1. metamorphic Ceol., 1990, 8, 401411

Chronology of Sanbagawa metamorphism Y. ISOZAKI Department of Geology and Mineralogical Sciences, Yamaguchi University, Yamaguchi, lapan 753

T. ITAYA Hiruzen Research Institute, Okayama University of Science, Oka yama, Japan 700

ABSTRACT By collating age data based on the fossil age of the protoliths, radiometric dating of the metamorphic minerals, and sedimentary records of erosion at the earth's surface, the history of the Sanbagawa metamorphism can be summarized as follows. (1) The pre-metamorphic sedimentary rocks (Carboniferous-Jurassic + Early Cretaceous?) became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction-related tectono-sedimentary processes, probably in Early Cretaceous time (c. 130-120Ma). (2) The subducted protoliths underwent progressive metamorphism reaching a maximum depth of c. 30 km in late Early Cretaceous time (c. 116 f 10 Ma). (3) The high-P / T metamorphic rocks began to rise toward the surface (during the interval 110-50 Ma) with minimum estimates for the average cooling rate around 9-12"C/Ma and an average uplift rate around 0.60.5mm/year. (4) Finally, at some stage after reaching the erosional surface, the high-PIT metamorphic rocks were covered unconformably by the middle Eocene (c. 50-42 Ma) Kuma Group.

On the basis of the present chronological summary of the Sanbagawa metamorphism, the areal extent of the Sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction-accretion complex of the next tectonic belt to the south, the Northern Chichibu belt.

Key w o n k high-P/T metamorphism; microfossil; radiometric age; surface erosion; upliftcooling.

INTRODUCTION

In accordance with the general understanding of high-P/T metamorphic rocks preserved and exposed on land (e.g. Thompson & Ridley, 1987; Emst, 1988). the Sanbagawa schists of south-west Japan represent a subduction-related high-PIT metamorphic complex that formed along the Late Mesozoic convergent margin of East Asia due to the interaction between the Asian continent and an ancient oceanic plate, the Kda-Izanagi Plate (Miyashiro, 1973; Uyeda & Miyashiro, 1974; Maruyama & Seno, 1986). The thermal history of this region has been well documented using integrated petrological studies (Seki. 1958; Banno, 1W; Banno & Sakai, 1989). Although various suggestions have been made about the possible large-scale structure and tectonic evolution of the Sanbagawa belt (e.g. Emst, Seki, Onuki & Gilbert, 1970; Hara, Hide, Takeda, Tsukuda, Tokuda & Shiota, 1977; Taira, Katto & Tashiro, 1979; Ono, 1980; Faure, 1983) many aspects still remain a matter for debate. In particular, the uplift mechanism responsible for bringing the Sanbagawa schists to the eartb's surface and timing constraints on the various tectono-metamorphic events are vital pieces of information that have been lacking.

In the last decade. microfossil studies have helped determine the sedimentary age of the Sanbagawa high-P/T rocks (e.g. Matsuda, 1978). In addition, the number of

radiometric age determinations using a variety of methods has increased considerably. and in some cases, in the higher-grade zones, average cooling rates can be estimated (Itaya & Takasugi, 1988). Another recent contribution to chronological studies of the Sanbagawa metamorphism follows from the reexamination of schist clasts in Mesozoic and Tertiary conglomerates overlying or in close proximity to the Sanbagawa belt (Isozziki & Itaya, 1989; Yokoyama & Itaya, 1990). This work also helped resolve an apparent paradox of the Sanbagawa metamorphic evolution; i.e. a contradiction between the alleged mid-Cretaceous age of the belt's first exposure at the surface and the younger Late Cretaceous radiometric ages for metamorphism.

On the basis of the available data, this paper aims to give the latest constraints on the age of the Sanbagawa metamorphism, which will provide a new chronological basis for constructing more sophisticated and feasible working hypotheses for the tectonic evolution of the Sanbagawa belt.

GENERAL R E M A R K S

As the Sanbagawa high-P/T rocks are best understood as the products of an ancient subduction zone, the tectono-metamorphic evolution should be scrutinized through a comparison and analogy with modem

101

#Z Y. ISOZAKI & T . I T A Y A

subduction-related processes working in an active arc- trench regime. Although the subduction process that generated blueschist must have continued over a long period (Peacock, 1987). the high-PIT rocks presently exposed on land record only a fragmentary history of the long-lasting process. Our discussion of the chronology of the Sanbagawa metamorphism will concentrate on the endem'c history of material transport through various PIT conditions in the tectonic framework of the Late Mesowic Sanbagawa subduction wne, and the successive order of teeono-metamorphic events.

