Masaki Enami

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Sodic pyroxene (jadeite content Xjd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average XAcm=0.26–0.50) than that in the epidote-free samples (XAcm=0.45–0.47). XJd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average XJd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing XJd from core to rim, the other type by decreasing XJd from core to rim. Both types of zoned pyroxenes show an increase in XFe2+[=Fe2+/(Fe2++Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.

Occurrence and field relationships of ultrahigh-pressure metagranitoid and coesite eclogite in the Su-Lu terrane, eastern China

Coesite eclogite is associated with metagranitoid in a 50×100 m2 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.

Decompression P–T path of coesite eclogite to granulite from Weihai, eastern China

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

Sr-bearing zoisite and epidote in ultra-high pressure (UHP) metamorphic rocks from the Su-Lu province, eastern China; an important Sr reservoir under UHP conditions

Abstract 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 (<0.1 wt%). Preliminary rare earth element (REE) analyses of epidote give La2O3 (up to 2.9 wt%), Ce2O3 (5.9 wt%), and Nd2O3 (3.0 wt%). REE contents of zoisite are distinctly lower (La2O3 up to 0.16 wt%, Ce2O3 up to 0.26 wt%, and Nd2O3 up to 0.16 wt%) than coexisting epidote. Apatite is always more depleted in SrO (0.10-0.59 wt% on average) than coexisting zoisite and epidote, and Sr-Ca partition coefficients for zoisite and epidote and apatite [(Sr/Ca)zo/ep-ap] range from 5 to 20. SrO content of K-white mica (0.012-0.044 wt%) is an order of magnitude lower than that of apatite. An evaluation of the SrO content in zoisite and epidote and their modal abundances in seven samples indicates that >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.

Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures

Abstract 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 ≈ kyanite > omphacite ≈ 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°C), through the epidote-amphibolite facies (0.8-1.1 GPa/470-635°C) to the quartz-eclogite facies (2.1-2.5 GPa/660-710°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

Subduction-stage pressure-temperature path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction

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 °C; the dP / dT is >7.1 kbar/100 °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.

The Sulu UHP Terrane: A Review of the Petrology and Structural Geology

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ∼120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.

A mechanism for Na incorporation in garnet; an example from garnet in orthogneiss from the Su-Lu Terrane, eastern China

Abstract To calibrate infrared (IR) spectroscopy for quantitative analysis of trace structural OH in specific minerals, we have determined concentrations of H in pure separates of mantlederived pyrope garnet (56 ± 6 ppm H2O by weight), augite (268 ± 8, ppm H2O), and enstatite (217 ± 11 ppm H2O) by manometry after heating the samples and extracting H2 gas under vacuum. IR spectroscopy confirmed the presence of intrinsic OH in these samples prior to extraction and indicated between 86 and 100% removal of H during the extraction procedure. The integral specific absorption coefficients of 1.39 ± 0.14 (1σ),7.09 ± 0.32, and 15.6 ± 0.94 /(ppm H2O·cm2) for pyrope, augite, and enstatite, respectively, allow precise spectroscopic determination of the OH content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million. Uncertainties in accuracy depend on mineral composition and characteristics of the OH absorption spectrum and are estimated to range between ± 10 and ± 50%.

Areal extent of eclogite facies metamorphism in the Sanbagawa belt, Japan: New evidence from a Raman microprobe study of quartz residual pressure

Application of Raman spectrometry to determine residual pressure retained by quartz grains sealed in garnet reveals significant differences between eclogite facies rocks and lower-grade schistose rocks of the Sanbagawa metamorphic belt in central Shikoku, Japan. Garnet in the eclogitic lithologies commonly exhibits two chemically distinct growth stages. The inner segment commonly includes quartz grains with higher residual pressures than those in the outer segment, suggesting that the former represents prograde eclogite facies and the latter formed during exhumation. Regional variation of the quartz residual pressure suggests that the eclogite unit has a far greater extent than previously recognized, and reveals a large pressure gap between the newly proposed eclogite and noneclogite units.

Evaluation of residual pressure in an inclusion–host system using negative frequency shift of quartz Raman spectra

Abstract Raman spectra of quartz inclusions in garnet hosts of low-pressure/temperature metamorphic rocks from the Yanai district in the Ryoke belt (around 0.1-0.3 GPa/500-600 °C), Southwest Japan, exhibit frequency (peak position) shifts toward lower wavenumbers as compared to those of a quartz standard measured at ambient conditions. The observed negative frequency shifts indicate that tensile normal stress is exerted on the quartz-garnet boundary and therefore, quartz inclusions are subjected to negative residual pressure. Elastic modeling that assumed the constant elastic properties of minerals cannot explain this negative residual pressure. This study estimated the residual pressure based on a new scheme of elastic modeling with equation of state (EOS) of quartz and garnet, which takes into account the pressure- and temperature-dependency of compressibility and expansivity. The calculated residual pressure was converted into frequency shifts of quartz Raman spectrum based on the experimentally determined relation. The results showed that the quartz inclusions in garnets retain residual pressure of about -0.3 GPa, and logically reproduced the observed frequency shifts in the direction of lower wavenumbers. The new elastic modeling also simulates positive frequency shifts retained by quartz inclusions in garnets of high-pressure metamorphic rocks from the Sambagawa metamorphic belt in Southwest Japan, and from the Motagua fault zone in Guatemala. The degree and direction of Raman frequency shifts of quartz inclusion in garnet depend on metamorphic conditions when the quartz was included in the host garnet. Conversely, the metamorphic conditions prevailing when a set of a quartz inclusion and garnet host was recrystallized can be inferred from Raman frequency shifts of quartz inclusion in garnet. The proposed Raman spectroscopic analysis should be a powerful and useful tool to decipher information at earlier stage of garnet growth even in samples of highly recrystallized matrix phases during exhumation and retrograde stages.