Sanbagawa Metamorphic Research Articles

Brief history of petrotectonic research on the Sanbagawa Belt, Japan

Abstract Petrological study of the Sanbagawa schists was initiated by B. Koto (1856–1935) and extensive petrographic works were performed by J. Suzuki (1896–1970) and Y. Horikosi (1905–1992), who studied in the Besshi area of central Shikoku. Petrological work based on the mineral facies concept of P. Eskola (1883–1964) was initiated in Japan in the 1950s by A. Miyashiro on the low pressure/temperature (P/T) Abukuma complex, and then by Y. Seki and S. Banno on the high P/T Sanbagawa Metamorphic Belt. A unique inverted thermal structure was established by researchers in the 1970s. Therefore, mainly geological and petrographic features of the Sanbagawa Belt were established by the 1990s, and contemporary researchers are now testing the classical images using the new and quantitative viewpoints of geochronological, structural, tectonic, and thermal modeling.

Strain geometries in the Sanbagawa Metamorphic Belt inferred from deformation structures in metabasite

Abstract Ductile deformation in the Sanbagawa Metamorphic Belt has been considered to be characterized by uniaxial elongation parallel to the east–west‐trending lineation, based on strain analysis of radiolarian fossils in metachert. However, only limited data were available to test this idea in strongly recrystallized schists. Strain measurements on deformed pillow structures in central Shikoku show uniaxial flattening, whereas pressure shadows around pyrite in the Kanto Mountains indicate constrictional strain. In addition, in the low‐grade part of the Sanbagawa Metamorphic Belt belonging to the Northern Chichibu Belt in the Kanto Mountains, deformed amygdules in pillow lava show flattening strain, which is consistent with the results of strain measurements using radiolarian fossils in the same area. These observations, together with the results of other strain analyses reported so far, indicate that the strain field is not uniform and that the east–west‐trending shearing and stretching were not pervasive in the Sanbagawa Metamorphic Belt.

The parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt, Shikoku, southwest Japan, based on whole-rock geochemistry

In order to identify the parentage of low-grade metasediments in the Besshi unit of the Sanbagawa Metamorphic Belt in Shikoku, southwest Japan, we have studied the bulk chemistry of psammitic and pelitic rocks in the Sanbagawa Belt and in related nonmetamorphosed Middle/Late Jurassic–Cretaceous accretionary complexes (Southern Chichibu and Northern Shimanto Belts). Tectonostratigraphic relationships and paleontological evidence suggest a post-Middle Jurassic depositional age for the protoliths of the Sanbagawa psammitic/pelitic schists. Zr/TiO 2 ratios of metasandstones in the Besshi unit differ from those of the Middle/Late Jurassic sandstones in the Southern Chichibu Belt. Besshi metasandstones from the Iya–Inouchidani area have chemical affinities with sandstones from the KSII unit (Coniacian–Campanian) of the Northern Shimanto Belt. The Besshi unit in the Asemi–Tachikawa area can be subdivided into northern and southern parts based on the geochemistry of the metasandstones and metapelites. The geochemistry of the metasediments suggests that the parents of the northern and southern parts are the KSII unit and KSIc unit (Late Albian–Early Coniacian), respectively. The depositional ages of the protoliths of the Besshi metasandstones and metapelites are thus inferred to be Late Albian–Early Coniacian (southern part of the Asemi–Tachikawa area), and Coniacian–Campanian (Iya–Inouchidani area and the northern part of the Asemi–Tachikawa area). These conclusions are consistent with the radiometric ages of metasediments from Asemi–Tachikawa, and imply that most of the low-grade metasediments in the Sanbagawa Belt are an underplated, deeper facies of the Cretaceous Shimanto accretionary complex. Protoliths of the Besshi unit represent muddy facies with greenstones, whereas the KSIc and KSII units in the Shimanto Belt mainly consist of sandy facies of probable trench-fan and trench-wedge origin. The lithologic contrast between the KSIc/KSII and Besshi units may be explained by selective underthrusting and underplating of hemipelagic mud deposited outboard of a trench, and as offscraping of turbidite sequences deposited within a trench.

