© 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>Eleven localities of staurolite-bearing metapelites and pelitic bands in metachert, are reported from the Ryoke metamorphic belt in the Hongu-san area (Aichi Prefecture, Japan). Staurolite occurs as inclusions within andalusite porphyroblasts and within mica-quartz aggregates in metapelites. On the other hand, staurolite in pelitic bands in metachert, occurs not only as inclusions within mica-quartz aggregates but also as porphyroblast. Petrographic observations indicate that the mica-quartz aggregates were formed by replacing staurolite porphyroblasts during prograde stage of the Ryoke regional metamorphism. Based on the widespread spatial distribution of the mica-quartz aggregates, it is suggested that staurolite was widely produced in the area. During subsequent stages of prograde and peak metamorphism, staurolite became unstable in some rocks while remaining stable in others. Thermodynamic calculations reveal that the Ti content of biotite and the Zn content of staurolite affect the stability of staurolite, thus explaining its heterogeneous preservation in the study area.</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;Schistose staurolite-bearing pelites occur in the Ryoke Metamorphic Belt, Dando-san area, Aichi Prefecture, Japan. These rocks contain andalusite porphyroblasts and coarse-grained elongate mica-quartz aggregates composed of muscovite, biotite, and quartz in a matrix of muscovite, biotite, quartz, and fibrolite. Staurolite occurs as inclusions in andalusite porphyroblasts and in the minerals of the micaquartz aggregates. Detailed microscopy indicates that a staurolite-consuming and andalusite-producing reaction occurred before the deformation causing the schistosity, such that staurolite grains apart from andalusite porphyroblasts have been replaced by mica-quartz aggregates, and those adjacent to the porphyroblasts have been replaced by andalusite. It is possible that the prograde &lt;i&gt;P&lt;/i&gt;-&lt;i&gt;T&lt;/i&gt; paths of the staurolite-bearing pelites of the Dando-san and Hazu-Hongu-san areas lie on the high-pressure side of those of staurolite-free pelitic rocks with similar metamorphic grades in other areas of the Ryoke Belt.&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.

In Aichi Prefecture of Central Japan, metamorphic rocks of the Ryoke Belt are intruded by the Inagawa Granodiorite southeast of the town of Asuke. The pluton extends in a NE-SW direction for ca. 120 km with a width of ca. 25 km at its southwestern end and consists of four distinct intrusive units. A new CHIME monazite age of 83.3±1.3 Ma for the third unit is within error of a previously reported age of 83-82 Ma for the forth unit, thereby supporting the proposal that the whole of the granodiorite intrusion solidified over a shorttime span. Geological and petrological evidence, including metamorphic zones, the occurrence of prismatic sillimanite after andalusite, and the spatial distribution of migmatitic rocks, shows that the metamorphic grade increases gradually towards the northwest as the contact with the granodiorite is approached. The width of the thermal aureole recognized during mapping reaches 8 km, whereas its true width is estimated to be ca. 5 km after correcting for the dip of the contact between the pluton and country rocks. The wide thermal aureole around the Inagawa Granodiorite is ascribed to the large heat capacity of the intrusion and the elevated geothermal gradient that existed in the region at the time, owing to the active magmatic processes during the successive emplacement of the Younger Ryoke granitoids.

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.