Abstract 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.

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．

Abstract Aragonite and omphacite‐bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite‐rich and graphite‐poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite‐poor layer contains a 1.5‐cm ivory‐colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO3 end‐member: the FeCO3 and MnCO3 components are both less than 0.3 mol% and the SrCO3 component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P > 1.1–1.3 GPa and T = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite‐bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized.

Reply to “Comment on ‘Metamorphic P– T–time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation’” by S. R. Wallis and S. Endo

Comment on ‘Metamorphic P–T–time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation’ by K. Aoki, K. Kitajima, H. Masago, M. Nishizawa, M. Terabayashi, S. Omori, T. Yokoyama, N. Takahata, Y. Sano, S. Maruyama [Lithos 113 (2009) 393–407

The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance > 800 km. However, Aoki et al. (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after ca. 90-80 Ma. Furthermore, Aoki et al. (2008) showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional Sanbagawa belt; the structurally upper part is the Sanbagawa metamorphic belt (sensu stricto). It formed as an accretionary complex at ca. 140-130 Ma and subsequently experienced BS-EC facies metamorphism at ca. 120-110 Ma (Okamoto et al., 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after ca. 90-80 Ma and experienced peak metamorphism at ca. 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age (Tsutsumi et al., 2009), clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (Otoh et al., 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (ss) is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shimanto metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama et al., 1996). In SW Japan, the Sanbagawa belt (ss) is paired with the Ryoke low-P/T metamorphic belt and with the ca. 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea ca. 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (ss) and the Ryoke belts.

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.

Metamorphic P–T–time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation

Research Article| June 01, 2008 Areal extent of eclogite facies metamorphism in the Sanbagawa belt, Japan: New evidence from a Raman microprobe study of quartz residual pressure T. Mouri; T. Mouri 1Department of Earth and Planetary Sciences, Nagoya University, Chikusaku, Nagoya 464-8601, Japan Search for other works by this author on: GSW Google Scholar M. Enami M. Enami 1Department of Earth and Planetary Sciences, Nagoya University, Chikusaku, Nagoya 464-8601, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information T. Mouri 1Department of Earth and Planetary Sciences, Nagoya University, Chikusaku, Nagoya 464-8601, Japan M. Enami 1Department of Earth and Planetary Sciences, Nagoya University, Chikusaku, Nagoya 464-8601, Japan Publisher: Geological Society of America Received: 26 Nov 2007 Revision Received: 19 Feb 2008 Accepted: 01 Mar 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (6): 503–506. https://doi.org/10.1130/G24599A.1 Article history Received: 26 Nov 2007 Revision Received: 19 Feb 2008 Accepted: 01 Mar 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation T. Mouri, M. Enami; Areal extent of eclogite facies metamorphism in the Sanbagawa belt, Japan: New evidence from a Raman microprobe study of quartz residual pressure. Geology 2008;; 36 (6): 503–506. doi: https://doi.org/10.1130/G24599A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Application of Raman spectrometry to determine residual pressure retained by quartz grains sealed in garnet reveals significant differences between eclogite facies rocks and lower-grade schistose rocks of the Sanbagawa metamorphic belt in central Shikoku, Japan. Garnet in the eclogitic lithologies commonly exhibits two chemically distinct growth stages. The inner segment commonly includes quartz grains with higher residual pressures than those in the outer segment, suggesting that the former represents prograde eclogite facies and the latter formed during exhumation. Regional variation of the quartz residual pressure suggests that the eclogite unit has a far greater extent than previously recognized, and reveals a large pressure gap between the newly proposed eclogite and noneclogite units. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

In order to make it clear the mode of occurrence of the Sanbagawa belt, we carried out in situ U-Pb isotope analyses of igneous zircon grains from the Oboke area that was a type area of the Sanbagawa belt in central Shikoku, Japan. Analyzed igneous zircons were separated from psammitic schist in the Minawa and Kawaguchi Formations and from igneous cobbles in the Koboke Formation. Spot analyses were performed on the laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS). The youngest U-Pb ages of zircon grains from the Koboke Formation and the Kawaguchi Formation showed 92±4 Ma and 82±11 Ma, respectively. On the other hand, zircons from the Minawa Formation yielded remarkably older ages clustered around 1900-1800 Ma. There is a large chronological gap between protolith sedimentary clasts of the Minawa and those of the other two formations. The protolith sedimentary ages of the Sanbagawa belt have been well constrained as older than 130 Ma based on fossil and U-Pb isotopic ages. The peak metamorphism occurred in 120-110 Ma. Therefore, both Koboke and Kawaguchi Formations must not belong to the Sanbagawa belt, because the timing of formation of accretionary complex must be later than 92±4 Ma for the Koboke Formation and 82±11 Ma for the Kawaguchi Formation. Both the Koboke and Kawaguchi Formations correspond to the late Cretaceous accretionary complex, and they are equivalent to the Northern Shimanto belt. The tectonic boundary between the Sanbagawa and the Northern Shimanto belts is reverse fault and the Northern Shimanto belt appears as a tectonic window in the Sanbagawa belt, central Shikoku. The whole package of the Sanbagawa and underlying Shimanto belts are deformed by the secondary fault movement and doming after the tectonic juxtaposition at the mid-crustal levels.

Abstract 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.

K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics

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.

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

221 A widespread occurrence of talc-phengite in piemontite quartz schist of the Sanbagawa metamorphic belt in central Shikoku

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

Actinolite and hornblende commonly coexist in metabasite of upper greenschist-lower amphibolite facies. Two contradictory interpretations for the coexistence of two calcic amphiboles are suggested, that is, miscibility gap and transitional loop between actinolite and hornblende. We determined the space group of actinolite obtained from a greenschist of the Sanbagawa belt in central Shikoku. The greenschist was taken from lower metamorphic temperature area than that of the hornblende first appearance. Weissenberg and precession photographs indicate that the actinolite have C-lattice, and the existence of the center of symmetry is confirmed by treating the reflected X-ray intensities obtained by 4-circle diffractometer with statistical method (Nz method). The space group of the actinolite is C2/m. Since the actinolite has the same space group as that of hornblende, no crystallographic transformation can be considered in calcic amphibole. Hence, the coexistence of actinolite and hornblende should be explained by the miscibility gap between them.

The mineral assemblages of hematite‐bearing basic schists in intermediate high‐pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four‐component system, without omitting or grouping major components.In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite‐bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low‐T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P‐T trajectory which passes on the lower pressure side of the prograde one.

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.