Geochemistry and origin of piemontite-quartz schists in the Sanbagawa Metamorphic Belt, central Shikoku, Japan

In the Sanbagawa Metamorphic Belt of central Shikoku, geochemical characteristics are used to demonstrate that piemontite-quartz schists had a manganiferous sedimentary protolith. The Sanbagawa Metamorphic Belt was formed during a Cretaceous regional intermediate high-pressure type of metamorphism which is present throughout Southwest Japan. Based on a detailed mineralogical study of the Asemi-gawa and Besshi areas in central Shikoku, the Sanbagawa piemontite-quartz schists can be divided into talc-bearing and talc-free types. Phlogopite occurs only in talc-bearing assemblages and the modal percentage of piemontite in talc-bearing types is higher than in talc-free types. Plotting analyses of piemontites and garnets from talc-bearing and talc-free types in the Al 2O 3–Fe 2O 3–Mn 2O 3–MnO(Al–Fe 3+–Mn 3+–Mn 2+) diagram, for the Asemi-gawa and Besshi areas separately, shows that the piemontite which accompanies talc is richer in Mn 3+ and poorer in Fe 3+. Also, garnets from talc-bearing assemblage contain a slightly higher amount of Mn 2+ and a lower amount of Al than talc-free types. The Al 2O 3–SiO 2–Fe 2O 3+FeO plot shows that Sanbagawa piemontite-quartz schists have a composition between deep-sea pelagic clay and pure silica. By the use of Pearson's product moment correlation coefficient method, and plotting elements against each other after removing silica and volatiles, it is possible to distinguish four components which are able to explain the variance of the samples. These components are: (first component: Al 2O 3, TiO 2, MgO, Na 2O, K 2O, Zr, Rb, Y, Nb and to lesser degree Fe 2O 3, FeO, Ni, Th, Ba); (second component: SiO 2); (third component: MnO) and (fourth component: CaO). Al 2O 3, TiO 2, Zr and other elements with positive coefficient relations with them have a detrital origin. Biogenic origins are considered for CaO and SiO 2 and it was concluded that MnO was derived from an hydrothermal source. Because the Sanbagawa Metamorphic Belt has been subjected to much detailed research and also contains talc-bearing and talc-free piemontite-quartz schists, it provides the best opportunity to find an explanation for the occurrence of talc in some types of piemontite schist and its absence from others. Mineralogical and geochemical evidences show that the oxygen fugacity for talc-bearing and talc-free types is similar. Oxygen fugacity is so high that almost all of the iron is present as Fe 3+ in hematite. Pressure and temperature is considered to have been constant for talc-bearing and talc-free types in each locality because the samples were collected from a known metamorphic context. The talc absent/present problem is discussed using phase relations of the samples with the greatest number of phases in each locality. Consideration of talc, clinochlore, piemontite and garnet in the system Al 2O 3–Mn 2O 3/MnO–MgO–CaO(ACMMn), with excess phases albite, quartz, hematite, phengite and water, shows that talc coexists with piemontite and garnet which are richer in the piemontite and spessartine end-members, respectively.

An Effect of the Method of Structural Improvement by Vertical Forepoling on the Ground of the Sanbagawa Metamorphic Belt in Shikoku

An Effect of the Method of Structural Improvement by Vertical Forepoling on the Ground of the Sanbagawa Metamorphic Belt in Shikoku

A re‐evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe

Known eclogite occurrences in the Sanbagawa metamorphic belt of SW Japan are dominantly in metagabbro bodies which have complex polyphase metamorphic histories. These bodies are generally described as tectonic blocks and their relationship to the Sanbagawa metamorphism is unclear. New findings of foliated eclogite in the Seba and Kotsu areas show that eclogite facies metamorphism is much more widespread than generally thought. Evidence that the foliated eclogite units originated as lavas or sediments implies that these units can be treated as a high‐grade part of the subduction‐related Sanbagawa metamorphism. Although separated by an along‐strike distance of 80 km, the Seba and Kotsu eclogites have very similar garnet and omphacite compositions, suggesting that they were formed under similar metamorphic conditions. However, differences in the associated retrograde assemblages (epidote–amphibolite in the Seba unit and epidote–blueschist in the Kotsu unit) suggest contrasting P–T paths. In both units, the eclogite rocks occupy the highest structural level of the Sanbagawa belt and overlie rocks metamorphosed at lower pressure. The lower boundary to the eclogite units is therefore a major tectonic discontinuity locally decorated with lenses of exotic material. These features can help trace the boundary into other areas. The previously known outcrops of eclogite show enough similarities with the newly found areas to suggest that all the eclogite facies rocks in the Sanbagawa belt constitute a single nappe that lies at the highest structural levels of the orogen.

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.

