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Aluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD. The mantle amphibole occurs between garnet and symplectite or quartz. A set of plagioclase and aegirinediopside/argirine-hedenbergite thin monomineralic bands forms at the boundary between the mantle amphibole and matrix quartz. However, these monomineralic bands do not occur at the mantle amphibole-symplectite boundary. These textural diferences indicate that the recrystallization of the aluminous amphibole around garnet was controlled by significant local reactions, and the size of equilibrate domains was probably several tens of micrometers or less. The mantle amphibole is composed of inner (garnet-side) and outer (matrix-side) zones. The inner zone is compositionally homogeneous, and its atomic Al/Si value is ~0.63–0.66 and similar to that of garnet. Atomic Ca/Si value in the inner zone is also almost uniform and is generally identical to that of garnet. The outer zone exhibits a monotonic decrease in the Al/Si and Ca/Si values outward, and its composition at the outermost margin is similar to that of the symplectitic amphibole. The crack-filling amphibole has a composition similar to the inner zone of the mantle amphibole. The CPO pattern of the crack-filling amphibole is diferent from that of the adjacent mantle amphibole, showing that the crack-filling amphibole is cut by the mantle amphibole. The textural relationship between the mantle and crack-filling amphibole phases and their compositional characteristics imply that: (1) the mantle type is a slightly later stage product than the crack-filling type, and (2) the boundary between the inner and outer zones of the mantle aluminous amphibole probably corresponds to the initial surface of the porphyroblastic garnet. The inner zone is considered to have grown inward by simple substitution of garnet, using the tetrahedral and octahedral cations of the garnet as the basic framework. On the other hand, most of the outer zone of the mantle-type amphibole grew outward in the matrix from the initial surface of the garnet porphyroblast. The mantle amphibole shows a CPO similar to that of amphibole in the adjacent symplectite domain, suggesting that these two types of amphibole formed almost simultaneously, sharing crystallographic orientation with each other. The formation of crack-filling aluminous amphibole was probably promoted by the hydraulic microfracturing process at an early stage of exhumation and hydration. The mantle and symplectitic amphibole phases formation was promoted by the subsequent infiltration of metamorphic fluid. The aluminous amphibole in the SiO2 phase-bearing eclogites probably recrystallized with the formation of a localized SiO2-undersaturated reaction domain because of rapid exhumation and subsequent rapid cooling of the Sulu UHP metamorphic belt. [ABSTRACT FROM AUTHOR]

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]

The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1 and <8.5 cm−1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength. [ABSTRACT FROM AUTHOR]

The high temperature ( T )/pressure ( P ) regional Mogok metamorphic belt is situated in central Myanmar, and is mainly composed of pelitic gneisses, amphibolites, marbles, and calc-silicate rocks. The garnet–biotite–plagioclase–sillimanite–quartz assemblage and its partial system suggest equilibrium P / T conditions of 0.6–1.0 GPa/780–850 °C for the peak metamorphic stage, and 0.3–0.5 GPa/600–680 °C for the exhumation and hydration stage. Monazite grains show complex compositional zoning consisting of three segments—I, II, and III. Taking into consideration the monazite zoning and relative misfit curves, the calculated chemical Th–U–total Pb isochron method (CHIME) monazite age data (284 spot analyses) indicated four age components: 49.3 ± 2.6–49.9 ± 7.9, 37.8 ± 1.0–38.1 ± 1.7, 28.0 ± 0.8–28.8 ± 1.6, and 23.7 ± 1.3 Ma (2 σ level). The ages of the Late Eocene and Late Oligocene epochs were interpreted as the peak metamorphic stage of upper-amphibolite and/or granulite facies and the postdated hydration stage, respectively. [ABSTRACT FROM AUTHOR]

This report describes an investigation of the composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments, and presents discussions of the possible distribution of eclogite facies lithologies in the Besshi region, central Shikoku, southwest Japan. Garnet grains usually show chemically composite zoning with resolved inner and overgrown outer segments, respectively. The inner segment usually contains paragonite as a sodic phase inclusion and rarely contains omphacite or glaucophane, whereas the outer segment rarely includes albite but no paragonite. The inner segment often includes quartz grains that preserve high residual pressures corresponding to the eclogite facies conditions, whereas the outer segment includes quartz grains that preserve lower residual pressures corresponding to the epidote-amphibolite facies conditions. For this composite-zoned garnet, the assemblage of sodic phase inclusions and the values of the residual pressure of quartz inclusions imply the following successive metamorphic path: prograde eclogite facies stage → decompression and hydration reaction stage → prograde epidote-amphibolite facies stage. Metasediments containing this composite-zoned garnet with such evidence for eclogite facies conditions are more widely distributed than the eclogitic lithologies that contain the assemblage of omphacite + garnet. These results imply that the combination of (i) the chemical composite zoning of garnet, (ii) the nature of sodic phase inclusions, and (iii) the residual pressure of quartz inclusions is useful as criteria to ascertain the true distribution of the lithologies that have experienced eclogite facies metamorphism for the Sambagawa metasediments, which was difficult to recognize in earlier studies. [ABSTRACT FROM AUTHOR]

