Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1989.043.01.50p519-532.

1989, v.43;Geological Society, London, Special Publications Shohei Banno and Chihiro Sakai metamorphic belt, JapanGeology and metamorphic evolution of the Sanbagawa

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Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Shohei Banno & Chihiro Sakai

SUMMARY: The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic 'Dr deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by 'D2' (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the 'D~' trend. These deformational events established the present thermal structure. The final regional deformation formed upright 'D3' folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P - T - t loops which progress at higher pressures and return at lower pressures to the surface.

The Sanbagawa (Sambagawa) metamorphic belt is the high-pressure part of the classic outer Japan pair of metamorphic belts, proposed by Miyashiro in his paper 'Evolution of metamorphic belts' published in 1961. Although the role of the Sanbagawa metamorphism in relation to the paired metamorphism in general, and the tectonic evolution of the Japanese Islands in particular, is still far from well understood, we summarize, herein, the present knowledge of the Sanbagawa belt mainly from the viewpoint of metamorphic petrology.

To reduce the number of references, the views generally accepted in Japan will be quoted from review papers rather than from the original work.

Geological outline

Areal extent

The Sanbagawa belt (Fig. 1) extends from the Saganoseki Peninsula in eastern Kyushu to the Kanto Mountains, north-west of Tokyo. Further eastwards, drilling has revealed its extension below the Pliocene-Pleistocene Kanto Plain (Tanaka 1978). The western end of the Sanbagawa belt was once placed in the Yatsushiro area, central Kyushu (Miyashiro 1961, 1973, Banno 1964), but Maruyama et al. (1984) have shown that glaucophane schists oc- curring to the west of the Saganoseki Peninsula belong to the next belt to the south, the Kurosegawa belt. In the east, the Sanbagawa

belt was considered to extend from the Kanto Plain to the eastern margin of the Abukuma Plateau (Miyashiro 1961, Banno 1964), but radiometric ages revealed that the high-pressure schists sporadically developed there were dis- tinctly older (300 Ma) than those of the Sanbagawa belt (Nozawa 1977). According to the present widely held view, these older schists occur along the suture between the Jurassic complex and accreted landmass (Banno 1986). We disagree with the notion that basic schists of the central Abukuma metamorphic terrain, of low-pressure metamorphism, are polymeta- morphosed Sanbagawa schists (Faure et al. 1986), as we see no supporting petrographical evidence (Tagiri et al. 1988).

All through south-west Japan, i.e. west of Fossa Magna (Fig. 1), the Sanbagawa belt occurs on the southern side of the Median Tectonic Line. Its southern boundary is, however, not well defined. It has been thought to be gradational to the northern sub-belt of the Chichibu belt, which was once regarded as a late Palaeozoic geosynclinal complex but is now believed to be part of the Jurassic complex, which comprises the major part of the basement rocks of the Japanese Islands. Faure (1983) and Guidi & Charvet (1987) consider t h a t some parts of the Chichibu belt are superficial nappes of unmetamorphosed formations which were thrust southwards over the Sanbagawa belt, and are distinct from that part of the Chichibu belt that grades into the Sanbagawa belt. However, where these formations suffered high-pressure metamorphism, we consider that they most

From DALY, J. S., CLIFF, R. A. & YARDLEY, B. W. D. (eds) 1989, Evolution of Metamorphic Belts, Geological Society Special Publication No. 43, p. 519-532.

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probably belong to the Sanbagawa belt. Parts of the Chichibu belt are free from the Sanbagawa metamorphism (Fig. 1), but in this paper we only consider the metamorphosed part of the Chichibu belt.

A few textbooks (Miyashiro 1973, Turner 1973) and the metamorphic facies map of Japan (Hashimoto et al. 1970), show the Sanbagawa metamorphism in central Shikoku extending southwards to the neighbourhood of the Kuro- segawa belt. Later studies, notably Maruyama et al. (1984), have shown, however, that the high-pressure schists near the Kurosegawa belt are not part of the Sanbagawa metamorphism. Thus the southern border may be placed in the northern part of the Chichibu belt, as shown on the large-scale geological map of Japan (1:1000000; Hirokawa 1978), where it is designated as unmetamorphosed Palaeozoic System.