98/01390 Fluid inclusion study of the Kirishima geothermal system, Japan: Sawaki, T. et al. Geothermics, 1997, 26, (3), 305–327

In order to identify the parentage of low-grade metasediments in the Besshi unit of the Sanbagawa Metamorphic Belt in Shikoku, southwest Japan, we have studied the bulk chemistry of psammitic and pelitic rocks in the Sanbagawa Belt and in related nonmetamorphosed Middle/Late Jurassic–Cretaceous accretionary complexes (Southern Chichibu and Northern Shimanto Belts). Tectonostratigraphic relationships and paleontological evidence suggest a post-Middle Jurassic depositional age for the protoliths of the Sanbagawa psammitic/pelitic schists. Zr/TiO2 ratios of metasandstones in the Besshi unit differ from those of the Middle/Late Jurassic sandstones in the Southern Chichibu Belt. Besshi metasandstones from the Iya–Inouchidani area have chemical affinities with sandstones from the KSII unit (Coniacian–Campanian) of the Northern Shimanto Belt. The Besshi unit in the Asemi–Tachikawa area can be subdivided into northern and southern parts based on the geochemistry of the metasandstones and metapelites. The geochemistry of the metasediments suggests that the parents of the northern and southern parts are the KSII unit and KSIc unit (Late Albian–Early Coniacian), respectively. The depositional ages of the protoliths of the Besshi metasandstones and metapelites are thus inferred to be Late Albian–Early Coniacian (southern part of the Asemi–Tachikawa area), and Coniacian–Campanian (Iya–Inouchidani area and the northern part of the Asemi–Tachikawa area). These conclusions are consistent with the radiometric ages of metasediments from Asemi–Tachikawa, and imply that most of the low-grade metasediments in the Sanbagawa Belt are an underplated, deeper facies of the Cretaceous Shimanto accretionary complex.Protoliths of the Besshi unit represent muddy facies with greenstones, whereas the KSIc and KSII units in the Shimanto Belt mainly consist of sandy facies of probable trench-fan and trench-wedge origin. The lithologic contrast between the KSIc/KSII and Besshi units may be explained by selective underthrusting and underplating of hemipelagic mud deposited outboard of a trench, and as offscraping of turbidite sequences deposited within a trench.

四国東部における三波川変成帯と黒瀬川地帯との構造的累重関係 徳島県上勝町北東部に分布する弱変成岩類のＫ‐Ａｒ年代

四国東部における三波川変成帯と黒瀬川地帯との構造的累重関係 徳島県上勝町北東部に分布する弱変成岩類のＫ‐Ａｒ年代

Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Summary The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic ‘D 1 ’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D 2 ’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D 1 ’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D 3 ’ folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.

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.

四国中央部三波川変成帯の超塩基性岩体の変成作用と起源

In the Sanbagawa metamorphic belt of central Shikoku, many dunitic and wehrlitic ultramafic bodies occur intercalated with the pelitic schists of the Upper member of the Minawa Formation and of the Ojoin Formation. They are metamorphosed peridotite (P-type) and metamorphosed serpentinite (S-type). Metamorphic zonal mapping was made on the metamorphosed serpentinites. Mineral assemblages are, in the order of increasing metamorphic grade, antigorite-brucite-diopside (zone I), olivine-antigorite-diopside (zone II), and olivine-antigorite-tremolite (zone III). The distribution of zones is consistent with the thermal structure of the metamorphic terrain. Relic mineralogy indicates that most of ultramafic bodies of the Sanbagawa metamorphic belt are originally cumulates formed at the spinel and garnet lherzolite facies in the upper mantle. One dunitic metamorphosed serpentinite (Shiraga body), of which chromite indicates Cr-Al substitution (Cr/(Cr+Al)=0.85 to 0.5) without Fe3+ and Ti variations, may have been originally a residue after partial melting of mantle peridotite. With regard to the origin and thermal history, there are at least 4 types of ultramafic bodies in the Sanbagawa metamorphic belt ; (1) P-type, cumulus garnet peridotite, (2) P-type, cumulus spinel peridotite, (3) S-type, cumulus spinel peridotite, and (4) S-type, residual spinel peridotite. The diversity of origin and thermal history indicates that the Upper member of the Minawa Formation and the Ojoin Formation were probably melange zones into which ultramafic bodies were tectonically emplaced. As metamorphic grade of ultramafic bodies is in harmoneous with that of their surrounding schists, tectonic emplacement of ultramafic bodies ceased before the maximum temperature of the Sanbagawa metamorphism was reached.

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.

Notes on petrography and rock-forming mineralogy (12). Chloritoid-bearing rocks from the pumpellyite-actinolite facies region of the Sanbagawa metamorphic belt in western central Shikoku.

Chloritoid-bearing rocks were newly found from the Nakatsu area, the northern part of the Chichibu belt that has suffered the Sanbagawa metamorphism. They occur in a lenticular bed in limestone. The observed assemblage chloritoid+chlorite+diaspore+hematite indicates that the rocks were derived from lateritic sediments being rich in Al2O3 and Fe-oxides and poor in SiO2.

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.