The protoliths have an initial history of formation and transport to the subduction zone. where they underwent a variety of sedimentary and tectonic processes causing mixing and the formation of a thickened accretionary complex (Cowan, 1985; Moore, Cowan & Karig, 1985). Rocks that were buried underwent progressive high-PIT metamorphism. Using the estimates for rates of plate convergence given by Engebretson, Cox & Gordon (1985), i.e. 5-10 cm/year in the last 175 Ma in the Pacific Ocean basin, the time gap between the onset of burial and the peak of metamorphism at depth will be of the order of a few million years, assuming an average dip of around 45" for the subduction wne.

After reaching the maximum depth, the Sanbagawa high-PIT metamorphic rocks were decompressed and cooled down during a phase of uplift, as shown in the P-T trajectory of Banno & sakai (1989). Uplift must have been a rapid process, in particular in the initial stage, because high-PIT mineral parageneses and an inverted metamor- phic gradient (Peacock, 1987) are clearly preserved in the Sanbagawa belt, without any evidence of a strong thermal overprint.

Finally, the high-PIT Sanbagawa schists appeared at the surface. An upper limit for the timing of t h i s stage is given by the age of unconfonnities above the high-PIT rocks and the sedimentary age of clasts derived from the high-PIT schists. If tectonic unroofing of the overburden is an important process, uplift and preservation of high-PIT metamorphic rocks do not necessarily require rapid erosion on the surface (e.g. Draper & Bone, 1981; Plan, 1986), so the phase of uplift should be discussed separately from any erosional event at the surface.

The sequence of events for the Sanbagawa schist can be summarized as:

(1) pre-metamorphic accumulation of protoliths at a subduction trench,

(2) peak metamorphism under high-PIT conditions and subsequent decompression and cooling through uplift, and

(3) post-metamorphic surface erosion. The ages of tbesc events can be determined separately on the basis of three independent sets of data, i.e. the fossil age of the protoliths. the isotopic age of the metamorphic rocks and the stratigraphic age of the unconformities.

The following sections will discuss chronological aspects of these three stages of the Sanbagawa metamorphism on the basis of up-to-date lines of evidence from palaeontol- ogy, geochronology, and stratigraphy. Tbe time-scale used

this paper is that of Snelling (1985).

AGES OF METAMORPHIC EVENTS

Pre-metamorphic accumulation The protoliths of the Sanbagawa schists were probably accumulated and mixed at an ancient trench and its environs before being buried and metamorphosed at depth in an accretionary complex, as proposed by Miyashiro (1961). The Sanbagawa schists consist mainly of metabasite, and psammitic and pelitic schist, with subordinate amounts of siliceous and calcareous schist, that were probably derived from basaltic-gabbroic greenstones, siliciclastic sandstone, mudstone, chert and limestone, respectively. The protoliths of the psammitic and pelitic schist are of continental affinity, and locally metaconglomerate occurs, containing clasts of granitic rock (Kano, 1960) probably derived from a continent or volcanic arc. The protoliths of the metabasite, siliceous and calcareous schist arc oceanic in origin. The lithological assemblage found in the Sanbagawa belt corresponds well to that of modern and ancient subduction complexes; the interminghng and complicated juxtaposition of oceanic rock types with continental or arc materials is one of the most important criteria for recognizing a subduction- accretion complex (e.g. Hamilton, 1969; Lash, 1985). Accordingly, it is difficult to believe that the protoliths of the Sanbagawa schists were derived from microcontinental fragments as suggested by Ernst (1988).

Until Matsuda (1978) found Middle-Late Triassic conodonts in calcareous schist of the pumpellyite- actinolite zone in the southern marginal wne of the Sanbagawa belt in Shikoku (Loc. 9 in Fig. l ) , there had been no reported Occurrence of diagnostic and datable fossils known from the Sanbagawa metamorphic rocks. Follom'ng his finding, Late Triassic conodonts have also been found in calcareous schists interleaved with basic and pelitic schists in the pumpellyite-actinolite wne at several localities in Shikoku (Kuwano, 199; Suyari, Kuwano & Ishida, 198Oa. b). Iwasaki. Ichikawa, Yao & Faure (1984) also found late Late Jurassic radiolarians, in a series of red volcanic phyllite interlayered with metabasite (Mikabu Greenstones) of the pumpellyite-actinolite wne in east Shikoku (Loc. 6). In addition to these sigmficant hds from the Sanbagawa metamorphic rocks sensu srricfo, there are several reports of Carboniferous, Permian, Triassic and Jurassic fossils from the weakly metamor- phosed rocks of the northernmost part of the Chichibu belt. This is the composite tectonic unit ( = Nc + Kr + Sb in Fig. lb ) immediately to the south of the Sanbagawa belt. These rocks are also regarded as part of the Sanbagawa metamorphic rocks scmu lnfo by many workers, because they belong to the same metamorphic grade (the middle pumpellyite-actinolite zone of Banno & Sakai, 1989) and have a similar radiometric age (discussed in a later section) to the southern part of the Sanbagawa schists. Table 1 lists all the diagnostic fossils found from the Sanbagawa metamorphic rocks semu strico in the Sanbagawa belt (in bold type) and those from the northern margin of the Chichibu belt. The often quoted report of 'Mesowic radiolarians' by Hutimoto (1938) is excluded

