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Journal of Petrology Volume 42 Number 7 Pages 1225-1248 2001
© Oxford University Press 2001
P–T–D Path of Eclogite from the Sambagawa Belt Deduced from Combination of Petrological and Microstructural Analyses
DEPARTMENT OF GEOLOGY AND MINERALOGY, GRADUATE SCHOOL OF SCIENCE, KYOTO UNIVERSITY, KYOTO 606-8502, JAPAN
Received November 19, 1999; Revised typescript accepted October 1, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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Both structural and petrological data can be used to constrain the P–T path of an eclogitic schist unit (the Seba basic schist) in the Sambagawa belt of SW Japan. The relationships between these two sets of data are well defined by porphyroblastic and other microstructures. The derived P–T path for the Seba basic schist has an overall clockwise trajectory with the decompression, or exhumation-related, path taking place under a lower P/T gradient than the burial, or subduction-related, path. The clockwise nature of the P–T path is qualitatively supported by chemical zoning of amphibole coexisting with eclogitic minerals. The significant feature of the P–T path is the presence of two temperature maxima, the first in the eclogite facies and the second in the epidote-amphibolite facies. The existence of two temperature maxima gives a simple explanation for the observation that metamorphic zonal boundaries postdating the eclogite facies metamorphism cross-cut the distribution of the main eclogite bodies in the Sambagawa belt. Estimates of metamorphic pressure using the jadeite content of clinopyroxene in the Seba area demonstrate the existence of a tectonic discontinuity between the eclogitic schist and surrounding non-eclogitic schist. Structural studies show that although these two units have experienced very different peak metamorphic conditions, they became juxtaposed during a single ductile deformation affecting both units. This deformation is related to exhumation of the eclogitic schist and subduction of the non-eclogitic schist, indicating that both were formed during the same subduction event. The presence of a major tectonic boundary between two units with a similar origin as subducted and accreted material, but contrasting metamorphic histories, can be interpreted in terms of nappe tectonics, and the existence of an ‘eclogite nappe’, the third nappe of the Sambagawa belt, is proposed.
KEY WORDS: deformation stage; dual thermal maxima; eclogite; P–T–D path; Sambagawa belt
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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Metamorphism of the Sambagawa belt of SW Japan was classified by Miyashiro (1973)
as an intermediate high-pressure facies series. Petrological, chronological and structural data suggest that the evolution of the Sambagawa belt is, in general, compatible with the predictions made for metamorphism in the deeper parts of an accretionary wedge (e.g. Banno & Sakai, 1989; Enami et al., 1994; Takasu et al., 1994; Wallis, 1998). The belt is composed mainly of metamorphosed sediments that have pelitic, basic or siliceous compositions. The mineral assemblages of most rocks are characteristic of metamorphism under greenschist, blueschist or epidote-amphibolite facies conditions. However, eclogite facies mineral assemblages have also been reported (e.g. Takasu, 1989), which occur dominantly within metagabbro bodies. Most of these eclogite-bearing bodies are located in the Besshi district (Fig. 1). The estimated P–T paths for the eclogite-bearing bodies are distinct from those estimated for the surrounding epidote-amphibolite-grade rocks and show considerable variation amongst themselves (e.g. Kunugiza et al., 1986; Takasu, 1989; Fig. 2). For this reason, most workers refer to the eclogite-bearing bodies as ‘tectonic blocks’ and the Besshi district has been treated as a tectonic mélange zone (e.g. Takasu, 1989). The general consensus is, therefore, that the highest grade of the Sambagawa regional metamorphism was in epidote-amphibolite facies (or low-temperature part of amphibolite facies; Fig. 2), and that the eclogite occurrences represent a disparate group of tectonic blocks with distinct metamorphic histories.
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However, recent studies (Aoya, 1998; Aoya & Wallis, 1999) have shown that eclogite is developed throughout a large meta-basaltic unit, the Seba basic schist (Figs 1 and 3), which is locally associated with metapelite. Eclogite in the Seba basic schist unit was previously interpreted as the result of solid-state contact metamorphism (Takasu, 1984). The new results indicate that at least part of the eclogite facies metamorphism affects metasediments and can be attributed to the same tectonic event as formed the rest of the Sambagawa belt, and that the highest grade of the Sambagawa metamorphism can be extended to include the eclogite facies. This also raises the possibility that the eclogite facies metamorphism recorded in the so-called tectonic blocks may represent the most extreme part of the Sambagawa metamorphism. The present work concentrates on establishing the P–T path for the Seba basic schist. The P–T path is important, first because it can give information on physical conditions of the earliest subduction, accretion and exhumation processes related to the Sambagawa metamorphism, and second because it can be helpful in studying the large-scale structure of the Sambagawa belt.
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The eclogite facies metamorphism of the Seba basic schist contrasts with the estimated peak P–T conditions for the surrounding pelitic schists, which lie within the epidote-amphibolite facies (Enami et al., 1994; Fig. 2). This difference suggests the existence of a major tectonic discontinuity between these two units. Determination of the peak P–T conditions for the eclogite can, therefore, provide a quantitative test of the existence of such a tectonic boundary. In addition, if the eclogite unit and the surrounding schist were tectonically juxtaposed early in their history, the P–T paths of the eclogite and the surrounding schist should be initially distinct, and eventually converge during the tectonic event that brought them together. This convergence should be reflected not only in the petrology but also the structure or deformation (D) of these units. Microstructures that reveal the relative growth timing of various metamorphic minerals are, therefore, important to study the relationships between the metamorphic and tectonic histories. In this work, I employ a range of meso- and microstructural data in combination with petrological studies to constrain the P–T path. In this sense, the P–T path presented in this paper can be termed a P–T–D path. The deformation history of the Seba basic schist has already been described in detail by Aoya & Wallis (1999). In this paper, I use the same classification of deformation stages and these can be employed as relative time-markers for studying the evolutionary history of the Seba eclogite. It will be shown later that the combination of structural and petrological analyses is a very powerful tool in studying the metamorphic history of schistose eclogites such as the Seba examples.