Dunites and serpentinites in the Sanbagawa metamorphic belt, central Shikoku and Kii Peninsula, Japan.

Large dunite-serpentinite masses of the Sanbagawa metamorphic belt occur in the axial zone of large scale recumbent fold. The texture and chemistry of olivines from the Higashi-Akaishi and Shiraga bodies (central Shikoku) and the Ryumon body (Kii peninsula) demonstrate the two types of dunite to be present in this belt. Dunites of the Higashi-Akaishi body had experienced two stages of metamorphism, resulting in slight compositional change of their olivines, but the dunites remained dunite, whereas those of the Shiraga and Ryumon bodies consist of olivines newly formed by the metamorphism of serpentinite. The newly formed olivines contain the minerals of serpentinite, such as magnetite, antigorite, chlorite, brucite and magnesite, as inclusions, and some of them have a positive correlation between XFe and NiO content being the reverse to the compositional trend formed by igneous crystallization. The presence of olivine+antigorite in the Shiraga and Ryumon bodies is in harmony with the thermal structure of the Sanbagawa terrain. This harmony and the widespread occurrence of newly formed olivine indicate that this type of dunite was formed by the metamorphism of serpentinite during the Sanbagawa metamorphism. From the view point of thermal history, not all the peridotites in the Sanbagawa metamorphic belt are simple dismembered masses of the Higashi-Akaishi-type peridotite.

Notes on petrography and rock-forming mineralogy (8) margarite-bearing metagabbro from the Iratsu mass in the Sanbagawa belt, central Shikoku.

Margarite-bearing metagabbro was found from the iratsu epidote amphibolite mass in the Sanbagawa metamorphic belt. The sample is a metamorphosed anorthosite layer and consists of zoisite, paragonite, kyanite, margarite, chlorite, quartz and clay mineral (kaolinite or dickite). The unit cell parameter of margraite is as follows: a0=5.11 A, b0=8.79 A, c0=19.15 A. β=95.3°. Muscovite component in margarite is almost negligible, but (CaO=0.4-1.1 wt%) and Ca-poor (CaO=0.2-0.4 wt%) types. Within the constitutent of plagioclase in original anorthosite, and represent the equilibrium assemblage at the highest temperature stage during the Sanbagawa metamorphism. Margarite and Ca-rich paragonite were formed during the retrogressive metamorphism within the Sanbagawa stage.

New data on the K-white micas from the Sanbagawa Metamorphic Belt, and their petrologic significace.

New b0 values were measured for 123 K-white mica specimens from the Sanbagawa Metamorphic Belt. The rather high b0 values have been obtained on 43 specimens from the quartzose schists and metabasites and the values are considered to be related to the bulk rock compositions. On 80 b0 values which has been obtained from the petapelites, we obtained the following remarks: i) the b0 values decrease significantly with increasing metamorphic temperature; ii) the physical conditions during the Sanbagawa metamorphic processes has been changed significantly. It has become evident that the metamorphic temperature probably increased under substantially unchanged pressure conditions during the metamorphism.

四国中央部，三波川変成帯の藤原苦鉄質‐超苦鉄質複合岩体 Ｉ 岩石化学と造岩鉱物

The Fujiwara plutonic complex emplaced as largely solid state at the present position in the Sanbagawa metamorphic belt of central Shikoku, about 400m long and 200m wide, consists of a mafic unit and an ultramafic unit. The former appears to be in contact with the latter also tectonically. In the mafic unit the main rock-type is a Mg-rich medium-grained gabbro which is penetrated by iron-rich fine-grained gabbro. The ultramafic unit comprises serpentinites derived from dunite and wehrlite. The Sanbagawa metamorphism in epidote amphibolite facies has intensely superimposed on the whole complex. The metamorphic overprinting is characterized by associations with hornblende, zoisite, albite, garnet and chlorite. Not only wehrlitic serpentinite but also most gabbroic rocks still contain some relic clinopyroxenes. 18 whole rock chemical analyses were performed. The chemical compositions of olivines, clinopyroxenes, hornblendes, garnet and albite were determined by means of EPMA and the classic wet method. The whole rock chemistry suggests that the complex is mainly composed of meta-cumulates and the minor fine-grained gabbro represents a part of residual liquid. Probably some gabbroic rocks primarily contained a considerable amount of olivine. The ultramafics are relatively rich in iron and Al2O3. A significant gap in Mg/(Mg+Fe) ratio between olivine and its host rock is notable in the serpentinite and this fact could be indicative of the recrystallization of this mineral. The analysed iron-rich hornblendes are relatively rich in Na in M4, but they are also rich in (Na+K) in A. This peculiarity may be a reflex an iron-rich environment and a low oxidation state during metamorphism.

Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan

Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan