JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 6
PAGES 1029–1047
2002
Remelting of an Andesitic Crust as a Possible
Origin for Rhyolitic Magma in Oceanic
Arcs: an Example from the Izu–Bonin Arc
YOSHIHIKO TAMURA1∗ AND YOSHIYUKI TATSUMI
INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN MARINE SCIENCE AND TECHNOLOGY
CENTRE ( JAMSTEC), YOKOSUKA 237-0061, JAPAN
RECEIVED FEBRUARY 20, 2001; REVISED TYPESCRIPT ACCEPTED JANUARY 8, 2002
The Izu–Bonin volcanic arc is an excellent example of an intraoceanic convergent margin. A total of 1011 chemical analyses of
17 Quaternary volcanoes of the arc are reviewed to estimate relative
proportions of magmas erupted. Basalt and basic andesite (SiO2
<57 wt %) are the predominant eruptive products of the Izu–Bonin
arc, and rhyolite (SiO2 >70 wt %) forms another peak in volume.
Such rhyolites possess compositions identical to those of partial
melts produced by dehydration-melting of calc-alkaline andesites at
low pressure (<7 kbar). Meanwhile, the major element variation
of the Shirahama Group Mio-Pliocene volcanic arc suite, Izu
Peninsula, completely overlaps that of the Quaternary Izu–Bonin
arc volcanoes, and groundmasses of Shirahama Group calc-alkaline
andesites have compositions similar to those of Izu–Bonin rhyolites.
Moreover, phenocryst assemblages of calc-alkaline andesites of the
Shirahama Group resemble restite phase assemblages of dehydrationmelting of calc-alkaline andesite. These lines of evidence suggest
that the rhyolite magmas may have been produced by dehydrationmelting of calc-alkaline andesite in the upper to middle crust. If
so, then the presence of large amounts of calc-alkaline andesite (3–5
times more abundant than the rhyolites) within the oceanic arc crust
would be expected, which is consistent with a recently proposed
structural model across the Izu–Bonin arc. The calc-alkaline
andesite magmas may be water saturated, and would crystallize
extensively and solidify within the crust. The model proposed here
suggests that rhyolite eruptions could be triggered by an influx of
hot basalt magma from depth, reheating and partially melting the
calc-alkaline andesite component of the crust.
INTRODUCTION
The northern Izu–Bonin volcanic arc extends for 550 km
from Izu Peninsula, Japan, to near the Nishinoshima
Trough or Sofugan Tectonic line (Yuasa, 1985) (Fig. 1).
This is the northernmost segment of the Izu–
Bonin–Mariana arc system, which extends 2500 km from
the Izu Peninsula to beyond Guam (e.g. Taylor, 1992).
The Izu–Bonin–Mariana volcanic arc formed by subduction of the Pacific plate beneath the Philippine Sea
plate, and is an excellent example of an intra-oceanic
convergent margin. Here, we show that modern magmatism at the northern Izu–Bonin arc is bimodal with
basalt and rhyolite predominating. The origin of rhyolite
in oceanic arcs is a matter of considerable interest. We
suggest that this rhyolite is a partial melt of calc-alkaline
andesite occurring at depth within the oceanic islandarc crust. Our study thus casts doubt on the importance
of the partial melting or ultimate fractionation of basalt
in producing such rhyolites and suggests the fundamental
role of calc-alkaline andesite in the formation of islandarc crust. An original calc-alkaline andesite magma is
likely to be water saturated and will therefore solidify in
the crust, forming an andesite source region at depth,
which could be reheated and remobilized by influxes of
basalt.
THE IZU–BONIN ARC
Geological setting and submarine eruptions
rhyolite
The Izu–Bonin arc forms more than half of the Izu–
Bonin–Mariana arc system, and is divided into north
∗Corresponding author. Telephone: +81-468-67-9761. Fax: +81468-67-9625. E-mail: [email protected]
 Oxford University Press 2002
KEY WORDS:
bimodal magmatism; calc-alkaline andesite; oceanic arcs;
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Fig. 1. Map showing the 11 Quaternary volcanoes and eight Quaternary submarine caldera volcanoes with which this study is concerned and
the location of the Mio-Pliocene Shirahama Group and the Miocene Tanzawa plutonic complex. Numbered dots indicate sites drilled on the
Philippine Sea plate in the Izu–Bonin region during ODP Legs 125 and 126. The location map (lower left) shows the structure of the
Izu–Bonin–Mariana arc system (Taylor, 1992). Double lines indicate spreading centres, active in the Mariana Trough and relic in the Shikoku
and Parece Vela Basins. The Izu–Bonin, West Mariana and Mariana arcs are outlined by the 3 km bathymetric contour, and other basins and
ridges are outlined by the 4 km contour.
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TAMURA AND TATSUMI
ANDESITE REMELTING IN ISLAND-ARC CRUST
and south segments, based on submarine topography,
chemical composition of volcanic rocks, and distributions
of hypocentres and back-arc rifts (Yuasa, 1985). It appears
that subduction along the northern Izu–Bonin segment
began several million years earlier than along the southern
segment, and the difference in alkalinity of modern
volcanic rocks between the two segments reflects differences in crustal thickness and thermal structure of the
mantle wedge below the crust (Yuasa, 1992). We will here
focus on the northern Izu–Bonin volcanic arc (>550 km)
(Fig. 1) to avoid complexities related to the along-arc
geological variations of the overall Izu–Bonin–Mariana
arc system.
Six Quaternary island volcanoes and nine Quaternary
submarine silicic calderas lie along the volcanic front of
the Izu–Bonin arc between latitudes 35°N and 30°N
(Nagaoka et al., 1991; Yuasa et al., 1991; Iizasa et al.,
1999; Fig. 1). Before these calderas were recognized,
basalts were believed to be the predominant eruptive
products in the Izu–Mariana arc (Aramaki & Ui, 1978).
However, samples recently dredged from two large submarine caldera volcanoes (Omurodashi and Kurose Hole)
are mostly rhyolite (Uto, 1983) as is the thick syn-caldera
pumice deposit at Myojin Knoll Caldera (Fiske et al.,
2001). Modern volcanism at the Izu–Bonin arc thus
contrasts sharply with that of the NE Japan arc, which
is dominated by andesites (Aramaki & Ui, 1978).
Suga & Fujioka (1990) used topography to determine
the distribution and volumes of volcanoes along the
northern Izu–Bonin arc. Back-arc seamount chains, made
up of individual Mio-Pliocene volcanoes having volumes
>100 km3, extend obliquely to the volcanic front (Ishizuka
et al., 1998). However, a greater volume of modern
volcanic material lies along the volcanic front, and this
is contained in basalt-dominant island volcanoes and
rhyolite-dominant submarine calderas. Myojin Knoll
Caldera, which was the first of these silicic structures to
receive detailed submersible-based study, was a site of a
submarine eruption that produced 35–40 km3 of rhyolite
tephra (Naka et al., 1995; Yuasa, 1995; Fiske et al., 2001).
A post-caldera hydrothermal system is now producing a
modern Kuroko-type polymetallic sulphide deposit (Iizasa
et al., 1999).
Fiske et al. (1998) documented the unique characteristics
of the shallow submarine 1952–1953 eruption of Myojinsho. The flushing action of water convecting through the
hot rubble at the volcano’s summit removes fine-grained
matrix vigorously and persistently, resulting in the finesdepleted characteristics of proximal deposits and the
dispersal of fine ash over wide areas by ocean currents
(Fiske et al., 1998). Tephras at Sites 787, 792, and 793
of Ocean Drilling Program (ODP) Leg 126, which might
actually result from such tephra dispersal from arc submarine volcanoes, clearly show the bimodal nature of
magmatism in the Izu–Bonin arc (Fujioka et al., 1992).
Turbidites, which have been delivered to the forearc
largely through submarine canyons, provide a more
complete record of arc volcanism than arc lavas, and are
more voluminous and proximal than ashes (Gill et al.,
1994). Hiscott & Gill (1992) and Gill et al. (1994) characterized the turbidite geochemistry of the Izu–Bonin
arc by using 271 samples of volcaniclastic sand and
sandstone collected from cores at the six ODP Leg 126
sites (787, 788, 790, 791, 792 and 793). These turbidites
are andesitic on average (>60 wt % SiO2) but are
bimodal in detail (Gill et al., 1994).
Bimodal volcanism
Relative proportions of erupted magmas
A total of 1011 chemical analyses of samples from 17
Quaternary volcanoes of the Izu–Bonin arc (30°N–35°N)
were reviewed to estimate the relative proportions of
erupted magmas (Table 1). All discussions in this paper
refer to analyses that have been normalized to 100% on
a volatile-free basis with total iron calculated as FeO.
Figure 2a shows the frequency distribution of the SiO2
content of samples. The histogram showing the number
of analyses is converted into volume-weighted histograms
(Fig. 2b and c) by the method of Aramaki & Ui (1978).
If, for example, there are 10 analyses available for a
volcano with a volume of 20 km3, a volume of 2 km3 is
allotted to each analysis. Volumes of volcanoes are from
data provided by Suga & Fujioka (1990) and the Committee for the Catalog of Quaternary Volcanoes in Japan
(1999). Myojinsho caldera (326 km3) was not included in
this review, because safety restrictions have prevented
dredging and submersible study of this area after the
tragedy of the 1952 Myojinsho eruption (Fiske et al.,
1998). Erupted materials from this eruption, however,
were mostly dacitic in composition.
The silica frequency relationship based on the number
of analyses (Fig. 2a) is similar to that of the volumeweighted histogram (Fig. 2b). Basalt and basic andesite
(<57 wt % SiO2) are clearly the predominant eruptive
products of the Izu–Bonin arc, but rhyolite (>70 wt %
SiO2) also forms a major mode. The shift of the rhyolite
peak from Fig. 2a relative to Fig. 2b reflects the relatively
low silica contents of rhyolites forming the large submarine calderas. Volumetrically, the Izu–Bonin magmas
do not simply decrease linearly from basalt to rhyolite,
but pass through the minimum in acid andesite and
increase again in dacite and rhyolite (Fig. 2c). The latter
peak would be emphasized even more strongly if (1) the
Myojinsho caldera (326 km3) actually consists of dacite
and rhyolite and (2) pumices from submarine calderaforming eruptions, dispersed far from their source volcanoes, were taken into consideration. For example, more
than half of the sediment layers drilled in the Sumisu
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Table 1: Sources of analytical data of the Quaternary volcanoes of the Izu–Bonin arc
Volcano (number of analyses)
References
Hakone (101)
Kato (1985), Arculus et al. (1991)
Higashi–Izu (85)
Hamuro (1985), Miyajima et al. (1985)
Higashi–Izu Submarine (56)
Hamuro et al. (1980, 1983), Nakahara et al. (1992)
Izu–Oshima (206)
Soya (1976), Isshiki (1984), Nakano & Yamamoto (1987, 1991), Fujii et al. (1988), Nakano et al. (1988b),
Kawanabe (1991)
Omurodashi Caldera (2)
Hamuro et al. (1983)
Toshima (16)
Isshiki (1978), Iwabuchi et al. (1989), Tokyo-To (1992)
Niijima (84)
Koyaguchi (1986), Isshiki (1987), Tokyo-To (1992)
Kozushima (27)
Isshiki (1982), Taniguchi et al. (1990), Tokyo-To (1992)
Miyakejima (84)
Aramaki & Hayakawa (1984), Fujii et al. (1984), Soya et al. (1984), Aramaki et al. (1986), Sato et al.
(1996), Amma-Miyasaka & Nakagawa (1998)
Mikurajima (18)
Tokyo-To (1992)
Kurose Hole (Caldera) (3)
Yuasa & Nohara (1992), M. Yuasa (unpublished data, 2000)
Hachijojima (134)
Tsukui et al. (1993), Nakano et al. (1997), Hirata et al. (1999)
Aogashima (106)
Takada et al. (1992), Tokyo-To (1992)
Higashi–Aogashima Caldera (9)
Yuasa & Nohara (1992), Takada et al. (1994), Morita (1998), Hochstaedter et al. (2000)
Myojin Knoll Caldera (11)
Yuasa & Nohara (1992), Yuasa (1995), Morita (1998)
Sumisu Caldera (16)
Yuasa & Nohara (1992), Morita (1998), Hochstaedter et al. (2000)
South Sumisu Caldera (5)
Yuasa & Nohara (1992), Morita (1998)
Torishima (48)
Tokyo-To (1992)
Rift during ODP Leg 126, at Sites 790 and 791 (Fig. 1),
were made up of thick layers of two-pyroxene rhyolite
pumice derived from nearby arc volcanoes (Nishimura
et al., 1992).
Average magma composition
Plots of major element abundances vs SiO2 content for
the Izu–Bonin arc Quaternary volcanoes are shown in
Fig. 3. Although the eruptive products are volumetrically
bimodal, magmas range from basalt, through andesite
and dacite, to rhyolite. It should be stressed that the
average turbidite of Gill et al. (1994) plots in the centre
of the eruptive products of the Quaternary Izu–Bonin arc.
A large amount of silicic magma production, comparable
with the volumes of basalt and basic andesite produced
in the arc, is required to yield an average SiO2 content
of 60 wt %.
Relatively dry basalt
Basalt magmas on the volcanic front of the Izu–Bonin
arc are either anhydrous or contain very little water
(Aramaki & Fujii, 1988; Fujii et al., 1988; Nakano et al.,
1988a, 1991; Nakano & Yamamoto, 1991; Takada et al.,
1992). Fujii et al. (1988) showed that aphyric basalt of
the 1986 Izu–Oshima eruption has a composition close
to an anhydrous 1 atm cotectic line projected on the
pseudoternary normative diagram Pl–Opx–(Qz + Or).
Lower H2O contents in basaltic melts will result in higher
melt density so that plagioclase crystals will be likely to
float rather than sink (Aramaki & Fujii, 1988; Nakano et
al., 1988a, 1991; Nakano & Yamamoto, 1991). There
is, for example, petrological evidence that plagioclase
phenocrysts have accumulated in the upper parts of
magma chambers beneath Izu–Oshima volcano where
plagioclase phenocryst contents in lavas vary between 0
and 20 vol. % and mafic phenocrysts are rare, yet all
the lavas show similar groundmass (melt) compositions.
For plagioclase to float in a basaltic liquid, the H2O
content must be <0·7% (Aramaki & Fujii, 1988; Fujii et
al., 1988; Nakano et al., 1988a; Nakano & Yamamoto,
1991). Hachijyojima and Aogashima volcanoes are also
characterized by magmas in which the composition is
controlled by the abundance of plagioclase phenocrysts,
which ranges from 1 to 43 vol. %, with groundmass
(liquid) compositions being constant across the range of
rock compositions (Nakano et al., 1991; Takada et al.,
1992). Aramaki & Fujii (1988) demonstrated that an
average Izu–Oshima basalt can be generated by fractionation of a primary olivine basalt by removing
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TAMURA AND TATSUMI
ANDESITE REMELTING IN ISLAND-ARC CRUST
Izu–Bonin arc. These ashes are made up of volcanic glass,
strictly representing liquid compositions. Groundmass
compositions of volcanic rocks can also be used to suggest
liquid lines of descent for arc magmas (Tamura, 1995).
Figure 4 shows Miyashiro diagrams (FeO∗/MgO vs
SiO2) of the Izu–Bonin arc ashes (Arculus & Bloomfield,
1992) and groundmasses of the Shirahama Group (Tamura, 1995). Interestingly, there is a striking similarity
between these two liquid trends. First, in both cases,
liquid compositions range from basalt to rhyolite. Second,
tholeiitic andesites (FeO∗/MgO >0·156 × SiO2 – 6·68)
exist in a liquid state, but there are no analyses to indicate
that the calc-alkaline andesites (FeO∗/MgO <0·156 ×
SiO2 – 6·68) were erupted in a liquid state. Similarly,
glasses from the Mariana Trough fallout tephra contrast
with the contemporaneous basaltic to dacitic lavas of the
Mariana arc volcanoes (Straub, 1995); all compositions
of glasses with SiO2 <57% plot in the tholeiitic field,
whereas FeO∗/MgO vs SiO2 variation in the Mariana
arc volcanoes shows a complete chemical gradation from
the tholeiitic to calc-alkaline fields (Straub, 1995).
One possible explanation for the curious lack of calcalkaline andesitic melts in the Izu–Bonin and Mariana
arcs is that such liquids were originally water saturated.
Water-saturated liquidi have negative slopes in P–T space
and this results in the crystallization of these magmas
before they can be erupted at the surface. For example,
a water-saturated andesitic composition has a liquidus
temperature of 970°C at 5 kbar, but the liquidus temperature rises to 1200°C at 1 atm (Green, 1982).
Rhyolites in the Izu–Bonin arc
Crustal partial melts
Fig. 2. Histograms of SiO2 content from 17 Quaternary volcanoes in
the Izu–Bonin arc based on 1011 chemical analyses. (a) Number-ofanalyses histogram. (b) Volume-weighted histogram converted from
the number-of-analyses histogram for the Quaternary Izu–Bonin arc
volcanics. (c) Volume-weighted histograms of rock type for the Quaternary Izu–Bonin arc, showing a bimodal basalt–rhyolite profile.
>50 wt % crystals. Consequently, <0·4% H2O would
have been present in such primary basalts.
Wet calc-alkaline andesite
Arculus & Bloomfield (1992) studied ashes recovered
during ODP Leg 125 (Sites 782, 784 and 786) from the
Beard & Lofgren (1991) showed that dehydration-melting
of basaltic and andesitic rocks at 1, 3 and 6·9 kbar
yields partial melts with compositions similar to islandarc tonalites and dacites. Their starting materials had
major element compositions close to Izu–Bonin arc basalts and andesites, and it is therefore informative to
compare the compositions of these dehydration melts
with silicic rocks (60–79 wt % SiO2) from the Izu–Bonin
arc Quaternary volcanoes (Fig. 5). Comparison can also
be made with melts from experiments at higher pressure
(10 kbar) carried out by Wolf & Wyllie (1994) and Nakajima & Arima (1998). The starting materials in these
experiments were similar Izu–Bonin basalts and the experiments were carried out under water-deficient conditions.
