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.

Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet-clinopyroxene geothermometer based on 333 garnet-clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe 3+ content of clinopyroxene. For the calibration, a published subregular-solution model was adopted to express the non-ideality of garnet. The magnitude of the Fe-Mg excess interaction parameter for clinopyroxene (W FeMg Cpx ), and differences in enthalpy and entropy of the Fe-Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe-Mg excess interaction parameter of clinopyroxene (W FeMg Cpx = ) 3843 J mol )1 ). The pressure correction is simply treated as linear, and the difference in volume of the Fe-Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be )120.72 (J kbar )1 mol )1 ). The regressed and obtained thermometer formulation is as follows:Tð CÞ ¼f2784 þ 14:52 P þ ð2601 þ 1:44 PÞð2X grs X prp À AÞ þ ð1183 þ 6:98 PÞðX grs 2 À AÞÀ 105ð2X grs X alm þ BÞ þ ð814:6 þ 3:61PÞðX grs 2 þ BÞ À ð254:6 þ 8:42 PÞð2X prp X alm À X alm 2 þ CÞ À 83:6ðX prp 2 À 2X prp X alm þ CÞ þ 1388 X sps À 462ðX Mg Cpx À X Fe Cpx Þg=flnK D þ 1:431 þ 0:695ð2X grs X prp þ X grs 2 À 2AÞ þ 0:203ðX grs 2 À 2X grs X alm Þ þ 0:922X sps g À 273;where T = temperature, P = pressure (kbar), A = 0.5 X grs (X prp ) X alm ) X sps ), B = 0.5 X grs (X prp ) X alm + X sps ), C = 0.5 (X grs + X sps ) (X prp ) X alm ), X prp = Mg ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X alm = Fe ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X sps = Mn ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X grs = Ca ⁄ (Fe 2+ + Mn + Mg + Ca) Grt , X Mg Cpx = Mg ⁄ (Al + Fe total + Mg) Cpx , X Fe Cpx = Fe 2+ ⁄ (Al + Fe total + Mg) Cpx , K D = (Fe 2+ ⁄ Mg) Grt ⁄ (Fe 2+ ⁄ Mg) Cpx , Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800-1820°C), with a standard deviation (1 sigma) of 74°C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100°C at >1300°C and overestimate by 100-200°C at <1300°C. In addition, they tend to overestimate temperatures for high-Ca garnet (X grs % 0.30-0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20-100°C lower than previous formulations.

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Coesite eclogite is associated with metagranitoid in a 50×100 m 2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.