Geodynamic setting of volcano-plutonic rocks in so-called “paleo-accretionary prisms”: Fore-arc activity or post-collisional magmatism? The Shimanto belt as a case study

E L S E V I E R Lithos, 33 (1994) 85-107

L I T H O S

Geodynamic setting of volcano-plutonic rocks in so-called "paleo-accretionary prisms": Fore-arc activity or post-collisional

magmatism? The Shimanto belt as a case study

G. Stein a, J. Charvet a, H. Lapierre b, O. F a b b r i c

a URA-CNRS 1366, GDR G09690, Laboratoire de GOologie Structurale, UniversitO d'Orl~ans, B.P. 6759, 45067 OrlOans Cedex 2, France

bURA-CNRS 69, UniversitO Joseph Fourier, Institut Dolomieu, 15 rue Maurice Gignoux, 38031 Grenoble Cedex, France ¢Construction Technical Institute Ltd., Fukuoka Branch, Fukuoka 810, Japan

Received 30 September, 1992; revised and accepted 21 June, 1993

Abstract

The Cretaceous-Tertiary Shimanto belt of southwestern Japan is usually considered to be a simple accretionary prism (Taira et al., 1982; Ogawa, 1985), whereas, based on structural and sedimentological evidences, Charvet and Fabbri (1987) and Charvet et al. (1990) proposed an alternative model in which a collision with an unknown microblock followed the formation of the accretionary prism, and induced the Early Miocene main tectonism. One characteristic of the Shimanto belt is the occurrence of well developed magmatic bodies of Middle Miocene age ( 14 + 1 Ma) which can be used for testing the model. They show numerous peculiarities. ( 1 ) They consist predom- inantly of acidic volcano-plutonic complexes. Mafic sills or lavas are present in a lesser amount. (2) With respect to the supposed position of the subduction trench during Miocene time and to the northern location of the Setouchi volcanics, considered as a volcanic front ca 13 Ma, this magmatism is a near-trench magmatism. (3) Various tectono-magmatic affinities are present. The basaltic sills or lavas have mostly T- to E-MORB type affinities. An alkaline complex is present at Ashizuri cape. The most abundant magmatism consists of calc-alkaline volcano- plutonic bodies, typically peraluminous in the southern part of Shimanto and metaluminous near the northern border of the belt. (4) Metamorphic enclaves inter alia are often included in the calc-alkaline complexes. Their highest P - T conditions range between 0.7-0.8 GPa and 630-860°C. A simple subduction model is not able to explain all these features. The first necessity is to find a mechanism which develops a high thermal anomaly allow- ing the formation of magmas close to the trench. According to thermal modelling, the subduction of an oceanic plate, even young, does not provide enough heat. Secondly, in well known mature accretionary prisms (e.g. the Barbados one ), the thickest part of the complex does not exceed 15 km. In such conditions, it is difficult to obtain pressures ranging from 0.7 to 0.8 GPa. This is much more difficult if we consider that some granitic rocks were formed in the youngest part of the so-called prism, in the thinnest portion. Finally, a simple subduction model does not explain the various magmatic affinities. Geochemical data show that the magmatic source of the Ashizuri alkaline complex is likely to be an enriched mantle comparable to OIB sources. On the other hand, the calc-alkaline rocks may have been derived by mixing between mantellic components and crustal ones, or in some cases by pure anatectic processes. The upwelling of a hot asthenospheric mantle, from which the Ashizuri suite could have been derived, may represent a heating source strong enough to make crustal materials melt. The proposed collision model could explain (1) the heating source, (2) the various magmatic affinities and also (3) the 0.7-0.8 GPa pressure invoking crustal thickening induced by the collisional event.

0024-4937/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10024-4937 ( 94 ) 00020-3

86 G. Stein et al. / Lithos 33 (1994) 85-107

1. Introduction

The Cretaceous to Lower Miocene Shimanto belt extends along the Pacific Ocean side of the Outer zone of southwestern Japan (OZSWJ, Fig. 1 ). It is the youngest main orogen of Japan and the timing of deformation is rather well constrained. In various localities, the units of the Shimanto belt are unconformably covered by non-deformed shallow water sandy deposits with ages ranging from late Early Miocene (17 Ma) to Upper Miocene. The Shimanto units are also cross-cut by volcano-plutonic complexes which are found all along the belt (Fig. 2 ). These complexes yield radiometric ages which are remark-

ably clustered at 14+ 1 Ma (Shibata, 1978),just after the 21-17 Ma tectonic hiatus and coin- cided with the climax of the opening of the Japan Sea.

However, the building of the Shimanto belt is still a matter of debate. Among the two cate- gories of models proposed up to now, the first one, and most popular, states that the mountain building is the result of accretion of trench fill sediments and minor ocean-derived lithologies during a more or less continuous subduction of oceanic crust beneath the southwestern Japan arc since Cretaceous time (Pacific Type Orogeny; Taira et al., 1982; Ogawa, 1985). The second model introduces a major modification to the

Out r * Zone of SW Japan

t .

" 14

.,']~15 7 " .-" ~ o ~ ... 32ON

:'4 , ~ / ~ o 3 J , ' i 3 (.~ff.)/') f J ~ , t ~ O ~ " ~,O 'b'~ 136°E136°E

"'" 7~"J~ ~F'2 . . - ~ ~ ~ OZSWJ Middle Miocene igneous bodies Igneous bodies suggested by acoustic data

Structural high

100 Km • = Magnetic anomalous bodies

t iiiiiiiiiil s,,m nto

Fig. 1. Index map. MTL, Median tectonic line; BTL, Butsuzo tectonic line. Numbers refer to the names of the igneous bodies; I, Yakushima; 2, Minami-Osumi; 3, Takakuma; 4, Satsuma Peninsula; 5, Shibisan; 6, Ichifusa; 7, Osuzu, 8, Okue; 9, Uwajima; 10, Kashiwajima-Okinoshima; 11, Ashizuri; 12, Muroto; 13, Shionomisaki; 14, Kumano; 15, Omine.

G. Stein et al. / Lithos 33 (1994) 85-107 87

SE NW Upper I Cretaceous sub-belt I Paleogene sub-belt J It~llrl~lla It~ll^~n Jurassic I I I ................

Olistostrome I BT ~ I NTL I =- z ~ 2 : z ~ - 7 ~ i ~ i i ~ :: : i~i~:i~ ' : i~:~:~:~[- :i~:~:!::"~i: . . . . . . . " : f : * ; : f : ' . , :~,-,,~:::. :~ : : :~ , ;~ : : ,~ . - .~ :~_ : : : : : : : : : : : : : : : : : : : : : : : : : . ~ . . ~ : : : : . • , , , ,

~ l l ~ l ~ + ~'.~::::~ i>,:~ ............ ~ :~:~i:i:i~:i:i~: i:~ ~: ]q- ? q " ~ . . ~ i : ~ " l - -I- ~:~:~.~i~i: . . . . . "~:'~I

0 10 krn

Fig. 2. Simplified cross-section showing the different units of the Shimanto belt. BTL, Butsuzo tectonic line; NTL, Nobeoka tectonic line.

first one because it calls for a collision of a buoyant microblock with southwestern Japan (Charvet, 1980; Charvet and Fabbri, 1987; Charvet et al., 1990) between 22 Ma and the 17 Ma unconformity with the overlying sediments.

There is no direct evidence of the colliding microblock at the surface. Moreover, structural sec- tions based on geophysical investigations made offshore (e.g. Yoshii et al., 1973), concerning mainly the Nankai prism, and onshore (e.g. Ki- mura and Okano, 1980), displaying a view of the Japanese continental crust, do not provide infor- mation on the Shimanto belt basement.

One way to test both accretion and collision models lies in addressing the problem of the nature and genesis of the Middle Miocene magmatism. Indeed, whether the crust underlying the Shimanto belt in Middle Miocene time was oceanic (accretion model) or continental (collision model) should be reflected in the petrological and geochemical affinities of the magmatism which may retain the trace of the cross-cut lithologies.

This paper summarizes the results of petrological and geochemical investigations in order to discuss the petrogenesis of the various complexes and finally to integrate all the data in a geodynamic model for the evolution of southwestern Japan during Miocene time.