CHRONOLOGY OF SANBAGAWA METAMORPHISM 143

Fig. 1. Index map of the Sanbagawa belt, south-west Japan (a), with an enlargement of Shikoku Island (b. compiled from Katto eI d., 19TI; Isozaki, 1987; Banno & Sakai, 1989). showing the fossil localities and radiometric dating listed in Table 1 and Fig 2 and 3. M.T.L. = Median Tectonic Line, T.T.L. = Tanakura Tectonic Lne. I-S.T.L. = Itoigawa-Shizuoka Tectonic Line, B.T.L. = Butsuu, Tectonic Line (teeth on upper plate), Nc = Northern Chichibu belt, Kr = Kuroscgawa belt, Sb = Sanbosan belt. The broken h e (partly with teeth on upper plate) in the Northern Chichibu belt represents a probable southern Limit of the Sanbagawa high-PIT metamorphic rocks (see text for details).

4 - 4 LOW-P/T Ryoke metamorphic belt

E Hlgh-P/T Sanbagawa metamorphic belt CI'

/ 130'E

/ 135'E

Tortl8ry covor, '>

Sanbagawa mot. rocks llDLy hM . ._

YY 50 km .-. -'-\ I - I,-

from this table, because re-evaluation of this finding is difficult at present owing to the disappearance of the outcrop. The oldest and youngest known protoliths of the Sanbagawa metamorphic rocks sensu lato are the Carboniferous limestone, and the Late Jurassic red volcanic phyllite and bedded chert, respectively (Table 1).

It is noteworthy that all the Listed fossils occur in schist derived from oceanic rock types. For example, calcareous schist associated with metabasite was probably derived from reef limestone that formed an atoll or carbonate mound capping ancient seamounts. The red volcanic phyllite intercalated with thick greenstones is very fine-grained and completely devoid of coarse-grained terrigenous clastics. The bedded radiolarian chert is a typical ancient pelagic sediment with unusually low sedimentation rates (Matsuda, Isozaki & Yao, 1980; Isozaki, 1987). As mentioned above, fragments or slices of subducting

oceanic plate are mixed with land-derived trench-fill deposits, mostly turbiditic sandstone and mudstone of terrigenous nature, through a variety of sedimentary and tectonic processes. Because the oceanic rocks were already formed and partly consolidated on the oceanic plate before

their arrival at a trench, they are naturally older than the land-derived material. Hence, the diagnostic age for the arrival of this material at the trench and the onset of burial are best approximated, not by the fossil age of the oceanic rocks but by those of the trench-fill sandstone and mudstone.

As far as the Sanbagawa schists are concerned, all the fossil-bearing rocks are of oceanic origin and no fossil has ever been found from psammitic and pelitic schist derived from tenigenous clastics. The latest Jurassic age (c. 139-135 Ma) of the red volcanic phyllite, therefore, is not likely to represent the age of onset of burial of the Sanbagawa protoliths at the ancient trench. The burial of the Sanbagawa protoliths probably took place later than the latest Jurassic (139-135 Ma). The authors assume that the protoliths of the Sanbagawa schists were accumulated at the trench around 130-120 Ma, because the subsequent peak metamorphism at the depth of the subduction zone took place probably around 116f10Ma, as will be discussed in the next section.

Judging from the lithological assemblage and age of the protoliths, the Sanbagawa schists are best correlated with the weakly metamorphosed subduction-accretion complex

49) Y. ISOZAKI & T. I T A Y A

Tabk 1. List of diagnostic fossils from the -ty Area Age Rock M.ZOM Fosil Reference Sanbagawa metamorphic rocks semu kuo.

See Rg. 1 for locations. M. zone = metamorphic zone. P-A = pumpellyite- bJ (198)) 1. ososc

(Saitama) 2. M.Buko (SC.nu) 3. M.Buko (Sli tma) 4. Kasbiwagi

Mikabo (Gunma) 5. M.Ni~hi-

Pcm.

