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Xray mapping images of phengite in a kyanitequartz eclogite (ME75043008) from the Gongen area. Abbreviations are defined in Table 2.
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Kyanite-bearing edogitic assemblages occur in the highest-grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/ pargasite), phengite and paragonite occur as inclusions in garne...
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Eclogites are high pressure and high temperature metamorphic rocks found at various locations originating from different types of formations. Although their essential mineralogy is quite simple, containing just two minerals, a pyroxene and a garnet, the accessory minerals that can be found within its lattice can be highly varied indeed. To better u...
This contribution presents a geological and structural study of the Val d’Ala metaophiolite complex (Lower Piemonte Zone) at the southern boundary of the Gran Paradiso Massif. Although this area was known since the 18th century for mining activity, a complete and modern multiscale structural study of the metaophiolite is still lacking. For this rea...
Metaeclogites from the Devesil Unit, East Rhodopes are studied. Well preserved eclogite paragenesis is presented by omphacite+garnet+rutile and is obliterated by still high temperature retrograde assemblage of newly formed garnet, diopside, amphibole, plagioclase, titanite, and quartz. The P-T conditions of the high pressure metamorphism are in the...
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...
Peridotites and different types of eclogites occurring in the Monte Duria area (Adula–Cima Lunga unit, Central Alps, Italy) share a common eclogite-facies peak at P = 2.6–3.0 GPa and T = 710–750 °C, constrained by conventional thermobarometry and thermodynamic modelling. High-pressure minerals are replaced both in peridotites and in eclogites by lo...
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... Phase equilibrium modeling of jadeite-and/or chloritoid-bearing metapelites and metagreywackes in the same unit also indicated they underwent UHP metamorphism at 25-31 kbar and 430°C-510°C (Lü et al., 2012a). Similar bulk rock compositions were reported in the Sulu metamorphic belt, China Zhang et al., 2004) and the Sanbagawa metamorphic belt, Japan (Miyamoto et al., 2007). These rocks were called eclogitic gneiss , high-Si eclogite , and quartz eclogite (Miyamoto et al., 2007). ...
... Similar bulk rock compositions were reported in the Sulu metamorphic belt, China Zhang et al., 2004) and the Sanbagawa metamorphic belt, Japan (Miyamoto et al., 2007). These rocks were called eclogitic gneiss , high-Si eclogite , and quartz eclogite (Miyamoto et al., 2007). In fact, they are not typical eclogites, but eclogitefacies felsic rocks, in which UHP index minerals were not found. ...
Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.
... These rocks are extensively distributed in the Besshi region, central Shikoku, and are divided into the eclogite and noneclogite units (Wallis and Aoya 2000;Kouketsu et al. 2014). The eclogite unit, which was equilibrated at 1.9-2.5 GPa/525-740 °C (e.g., Miyamoto et al. 2007;Endo 2010;Endo and Tsuboi 2013), was juxtaposed onto the subducted noneclogite unit during exhumation (e.g., Wallis and Aoya 2000;Kouketsu et al. 2014). These two units subsequently recrystallized and joined together, during prograde metamorphism, up to epidote-amphibolite facies conditions, forming the regional thermal structure of the Besshi and neighboring regions (cf. ...
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.
... On the other hand, to explore the HP evolution of Sanbagawa belt, we selected a kyanitebearing quartz eclogite from the Gongen area (Besshi area). The bulk-rock composition used for calculating the phase equilibrium models correspond to sample GE1501a from Miyamoto et al. (2007) and Enami et al. (2019). All bulk-rock compositions are given in Table 1. ...
... Mineral compositions used to constrain the P-T evolution of the samples from the Renge belt are from Shinji and Tsujimori (2019) and Tsujimori (2002), whereas for the Sanbagawa sample correspond to Miyamoto et al. (2007). Models were constructed from 0.8-2.5 GPa and 450-650 C for samples from the Renge belt whereas for the eclogite from the Sanbagawa belt, the model was calculated from 1.3-3.0 ...
... This array has been interpreted as an accretionary complex formed during subduction at the eastern margin of Eurasia (Wallis et al., 2009). The highest-grade portions of the Sanbagawa metamorphic belt are exposed in the Besshi area, in central Shikoku; the eclogite-facies portion in the Besshi area consists of the Iratsu and the ultramafic Higashi-Akaishi bodies, and the Seba and Tonaru metabasites and schists (Aoki, Aoki, Chiba, et al., 2020;Aoki, Aoki, Tsujimori, et al., 2020;Aoya, 2001;Enami et al., 2004;Hattori et al., 2010;Kouketsu et al., 2014;Miyagi & Takasu, 2005;Miyamoto et al., 2007;Ota et al., 2004;Weller et al., 2015). ...
