JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
PAGES 3–16
2002
The Petrology and Geochemistry of CalcAlkaline Andesites on Shodo-Shima Island,
SW Japan
Y. TATSUMI1∗, T. NAKASHIMA2 AND Y. TAMURA1
1
INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION, JAPAN MARINE SCIENCE AND TECHNOLOGY
CENTER, YOKOSUKA 237-0061, JAPAN
2
INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 874-0903, JAPAN
RECEIVED OCTOBER 15, 2000; REVISED TYPESCRIPT ACCEPTED JUNE 18, 2001
Petrographical and geochemical characteristics of calc-alkaline
andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented.
The following petrographic observations suggest a role for magma
mixing in producing such andesite magmas: (1) two types of olivine
phenocrysts and spinel inclusions, one with compositions identical
to those in high-Mg andesites and the other identical to those in
basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+
relations, respectively; (2) the presence of orthopyroxene phenocrysts
with mg-number >90 suggests the contribution of an orthopyroxenebearing high-Mg andesite magma to production of calc-alkaline
andesites; (3) reversely zoned pyroxene phenocrysts may not be in
equilibrium with Mg-rich olivine, suggesting the involvement of a
differentiated andesite magma as an endmember component; (4) the
presence of very Fe-rich orthopyroxene phenocrysts indicates the
association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calcalkaline andesite genesis can also provide a reasonable explanation of
the Pb–Sr–Nd isotope compositions of such andesites.
The continental crust possesses an andesitic composition
broadly identical to calc-alkaline andesites (Christensen
& Mooney, 1995; Rudnick & Fountain, 1995; Taylor &
McLennan, 1995) that are produced at convergent plate
boundaries and are characterized by lower FeO∗/MgO
ratios than tholeiitic andesites (Miyashiro, 1974). An
understanding of the origin of such orogenic, calc-alkaline
andesites is thus essential for comprehending not only
the magma flux in the modern Earth but also the role
of subduction-related magmatism in the evolution of the
solid Earth. Since the pioneering work of Eichelberger
(1975, 1978), Anderson (1976), Sakuyama (1979, 1981,
1984), and Luhr & Carmichael (1980), the majority of
petrologists have considered mixing of mafic and felsic
magmas to be one of the major mechanisms of generation
of calc-alkaline andesites. Continental crust formation,
however, may not be elucidated solely by such magma
mixing processes. The reasons for this are twofold. First,
calc-alkaline andesite magmatism typifies continental arcs
at least on the modern Earth, suggesting that the continental crust itself plays an important role in such andesite
formation, possibly as the source of the felsic endmember
component. Second, there is a slight but significant
difference in MgO contents between the bulk continental
crust and the typical calc-alkaline andesite (e.g. Kelemen,
1995), i.e. 4·4 and 3·2 wt % MgO at >60 wt % SiO2,
respectively (Gill, 1981; Taylor & McLennan, 1995).
Such Mg-rich andesites may not be formed by simple
differentiation of basaltic magmas (Kelemen, 1995), so
that some additional mechanisms must have been operational.
One of the processes responsible for the production
of Mg-rich continental crusts may be differentiation of
mantle-derived andesitic, not basaltic, magmas (Shirey
∗Corresponding author. Telephone: 81-468-67-9760. Fax: 81-467-669625. E-mail: [email protected]
 Oxford University Press 2002
KEY WORDS: calc-alkaline
andesites; high-Mg andesites; magma mixing;
continental crust; SW Japan
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 43
& Hanson, 1984; Stern et al., 1989; Stern & Hanson,
1991). Observations supporting this mechanism include
the following: (1) andesitic melts can be generated by
partial melting of peridotites at mantle pressures under
hydrous conditions (e.g. Kushiro, 1969; Mysen &
Boettcher, 1975; Hirose, 1997); (2) particular natural
andesites with high MgO contents and high Mg/(Mg +
Fe) ratios, which have been referred to as high-mg-number
andesites (HMAs), may represent such unfractionated,
mantle-derived melts (e.g. Tatsumi & Ishizaka, 1981,
1982; Umino & Kushiro, 1989); (3) the compositional
variation of Archean monzonite–granodiorite suites can
be explained by crystallization differentiation of mantlederived andesitic magmas (e.g. Stern & Hanson, 1991).
It is thus interesting to examine the genetic linkage
between HMAs and calc-alkaline andesites.
This paper presents petrographical and geochemical
characteristics of porphyritic and plagioclase-phyric calcalkaline andesites from Shodo-Shima Island, SW Japan,
where mantle-derived HMAs and basalts are also distributed, and provides constraints on the genesis of such
calc-alkaline andesites.
NUMBER 1
JANUARY 2002
southward drift of the SW Japan arc caused by the
clockwise rotation of the arc sliver resulted in the subduction of very young (>17 Ma; Okino et al., 1994)
oceanic crust from the Shikoku Basin beneath the arc
lithosphere (Tatsumi & Maruyama, 1989). On the basis
of numerical simulations, Furukawa & Tatsumi (1999)
demonstrated that the temperature at the surface of such
a young subducting slab is high enough for partial melting
of both sediments and oceanic crust, thereby providing
support for the proposal by Shimoda et al. (1998) and
Shimoda & Tatsumi (1999) that the Setouchi magmatism
was triggered by partial melting of subducting sediments.
Shodo-Shima Island, which is located to the NE of
Shikoku (Fig. 1), is distinctive for the collective occurrence
of major types of Setouchi volcanic rocks ranging from
phenocryst-poor andesites (including HMAs) to basalts
referred to as sanukitoids (Tatsumi & Ishizaka, 1981),
porphyritic and plagioclase-phyric calc-alkaline andesites,
garnet-bearing dacites or rhyolites, and pitchstones (Tatsumi, 1983b). Because HMAs in the Setouchi volcanic belt
are distinct from boninites, well-known HMAs originally
described from the Bonin Island, in the absence of
clinoenstatite phenocrysts, the presence of plagioclase in
a groundmass, and the enriched trace elements and
isotopic signatures, Setouchi HMAs are referred to as
HMA sanukitoids hereafter.
The older Setouchi volcanic rocks (Uchinomi Formation), which cover basement granites and gneisses, are
composed of felsic lava flows, lava domes, sheets, dykes,
and volcaniclastic rocks, whereas the younger group
(Kankakei Formation) consists of volcanic rocks of intermediate to mafic compositions (Fig. 1). K–Ar dates for
Setouchi volcanic rocks on Shodo-Shima Island (Tatsumi
et al., 2001) yielded the mean ages of 12·82 ± 0·12 Ma
and 13·78 ± 0·17 Ma for Kankakei and Uchinomi
Formations, respectively, which is consistent with the
stratigraphic relations. The present volume of Setouchi
volcanic rocks on Shodo-Shima Island is estimated to be
>30 km3 (Tatsumi, 1983b). Tatsumi (1983b) suggested
that most lavas and volcaniclastic rocks on Shodo-Shima
Island were formed under subaqueous conditions. Some
samples show typical water-chilled structure with cracks
perpendicular to the rock surface.
The andesite samples used in the present study are
lavas and volcanic breccias from the Kankakei Formation
(Fig. 1). To evaluate the role of granitic basements
in the formation of calc-alkaline andesite magmas, we
analyzed four representative basement rocks on ShodoShima Island.
GEOLOGY
The Philippine Sea plate is actively subducting beneath
the Eurasian plate to form the arc–trench system of SW
Japan (Fig. 1). Although the clearly defined seismicity
and active volcanism in Kyushu does not continue to
Honshu and Shikoku, a study of the ScSp phase (Nakanishi, 1980) suggests that the leading edge of the subducting slab may have reached the upper mantle beneath
the Quaternary volcanic front in SW Honshu (Fig. 1).
Miocene igneous rocks in SW Japan are located in the
present forearc region, more than 80 km trenchward of
the Quaternary volcanic front (Fig. 1). In the near-trench
region of the SW Japan arc, felsic volcano-plutonic
complexes, which intruded into an accretionary prism
built in the Cretaceous to Miocene period, are distributed
(Fig. 1). The K–Ar ages of these complexes concentrate
remarkably at 14 ± 1 Ma (Shibata, 1978).
The Setouchi volcanic belt extends for >600 km along
the SW Japan arc, with five major volcanic regions (Fig.
1). Formation of the Setouchi volcanic belt parallel to
the arc–trench system may indicate the involvement of
the subducting lithosphere of the Philippine Sea plate in
producing Setouchi magmas. New K–Ar age data for
Setouchi lavas (Tatsumi et al., 2001) confirmed an earlier
suggestion (Tatsumi, 1983a) that the Setouchi magmatism
took place within the short period of 13 ± 1 Ma. This
is largely synchronous with the timing of a 40–50°
clockwise rotation of the arc sliver of SW Japan or the
major mode of rifting of the Japan Sea backarc basin at
14–16 Ma (Otofuji et al., 1991). It is thus suggested that
EXPERIMENTAL METHODS
Major and trace element compositions were measured by
RIGAKU Symaltics 3550 and 3070 X-ray fluorescence
4
TATSUMI et al.
CALC-ALKALINE ANDESITES, SHODO-SHIMA, SW JAPAN
Fig. 1. Tectonic setting of the Setouchi volcanic belt and a simplified geological map of Shodo-Shima Island after Tatsumi (1983b). The
Setouchi volcanic rocks (open stars) and coeval felsic volcano-plutonic complexes (Β) are distributed in the present forearc region, to the trench
side of the Quaternary volcanic front (dashed lines). Setouchi volcanic rocks on Shodo-Shima Island, which cover mainly granitic basement
(cross), are divided into two groups, the Kankakei (1–11) and Uchinomi (12) Formations in descending order. EUR, Eurasian plate; PHS,
Philippine Sea plate; PAC, Pacific plate; 1, Seiho lavas; 2, Toho lavas; 3, Utsukishi-nohara lavas; 4, Utsukushi-nohara volcaniclastic rocks; 5,
Dan-yama lavas; 6, Kaerugo-ike volcaniclastic rocks; 7, Kiyotaki stock; 8, Choshikei lavas; 9, Kamikake-yama volcaniclastic rocks; 10, basaltic
sanukitoid lavas; 11, andesitic sanukitoid lavas; 12, Uchinomi group.
Pb/204Pb = 15·445 ± 0·005, and 208Pb/204Pb =
36·553 ± 0·017 were obtained for NBS SRM 981 lead.
The correction factors per a.m.u. for 206Pb/204Pb, 207Pb/
204
Pb, and 208Pb/204Pb were 0·10%, 0·10%, and 0·11%,
respectively. Total analytical blanks were below 100 ng
for all elements.
Mineral compositions have been analyzed using a
JEOL XMA-8800 electron-probe micro-analyzer. Analyses were made using an excitation potential of 15 kV
(25 kV for olivine analyses), specimen current of 1·2 nA
(2·0 nA for olivine analyses) and analytical time of 20 s
(200 s for olivine analyses). The correction procedures
followed those of ZAF.
207
(XRF) spectrometers, on fused glass beads and pressed
powder pellets, respectively. Detailed analytical procedures were described by Goto & Tatsumi (1994, 1996).
Sr, Nd, and Pb isotope compositions were analyzed
with a Thermoquest MAT 262 mass spectrometer. The
measured 143Nd/144Nd ratios were normalized to a 146Nd/
144
Nd value of 0·7219. The average value for the JNdi1 standard supplied from the Geological Survey of Japan
during this study was 143Nd/144Nd = 0·512100 ± 12
(n = 23; 2), which corresponds to a value for La Jolla
of 0·511842 (Tanaka et al., 1996). For the NBS SRM 987
standard, we obtained a value of 87Sr/86Sr = 0·710259 ±
10 (n = 32). The values 206Pb/204Pb = 16·902 ± 0·004,
5
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
JANUARY 2002
reversely zoned clinopyroxene phenocrysts and of dusty
plagioclase phenocrysts. An andesite SD936 also does
not include disequilibrium phenocrysts, although only
minor amounts of reversely zoned orthopyroxene are
found (Fig. 6). To emphasize those distinctive petrographic features, such andesites are called ‘evolved
andesites’ hereafter. The origin of evolved andesites will
be examined geochemically in the next section.
RESULTS
Major and trace element, and isotopic compositions of
Shodo-Shima lavas are listed in Table 1 together with
modal compositions of phenocrysts. Porphyritic and
plagioclase-phyric andesites on Shodo-Shima Island possess a wide range of compositions with SiO2 contents
from 52 to 65 wt % and show a broad calc-alkaline trend
(Fig. 2). In connection with Fig. 3, the bulk continental
crust compositions are found to lie largely within the
geochemical trend formed by Shodo-Shima andesites,
except for samples of rather high concentration of Al2O3
in Shodo-Shima andesites. Further, calc-alkaline andesites on Shodo-Shima Island show incompatible element
patterns which typify both subduction zone magmas and
bulk continental crust compositions; i.e. relative depletion
and enrichment in Nb and Pb, respectively, when compared with N-MORB compositions (Fig. 4).
Figure 3 demonstrates that HMA sanukitoids and
porphyritic andesites, except for ‘evolved’ andesites described below, form broadly a single compositional trend.
Such trends were also reported from Archean monzodiorite and trachyandesite suites in Superior Province
and were explained by crystallization differentiation of
mantle-derived HMA magmas (Shirey & Hanson, 1984;
Stern & Hanson, 1991).
Porphyritic calc-alkaline andesites on Shodo-Shima
Island show more enriched isotopic characteristics than
both basalts and HMA sanukitoids (Fig. 5), suggesting
the involvement of enriched endmember components
such as rhyolitic magmas and granitic basements (Fig.
5) in their formation.
Most porphyritic andesites exhibit the following disequilibrium petrographic characteristics: (1) the presence
of plagioclase phenocrysts with a dusty zone containing
fine melt inclusions and with wide range of compositions
(Fig. 6); (2) the presence of reversely zoned pyroxene
phenocrysts with rounded cores mantled by rims of higher
mg-number (Fig. 6); (3) the presence of subhedral to
rounded olivine phenocrysts rimmed by pyroxenes. These
petrographic observations suggest the role of magma
mixing processes in formation of such magmas
(Eichelberger, 1975; Sakuyama, 1979; Bloomfield & Arculus, 1989; Kawamoto, 1992; Yang et al., 1999). It is
thus confirmed that porphyritic calc-alkaline andesite
magmas are not produced by simple crystallization
differentiation processes from HMA sanukitoid magmas,
although they form a single compositional trend (Fig. 3).
On the other hand, particular porphyritic andesites to
basaltic andesites show little evidence for the above
disequilibrium features. A basaltic andesite SD931, as
shown in Fig. 6, contains olivine phenocrysts, which are
not rimmed by pyroxenes and are in equilibrium with
the bulk composition in terms of Fe–Mg exchange partitioning, and is characterized by the absence both of
DISCUSSION
Assimilation and fractional crystallization
Evolved andesites possess geochemical characteristics distinct from other mixed andesites. First, they occupy the
most silica-deficient part of the andesite trend and show
a gentle slope close to the boundary between calc-alkaline
and tholeiitic trends in an FeO∗/MgO–SiO2 diagram
(Fig. 2). Second, basalts and evolved andesites form major
and trace element trends distinct from those of HMAs
and mixed andesites (Fig. 3). It may be thus suggested,
based on such distinctive chemistry together with equilibrium mineralogy in those evolved andesites, that
evolved andesite magmas are derived originally from
basalt magmas not from HMA sanukitoid magmas.
