Sanbagawa metamorphic rocks in central Shikoku.

Unusual garnets, exceedingly close to the mid-point between the two end members almandine and grossular, have been found in metagabbros of the epidote amphibolite facies in the Sanbagawa metamorphic terrain. The major minerals associated with them are subcalcic hornblende and zoisite. The chemical compositions of these garnets range from Alm38Spe12Pyr1Gro49And0 to Alm54Spe3Pyr3Gro39And1, thus filling the previously empty gap in the pyralspite-ugrandite series of glaucophanitic metamorphic rocks. The reactions concerned in the formation of these minerals, assuming different environments, are given. Most appropriate equations can not, however, be definitely determined because of no exact mineralogical information on the intrusive rocks before metamorphism. Each of the solid solution spessartine-grossular and almandine-grossular covers, at least, the range from pyralspite end member to about 50 molecular per cent grossular under the conditions of glaucophanitic metamorphism. Ca-rich pyralspites are widespread in, but not restricted to, glaucophanitic metamorphic rocks.

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.

Biotite zone of Sanbagawa metamorphic terrain in the Siragayama area, central Shikoku, Japan

Biotite zone of Sanbagawa metamorphic terrain in the Siragayama area, central Shikoku, Japan

Low-grade Progressive Metamorphism of Pelitic Schists of the Sazare area, Sanbagawa Metamorphic Terrain in central Sikoku, Japan

Low-grade Progressive Metamorphism of Pelitic Schists of the Sazare area, Sanbagawa Metamorphic Terrain in central Sikoku, Japan

GEOLOGIC STRUCTURE AND METAMORPHIC ZONING OF THE NORTHERN EXTREMITY OF THE SANBAGAWA METAMORPHIC TERRAIN IN THE KANTO MOUNTAINS

The studies on the geologic structure and the metamorphic zoning of the crystalline schists which expose in the northern extremity of the Sanbagawa metamorphic terrain of the Kanto Mountains, central Japan have been carried out by the present writers. The geologic structure in this area is gentle, being usually flat without large-scale folding. On the basis of the stability relation of metamorphic minerals, this district may be divided into three zones. Zone I is characterized by the occurrence of pumpellyite and alkaliamphibole. In zone II, pumpellyite and alkali-amphibole are not stable and “green biotite” appears. Common hornblende and “brown biotite” are observed in zone III. In term of mineral facies concept, zones I, II and III correspond to the glaucophane schist facies (lower pressure side), the green schist facies and the epidote-amphibolite facies, respectively. The metamorphism of zones I, II and III is progressive one, and the metamorphic grade increases southwards. Consequently, the present writers think the existence of the center line which attains to the highest temperature of the Sanbagawa metamorphism in the Kanto mountains.