2. Geological setting

The Shimanto belt extends over 1500 km along the Pacific Ocean side, from Tokyo to the east to Okinawa in the Ryukyu islands to the west (Fig. 1 ). It is separated from Jurassic and older units lying to the north by a regional north-dipping

thrust, the Butsuzo Tectonic Line (BTL, Fig. 1 ), and is further separated into a Cretaceous sub- belt and a Paleogene to Lower Miocene sub-belt by another regional thrust. Units of the Shi- manto belt are made of thick marine clastic strata with minor oceanic crust-derived olistoliths (basaltic pillow-lavas, hyaloclastites). The whole is severely deformed (Taira et al., 1982; Ogawa, 1985; Fabbri and Charvet, 1987; Fabbri et al., 1987; Fabbri et al., 1990). Seaward verging thrusts and folds, as widely described in modern accretionary prisms, but also a few landward thrusts and folds and early synschistose shearing lead to a complicated structure. Most of the studied igneous complexes lie to the south of the BTL (Fig. 1 ). K-Ar (mineral and whole-rock) ages display a narrow range of 14+ 1 Ma (Shi- bata, 1978). Ages of the intruded strata range between Silurian in the Kurosegawa zone, just to the north of the Shimanto belt, up to Aquitanian in the southernmost part of the Shimanto belt.

With respect to the basalts, high-magnesian andesites and porphyritic andesites of the Seto- uchi zone (Fig. 1 ) considered as representing the volcanic front at 13 Ma (Tatsumi and Ishizaka, 1982), the Shimanto magmatic province has a forearc position. Moreover, the different plutono-volcanic complexes or plutonic bodies show a rough geographic distribution with respect to their magmatic affinities. The alkaline Ashizuri complex, and the Muroto and Shionomisaki E- to T-Morb are located far south along the Pacific coast. The I-type complexes crop out in the northmost areas of the Shimanto belt. The S-type complexes are located in between the former two assemblages.

The complexes show various modes of occurrence. Shibisan, Takakuma, Ichifusa, Uwajima


and Omine complexes are formed of one or several small stocks or plutons which do not crop out over more than 40 km 2. Yakushima and Minami-Osumi are larger plutons (about 600 km2). In several complexes (Okue, Osuzu, Shionomisaki, Kashiwajima-Okinoshima) early volcanic piles are intruded by plutonic rocks. These plutonic or volcano-plutonic bodies are often associated with cauldron structures or collapse basins. Finally, geophysical investigations (Geological Survey of Japan, 1982) suggest the offshore continuation of this magmatic province (Fig. 1 ).

3. Middle Miocene magmatism: petrological features and magmatic affinity

The Middle Miocene magmatism is highly diversified. It consists mainly of felsic plutonic or plutono-volcanic complexes of calc-alkaline magmatic affinity and generally classified into I- and S-type granitoids. Nevertheless, mafic rocks also exist. At Ashizuri, an alkaline complex has been described (Murakami and Matsuo, 1963; Matsuura et al., 1988; Murakami et al., 1989; Stein et al., 1992, 1994); at Muroto pillow basalts and doleritic intrusions of arc tholeiite affinity occur (Hibbard and Karig, 1990a); and finally, at Muroto and Shionomisaki, transitional to enriched middle oceanic ridge basalts (T- to E-MORB) are exposed (Miyake, 1985, 1988; Hibbard and Karig, 1990a). We will focus our attention on the major petrological and geochemical characteristics of the alkaline, peraluminous (S-type) and metaluminous (I-type) rocks, and to a lesser extent of the arc tholeiites and T- to E-MORB, in order to constrain the geodynamic setting prevalent to the building up of the Shimanto belt during Miocene time.

3.1. Analytical procedures and accuracy

Most major and trace elements analyses presented in Table 1 were carried out by ICP at the CRPG (Nancy), with analytical errors of __ 0.5 ppm for contents less than 10 ppm (except REE ) and 5% for contents more than l0 ppm. Preci- sion for the REE is estimated at 5% when chon-

drite normalized concentrations are more than 10 ppm and at 10% when they are lower. Sam- ples TO34 and TO48 major and trace elements (except REE) were carried out by XRF (Ku- mamoto University) and REE were analyzed by ICP (University of Marseilles III). Sr and Nd isotopes presented in Table 1 were performed at the CRPG (Nancy). The average value for La Jolla standard and E and A SrCO3 standard obtained at the CRPG were respectively 0.511832 (2a, 11 runs) and 0.708032 (2a, 24 runs). All initial ratios and e values were corrected for de- cay since time of crystallization. Stable oxygen isotopes were measured at the BRGM (Orlrans) on separated quartz. The values are expressed in units of~ {fi= [ (RE/RS) - 1 ]*1000} which are given relative to SMOW. Analytic reproducibil- ity is 0.2 per rail.

3.2. Alkaline magmatism

Petrography The Ashizuri complex, ranging from gabbro to

syenite and biotite-granite, is the only alkaline complex exposed in the Shimanto belt. Detailed petrographic studies are available in Murakami and Matsuo (1963), Matsuura et al. (1988) and Murakami et al. ( 1989 ). The gabbro, rich in ferromagnesian minerals and often displaying a doleritic texture, is mainly composed of euhedral laths of zoned plagioclase ranging in composition from An6o-7o in the core to An~5_25 in the rim, interstitial tremolitic hornblende including relics of Ti-rich (TiOz~> 1%) augite or salite (Wo44_asEn33_a4FSlo_2o) and biotite. Biotite and tremolite, often clustered, seem to develop after clinopyroxene. Syenitic rocks, including syenite, Qtz-syenite and Rapakivi granite, are medium to coarse-grained rocks mainly formed of quartz, perthitic orthoclase, ferroedenite to ferroedeni- tic hornblende and Fe-rich biotite (0.91 ~< XFe ~< 0.99 ). The biotite granite, locally porphyritic, is composed of quartz, displaying often granophyric intergrowths with alkali feldspar, plagioclase (An~o_25) and Fe-rich biotite (0.74 ~< XFe ~< 0.91 ). Apatite, ilmenite and mag- netite are ubiquitous accessory minerals. The gabbro also includes titanite whereas syenite and granite display allanite, zircon and fluorite.

Tab

le 1

R

epre

sent

ativ

e m

ajor

ele

men

t, tr

ace

elem

ent

and

isot

ope

anal

yses

of a

lkal

ine

rock

s (A

shiz

uri)

, per

alum

inou

s ro

cks

(Uw

ajim

a, O

suzu

and

Oki

nosh

ima)

an

d m

etal

umin

ous

rock

s (O

kue)

T

otal

Fe

as F

e203

. LO

I, L

oss

on ig

nitio

n. G

b, g

abbr

o; S

y, s

yeni

te;

Gd,

gra

nodi

orite

; G

, gra

nite

; rh

, rh

yolit

e; r

d, r

hyod

acite

; Q

z, q

uart

z; B

i, bi

otit

e; C

o,

cord

ieri

te;

Opx

, ort

hopy

roxe

ne; A

mp,

am

phib

ole

Smpl

e n

xt

type

Ash

izur

i alk

alin

e co

mpl

ex

A23

A

I4

AI7

A

I3

A1

Gb

C__,b

Q

z-Sy

Q

z-Sy

G

Uw

(jim

a

A7

A9

A91

Bt-

G

Co

-Op

x-G

d~

-Gd

Osuz

u l

Kas

hiw~

fjim

a ] O

kmo~a

T O

kue

OS-

9 O

S-4

OS-

2 /

KW

-55

K

W-5

6 K

W-7

4 O

KS

-80

KS

-12

] O

K-2

H

NI6

T

O34

T

O48

Co-

Opx

-rd

CoO

px-~

C

oOpx

-dl[

CoO

Im-G

d C

oOpx

-Gd

Gt-

G J

G

t-G

G

t-G

IB

i-A

mp-

GdB

i-A

mp-

GdB

i-A

mpA

3d

Bi-

G

SiO

2(w

t%)

53

.59

56

.90

65.9

9 6

8.2

8

76.1

6 73

.85

67.3

5 61

,84

67.4

2 75

.78

66,9

6 68

,96

67

,71

73

,49

76

.81

74

.09

66,5

7 67

,56

72,3

7 76

,89

A12

03

15.5

1 1

4.7

1

15

.85

1

5.7

3

12

,07

12

.95

14.8

0 15

,19

14.3

9 11

.30

14,(D

14

,38

14

,55

1

3,4

8

12

.50

13

.51

15,O

8 14

,83

13,4

6 13

,04

Fe20

3*

8.25

8.

19

4.04

2.

62

2,11

1.

63

4.89

5,

73

5.04

1.

69

4,75

4,

57

4,94

2,

47

1.41

2.

20

4.57

4,

39

MgO

5.

12

2.95

0.

56

0.14

0.

17

0.41

1.

82

1,77

1.

32

0.25

1,

05

1,29

1,

37

0.20

0.

00

0.02

1,

38

1,39

C

.aO

8.

48

5.67

1.

72

0.83

0.

57

1.31

2.

82

3,71

2.

61

0.35

2,

40

2,36

2,

37

1,53

0.

78

1.16

3,

45

3,05

N

a20

3.40

4.

00

3.97

4.

75

3.40

2.

90

2.88

3,

42

3.00

1.

94

2,91

2,

87

2,95

3,

59

3.32

3.

69

2,88

3.

23

K20

2.

54

3.59

6.

25

5.94

4.

85

5.34

3.

63

3,04

3.

39

5.01

3,

41

3,52

3.

40

4,12

4.

32

4.12

3,

92

3,75

T

i02

1.43

1.

52

0.55

0.

38

0.18

0.

26

0.74

1,

14

0.83

0.

18

0,84

0,

68

0,70

0,

20

0.08

0,

17

0,63

0,

62

MnO

0.

14

0.17

0.

06

0.05

0.

04

0.04

0.

07

0,06

0.

09

0.05

0,

09

0,07

0,

08

0,04

0.

02

0.03

0,

08

0,08

P

205

0.36

0.

44

0.20

0.

15

0.01

0.

01

0.11

0,

27

0.12

0.

10

0,14

0,

17

0,16

0,

06

0.02

0.

05

0,13

0,

13

LO

I 0.

69

0.73

0.

51

0.88

0.

59

0.99

1.

26

2,79

2.

29

2.72

3,

28

0,85

0,

91

0,56

0.

54

0.73

0.

56

Tot

al

~,fl, rln)

Rb

V 7/

Cr

Co

Ni

Cu

Zu

Nb

Y

Th

2,83

1,

12

0,76

0,

14

2,00

1,

06

3,08

3.

25

4,15

3,79

~.

~ 0,34

0,13

• 0,06

o,03

0,08

0,03

~.

99.5

1 98

.87

99.7

0 9

9.7

5

10

0.1

5

99.6

9 1

00

.37

9

8,9

8

10

0.5

0

99.3

7 99

,86

99,7

2 99

,14

99,7

4 99

.80

99.7

7 99

,25

99,4

8 99

,66

100,

43

618

749

639

323

218

299

524

381

531

332

544

571

565

599

422

521

563

605

412

205

IO7

145

281

283

323

200

127

144

123

232

150

138

135

151

166

166

163

163

201

169

'~

336

279

124

30

46

95

205

237

167

47

200

160

157

93

37

59

215

214

135

51

233

214

14

<5

23

20

10

124

91

20

120

51

61

8 <5

<5

96

81

19

<

5 t.~

20

0 20

7 54

1 42

7 21

2 11

9 20

9 21

2 31

8 78

30

0 21

6 22

5 15

3 10

7 16

9 19

6 18

2 14

2 98

'-~

23

8

8 <

5 10

16

65

37

45

10

54

49

54

18

5

8 20

15

6

<5

',o

20

20

56

50

43

40

19

34

23

15

19

16

27

130

65

121

49

35

33

54

~:>

32

II

<5

<5

10

<i0

30

26

21

<

10

15

16

15

7 <

5 <5

17

7

<5

<5

Oo

53

26

10

8 10

<

iO

27

23

54

62

>15

0 37

32

14

<

5 9

17

<5

7 <5

71

10

6 64

10

5 83

10

7 62

59

62

58

32

10

I

65

72

105

142

52

14

16

26

16

11

16

I1

12

11

10

12

19

21

17

12

"--.I

32

38

58

64

47

35

27

32

32

30

35

29

33

37

39

41

29

28

33

40

11

21

40

40

8 9

10

I0

13

La

Ce

Iqd

Sm

Eu

C,d

D

y Ho

E

r Y

b L

e ~N

di

eSfi

81

8 0

47.6

8 67

.75

118.

85

11

9.8

4

65.5

0 33

.79

32.9

7 85

.46

11

4.8

3

19

9.0

9

17

9.6

8

11

7.2

4

68.8

7 67

.17

34.3

5 42

.86

62.2

6 6

1.1

7

42

.38

27

.52

29.1

9 6.

96

8.3

1

11

.78

11

.65

8.08

5.

82

5.92

1.

62

1.81

1.

27

1.07

0.

76

0.72

1.

04

6.66

7.

64

9.26

8.

77

6.12

4.

68

4.89

6.

03

6.71

8.

59

8.70

6.

62

4.78

4.

14

3.30

3.

66

5.02

5.

70

4.01

2.

71

2.19

3.

0G

3.63

4.

99

6.40

5.

13

3.13

2.

46

0.52

0.

65

0.80

1.

12

0.74

0.

43

0.33

1.

83

2.56

0.

40

1.9

6

-0.5

9

-4.5

0

-5.1

2 -9

.18

-8.8

3

-5.0

7

-3.8

5 2.

90

34.8

2 50

.82

9.80

14,40

36,1

9 36

.03

9.72

33

,12

34,9

2 34

,76

30,7

8 23

.58

31.6

4 33

,98

35,6

71

,97

72.5

2 21

.61

67,3

0 67

,38

70,8

5 62

.65

51.0

0 65

.99

63.5

0 69

,6

30,7

3 33

.80

9.05

30

,65

31,0

4 31

,48

28,9

6 2

2.8

1

29,0

8 26

,85

28,1

6,

47

7.02

2.

99

6,61

6,

75

6,93

6,

79

6.09

7,

09

5,31

4,

93

1,49

1.

45

0.36

1,

41

1,24

1,

12

0,80

0.

54

0,70

1,

13

I 5,

34

5.93

2.

99

5,52

5,

74

6,15

6.

02

5.99

6.

39

4,43

4,

16

4,64

5.

32

4.13

5,

09

4,99

5.

57

6.22

6.

34

6.82

4,

15

4,35

0,

87

2,47

2.

82

2.32

2,

74

2,22

2,

73

2,92

3.

13

3.14

2.

23

2,79

3.

21

2.82

3,

16

2,44

2,

95

3,30

3.

49

3.64

2,

64

2.27

0,38

0.45

0.43

0,46

0,30

0,39

0,42

0.44

0.46

0,40

0,3

-5.18

-6.04

74.3

9 84

.44

13.1

0 12

.30

38,1

27

78

,4

58,7

29

,9

26,2

5,

15

5.22

0,

62

0,36

3.

82

3.24

4.

42

8,33

0,

93

0,4

2,66

0,5

0,3

0,05


Geochemistry Petrography and geochemistry of the Ashizuri

complex are typical of alkaline complexes (Mu- rakami et al., 1989; Stein et al., 1992, 1994). Among their metaluminous chemical compositions and high contents in total alkalies (Table 1 ), all the components of the Ashizuri complex are characterized by ( 1 ) high enrichment in total REE and particularly in LREE (Fig. 3 ), (2) enrichment in Large Ion Lithofile (LIL) elements, (3) enrichment in High Field Strength (HFS) elements with particularly high Nb contents and low Ba/Nb and Ba/La ratios (Table 1 ). Both mafic and felsic rocks exhibit character-

, • 1 0 0

t - O

o 0 ~ 10

Peralumlnous rocks

0S-4

I I I I I I I I I I I I I I I

" i , , I o , . .

O e -

(J O O rr

100

10

Okue metaluminous

T0-48 ~ . ~ I I I I I I i i L I I I i i "~ '~o

100 "L . I D I : O

. C o ( . I O rr

1 0

Ashizuri alkaline rocks

T

I I 1 t I I I I I I I I I I I

La Ce Nd SmEu Gd Dy Ho Er Yb Lu

Fig. 3. Chondrite normalized REE patterns of the peraluminous metaluminous and alkaline rocks.

istic features of ocean island basalt (OIB) type magma when plotted in spidergrams (Figs. 4, 5 ) normalized to MORB (Pearce, 1983) and to ocean ridge granite (ORG, Pearce et al., 1984) respectively. In geotectonic diagrams (Fig. 6) opposing Nb to Y or Rb to N b + Y (Pearce et al., 1984), every unit of the suite displays a marked within-plate character. Sr and Nd isotopic data [ -0 .59~<eNd(T=14 Ma)~<2.56; -9.19~<cSr (T= 14 Ma)~2.90] differ strongly from those obtained for other igneous rocks of southwestern Japan; eNdi and eSri are respectively ( 1 ) lower and higher than the Shionomisaki basalts and (2) higher and lower than all the other acidic rocks in the Shimanto belt (Fig. 7 ). Slight decrease of eNdi and increase of eSri from basalt to granite is linked to combined assimilation and frac- tional crystallization (AFC process). Stein et al. (1994) showed that these isotopic features are in agreement with OIB's known from the litera- ture. They attributed the source of Ashizuri suite to an OIB-type mantle located deep in the con- vective asthenosphere.