Post-metamorphic tectonic elements

The areal extent and the present trend of the

Sanbagawa belt shown in Fig. 1 are the result of the complex tectonic history of the Japanese Islands, which is summarized below.

1 The transition of the boundary between the Eurasia and North American plates from central Hokkaido to Fossa Magna took place about 0.5 to 1 Ma ago (K. Nakamura 1983).

2 The collision of the Izu Peninsula with the Japanese Islands occurred at about 5 -10 Ma ago and bent the Sanbagawa belt in the Fossa Magna region (Niitsuma & Akiba 1985).

3 The 60 ° clockwise rotation of south-west Japan to open the Japan Sea took place at around 15 Ma ago (Otofuji & Matsuda 1984).

4 The formation of the accretionary complex of the Shimanto terrane continued from Cretaceous to Oligocene (Taira et al. 1983).

5 The Kurosegawa belt, a mature island-arc or microcontinent, once situated in an equatorial region, collided with the Japanese islands in late Jurassic or early Cretaceous time and was fragmented by transcurrent faulting (Taira et al. 1983, Maruyama et al.

Geology & metamorphic evolution of Sanbagawa belt, Japan 521

1984). This could be, however, earlier or contemporaneous with the Sanbagawa metamorphism.

Thus, when discussing tectonics before the opening of the Japan Sea, such as the Sanbagawa metamorphism, we should use coordinates whereby the Japanese Islands had a trend of N30°E and were situated on the eastern margin of the Asian continent. As the details of the reconstruction of the geology before that period is still in dispute, we will herein refer to the present orientation of south-west Japan, i.e. nearly east-west.

The nature of the original rocks

The Jurassic complex in south-west Japan is generally composed of Jurassic shale in which various rock types of oceanic derivation and of different ages are incorporated as olistoliths: Permo-Carboniferous limestone, greenstone and chert, and Triassic chert.

The Jurassic complex of the outer zone of south-west Japan, i.e. the southern side of the Median Tectonic Line, may be divided into three belts from south to north, the Chichibu, Mikabu and Sanbagawa (Fig. 1, see also Fig. 3). The Chichibu belt includes virtually unmetamorphosed to slightly metamorphosed rocks (the pumpellyite-actinolite facies), and the Mikabu belt is a summary name for lensoid areas of greenstones that occur intermittently near the boundary between the Sanbagawa and Chichibu belts.

The Sanbagawa belt is composed of schists of various lithologies such as pelite, psammite, basite, psephite, calcareous rocks and chert, and is accompanied by ultramafic and mafic tectonic blocks of deep-seated origin.

From the viewpoint of metamorphic facies series, the Chichibu, Mikabu and Sanbagawa belts are essentially gradational, but the Mikabu belt was a separate geological unit from the others at the time of its accretion.

The Chichibu belt contains tholeiitic to slightly alkalic basalts, many of which occur as pillow lavas (Maruyama & Yamasaki 1978). The Mikabu belt principally comprises lavas and gabbros with clastics derived from them, but also contains ultramafic masses, and amphibolite formed by ocean-floor metamorphism (Iwasaki 1979, Y. Nakamura 1981). The Mikabu complex may be the remnants of seamounts (Shikano 1981). The basaltic rocks of the Sanbagawa belt include alkali basalt as indicated by the occurrence of relict alkali pyroxene and Ti-augite.

Metasediments in the schist are mostly of shale or siltstone origin, and contain carbonaceous materials. The structurally lowest part in central Shikoku and the Kii peninsula to the east of Shikoku (Fig. 1) is occupied by a thick psammitic schist formation occasionally con- taining pebbles of quartz porphyry, i.e. continental derivatives. Usually, the superposition of pelitic schists and metabasites on the psammitic schists of the Koboke Formation in central Shikoku, is interpreted as conformable (Kenzan Research Group 1984), but Faure (1983) proposed that the overlying formation composed of oceanic materials is thrust over the psammitic formation, which was a part of the accreted Kurosegawa continent. In any event, the metamorphism is continuous over this contact.