To clarify the origin of evolved andesites and to examine the role of crystallization differentiation of basaltic
magmas in their formation, the fractionation trend of a
Mg-rich, primitive basalt magma was formulated by
MELTS calculations (Ghiorso & Sack, 1995). Figure 7
indicates the fractional crystallization trends obtained for
the basalt SDSYB (Table 1) at 0·3 GPa in the presence of
1·5 wt % H2O under the QFM buffer. The crystallization
sequence of silicate phases under those conditions is
olivine → clinopyroxene → plagioclase → orthopyroxene, which is consistent with the petrographic observations. Although the compositions of the evolved
andesites can be largely explained by fractional crystallization of a basalt magma, the significant difference
between inferred and observed andesites, especially in
CaO and Na2O, should be stressed (Fig. 7). One of the
possible processes responsible for such compositional
changes would be assimilation of granitic upper crust. If
we assume the basement granites as such an assimilant,
then evolved andesite magmas can be produced by a
process of assimilation–fractional crystallization (AFC) as
intuitively shown in Fig. 7. Further examination of the
role of AFC processes in evolved andesite genesis is not
easy, because of the presence of a variety of basement
rocks (e.g. gabbro, gneiss, granite, sedimentary rocks,
etc.). However, the isotopic compositions of basalts and
evolved andesites support such AFC processes; that is,
more radiogenic isotopic ratios are observed for those
rock series with increasing degrees of crystallization (i.e.
6
SD511
Sample:
1
7·42
0·13
5·23
7·34
3·15
1·75
0·24
99·22
Fe2O3∗
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
7
0·512637
2·2
1·0
clinopyroxene
hornblende
0·2
8·9
84·0
opaque
plagioclase
groundmass
biotite
2·6
1·1
orthopyroxene
—
38·644
15·612
olivine
vol. %
Pb/204Pb
208
Pb/204Pb
Pb/204Pb
207
206
18·380
0·70517
Nd/144Nd
143
117
Zr
( 87Sr/86Sr)ib
15
Y
0·705305
4
Th
Sr/86Sr
407
Sr
87
9
94
Rb
54
Ni
Pb
5
Nb
Ba
256
0·24
17·20
Al2O3
ppm
1·89
0·78
0·1
76·8
14·2
—
4·0
1·9
2·1
0·9
135
21
5
409
101
13
28
5
310
99·26
3·41
6·60
3·63
0·13
6·83
17·85
0·71
55·96
SiO2
57·98
SD946
PA
TiO2
wt %
1
PA
graphy:a
Rock type:
Strati-
0·2
78·9
14·7
—
2·0
1·5
2·1
0·6
38·591
15·589
18·383
0·70542
0·705606
121
21
6
387
128
14
61
5
312
98·79
0·20
2·24
3·34
5·59
4·27
0·11
6·11
17·04
0·61
59·29
SD941
PA
1
0·1
70·6
19·4
—
2·2
3·1
4·5
0·1
38·551
15·586
18·350
0·512651
0·70502
0·705234
132
22
6
364
133
13
15
6
298
99·26
0·21
2·29
3·34
6·18
3·20
0·12
6·36
17·49
0·66
59·41
SD512
PA
1
0·4
67·5
17·1
—
4·3
1·0
6·4
3·3
38·636
15·603
18·390
0·512614
0·70537
0·705537
122
13
7
444
128
17
81
5
305
99·24
0·21
2·39
3·54
5·60
5·08
0·10
5·75
16·26
0·63
59·68
SD516
PA
1
0·7
0·1
7·4
4·5
57·6
29·7
—
—
116
15
5
405
113
12
23
6
302
99·22
0·23
2·04
3·13
6·56
4·28
0·12
7·20
17·93
0·80
56·95
SD515
PA
2
0·1
8·2
4·2
59·2
28·3
—
—
—
38·608
15·599
18·375
0·512608
0·70549
0·705649
115
13
5
403
114
11
24
5
311
100·60
0·23
2·06
3·19
6·81
4·52
0·13
7·91
17·85
0·77
57·13
EHJ
PA
2
0·2
77·5
15·0
—
0·7
0·6
1·8
4·2
38·635
15·603
18·388
0·512567
0·70545
0·705612
123
43
5
375
108
12
54
4
347
98·67
0·20
2·04
3·27
5·98
4·23
0·12
6·42
16·99
0·65
58·77
SD513
PA
3
0·1
8·6
1·3
75·7
14·3
—
—
—
38·560
15·584
18·361
0·512609
0·70512
0·705342
134
31
7
384
149
15
11
6
307
98·72
0·25
2·77
3·08
6·01
2·99
0·12
5·99
18·15
0·61
58·74
SD503
PA
5
0·3
65·8
16·3
—
10·6
2·3
2·4
2·3
124
12
7
394
127
14
56
5
288
99·51
0·19
2·23
3·51
6·06
4·25
0·12
6·09
16·97
0·60
59·48
SD902
PA
5
0·2
2·4
2·2
3·4
66·5
25·3
—
—
135
14
6
421
120
20
15
5
303
99·81
0·22
2·19
3·90
6·00
3·07
0·11
5·83
18·11
0·65
59·73
SD901
PA
5
7·5
0·3
3·5
3·6
85·1
—
—
—
102
17
4
399
67
6
32
5
215
98·22
0·27
1·43
2·84
9·52
5·78
0·14
8·00
17·20
1·00
52·03
SD932
EA
6
8·3
0·1
4·3
5·3
82·0
—
—
—
101
18
3
397
66
6
33
5
213
98·50
0·27
1·44
2·91
9·60
5·82
0·15
7·98
17·21
1·00
52·13
SD933
EA
6
8·2
0·3
3·6
4·2
83·7
—
—
—
38·511
15·580
18·342
0·512695
0·70475
0·704844
100
16
3
394
68
7
33
5
204
98·70
0·26
1·43
2·91
9·68
5·88
0·15
7·90
17·12
0·99
52·37
SD931
EA
6
0·4
5·1
6·2
2·5
50·8
35·0
—
—
38·648
15·605
18·392
0·512576
0·70524
0·705383
128
18
4
371
93
11
20
7
293
99·15
0·22
1·93
3·33
6·85
4·22
0·12
6·98
17·61
0·82
57·08
SD929
PA
6
0·1
5·7
5·4
2·1
57·5
29·2
—
—
38·606
15·594
18·384
0·512598
0·70547
0·705643
112
12
5
379
111
15
28
5
282
98·62
0·22
1·96
3·17
6·71
4·55
0·13
6·79
17·40
0·68
57·00
KTV
PA
7
0·5
67·9
21·2
—
0·9
2·2
6·4
0·9
38·563
15·585
18·364
0·512643
0·70493
0·705140
103
14
5
340
123
11
18
5
269
98·70
0·24
2·14
3·10
6·75
3·67
0·12
6·61
18·16
0·65
57·24
SD502
PA
8
0·1
72·3
19·2
—
0·1
3·4
4·5
0·4
118
16
5
367
119
13
9
5
293
98·92
0·21
2·10
3·22
6·47
3·55
0·14
6·79
18·17
0·73
57·56
SD505
PA
8
Table 1: Major and trace element, Sr–Nd–Pb isotopic, and modal compositions of calc-alkaline and basement rocks on Shodo-Shima Island
9
0·1
7·8
2·9
59·5
29·7
—
—
—
132
12
4
424
105
14
49
8
320
97·36
0·25
1·96
3·52
6·61
3·32
0·07
6·54
18·07
0·85
56·17
SD913
PA
TATSUMI et al.
CALC-ALKALINE ANDESITES, SHODO-SHIMA, SW JAPAN
PA
SD908
Rock type:
Sample:
0·21
7·75
2·89
2·10
0·21
98·30
CaO
Na2O
K2O
P2O5
Total
8
0·512625
0·3
53·0
40·8
—
0·4
2·6
2·4
0·5
132
14
5
422
0·1
8·1
0·1
67·8
23·9
—
—
—
38·564
15·588
18·359
0·512646
0·70498
0·705145
127
13
6
445
128
15
6
5
269
98·88
0·22
2·17
3·43
6·10
2·60
0·3
5·4
0·5
75·6
18·2
—
—
—
118
12
5
450
119
16
6
5
281
99·14
0·23
2·14
3·48
6·21
2·39
0·12
5·96
18·75
0·55
59·31
SD935
EA
9
0·1
6·3
2·5
66·1
25·0
—
—
—
38·573
15·587
18·367
0·512651
0·70493
0·705101
136
12
6
427
126
16
28
6
309
98·48
0·21
2·32
3·49
6·00
3·59
0·11
5·42
17·42
0·58
59·34
SD907
PA
9
0·2
6·9
0·0
69·8
23·1
—
—
—
126
18
7
436
130
16
33
7
307
99·26
0·22
2·36
3·72
6·07
3·01
0·09
5·92
17·62
0·63
59·62
SD937
PA
9
0·3
71·1
15·5
—
4·9
3·9
2·1
2·2
139
11
7
402
132
18
45
5
301
97·86
0·20
2·28
3·54
5·73
3·55
0·11
5·35
16·90
0·55
59·66
SD912
PA
9
0·2
7·6
1·3
3·5
60·8
26·6
—
—
121
11
6
382
126
18
35
5
275
98·44
0·18
2·32
3·45
5·93
3·78
0·12
5·44
17·00
0·51
59·71
SD906
PA
9
0·2
1·9
7·9
0·5
69·8
21·6
—
—
38·583
15·585
18·373
0·51259
0·70539
0·705613
153
9
8
414
158
22
4
6
383
99·11
0·21
2·70
3·94
4·19
1·48
0·09
3·85
18·28
0·37
64·01
SD923
PA
9
0·5
87·3
—
—
—
—
—
12·2
38·469
15·566
18·315
0·51273
0·70442
0·704491
99
18
4
277
33
8
207
4
219
98·60
0·27
1·20
2·57
8·75
11·79
0·17
9·19
14·69
1·02
48·95
0·8
2·5
96·7
—
—
—
—
—
38·555
15·583
18·367
0·512684
0·70488
0·705128
80
15
5
267
114
17
148
5
195
97·16
0·17
2·25
2·84
7·04
6·89
0·17
6·23
15·55
0·65
55·37
HMA
SD261c
SDSYBc
11
basalt
11
b
Numbers correspond to those in Fig. 1.
Corrected for 13 Ma.
c
Date from Shimoda et al. (1998).
PA, porphyritic andesite; EA, evolved andesite; HMA, high-mg-number andesite; Fe2O3∗, total iron as Fe2O3.
a
32·5
60·4
16·8
69·0
plagioclase
groundmass
0·5
3·0
2·8
0·8
—
—
0·2
biotite
opaque
—
5·2
clinopyroxene
hornblende
7·1
1·7
olivine
orthopyroxene
—
38·560
15·584
116
14
5
410
122
14
15
5
297
98·13
0·24
2·23
3·48
6·37
3·69
0·13
5·93
18·62
0·54
59·15
SD936
EA
9
38·677
15·608
18·515
0·512358
0·70780
0·707874
140
17
3
385
51
11
2
7
212
99·58
0·28
1·05
3·39
8·50
3·95
0·18
9·47
17·89
1·29
53·59
SD942
gabbro
Basement
gneiss
38·804
15·618
18·565
0·512361
0·70810
0·708257
243
27
9
316
85
14
0
10
517
99·62
0·24
2·30
3·71
5·09
0·70
0·10
5·71
17·13
0·71
63·93
SD944
granite
38·829
15·610
18·536
0·512366
0·70792
0·708104
176
16
12
257
83
15
0
7
1052
99·18
0·07
2·95
3·37
3·14
0·39
0·04
2·44
15·12
0·23
71·41
SD927
granite
38·749
15·613
18·582
0·512366
0·71077
0·711425
118
22
12
133
151
27
1
8
607
99·99
0·04
4·20
3·88
1·34
0·07
0·04
1·37
14·00
0·08
74·97
SD903
NUMBER 1
vol. %
Pb/204Pb
Pb/204Pb
208
207
Pb/204Pb
206
18·360
143
Nd/144Nd
0·70498
115
Zr
( 87Sr/86Sr)ib
14
Y
0·705168
5
Th
Sr/86Sr
340
Sr
107
13
37
4
293
98·58
3·41
7·16
3·78
0·10
6·17
17·99
0·73
57·13
SD911
PA
9
VOLUME 43
87
11
113
55
Ni
Rb
5
Nb
Pb
262
Ba
ppm
1·93
4·75
MgO
7·20
0·14
7·11
17·32
0·15
16·39
Al2O3
0·77
56·65
Fe2O3∗
0·75
SD909
PA
9
MnO
56·22
SiO2
TiO2
wt %
9
graphy:a
Strati-
Table 1: continued
JOURNAL OF PETROLOGY
JANUARY 2002
TATSUMI et al.
CALC-ALKALINE ANDESITES, SHODO-SHIMA, SW JAPAN
than those in HMAs (Fig. 8). It is shown in Fig. 8 that
olivine and spinel in andesites SD512 and SD513 may
be derived from HMA sanukitoid, rather than from basalt
magmas. On the other hand, the compositions of these
minerals in the andesite SD516 require the contribution
of basalt, in addition to HMA sanukitoid magmas, in the
formation process. It is thus likely that both basalt and
HMA sanukitoid magmas are mafic endmember components for the mixed calc-alkaline andesites on ShodoShima Island. If so, then rather Mg-rich, normally zoned
clinopyroxene in porphyritic andesites may also be derived from HMA sanukitoid and/or basalt magmas. It
is hard to identify the original magma for such clinopyroxenes because of little compositional difference in
major element compositions of pyroxenes between HMA
sanukitoids and basalts (Tatsumi & Ishizaka, 1981; Nakashima et al., 1999). Further examination of the trace
element compositions of clinopyroxene phenocrysts (e.g.
Yogodzinski & Kelemen, 1998) is needed, to clarify the
origin of such pyroxenes.
It has been documented that two types of HMA
sanukitoids, cpx-HMA and opx-HMA sanukitoids, are
present in the Setouchi volcanic belt (Tatsumi, 1982).
Cpx-HMA and opx-HMA sanukitoids crystallize clinopyroxene and orthopyroxene after olivine, respectively.
The melting phase relations at high pressures for these
two types of HMA sanukitoids suggest that opx-HMA
magmas are produced at higher degrees of partial melting
than cpx-HMA, because they are multiply saturated
with harzburgitic phases (olivine + orthopyroxene), and
lherzolitic phases (olivine + orthopyroxene + clinopyroxene), respectively, at upper-mantle pressures and
temperatures (Tatsumi, 1981, 1982). Differences in the
degree of partial melting may be supported by the
compositions of the constituent minerals in these HMA
sanukitoids; olivine is more enriched in Mg and Ni, and
spinel is more chromian in opx-HMA than in cpx-HMA
sanukitoids (Fig. 8).
The andesite SD516 contains olivine phenocrysts and
spinel inclusions with compositional characteristics identical to those in opx-HMA sanukitoids (Fig. 8). Furthermore, the presence of very Mg-rich orthopyroxenes in
SD516 (mg-number >90; Fig. 6) may also imply that
the opx-HMA sanukitoid magma is one of the mafic
endmembers. Although opx-HMA sanukitoids are not
found on Shodo-Shima Island, the generation and involvement of such HMAs in the production of calcalkaline andesites may reasonably explain the petrographic characteristics of porphyritic andesites. An opxHMA sanukitoid is, on the other hand, distributed in the
Takamatsu region, NE Shikoku (Fig. 1), >30 km to the
SW of Shodo-Shima Island (Kushiro & Sato, 1978).
