Geochemical Journal, Vol.16, pp. 311 to 320, 1982
S r/ C a- B a / C a sy ste m atics in H ig a s hi-Iz u m o n o g e n etic
v olc a n o g r o u p, Iz u P e n in s ula, J a p a n
M ASATAKA HIRAN0,1* ICAZUCHIKA H AM UR02 and N AOKI O NUMA3
Insitutute of Chemistry, University of Tsukuba, Sak ura-m ura, Niih ari-gu n, lb arak i 305,1
Ocean Division, Econom ic Affairs Bureau,Ministry of Foreign Affairs,2-ch jm e, Kasum igaseki,
Chiyoda-ku, Tokyo 1002 and
Depart m ent of Earth Science, lb arak i University,.B unk y ,2-chom e, Mito 310,3 Japan
(R eceivedFebru ary 17, 1982:Accepted Novem ber11, 1982)
Ca, Sr and Ba contents of various volcan ic rocks (basalts, andesites, dacites and a granitic xenolith)
from the Higash i-iz u m onogenetic volcano gr oup, the lzu Peni nsula, Japan, have been determined by an
ICP-OES method. Th e results in dicate that the m onogenetic volcano group m ak es a Sr/Ca-Ba/Ca system atics which is sim ilar to that defined by a stratovolcano. The SB system atics suggests that there are
two different high alum in a basalt m agm as in the Higash i-Izu monogenetic volcano group: one (basaltI) is
a prim itive basalt magma derived from the m antle with rather small degree ofpartial melting and the oth er
(basalt II) is a m ix ture of the prim itive basalt magm a and a cum ulus phase enriched in plagioclase. The
high alumina basalt series (basal t (1)- andesite-dacite series) is considered to be established, not by cry stal
fractionation process in magma cham ber, but by assim ilation process of the prim ary basalt m agma with
the granitic crust. The crust under the iz u Penin sula m ay have a chemical com position sim ilar to that
ofthe dacite(1) from Kaw agodaira.
INTRDUcr ION
A s show n in Fig. I, there ar
. e m any Q uaternary volcanoes in the lzu Peninsula. T he basem ent of the lzu Peninsula is com posed of subm arine volcanic products called the Y ugashim a
Group and the Shiraham a G roup of M iocene
age. O n the basem ent, subaerial stratQvolcanoes
had been built in Q uaternary age, that is, Taga
volcano, U sam i volano, Tenshi volcano, A m agi
volcano on the east side, and Ida volcano,
D aru m a volcano, Tanaba volcano, Jaishi volcano
on the w est side, and N anzaki volcano at the
south end of the Peninsula. A fter those volcanoes had finished their activities, the m ost
recent volcanism initiated in the eastern part of
the Peninsula about m ore than 30,000 years ago.
The volcanic activity provided m ore than 70
sm all volcanoes in the area of 350km 2.
This sm all volcano group is nam ed "Higashilzu m onogenetic volcano gr oup" by A RAMAKI
and H AM URO (1977). A m onogenetic volcano is
a sm all one produced by one eruptive cycle,
w hile a stratovolcano is a larger one produced by
repeated eruptive cycles.
T he H igashi-Izu
m onogenetic volcano group is characterized by
the bim odal volcanism with relatively large
volum e of basic m agm a and sm all volum e of
acidic m agm a, w hile the other Q uaternary
stratovolcanoes are characterized by the
unim odal volcanism m ostly consisted of andesitic m agm a, typically observ ed in island arcs.
Recently, A RAM AKI and H AM URO (1977)
and H AM URO (1978, 1 982) have carried out a
geological study of the Higashi-Izu m onogenetic
volcano group in detail. A ccording to them , the
volcanic activity of the basic m agm a (basaltandesite) had started about m ore than 30,000
years ago, w hile the volcanic activity of the
acidic volcanism (dacite), started about 3,000
years ago.
H AMURO (1978, 1982) also has clarified the
petrology of the Higashi-Izu m onogenetic vol-'
cano group. Silica contents of the volcanic
rocks from this volcano group show the bim odal
distribution separated into the range from 49 to
*present address.' Central Research Laboratory , Sumitomo M etal M ining Co., Ltd., 3-18-5. Nakakokubun,
lchikawa, Chiba272,Japan
311
312
M.HIRANO etal.