M.-L. Tri.s.

KantoMtr Late Jur.

Trips. JW?

!+!C

L.tc J.r. L.-M. Trips. C u b o n .

Carbon.

M.-L. W o k u T k

M A . T k M.-L. ttir. Late Trips. La T k late T&

Lt.

Ls.

0.

Ls.

Volc. ms.

V o l c

Ls.

Ls.

Lr.

Lr.

L.

L.

Lr.

L.

L.

m.

? amodoo1

? conodont

? radiolaria bivalve

P-A conodont

P-A radiolaria

P-A ndhkrfa

P-A? conodont

P-A? amodont

P-A conodont

?-A CQd0.t

?-A -1

P-A EoIolo.1

? mnodont

actinotitc zone. LS. = ~imatone,- ~ h . = chert, Volc. ms. = volcanic mudstone. Igo (1972). Tarnun

Hiss& n d.

sat0 CI d.

Guidi cf d.

cr d. (1978)

( 1 9 ~

(1985)

I W u d ad. (M)

Suyari ct a!. (1980.)

suyari n d. (1980.)

Suyari CI d. (1980.)

% p i ad. (-1

mbd. (m)

Y.rrr0 (197))

Suyari CI al. (19804

Sqdarl. (I-)

%yui el d. (I-)

exposed in the southern marginal part of the Chichibu belt ( = t h e Sanbosan belt, Sb in Fig. lb) and/or that of the northernmost Shimanto belt. Accordingly, Isozaki (1988,1989) pointed out that the Sanbagawa schists are metamorphosed equivalents of the Sanbosan (+partly Shimanto?) complex accumulated at an ancient subduction zone along the southern margin of the Kurosegawa belt, an accreted arc or microcontinent (Maruyama, Banno, Matsuda & Nakajima, 1984; Kr in Fig. lb). and that the Sanbagawa belt is now exposed on the continental side of the latter, forming a tectonic window more than 800 km

N t Smbagawa met. rockr +)mu lat~-

Rwke b. I Sanbaaawa b. I

long (Figs 1 and 2). Details of the tectonic evolution of the Sanbosan-Sanbagawa belts are not the subject of the present article and will be discussed elsewhere.

Peak of metamorphism and subsequent cooling

Petrological studies suggest that the Sanbagawa protoliths were metamorphosed in a subduction zone, under intermediate high-P/T conditions reaching a peak of about 7kbar and 400°C (e.g. Brothers & Yokoyama, 1982; Banno, S a k i & Higashino, 1986). The age of peak

S

Chichibu b. I Shiman to b.

I Late Cr et a ceou8 I u bduction complex 10 km

Ftg. 2. Schematic cross-stcti on of the Outer zone of south-west Japan (modified from Isozaki, 1988, 1989). based mainly on the geolog~cal structure of ocntral Shikoku but vertically exaggerated and not balanced. The solid arrow represents the southern margin of the Svlbsgawa metnmorptuc rocks on the surface.

C H R O N O L O G Y OF S A N B A G A W A M E T A M O R P H I S M 485

metamorphism can be roughly determined using a Rb-Sr whole-rock isochron method. Yamaguchi & Yanagi (1970) reported an isochron age of 110f20Ma (Rb decay constant 1.39 x lO-"/year) from three samples of pelitic schist from the highest grade zone in the central Kii area (Fig. la). Minamishin, Yanagi & Yamaguchi (1979) presented an age of 116 f 10 Ma (Rb decay constant 1.12 x lO-"/year) from eight samples of pelitic schist from the biotite zone in central Shikoku. These ages were determined using the same laboratory method. Recently, Shibata & Takagi (1988) determined an age of 69 f 10 Ma (Rb decay constant 1.42 X lO-"/year) from seven samples of pelitic schist of the middle grade mne in the Bungui Pass area (Fig. la), central Japan, located 300 and 600 km east of the central Kii and central Shikoku areas, respectively. The significantly younger age suggests a younger age for peak metamorphism in the eastern region of the Sanbagawa belt (Fig. la), assuming that the whole-rock isochron age dates the peak of metamorphism.

