The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 123, Issue 9
Displaying 1-9 of 9 articles from this issue
Special Issue Recent Progress and Future Perspectives of Metamorphic Studies in Japan. Part 1
Preface
Review
  • Masaki Enami, Takao Hirajima
    2017 Volume 123 Issue 9 Pages 661-675
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    This paper provides an overview of recent progress in the study of high- and ultrahigh-pressure metamorphic rocks, which record the dynamics and material interaction at subduction and continental collision zones. The focus is on ultrahigh-pressure metamorphic terranes worldwide for which Japanese scientists have contributed to petrological studies, and high-pressure metamorphic rocks, particularly eclogite and related lithologies, from the Sanbagawa, Renge, and other high-pressure metamorphic regions in Japan.

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  • Mutsuki Aoya, Shunsuke Endo
    2017 Volume 123 Issue 9 Pages 677-698
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    Recent petrological studies on the Sambagawa metamorphic belt in Shikoku have recognized that the coarse-grained eclogite-bearing lithologies (so-called ‘tectonic blocks’ in earlier studies) in the Besshi area exclusively preserve evidence for the ‘early’ Sambagawa metamorphism, which can be related to onset of the Sambagawa subduction system during Early Cretaceous (c.116Ma). Geological mapping and associated multidisciplinary studies on the regional (spatially widespread) Sambagawa metamorphism (both the eclogite-facies and main metamorphisms) have revealed the tectonic framework of the Late-Cretaceous Sambagawa subduction zone as follows: (i) a spreading ridge was approaching close to the trench; (ii) the subducting slab was coupled with the convective mantle at depth of >65 km; (iii) thickness of the hanging-wall continental crust was 30-35 km; and (iv) the forearc mantle wedge (30-65 km depth) was largely serpentinized. These features allow us to draw a semi-quantitative cross-section of the Sambagawa subduction zone at around 89-85Ma, implying that boundary conditions for thermo-mechanical modeling aiming to simulate exhumation of high-P/T metamorphic rocks are now well constrained. It has also become clear that ultramafic blocks dispersed in the higher-grade part of the Sambagawa belt were derived from the mantle wedge, i.e. the corresponding part of the belt has been re-evaluated as a ‘fossil subduction boundary’ of a relatively warm subduction zone. Field-based petrological studies in the Sambagawa belt can, therefore, have potential to provide invaluable information on material behaviors at the slab-mantle wedge interface including domains of episodic tremor and slip (ETS) in present-day warm subduction zones.

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  • Numerical method of P-T evaluation
    Toshisuke Kawasaki
    2017 Volume 123 Issue 9 Pages 699-706
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS
    Supplementary material

    This study describes numerical estimates of P-T conditions based on chemical reactions among metamorphic minerals. Firstly, the availability of the P-T curve of the chemical reactions among minerals for geothermobarometry is discussed by assessing the slope of the curve. Steeply sloping and shallowly sloping P-T curves are suitable for use as geothermometers and geobarometers, respectively. Secondly, the uncertainty in the metamorphic conditions inferred from the intersection of two P-T curves and the difference in their dP/dT is discussed. If the difference in the slopes is significant, the uncertainties in the estimated pressure and temperature are low. In addition, uncertainty increases with decreasing difference in the slope of the curves. Furthermore, the intersection of two curves with similar slopes may yield an unrealistic estimate of metamorphic conditions, even if the two curves pass through the probable P-T field estimated by other methods. This result arises from improper thermodynamic modelling of the chemical reaction, an error in the P-T curve, or the use of a nonequilibrium composition and/or a nonequilibrium assemblage. Thirdly, the systematic least squares calculation is presented and the reliability of the estimated P-T conditions is assessed from the statistical point of view. The result of the weight-free least squares analysis is plotted near the intersection of the steepest and shallowest P-T curves and is located at a considerable distance from the weight-free average of the intersections of the curves. The weighted least squares show that the calculated P-T conditions can be used to plot near the curves with the highest and second-highest weights and plot near the weighted average of the intersections of the P-T curves.

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  • Geothermometer based on the trace element partitioning
    Toshisuke Kawasaki
    2017 Volume 123 Issue 9 Pages 707-716
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    This study concerns the thermodynamics of trace element partitioning and its application to the P-T estimations of metamorphic rocks. Firstly, the application of Henry's law and Raoult's law, which are commonly used for the infinite dilute solutions to analyse trace element partitioning in geothermometry, is discussed. Secondly, P-T estimates based on trace element partitioning, including Zr-in-rutile, Ti-in-zircon and Ti-in-quartz geothermometers, are critically reviewed from practical and thermodynamical viewpoints. In particular, the Ti-in-quartz geothermometer is discussed in detail from the following perspectives: (1) the validity of using ppm amounts for Ti solubility of quartz; (2) the applicability of Nernst's distribution law to Ti partitioning between quartz and rutile containing high levels of impurities; (3) stable phase assemblages derived from the relation between the Gibbs' free energy and the bulk composition in the SiO2-TiO2 system; (4) variations in Ti content in quartz with temperature, as predicted from the SiO2-TiO2 phase diagram; and: (5) activity of the TiO2 component in quartz containing other impurities.

