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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 CO.sub.2-rich fluid [X(CO.sub.2) = CO.sub.2/(CO.sub.2 + H.sub.2O)] 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(CO.sub.2) 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(CO.sub.2) 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 CO.sub.2-rich fluid in the alternating layers, especially in the semi-pelitic band, was probably generated by a redox reaction between Fe.sup.3+-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., Author(s): Masaki Enami [sup.1], Yuki Wakasugi [sup.1], Motohiro Tsuboi [sup.2] Author Affiliations: (1) grid.27476.30, 0000 0001 0943 978X, Institute for Space-Earth Environmental Research, Nagoya University, , Chikusa-ku, 464-8601, Nagoya, Japan [...]

A Laser Raman microspectrometry method was applied to metamorphic quartz in quartz-eclogite-, epidote-amphibolite-, and amphibolite-facies rocks to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnet and other phases have a higher frequency shift than those in the matrix. Furthermore, the quartz inclusions show a frequency shift specific to the individual host crystals in eclogites (garnet [approximately equal to] kyanite > omphacite [approximately equal to] epidote). These observations imply that the residual pressures retained by quartz inclusions depend on elastic parameters of the host Crystals, as discussed by previous researchers. The Raman frequency shift of quartz inclusions in garnet systematically increases with increasing peak metamorphic pressures from the amphibolite facies (0.30-0.55 GPa/470-570 [degrees]C), through the epidote-amphibolite facies (0.8-1.1 GPa/470-635 [degrees]C) to the quartz-eclogite facies (2.1-2.5 GPa/660-710 [degrees]C). Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1-0.2, 0.4-0.6, and 0.8-1.0 GPa for these three groups of metamorphic rocks, respectively. Normal stresses (internal pressures) of quartz inclusions in garnet, numerically simulated with an elastic model, and inferred pressure-temperature conditions at peak metamorphic stage are compatible with the residual pressures estimated from the frequency shifts. Laser Raman microspectroscopic analysis of quartz is a simple and effective method for (1) comparison of pressure conditions in metamorphic rocks formed under various pressure-temperature conditions, and (2) detection of a higher-pressure signature in metamorphic rocks extensively recrystallized during the subsequent exhumation and hydration stage. Keywords: Raman shift, quartz, residual pressure, metamorphism, elastic model

Petrologic studies of the Kotsu eclogite from the Sambagawa high-pressure (P), low-temperature (T) metamorphic belt show that the eclogite has recorded a very steep subduction-stage P-T path reaching 20 kbar and 600 [degrees]C; the dP/dT is >7.1 kbar/100 [degrees]C. Compilation of previously derived subduction-stage P-T paths from the Sambagawa belt and the result of this study show that these P-T paths are not on a straight line from the origin, but define a curve with dP/dT increasing with pressure. A new model incorporating the progressive approach of an oceanic ridge to a subduction zone shows that the Sambagawa P-T paths are best explained by a setting in which a ridge is close to being subducted at a slow rate relative to the plate-subduction rate. Exhumation of high-P, low-T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges. Keywords: pressure-temperature path, subduction, oceanic ridge, modeling, eclogite.

Chromian dissakisite-(Ce), a Cr-enriched Mg analogue of allanite-(Ce), is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. Major minerals of the lherzolite are olivine [[X.sub.Mg] = Mg/(Mg + Fe) = 0.92], pyrope-rich garnet ([Prp.sub.70.1-73.2] [Alm.sub.11.3-13.4][Grs.sub.6.9-8.3]Uvr.sub.3.6-5.[Anr.sub.0-0.4][Sps.sub. 0.7-1.7]), orthopyroxene ([X.sub.Mg] = 0.92-0.93), and clinopyroxene (X.sub.Mg = 0.92-0.95). The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains. It is partly metamict, and there is intensive radial cracking in the surrounding clinopyroxene. Its composition is represented by the averaged formula: [([Ca.sub.0.86][Na.sub.0.03]).sub.0.89][([REE.sub.0.86] [Th.sub.0.04]).sub.0.90] ([Mg.sub.0.98][Fe.sup.2+sub.0.20][Fe.sup.3+.sub.0.21] [Cr.sub.0.36][Al.sub.1.39] [Ti.sub.0.04])[Si.sub.3.03][O.sub.12](OH), with [Ce.sub.2][O.sub.3], MgO and [Cr.sub.2][O.sub.3] contents up to 13.1, 7.3, and 5.4 wt%, respectively. Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and [Fe.sup.2+]., [Fe.sup.3+] and Cr can only occupy the smaller M1 site. Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt, Japan, which contains up to 0.547 Cr atoms per formula unit (apfu). Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members, 'CeCa(Mg,Fe)CrAl[Si.sub.3][O.sub.12](OH),' for the epidote-group minerals, which may form solid solution with [Ca.sub.2][Al.sub.3][Si.sub.3][O.sub.12](OH) through coupled substitutions of REEMg[Ca.sub.-1][Al.sub.-1], REEFe[Ca.sub.-1][Al.sub.-1], and Cr[Al.sub.-1]. The rare earth elements (REE), Ca, and Na total 1.79 apfu, implying that Mn and [Fe.sup.2+] are also present on the nine-coordinated Al site. The high REE abundance, the presence of minor Na, and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crustderived melt/fluid at UHP conditions. Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene, Dissakisite(REE) in subducted ultramafic rocks can transport rare earth elements into the Earth's mantle.