Petrological modeling is a powerful technique to address different types of geological problems via phase‐equilibria predictions at different pressure‐temperature‐composition (P‐T‐X) conditions. In this contribution, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high‐pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan; and by (2) forward‐modeling the mineral–fluid–melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). In particular, we calculated the peak metamorphic conditions at ~2.0–2.3 GPa and ~550–630 °C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5–2.8 GPa and ~640–750 °C. According to our models, the quartz eclogite experienced partial melting just after peak metamorphism. On the other hand, in terms of the petrological evolution of the basaltic uppermost portion of the oceanic crust during subduction along the warm Nankai geotherm, our models show that fluid release occurs at ~20‐60 km, likely promoting high pore‐fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown. Our petrological models predict partial melting at > 60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at the considered conditions. Modeled melt compositions correspond to high‐SiO2 adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in SW Japan. We posit that, while at the considered conditions melt migration may not occur, the modeled melts may serve as an analogue to explain adakite petrogenesis.
... These eclogite facies lithologies might be grouped into kyanite-eclogite, common mafic eclogite, and associated schistose rocks of basic and pelitic lithologies. The kyanite-eclogite, in which protoliths might be mixtures of pelitic and basic volcaniclastic rocks (Takasu, 1989;Takasu et al., 1994;Utsunomiya et al., 2011;Enami et al., 2019), records P/T conditions of 2.3-2.5 GPa/570-740°C for the eclogite facies stage (Miyamoto et al., 2007;Endo and Tsuboi, 2013). However, common mafic eclogite retains a slightly lower P/T equilibrium of 1.8-2.1 GPa/520-590°C (e.g., Endo, 2010;Kabir and Takasu, 2010b). ...
The Tonaru epidote–amphibolite is one of the large metagabbro dominated bodies and occurs in schistose rocks of the Sanbagawa metamorphic belt, central Shikoku. This body locally retains mineral parageneses of eclogite facies equilibrium prior to the epidote–amphibolite facies stage. The lithologic boundary between the epidote–amphibolite and the surrounding schistose rocks was well observed along the Kokuryo River in the western part of the Besshi region in central Shikoku. Boundary zone of 1.5–2.5 m wide is developed between the epidote–amphibolite and pelitic schist. This zone is composed of a basic layer and alternating layers consisting of thin amphibole–rich and mica–rich bands, which occupy the epidote–amphibolite and pelitic schist sides, respectively. The basic layer has a chondrite–normalized rare–earth element (REE) pattern with slight enrichment of light REEs, which corresponds to epidote–amphibolite. By contrast, the amphibole–rich band has a flat REE pattern similar to the basic schist of the Sanbagawa belt.
The basic layer in the boundary zone and epidote–amphibolite have composite–zoned garnet, showing a compositional discontinuity between the core and mantle parts, similar to that in the Sanbagawa eclogite unit. Garnet in the amphibole–rich and mica–rich bands of the alternating layers and pelitic schist shows simple normal zoning, which commonly occurs in the Sanbagawa non–eclogite unit. Sodic plagioclase occurs as inclusions in the mantle part of the composite–zoned garnet and normally zoned garnet as well as in a matrix phase. These lithologies belong to the oligoclase–biotite zone with equilibrium pressure/temperature conditions of 1.1–1.2 GPa/595–625 °C; discontinuity of metamorphic grade is not detected throughout the outcrop for the epidote–amphibolite facies stage.
These data suggest that (1) the basic layer is the fractured part of the epidote–amphibolite and (2) the tectonic boundary between the eclogite and non–eclogite units corresponds to the lithologic boundary between the basic layer and alternating layers of thin amphibole–rich and mica–rich bands in the boundary zone.
... From decades of experimental work on mantle melting we know that the diversity of magmas compositions produced at arcs is primarily driven by mantle bulk composition and water content in the melting region (Sisson & Bronto, 1998;Grove et al., 2012;Mandler et al., 2014;Mitchell & Grove, 2015;Till, 2017). For example, primary melts with lherzolitic residues are more enriched in Ca, Na and Al (i.e. ...
Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths, and as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0.94, Mg# olivine = 0.88), high NiO in olivine (∼0.26 wt.%) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs. SiO2 wt.%, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure-temperature at 2.6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE & Ce enrichments and strong Nb & Zr depletions. The trace elements and the pressure-temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.
... Regarding barroisite, an open question is its P-T environment of formation. There are two views about the metamorphic stage at which barroisite forms: as part of a prograde and even peak metamorphic assemblage coexisting with the eclogite phases garnet and omphacite (Chatterjee & Ghose, 2010;Di Vincenzo, Horton, & Palmeri, 2016;Gómez-Pugnaire et al., 1997;Kim et al., 2019;Weller et al., 2015;Yokoyama, Brothers, & Black, 1986) versus formation at the epidote-amphibolite facies as the product of later decompression (Bucher, 2005;Caby et al., 2008;Kabir & Takasu, 2016;Li, Klemd, Gao, & Meyer, 2012;Lombardo, Rolfo, & Compagnoni, 2000;Miyamoto, Enami, Tsuboi, & Yokoyama, 2007;Okay & Whitney, 2010). Singh, Pant, Saikia, and Kundu (2013) ...
The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist‐facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist‐facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane + paragonite to jadeite + pyrope + quartz + H2O over the range of 2.2–3.5 GPa and 650–820 °C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane + paragonite was observed at the intersectional stability field of both paragonite and jadeite + quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na2O–MgO–Al2O3–SiO2–H2O. The second set of experiments involved unreversed experiments using glaucophane + zoisite ± quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780 °C. These produced omphacite + paragonite/kyanite + H2O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium‐calcium amphiboles such as winchite (⃞(CaNa)(Mg4Al)Si8O22(OH)2) and barroisite (⃞(CaNa)(Mg3Al2)(AlSi7)O22(OH)2). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane + zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H2O. It also indicates that sodium‐calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction‐zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850 °C involving a high‐Fe (Fe# = Fetotal/(Fetotal + Mg) ≈ 0.36) natural glaucophane, synthetic paragonite, and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet + omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane + zoisite is instrumental in controlling dehydration processes at the blueschist‐eclogite transition during warm subduction.
... The biotite zone schists in the Asemi-gawa unit and the Iratsu quartz eclogite have recorded the epidoteamphibolite facies overprint at 86 Ma (Aoki et al., 2009) and the amphibolite facies overprint at 109 Ma , respectively. The exhumation rates from the deepest level to the lower crust were 6 mm/y for the Asemi-gawa unit and 4.5 mm/y for the Iratsu quartz eclogite, which are calculated from the peak P-T conditions estimated by Aoki et al. (2009) and by Miyamoto et al. (2007), respectively (Fig. 7A). The exhumation rates of the garnet and chlorite zones were less than 6 mm/y, as deduced from the long-term deformation during the exhumation; in particular, the exhumation rate was extremely low in the chlorite zone. ...
The reported discordant and anomalously old K–Ar (⁴⁰Ar/³⁹Ar) phengitic white mica ages from collisional orogenic belts are due to the fact that white micas in continental lithologies are not reset completely during high– to ultrahigh–pressure (HP–UHP) metamorphism because the closure temperature of white mica is much higher than the generally accepted value, approximately 600 °C. On the other hand, phengites in HP–UHP schists experience deformation–induced recrystallization during exhumation of the host lithology. The radiogenic argon is released from the deformed phengite, as documented by comparison of the in situ ⁴⁰Ar/³⁹Ar dating of phengite included in rigid garnet and of stretched phengite in the matrix. These non–resetting and argon–release phenomena give inconsistent phengitic white mica ages in metamorphosed continental lithologies. The heterogeneity in the deformational process due to differences in lithological compositions, local domains or even within single mica crystals results in inconsistent ages, as documented from the in situ ⁴⁰Ar/³⁹Ar dating of the deformed micas. The Sanbagawa HP schist belt and Lago di Cignana HP–UHP units both consist of metamorphosed oceanic lithologies that usually record only a single metamorphic cycle and have phengites without any inherited excess argon. The duration of deformation during exhumation spans from the peak metamorphism to the end of deformation in the crust, making it possible to estimate the exhumation rates of the metamorphic sequences. The low exhumation rates (<6 mm/y) of the Sanbagawa belt suggest a slow strain rate during rock deformation, resulting in a ‘slow schist’ sequence with a recumbent fold structure. The rapid exhumation rates (<26 mm/y) of Lago di Cignana suggest a high strain rate during rock deformations, resulting in a ‘fast schist’ sequence consisting of several units with fault–bounded contacts. The Lago di Cignana UHP unit, which underwent the highest exhumation rate, could indicate a subsequent continental collision event, whereas the Sanbagawa belt did not experience a subsequent continental collision event.
... The eclogitic rocks of the Sambagawa belt have yielded a relatively wide range of P-T estimates of metamorphic conditions of ~500-800 °C and 1.4-2.5 GPa (Takasu, 1989;Aoya, 2001;Matsumoto et al., 2003;Ota et al., 2004;Miyagi and Takasu, 2005;Miyamoto et al., 2007;Aoki et al., 2009;Kouketsu et al., 2010Kouketsu et al., , 2014Kabir and Takasu, 2010). Peak ultrahigh-pressure conditions estimated for a garnet clinopyroxenite within the Higashiakaishi ultramafi c body of 700-810 °C at 2.9-3.8 ...