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  • New application of the singular value decomposition method
    Tadao Nishiyama, Yosuke Moribe, Yasushi Mori, Miki Shigeno, Takashi Yu ...
    2017 Volume 123 Issue 9 Pages 717-731
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    This paper reviews the application of the singular value decomposition (SVD) method to the analysis of chemical reactions in a multicomponent system, emphasizing the power and generality of the SVD method. Furthermore a new development of application of the SVD method to the analysis of open-system reactions and mass transfer is shown, taking as an example a retrogressive reaction in an epidote glaucophanite in a serpentinite mélange from the Nishisonogi metamorphic rocks.

    The epidote glaucophanite occurs as a tectonic block surrounded by a selvage of metabasite in a serpentinite mélange, representing a retrogressive reaction during the exhumation. The original assemblage of the epidote glaucophanite is epidote + glaucophane + winchite + phengite + chlorite + albite, which is replaced by the assemblage of epidote + winchite/barroisite + phengite + chlorite + albite + K-feldspar in the selvage. K-felspar occurs with winchite in small amounts. Because the occurrence of K-feldspar in basic rocks of epidote-blueschist facies is conspicuous, the K-feldspar-forming reaction was assessed with the SVD method. No reaction forming K-feldspar was found in the closed system (i.e., the SiO2–Al2O3–Fe2O3– FeO–MgO–CaO–Na2O–K2O system). Next, only one component was assumed to be mobile in the same system. An arbitrary choice of mobile component from the eight components did not give a correct K-feldspar-forming reaction that is consistent with the relative abundances of winchite and K-feldspar. Finally, mobile components and inert components were discriminated from the isocon diagram. In the case of an isocon defined by SiO2, MgO and Na2O, we found a total reaction by combining a K-feldspar-forming reaction and winchite-forming reaction that is consistent with the result of the isocon analysis such that Al2O3, CaO and K2O are evolved from the system. Thus the SVD method, combined with the isocon analysis, is a powerful tool to derive the reaction relations in open systems.

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  • Forward models and inversion analyses
    Atsushi Okamoto, Tatsu Kuwatani
    2017 Volume 123 Issue 9 Pages 733-745
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    Metamorphism refers to the reactions that proceed in rocks in response to the dynamic environmental factors of the Earth's interior, which include temperature, pressure, and chemical composition. Because it is difficult to obtain time-series data of metamorphic processes directly, it is necessary to extract information from the spatial patterns of the final states of the rocks (e.g., the textures of metamorphic rocks) to understand the conditions and processes of metamorphism. The complications of metamorphic rock textures range widely, from simple problems that can be modeled based on rigid theoretical backgrounds, to complex problems in which several processes interact in nonlinear ways, and forward models themselves are still being developed based on combinations of empirical laws. In this paper, we review recent progress in the analysis and modeling of metamorphic rock textures, with a particular focus on the thermodynamic analysis of zoned minerals and forward modeling of reaction-induced fracturing, employing the distinct element method (DEM). We show that stochastic inversion analyses based on Bayesian inference can be a powerful tool for solving various petrological problems characterized by parameters with undefined values and noise. By effectively using algorithms of machine-learning, the approach of data assimilation, which combines numerical simulations and observed data, is likely to yield a breakthrough in terms of deciphering the complex textures of metamorphic rocks.

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Regular Section
Articles
  • Keiichi Hayashi, Saki Fujita, Kazuto Koarai, Masaki Matsukawa
    2017 Volume 123 Issue 9 Pages 747-764
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS

    The Cretaceous non-marine Sasayama Group of Hyogo Prefecture, Japan, has been newly subdivided into the Ohyamashimo and Sawada formations, in ascending order, and this group is described based on the International Stratigraphic Guide. The lower and upper members or formations have been used as informal stratigraphic units. The group can be divided into four lithofacies assemblages reflecting deposition in a gravel bar, sand bar, flood-plain, and crevasse splay/natural levee. The assemblages assigned to the Sasayama Group are interpreted as follows: (1) the group was deposited in an inter-montane basin, as river channel deposits occur in the basin margin, and flood-plain deposits dominate the central area; (2) gravelly river channel environments were common in the early stages of deposition, and these were succeeded by meandering river environments that produced prominent flood-plain and natural levee deposits; and (3) the detection of the clay mineral kaolin by X-ray diffractometry indicates that the reddish-colored soil deposits of the upper Ohyamashimo Formation formed under a semi-arid to semi-humid climate.

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  • Akira Furusawa
    2017 Volume 123 Issue 9 Pages 765-776
    Published: September 15, 2017
    Released on J-STAGE: December 25, 2017
    JOURNAL FREE ACCESS
    Supplementary material

    The Towada Ofudo and Hachinohe tephras distributed in the northern part of Tohoku district are petrographically similar. Major element compositions of volcanic glass shards of both tephras are also similar. Only a difference in the trace element zirconium content in volcanic glass shards has been observed; however, both tephras are mainly identified by the presence of green hornblende. The content of the green hornblende in the Hachinohe tephra is generally higher than Ofudo tephra. The Ofudo tephra contain a small amount of green hornblende. In addition, a low content unit of green hornblende is also included in the Hachinohe tephra. For this reason, the two tephras cannot be identified only by this content.

    In this study, several Ofudo and Hachinohe tephras were sampled from a wide area, and the content of amphibole, major elements of volcanic glass, trace elements of volcanic glass were analyzed by EDX and laser ablation (LA)-ICP-MS. As a result, it is clear that the both tephras cannot be clearly identified by the green hornblende content, and major element chemical composition of volcanic glass shards. Trace element contents of both tephras closely resemble, but there is a clear difference in the Pb content. The two tephras can be discriminated by the Pb content.

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