3.3. Ca~c-alkaline magmatism

Petrography Three main lithologies of peraluminous rocks

can be recognized in the Shimanto belt. They are all SiO:-saturated rocks and consist predomi- nantly of ( 1 ) cordierite-orthopyroxene-bearing

1000

I O0

m 10 o

2 1

0.1

0.01 I I I

, , , , , , , , ~ , , , s~.~o2y srKZORb Ba Th Nb Ce 2Oszr Ylo

Fig. 4. Multi-element patterns of the Ashizuri alkaline mafic rocks normalized to MORB (Pearce, 1983 ).


100

10- (.9

O "e O O i1-

1

Peraluminous rocks

0.1 I I I I I i i i i i i i

(9 rr o O rr

10 -

0.1

TO-48

Okue metaluminous rocks

Ashizuri alkaline rocks

O rr o t j O rr

10-

0 . 1 ~ K20 Rb Ba Th Nb Ce Zr Sm Y Yb

Fig. 5. Multi-element patterns of the peraluminous, metaluminous and alkaline siliceous rocks normalized to ocean ridge granite (Pearce et al., 1984).

rocks, (2) garnet-biotite-bearing monzogranite and (3) biotite monzogranite. They tend to be located to the south of the belt whereas the metaluminous complexes are clustered to the north. Fig. 8 summarizes the main mineralogical assemblages of all the calc-alkaline complexes.

The Co-Opx bearing rocks consist of rhyodacitic welded tufts, lava flows, granodiorite porphyry (Osuzu, Kashiwajima-Okinoshima, Shionomisaki), or granodiorite (Uwajima). The most common mineral assemblage comprises quartz, plagioclase, cordierite and orthopyroxene. But cordierite or orthopyroxene may sometimes lack whereas biotite and, more rarely, alkali feldspar may appear. Ilmenite, zircon, apatite and tourmaline are accessory minerals. Mega- crysts of garnet and sillimanite, always rimmed by a reaction corona, occur sporadically in all the Co-Opx occurrences. Plagioclase, generally euhedral, is characterized by normal oscillatory zoning with a large range of anorthite content (Ane6_78). Corroded-like cores are common. Decreasing of An content can be correlated to silica enrichment in the whole rock. Cordierite phenocrysts are generally clear, euhedral, devoid of any reaction rim, and contain rare apatite or ilmenite. Compositionally, they are relatively rich in Mg (0.31~<XFe~<0.44). All these features are characteristic of magma-derived cordierite rather than xenocrysts. Orthopyroxene phenocrysts are euhedral, display a pink to pale green pleochroism and consist compositionally of ferrohypersthene to hypersthene. Primary biotite is rare. It is relatively poor in Fe (0.51 ~<XFe~<0.65) and total AI (1.389~<A1T~< 1.435) which can be considered as an hypoaluminous feature (Nachit, 1986). Coexistence between rhyolitic lavas and grano- dioritic rocks of same mineralogical compositions, and abundance of miarolithic cavities favor a crystallization pressure lower than 0.2 GPa. Magmatic temperatures estimated from Stormer (1975) and Bishop (1980) geothermometers range between 800 and 870°C (Nakada, 1983; Stein, 1993).

The garnet-biotite-bearing monzogranites are restricted to Kashiwajima and Okinoshima complexes. They intrude the Co-Opx rhyodacite and granodiorite porphyry. They consist generally of medium-grained rocks mainly composed of quartz, plagioclase, alkali feldspar, biotite, garnet and tourmaline. Accessory minerals are represented by ilmenite, zircon and apatite. Mia- rolithic cavities containing quartz, tourmaline, garnet, stilbite and chlorite are common. Plagio-

1000

100

Nb (ppm)

10

1000

WPG .f . ,J f . , c ' f

f J" j . / "

VAG ~ ~ i C ORG syn-COLG

. . . . . . . . I . . . . . i . . . . . . .

lo y (ppm) lOO lOOO

/ Ashizuri alkaline complex 10 samples

Okue metaluminous complex 36 samples

Peraluminous bodies 34 samples

100

Rb (ppm)

10

VAG

syn-COLG

ORG


r

. . . . . . i 100

Nb+Y (ppm)

. . . . . . . . I . . . . . . . .

10 1000

Fig. 6. Nb vs. Y and Rb vs. N b + Y geotectonic diagram (Pearce et al., 1984). I,~4G, voLcanic-arc granite; syn-COLG syn-collisional granite; WPG, within-plate granite; ORG, ocean ridge granite.

clase is euhedral and often clustered. It displays a strong and regular normal zoning pattern ranging between An35 and An6 from the core to the rim of the crystals. Large poikilitic Carlsbad- twinned alkali feldspar, roughly euhedral, or small interstitial alkali feldspar are characterized by perthitic texture and Or content ranging between Or52 and Or98. Biotite is usually euhedral

and iron rich (0.73~<XFe~<0.85). Low total AI is characteristic of subalkaline to calc-alkaline affinity (Nachit, 1986). Despite a rough euhedral shape, garnet crystals are poikilitic containing plagioclase and rare biotite. They are often associated with Fe-rich tourmaline, chlorite and stilbite. Compositionally, they are almandine- rich and display normal zoning from core to rim


ENdi

10

5

0

.5

-10 L -40

Shic +

~omisaki T-MORB

Ashizuri alkaline suite

Peraluminous and metaluminous acidic rocks Shimanto

Sedimentary rocks old crustal rocks in the OZSWJ

-20 0 20 40 60 80 100 120

Esa Fig. 7. ~Ndi-~Sri diagram (initial isotopic ratios back-calculated to a model age of 14 Ma). Fields of the peraluminous and metaluminous and the sedimentary and old crustal rocks arc drawn after Terakado et al. (1988).

(Aim77 to Aim86 ). Such garnets are believed to represent late crystallizing phase in a peraluminous environment.

Biotite-bearing monzogranites are found as small stocks (e.g. South of Uwajima) but also form a large pluton (Yakushima) which we se- lect as an example. Petrography of this body and particularly of the K-feldspar megacrysts has been extensively studied (Sato, 1977; Kawachi and Sato, 1978). It is characterized by parallel oriented K-feldspar megacrysts (6 to 9 modal %, l 0 cm maximum length), in a groundmass composed of quartz, interstitial K-feldspar, subhed- ral to euhedral plagioclase and euhedral biotite. Zircon, apatite and allanite are accessory minerals. Alkali feldspar megacrysts include all the other rock forming minerals. They consist of

magma-derived (Kawachi and Sato, 1978) mi- croperthite devoid of any marked zoning pattern and with an average composition of OrsoAb2oAno. Plagioclase displays a normal oscillatory zoning with An45 in the core and An~o_2o in the rim. Biotite is compositionally relatively homogeneous with XFe ranging from 0.63 to 0.66. Following Nachit's (1986) classification, it is typical of calc-alkaline affinity.

Metaluminous igneous bodies are mainly located in the northern part of the Shimanto belt, south of the Butsuzo tectonic line (BTL, Fig. 1 ). They are exposed from west to east in Satsuma peninsula, Minami-Osumi, Shibisan, Okue, Ish- izuchi, and northern Omine. They consist mainly of granodiorite and granodiorite porphyry with a lesser amount of dacitic to rhyodacitic tufts or


I Qz+PI+Kf+Bi

I Qz+PI+Kf+Bi Gd Qz+PI+Kf+Bi+Hbl G d

#a

/ t

oS

?

