The Raman spectra of carbonaceous material (CM) from 19 metasediment samples collected from six widely separated areas of Southwest Japan and metamorphosed at temperatures from 165 to 655°C show systematic changes with metamorphic temperature that can be classified into four types: low-grade CM (c. 150-280°C), medium-grade CM (c. 280-400°C), high-grade CM (c. 400-650°C), and well-crystallized graphite (> c. 650°C). The Raman spectra of low-grade CM exhibit features typical of amorphous carbon, in which several disordered bands (D-band) appear in the first-order region. In the Raman spectra of medium-grade CM, the graphite band (G-band) can be recognized and several abrupt changes occur in the trends for several band parameters. The observed changes indicate that CM starts to transform from amorphous carbon to crystallized graphite at around 280°C, and this transformation continues until 400°C. The G-band becomes the most prominent peak at high-grade CM suggesting that the CM structure is close to that of well-crystallized graphite. In the highest temperature sample of 655°C, the Raman spectra of CM show a strong G-band with almost no recognizable D-band, implying the CM grain is well-crystallized graphite. In the Raman spectra of low-to medium-grade CM, comparisons of several band parameters with the known metamorphic temperature show inverse correlations between metamorphic temperature and the full width at half maximum (FWHM) of the D1-and D2-bands. These correlations are calibrated as new Raman CM geothermometers, applicable in the range of c. 150-400°C. Details of the methodology for peak decomposition of Raman spectra from the low to medium temperature range are also discussed with the aim of establishing a robust and user-friendly geothermometer.

The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature (T) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low-pressure (<3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact-metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c-axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532-nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5-nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2-T correlations shown by contact and regional metamorphic rocks: the former are slightly bettercrystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of ±50°C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532-nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532-nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to $ ±30°C.

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Omphacitebearing metapelite was found from the Seba area of the Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. Omphacite occurs as inclusions in garnet together with quartz, sodic amphibole, phengite, and paragonite. The major matrix phases are quartz, albite, phengite, chlorite, subcalcic amphibole, calcite, dolomite, and graphite. Garnet shows a prograde zoning and comprises three segments in order from the crystal center to the margin: the core, inner mantle, and outer mantle. The garnet core shows monotonous decrease of MnO content outward, and includes sodic phases: paragonite and glaucophane. The garnet mantle is substantially homogeneous in composition, and poorer in MnO and richer in MgO than the core. The inner mantle of garnet includes omphacite, paragonite and glaucophane. The outer mantle includes omphacite, but paragonite and glaucophane grains are absent. The jadeite content of omphacite inclusions (X Jd ) increases slightly from 0.55 in grains included in the inner mantle to 0.62 in the outer mantle of garnet. The systematic distribution of sodic minerals in garnet documents a prograde evolution of metamorphism from the blueschist to eclogite facies conditions. The occurrence of omphacitebearing metapelite in the Seba area of the Besshi region is direct evidence of: (1) at least some of the Sambagawa metapelites in the Besshi region certainly experienced eclogite facies metamorphism, and (2) eclogite facies metamorphism extends beyond the previously assumed eclogite facies area in the Sambagawa belt.