Quartz schists, i.e. manganiferous and fer- rugenous metachert which offers fascinating mineralogy with a variety of N a - C a amphibole and Mn-rich minerals, are common in the schist.

Age of the sediments

It has been established that the Jurassic complex in south-west Japan is composed of Jurassic shales that host exotic sediments, presumably olistoliths. The Jurassic age of the Sanbagawa schists depends solely on the radiolaria found from the Mikabu belt in eastern Shikoku (Iwasaki et al. 1984). Further, the presence of Triassic limestone in the pumpellyite- actinolite zone is consistent with the lithology of the Chichibu belt, which is part of the Jurassic complex according to radiolarian biostratigraphy.

Metamorphic facies series

By metamorphic facies series, we mean the P - T trajectory of metamorphism at which the metamorphic recrystallization was most ex- ensive, as expressed by the series of mineral facies (Miyashiro 1961). This corresponds to the 'metamorphic geotherm' of England & Richardson (1977). The facies series of the Sanbagawa metamorphism is remarkably uniform for the whole distance of 700 km from eastern Kyushu to the Kanto Mountains.

Zonal mapping

At present, the zonal mapping (e.g. Fig. 2) is based on a four-fold division of pelitic schists into the chlorite, garnet, albite-biotite and oligoclase-biotite zones. None of the bound-

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aries between them is defined by a discontinuous reaction, and thus all of them are subject to bulk compositional effects. The garnet isograd, i.e. the first appearance of garnet in pelitic schists (usually as fine grains, 0.02 mm across with AM. = 0.25 and Xca = 0.38), occurs first in Mn-rich and Ca-rich schists. In the Asemigawa area, it corresponds to garnet-chlorite equilibria with the apparent F e - M g partition coefficient K' = 0.02-0.03, where K' = (Mg/Fe)g~t(Mg/Fe) chl. The entry to the albi te- biotite zone is defined by the first appearance of biotite with K' = 0.10, and that to the oligoclase-biotite zone by the appearance of oligoclase at which K' = 0.12. The K' values are calculated using pairs of chlorite cores (if zoned) and garnet rims, but are only used when Mn zoning in the garnet is simple, i.e. bell- shaped.

The four-fold zonal classification is practical, as the pelitic schists are widespread except in the Mikabu belt. The zonal mapping based on the appearance of index minerals presumes the isochemical nature of rocks concerned. Even though a comprehensive survey of bulk composition is not yet available, the variation of the Mn content of chlorite (Fig. 3) suggests that the most important parameter stabilizing garnet is statistically uniform over the area concerned. Table 1 compares the zones defined for the pelitic schists with the critical mineral assemblages of other rock types, namely haematite- bearing and haematite-free basic schists. The former are more sensitive to metamorphic grade, but their occurrence is too localized for areal mapping.

The chlorite zone is the widest zone in the Sanbagawa metamorphic belt, and we can divide it into four subzones wherever basic schists of appropriate compositions are available: the epidote-actinolite zone, the Al-rich epidote- pumpellyite-actinolite zone (high-PA zone), Fe3+-rich epidote-pumpellyite- actinolite zone (middle-Pa zone), and the pumpellyite- actinolite-haematite zone (low-PA zone). Zonal mapping based on these subzones has been done only in limited areas of central Shikoku. In Fig. 2, the epidote-actinolite zone and Al-rich epidote-pumpellyite-actinolite zone are jointly shown as higher PA zone.

Metamorphic fades series

Prograde changes of mineral assemblages of various rock types, as shown in Table 1, are typical of the standard metamorphic facies series of the whole Sanbagawa belt. To this a few added comments are necessary.