G

Qz+PI+Kf+Bi G Qz+PI+Kf+Bi+HbL+Cpx Gd Qz+PI+Kf+Bi+Hbl Gd Qz+Pl+Kf+Bi+Hbl_+Cpx Gd Qz+PI+Kf+Bi+Opx+Co G Qz+Pl+Kf+Bi+Co+Opx Gd Qz+PI+Kf+Bi+Co G . . . . t ~ ::::::::::::::::::::::: ~:

[ Qz+PI+Kf+BL+Cd G I

Qz+PI+Kf+Bi+AIm G Qz+Pl+Kf+Bi+Opx+Co+Alm G

[ Q z + P I + K f + B ~ Qz+Pl+Kf+Bi-~Opx+Co+Ilm Gd

Qz+Pl+Kf+Opx+Co+IIm Qz+Pl+Bi+Co+Ilm rh

rh

Qz+Pl+Kf+Bi+Co+Ilm rd Qz+Pl+Kf+Bi+Opx+Co+Ilm rd Qz+Pl+Opx+Co+Ilm Gd, rd

Qz+PI+Kf+Bi G I Qz+PI+Kf+Bi+Hbl Gd

] Pl-~o+Opx+Ilm rd dd I Qz+Pl+Opx+Co+Ilm Gd, rd Qz+Pl+Bi+Opx+Co+Ilm Gd, Qz+Pl+Kf+Bi+Opx+Co+Ilm Gd, rr

Qz+PI+Kf+Bi+AIm G

Fig. 8. Map showing the main mineralogical assemblages of all the calc-alkaline igneous rocks within the Shimanto belt. Qz, quartz; PI, plagioclase; Kf, K-feldspar; Bi, biotite; Co, cordierite; Opx, orthopyroxene; Hbl, hornblende; llm, ilmenite; Alto, almandine; G, granite; Gd, granodiorite; rd, rhyodacite; rh, rhyolite.

lavas. They are characterized by the occurrence of hornblende. Okue complex, widely described elsewhere (Nozawa and Takahashi, 1960; Oba, 1977; Arakami et al., 1977; Takahashi, 1986), has been chosen to exemplify their petrography and geochemistry. Takahashi (1986) considered this complex as a typical example of Valles- type caldera and proposed the following volcano-plutonic succession: early lava or ash flows of rhyolite, high K-andesite and dacite and their related vent-filling intrusives have been partly downfaulted by successive cauldron subsidences before the solidification of the resurgent magma chamber forming the Okue batholith. Detailed petrographic description of each unit can be found in Takahachi (1986). We will just summarize the main characteristics of the hornblende biotite-beating granodiorite forming most of the batholith. It is a medium-grained grano-

diorite formed of quartz, alkali feldspar, plagioclase, biotite and amphibole. Proportion of minerals change with altitude; plagioclase and total amount of mafic minerals increase downward from the roof while alkali feldspar and quartz decrease. Locally, enclaves of dioritic composition including augite and sometimes orthopyroxene may represent up to 60% of the exposed rock surface. Mixing features are numerous - - e.g., dif- fuse contact between the felsic host rock and the mafic enclaves or ocelli of alkali feldspar and quartz in the enclaves. Such observations imply that derivation of all the terms of the Okue complex through a simple, closed system crystal fractionation process as suggested by Takahashi ( 1986 ) is unlikely. The operating process is more complex. Mixing processes between two or more components have played a major role in the genesis of the complex. Plagioclase displays a strong


oscillatory zoning ranging from labrador (An57) in the core to oligoclase in the rim (Ana7). More calcic (AnTo) corroded-like cores can be observed and are believed to represent xenocrysts derived from the mafic enclaves magma. Biotite is relatively homogeneous (0.60 ~< XFe ~< 0.65 ). It is poor in total AI ( 1.159 ~< Al-r ~< 1.227 ) and is located on the boundary between subalkaline and alkaline biotite fields after Nachit's ( 1986 ) classification. Most of the amphiboles are typical magnesio-hornblendes following Leake's ( 1978 ) classification. Some of them grade into the actinolite hornblende and actinolite fields. The maximum pressure indicated by the amphibole geo- barometers (Hammarstrom and Zen, 1986; Hollister et al., 1987) does not exceed ~<0.15 GPa confirming the shallow-depth ( ~ 3 km) crystallization and solidification of the body.

Geochemis try

SiO2 contents of all the calc-alkaline, peraluminous and metaluminous igneous rocks exhibit a relatively narrow range of composition (62 ~ wt.%SiO2 ~ 75). Peraluminous rocks are corundum-normative, while metaluminous rocks are diopside-normative, reflecting their respec- tive mineralogical assemblages. Despite the distinctive features in the petrography and the major element chemistry, the trace element chemistry of peraluminous and metaluminous rocks is rather homogeneous. REE patterns (Fig. 3) for both rock-types have a comparable enrichment in LREE [2.51 ~< (La/Sm)N~<4.16], a marked negative europium anomaly (0.33 ~< Eu/ Eu* ~< 0.89) and a slight fractionation in HREE [1.16~<(Gd/Yb)N~<2.08]. However, compared to the aforementioned Ashizuri alkaline suite, the LREE enrichment and the Eu anomaly are much less important. Both peraluminous and metaluminous rocks display a marked enrichment in K20, Rb, Ba and Th relative to Zr, Sm, Y, and Yb in the spidergrams normalized to ORG (Fig. 5 ). The Okue metaluminous rocks exhibit however a distinctive positive Th anomaly ( 100 times ORG-normalizing value). All these features, and particularly the negative Nb anomaly and the low content in Y and Yb, are typical of volcanic arc or syn-collisional granite (Pearce et al., 1984). In Nb vs. Y and Rb vs. N b + Y (Fig.

6), the analyzed samples are related to volcanic arc granites. Sr and Nd isotopic data (Terakado et al., 1988 and Table 1 ) display a large range of compositions [ - 6.6 ~< ENd ( T= 14 Ma) ~< - 0.6; 12.6 ~< ESr( T= 14 Ma) ~< 84.4, Fig. 7 ]. However a distinction can be made between metaluminous rocks [ -2 .9~<~Nd(T=14 Ma)~<-0.6; 12.6~ESr(T= 14 Ma) ~<29.5, Fig. 7] and peraluminous rocks [ - 6.6 ~< ENd( T= 14 Ma) ~< - 2.3; 34.8 ~< ESr( T=14 Ma)~<84.4, Fig. 7]. Oxygen isotopic values for peraluminous rocks (Uwa- jima and Osuzu bodies, Table 1 ) obtained on separated quartz crystals are elevated with ~180 ranging from + 12.3 to + 14.4 (Table 1 ). They are slightly higher than data formerly obtained on whole rocks (+9.5~<~180~<+12.2, Oka- moto and Honma, 1983). Such values are characteristic of sediment involvement in the genesis of these rocks. ~ 8 0 values obtained on metaluminous rocks (Okamoto and Honma, 1983) are markedly lower ( + 9 ~< ~ ~ 80 ~< + 10) and reveal a less important contribution of sediments in the genesis of the metaluminous rocks or the involvement of crustal material different from sediment (e.g. lower crust). Terakado et al. (1988) postulated that the genesis of all the felsic rocks exposed in the Shimanto belt was linked to mixing between a mantellic component and a sedimentary one. Although the sedimentary component is well defined, there still remains a problem in the definition of the mantellic component. Shionomisaki or Muroto basalts are not likely to represent such a mantellic component (see Ter- akado et al., 1988). Stein et al. (1994) showed that the elemental chemistry of the Ashizuri alkaline gabbro is also incompatible. Nevertheless, although such a mixing process played certainly a major role in the genesis of both peraluminous and metaluminous rocks, some peraluminous rocks such as the Osuzu rhyodacite may be the result of pure anatexis. Contribution of the mantellic component would be restricted in that case to heat energy supply. Experimental fluid-absent melting of greywacke lead to formation C o + O p x + B i in a granitic melt under conditions ranging between 0.1 to 0.3 GPa and 810- 860 ° C (Vielzeuf and Montel, 1991 ). Such rocks and conditions correspond well to the Osuzu


cordierite-orthopyroxene-rhyodacite protolith and formation conditions.

3.4. Arc-tholeiites and T- to E - M O R B

Shionomisaki and Muroto, located along the southern fringe of the Shimanto belt (Fig. 1 ), are the only exposures of basalt. They represent about 1% of the surface area of the igneous rocks exposed in the Shimanto belt.

Petrography At Shionomisaki, basaltic lavas and doleritic

dykes are associated with cordierite-orthopyroxene lavas and pyroclastic rocks. Their occurrence is linked to a Middle Miocene topographic and structural high similar to the present one (Fig. 1 ) and thus, near to the subduction trench in a so-called fore-arc position (Miyake, 1985, 1988). Detailed descriptions of these rocks are given in Miyake (1985, 1988). The basaltic lavas which commonly display a pillow structure are aphyric or plagioclase-phyric. The dolerite consists mainly of plagioclase laths, Ca-rich pyroxene, orthopyroxene, hornblende and biotite. The gabbro is composed of Mg-rich olivine (Fo72-88) enclosing Cr-spinel and Ca-rich plagioclase (An8o) laths cemented by augite and bronzite.

At Muroto, two types of basaltic rocks are found, (1) thick layered gabbro associated to doleritic and basaltic dykes and, (2) pillow basalts, massive basalts and mafic breccia in- terbedded with mudstone and sandstone. Al- though the former are clearly intrusive in the surrounding strata and autochthonously formed, the latter may represent olistoliths and thus, are not primordial for our purposes. Descriptions of these rocks have been published by Yajima ( 1972 ). The doleritic and gabbroic dykes mainly contain olivine (Fo61_67) phenocrysts and plagioclase laths (Anso_70) cemented by augite (En42Fs18Wo40).