Aragonite and omphacite-bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite-rich and graphite-poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite-poor layer contains a 1.5-cm ivory-colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO3 end-member: the FeCO3 and MnCO3 components are both less than 0.3 mol% and the SrCO3 component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P > 1.1–1.3 GPa and T = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite-bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized. [ABSTRACT FROM AUTHOR]

Jadeite-bearing eclogites and associated blueschists locally crop out in a greenschist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca-poor/Fe-rich core and Ca-rich/Fe-poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (Xjd = 0.34–0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre-exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure ( P)-temperature ( T) conditions of 0.9 GPa/390°C−1.4 GPa/560°C for the stage of the garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation, and 2.2 GPa/495°C-2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix. The estimated P-T conditions and continuous changes of mineral parageneses imply a counterclockwise P-T path which is a combination of (i) an early prograde stage of high-pressure/low-temperature ( HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax-of-subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle–slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge. [ABSTRACT FROM AUTHOR]

Abstract: Two-mica granites from the Nkambe area (West Cameroon) comprise two-mica granite and magnetite-rich two-mica granite. They are composed of K-feldspar, phengite, biotite and quartz. Megacrysts of magnetite surrounded by pink-colored patches are present locally. They are peraluminous with a ferroan affinity and exhibit normative corundum. These two-mica granites were generated at the minimum temperature and pressure of crystallization of 640°C/600MPa and later on evolved in the late stage of differentiation to give magnetite-rich two-mica granite. During the differentiation, hydrothermal events took an important role and led to the genesis of megacrysts of magnetite and leaching of alkalis, calcium, iron and magnesium out of some minerals. However the substitution of Fe3+ by Alvi has been more prominent in magnetite-rich two-mica granite than in two-mica granite leading the formation of muscovite adversely to celadonite during the hydrothermal process. This process lasted about 50m.y and was associated to the cooling path that reached lower pressures with the formation of chlorite at temperatures below 400°C. [Copyright &y& Elsevier]

Presents a study that examined the chromian dissakisite-(Ce) in a garnet lherzolite from the Su-Lu ultrahigh pressure metamorphic terrane in China. Description of the sample; Mineral composition of the sample; Origin of the chromian dissakisite-(Ce).

Discusses a study which examined the partitioning of strontium between coexisting minerals of the hollandite- and piemontite-groups in a quartz-rich schist from the Sanbagawa belt in Japan. Mode of occurrence and sample description; Mineral composition; Conclusions and recommendations.

Focuses on a study of Sr-bearing zoisite and epidote in ultra-high pressure (UHP) metamorphic rocks, which are common constituents of eclogites and associated paraschists throughout the Su-Lu ultra-high pressure (UHP) province, in eastern China. Geological background and petrography of the Sr-bearing zoisite and epidote; Identification of the two distinct sites of monoclinic epidote-group minerals; Discussion pertaining to the study.

Presents a paper by the authors that reports an occurrence of sodian and yttrian garnets, with up to 0.37 wt% Na2O, 2.1 wt% Y2O3 and 0.48 wt% Yb2O3, in orthogneisses from Su-Lu terrane of eastern China and discusses the substitution relations of Na and Y + Yb into garnet. Details of the study and its results; Discussion on the results.

Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is the most common and sometimes coexists with other types of garnet in a thin section. The coexistence of the different types of garnet is probably due to deformation‐induced lithological mixing at various points during the prograde eclogite facies stage. [ABSTRACT FROM AUTHOR]

Abstract: Here, we report the first discovery of an amorphous SiO2 phase (APSI phase) in a pseudomorph after coesite included in garnet from an ultrahigh‐pressure (UHP) eclogite from the Su–Lu metamorphic belt, eastern China. Using transmission electron microscopy, Raman spectroscopy and selected area electron diffraction, we show that the internal structure of the pseudomorph consists of an APSI phase with nano/submicrocrystalline particles of quartz and a polycrystalline K‐bearing fibrous sheet‐silicate phase (KFSS phase). The APSI phase‐bearing aggregates included in the garnet might have formed by reactions involving a supercritical fluid during exhumation by the following processes: (1) the development of radial cracks within the host garnet by the phase transition of coesite to quartz; (2) the decomposition of a part of the pseudomorph following infiltration of supercritical fluid; (3) the precipitation of the KFSS phase from the fluid phase during subsequent exhumation and cooling, which was likely promoted by a change in the metamorphic fluid from supercritical and/or subcritical to aqueous fluid; and (4) the rapid precipitation of the APSI phase under a metastable (non‐equilibrium) state, such as quenching, during a later stage of the exhumation. Whether the APSI phase generally formed during exhumation and survived widely throughout the Su‐Lu terrane is unknown. However, the presence of the APSI phase in a UHP eclogite provides new insight into the geodynamic phenomena occurring at continental collision zones. [ABSTRACT FROM AUTHOR]

