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Alternating layers of pelitic and basic bands with occasional semi-pelitic band, millimeter to centimeter in width, occur between the epidote–amphibolite (metagabbro) and the pelitic schist in the epidote–amphibolite facies region of the Sanbagawa metamorphic belt, central Shikoku. The whole-rock major, trace, and rare earth element compositions of the semi-pelitic band are intermediate between those of the basic and pelitic bands. The peak metamorphic conditions were estimated at 1.0–1.2 GPa/600–630 °C for the mineral assemblage of the pelitic and semi-pelitic bands. The evolution of the CO2-rich fluid [X(CO2) = CO2/(CO2 + H2O)] at the lithologic boundary between the epidote–amphibolite and the pelitic schist, during the Sanbagawa prograde metamorphism, is discussed in the context of a titanite, rutile, calcite, dolomite, and quartz assemblage. The X(CO2) of the semi-pelitic band and basic and the pelitic bands increased during prograde metamorphism from the stability field of titanite to those of rutile + dolomite + amphibole + quartz and rutile + calcite + amphibole + quartz, respectively. The X(CO2) values of the metamorphic fluid at the peak metamorphic stage estimated by the matrix assemblages were higher in the order of the pelitic schist and epidote–amphibolite (less than 0.12–0.23), basic and pelitic bands (0.23–0.38), and semi-pelitic band (0.38–0.57), suggesting variations in the fluid compositions on a millimeter to centimeter scale. The CO2-rich fluid in the alternating layers, especially in the semi-pelitic band, was probably generated by a redox reaction between Fe3+-bearing silicate phases, such as amphibole and epidote in the basic band and carbonaceous material in the pelitic band. This reaction is thought to have been triggered by the chemical and/or mechanical mixing of these two bands during prograde metamorphism, resulting in the formation of the semi-pelitic band. [ABSTRACT FROM AUTHOR]

Investigation of an eclogite xenolith, discovered in a Cretaceous granite from the Central Domain of the Dabieshan massif in eastern China, yields new petrological insights into the high to ultrahigh-pressure metamorphism, experienced by the Qinling-Dabie orogen. Prior to inclusion as a xenolith in the granite during the Early Cretaceous, this eclogite xenolith had recorded a complex metamorphic evolution that complies with subduction and exhumation processes experienced by the continental crust of the South China Block. Well-preserved mineral parageneses substantiate the prograde and retrograde stages revealed by inclusions in porphyroblastic garnet and zoned minerals such as garnet, omphacite and amphibole in the matrix. The relatively low P/T re-equilibration during a late metamorphic stage was textually inferred by the presence of aluminous and calcic-subcalcic amphiboles such as katophorite, edenite, taramite and pargasite as main matrix phases. According to our U/Pb, Rb/Sr and new 40Ar/39Ar geochronological results, namely109 ± 1 and 112 ± 2 Ma plateau ages for muscovite and amphiboles, respectively, two successive but distinct cooling stages account for the thermal history of the granite–migmatite gneiss dome that forms the Central Dabieshan Domain. We argue that prior to the Cretaceous doming, the Central Dabieshan Domain experienced a tectono-metamorphic evolution similar to that observed in the high-pressure to ultra high-pressure units developed in the Southern Dabieshan Domain and Hong’an massif. [ABSTRACT FROM AUTHOR]

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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. [ABSTRACT FROM AUTHOR]