The melts produced by dehydration-melting at lower
pressures (<7 kbar) by Beard & Lofgren (1991) have major
element compositions identical to those of Izu–Bonin arc
dacites and rhyolites except that Na2O contents are lower.
Sodium is thought to have been lost during microprobe
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Fig. 3. Major element Harker diagrams for rocks of the Quaternary Izu–Bonin arc volcanoes ( Β ) and average Izu–Bonin arc turbidite (large
Β, Gill et al., 1994). These turbidites are andesitic on average but bimodal in detail (Gill et al., 1994), providing another indication of bimodal
volcanism in the Izu–Bonin arc. All samples plotted have been normalized to 100% volatile free with total Fe calculated as FeO.
analysis. Interestingly, the melts from the higher-pressure
(>10 kbar) experiments of Wolf & Wyllie (1994) and
Nakajima & Arima (1998) have calc-alkaline dacite or
rhyolite compositions, but these differ from those of most
Izu–Bonin arc dacites and rhyolites in having lower
TiO2 and FeO and higher Al2O3. Moreover, the phase
assemblages produced during the 10 kbar experiments
contain hornblende and garnet (Wolf & Wyllie, 1994),
but these differ from the phenocryst assemblages of
Izu–Bonin rhyolite pumices (Nishimura et al., 1992).
Partial melting of calc-alkaline andesite
Figure 6 and Table 2 show modal variations and melt
composition as a function of temperature for the dehydration-melting experiments carried out by Beard &
Lofgren (1991) at 3 kbar for their composition 557 (lowK calc-alkaline andesite). The data show that rhyolitic
melts can be produced by <35% partial melting of calcalkaline andesite at depths equivalent to the middle crust.
The dehydration-melting experiments yielded rhyolitic
melts coexisting with the anhydrous restite assemblage
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ANDESITE REMELTING IN ISLAND-ARC CRUST
Fig. 4. Liquids comparable with calc-alkaline andesites (low FeO∗/MgO andesites) are missing in the Izu–Bonin arc. Tholeiitic and calc-alkaline
boundary (FeO∗/MgO = 0·156 × SiO2 – 6·68) after Miyashiro (1974). (a) Ashes from Sites 782, 784 and 786 of ODP Leg 125 (Χ) plotted
on FeO∗/MgO vs SiO2 (Arculus & Bloomfield, 1992). Β, Quaternary Izu–Bonin volcanic rocks. (b) Tholeiitic groundmasses (Χ) and calcalkaline groundmasses (Ε) of the Shirahama Group plotted on FeO∗/MgO vs SiO2 (Tamura, 1995).
plagioclase + orthopyroxene + clinopyroxene + magnetite (Fig. 6, Table 2). This assemblage, without amphibole, is consistent with the phenocryst assemblage of
rhyolite pumices cored from the Sumisu Rift (Nishimura
et al., 1992). Rhyolite pumices from Myojin Knoll, Sumisu
and South Sumisu Calderas also have a two-pyroxene
assemblage (± hornblende ± quartz), but quartz phenocrysts are commonly resorbed (Yuasa & Nohara, 1992;
Yuasa, 1995), probably caused by reheating of the magma.
TERTIARY SHIRAHAMA GROUP, IZU
PENINSULA
The Mio-Pliocene Shirahama Group medium-K volcanic
arc suite of the Izu Peninsula is characterized by the
occurrence of both a tholeiitic series (basalt–dacite) and
a calc-alkaline series (andesite–dacite) (Tamura, 1994,
1995; Tamura & Nakamura, 1996). Magmatic temperatures inferred by the two-pyroxene thermometer
show unambiguous differences between the two. Generally, temperatures of 950–1100°C are obtained from
tholeiitic samples, whereas 800–900°C is indicated for
calc-alkaline samples (Tamura, 1994). Chemical variations in the tholeiitic series and the calc-alkaline series
are consistent with crystal fractionation from basalt and
assumed magnesian andesite, respectively (Tamura,
1994). Both the tholeiitic series and the calc-alkaline
series are isotopically identical (Tamura & Nakamura,
1996). Although rocks of the Shirahama Group were
classified into the tholeiitic series and the calc-alkaline
series on the basis of the fractionation models (Tamura,
1994), most of the tholeiitic series rocks and all of the
calc-alkaline series rocks satisfy Miyashiro’s (1974) requirements of FeO∗/MgO >0·156 × SiO2 – 6·68 and
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Fig. 5. Quaternary Izu–Bonin arc volcanics (60–80 wt % SiO2) compared with dehydration experimental data for basaltic and andesitic
compositions. Data sources are Beard & Lofgren (1991) at 1, 3 and 6·9 kbar; Wolf & Wyllie (1994) at 10 kbar; Nakajima & Arima (1998) at
10 kbar. Rhyolites in the Izu–Bonin arc have major element compositions similar to melts produced at 1, 3 and 6·9 kbar by Beard & Lofgren
(1991). Melts produced by water-deficient partial melting experiments for typical island-arc low-K tholeiite at 0·7–1·5 GPa (Nakajima & Arima,
1998) as well as melts produced by dehydration-melting of low-K basaltic amphibolite at 10 kbar (Wolf and Wyllie, 1994) have compositions
different from those of Izu–Bonin rhyolites.
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Table 2: Starting composition, glass analyses,
and calculated modes for dehydration-melting
runs of calc-alkaline andesite (557) at 3 kbar
by Beard & Lofgren (1991)
Temperature (°C)
850
900
950
1000
wt %
SiO2
57·56
76·18
72·55
TiO2
0·61
0·40
0·74
69·08
0·90
Al2O3
15·53
13·03
14·54
15·95
FeO∗
8·09
2·29
3·44
4·55
MnO
0·17
0·08
0·05
0·08
MgO
5·57
0·60
0·69
0·51
CaO
9·29
2·67
3·13
3·71
Na2O
2·56
3·01
3·13
3·46
K2O
0·44
1·66
1·48
1·52
P2O5
0·18
0·08
0·25
0·25
Total
100·00
100·00
100·00
100·00
5·20
3·41
4·31
H2O∗
LOI
1·56
Fig. 6. Calculated modes vs temperature for composition 557 (low-K
calc-alkaline andesite) at 3 kbar for the dehydration-melting experiments
after Beard & Lofgren (1991).