Geochemistry We did not complete any analyses of Shion-

omisaki or Muroto basalts. However, to get a general view of the Middle Miocene magmatism, we will summarize results concerning both oc-

currences. Based on trace element geochemistry, and particularly the fiat or slightly fractionated REE patterns ( 1.4 ~< CeN/YbN ~< 1.6), relatively high Cr contents ( 1.01 ~<TiO2~< 1.72; 160 ~< Cr ~< 300) and high Th/Hf ratios, Miyake ( 1985 ) claimed a T- to E-MORB affinity for the Shionomisaki basalts and related mafic rocks. At Muroto, basalts from the Maruyama complex display similar features (Hibbard and Karig, 1990a). Sr and Nd isotopic compositions of Shionomisaki basalt [ENd(T= 14 Ma) = +8.2; cSr(T= 14 Ma) = - 14.2; Terakado et al., 1988] are characteristic of MORB-like asthenosphere- derived magma.

3.5. Xenoliths

Three main types of inclusions are found in the magmatic rocks of the Shimanto belt: (1) xenoliths of sedimentary rocks, (2) mafic inclusions with igneous-like texture and (3) metamorphic xenoliths. In the peraluminous and metaluminous complexes all the types are represented. However, the metaluminous complexes are characterized by an abundance of mafic igneous-like enclaves compared to the peraluminous complexes. In the Ashizuri alkaline suite and T- to E-MORB's metamorphic xenoliths are rare or absent.

1 - Xenoliths of sedimentary rocks show generally subangular outlines and resemble the surrounding sedimentary hornfels. Their size ranges from a few centimeters to 100 m or more. They are mainly located close to the contact with the country rocks from which they were dislodged before sinking in the plutons.

2 - Mafic inclusions are typically small-sized (maximum length of 15 cm) and rounded. In thin section, they display magmatic textures. They range compositionally from monzogranite rich in ferromagnesian minerals to diorite or gabbro. Occurrence of such basic enclaves is a major argument for contribution of mixing process during the genesis of the metaluminous and in a lesser amount of the peraluminous rocks. At Osuzu, Nakada (1983) estimated an equibra- tion temperature of at 1050-1100°C, using the pigeonite geothermometer, for a quenched an- desitic enclave and at 990°C, using the ilmen-

G. Stein et al. / Lithos 33 (1994) 85- lO 7 97

ite-orthopyroxene geothermometer for the crystallization temperature of noritic enclaves.

3 - Small-sized, rounded ( 1-30 cm maximum length ) metamorphic xenoliths displaying schistose or gneissose texture are described in all the calc-alkaline occurrences (Nozawa and Taka- hashi, 1960; Sato, 1977; Furugori, 1986; Taka- hashi, 1986; Tateishi et al., 1986; Ishikawa et al., 1992; Komatsu and Tanaka, 1992). Compila- tion of the main mineralogical assemblages gives:

For pelitic rocks: Qz-P1-Kf-Bi-Gt Qz-P1-Kf-Bi-Co-Sill Qz-P1-Kf-Bi-Gt-Co-Sill For psammitic rocks: Qz-Pl- ( Kf )-Bi-Gt Qz-Pl- ( Kf )-Bi-Gt-Opx Qz-Pl- ( Kf )-Bi-Gt-Opx-Co. Although most of the estimated maximum P-

T conditions for these enclaves are between 0.45- 0.67 GPa and 630-860°C some display pressures reaching 0.78 GPa (Komatsu and Tanaka, 1992).

Rb-Sr whole-rock age of a Qz-P1-Kf-Bi-Gt- Opx-Co assemblage has been estimated at 41.8 +_ 6 Ma (Komatsu and Tanaka, 1992 ). Such metamorphic grades are much higher than the metamorphic conditions recorded by the Shi- manto belt rocks. Recent studies on organic matter throughout the Shimanto belt, and particularly in the vicinity of the Muroto peninsula, reveal maximum paleo-temperatures ranging between l l0°C and 350°C (e.g. DiTullio and Hada, 1993; Hibbard et al., 1993; Laughland and Underwood, 1993; Underwood et al., 1993). Moreover, based on inorganic mineral phases analysis, Toriumi and Teruya (1988) show that the highest metamorphic conditions, affecting the oldest rock (Cretaceous) rocks of the belt, do not exceed the greenschist facies metamorphic conditions.

4. D i scuss ionmpropos i t i on of a geodynamic model

Taking into account all the results from the pe- trogenetic study of the Middle Miocene magmatism which cross-cuts the Shimanto belt, we will

try to constrain the geodynamic models cur- rently discussed for the building of the belt. Global understanding of this orogenesis requires integration of the following facts:

1 - First of all, the model must explain the emergence of the Shimanto accretionary prism. Usually, accretionary prisms are not directly ac- cessible onshore; they are always below the sea level. Thus, we must explain the process of prism emergence.

2 - It must also take into account the structural peculiarities of the Shimanto orogeny. Sev- eral important characteristics are atypical of modern accretionary prisms (Charvet and Fab- bri, 1987; Charvet et al., 1990); e.g. a schistose Cretaceous metamorphic unit overthrusting the Paleogene unmetamorphosed units, presence of northward folds even in the Lower Miocene unit, oceanward thrusting of the fore-arc and re- thrusting of the older orogens to the north of the Shimanto, implying a very wide compressional stress system.

3 - Finally, it must account for the presence, mode of occurrence, anomalous near-trench location and diversity of the Shimanto belt Middle Miocene magmatism.

4. I. Simple accretionary prism model

As previously stated, the Shimanto belt has been widely interpreted as a result of ocean-derived sediment accretion above an oceanic lithosphere subducting more or less continuously beneath southwestern Japan since Cretaceous times. Such a model is not likely to explain the magmatic characteristics of the Shimanto belt.

1 - Presence and location of the magmatism Accretionary prisms of Barbados, Cascade, etc.

lack magmatic activity equivalents of the Shi- manto prism. A strong thermal anomaly must have developed beneath the Shimanto prism. This high-T event is also supported by the temperature (630-860°C) deduced from the mineral assemblages of metamorphic xenoliths included in the calc-alkaline complexes. One way to reconcile this "high-T constraint" with a simple accretionary prism model, is to relate the Shimanto Middle Miocene magmatism and


high-T metamorphic enclaves to the subduction of the young hot Philippine Sea oceanic lithosphere (Kobayashi, 1979; Takahashi, 1986 ). But, if the mantle-derived melts were provided by the subduction of the Shikoku oceanic lithosphere, boninites, adakites and/or their intrusive equivalents: tonalites and trondhjemites, should occur in the Shimanto belt (Defant and Drummond, 1990; Defant et al., 1992 ).

Indeed, boninites are widely considered as typical of fore-arc magmatism, due to melting of hydrous peridotite at the initiation stage of subduction (e.g. review in Pearce et al. 1992 ). Many authors emphasize the necessity of subduction of a very young lithosphere, that is capable to melt at rather shallow depths and lead to metasoma- tism of the mantle wedge (van der Laan et al., 1989; Pearce et al., 1992). The end member model is the ridge subduction, even beneath a thermally mature lithosphere, which may cause an anomalous near-trench magmatism (De Long et al., 1979). That situation could fit the case of the Shikoku basin and ridge subduction initiation. But in the case of the Shimanto magmatic province, no boninites are known. In contrast, the high magnesium andesites named sanukitoids, more or less equivalent to boninites but affected by crustal contamination, were erupted further west in the Japan volcanic arc, slightly after the emplacement of the Shimanto diversified igneous rocks, in relation with rifting in the Seto- uchi area (Tatsumi, 1982 ).

Adakites are considered to derive directly from the partial melting of the subducted slab of a young and hot lithosphere (Defant and Drum- mond, 1990; Defant et al., 1992 ). Neither adakites, nor tonalites and trondhjemites are found in the Shimanto belt. Calc-alkaline rocks of in- termediate to acidic composition prevail in the Shimanto belt, outward with respect to the sanukitoids. Thus subduction of the hot lithosphere of the Shikoku basin may well explain the eruption of the Setouchi sanukitoids, but not this slightly older calc-alkaline magmatism.