Sr-bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the Su-Lu ultra-high pressure (UHP) province, eastern China. The SrO content of prograde zoisite and epidote reaches 3.2 wt% in crystal cores and generally decreases toward crystal margins. Retrograde epidote is poorer in SrO (70% of the whole-rock SrO is contained in these minerals. Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Zoisite and epidote are thus regarded as the most important Sr reservoirs at UHP conditions where calcic plagioclase and titanite are unstable.

The molecular structure of garnet in samples from orthogneiss metamorphic rock from eastern China were analyzed. Garnets in some pegmatites were also analyzed. The Al-rich garnets and the Fe-rich garnets were compared for their incorporation of the elements sodium (Na), ytterbium (Yb)and yttrium (Y). Results indicated that these elements were incorporated more readily at higher pressure in Al-rich garnets than in Fe-rich garnet.

Estimates of jadeite content of sodic pyroxene associated with quartz and albite in the Sanbagawa belt, Japan, quantifies metamorphic pressure. Local zoning is exhibited by these pyroxene grains and the estimation of zoning enables the determination of the pressure temperature (P-T) paths. The P-T paths obtained are consistent with P-T paths obtained from garnet-zoning patterns.

Coexisting minerals of the hollandite-cryptomelane series, piemontite-group, and abswurmbachite-braunite series occur as a small nodule in a quartzose schist (metachert) from the epidote-amphibolite facies area of the Sanbagawa metamorphic belt, central Shikoku, Japan. Strontium, Ba, K, and Pb contents of the hollandite-cryptomelane minerals are 0.00-0.42, 0.07-0.99, 0.00-0.75 and 0.00-0.05 apfu (atoms per formula unit: O = 16), respectively. The piemontite-group minerals can be divided into common piemontite (Ca [is greater than] Sr in the ten-coordinated A2-site) and strontiopiemontite (Sr [is greater than] Ca). Strontium, Ba and Pb contents of the strontiopiemontite reach 0.71, 0.26 and 0.20 apfu (O = 12.5), respectively, suggesting the existence of two possible new end-members for epidote-group minerals: 'Ba-piemontite [BaCa[([Mn.sup.3+], [Fe.sup.3+], Al).sub.3][Si.sub.3][O.sub.12](OH)]' and 'Pb-piemontite [PbCa[([Mn.sup.3+], [Fe.sup.3+], Al).sub.3][Si.sub.3][O.sub.12](OH)].' The abswurmbachite-braunite series minerals (Cu = 0.30-0.53 apfu: O = 12) contain up to 0.34 apfu Mg, indicating the exchange Mg[([Mn.sup.2+], Cu).sub.-1]. Distribution coefficients for Sr between the hollandite-cryptomelane series and the piemontite-group minerals are up to 20, implying that the hollandite-type structure with its large open tunnel is more suitable for incorporation of Sr than the A2-site of the piemontite-group minerals. A synthetic aluminosilicate analogue of the hollandite-cryptomelane compound is stable under P [is greater than] 8-10 GPa, and suggests the hollandite-type aluminosilicate may be a reservoir of Sr and other large-ion-lithophile elements in the upper mantle.

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation vein in metabasite, and another, a metabasite, both from Ward Creek, Cazadero area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highest-grade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are very heterogeneous and cannot be ascribed to crystallization under conditions of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz.

A minor error in calculation of geobarometric estimates from laser Raman microspectrometry of quartz inclusions in garnet appears in Enami et al. (2007). Correction of this error eliminates anomalous results for grossular-rich garnet, inspiring greater confidence in the validity and applicability of this geobarometric technique. Keywords: Geobarometry, Raman spectrometry, quartz, garnet, elastic model