One of the distinctive petrographic features of calcalkaline porphyritic andesites on Shodo-Shima Island is
Fig. 2. FeO∗/MgO vs SiO2 diagram for Shodo-Shima volcanic rocks.
Porphyritic andesites on this island exhibit a broad calc-alkaline trend
based on Miyashiro’s (1974) definition. Data are from this work and
Shimoda et al. (1998). Porphyritic andesites that show little evidence
for magma mixing are called ‘evolved andesites’ (see text).
from basalt, via basaltic andesite, to andesite), as shown
in Fig. 5.
Mafic to intermediate components
Porphyritic calc-alkaline andesites on Shodo-Shima
Island, except for evolved andesites, may be produced
by mixing of magmas with different compositions, as
suggested by petrographical observations. We will further
examine the endmember components for such mixed
magmas based mainly on mineral compositions.
Possible mafic endmember components for ShodoShima porphyritic andesites are basalts and HMA sanukitoids, which were formed in close spatial and temporal
relation to the andesites on this island (Fig. 1). Thus, we
first examine the contribution of such mafic magmas to
the production of calc-alkaline andesite magmas. Olivine
phenocrysts in Shodo-Shima andesites have mg-numbers
greater than those obtained by assuming equilibrium
Fe–Mg partitioning between olivine and the bulk magma
(Fig. 6). One possible explanation for this observation
may be the derivation of disequilibrium olivine from a
mafic magma. Tatsumi & Ishizaka (1981) and Nakashima
et al. (2000) demonstrated the compositional difference
of olivine phenocrysts and spinel inclusions in olivine
between basalts and HMA sanukitoids on Shodo-Shima
Island. Olivines and spinels in basalts are more enriched
in NiO at a given mg-number and Fe3+, respectively,
9
JOURNAL OF PETROLOGY
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NUMBER 1
JANUARY 2002
Fig. 3. SiO2 variation diagrams for selected major and trace element concentrations in Setouchi volcanic rocks on Shodo-Shima Island. Data
are from this study, Shimoda et al. (1998) and Shimoda & Tatsumi (1999). The bulk continental crust compositions (Christensen & Mooney,
1995; Rudnick & Fountain, 1995) plot largely within the geochemical trend formed by Shodo-Shima andesites, except for the rather lower
concentration of Al2O3 in the bulk continental crust. Compositions of porphyritic andesites can be explained reasonably by mixing of at least
five endmember magmas: basalt, HMA sanukitoids (cpx-HMA and opx-HMA), evolved andesites, and rhyolites.
the presence of reversely zoned pyroxenes with mg-number >70 (Fig. 6). Such Mg-poor pyroxenes are observed
neither in HMA sanukitoids nor in basalts. Furthermore,
HMA sanukitoids and basalts do not contain plagioclase
phenocrysts, whereas plagioclase is the major phenocryst
phase in porphyritic andesites. These observations may
10
TATSUMI et al.
CALC-ALKALINE ANDESITES, SHODO-SHIMA, SW JAPAN
Fig. 4. N-MORB normalized (Sun & McDonough, 1989) incompatible
element compositions of Shodo-Shima porphyritic andesites and the
bulk continental crust (Rudnick & Fountain, 1995). Both andesites
show characteristics that typify subduction zone magmas.
suggest the involvement of differentiated magmas.
Evolved andesites with little evidence of magma mixing
may be produced by fractional crystallization of basalt
magmas with a certain amount of crustal assimilation
as shown in the previous section. Although phenocryst
compositions, especially those of plagioclase, in a particular evolved andesite SD936 do not completely match
compositions of inferred phenocrysts in differentiated
magmas, the presence of evolved andesites on ShodoShima Island reinforces the involvement of variably
differentiated and assimilated, basalt-derived magmas in
the magma mixing processes.
Felsic component
The porphyritic andesite SD513 contains reversely zoned
orthopyroxenes with Fe-rich compositions (mg-number
>50; Fig. 6). This andesite does not contain clinopyroxene, which should be in equilibrium with such Ferich orthopyroxene (Fig. 6). This may suggest that a
magma containing only orthopyroxene as a mafic phase
contributed to the formation of this particular andesite.
In the Setouchi volcanic belt including Shodo-Shima
Island, dacites or rhyolites containing such Fe-rich orthopyroxene phenocrysts are present (e.g. Shimoda & Tatsumi, 1999). Compositions of this type of rhyolites are
shown in Fig. 3, suggesting that such rhyolite magmas
may be a possible felsic endmember component. Also,
Sr–Nd–Pb isotope compositions of rhyolites in the Setouchi volcanic belt can reasonably explain the isotopic
trends of calc-alkaline andesites (Fig. 5).
Fig. 5. Isotopic compositions of Setouchi volcanic rocks and basement
on Shodo-Shima Island. It is suggested that mixing of basalt, HMA
and enriched rhyolite magmas can produce porphyritic andesite
magmas.