This paper is a testimony of how electron microprobe dating of monazite was developed in the early days of this technique, in the 90's. Five contributions have been collected from Nagoya, Clermont-Ferrand, Orléans, Salzburg, and Amherst. The technique was designed in Nagoya by Professor Suzuki, using an old JXA-5A JEOL electron microprobe. The first results were published in 1991, with great difficulty. Professor Suzuki also created the isochron-like Pb-Th* diagram to present the data. Progressive improvement of the method finally allowed the Nagoya team to date Cenozoic monazite. The technique was re-discovered in Clermont-Ferrand using a Cameca Camebax microprobe. This team demonstrated the accuracy of the method, and proposed statistical tools to improve the precision and to handle multimodal populations. The first results were published in 1993. In Orléans, the French Geological Survey (BRGM) developed the method with a Cameca SX50 to support mapping projects. They developed a new method for calculating the age, and showed that this technique was very efficient for dating monazite in migmatites. In Salzburg, the first results were obtained in 1996, using a Jeol JX 8600. It proved to be very efficient in the Alps to distinguish Cadomian, Ordovician, Variscan and Alpine events. It allowed this team to directly date a monazite-allanite reaction they described for the first time. Finally, the Amherst team investigated in detail the technical issues of electron microprobe monazite dating. They showed that the best analytical procedures must include at least: detailed chemical mapping, several interference corrections and special care for background measurements. With the help of Cameca they designed a specific microprobe for dating, named “Ultrachron”. With this machine, high precision ages can be obtained on well characterized individual grains in thin section, giving birth to a new era in geochronology of complex metamorphic rocks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Indentation testing of natural single-crystal a-quartz parallel to the crystallographic c axis, using a triangular pyramidal diamond indenter at a maximum load of 500 mN, produced a very small residual volume of less than 1 µm³ in which α-quartz is highly stressed. Laser Raman microspectroscopy across the indentation at room temperature and pressure revealed a shift in the Raman bands, interpreted to reflect the residual stress field generated within the α-quartz. Based on the observed Raman shift, we identified a steep gradient in the residual non-hydrostatic stress field after complete unloading in quartz near the impression formed by indentation. At the center of the indentation, the maximum compressive stress and tensile stress were inferred to be higher than 2.2 and 0.1 GPa, respectively. [ABSTRACT FROM AUTHOR]

The Higashi-akaishi garnet-bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic-type ultrahigh-pressure metamorphism. The body of 2 km × 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene-rich rocks. Dunite samples contain high Ir-type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene-rich rocks contain low concentrations of Ir-type PGE and Cr, high concentrations of fluid-mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al2O3. The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine-dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene-rich rocks are cumulates of subduction-related melts. Thus, the Higashi-akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form. [ABSTRACT FROM AUTHOR]

The chemical Th-U-total Pb isochron method (CHIME) has been applied to determine the ages of monazite grains from metasediments of greenscshist-amphibolite facies in the Altai orogen, northwest China. The area of distribution of these metamorphic rocks is divided into the Permian (261–268 Ma) central-western and Devonian (377–382 Ma) eastern units on the basis of their metamorphic ages. The Devonian CHIME ages are consistent with the Pb–Pb ages of granitoid in the eastern unit, and support the idea that emplacement of the granitoids was synchronous with regional metamorphism at deep levels. The Permian metamorphic ages (the present study) and igneous ages previously reported from the central-western unit can be interpreted in terms of subduction of crustal material and oceanic plate, and rapid exhumation. [ABSTRACT FROM AUTHOR]

Raman CM geothermometry applied to 126 samples of pelitic schists collected over an area of 11 km × 7 km reveals the thermal structure of the Asemigawa region of the Sanbagawa metamorphic belt, southwest Japan in unprecedented detail. In general, the estimated temperatures gradually increase from south to north in the range of 288–553°C. However, a temperature gap from ~380 to ~440°C is identified near the boundary between the chlorite and garnet zones. This temperature region matches the depth of the continental Moho of the Sanbagawa subduction zone. The temperature gradient in the higher‐temperature domain is higher than that in the lower‐temperature domain, and large‐scale tight folds that affect the thermal structure are developed in the high‐grade units and in the vicinity of the temperature discontinuity. These geological structures probably reflect that the exhumed slab units was dammed at the Moho depth due to the upward movement being impeded by increase in the coupling strength of the overlying rocks associated with exhumation from beneath serpentinite rocks to a shallower domain overlain by crustal rocks. Changes in the coupling strength along the subduction boundary led the strong folding at the higher‐temperature domain and the pre‐formed foliation developed at the Moho depth may have acted as the tectonic boundary, resulting in a temperature discontinuity. These results will contribute to elucidating various geological phenomena occurring in the forearc regions of modern subduction zones. [ABSTRACT FROM AUTHOR]

To understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~109 cm−2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~108 cm−2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~108 cm−2, but a high dislocation density region of ~109 cm−2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤108 cm−2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]