Maruyama & Liou (1985) have stated that a facies characterized by the assemblage chlorite-sodic-augite exists in the Osugi area of the Mikabu belt of central Shikoku. The assemblage chlorite-sodic-augite is also common in the Mikabu greenstone belt of the Kanto Mountains (Hirajima 1983), where, however, it never occurs in quartz-bearing metabasites. As to the Osugi assemblage, Maruyama & Liou (1985) are not persuasive in demonstrating the presence of quartz in the assemblage in question. The examples from other areas they quote are in fact either quartz-free or contain aegirine-quartz assemblage, which is irrelevant to the present discussion. Previously, a quar tz - jadeite assemblage was reported in the Kanto Mountains, but Hirajima (1983) showed this to be incorrect.

Another problem with the facies series is the relationship between the oligoclase-biotite zone and eclogites. Eclogitic rocks, both prograde and retrograde, occur in the albi te- biotite and oligoclase-biotite zones in the Bessi (Besshi) area (Kunugiza et al. 1986, Takasu 1989). We accept Takasu's notion that prograde hornblende schists were transformed to eclogitic hornblende schists by prograde metamorphism. Such a transformation did not take place in the oligoclase zone, but did so in the higher grade part of the albite-biotite zone. It is then possible that the formation stages of the prograde eclogites and the oligoclase-biotite zone are different. The formation of regional oligoclase could have post-dated the eclogite formation, and was perhaps related to the uplift of the Sanbagawa regime as suggested by Otsuki (1980).

Tectonic blocks

Kunugiza et al. (1986) summarized our under- standing of the tectonic blocks within the Sanbagawa belt and concluded as follows.

1 Tectonic blocks were originally deep-seated rocks such as granulitized metagabbro, eclogi- tized metagabbro and ultramafic rocks carry- ing garnet-clinopyroxenite.

2 They were mostly or partly recrystallised under biotite-zone conditions of the Sanba- gawa metamorphism.

3 No relics of prograde metamorphism from the chlorite zone conditions of the Sanba- gawa metamorphism have been found.

4 Some blocks (at least the Sebadani body described by Takasu (1984)) were intruded at higher temperature than the host Sanbagawa schists and contact-metamorphosed them to eclogitic assemblages.

524 S. Banno & C. Sakai

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Geology & me~urnorphic evolution of Sanbagawa belt, Japan

TABLE 1. Comparison of metamorphic zones defined for different rock types

525

Chlorite-bearing Haematite-bearing Haematite-free pelitic schists basic schists basic schists

Oligoclase- Oligoclase- Oligoclase- biotite zone hornblende zone hornblende zone

(oligoclase) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(albite) Albite- Albite- hornblende zone hornblende zone

Albite- biotite zone ....................................................................................... ....................................................... Barroisite zone Barroisite zone

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Garnet zone

Chlorite zone

Crossite zone Epidote- actinolite zone . . . . . . . . . . . . . . . . . . . . . . . .

Winchite zone Pumpellyite- ............................. epidote- Haematite- epidote- actinolite zone actinolite zone

Haematite- Pumpellyite- pumpellyite- stilpnomelane- actinolite zone zone

On this evidence, the solid-state emplacement of the tectonic blocks into the albite- and oligoclase-biotite regime was established. Recently, Takasu (1989) has shown that the geology of the tectonic blocks is more complex than summarized by Kunugiza et al. (1986), but their general scheme may be accepted.

Metamorphic temperature as a continuum

The mineral zones are defined by the change of mineral assemblages from one zone to another. They do not tell by themselves whether or not the temperature changes gradually in each of the mineral zones. In other words, temperature is not necessarily treated as a continuous quan- tity in a mineral zone. Metamorphic temperature as a continuum is monitored only by sliding or continuous reactions such as:

1 The apparent partition coefficients of Fe and Mg between garnet and chlorite, K'. This is independent of the fugacity of water, but has drawbacks in that Ca-rich pyralspite garnet is non-ideal and that the role of Tschermak's substitution in chlorite is not incorporated quantitatively.