On the other hand, this process cannot ac- tually provide enough heat to explain the magmatism and high-T metamorphic xenoliths:

(i) The oldest Shimanto prism accreted strata are of Cretaceous age. Low greenschist facies is

the maximum metamorphic grade they exhib- ited (Toriumi et al., 1986; Toriumi and Teruya, 1988). It seems difficult to correlate this low grade metamorphism with the high grade one displayed by the metamorphic xenoliths. One granulitic metamorphic enclave has been dated as Eocene (e.g. 41.8_+ 6 Ma; Komatsu and Tan- aka, 1992) and thus, cannot be related to the subduction of the Shikoku basin which opened during 23 to 12 Ma. Alternatively, either this rock was formed by another tectonothermal event ca 42 Ma ago, which affected southwestern Japan, or derives from a microblock with an Eocene metamorphic basement underthrust during the lower Miocene event. The former hypothesis poorly constrained, could be related to an Eocene collision between southwestern Japan and an exotic block (Charvet and Fabbri, 1987; Komatsu and Tanaka, 1992). The latter one is also plau- sible, as the northwestern Philippine basin comprises several features with an Eocene and even Cretaceous basement (Shiki, 1985; Pearce et al., 1992). In both cases, the simple accretionary prism model lying on an oceanic crust is ruled out.

(ii) Existence of the Middle Miocene to Qua- ternary Nankai accretionary complex implies that the present location of the Nankai trough lies further to the south, as a consequence of the growth of the prism, compared to the supposed position of the Middle Miocene subducting trench. Assuming a trench position identical to the present one, together with the 13 Ma volcanic-arc front located in the Setouchi area (Fig. 1 ), it follows that the southernmost Middle Mio- cene igneous exposures developed at a maximum of 20 km above the Middle Miocene subducting slab. In fact, the slab was shallower, due a gentle dip, suggested by sanukitoids generation, and which still exists. Several authors (Wang and Shi, 1984; James et al., 1989; Dumi- tri, 1991 ) have shown that the thermal structure of an accretionary prism is affected by the amount of heat conducted across the base of the wedge. James et al. (1989) show that subduction of an oceanic crust must be less than 3 Ma old to rise the isotherms. At around 20 km depth, which is probably an over-estimate, an 1.5 Ma old subducting oceanic crust would lead to a


temperature close to 400 °C on the accretionary drcollement surface. Such modelling suggests that the overheat energy provided by young subducting plate of the Shikoku basin would not be high enough to generate (1) the high-T metamorphism upon the subducting slab, (2) the granitic melts. According to the modelling of Pearce et al. (1992), the temperature of 400 ° C should be reached at depth of 15 to 20 km, with a 45 ° dipping slab, only in the case of subduction of a ridge beneath a young oceanic lithosphere, within 2 Ma. Otherwise, it should be much colder, like in the Shimanto case, which did not satisfy these required conditions. The alternative of the subduction of the Shikoku basin accretion ridge as a potential heat source has been considered (Marshak and Karig, 1977). De- pending on the migration or not of this ridge, we should observe: (1) an evolution in the age of the magmatism or (2) a magmatic activity restricted to a very limited area. None of them are observed. As previously stated, the magmatic rocks are found throughout the Shimanto belt and the ages of the complexes are homogeneous. These facts contradict this hypothesis, in addition to the recent abovementionned thermal modelling.

2 - Pressure conditions o f the metamorphic enclaves

Temperature is not the only critical point. Maximum pressures obtained from the study of the metamorphic xenoliths range between 0.7- 0.8 GPa. The only well-known cross-section of a mature accretionary prism, which is the Barba- dos one, displays a maximum depth of 15 km (Westbrook, 1988 ). Pressures of 0.7 to 0.8 GPa represent almost twice this maximum depth. Moreover, enclaves displaying 0.6 to 0.8 GPa are found in igneous complexes located rather south of the belt, that is to say, relatively close to the frontal thin part of the accretionary prism. Fi- nally, crystallization pressures obtained from the host rocks mineralogical assemblages or from experimental investigations (Vielzeuf and Montel, 1991 ) display values ranging from 1 to 3 kb. Thus the average pressure of the included xenoliths is higher than the crystallization pressure of the calc-alkaline magmas. Such apparent inconsis-

tency cannot be explained by a simple accretionary prism model.

3 - Diversity and type of the magmatism One characteristic of the Shimanto belt mag-

matism is the occurrence of mantle-derived rocks (Ashizuri, Shionomisaki, Muroto) and the involvement of upper mantle (asthenospheric) components in the genesis of the calc-alkaline rocks. However, simple accretionary prisms are never directly underlain by the mantle. Back- ward they are underlain by the continental margin or by the arc basement. Foreward they are directly underlain by the subducting oceanic crust. So there is always a physical barrier to the uprise of mantle-derived products. One could argue that these products could be provided by the subduction of the Shikoku Basin-related oceanic ridge (Marshak and Karig, 1977; Hibbard and Karig, 1990a, b). But, as previously stated, such a process would lead to an evolution in the age of the magmatism or to a magmatic activity restricted to a small point, which is not the case. Moreover, subduction of an active accretion ridge may explain the local occurrence of MORB- like rocks like in the Taitao peninsula of Chile (Forsythe and Prior, 1992) but could not account for the alkaline magmatic rocks observed at Ashizuri.

4 - Tectonic implications It is beyond the scope of this paper to develop

the tectonic aspects of the debate. We would just recall the main points (Charvet and Fabbri, 1987; Charvet et al., 1990). The Shimanto orogeny affected, by thrusting and folding, a wide area involving different zones located in the inner side of the former prism, thrusting metamorphic rocks over non-metamorphic sediments and leading to a marked and rapid uplift, before the Lower to Middle Miocene general unconformity. A review of modern oceanic subduction zones with accretionary prisms shows that such a se- quence of events is occuring nowhere; the com- pressive deformation is usually restricted to an area within the inner trench wall and reaches only exceptionally the outer edge of the fore-arc basin as in the Barbados example (von Huene, 1984; Hamilton, 1988; Charvet and Ogawa, 1993).


When the stress system is more widely and continuously compressional from the trench inner wall to the fore-arc basin inner edge or even to the arc and beyond the arc, the underthrusting lithosphere is no more oceanic in the presently working examples, but a more buoyant one. In other words, the geodynamic context is collisional. Best known examples are provided by Taiwan and Banda-Timor collision zones. In addition, numerical modelling clearly shows that subduction of a buoyant feature (e.g. with a light root) is the only mechanism able to induce an uplift of kilometric order in the overriding plate, due to isostatic unbalance (Moretti and N'Gok- wey, 1985 ) .This mechanism works with underthrust blocks of different sizes, even only 100 km wide, like the Entrecasteaux ridge colliding with the New Hebrides arc. On the contrary, subduction of asperities without buoyancy will produce

limited effects and an uplift (if any) of about 200 m. This prediction is in good agreement with the observation of recent to present subduction of asperities (see below). In summary, the simple accretionary prism model is unable to explain the magmatism and its diversity, the nature of the xenoliths, and the tectonic peculiarities of the Shimanto belt. Another model, in which the accretion stage linked with the Pacific plate subduction has been interrupted by the collision with the Philippine Sea plate and the subsequent onset of the Philippine Sea plate subduction is preferred.

4.2. Collisional model

The link between the Takachiho orogeny (Lower Miocene Shimanto orogeny) and the

~, ~ PACIFIC PLATE

G. Stein et aL / Lithos 33 (1994) 85 - I07 101

PA~CIFIC PLATE

1000 Km

Fig. 9. Hypothetical geodynamic reconstruction at 20 and 15 Ma. See text for explanation.

change from Pacific subduction to Philippine Sea plate subduction beneath southwestern Japan is now rather widely accepted. In other words, the Shimanto orogeny was the result of the first contact (or "collision") between the Philippine Sea plate and southwestern Japan. After Charvet and Fabbri ( 1987 ), several models based on this as- sumption have been proposed (Sakai, 1988; Charvet et al., 1990; Hibbard and Karig, 1990a, b). They have to be tested by examining how they satisfy the tectonic, geodynamic and magmatic constraints.

The model proposed by Sakai (1988) assigns this event to the initiation of strong coupling between the hot and "buoyant" lithosphere of the Shikoku basin and southwestern Japan fore-arc. Besides the buoyancy problems (see below ), this model is facing a major geodynamic difficulty,

as it is based on the anchored slab model, in con- flict with all the paleomagnetic data.