Minamishin ef al. (1979) concluded that the good linearity of the Rb-Sr isochron for samples of pelitic schist, when compared with the widely dispersed data for quartz schist and psammitic schist, warrants the use of a whole-rock dating method using samples of this material. The homogenization of strontium isotopes during sedimen- tation may be achieved by sea water-clay interaction. However, such a homogenization process is unlikely to be effective for coarse-grained material, and the sediments on the modern ocean floor have detectably heterogeneous Sr isotope distributions (Biskaye & Daxh, 1971). Therefore, the process responsible for producing homogeneous initial Sr isotope ratios and therefore defining an isochron has to be sought in post-depositional processes, i.e. during diagenesis/metamorphism. Shibata & Mizutani (1982) have shown that the diagenesis of mudstone of a Jurassic accretionary complex in the Inner zone of south-west Japan homogenized the Sr isotopes after deposition. As the homogenization and equilibration of Sr isotopes among coexisting silicates requires considerable mobility, this may be even more easily realized during prograde metamorph- ism, in which recrystallization associated with dehydration reactions takes place, causing large-sale flow of water. It follows that the Rb-Sr isochron clock was set when the migration of water ceased, i.e. after the peak metamorph- ism was past. Thus, the whole-rock isochron obtained by Minamkhin ef al. (1979) may date the time when the migration of water ceased. Retrograde metamorphism took place in the Sanbagawa belt, but not to any great extent. Thus, we take the Rb-Sr isochron age of 116 f 10Ma as dating peak metamorphic conditions in central Shikoku. After peak metamorphic conditions were reached, the higher grade schist cooled passing through various closure temperatures: 550°C for the K-Ar amphibole system, 500" C for the Rb-Sr muscovite. 350" C for the K-Ar muscovite, and 300°C for K-Ar and Rb-Sr in biotite (Jlger, 1979). Since the data of Banno & Miller (1%5), more than 30K-Ar, Ar-Ar and Rb-Sr mineral ages have been reported by Hayase & Ishizaka (1967). Yamaguchi & Yanagi (1970), Ueda, Nozawa, Onuki &

Shibala b TWagi I19881 Minamishin el ol IS791 - - - Rb-Sr r d Yamagucni b Yanagi IS701

60 80 100 120 Ma

Ar-Ar

60

K -Ar

. . . . . .

60 80 100 1ZOMa

Fig. 3. Frequency distribution for radiometric a g a reported in the whole Sanbagawa belt from the Kanto mountains to tactern Kyushu via Shikoku. Hatched squares are from the Shikoku area. The Rb-Sr mineral age of 110 Ma is from scricite in weakly metamorphosed siliceous slate. The K-Ar biotite age of 128 Ma is from omphacite-bcaring amphibolite and may be due to excess argon.

Kawachi (1977). Watanabe, Yuasa & Goto (1982), Monie, Faure & Maluski (1987) and Shibata & Takagi (1988) with different age determinations. Figure 3 shows the frequency distribution of the reported mineral ages. In this figure, the Rb-Sr whole-rock isochron ages reported by Yamaguchi & Yanagi (1970), Minamishin ef af. (1979) and Shibata & Takagi (1988) are also shown, as well as the range of 70K-Ar muscovite ages reported by Itaya & Takasugi (1988). K-Ar whole-rock ages of Ueda er al. (1977) and K-Ar mineral ages of the fault gouge materials along the Median Tectonic Line reported by Shibata & Takagi (1988) are excluded. The former is not appropriate for the dating of schist and the latter dates the time of fault movement. Exapt for one Rb-Sr age (110 Ma) of sericite in weakly metamorphosed siliceous slates (Yamaguchi & Yanagi, 1970) and one K-Ar biotite age of omphacite- bearing amphibolite (Ueda et al., 1977; 128 Ma, probably due to excess argon), the mineral ages range from 105 to 65 Ma. This range exceeds the error of the radiometric age determinations. The different ages in different arcas, and in different grades in the same area, must be ascribed to local differences in the thermal and deformational history of the Sanbagawa metamorphic rocks.

If all the Sanbagawa metamorphic rocks had a uniform cooling rate, and if they started cooling at the Same time,

4 6 Y . ISOZAKI 81 T . I T A Y A

0 10 2okm Mineral zone

M i a n Tectonic

go:-- L s u

Y . s - - - - 1M

q - 1 0 0

N 4 110-

0 v 2 100- 0,

8 90- 8 $ Y , . -e -*b.e-8- ,

southern mclrpinal belt

higher-grade rocks should have younger mineral ages than lower-grade rocks that passed the closure temperature earlier. However, systematic younging with metamorphic grade has not so far been recognized. Itaya & Takasugi (1988) determined K-Ar muscovite ages from 70 samples of schist collected systematically from the Asemigawa area, central Shikoku, and showed that there is a correlation between metamorphic age and grade of metamorphism but that the highest grade material has the greatest age, i.e. the opposite to what would be expected for a region with a simple and uniform cooling history.