2 The compositions of chlorite and garnet in pelitic schists in buffered assemblages. The Xug of chlorite is buffered by chlori te- biotite-garnet (Mn-poor)-clinozoisite in the albite-biotite and oligoclase-biotite zones.

3 Epidote and pumpellyite compositions in ep idote- pumpellyite- actinolite assemblage (Nakajima et al. 1977).

4 Amphibole composition in the haemat i te- ep ido te -amphibole - chlorite assemblage. The amphibole changes from actinolite, through winchite (= ferriwinchite) and crossite, to glaucophane, which is then replaced by barroisite with further increase in temperature (Otsuki 1980, Hosotani & Banno 1986).

5 Though not strictly a thermodynamic parameter, the degree of graphitization of carbonaceous materials is correlatable with the grade defined by silicate equilibrium (Itaya 1981).

In Fig. 3, the distribution of the apparent partition coefficients K', the atomic ratio of Mn in chlorite and basal spacing of graphite-like phases d(002) are shown on the cross-section of the Asemi-gawa area. Note that the sample location does not lie exactly on the cross-section.


The temperature of metamorphism increases from south (lower structural level) to north (progressively higher structural level) up to the highest grade, oligoclase-biotite zone, which is reached in a middle structural level. Further north, the temperature decreases at progressively higher structural levels.

In several places K' deviates from a smooth trend across the section. The anomaly at the boundary between the biotite zone and the underlying garnet zone is tectonic, and that between the oligoclase-biotite and the underlying albite-biotite zones may be so as well, but the other anomalies are due to local structural disturbances. The tectonic boundaries mentioned above agree with the structural breaks shown by the Research Group of Sanbagawa Belt (1981). The temperature of metamorphism gradually increases from the chlorite to the garnet zones, as revealed by the epidote- pumpellyite- actinolite equilibria (Nakajima 1982) and haemat i te -ep idote- amphibole equilibria (Otsuki 1980, Hosotani & Banno 1986). Also, the temperature decreases continuously from the oligoclase-biotite zone to the garnet zone. A gradual increase of temperature from the chlorite zone to the garnet zone was also confirmed in the Bessi area (Hosotani & Banno 1986).

Deformational stages

Three-fold division of deformational stages

Our view of the structure of the Sanbagawa belt is based on Sakai's work (unpublished) combined with our critical evaluation of the views of previous workers, e.g. Research Group of Sanbagawa Belt (1981), Faure (1983) and Toriumi (1985).

The deformational stages in Shikoku are divided into D~, D2 and D3, each associated with fold F~, schistosity $1 and lineation L1, etc. Each of these may have several substages.

D 1 results from the flow of the metamorphic complex in an east-west direction, which formed rootless isoclinal folds (F0, the S1 schistosity mostly parallel to original bedding, and an L~ mineral lineation of amphiboles, epidote and layered silicates. It is accompanied by sheath folds first found in the Sanbagawa belt by Faure (1983). Porphyroblasts such as garnet and albite (including oligoclase) rotated with the axis parallel to north-south, i.e. normal to simple shear. Also, pull-apart textures of amphibole and epidote are controlled by D1. The albite porphyroblasts formed during DI

contain retrograde minerals such as Al-poor amphibole and retrograde ilmenite, but not chlorite replacing garnet. Garnets with rims representing the highest temperature (least MnO content) have given rise to pressure shadows during the D1 plastic flow. It then follows that D1 included all the stages of porphyroblast growth and some stages of retrograde mineral formation.

Toriumi (1985), using deformed radiolaria as strain markers, has concluded that the first stage deformation was north-south compression, by which spherical radiolaria changed to rods elongated east-west. This model was accepted by Kunugiza et al. (1986). However, we now regard his model to be geometrically impossible as it requires independent rotation of domains within the schists as small as radiolaria which necessarily disrupts the schistosity.

