© 2019, The Author(s). The microtexture of graphite exposed on the polished surface was studied using confocal laser scanning microscopy, laser Raman spectroscopy, and focused ion beam–transmission electron microscopy (FIB–TEM) to elucidate the effect on surface condition and crystallinity of graphite by polishing process. The polished surface of the graphite was divided into a flat part with no irregularities and a grooved band with a width of < 1 μm and a depth of < 100 nm. Raman analyses revealed that the original structure of the graphite covered by the host mineral was a well-ordered graphite, whereas the polished graphite at the surface had a reduced crystallinity, particularly in the flat part of the sample. Based on scanning TEM observations of an ultra-thin FIB section, fractures that developed during sample preparation were concentrated in the region extending from the surface to a depth of 1 μm. Furthermore, graphite sheets were peeled away by shearing, with scraped graphite sheets filling in the gap. Our results demonstrate that the original microtexture of graphite was easily deformed by shearing during polishing, and careful attention should be paid to sample preparation. In addition, we also need to pay more attention to the effects of natural shearing such as faulting on the graphite or sheet-like minerals. [Figure not available: see fulltext.].

The present paper reports, for the first time, the occurrence of an omphacite-bearing mafic schist from the Asemi-gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite-biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde-type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system-transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm(-1), and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Delta omega(1) > 8.5 cm(-1), corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet-clinopyroxene geothermometry gives peak pressure-temperature (P-T) conditions of 1.7-2.0 GPa and 440-540 degrees C for the mafic schist. The peak P-T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P-T histories along the Late Cretaceous subduction zone.

© 2019 John Wiley & Sons Ltd To understand the preservation of coesite inclusions in ultrahigh-pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub-grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~10 9  cm −2 . A high-resolution TEM image and a fast Fourier transform-filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~10 8  cm −2 , being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite-bearing matrix kyanite is ~10 8  cm −2 , but a high dislocation density region of ~10 9  cm −2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤10 8  cm −2 . Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks.

<p>愛知県本宮山地域の領家変成帯の11地点の紅柱石を含む泥質変成岩およびメタチャート泥質部から十字石を見出した．十字石は，泥質変成岩では紅柱石や「雲母-石英集合体」の包有物として産するのに対して，メタチャート泥質部では「雲母-石英集合体」内の包有物のほかに，基質鉱物と接する斑状変晶としても産する．偏光顕微鏡による観察から，「雲母-石英集合体」は広域変成作用のプログレード期に仮像化した十字石を起源としていると解釈した．地域内の「雲母-石英集合体」の分布から，十字石は広く生成されたと考えられる．その後ピーク変成時までに，十字石が安定に存在した岩石と不安定になった岩石があった．十字石を含む鉱物組み合わせの岩石に対して，熱力学的計算による検討を行った．本研究地域の十字石の安定性の違いは（したがって産状の違いは），構成鉱物の化学組成の違い（特に黒雲母中のTiおよび十字石中のZn含有量）によって説明できる．</p>

© 2018 John Wiley & Sons Ltd Here, we report the first discovery of an amorphous SiO2 phase (APSI phase) in a pseudomorph after coesite included in garnet from an ultrahigh-pressure (UHP) eclogite from the Su–Lu metamorphic belt, eastern China. Using transmission electron microscopy, Raman spectroscopy and selected area electron diffraction, we show that the internal structure of the pseudomorph consists of an APSI phase with nano/submicrocrystalline particles of quartz and a polycrystalline K-bearing fibrous sheet-silicate phase (KFSS phase). The APSI phase-bearing aggregates included in the garnet might have formed by reactions involving a supercritical fluid during exhumation by the following processes: (1) the development of radial cracks within the host garnet by the phase transition of coesite to quartz; (2) the decomposition of a part of the pseudomorph following infiltration of supercritical fluid; (3) the precipitation of the KFSS phase from the fluid phase during subsequent exhumation and cooling, which was likely promoted by a change in the metamorphic fluid from supercritical and/or subcritical to aqueous fluid; and (4) the rapid precipitation of the APSI phase under a metastable (non-equilibrium) state, such as quenching, during a later stage of the exhumation. Whether the APSI phase generally formed during exhumation and survived widely throughout the Su-Lu terrane is unknown. However, the presence of the APSI phase in a UHP eclogite provides new insight into the geodynamic phenomena occurring at continental collision zones.

&lt;p&gt;愛知県段戸山地域の領家変成帯の泥質片岩から十字石を発見した．これらの泥質片岩には白雲母，黒雲母，石英，珪線石等の基質鉱物，紅柱石斑状変晶，および基質より粗粒な白雲母，黒雲母，石英から構成される扁平な形状の集合体（雲母-石英集合体）が含まれている．十字石は，紅柱石斑状変晶に包有されるものと，雲母-石英集合体内の石英や白雲母に包有されるものがある．偏光顕微鏡による詳細な観察から，紅柱石は十字石斑状変晶を消費しながら成長し，その過程で紅柱石から離れた十字石斑状変晶は，雲母-石英集合に置換され，紅柱石に接した十字石斑状変晶は紅柱石に置換されたと解釈した．段戸山地域や幡豆-本宮山地域の泥質岩に十字石が出現するのに対して，他地域の領家変成帯の同程度の変成度の泥質岩に十字石が見つからないのは，プログレード期の&lt;i&gt;P&lt;/i&gt;-&lt;i&gt;T&lt;/i&gt;経路がより低圧であったため，十字石が安定な&lt;i&gt;P&lt;/i&gt;-&lt;i&gt;T&lt;/i&gt;領域を通らなかったせいである可能性が高い．&lt;/p&gt;

The prograde metamorphic history of the Sulu ultrahigh-pressure metamorphic terrane has been revealed using Raman-based barometry of the SiO2 phases and other mineral inclusions in garnet porphyroblasts of a coesite eclogite from Yangzhuang, Junan region, eastern China. Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO2 phase inclusions are -quartz with no coesite or relict features such as radial cracks. The residual pressures retained by the quartz inclusions systematically increase from the crystal centre to the margin of the inner segment. The metamorphic conditions estimated by calculation from the residual pressure and conventional thermodynamic calculation range from 500 to 630 degrees C and 1.3 to 2.3GPa for the stage of the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment of the garnet and matrix omphacite. This occurrence of coesite is consistent with the pressure and temperature conditions of 660-725 degrees C and 3.1GPa estimated by conventional geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under -quartz-stable conditions and survived phase transition to coesite at the peak metamorphic stage. The SiO2 phases and other inclusions in the garnet have retained evidence of the pre-eclogite prograde stage even during exhumation stage. The combined Raman spectroscopic and petrological approaches used here offers a powerful means for obtaining more robust constraints prograde stages involving garnet growth where different SiO2 phases are present as inclusions.

愛知県足助南東部地域には領家変成岩類に巨大な伊奈川花崗閃緑岩体が貫入している．貫入部付近の伊奈川花崗閃緑岩のCHIMEモナズ石年代測定を実施したところ，以前に測定されていた別の地点の年代にほぼ一致し（83.3±1.3Ma），巨大な岩体はほぼ同時に固結したという解釈を支持する結果になった．伊奈川花崗閃緑岩体の南東側に分布する変成岩類を地質学的・岩石学的に調査した結果，（1）変成分帯の配列，（2）泥質岩中の紅柱石仮像中の柱状珪線石の出現分布，（3）ミグマタイトの分布，はいずれも伊奈川花崗閃緑岩に近いほど高温の変成作用を経験したことを示した．接触変成帯の水平幅は約8kmに達し，花崗岩からの垂直距離は約5kmに及ぶ．このような広大な接触変成帯が形成されたのは，伊奈川花崗閃緑岩体の熱容量が大きかったことに加えて，新期花崗岩類に関係するマグマ活動によって，中部地方領家帯における地温勾配が高くなっていたことが重要な要因であったと解釈した．