Their results are quoted in Fig. 4, along with the new muscovite K-Ar ages of the pclitic schists of the Omoiji area in the south of the Asemigawa area (Fukui & Itaya, 1989). The Ornoiji area is located in the Southern Marginal zone of the Sanbagwa belt and is underlain by the chlorite zone schists of the pumpellyite-actinolite facies (Nakajima, Banno & Suzuki, 1977; Banno & Sakai. 1989). The new data confirm the notion that this area belongs to the Sanbagawa belt. These data also show that the radiometric age within the chlorite zone at least is dependent on geographical location. The most northerly section has an average age of 65 Ma, while in the south of

the Asemigawa area this changes to 75 Ma, and the highest ages are found in the Southern Marginal belt. mostly around 85Ma with one exception of 108Ma. The southward younging polarity of recorded ages within the chlorite zone probably can be explained in terms of increasing uplift rates from north to south (Fig. 5). However, the general decrease of age with metamorphic grade in the same region (upward convex pattern in Fig. 4) requires a separate explanation.

Itaya & Takasugi (1988) suggested that uplift-related deformation was concentrated in the chlorite zone and explainedthe above phenomenon as follows. K-Ar ages in the higher-grade material may approximate the time of passing the real blocking temperature, while ductile deformation within the chlorite w n e may have caused continuous depletion of Ar even below the blocking temperature. As brittle deformation docs not appear to affect the K-Ar age significantly, the K-Ar ages from the chlorite zone in the Asemigawa area probably reflect the time when ductile deformation ceased and brittle deformation began, and this ductile/brittle transition took place considerably later than the time when rocks of high-grade wnes passed the blocking temperature (Fig. 5) .

CHRONOLOGY OF SANBACAWA METAMORPHISM y7

600 -, L

Eocene - Palaeo- Cretaceous cene 1 m i t e

zone .... .. deformation during uplift

/

Fig. 5. Schematic thermal histories of schist from the main Sanbagawa belt and the Southern Marginal belt in central Shikoku. Cooling rates in the northern part (broken lines) and southern part (solid lines) of the Asemigawa area in the main Sanbagawa belt are different, and thox of the Southern Marginal belt (dotted lines) are higher than thosc in the main Sanbagawa belt. A closure temperature is assumed for retention of argon in muscovite. A tentative ductile-brittle deformation boundary and argon depletion temperatures during ductile deformation (hatched area) have been drawn (Itaya & Takasugi, 1988). Squares show apparent resetting of K-Ar ages of mumvites in the pelitic schist from chlorite, garnet and biotite zones.

- 400- - Clowre temp.. 350'C E

/ U

r / 3

Surface exposure

The time at which the Sanbagawa metamorphic rocks reached the topographic surface can be inferred using sedimentary phenomena such as the age of an unconfor- mity above metamorphosed material or the occurrence of schist clasts in conglomerates. The Upper Cretaceous Onogawa Group in east Kyushu is a thick unmetamorphosed turbidite sequence with local conglomeratic horizons (Teraoka, 1970) that contain large (<5m in diameter) clasts of schist. The Occurrence of these schist clasts in the Santonian (86-83Ma) portion of the group, in close proximity to the present-day exposure of the Sanbagawa belt, has been taken as evidence for the first surface exposure of the Sanbagawa schists (Kobayashi, 1951; Teraoka, 1970). However, Isozaki & Itaya (1989) recently obtained 199-182Ma (Early Jurassic) K-Ar ages on muscovite from the schist clasts. These ages predate not only the peak metamorphism but even the tectono- sedimentary accumulation of protoliths of the Sanbagawa schists. The supposed Sanbagawa origin of these schist clasts has, therefore, been refuted.

An unconformity between the middle Eocene (c. 52-40Ma) Kuma Group and the Sanbagawa schists in western Shikoku (Nagai, 1972) indicates that high-grade schist was exposed at the surface at least by middle Eocene time. Hence, the Kuma Group is the oldest known geological unit that records the surface exposure of the Sanbagawa schists. Yokoyama & Itaya (1990) have shown that low- to high-grade Sanbagawa schists were exposed at the surface during middle Eocene time, and that K-Ar ages of these schist clasts correspond well with those of the Sanbagawa belt.

The age of p e a k metamorphism (120-110Ma) and the middle Eocene (52-40Ma) unconformity provide us with a maximum duration for uplift of the Sanbagawa schists and, together with geobarometry, a minimum estimate for

40 60 80 100 120 Time (Ma)

the average uplift rate. The age difference between the two events is approximately 80-58 million years. This is the maximum time that could be taken by the Sanbagawa metamorphic rocks to be uplifted from the deepseated metamorphic environment to the surface. Judging from the geobarometric estimates of the maximum pressure, around 7 kbar for the biotite zone schist of Sanbagawa (Banno & Sakai, 1989), the depth of burial of the Sanbagawa schists is estimated to be up to 28 km (1 kbar = 4 km). Therefore, a minimum estimate for the average uplift rate for the highest grade rocks of the Sanbagawa can be roughly calculated as 0.4-0.5mm/year, which is concordant with the average uplift rate for high-PIT metamorphic rocks given by Draper & Bone (1981).