D2 deformation was north-south compression under brittle-ductile conditions, which gave rise to tight folds with southward vergence. The garnet porphyroblasts were rotated by F2, but did not grow during this stage. No major minerals seem to have grown during D2. How- ever, the Fe folding is responsible for the recumbent folding and thrusting which produced the major thermal structure in central Shikoku. The discontinuity of metamorphic grade at the boundary between the albite-biotite zone and underlying garnet zone is due to the thrust developed on the lower limb of the recumbent fold.

The last major deformation, D3, resulted in upright folding with an east-west striking vertical cleavage.

The emplacement of the tectonic blocks occurred at least in part during D1 deformation, since, e.g. in the Tonaru area, the epidote amphibolite derived from eclogite-bearing- metagabbro has a hornblende lineation parallel to that of surrounding hornblende schists. On the other hand, in the contact aureole of the Sebadani eclogitic metagabbro, the schistosity of prograde hornblende schists was obliterated, suggesting rather static conditions of the contact aureole (Takasu 1984).

Thermal structure

Faure (1983) claimed that D2 (in our notation) resulted in the large nappe, by which high- grade schists ('spotted schists') were thrust over the low-grade schists ('non-spotted schists') and produced the fundamental structure of the Sanbagawa belt in Shikoku. We disagree with this in several respects. Firstly, the Sanbagawa schist cannot be unequivocally divided into

(a)

Geology & metamorphic evolution of Sanbagawa belt, Japan

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Fro. 4. Some characteristic textures of the Sanbagawa schists, a, albite porphyroblast in a basic schist of the albite-biotite zone (Otsuki 1980); b, amphibole zoning in a basic schist of the albite-biotite zone (Otsuki 1980); c, garnet in a pelitic schist of albite-biotite zone that grew during D~, bc section; d, garnet in a pelitic schist of the albite-biotite zone that rotated but did not grow during D2, ab section. Scale bars: 0.1 mm.

'spotted' and 'non-spotted' formations, as the albite porphyroblasts are common even in Faure's non-spotted formation. The difference between them is only in grain size. More essential is the fact that the thrust he showed certainly gave rise to the discontinuity of grade as shown in a preceding section, but the difference of grade is small, as judged from K' (Fig. 3) as well as the phase relations of hornblende on both sides of that thrust. Also the gap of equilibrium temperature on both sides of another possible discontinuity, at the lowermost level of the ol igoclase-biot i te zone in the Asemi-gawa section, is small, corresponding to K' from 0.10 to 0.12. Thus, even though there are thrusts, the grade of metamorphism in central Shikoku may be treated as essentially continuous rather than discontinuous.

Nevertheless, though Banno et al. (1978) concluded that no discontinuity of grade occurred in central Shikoku, within the resolution

of techniques available at that time, later progress in petrological phase studies has revealed that there are indeed discontinuities in grade.

Thermal history

Chemical heterogeneity of minerals and the record of P - T conditions

Zoning of pelitic garnets with bell-shaped Mn variation from core to rim is common. There are some fine-structures which are smoothed out in general interpretation, but are not yet properly explained; such as the anomalous shoulder described in fig. 2 of Banno et al. (1986). Reversed zoning in regard to Mn has been known from the sheared zone of the structurally lower garnet zone.

Amphibole in basic schists is generally zoned with A1203 decreasing from core to rim. This is

528 S. B a n n o & C. S a k a i

primarily due to retrograde resorption. The Al203-poor rims suggest movement towards lower temperatures and pressures, where actinolite is stable. The retrograde amphibole in haematite-bearing basic schists never forms glaucophane-crossite in the rims, an amphibole stable in prograde metamorphism, thereby suggesting that the retrograde P - T trajectory passed through lower pressure paths than the prograde one (see Fig. 5).