Coexistence of jadeite (Jd(92)) and quartz was newly found in an inner segment of a composite-zoned garnet from a metapelite in the southern albite-biotite zone of the Asemi-gawa region, Sanbagawa belt in central Shikoku, Japan. The assemblage of jadeite + quartz in garnet gives a minimum metamorphic pressure of 1.4-1.9 GPa at 500-700 degrees C, which was significantly higher than that previously proposed for epidote-amphibolite facies conditions in the albite-biotite zone. Garnet includes quartz retaining high residual pressure (Delta omega(1) values up to 11.7 cm(-1)), which is comparable to the residual pressure reported in eclogite and metapelite within the eclogite unit in central Shikoku. These results imply that (1) the inner segment of the composite-zoned garnet records prograde recrystallization under high-pressure blueschist to eclogite facies conditions and (2) at least a part of the southern albite-biotite zone of the Asemi-gawa region recrystallized under higher pressure conditions prior to the regional metamorphism from the greenschist to epidote-amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

Compositional and textural characteristics of the zonal structure and inclusions of garnet in metapelites from areas of high-grade greenschist facies (garnet zone) to areas of epidote-amphibolite facies (albite-biotite and oligoclase biotite zones) located along the Asemi-gawa route were examined using EPMA and Raman spectroscopy to investigate evolution of the P-T history of Sanbagawa metamorphism. Garnet grains exhibit two types of compositional zoning patterns: bell-shaped and composite types with spessartine variation. Bell-shape zoning is characterized by a monotonous decrease in the spessartine component from the crystal core towards the margin. Almandine and pyrope components exhibit the opposite trend. The grossular content reaches a maximum at an intermediate position between the core and the rim, and then decreases towards the outermost rim. The composite-zoned garnet is divided into core and mantle parts and it exhibits discontinuous compositional variations according to the associated boundary, evidencing resorption of the core and overgrowth of the mantle during crystallization; implying a two-stage growth of garnet during the Sanbagawa metamorphism. Composite-zoned garnet is characteristically observed in metapelites from a part of the albite-biotite zone, and quartz grains included in its core retain a higher residual pressure than those in garnet from other mineral zones. Isolated paragonite crystal occurs particularly as an inclusion in the core part of this composite-zoned garnet. These data suggest that part of the Sanbagawa metamorphic rocks in the Asemi-gawa region recrystallized under higher-pressure conditions up to the eclogite facies, prior to regional metamorphism from the greenschist, to the epidote-amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

Chromium-rich calc-silicate rocks in the Archaean Bandihalli supracrustal belt in the Dharwar Craton, India, show a characteristic mineral assemblage that includes euhedral and anhedral (resorbed) green garnet associated with chromite, in addition to diopside, quartz, clinozoisite and calcite. Two distinct types of green garnets and a brown variety were identified in the calc-silicate rocks. These garnets show complex chemical compositional variations that range between the endmembers of uvarovite, grossular, and spessartine. Chromite grains in the matrix are euhedral or subhedral, whereas inclusions in garnet are rounded or amoeboid in shape. The two textural varieties of chromite from Bandihalli are separated by a compositional gap. Furthermore, chromites from the study area have high Cr# (0.80-0.97) and low Mg# (0.01-0.04), which is similar to the chromite in serpentinites and are in contrast with chromite compositions reported from igneous rocks. Therefore, the chromite in the calc-silicate rocks may have been derived from serpentinized ultramafic rocks. Textural and compositional data presented here provide evidence for chemical changes of the primary chromite with increasing metamorphic grade, presumably associated with primary hydrothermal alteration that resulted in serpentinization and carbonation of the ultramafic protolith. Therefore, Cr-rich calc-silicate rocks can be considered as products of metamorphosed carbonated and serpentinized mafic or ultramafic igneous rocks.