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GEOLOGICAL AND STRUCTURAL FRAMEWORK OF THE SEBA BASIC SCHIST |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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Geological setting
In general, the Sambagawa metamorphism is non-eclogitic and can be discussed in terms of four metamorphic zones based on the appearance of key metamorphic minerals in metapelite (e.g. Higashino, 1990
; Fig. 1). In order of increasing grade these are: the chlorite, garnet, albite–biotite and oligoclase–biotite zones. The Seba basic schist unit, the focus of this paper, occurs in the central part of the Besshi district and near its centre contains a small eclogite-bearing metagabbro body (the Sebadani metagabbro; SB in Fig. 1) with outcrop dimensions of 260 m x 180 m. The Seba basic schist unit is surrounded by rocks of the albite–biotite zone including pelitic, basic and siliceous schist (Fig. 3a). Eclogitic schists of various textural types are widely distributed in the Seba basic schist (Fig. 3b). Aoya & Wallis (1999) have recently shown that formation of all textural types of eclogite in the Seba basic schist can be accounted for by a single subduction-related metamorphism.
In contrast, in the eclogitic Sebadani metagabbro, Takasu (1984) proposed that there were two separate phases of eclogite facies metamorphism. This is based on abrupt changes in chemical composition of garnet with an Mg-rich core and an Mg-poor rim. Takasu (1984) proposed that an initial high-temperature eclogite stage (shown in Fig. 2) was followed by a second low-temperature stage, and that rapid exhumation between the two stages, with tectonic emplacement of the metagabbro into the Seba basic schist, caused solid-state contact metamorphism forming the eclogitic basic schist near the eclogitic metagabbro (Fig. 2). However, the structural study of Aoya & Wallis (1999) demonstrated that emplacement of the metagabbro into the Seba basic schist took place earlier than, or simultaneous with, a deformation associated with the subduction of the Seba basic schist to eclogite facies. In addition, omphacite in the eclogitic metagabbro has no clear chemical zoning that could correspond to the zoning in garnet (Takasu, 1984). There is, therefore, no clear evidence for the proposed two eclogite stages in the metagabbro. Formation of eclogite within the Sebadani metagabbro can be more simply explained in terms of a single eclogite facies metamorphism related to the same subduction event that caused the formation of the Seba basic schist eclogite (Aoya & Wallis, 1999). The dominant focus of this paper is on eclogite from the Seba basic schist, and does not consider possible differences in the metamorphic histories of the Sebadani metagabbro and the Seba basic schist. In this paper I shall refer to the eclogite occurrences within the Seba basic schist as ‘the Seba eclogite’.
The P–T path for the albite–biotite zone schists in the Besshi district, surrounding the Seba basic schist, was reported by Enami et al. (1994). Using the zoning patterns of rare sodic pyroxene in siliceous schist that coexists with albite and garnet, these workers demonstrated that the early stage of decompression is associated with a temperature rise. This implies that the albite–biotite zone schist has a clockwise P–T path (Fig. 2). Similar clockwise P–T paths have been proposed for the higher-grade parts of the non-eclogitic Sambagawa schist based on compositional zoning of amphiboles in hematite-bearing basic schists (Otsuki & Banno, 1990; Nakamura & Enami, 1994). The peak T metamorphic conditions of albite–biotite zone surrounding the Seba basic schist are estimated to be 520±25°C, 8–9·5 kbar (0·8–0·95 GPa) (Enami et al., 1994), which correspond to the epidote-amphibolite facies (Fig. 2). This estimate suggests that there is some P–T gap, or tectonic discontinuity, between the Seba eclogite and the surrounding non-eclogitic schists.
Structural framework of the Seba basic schist
Detailed structural analysis in the Seba basic schist was carried out by Aoya & Wallis (1999) and these workers showed that three deformation stages can be recognized on the outcrop scale: D0, DA and DB in chronological order (Fig. 4). Mesostructural relationships between these three deformation stages are illustrated in Fig. 4c. The dominant deformation stage is DA and schistosity formed during DA (SA) can be observed throughout most of the Seba basic schist (Fig. 4a). Development of SA is, in general, stronger in the outer part of the Seba basic schist and weaker in the inner part. The effect of the DA phase of deformation is weakest in the area immediately adjacent to the Sebadani metagabbro, where an old schistosity (S0) formed during the D0 phase of deformation is preserved (Fig. 4a). The existence of the two different schistosities is best shown in a stream section with continuous exposure from S0- to SA-dominant areas in the eastern part of the Seba basic schist. The transition from S0 to SA is shown by the development of DA folds (FA), which fold S0 with the development of a newly formed SA along their axial planes (Fig. 4a and c). DB is a post-DA phase of deformation recognized most readily by the presence of kink-like mesoscopic folds that fold SA (Fig. 4c). The existence of an independent DB phase of folding is demonstrated by local preservation of interference structures between FA and FB. FB folds are observed throughout the Seba basic schist on a scale of centimetres to 10 m.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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The Seba basic schist is dominantly composed of epidote amphibolite containing barroisitic amphibole, epidote, phengitic mica and lesser amounts of garnet, quartz and rutile. In addition, rare but spatially widespread omphacite is also found (Fig. 3a). The common development of garnet means that samples of the Seba basic schist that contain omphacite can be termed eclogite in a broad sense. The petrography of the Seba eclogite including both microstructural relationships and mineral chemistry is described below. The mineral analyses were carried out using a Hitachi S550 electron microscope with a Kevex Quantum Detector, at Kyoto University, following the procedure of Mori & Kanehira (1989) and Hirajima & Banno (1991)
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Textural variation of eclogite and chemical compositions of associated omphacite
The existence of the two different schistosities (S0 and SA) in the Seba basic schist is shown not only by mesostructural studies but also by microtextural observations of omphacite (Aoya & Wallis, 1999). Increasing strength of DA is associated with a transition from S0 to SA. This transition is also reflected in changes in the microtexture of omphacite. In eclogite samples from the area immediately adjacent to the Sebadani metagabbro, omphacite is coarse grained and randomly oriented, cross-cutting a pre-existing schistosity, S0. This type of eclogite is referred to as R(random)-type eclogite (Fig. 3). On the other hand, in eclogite samples from the outermost part of the Seba basic schist, omphacite is fine grained and strongly aligned within the SA schistosity forming a mineral lineation, LA. This type of eclogite is referred to as L(lineated)-type eclogite (Fig. 3). The textural difference between R- and L-type eclogites can be explained by a single-stage growth of omphacite at some time after D0 but before the completion of DA and by a textural evolution from R- to L-type with increasing strength of DA deformation. The existence of intermediate textures in areas where both S0 and SA are well developed (I-type eclogite; Fig. 3) strongly supports the idea of the textural evolution from R- to L-type eclogites.