Calculated mode (wt %)
Plagioclase
47·5
38·9
38·0
30·3
1·0
25·4
21·5
21·1
Orthopyroxene 6·8
7·6
7·4
9·7
Amphibole
29·3
0
0
0
Quartz
12·3
Clinopyroxene
0
0
0
Magnetite
2·4
4·6
5·5
3·8
Ilmenite
0·7
0
0
0
Glass
0
23·4
27·7
35·3
FeO∗, all Fe as Fe2+; H2O∗, estimated water content from
summation difference; LOI, loss on ignition.
FeO∗/MgO <0·156 × SiO2 – 6·68, respectively (Tamura, 1994).
Figure 7 shows that the major element variation of the
Shirahama Group lies within that of the Quaternary
Izu–Bonin arc volcanoes; the latter extends to highersilica rhyolite (>78 wt % SiO2) and more magnesian
basalt (9 wt % MgO).
Groundmasses of the Shirahama Group
The SiO2 contents of the tholeiitic and calc-alkaline rocks
of the Shirahama Group, classified on the basis of the
fractionation models of Tamura (1994), range from 47
to 67 wt % SiO2 and from 61 to 69 wt % SiO2,
respectively. Mixed magmas have SiO2 abundances ranging from 54 to 60 wt % SiO2 (Tamura, 1994). The
Shirahama Group is a shallow submarine sequence of
volcaniclastic deposits, lava flows and intrusive bodies
(e.g. Cashman & Fiske, 1991; Tamura et al., 1991); all
of these rocks have rapidly cooled groundmasses. Thus
late-stage microlite growth and devitrification of glass,
observed in equivalent subaerial eruptions (e.g. Sparks et
al., 2000) were minimal. Tamura (1995) determined the
groundmass compositions by electron probe microanalysis (EPMA) using the following procedure: (1)
squares with sides ranging from 200 to 500 m, which
included microlites (<50 m), were taken to represent
groundmasses; (2) 49 points were arranged equally in the
square of each sample and measured by EPMA (grid
analyses) by using a beam of 20 m diameter; (3) average
values were calculated as the groundmass compositions.
Representative whole-rock major element analyses,
modal composition (wt %) and groundmass analyses of
calc-alkaline andesite and dacite in the Shirahama Group
are given in Table 3. Most calc-alkaline magmas in the
Shirahama Group (61–69 wt % SiO2) consist of rhyolitic
groundmasses (70–78 wt % SiO2) containing phenocrysts
of augite, hypersthene, plagioclase and titanomagnetite;
minor quartz and/or hornblende can be found in about
half of these (Tamura, 1995). Figure 8 shows plots of
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Fig. 7. Harker diagrams of major element variations in rocks of the Shirahama Group (Χ) and the Quaternary Izu–Bonin arc volcanoes (Β).
Major element compositions of the Shirahama Group are encompassed by those of the Quaternary Izu–Bonin arc volcanoes. The latter extend
to both more magnesian basalts and high-silica rhyolites.
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Table 3: Representative whole-rock major element analyses, modal composition (wt %) and groundmass
analyses of calc-alkaline samples of the Shirahama Group
283
330
295-1
319-2
309-2
266
327
303-2
324
Whole-rock major element abundance (wt %)
SiO2
64·13
67·39
62·79
64·91
66·27
63·69
64·32
61·27
TiO2
0·62
0·51
0·67
0·60
0·62
0·61
0·63
0·70
63·69
0·63
Al2O3
16·06
15·54
16·55
16·45
16·09
16·12
16·31
16·43
16·44
FeO∗
5·92
4·41
6·00
5·13
4·76
6·03
5·64
6·81
5·84
MnO
0·15
0·11
0·16
0·12
0·12
0·09
0·13
0·15
0·15
MgO
2·28
1·53
2·20
1·76
1·43
2·48
1·95
3·00
2·17
CaO
5·99
5·04
6·38
5·41
4·53
5·73
5·80
7·20
5·95
Na2O
3·46
4·07
3·66
3·89
4·50
3·61
3·75
3·01
3·60
K2O
1·25
1·28
1·41
1·57
1·49
1·51
1·32
1·33
1·33
P2O5
0·13
0·11
0·19
0·15
0·17
0·13
0·15
0·11
0·17
Total
100
100
100
100
100
100
100
100
100
Modal abundance (wt %)
Orthopyroxene 1·4
2·0
0·3
1·6
1·5
1·2
0·5
2·0
Clinopyroxene 6·4
2·9
1·2
1·8
1·5
2·5
2·9
3·6
3·4
29·2
21·9
26·7
21·0
15·6
26·9
16·2
17·1
12·1
Fe–Ti oxides
2·9
2·0
1·0
1·4
1·4
2·8
0·6
1·2
1·4
Hornblende
0·0
0·4
6·2
0·1
0·0
0·0
0·0
0·0
0·0
Quartz
2·0
0·0
0·4
0·0
0·0
0·3
0·0
0·0
0·0
Plagioclase
Groundmass
Total
58·1
100
70·8
100
64·2
100
74·1
80·0
100
100
66·3
100
79·8
100
76·1
100
1·8
81·3
100
Groundmass major element abundance (wt %)
SiO2
78·53
77·90
77·24
74·56
74·46
73·20
71·66
70·97
TiO2
0·23
0·23
0·27
0·23
0·26
0·28
0·45
0·56
0·52
Al2O3
11·30
12·00
11·31
13·68
14·05
13·04
14·39
13·59
15·40
FeO∗
1·76
1·78
2·35
1·83
1·76
3·10
3·24
4·30
3·53
MnO
0·07
0·05
0·04
0·05
0·04
0·04
0·08
0·11
0·09
MgO
0·47
0·36
1·09
0·44
0·34
1·55
0·79
1·39
0·97
CaO
2·08
2·06
2·30
2·45
2·75
2·64
3·89
3·97
4·22
Na2O
3·01
3·53
3·09
4·02
4·40
3·35
3·93
3·04
3·88
K2O
Total
2·55
100
2·09
100
2·31
100
2·74
1·94
100
100
2·80
100
1·57
100
2·07
100
69·67
1·72
100
FeO∗, all Fe as FeO. All analyses are recalculated to 100% total.
major element abundances against wt % SiO2 for tholeiitic and calc-alkaline groundmasses (liquids) of the Shirahama Group. Rhyolitic groundmass compositions of
the calc-alkaline series contrast sharply with the wide
range of compositions shown by tholeiitic liquids (49–
71 wt % SiO2). These calc-alkaline liquids resemble
rhyolites from the Quaternary Izu–Bonin arc volcanoes
(Fig. 8, Table 3) except that Na2O contents are lower; this
feature suggests loss of this component during microprobe
analysis. Moreover, the phenocryst assemblages of calcalkaline andesites of the Shirahama Group are similar
to restite phase assemblages of dehydration-melting of a
calc-alkaline andesite (Fig. 6).
Figure 9 shows modes of phenocrysts (wt %) of calcalkaline andesites and dacites of the Shirahama Group
in descending order of host liquid SiO2 content. Although
the Shirahama calc-alkaline bulk-rock samples have SiO2
contents between 61 and 69 wt % in bulk composition,
Fig. 9 corresponds approximately to the dehydrationmelting experiments of a single calc-alkaline andesite
shown in Fig. 6. In the Shirahama Group calc-alkaline
andesites and dacites, quartz and/or hornblende exist at
lower temperature (<850°C) and the wt % SiO2 of the
liquid decreases and the fraction of liquid increases as
the magmatic temperature rises (Fig. 9). Interestingly,
liquid compositions, phenocryst assemblages and mag-
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Fig. 8. Harker diagrams of major elements of tholeiitic groundmasses (Χ) and calc-alkaline groundmasses (Ε) of the Shirahama Group and
rocks of the Quaternary Izu–Bonin arc volcanoes (Β). A characteristic of the calc-alkaline series magmas is the development of rhyolitic liquids
(Tamura, 1995), which are indistinguishable from the Quaternary Izu–Bonin rhyolites.
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TAMURA AND TATSUMI
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Fig. 9. Weight percent modes, melt (groundmass) compositions, bulk compositions, and magmatic temperatures of the calc-alkaline andesites
and dacites of the Shirahama Group in decreasing order of SiO2 content in melt (groundmass) after Tamura (1995).
matic temperatures are related to each other, but they
are not related in a simple way to bulk-rock compositions
(Fig. 9). Hindered crystal fractionation of a cooling
magma body or remobilization of a reheated and partially
melted pluton may result in variable liquid compositions
irrelevant to bulk-rock compositions.
Phases to be subtracted from the assumed parental
magma to produce calc-alkaline series rocks in the Shirahama Group are olivine + orthopyroxene + clinopyroxene + plagioclase (Tamura, 1994), which are
different from phenocryst phases actually found in the
calc-alkaline rocks (Fig. 9, Table 3). This discrepancy,
not explained by Tamura (1994), can be reconciled by
a partial melting model.
Figure 10a shows compositional relationships between
bulk rocks and their groundmasses for the Shirahama
Group. There is no correlation between these in the
calc-alkaline series magma. On the other hand, tholeiitic
andesites and dacites do not develop high-silica rhyolites
(75–79 wt % SiO2) (Fig. 10a), but they show a positive
correlation between bulk-rock and liquid compositions
in the range of 50–70 wt % SiO2 (Tamura, 1995).
Figure 10b shows wt % SiO2 of liquids vs magmatic
temperatures from Tamura (1995). In the calc-alkaline
liquids of the Shirahama Group, there is a negative
correlation with temperature, similar to the results
obtained in the dehydration experiments of Beard &
Lofgren (1991) (Fig. 10b). The significantly higher
temperature of the experimental results is probably
due to the lower SiO2 content of their starting material
(57 wt % SiO2).
DISCUSSION
Genesis of rhyolite
Does melting of basalt or andesite produce rhyolite?
Hydrous basalt and/or andesite are likely source rocks
from which rhyolites could be produced by partial melting. It is commonly accepted that rhyolites form by the
melting of hydrous basaltic rocks in the crust (Beard,
1995), but basaltic magmas along the volcanic front of
the Izu–Bonin arc are inferred to be almost anhydrous,
and fractional crystallization would be inevitable within
their crustal magma chambers. In other words, these
basalt magmas cannot solidify within the crust without
undergoing significant differentiation. On the other hand,
the absence of calc-alkaline andesite glasses along the
Izu–Bonin arc (Fig. 4) and the Mariana volcanic arc
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hydration-melting of calc-alkaline andesite at low pressure
(<7 kbar) (Figs 5 and 6, Beard & Lofgren, 1991). Moreover, compositions of these rhyolites are close to groundmass compositions of calc-alkaline andesites of the
Shirahama Group, Izu Peninsula (Figs 8 and 9, Tamura,
1995). These lines of evidence support the concept that
rhyolites are produced by 20–30% dehydration-melting
of calc-alkaline andesites in the upper to middle crust.
The foregoing argument does not necessarily rule out
a fractionation origin for rhyolites, but we prefer a melting
origin because (1) fractional crystallization from basalt
magma (tholeiitic series) does not develop high-silica
(>75% SiO2) rhyolite in the Shirahama Group (Fig. 8,
Tamura, 1995); (2) hydrous calc-alkaline andesite magma
of the Izu–Bonin arc would solidify within the crust rather
than develop rhyolite through fractional crystallization.
Concealed calc-alkaline andesite
Fig. 10. (a) Plots of wt % SiO2 of bulk rocks against wt % SiO2 of
liquids. Rhyolites are very common in Shirahama calc-alkaline andesites
and dacites as interstitial liquids; the chemical variation in the rhyolitic
liquids does not correlate with that observed in the bulk rocks. (b) Plots
of magmatic temperature against liquid SiO2. There is a negative
correlation between temperatures inferred from two-pyroxene thermometry in the Shirahama rocks and liquid SiO2.
(Straub, 1995) suggests that liquids with these compositions could be water saturated, causing them to
solidify at depth and never erupt to the surface. Thus,
the partial melting of solidified calc-alkaline andesites
rather than basalts might play an important role in
producing rhyolite. The fundamental role of basalt
magma in the genesis of rhyolite would therefore be to
provide heat to masses of solidified calc-alkaline andesite
at depth.
Calc-alkaline andesite is abundant in evolved island arcs
having crustal thicknesses >30 km, and in continental
marginal regions (Green, 1982). The volcanic front of
the Izu–Bonin arc, with a crustal thickness of >22 km
(Suyehiro et al., 1996), is characterized by bimodal volcanism, which produces large amounts of rhyolite magma
as well as basalt and basic andesite magmas (Fig. 2c).
The genesis of rhyolite in the Izu–Bonin arc leads us to
an interesting conclusion that a much larger amount of
calc-alkaline andesites (3–5 times greater than the rhyolites) is concealed at depth within the oceanic arc crust.
A detailed structural model across the Izu–Bonin arc
along 32°15′N (Suyehiro et al., 1996) indicates that the
oceanic arc contains a middle crust with a P-wave velocity
of >6 km/s and occupying >25% of the crustal volume.
Moreover, this middle-crustal component is confined
beneath the arc and is absent beneath the Shikoku backarc basin. The relatively low velocity (>6 km/s) and its
small increase with depth in the middle crust beneath
the arc might be attributable to granitic rocks (Suyehiro
et al., 1996). On the basis of the geological and petrological
studies, Kawate & Arima (1998) suggested that the middle
crust of the Izu–Bonin arc would be similar to the
Miocene Tanzawa plutonic complex, central Japan,
which is a tonalitic suite exposed at the northern end of
the Izu–Bonin arc system (Fig. 1). The most voluminous
intrusions in this suite comprise rocks with >60 wt %
SiO2 (Kawate & Arima, 1998). It is thus possible that
voluminous calc-alkaline andesites may have accumulated in the oceanic arc middle crust even though
it is generally accepted that andesitic volcanism typifies
continental arcs.
Partial melting of calc-alkaline andesite
Remobilization of calc-alkaline andesite
Rhyolites of the Izu–Bonin arc possess major element
compositions similar to those of melts produced by de-
If calc-alkaline andesite magma is water saturated, it will
solidify in the crust. Additional heat is necessary to
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TAMURA AND TATSUMI
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reheat and remobilize this highly crystalline calc-alkaline
material. Reheating and remobilization of calc-alkaline
magmas has been envisaged to have occurred in the
Adamello massif, Italy (Blundy & Sparks, 1992), the
Lascar volcano, Chile (Matthews et al., 1999) and the
Soufriere Hills volcano, Montserrat (Murphy et al., 2000).
Murphy et al. (2000) concluded that crystal-rich calcalkaline andesite of the 1995–1999 eruption represents
highly crystalline magma with physical properties that
are more probably related to a partially molten but rigid
and relatively immobile pluton rather than a mobile body
of magma. The current eruption was triggered by influx
of hot mafic magma, which reheated and softened the
andesite magma body (Murphy et al., 2000), permitting
it to erupt (Sparks et al., 2000).
Were the calc-alkaline andesites and dacites of the
Shirahama Group also the result of heating and mobilization caused by mafic magma? The silicic liquids
(groundmasses) contained in calc-alkaline andesites and
dacites do not overlap the whole-rock calc-alkaline compositional trends (Fig. 8). The tholeiitic series of the
Shirahama Group formed via fractionation of crystallizing phases (phenocrysts) from basalt through andesite
to dacite (Tamura, 1994), and tholeiitic liquids show a
wide range of composition, which overlaps the wholerock trends (Tamura, 1995). Calc-alkaline series rocks
lack both of these characteristics. Further, the calcalkaline series includes a wide range of rhyolitic liquids
(Fig. 10a). For example, calc-alkaline rocks with 64 wt %
SiO2 are associated with rhyolitic liquids with SiO2 contents ranging from 69 to 79 wt % (Fig. 10). These lines
of evidence suggest that calc-alkaline magmas in the
Shirahama Group represent magmas formed by partial
melting and consist of low- to high-percentage partial
melts with restite crystals.
Segregation of partial melt from restite crystals would
produce a magma of rhyolitic composition, and complete
mobilization would produce a magma of the same composition as the source rock. This interpretation of some
orogenic andesite-to-rhyolite sequences as the products of
partial melting is increasingly supported by accumulating
evidence, such as reversed zoning of orthopyroxene
phenocrysts in terms of temperature (Matthews et al.,
1999; Murphy et al., 2000) and hydrogen isotopic zoning
of some amphiboles (Harford & Sparks, 2001).
part of the mantle wedge above the slab and ascends
buoyantly through the mantle wedge; (2) this diapir is
heated during ascent through the hot and dry mantle
wedge, which is consistent with the model of Tatsumi et
al. (1983) for the thermal structure; (3) finally, the heated
diapir, which still has a wet and cool interior and heated
dry and hot rind, produces both magnesian andesite and
basalt, respectively (Tamura, 1994).
Hirose’s (1997) experiments are consistent with this
hypothesis. He showed that wet magnesian andesite
magma (54·4 wt % SiO2 and 6 wt % MgO on anhydrous
basis) containing 6·3 wt % H2O is produced by melting
of lherzolite KLB-1 with 1 wt % H2O at 1 GPa and
1050°C. The same peridotite produces basalt (50·5 wt %
SiO2 and 10·1 wt % MgO) at 1 GPa and 1300°C under
dry conditions (Hirose & Kushiro, 1993). The degrees of
melting are 16 wt % and 12 wt %, respectively.