Summary of the Sanbagawa history

We propose a new chronological summary of the Sanbagawa metamorphism in Fig. 6. The geological events related to the Sanbagawa metamorphism are summarized as follows. (1) Pre-metamorphic sedimentary rocks (Carboniferous-Jurassic + Early Cretaceous?) were in- corporated into an accretionary complex by a variety of subduction-related tectono-sedimentary processes prob- ably in Early Cretaceous time (c. 130-120Ma). (2) The subducted protoliths suffered peak metamorphism, reacb- ing a maximum depth of burial (c. 30 krn) in the late Early Cretaceous (c. 116f 10Ma). (3) The high-PlT metamorphic rocks began to rise toward the surface (during the interval 110-50 Ma) with an average cooling rate estimated at 9-12"C/Ma. (4) Finally, the high-P/T metamorphic rocks reached the erosional surface and were covered unconformably by the middle Eocene (c. 5 2 4 Ma) Kuma Group.

In this chronological summary the whole of the Sanbagawa metamorphic rocks are regarded as forming a

400 Y. ISOZAKI & T . I T A V A

1 -

/' averipe rate VOIC. ms. !

chert i \ llrneatone j

1 i \ / subduction uplift-cooling

-

: v ; Sanbagawa belt

: (+northernmost part j of N, Chichlbu belt) : peak metamorphism

:I116 M I : Rb-Sr whole rock1 I conodonl

. ~ , tM Hauterivian ' Yuam Formatlon

radiolaria rad. ..'.I: :.. . .. . ... i::. , .. : f radiolaria .......

! Northern Chichibu belt bedded chert sil. mr. rnrlss (southern parts)

t - 8 .

? ;

)ePW

0

10

20

30 k m

0

10

20 km

Fq. 6. Chronological summary of the Sanbagawa metamorplusm, in comparison with that of the Northern Chichibu (Nc) complex. ms = mudstone, ss = sandstone.

single packet that had experienced a uniform coeval episode of high-P /T metamorphism because chronological resolution is still not high enough to document the spacio-temporal variation of the Sanbagawa metamorph- ism. However, subduction-related growth of an accretion- ary complex, in stepwise and/or succcssive manner, generally continues for a certain time span, and the subduction process itself has to be long-lasting and quasi-steady in order to generate and preserve blueschist (Peacock, 1987). Therefore, temporal variations in tectono-metamorphic events throughout the Sanbagawa belt, such as a diachronous relationship between lithology, metamorphic grade and age, probably may be recognized through further chronological study with more enhanced resolution. In comparison with other subduction-related high-P / T metamorphic rocks, however, the chronological development of the high-PIT metamorphic events in the Sanbagawa metamorphic belt is fairly well documented. The youngest high-PIT schist exposed on land in New Caledonia is another example where the metamorphic evolution has been well documented in terms of the age of the protoliths, radiometric dates, and the timing of uplift (Brothers & Blake, 1973; Brothers & Lillie, 1988). The Franciscan metamorphic complex in California has relatively good data on the timing of events from both fossil and radiometric age data (q. Bailey, Irwin & Jones, 1964; Blake, Howell & Jayko, 1984; Cloos, 1986). However, the final stage of uplift, marked by exposure at

the earth's surface and erosion, is only poorly constrained by stratigraphic evidence (Page, 1981; Cloos, 1986), because there is neither an unconformity observed directly above the blueschist nor radiometric dates of schist clasts. Moreover, the polygenetic nature of the Franciscan complex and later strike-slip faulting often obscures critical relationships concerning metamorphic events. The high- PIT metamorphic rocks in New Zealand have been discussed from the viewpoint of radiometric ages (e.g. Harper & Landis, l%7; Adams & Gabites, 1985) and their relationship to strike-slip faulting (Scholz, Beavan & Hanks, 1979). but as yet there is little data on the protoliths and on their first appearance at the surface.