The idea that all textural types of eclogite formed by a single metamorphism can be further assessed by comparing chemical compositions of omphacite within these three types of eclogite. In most eclogite samples including R-, I- and L-types, omphacite has no clear chemical zoning although some L-type eclogites rarely contain omphacite with an aegirine-rich core. Chemical compositions of omphacite are plotted in Fig. 5 and several data are listed in Table 1. Plots in Fig. 5a and b show that omphacite in the three different types of eclogite has almost identical composition around Xjd [= Al/(Na + Ca)] = 0·35. These data strongly support the proposal that the genesis of all omphacite is related to a single stage of metamorphism.
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Synchronous growth of garnet and omphacite during DA deformation
The stability field of garnet for basic lithologies in P–T space is, in general, larger than that for omphacite. Indeed, in the highest-grade part of the non-eclogitic Sambagawa belt, the occurrence of garnet is reported in some basic rocks of epidote-amphibolite facies grade. It is, therefore, necessary to verify synchronous growth of garnet and omphacite before it is possible to discuss the eclogitic P–T conditions for the Seba basic schist. Synchronous growth is shown most clearly by the presence of textures indicating both the formation of garnet after omphacite and the formation of omphacite after garnet. In addition, if the chemical composition of garnet and omphacite of both textural types is identical, the only way to reconcile these observations is to conclude that omphacite and garnet developed contemporaneously. Textures showing growth of omphacite after garnet are mainly observed in R- and I-type eclogites, where fine-grained euhedral garnet is overgrown by relatively coarse-grained omphacite (Fig. 6a). L-type eclogite commonly contains coarse-grained porphyroblastic garnet (Fig. 6b and c). This type of garnet contains abundant inclusion minerals, including omphacite, showing growth of garnet after omphacite. However, growth of omphacite after garnet can also be observed in L-type eclogite: coarse garnet in L-type is locally fractured and in some cases, these fractures are filled with omphacite (Fig. 6c). Omphacite filling these fractures clearly grew after garnet. It has already been shown that omphacite from all textural types has nearly the same composition (Fig. 5). In addition, microprobe data for coarse-grained garnets in L-type eclogite and fine-grained garnet overgrown by omphacite in R- and I-type eclogite all plot in the same compositional range (Fig. 7a and c; Table 2). These observations indicate that garnet and omphacite in the Seba eclogite grew synchronously and that the garnet grew, at least in part, during eclogite facies metamorphism.
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In addition to the synchronous growth of garnet and omphacite, the microstructure of coarse-grained garnet in L-type eclogite also gives information on the chronological relationship between the different deformation stages and the eclogitic metamorphism. Microstructural features of the coarse-grained garnet can be summarized as follows:
- inclusion trails in the coarse-grained garnet commonly show a sigmoidal curvature. In addition, fine-grained straight inclusion trails are also rarely observed in the core of the garnet. In this case, the garnet grains contain a composite inclusion trail that includes a straight segment in the core and a sigmoidally curved segment in the outer part (Fig. 6b).
- The inclusion trails in the outermost rim of garnet lie subparallel to the schistosity of the surrounding matrix, SA. Omphacite inclusions occur in the outer part of garnet and lie parallel to other inclusions, which together constitute the sigmoidally curved inclusion trails (Fig. 6b).
The observations given in (2) indicate that the sigmoidal inclusion trails observed in the garnet rim are related to DA. Moreover, the sigmoidal arrangement of SA indicates that the garnet rotated as it grew during the formation of the rim, which indicates non-coaxial deformation during DA (e.g. Passchier & Trouw, 1996). The rare straight and fine-grained inclusion trails found in the garnet core can, therefore, be interpreted as an older schistosity (S0) and growth of the core is likely to have occurred during a non-deformational stage between D0 and DA. The important point here is that the main growth of garnet and omphacite occurred synchronously with DA deformation and consequently that DA occurred mainly in the eclogite facies.
Occurrence of albite and tectonic interpretation of DA
Albite is only rarely found in the Seba eclogite. It is, however, a common mineral in the epidote amphibolite samples from the Seba basic schist. The main modes of occurrence of albite can be classified into the following three types: (1) a constituent mineral of albite–hornblende symplectite formed after omphacite (Fig. 6d); (2) porphyroblasts with highly irregular outlines that grew across SA (Fig. 6e); (3) albite veins that cross-cut SA and are rarely associated with sodic pyroxene (Fig. 6f). These textures indicate that the main growth of albite in the Seba basic schist occurred after DA, and that albite did not coexist with omphacite (Aoya & Wallis, 1999). As demonstrated in the previous section, DA deformation occurred mainly in the eclogite facies. Post-DA growth of albite, therefore, suggests that DA can be related to the decompression (or exhumation) from the eclogite facies with albite absent to conditions where albite was stable.