The M1 blueschist to epidote amphibolite metamorphism that defi nes the named metamorphic zones of the Sambagawa belt of Japan and coeval ductile D1 deformation overprinted and replaced formerly more extensive eclogite-facies rocks and obscured the original subduction-accretion architecture. Based on new fi eld, structural , and petrographic observations, integrated with published geochronologic, structural, and metamorphic petrologic data, we propose that the eclogites were emplaced both as intact slabs as well as blocks-in-mélange. Some of the latter may record earlier eclogite burial, exhumation to the surface, sedimentation, and resub-duction to eclogite-facies conditions. Syneclogitic D0 fabrics include widely distributed granoblastic fabrics, as well as fabrics defi ned by planar and linear preferred orientations. These eclogitic fabrics collectively indicate strain localization along the subduction interface at the depth of eclogite metamorphism (~50-80 km). Elongate bodies of metamorphosed pelagic sediments associated with mafi c rocks and trench-fi ll turbidites show that coherent imbricates and duplexes with subordinate mélange characterized the original subduction complex architecture of the Sambagawa belt. Eclogite-facies metamorphism spans a range of ages that may defi ne discrete pulses at ca. 120-110 Ma and ca. 90 Ma or more temporally intermediate subduction-accretion events associated with an extended period of subduction. D1 exhumation fabrics exhibit a west-vergent sense of shear antithetic to the rarely preserved east-vergent early (shallow) subduction fabrics (D-1). These early fabrics may have been rotated since their development. D1 fabrics are overprinted by south-vergent D2 brittle and brittle-ductile structures associated with an internal extrusional wedge that was subsequently cut by a major out-of-sequence fault, duplexed, and folded. Exhumation of the eclogite to the depth of the M1 overprint (0.5-1.5 GPa pressure difference between M0 and M1) may have taken place as extruded slabs accommodated by D1 Osozawa, S., and Wakabayashi, J., 2016, Variety of origins and exhumation histories of Sambagawa eclogite interpreted through the veil of extensive structural and metamorphic overprinting, in Bianchini, G., Bodinier, J.-L., Braga, R., and Wilson, M., eds., The Crust-Mantle and Lithosphere-Asthenosphere
... The Sanbagawa eclogite facies rocks were metamorphosed at pressures between 1.5 and 2.6 GPa (e.g. Aoki et al., 2009;Aoya, 2001;Endo & Tsuboi, 2013;Matsumoto et al., 2003;Miyamoto, Enami, Tsuboi, & Yokoyama, 2007;Tsuchiya & Hirajima, 2013;Ko, Enami, & Aoya, 2005). Currently, in this field, it cannot be determined if fine-grained metasedimentary lithologies underwent eclogite facies metamorphism; therefore, the areal distributions of the eclogite unit in the Besshi region have been deduced based on the combined study of the (i) compositional zoning in garnet, (ii) Na phase inclusions such as paragonite, glaucophane, and omphacite/jadeite in garnet, and (iii) the residual pressure retained by quartz inclusions within the garnet . ...
The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm ⁻¹ , and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P 2/ n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω 1 > 8.5 cm ⁻¹ , corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature ( P – T ) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.
... Subsequently, detrital zircon younger than 90 Ma has been found at a number of sites considered to be part of the Sanbagawa metamorphics (Knittel et al., 2014;Endo et al., 2018). The eclogite unit (nappe) records the highest metamorphic grade (Wallis and Aoya, 2000;Miyamoto et al., 2007;Endo and Tsuboi, 2013) but eclogite facies metamorphism appears to have not been restricted to the eclogite unit as relicts of such metamorphism were also detected in parts of the Besshi unit. The units traditionally considered as eclogite unit consist largely of mafic and ultramafic rocks. ...
The high-P/low-T Sanbagawa Metamorphic Belt that traverses SW Japan, has been subdivided into two belts thought to have been metamorphosed at ca. 120 Ma and at ca. 65 Ma ('Sanbagawa Metamorphic Rocks' and 'Shimanto Metamorphic Rocks'). The subdivision was based on the assumption that metamorphism occurred at ca. 116 Ma, largely based on an early Rb-Sr isotope study and zircon data obtained for the eclogite unit of the Sanbagawa Belt, whereas in some parts of the belt detrital zircons of late Cretaceous age (90-80 Ma) were discovered. Analysis of detrital zircons sampled from two sites within the area considered to expose the older 'Sanbagawa Metamorphic Rocks', including the area investigated by the Rb-Sr study, reveals the presence of zircons younger than 95 Ma in all samples and some grains as young as 80 ± 4 Ma. It is therefore concluded that the Sanbagawa Belt is one single tectonic entity that formed in the Late Cretaceous though it contains older components, including fossiliferous clasts, older basic meta-volcanics and eclogite units that may record earlier metamorphic events.