AREAL E X T E N T O F T H E S A N B A C A W A M E T A M O R P H I S M

The Sanbagawa belt, where the Sanbagawa schsts scmu stricto occur, forms a narrow (0-40 km) but long (800 km) zone extending from the Kanto mountains in the east to eastern Kpshu in the west, following the general trend of the Japanese Islands (Fig. la). In contrast with its well-defined northem boundary, the Median Tectonic Line, the southern limit of the Sanbagawa belt has remained controversial. The present chronological sum- mary of the Sanbagawa belt, together with a review of the data from the N o h e m Chichibu belt (Isozaki, 1988)

CHRONOLOGY OF SANBAGAWA METAMORPHISM 408

immediately to the south, can help define the southerly extent of the Sanbagawa metamorphic rocks senru lato.

The Northern Chichibu belt (Nc in Fig. lb) lies immediately to the south of the Sanbagawa belt and represents a weakly metamorphosed subduction-accretion complex (Isozaki, 1981. 1987). The boundary between the two belts has tentatively been drawn along a prominent lithological unit, i.e. along the southern margin of the Mikabu Greenstone complex. The Northern Chichibu (Nc) complex locally suffered low-grade high-Pl T regional metamorphism which reached a peak of middle pumpellyite-actinolite zone (e.g. Aiba, 1982; Banno & Sakai, 1989). This grade of metamorphism is directly comparable with that of the Mikabu Greenstone complex of the Sanbagawa, and the whole of the Nc complex has, therefore, traditionally been regarded as the weakly metamorphosed equivalent of the Sanbagawa schists (e.g. Seki, 1958; Banno, 1964; Toriumi. 1975).

However, the lowest grade part (prehnite-pumpellyite zone + lower pumpellyite-actinolite zone of Banno & Sakai, 1989) of the Nc complex, mostly in southern parts of the Nc belt, shows lithological associations distinct from the Sanbagawa schists. The Nc belt lacks some characteristic rock types of the latter, such as Late Triassic limestone and Late Jurassic red volcanic mudstone. Even though the same general kind of protolith is found in both, there is a clear age difference, e.g. the fossil ages of Oceanic rocks like the chert of the southern Nc complex are always older than Early Jurassic (Isozaki, 1981, 1987; Ishida, 1985; Yamakita, 1986), while those of the Sanbagawa belt and its equivalent in the northern parts of the Nc belt yield Late Jurassic fossils (Table 1). In addition, the age of the temgenous clastic rocks of the less metamorphosed Nc complex, probably ancient trench-fill sediments (Isozaki, 1987). is well determined by radidarians as late Middle Jurassic (Kurimoto. 1986; Yamakita, 1986). while that of the Sanbagawa is estimated to be some time in the Early Cretaceous as mentioned above. Furthermore, unconformable relationships between the Nc complex and Hauterivian (c. 120-116Ma. Early Cretaceous) shallow-water sediments have been reported from the western Kii Peninsula (Maejima, 1978). These lines of evidence indicate that the weak metamorphism (corresponding to the prehnite-pumpellyite zone + lower pumpellyite-actinolite zone) that affected southern parts of the Nc complex was a metamorphic event independent of the Sanbagawa metamorphism, and probably took place earlier than the Sanbagawa metamorphism at a site away from the Sanbagawa subduction zone scmu stricto.

We conclude that the Sanbagawa metamorphism af€ected certain northern parts of the Nc complex, but the southern part was completely free from Sanbagawa metamorphism. This is in agreement with Faure (1983) whose suggestion was based on a comparison of deformation features. Hence, the southern limit of the Sanbagawa metamorphism should be amended from the presently accepted line along the southem margin of the Nc belt (Banno & sakai, 1989) to a discontinuity, both metamorphic and structural, expected within the Nc belt

(Figs lb and 2). Our r e n t field study has clarified that this boundary in central Shikoku is a south-dipping normal fault (broken line with teeth in Fig. lb, teeth on upper plate), separating lower grade Nc rocks on the upper plate from the higher grade Sanbagawa schist on the lower plate. In east Shikoku, this south-dipping fault also separates the Sanbagawa schist from the overlying Kurosegawa belt (Kr in Figs lb and 2) which is an accreted arc complex (Maruyama el ol., 1984; Isozaki, 1987) allochthonously emplaced within the Chichibu belt. These observations suggest that post-metamorphic normal fault- ing (Platt, 1986) may have worked in the final stage of uplift in order to expose high-PlT Sanbagawa metamor- phic rocks at the surface as a tectonic window, removing older subduction-accretion complexes.

A C K N O W L E D G E M E N T S

Helpful criticism of the manuscript was given by Professor S. Banno, Dr S. Maruyama, Dr S. Wallis, Professor M. Hashimoto and Dr T. Matsuda. The present study was partly funded by the Grant-in-Aide from the Japanese Ministry of Education, Culture and Science (no. 635406 16).

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Received 1 March 1989; revision accepted 15 September 1989