Direct evidence for the syn-DA change in metamorphic conditions can be observed in some L-type eclogites. In most of the L-type eclogites in the Seba basic schist, coarse-grained omphacite is commonly preserved in the strain shadows developed adjacent to garnet (Aoya & Wallis, 1999; Fig. 6c). Microstructures and petrological data show that the omphacite grew before or during DA. The coarse-grained omphacite can, therefore, be regarded as either a relic of earlier coarse grains or as having grown during DA to fill the strain shadows. A further important observation is that these microstructures are locally associated with reaction zones developed between garnet and omphacite consisting of amphibole and a small amount of albite (Fig. 6g–i). Where the reaction zones form thin layers, these layers are developed subnormal to the DA stretching direction (Fig. 6g). In the other case, where there is greater separation between the garnet and omphacite, the amphibole grains defining the reaction zones are oriented parallel to LA (Fig. 6h). These microstructural features imply that although garnet and omphacite initially grew during DA and were in contact with each other, subsequently, but during the same DA deformation, these two minerals were pulled apart to form the reaction zones composed of albite and amphibole parallel to LA. Therefore, although the earlier stage of DA occurred in the eclogite facies, the final stage of DA was associated with metamorphism during which albite was stable. This clearly indicates that DA is related to decompression from the eclogite facies to the albite stability field.
Quartz is ubiquitous throughout the Seba basic schist and, therefore, the mole fraction of jadeite (Xjd) in omphacite can be used to study changes in metamorphic pressure at different stages of DA that are characterized by distinct microstructures. Where omphacite coexists with quartz but albite is absent, the maximum jadeite content of omphacite can be used to derive a minimum pressure. In the Seba eclogite, the maximum Xjd of omphacite is 0·38 (Table 1; Fig. 5). Conditions where albite is stable are represented by an albite vein (Fig. 6f) that cross-cuts SA and is associated with sodic pyroxene (Table 1). In this case the Xjd is only 0·15, and Xjd can be used to give a direct pressure estimate for a given temperature, because all three phases, albite, jadeite and quartz, coexist. The difference in jadeite component of Na-pyroxene associated with eclogitic and post-eclogitic microstructures (Xjd = 0·38 and Xjd = 0·15) shows, therefore, that the Seba basic schist experienced significant decompression associated with DA deformation. Sodic pyroxene coexisting with albite and with an Xjd of 0·12–0·18 has also been reported from the Sebadani metagabbro, where it occurs as a constituent mineral of symplectite after omphacite (Takasu, 1984).
Geothermometry: temperature rise during decompression
To study possible changes in temperature during DA, a coarse-grained garnet with abundant inclusion minerals was selected from a sample of L-type eclogite (Figs 6b and 8) for detailed microprobe analysis. The garnet is nearly euhedral with a hexagonal cross-section, suggesting it is cut near its core and bounded by {011} faces. In addition, the garnet contains composite inclusion trails that have beautiful sigmoidal patterns in the outer part and are straight in the core (Fig. 6b). From core to rim the inclusion trails trace out a shape resembling a sickle, and I shall refer to garnet with this type of microstructure as ‘sickle garnet’. It is rare to see the straight part of the inclusion trails in any given thin section, indicating that the full sickle shape is only observed when the section passes close to the centre of a grain.
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Inclusion minerals found in the sickle garnet are omphacite, quartz, barroisitic amphibole, epidote, mica, carbonate (calcite and ankerite), rutile, titanite, ilmenite and apatite. Chlorite is rarely found along cracks replacing garnet. The mineral assemblage of inclusions in the garnet is almost the same as that of matrix. One important difference, however, is the type of mica. Although compositions of mica both in matrix and in outermost rim of the garnet are phengite, mica grains included in the more central part of the garnet are paragonite (Fig. 8b, Table 3). Paragonite decomposes into jadeite + kyanite + H2O at high pressure (
24 kbar at 600°C) and the existence of paragonite can, therefore, be used to give an upper bound for the metamorphic pressure. Syn-DA growth of the sickle garnet deduced from the sigmoidal arrangement of inclusion trails suggests that the core of the garnet grew at an early stage of DA and, because DA took place during decompression, under higher pressure conditions than the rim. Assuming that exhumation will be reflected in penetrative deformation, the presence of paragonite in the central part of the sickle garnet also implies that the core of the garnet grew in the paragonite stability field.
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X-ray colour maps of the sickle garnet for Mn, Fe, Mg and Ca (Fig. 8a) show well-preserved concentric zoning patterns with hexagonal contours parallel to the edges of the garnet (notice that the intensity-colouring relationship is changed for each figure to emphasize the zoning patterns). These features suggest that the effect of later diffusion was minor and that chemical compositions of the garnet are close to those at the time of growth. Quantitative microprobe analysis of the garnet was carried out along the line A–B shown in Fig. 8b and the results are plotted in Fig. 8c. The main characteristics of the zoning pattern are: (1) the Mn content gradually decreases from core to rim showing a classic ‘bell-shape’ profile; (2) Mg content increases from core to rim. These features are often used as qualitative evidence for temperature rise during garnet growth (e.g. Takasu, 1984). Both features (1) and (2) are commonly observed in other garnet grains in the Seba eclogite including coarse garnet in L-type eclogite and fine-grained garnet in R- and I-type eclogites, although some garnet grains show an increase of Mn in the outermost rim (Fig. 7d). Garnets from the eclogitic basic schist of Takasu (1984), which corresponds to the R-type eclogite in this paper, also show the same type of chemical zoning with reference to Mn and Mg.
The sickle garnet includes abundant omphacite inclusions (Fig. 8b). Microprobe analysis of these omphacite inclusions shows that there is no major difference in the chemical compositions of the 43 analysed points (Fig. 5a). To obtain an estimate for the metamorphic temperature, the Fe–Mg distribution coefficient, KD (= [XFe/XMg]grt/[XFe/XMg]omp), for the omphacite inclusions and adjacent garnet was determined. Because the Mn content of the garnet used in these determinations is sufficiently low (up to 7 mol %; Fig. 8), its effect on KD values can be ignored. As mentioned above, the garnet can be considered to have preserved the composition at the time of growth and, therefore, omphacite inclusions are also expected to maintain their chemical compositions at the time of garnet overgrowth; i.e. the omphacite grains would be ‘sheltered’ by the continuously growing garnet. It should, therefore, be possible to assess the extent to which chemical equilibrium for the Fe–Mg exchange reaction was achieved for each garnet–omphacite pair by examining the change of KD from core to rim. The result is shown in Fig. 9a. KD values were calculated using three methods of estimating the Fe3+ content of omphacite (Fig. 9a). Irrespective of the method used, the KD values calculated for inclusions near the core of the garnet (Mn content >3%) show considerable scatter, locally showing a sudden increase or decrease. However, KD values calculated for inclusions near the rim of the garnet (Mn content <3%) have nearly constant values. These data suggest that the Fe–Mg exchange reaction between omphacite and garnet did not reach equilibrium during growth of the core of the garnet. In contrast, equilibrium for the same reaction is much more nearly achieved during growth of the rim of the garnet. This interpretation is supported by the zoning pattern of garnets taken from a number of spatially distinct eclogite samples in the Seba basic schist. Although the chemical compositions of the garnet cores show some variation probably as a result of the effect of differing bulk compositions, successive growth of garnet shows a gradual convergence towards the same compositional area (Fig. 7b and d), with the exception of rarely observed Mn-enriched outermost rims. These observations suggest that the composition of the garnet rims throughout the Seba basic schist is strongly controlled by physical factors such as temperature and pressure.