Origin of endmember magmas
Major and trace element, and isotopic compositions of
five endmember magmas and porphyritic calc-alkaline
andesites (Figs 3 and 5) support the present discussion
that mixing of five such magmas can reasonably account
11
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
JANUARY 2002
Fig. 6. Frequency distribution diagrams for mg-number or An mole percent of phenocryst phases in calc-alkaline andesites on Shodo-Shima
Island. Compositions of the phenocryst rims are shown with arrows. Filled and open boxes indicate compositions of normally and reversely
zoned phenocrysts, respectively. Groundmass pyroxene compositions are shown by arrows. Olivine compositions calculated by assuming Fe–Mg
exchange partitioning between olivine and the bulk rock [KD = 0·3, Fe2+/(Fe2+ + Fe3+) = 0·9] are shown by open arrows.
et al., 1989). High-pressure experiments on hydrous
peridotites confirmed that such HMA melts can be generated by partial melting of peridotites at mantle pressures
under hydrous conditions (e.g. Kushiro, 1969; Hirose,
1997). On the other hand, most researchers have favored
a mechanism including partial melting of the subducting
lithosphere and interaction of these slab-derived, hydrous
silicic melts with overlying mantle peridotites (Kay, 1978;
Pearce et al., 1992; Yogodzinski et al., 1994; Kelemen,
1995). Shimoda et al. (1998) demonstrated that Pb, Sr,
and Nd isotope compositions of HMAs in the Setouchi
volcanic belt are explained by a contribution of partial
melts derived from subducting sediments, rather than
aqueous fluids from the dehydrating slab. This mechanism was further supported by geochemical formulation
for the production of calc-alkaline andesites. The petrographical characteristics of the five potential endmember
magmas, which are required for the formation of calcalkaline andesites on Shodo-Shima Island, are summarized in Table 2. Although quantitative estimation of
the contributions of five endmember components to
the formation of a particular calc-alkaline andesite is
interesting, it is difficult because of the variable compositions of basalt-derived, differentiated magmas and
rhyolite magmas.
One of the characteristic endmember magmas for the
Shodo-Shima calc-alkaline andesites is an HMA magma,
which possesses high mg-number and hence can be in
equilibrium with upper-mantle peridotite (e.g. Sato, 1977;
Kuroda et al., 1978; Tatsumi & Ishizaka, 1981; Crawford
12
TATSUMI et al.
CALC-ALKALINE ANDESITES, SHODO-SHIMA, SW JAPAN
Fig. 7. Compositional trends formed by fractional crystallization of a primitive basalt SDSYB (Ο) with 1·5 wt % H2O at 0·3 GPa under the
QFM buffer. This process can largely explain the compositions of evolved andesites (Η). To fully explain the evolved andesite compositions,
assimilation of granite basements (oval fields) by crystallizing magmas may be needed.
Table 2: Petrographic characteristics of endmember magmas
Basalt
Opx-HMA
Cpx-HMA
Evolved
Rhyolite
andesite
olivine (mg-no.)
90–80
>90
90–85
none
none
orthopyroxene (mg-no.)
none
90
none
80–60
50–40
clinopyroxene (mg-no.)
90–80
none
90–80
80–70
none
chromite
Fe3+-rich
Cr-rich
Fe3+-poor
none
none
plagioclase
none
none
none
phyric
phyric
of partial melting of subducting sediments and subsequent
melt–mantle interaction (Tatsumi, 2001).
It has been widely accepted that generation of basalt
magmas in subduction zones is triggered by addition
of slab-derived fluids (e.g. Gill, 1981; Tatsumi & Eggins,
1995). A possible mechanism for production of basalt
magmas in the Setouchi volcanic belt is, on the other
hand, interaction of slab-derived sediment-melts with
overlying mantle peridotites (Shimoda & Tatsumi,
1999). Although the origin of basalt magmas is not
clearly understood, fractional crystallization of such
basalt magmas together with assimilation of basement
granitic rocks contributes to the formation of evolved
and least-mixed andesite magmas on Shodo-Shima
Island.
Setouchi rhyolites are characterized by their highly
radiogenic isotope signatures identical to those of trenchfill, terrigenous sediments, i.e. higher Pb and Sr, and
lower Nd isotope ratios relative to basalts and HMAs
(Fig. 5), which led Shimoda & Tatsumi (1999) to speculate
that such rhyolite magmas are produced by partial melting of subducting sediments and reach the surface with
little interaction with mantle peridotites. However, the
process that formed such an extremely enriched rhyolite
magma should be further examined.
CONCLUSION
Differentiation of mantle-derived HMA magmas has
been considered as a possible mechanism for continental
13
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
JANUARY 2002
andesites as one of the endmember components, are
produced by an AFC process from basalt magmas. In
other words, no andesite shows evidence for its derivation
from HMA magmas, suggesting that HMA magmas could
not differentiate to produce evolved magmas. Melting
experiments of HMA sanukitoids in the Setouchi volcanic
belt indicated that at least >7 wt % H2O is required for
generation of those magmas in the upper mantle (Tatsumi, 1981, 1982). Such H2O-rich magmas have experienced extensive crystallization with falling temperature and may solidify more rapidly within the crust
than H2O-poor basalt magmas.
ACKNOWLEDGEMENTS
We thank Takashi Sano and Hiroshi Shukuno for their
help in electron probe microanalyses and MELTS calculation, respectively. Positive reviews by Peter Kelemen,
Mark Defant, and Richard Arculus are greatly appreciated. This work was partially supported by Grantsin-Aid from the Ministry of Education, Science, and
Culture of Japan (08404029 and 09440177) and a Special
Coordination Fund for Promoting Science and Technology (Superplume) from the Science and Technology
Agency of Japan.
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