Hibbard and Karig ( 1990a, b) proposed a collision between southwestern Japan and the newly formed Shikoku basin ca. 15 Ma to explain the Muroto structural flexure and its associated magmatism, and the Shimanto orogeny. Before that, the Pacific oceanic plate would have separated southwestern Japan from the Shikoku basin. As the boundary between the latter two plates would have been a transform fault, this model presents some similarities with the initiation of a subduction along a former transform fault, as it is advocated for the Mariana arc (Hilde et al., 1977; Stern and Bloomer, 1992 ). It is also facing several difficulties, with respect to the magmatic, tectonic and geodynamic constraints. It cannot explain the nature and the di-


versity of magmatism within Shimanto, as stated above in the section devoted to the simple accretionary prism model. The location of the Yaku- shima pluton, in front of the northwestern cor- ner, that is the oldest part of the West Philippine basin, is another contradiction. If we take into account the recent change of kinematics which implies a more eastern location of the Palau- Kyushu ridge before 3 Ma (e.g. Angelier and Hu- chon, 1986), the plutonic bodies of southern Kyushu are unlikely linked to subduction of the Shikoku basin. Regarding the tectonic implications, recent works have shown that recent to present subduction of oceanic asperities (e.g. von Huene and Lallemand, 1990) as well as active oceanic ridges (Bangs et al., 1992) leads mainly to a conspicuous tectonic erosion and collapse of the margin, after some local and limited compressional effects, and not to the uplift of a thrust belt. These modern examples confirm the con- clusion drawn out from numerical modelling in the previous section. The subduction of young and hot Shikoku basin lithosphere cannot explain the tectonic phenomena as well as magmatic ones. By the way, a subduction initiation according to the model of Stern and Bloomer (1992), emphasizing the buoyancy factor, implies that the Shikoku basin lithosphere was less buoyant than the adjacent southwestern Japan one which is difficult if the "Shimanto prism" was still underlain by the old Pacific lithosphere. The last problem is the transform fault boundary between the Pacific and the Philippine Sea plate, prior to the collision, implied by this model. It requires a very peculiar combination of plate motion speeds in order to keep a stable boundary without convergence along the northwestern edge of the Philippine Sea plate, during its migration (Louden, 1977), and after the change of Pacific motion from northward to westward around 43 Ma. Moreover, the large concomitant clockwise rotation, not known in the Pacific (Louden, 1977; Koyama et al., 1992 ), makes this hypothesis unrealistic. Recent studies show a clockwise rotation of up to 70 or 80 ° of the Izu- Bonin arc since 27 Ma (Koyama et al., 1992) which implies that the leading edge of the Phil- ippine Sea plate, before that time, was a roughly E-W trending island-arc, active since 48-49 Ma

above a south-dipping subduction (Taylor, 1992). This is in agreement with conclusions of former works (e.g. Kobayashi, 1983) pointing out the difference between the northern and southern segments of Palau-Kyushu ridge, separated by a bend. The consequence is that the edge of the Philippine Sea plate colliding with SW Japan was likely an arc working since at least the Eocene, that is to say with a buoyant crust!

Although the exact configuration of this leading arc, now disappeared, is uncertain, and one may argue about its trend, the point is not critical. Because, as pointed out by several authors (Kobayashi, 1983; Pelletier et al., 1983; Shiki, 1985; Charvet and Fabbri, 1987; Jolivet et al., 1989; Charvet et al., 1990; Taylor, 1992), several highs of the northern part of the Philippine basin (Daito, Oki-Daito, Amami, Palau-Kyu- shu) show an island-arc and/or continental igneous or metamorphic Eocene to pre-Eocene basement. It seems thus reasonable to assume, whatever the exact trend of the boundary was, that further west the colliding front of the Phil- ippine Sea plate, now subducted, was made of or rich in features with a light crust. In summary, the balance of geodynamic considerations, inde- pendent from the study of the Shimanto belt it- self, is in favor of a Lower Miocene collision of buoyant lithosphere with southwestern Japan, initiating the process of present subduction of the Philippine Sea plate. The crustal structure can be imaged by the 15-20 km thick crust of the Izu- Bonin arc (Hotta, 1970), detached from the Pa- lau-Kyushu ridge by the opening of the Shikoku basin between 25 and 12 Ma (Chamot-Rooke et al., 1987) or 22 and 15 Ma (Taylor, 1992), and which is still colliding with central Japan in the Izu peninsula area. However, a restriction to the similarity is the fact that pre-Eocene basement rocks of continental affinity have not been found yet in this segment (Taylor, 1992).

A collisional model may also integrate the constraints provided by the study of the magmatic rocks. It is worth noting that the magmatic rocks of almost same age throughout the Shi- manto belt show striking similarities with the present igneous rocks in the Sunda arc, era- placed after the collisional event involving the Banda arc and the Australian continental plate

G. Stein et al. / Lithos 33 (1994) 85-107 103 W E

k m

o

50

1oo-

2>.-

+

Southwest Japan

+ + -

+ + -F +

Shimanto accretionary prism

North Philippine Sea block

detachment

The Shikoku Basin starts opening

= 25 M a ]

Southwest Japan

Shimanto belt Uplift and erosion

Post-coiL magmatism Nankal prism Shikoku Basin

Alkaline magmatism, T- and E MORB

Setonchi Shimanto belt volcanic ere Post-coll. magmatism N kaJ prism

..... ? ( V

Shikoku Basin

Fig. 10. Geodynamic model in cross-section of the Shimanto belt orogen (modified after Charvet et al., 1990). See text for explanations.


(Hamilton, 1979). The Sunda arc displays a large set of volcanic rocks, ranging in compositions from low K-tholeiites, through medium and high- K calc-alkaline andesites and dacites and to leu- cite basanites. After Stolz et al. (1990), at least 3 major source components are required to explain this magmatic activity; MORB-source, OIB-source and subducted sediments. This re- sembles interestingly to the magmatic sources involved in the case of the Shimanto belt magmatism.

Thus, a possible scenario, based on Charvet et al. (1990) model, could be the following one (Figs. 9 and 10):

- ca. 25 Ma, the Shikoku basin just starts opening (or is going to open) and migrates northward according to the model of trench re- treat and clockwise rotation of the Philippine plate (Seno and Maruyama, 1984; Huchon, 1985; Koyama et al., 1992). This plate carries the colliding microblock(s) on its northern- northwestern boundary. Collision is believed to occur between 21 and 17 Ma (Charvet and Fab- bri, 1987; Charvet et al., 1990). Such a collisional event may explain at least part of the high grade metamorphic xenoliths included in the Shimanto belt calc-alkaline rocks. In fact, the xenoliths displaying the oldest ages and the highest pressures could be easily explained if we consider their derivation from an exotic block dri- ven by the Philippine Sea plate. It may also explain the occurrence of xenoliths displaying higher pressure than their host rocks.

- After the collisional event and before the initiation of the new Philippine Sea oceanic plate subduction (around 15 Ma), asthenospheric upwelling may occur and generate the Ashizuri alkaline suite. Such asthenospheric injection has also been postulated to explain the Shionomisaki T- to E-MORB's (Shiki and Miyake, 1988) and is illustrated in the subduction model of Stern and Bloomer (1992). It may give a sense to the alignment and the southernmost location of Ashizuri, Muroto and Shionomisaki occurrences. Difference in extent of partial melting and in depth of generation may explain the occurrence of ocean-island like alkaline rocks together with T- or E-MORB's.

Subduction of the Philippine Sea oceanic plate

is starting, inducing a downdip migration of the asthenospheric upwelling. This "parallel trending" migration of asthenospheric material beneath the Shimanto belt may explain the generation of the calc-alkaline magmatism. It explains ( 1 ) the widespread distribution and the age ho- mogeneity of the calc-alkaline rocks inside the Shimanto belt, (2) the involvement of mantle- derived material in the calc-alkaline rocks genesis and (3) the heat energy required to melt sedimentary and/or low crustal material. Isostatic reaction following the collision and thickening will lead to a quick and extended uplift of the Shimanto belt.

- ca. 13 Ma, due to incipient subduction of the Philippine Sea oceanic plate, the hot Shikoku basin downgoing slab may have reached the required depth beneath southwestern Japan, inducing, together with extensional stress, the generation of sanukitoids erupting in the Setouchi area, back of the Shimanto belt. Range of time between the onset of the new subduction and the formation of the arc magmatism implies a subducting rate of about 7 cm/yr which seems reasonable.

Thus, a collisional event during Middle Mio- cene time followed by the onset of the Philippine Sea plate subduction and the associated asthenospheric upwelling more simply accounts for all the characteristics of the Shimanto belt; its emersion, its structuration and as demonstrated its related magmatic activity.

Acknowledgements

The authors are greatly indebted to Prof. A. Tsusue for his help and discussion in Kumamoto University. We thank also Prof. M. Faure and Dr. J. Pons for discussion, and J.B. Gill and an anon- ymous reviewer for their helpfull review of the manuscript. This work has been supported by the French Ministry of foreign affairs for G.S.

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Documents

Geodynamic setting of volcano-plutonic rocks in so-called “paleo-accretionary prisms”: Fore-arc activity or post-collisional magmatism? The Shimanto belt as a case study