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Estimation of the Fe3+ content of omphacite by assuming Fe3+ = Na –Al gives an average KD for the sickle garnet in the region where Mn <3% of 12·6 (Fig. 9b). Comparison with data from the rest of the garnet suggests this represents the peak metamorphic temperature for the Seba eclogite. The remaining data can also be used for a semi-quantitative discussion of the temperature change during the garnet growth. Average KD values for regions corresponding to Mn contents of 6–7%, 5–6%, 4–5%, 3–4% and <3% are listed in Fig. 9b, and these show a general trend of decreasing KD from core to rim. This suggests that decompression of the Seba basic schist in the eclogite facies was accompanied by a slight temperature rise (
50°C). One more important point about Fig. 9b is the result of using different methods to estimate the Fe3+ content of omphacite. In the core of the garnet, using the M4O6 method gives KD values that are systematically higher than those derived using Fe3+ = Na – Al method. However, in the region where Mn <3%, the results of both methods are in close agreement (12·6 and 12·7). This concordance shows that it is not necessary to take into account the Ca-Tchermak’s component of omphacite when estimating metamorphic temperature for the outermost rim. These data also suggest that using the Fe3+ = Na – Al method is likely to overestimate the true metamorphic temperature for the central part of the garnet. In addition to L-type eclogite that contains the sickle garnet, KD values were also determined for garnet + omphacite pairs in I- and R-type eclogite. Measurements were made in microstructural domains where euhedral fine-grained garnet is overgrown by omphacite showing the stable coexistence of these two minerals (Fig. 6a). The determined KD shows considerable scatter with values ranging from 21·5 to 13·1; however, this range is nearly identical to that of the L-type eclogite (Fig. 9c), indicating that all three types of eclogite formed at very similar temperatures. The KD values for I- and R-type eclogites have a tendency to be slightly higher than those for the L-type eclogite (Fig. 9c). This may suggest that although all types of eclogite were formed during the same metamorphism, the metamorphic temperatures for I- and R-types were slightly lower than that for the L-type eclogite.
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TIMING OF EMPLACEMENT OF THE SEBA BASIC SCHIST: CONSTRAINTS FROM STRUCTURAL STUDIES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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The classification of deformation stages of the Seba basic schist was carried out independently from that of the surrounding non-eclogitic schist. The deformation stages for the non-eclogitic Sambagawa schist based on Wallis (1990)
are listed in Fig. 10a with a schematic illustration showing the structural relationships. The non-eclogitic part of the Sambagawa belt is characterized by penetrative deformation with the development of a schistosity and associated orogen-parallel east–west stretching lineation. The main deformation stage for the non-eclogitic part is defined by this schistosity and is called Ds (e.g. Wallis, 1990) or D1 (e.g. Faure, 1983; Okamoto et al., 2000). To determine the timing of emplacement of the Seba basic schist into the surrounding non-eclogitic schists, it is important to correlate Ds with deformation stages D0, DA and DB that have been defined independently for the Seba basic schist.
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The area around the easternmost tail of the Seba basic schist shows a complicated lithological distribution with many rock types gradational between basic, pelitic and siliceous schist. This complexity is probably in part due to the existence of a hinge of a major fold in this area. In fact, many folds on micro- to mesostructural scales are observed throughout the area both in the Seba basic schist and the non-eclogitic schist, which is dominantly composed of pelitic and siliceous schist (Fig. 4a), and these folds can be regarded as structures formed during the same deformation stage.
In the non-eclogitic schist, Ds folds can be distinguished from similar structures formed during Dt and Du on the basis of the relationship with albite spots. Microstructural studies show that most of the growth of albite in the albite–biotite zone schist took place without accompanying deformation, between the subduction-related Dr and exhumation-related Ds (Wallis et al., 1992; Wallis, 1998). Therefore, in a typical region with a well-developed Ss, the albite spots are porphyroclastic with Sr preserved as straight inclusion trails that can be oriented at a high angle to the surrounding Ss (Fig. 11a). In regions where Fs folds and the associated Sr are preserved, the straight inclusion trails preserved in the albite spots will be continuous with Sr outside of the spot. In the case of the folds in the siliceous lithology around the eastern end of the Seba basic schist, continuity between inclusion trails and the folded schistosity is commonly observed and, therefore, these folds can be regarded as Fs (Fig. 11b).
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On the other hand, the folds in the Seba basic schist part can be regarded as FB. Figure 11c is a photomicrograph of albite spots from the Seba basic schist part and this shows that inclusion trails within the albite spots are nearly straight and are continuous with the surrounding folded schistosity. If the folds were FA, the folds themselves should be overgrown by albite spots because the growth timing of albite in the Seba basic schist is post-DA, as discussed above. The folds in the basic part, therefore, can be seen to be FB and both the folded schistosity and straight inclusion trails within albite spot are SA.