In both basic and pelitic schists, albite (and oligoclase) porphyroblasts become visible under the microscope in the middle chlorite zone, and they are recognizable with the naked eye in the high-grade part of it. Albite is zoned (Fig. 4) with inclusions of amphibole and epidote. Hornblendes included in the core are generally richer in A1203 than those in the rim, suggesting that the formation of albite porphyroblasts continued until an early stage of the retrograde metamorphism. Garnet is a common included mineral but is rarely seen chloritized. Otsuki (1980) interpreted the albite zoning (Fig. 5a) as resulting from growth during decreasing pressure, which stabilized albite over oligoclase.

In the pelitic schists, chlorite, muscovite and biotite are usually lepidoblastic and define the $1 schistosity. In addition, Higashino (1975) described biotites that replace hornblende along cracks, or replace chlorite that had replaced garnet, and suggested that hornblende and garnet had been unstable before the formation of the later biotite. He postulated that these textures recorded a decrease of pressure while the temperature was still high.

P--T conditions of metamorphism

There are not many assemblages or compositions which reliably constrain the P - T conditions.

Throughout the Sanbagawa belt, albite and calcite are stable. In the Bessi area, kyanite- zoisite-quartz-paragonite occurs in metao gabbro within tectonic blocks re-equilibrated during the Sanbagawa metamorphism. The quartz- lawsonite- albite assemblage con- strains the conditions of the lower grade part of the chlorite zone or middle pumpelliyite- actinolite zone (mPA in Fig. 5a), and the ab- sence of aragonite and jadeite + quartz is notable.

The assemblage quartz + albite + pyroxene is rare, unless the pyroxene is high in acmite component. Further we lack an adequate crystallochemical model to extrapolate the experimental results in this system to the P - T field of the Sanbagawa metamorphism. In the

oligoclase-biotite zone, two plagioclases co- exist but they do not serve for geothermometry. The estimation of pressure by the content of one or two components in solid solution, such as amphiboles or micas do not serve for quantitative discussion. Furthermore we believe that the sphalerite barometer lacks internal consistency.

For a few mineral pairs, the F e - M g partitioning is calibrated fairly well. The partitioning between garnet and omphacite, as calibrated by Ellis & Green (1979), gives 620°C (for 10 kbar) for the eclogite pair formed at the Sanbagawa stage (Takasu 1989). The F e - M g partitioning between garnet and biotite (Ferry & Spear 1978) gives 610°C for the oligoclase- biotite zone (Enami 1983). Oxygen isotope thermometry has not been tried, but carbon isotope thermometry using the calcite-graphite pair in metalimestones suggests 450°C for the garnet/albite-biotite zone boundary (Wada et al. 1984), although isotopic exchange may have continued to temperatures below those of the mineral equilibria.

Thus we assign 610°C and 10-12 kbars to the oligoclase-biotite zone, and 250-300°C and 5 - 6 kbars for the lower chlorite zone.

Slope of the P-- T path

Qualitatively, the slope of the P - T trajectory is estimated in four ways. First, the P - T field of the lower chlorite zone, 250°C and 6 kbars, and that of the oligoclase-biotite zone, 600°C and 10 kbars, require that the overall slope should be positive. Second, from the semi-quantitative phase relations among actinolite-winchite- magnesioriebeckite (crossite) in haematite- bearing basic schists, for which the stability of amphibole species is shown in Fig. 5b, the slope is positive (Banno et al. 1984). Third, Banno et al. (1986) concluded, based on the model consideration of garnet zoning of pelitic schists, that the higher grade rocks were formed at lower pressure than the lower grade schists, so far as the garnet to the oligoclase-biotite zones metamorphism is concerned. This, however, does not necessarily require a negative overall slope of the prograde P - T trajectory. Fourth, the analysis of retrograde mineralization of amphibole, garnet and others by means of chemical heterogeneity suggests that the temperature and pressure decreased together, or that pressure decreased at more or less similar temperature, at least at an early stage of retro- gression, as judged by the zonation of albite porphyroblasts.


