Progress of actinolite‐forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

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

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.

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.

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

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

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.

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

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.

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 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.

The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1 and <8.5 cm−1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.

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.

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.