Correlation of deformation stages is now possible. A schematic illustration showing structural relationships observed in the easternmost area is presented in Fig. 10b. Folds of the same deformation phase affect both the Seba basic schist and the non-eclogitic lithologies. These folds are Fs in the non-eclogitic part and FB in the Seba basic schist clearly indicating Ds = DB (Fig. 10b). A further important point is that the folded schistosity is Sr in the non-eclogitic part and SA in the Seba basic schist, showing apparent correspondence of the two deformation stages, Dr and DA (Fig. 10b). However, DA of the Seba basic schist occurred mainly in eclogite facies as discussed above, whereas Dr of the surrounding schist occurred associated with subduction and metamorphism in the blueschist or epidote-amphibolite facies (Wallis et al., 1992; Wallis, 1998; Fig. 10a). The fact that two deformations that occurred under different metamorphic grades appear to be identical in a restricted region indicates convergence of the two deformations during their final stages. Tectonic emplacement of the Seba basic schist into the surrounding non-eclogitic schist, therefore, can be interpreted to have taken place during the final stages of both DA and Dr. The growth of the albite spots mainly occurred during a non-deformational stage after this emplacement and the growth timing is the same for both the Seba basic schist and the surrounding non-eclogitic schist.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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P–T–D path of the Seba basic schist
The P–T–D path of the Seba basic schist can be drawn as shown in Fig. 12a. The corresponding microtextural evolution during this metamorphic history is illustrated in Fig. 12b. The main events in this path can be summarized as follows.
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- Subduction to eclogite facies. The Seba basic schist shows local gradation with pelitic rock types and also locally contains pillow structures. These features imply that the Seba basic schist formed close to the Earth’s surface and then underwent prograde metamorphism to eclogite facies associated with subduction (Aoya & Wallis, 1999). The presence of omphacite cross-cutting S0 in R-type eclogite suggests that D0 deformation is related to the subduction.
- Accretion and subsequent non-deformational stage. Peak metamorphic pressure was achieved during this stage. The pressure is unclear but is lower than the reaction curve paragonite = jadeite + kyanite + H2O. The existence of a non-deformational stage before DA is shown by straight S0 inclusion trails in the core of coarse garnet in L-type eclogite. The straight inclusion trails also suggest that growth of garnet began during this stage.
- Decompression associated with temperature rise. This stage corresponds to an early stage of DA deformation. Growth of garnet and omphacite mainly occurred during this stage. Syn-DA growth of garnet resulted in the formation of sigmoidal inclusion trails.
- Peak metamorphic temperature. The estimated metamorphic temperature using garnet–clinopyroxene Fe–Mg exchange geothermometer of Ellis & Green (1979) with a Ca content of garnet of 0·24 is 610–640°C (using a pressure range of 12–24 kbar).
- Decompression into the epidote-amphibolite facies and emplacement into non-eclogitic schists. The emplacement of the Seba basic schist occurred during the final stages of DA deformation. This can also be thought of as the accretion of the non-eclogitic schists to the Seba basic schist unit.
- Non-deformational stage after accretion of the non-eclogitic schists. Porphyroblasts of albite grew during this stage both in the Seba basic schist and non-eclogitic schist, overgrowing SA and Sr.
- Decompression together with non-eclogitic schist associated with DB (Ds). A temperature rise during the early part of this decompression stage was demonstrated by Enami et al. (1994) for the non-eclogitic schist.
Compositional change of amphibole: an indicator of a clockwise P–T path
The stability and compositional range of amphibole is very large and almost all metabasites contain some amphibole. The experimental work of Liu et al. (1996) showed that the stability of amphibole extends into the lower-pressure part of the eclogite facies for basic rocks of basaltic composition. In the Seba basic schist, textures showing the stable coexistence of barroisitic amphibole + omphacite are commonly observed (Fig. 6a). However, the chemical composition of amphibole coexisting with eclogitic minerals in different parts of the world shows considerable variation probably representing differences in the associated geothermal gradients, i.e. high or low P/T. As extreme examples, eclogite may be associated with Na-amphibole such as glaucophane representing cold eclogite formed under low geothermal gradients (e.g. Maruyama & Liou, 1988), or hornblende representing hot eclogite formed under relatively high geothermal gradients (e.g. Heinrich, 1986). In these two cases, the two types of eclogite can be considered to have formed under metamorphic conditions with a similar P/T ratio to those of the blueschist and amphibolite facies, respectively. Eclogite associated with barroisitic amphibole, intermediate between Na-amphibole and hornblende, may form under conditions with a P/T ratio characteristic of the epidote-amphibolite facies (e.g. Yokoyama et al., 1986).
The P–T–D path of the Seba basic schist eclogite shown in Fig. 12a is clockwise with the earlier stage of decompression accompanied by a temperature rise. One of the most significant characteristics of a clockwise P–T path is that the peak T occurs after peak P and petrological features formed around the peak P are, therefore, likely to be modified or even erased. The barroisitic compositions of amphibole in the Seba basic schist will not, therefore, indicate the P/T ratio of peak P but of peak T. This implies that the geothermal gradient associated with Pmax, and representing the physical conditions of the original subduction zone, is likely to have a higher P/T ratio than suggested by the presence of barroisite.
Information on the oldest part of the metamorphism in the Seba basic schist is expected to be preserved in the R-type eclogite where the oldest structures related to D0 are cross-cut by omphacite. In R-type eclogite, three distinct modes of occurrence of amphibole can be defined (Fig. 13a and b): (1) fine grained S0-forming amphiboles in and around porphyroblastic omphacite; (2) coarse-grained amphibole cross-cutting S0; (3) constituent mineral of symplectites after omphacite. Microprobe data of these three types of amphiboles are plotted in Fig. 13c (several data listed in Table 3). The first important point to notice about this plot is that compositional ranges of S0-forming and S0-cutting amphiboles are indistinguishable. This indicates that although S0 can be structurally recognized as an old foliation pre-dating the eclogite facies metamorphism, the petrological features of amphibole stable during D0 have been thoroughly overprinted during subsequent re-equilibration associated with rising temperature. The second point is that although nearly all the amphibole can be classified as barroisite, grains that cross-cut S0, in general, show a distinct compositional zoning from an Na-rich core to an Na-poor rim locally reaching compositions close to hornblende similar to that of the symplectite-forming amphibole. This zoning pattern, reflecting either progressive growth or secondary compositional change along grain boundaries by diffusion, represents a qualitative change of metamorphic conditions from high to low P/T. The development of amphibole cross-cutting S0 and attainment of symplectitic compositions only in the outermost rim show that the compositional zoning developed after subduction and accretion but before the epidote-amphibolite facies metamorphism. The zoning pattern of amphibole is, therefore, qualitative evidence supporting the idea that the P–T path has a clockwise trajectory as shown in Fig. 12a. A final important feature of the amphibole composition is that amphibole with a glaucophane composition is rarely found as inclusions of coarse-grained garnet in some localities of L-type eclogite (Fig. 13) although SA-forming amphibole in the matrix is dominantly barroisitic. This further suggests that prograde metamorphism of the Seba basic schist was associated with relatively high P/T conditions and may have passed through the blueschist facies.
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Significance of dual temperature maxima
Enami (1982) was the first to recognize the importance of oligoclase in the metamorphic zonation of the Sambagawa belt, and subdivided the previously defined biotite zone into albite–biotite and oligoclase–biotite zones in the Besshi district. One important result of the work by Enami (1982) is that the isograd defined by the appearance of oligoclase cross-cuts the Western Iratsu mass, the largest eclogite bearing body in the Besshi district (Fig. 1). This result is compatible with the P–T path of the Seba basic schist, which has two temperature peaks, the first in the eclogite facies and the second in the epidote-amphibolite facies together with the schists of the albite–biotite zone. Division of metamorphic zones in the Sambagawa belt is related to the second temperature maximum and has nothing to do with eclogitic metamorphism. The observation that the oligoclase–biotite zone cross-cuts the Western Iratsu mass can, therefore, be explained as being the result of overprinting during the second rise in temperature.
Takasu (1986) reported garnet with microstructures suggesting two stages of garnet growth punctuated by a phase of resorption within thin layers of pelitic schists sandwiched between the Sebadani metagabbro and the Seba basic schist. He considered that the growth of the resorbed core is related to the Sambagawa non-eclogitic metamorphism and the growth of the rim to high-pressure contact metamorphism caused by emplacement of the metagabbro into the Seba basic schist. However, Aoya (1998) has demonstrated that solid-state contact metamorphism is physically improbable and, moreover, Aoya & Wallis (1999) have used structural observations to show that this process is unnecessary to account for the formation of the Seba eclogite. The two-stage growth of garnet in the metapelite is the only feature unexplained by our model that could be taken as in support of solid-state contact metamorphism. The present work suggests that an alternative explanation is possible. Growth of the core could be related to the first temperature maximum in the eclogite facies and that of the rim to the second temperature maximum in the epidote-amphibolite facies. The reason for resorption between the two stages is still unclear. However, the reason is also unclear in the study by Takasu (1986) and I contend, therefore, that there is no reason to prefer the proposal of Takasu (1986) over the idea presented here.
Tectonic significance of Dr–DA convergence
As the P–T–D path shows, at some time the exhumation-related DA deformation in the Seba basic schist and the subduction-related Dr deformation in the non-eclogitic schist converge (Fig. 12a). This feature can be interpreted as a result of deformation along a common subduction boundary (Fig. 14). Two structural units on either side of a subduction boundary will suffer nearly identical deformation. In the case of the Seba basic schist and the surrounding non-eclogitic schist, DA for the exhuming eclogitic unit and Dr for the subducting non-eclogitic unit can be considered to have occurred along their mutual boundary until the accretion of non-eclogitic unit onto the hanging-wall side (Fig. 14).
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Dr–DA convergence has important implications for the position of the Seba eclogite in the evolution of the Sambagawa belt. First, the correlation of DA and Dr indicates the rough synchronicity of these deformations, and the facts that accretion of the Seba basic schist pre-dates DA and accretion of the non-eclogitic schist took place during or after Dr clearly indicate that the accretion of the Seba basic schist took place before that of the non-eclogitic schists. Second, growth of garnet + omphacite in the Seba basic schist has been demonstrated to have occurred synchronously with DA, which is synchronous with Dr. Dr is a subduction-related event for the non-eclogitic Sambagawa metamorphic unit (Wallis et al. 1992; Wallis, 1998), which shows that the eclogitic metamorphism of the Seba basic schist is related to the same subduction event that caused the non-eclogitic Sambagawa metamorphism.
Eclogite nappe: interpretation of tectonic boundary
As shown by the P–T–D path and discussion above, formation of the Seba eclogite is clearly related to the same subduction event that caused the non-eclogitic Sambagawa metamorphism. Similar eclogite is also found in the Western Iratsu mass (Fig. 1). The metamorphic history of the Western Iratsu mass has been qualitatively discussed by Takasu & Kohsaka (1987) and is very similar to that of the Seba basic schist with a peak metamorphic temperature of 650°C (Fig. 2). This suggests that the region of eclogitic metamorphism related to the Sambagawa subduction event can be extended to include the Western Iratsu mass. There are, however, significant differences in the peak P–T conditions between these eclogitic bodies and the surrounding schists. The peak P of the non-eclogitic schist is estimated at 8·5–9·5 kbar using Xjd of rare sodic pyroxene coexisting with albite and quartz (Enami et al., 1994). One of the sodic pyroxene-bearing samples of Enami et al. (1994) is located 300 m to the east of the Seba basic schist (TD1201; Fig. 3a) and the sodic pyroxene has an Xjd = 0·19. Comparison of this value with Xjd of omphacite in the Seba basic schist eclogite (maximum 0·38) and also the observation that in this case it does not coexist stably with albite, demonstrates a pressure gap of >2 kbar (>6 km in depth; Fig. 12a) within a horizontal distance of only 300 m. This clearly suggests the existence of a major tectonic discontinuity between the Seba basic schist and the surrounding schists.
The above relationships between the eclogitic and non-eclogitic parts of the Sambagawa belt, their identical origin and existence of a major tectonic boundary between them, can be interpreted in terms of nappe tectonics. In the non-eclogitic part of the Sambagawa belt, two nappes (the Besshi and Oboke nappes) have already been recognized on the basis of chronological and metamorphic differences between them (Takasu et al. 1994; Wallis, 1998). Here, I propose the existence of a third nappe, which I call ‘the eclogite nappe’, at the highest structural levels of the Sambagawa belt. I further suggest that the Seba basic schist and the Western Iratsu mass can both be regarded as members of the eclogite nappe and that the eclogitic metamorphism present in the Sambagawa belt is clearly related to the associated ‘Sambagawa’ subduction. Other eclogitic bodies such as the Tonaru and the Eastern Iratsu mass (Fig. 1) are in analogous structural positions to the Seba and Western Iratsu eclogite and could also be regarded as members of the eclogite nappe (Wallis & Aoya, 2000). A variety of P–T paths have been proposed for the eclogite-bearing bodies and these are distinct from those of the Seba basic schist and the Western Iratsu mass (Fig. 2). There are, however, a number of potential problems with the way these P–T paths have been derived. In particular: (1) in cases where two distinct stages of eclogite metamorphism are proposed this is based mainly on chemical zoning patterns of garnet, and although omphacite exists in these regions, there is no strong reason to believe that both stages of growth are due to eclogitic events; (2) the possible stability of amphibole in the eclogite facies is little considered and in most cases amphiboles are used simply as minerals representative of the epidote-amphibolite and the blueschist facies metamorphism. A re-evaluation of the eclogitic P–T conditions of these bodies is, therefore, an important goal for future studies, which would provide a test of the idea of a single eclogite nappe.
Significance of combining petrological and structural methods
The Seba eclogite is relatively poor in petrological information especially regarding the pre-peak T metamorphism. In general, metamorphic garnet is expected to include some mineral parageneses older than those in the surrounding matrix. However, as mentioned above, the mineral assemblage of inclusions in garnet in the Seba eclogite is almost identical to that in the matrix. In addition, amphibole forming the pre-eclogite schistosity, S0, has undergone significant re-equilibration during later metamorphism. I document garnet in some L-type eclogite that contains glaucophane. However, almost all amphibole included in garnet in the Seba basic schist eclogite has a barroisitic composition and, moreover, amphibole in the glaucophane-bearing garnet shows considerable compositional variation, with glaucophane, winchite, barroisite and taramite compositions all present (JSB28 in Table 3). This (and the scatter of KD value in Fig. 9a) suggests that chemical equilibrium was not attained during most of the growth of garnet.
A significant feature of the P–T–D path shown in Fig. 12a is that no single point is independently determined in P–T space even for the exhumation-related part. If the eclogite was analysed by petrological methods alone, all that could be known about the path would be that it passed through the eclogite facies within the shaded box in Fig. 12a and then experienced metamorphism in the epidote-amphibolite facies. This result would not include either the decompression and temperature rise in the eclogite facies or the second temperature peak in the epidote-amphibolite facies. It is, however, possible to place constraints on the shape of the P–T path as shown in Fig. 12a. The additional constraints are provided by combining microstructural data with the petrological results. The essence of this approach can be summarized as follows: (1) microstructures are fully exploited as markers to reveal the relative timing of growth of various metamorphic minerals; (2) the tectonic significance of the deformation stages (in particular the relationship to subduction and exhumation) is elucidated using a combination of microstructural and petrological observations; (3) the timing of juxtaposition of two different metamorphic units is determined by correlating their deformational histories. As shown in the present study, this approach has potential to place closer constraints on the P–T history of rocks than those given by petrological studies alone. In studying the metamorphic history, in particular the P–T path, of foliated rocks such as the Seba eclogite, it will, therefore, be useful to use a combination of both petrological and structural analyses.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND STRUCTURAL...PETROGRAPHYTIMING OF EMPLACEMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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The P–T–D path for the Seba basic schist eclogite of the Sambagawa belt was derived as shown in Fig. 12a using a combination of structural and petrological methods. From this P–T–D path I can draw a number of conclusions about the geological evolution of the Sambagawa belt, which are summarized below.
- The clockwise P–T path deduced from application of geothermometry to syn-DA garnet omphacite is compatible with the chemical zoning of barroisitic amphibole with an Na-rich core and Na-poor rim and represents a transition from high to low P/T conditions.
- Structural and metamorphic studies demonstrate that convergence of the P–T path of the Seba basic schist with that of the surrounding non-eclogitic schist took place during prograde metamorphism of the non-eclogitic schist. This indicates that the Seba basic schist experienced the second temperature maximum in the epidote-amphibolite facies together with the non-eclogitic schist. The existence of two temperature maxima during exhumation of the Sambagawa eclogite is compatible with the distribution of the oligoclase–biotite zone, which cuts across the boundary of the eclogitic Western Iratsu mass.
- Geothermobarometry shows quantitatively that there is a significant difference in the peak P–T conditions of the Seba basic schist and the surrounding non-eclogitic schist, demonstrating the existence of a major tectonic boundary between the two units. This tectonic discontinuity can be regarded as a nappe boundary and I make the new suggestion that the Sambagawa belt contains a large-scale eclogite nappe comprising at least the Seba basic schist and the Western Iratsu mass.
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ACKNOWLEDGEMENTS |
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I would like to express my sincere thanks to S. R. Wallis for his useful discussion and his careful review of this paper. X-ray colour mapping for the sickle garnet was carried out at Nagoya University with the help of M. Enami, to whom I am very grateful. I also thank J. Hermann and B. R. Frost for their reviews. I am particularly grateful to B. R. Frost for his constructive comments delivered with a streak of good humour. I also thank R. J. Arculus for editorial assistance. I acknowledge the financial support of JSPS Research Fellowship for Young Scientists.
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FOOTNOTES |
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*Telephone: +81 (0) 52-789-2529. Fax: +81 (0) 52-789-3005. Present address: Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan.
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M. Aoya, S.-i. Uehara, M. Matsumoto, S. R. Wallis, and M. Enami Subduction-stage pressure-temperature path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction Geology, December 1, 2003; 31(12): 1045 - 1048. [Abstract] [Full Text] [PDF]
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