Low H2O contents in pre-eruptive calc-alkaline andesite magmas, which are not saturated with water, are
commonly highlighted as one of the major problems
inherent in fractionation models from hydrous magnesian
andesite, because H2O contents are too low to have been
produced by crystal fractionation of H2O-rich mantlederived magmas (e.g. Tamura, 1995). Given solidification
and remobilization of calc-alkaline magma, H2O contents
can no longer be a constraint for genesis of calc-alkaline
andesite. Water-saturated magmas solidify in the crust.
Melting of such a solidified body produces water-deficient
magmas; partial melting of largely solidified pluton cannot
produce volatile-rich silicic magmas, because the volatile
content of the constituent minerals is too low (Matthews
et al., 1999).
Figure 11 shows the two-step model for genesis of calcalkaline magmas. The primary step is anhydrous or
hydrous melting of mantle peridotite, which produces
basalt or magnesian andesite, respectively. Tholeiitic
series rocks are produced by fractionation of basalt (Fig.
11b), but water-saturated magnesian andesite and/or
calc-alkaline series rocks, which are produced by fractionation of magnesian andesite, will solidify in the crust
(Fig. 11b). Thus, calc-alkaline magmas would not appear
on the surface without reheating and the second-stage
melting by hot and dry basalt magmas, which are produced in the same mantle diapir (Tamura, 1994, Fig.
11c). Tamura et al. (2000) presented evidence for supercooling of arc basalt at Daisen volcano, Japan; such a
process would be complementary to remelting.
Melt, solidify and melt again
Our contention is that mantle-derived hydrous magnesian
andesite, not basalt magmas, may be parental to the
calc-alkaline series rocks in the Shirahama Group (Tamura, 1994). Tamura (1994) developed this hypothesis
based on the following interpretations: (1) a mantle diapir
consisting of hydrous peridotite is formed in the lower
Implications for continental genesis
The andesitic major element composition of the continental crust can be formed only via a two-stage process
from the upper mantle: (1) generation of basalt (or
magnesian andesite); (2) differentiation of this protolith
(Arculus, 1999).
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Fig. 11. Model for evolution of mantle-derived basalt and magnesian andesite in higher-level magma chambers. Previous calc-alkaline magma
batches have partly solidified and are then remobilized and partially melted by later batches of basalt magmas in the same system. Numbered
magma batches evolving from (a) to (c). (a) A diapir, which has wet and cool interior and dry and hot rind, produces wet and cool magnesian
andesite and dry and hot basalt magmas, respectively (Tamura, 1994). (b) Tholeiitic series magmas are produced from dry basalt magmas, which
are superheated by decompression and then cool and evolve through fractional crystallization (1 and 4). These dry magmas can erupt. Wet
magnesian andesites magmas and their derivatives, however, become saturated and solidify within the crust (2, 3 and 5). New magma batches
produced in the mantle ascend through the crust (6–10). (c) Hot basalt magmas (8 and 10) are emplaced beneath the frozen andesite magma
bodies, which reheat and remobilize the andesite (3 and 5), triggering eruptions of calc-alkaline andesite, dacite and/or rhyolite. Some basalts
(10) could show evidence of supercooling (Tamura et al., 2000).
If continental crust formed mainly as a result of arc
processes, the results of this study could have a bearing
on the genesis of continental crust. In this study, the
second stage of differentiation, described by Arculus
(1999), involves both fractional crystallization of basalt
magma and partial melting and remobilization of solidified andesite (protolith) by basalt magma (Fig. 11).
Importantly, water-saturated andesite magma will solidify
in the crust not as a result of the sudden drop in
temperature, but as a result of decompression (Fig. 11b).
In that case, the solidified andesite magma bodies could
still have a high temperature (>800°C) before subjacent
basalt magmas are emplaced (Fig. 11c). Thus basalt
magma might ‘strike while the protolith is hot’, which
could make the resulting differentiation and continental
genesis both rapid and efficient.
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SUMMARY AND CONCLUSIONS
The Izu–Bonin arc is characterized by bimodal, basalt–
rhyolite, magmatism and it is commonly accepted that
the rhyolite forms by the partial melting of basaltic rocks
in the crust. However, basalt magmas of the Izu–Bonin
arc are anhydrous and if a reservoir of basalt magma
exists, it will not freeze within the crust, but will gradually
crystallize to fractionate into a layered intrusion. In
contrast, original calc-alkaline andesite could be saturated
in water and is more likely to freeze within the crust,
because the cooling path crosses the water-saturated
solidus (negatively sloping on a P–T plot). Rhyolites of
the Izu–Bonin arc could be produced by dehydrationmelting of the solidified hydrous calc-alkaline andesite,
which is also consistent with melting experiments in
terms of both composition and mineralogy. Thus the
fundamental role of basalt magmas in the generation of
calc-alkaline silicic rocks is to provide heat to the solidified
calc-alkaline magma bodies, thereby causing remelting
and remobilization. The middle crust of the Izu–Bonin
arc, a typical oceanic island arc, might consist of solidified
calc-alkaline andesite, which is being partially melted by
hot and dry basalt to produce rhyolite.
A two-stage process, involving mid-crustal solidification
of water-saturated calc-alkaline magmas followed by partial melting related to reheating by subjacent, relatively
anhydrous basaltic magmas, would generate melts that
would have equilibrated with a phase assemblage differing
significantly from that expected from direct fractional
crystallization from a parent. This new interpretation
complements Tamura’s (1994) fractionation models of
calc-alkaline series rocks from mantle-derived magnesian
andesite and provides a more complete explanation of
how calc-alkaline series rocks low in H2O are produced
from hydrous magnesian andesite.
ACKNOWLEDGEMENTS
We thank R. S. J. Sparks for a review of the draft version
of the manuscript. R. C. Price, T. H. Green and R.
Arculus are thanked for thorough and critical reviews.
R. S. Fiske helped with manuscript revisions. Many
thanks go to K. Aoike, K. Fujioka, T. Ishii, S. Kawate,
S. Morita, I. Moriya, K. Suga, A. Takada, T. Yamamoto,
M. Yuasa for their help in compiling Izu–Bonin data.
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