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Radiometric ages

Itaya & Takasugi (1987) have determined K - Ar ages of more than 70 muscovites from the Asemi-gawa section that range from 85 to 75 Ma. Previous studies gave a 116 Ma R b - S r whole-rock isochron for the pelitic schists of the Asemi-gawa area (Minamishin 1979), a 110 Ma R b - S r whole-rock isochron for pelitic schists and R b - S r model muscovite ages of 70 and 75 Ma for the Kii Peninsula, and a biotite model age of 87 Ma for the biotites from the pelitic schists of the Bessi area (Nozawa 1977). Pebbles of the Sanbagawa schists are first found in the Upper Cretaceous Onogawa Group.

Summary of the history of Sanbagawa metamorphism

The evidence introduced in the preceding sec- tions may be synthesized to give the following history of the Sanbagawa metamorphism.

The Sanbagawa metamorphism affected a part of the Jurassic complex in which olistoliths of Carboniferous to lower Jurassic rocks such as limestone, chert and greenstone, probably formed in equatorial regions, were incorporated. Sometime during the middle Cretaceous (116 Ma?) the Jurassic complex reached a depth of about 30 km. Tectonic movement was in an eas t -west direction or N30°E on the coordinates of the pre-Japan Sea Asian continent. The horizontal flow is necessary to explain the uniform metamorphic facies series, or pressure of metamorphism of the belt, from eastern Kyushu to the Kanto Mountains. Transpor- tation of the complex obliquely to the accretionary wedge could have produced this situation.

The horizontal flow gave rise to D~ deformation and prograde metamorphism. During D1, tectonic blocks were emplaced into the metamorphic regime, possibly from the lower- continental-crust/upper-mantle region, and they were re-equilibrated during the Sanbagawa metamorphism.

A little earlier than 87-85 Ma ago, horizontal

transport ceased and the belt started to uplift accompanied by D2 deformation. This took place at lower temperatures than the prograde metamorphism, and major mineral growth did not take place. Later 0 3 deformation took place probably before the late Cretaceous, by which time the Sanbagawa schists were already at the surface.

It is plausible that the metamorphism in the subduction zone was associated with the igneous activity on its continental side. However, if we regard the adjacent Ryoke metamorphic belt, a low-pressure metamorphic belt also derived from the Jurassic complex and accompanied by abundant granitic rocks, as representing such a zone of island-arc volcanism, then it is evident that a fairly wide region between the zones of the alleged igneous activity and of subduction, say 200 km in the present trench-volcanic-chain system, is missing. Thus we may envisage that this loss of landmass was related to transcurrent fault movement, which raises the question of whether or not the Ryoke belt, located just north of the Median Tectonic Line, was really situated on the northern side of the Sanbagawa belt at the time of metamorphism. On palaeon- tological grounds, the original association of the Ryoke and Sanbagawa belts is questionable (Ozawa et al. 1985). However, the fact that the Ryoke and Sanbagawa belts developed on the Jurassic complex cannot be fortuitous. Presum- ably the Ryoke and Sanbagawa belts developed separately from each other, but still within the Jurassic complex on the eastern margin of the Asian continent, and were then juxtaposed by transcurrent faulting, which developed before the deposition of the Onogawa Group in the late Cretaceous.

ACKNOWLEDGEMENTS: In preparing the manuscript, we benefited from quoting unpublished M.Sc. and Ph.D. theses of Mr K. Shikano, Mr H. Yoshizawa, and Dr M. Otsuki as well as the advice of Dr T. Itaya and Dr T. Nakajima on interpreting radiometric age data. Mr T. Higashino allowed us to read the manuscript of his paper on the mineral zones in central Shikoku. We also learned much from discussion with Dr Y. Ogawa. Comments of Dr S. Daly, Dr B. Yardley, and an anonymous reviewer are also appreciated.

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S. BANNO & C. SAKAI*, Department of Geology and Mineralogy, Kyoto University, Kyoto, 606 Japan. *Present address: Materials Testing Laboratory, Japan Sheet Glass Co. Ltd., Ibanai, 664 Japan.

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Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan