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

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).

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

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

A map of the graphitizing-degree has been drawn for the Sanbagawa metamorphic belt of central Shikoku, and compared with the metamorphic zones previously reported. The distribution of the graphitizing-degree (GD) is very consistent with the mineral zones of this district, showing minute thermal structures in the metamorphic zones, and suggests the extension of the oligoclase-biotite zone to the areas of the Tachikawa River and the Saruta River. The local disturbance of GD seems to be due to a tectonic event affecting rocks of the lower (or higher) metamorphic grade.

The geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO2 and Na2O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H2O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed.

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

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.

我々が直接には到達できない沈み込み帯深部の岩石を，何らかの地質学的過程の末に現在の地表に露出させているのが低温高圧型の三波川変成帯であり，三波川帯が擁する地下約15〜100kmの深さで形成した多様な岩石，しかも沈み込み境界の下盤側と上盤側，両者の岩石群をまとめて観察できるのが四国中央部の新居浜地域である．そういった岩石群が地下深部へもたらされ，その後地表に至るまでに行われたプレート間のせめぎ合いは，岩石が宿すに至った鉱物組成や変形構造，および岩石相互の露出位置関係という形で保存されている．本巡検では，造山運動や地殻-マントル相互作用に関する情報が数多く記録された新居浜地域三波川帯の地質を，おもに構造岩石学的な視点から概観する．地殻起源変成岩としては最浅部にあった中七番ユニットの砂質片岩や最深部にあった権現エクロジャイト等を，またマントルウェッジ起源の超苦鉄質岩類では最深部に達していた東赤石かんらん岩体を主な観察対象とする．白亜紀のユーラシア東縁沈み込み帯深部で起こっていた様々な地質現象，特に変形と化学反応に，短時間でなるべく濃密に触れることができるよう，本巡検を企画した．

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.

Journal Article Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan Get access TAKASHI NAKAJIMA, TAKASHI NAKAJIMA 1Department of Earth Sciences, Kanazawa UniversityKanazawa 920, Japan Search for other works by this author on: Oxford Academic Google Scholar SHOHEI BANNO, SHOHEI BANNO 1Department of Earth Sciences, Kanazawa UniversityKanazawa 920, Japan Search for other works by this author on: Oxford Academic Google Scholar TAKASHI SUZUKI TAKASHI SUZUKI 2Institute of Geology, Kochi UniversityKochi 780, Japan Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 18, Issue 2, May 1977, Pages 263–284, https://doi.org/10.1093/petrology/18.2.263 Published: 01 May 1977 Article history Received: 05 May 1976 Revision received: 22 July 1976 Published: 01 May 1977

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.

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