60w t.% (basalt-andesite) and the range from 68
to 74wt.% (dacite). M ost of the volcanic rocks
are distributed w ithin the high-alum ina basalt
series by K UNO (1960).
The basalts are classified in to tw o different
groups on the basis of petrological point of
view: basalt I has a phenocry st assem blage of
olivine and plagioclase and basalt 11 has a phenocry st assem blage of olivine, plagioclase and
augite. T he difference is also in dicated by the
slight difference in sodium and calcium contents. Basalt I is slightly depleted in sodium and
calcium com pared w ith basalt II. O ccurrence of
the tw o different basalts is also different. B asalt
I is distributed in the northern part of the area
(Fig. l), w hile basalt 11 is distributed in the
southern part of the area. Therefore, H AMURO
(1978, 1982) concluded that at least tw o different basaltic m agm a, though the difference is
slight,erupted in this area.
H AMURO (1978, 1 982) also pointed out that
the basalts of the m ost m afic com position had
already precipitated olivine cry stals during
ascent to the surface. O riginal basaltic m agm a
m ust have contained at least about 10 wi .%
M gO. A ndesite m agm a was produced by contam ination of the basalt m agm a with a granitic
rock constituting the crust under the lzu Peninsula. The dacite m agm a w as also produced by
rem elting of the granitic crust by heat of the
basaltic m agm a.
The purp ose of this report is to elucidate
the m agm atism characterized by the H igashi-Izu
m onogenetie volcan o group, on the basis of Sr/
Ca-Ba/Ca system atics w hi ch is one of the m ost
sensitive geochem ical indicator for m agm a
genesis, ori ginally proposed by O NUMA (1 980).
First, w e will discuss the origin of the tw o different basaltic m agm a. Second, we will try to
veri fy the contam ination m odel for the high
alum ina basalt series proposed by H AMURO
(1978, 1 982). T hird, a com parative study of
m agm atism betw een the lzu Peninsula and the
lzu Islands w ill be given, to clarify the nature
of the m antle m aterials and the cru st under the
lzu Peninsula.
EXPERIMENTAL
T he volcanic rocks from the H igashi-Izu
m onogenetic volcano group em ployed in this
w ork are collected m ainly by one of the authors
(K.H.). A basalt (J boshi, H.K. N o.) and an
andesite (K om uroyam a, H.K. N o.) are donated
by the late H. K UNO of U niversity of T okyo
through H. N AGASAW A of G akushuin U niversity.
Tw o dacites (Y ahazuyam a, R.K. N o.) ar e donated by R. K URODA of Chiba U niversity.
B asalts, andesites and dacites of the volcano
group are subdivided by H AMURO (1978, 1982),
on the basis of mineral assem blage of phenocryst: basalt I (OI + pl), basalt 11 (Ol, P1, A ug),
andesite I (OI i PD, andesite 11 (O1, Pl, A ug),
andesite 111 (Ol, P1, A ug, O px), dacite I (P1, O px,
H b, M t, Il, Qt), dacite 11 (P1, O px, H b, M t, Il,
O1, Cpx, Qt). In this w ork, 5 basalts (1), 6 basalts (II), 3 andesites (1), I andesite (II), 3 andesites (III), I dacite (1), 4 dacites (II) and 1
granitic xenolith brought by lw anokubo andesite (II) are used for chem ical analyses.
_ AKON
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Fig. 1. Locality m ap of the Higashi-Izu m onogenetic
volcanogroup, thelzu Peninsula,JapalL
Sr/Ca-Ba/Ca system atics
Ca, Sr an d Ba contents of the specim ens
were determ ined sim ultaneously by an inductively coupled plasm a-optical em ission spectrom etry (HIRANO et al., 1 980). The m ethod has
large dynam ic range (C a: I 04, Sr: 104, Ba: 104)
and low detection lim it (Ca: 13 ppb, Sr: 0.2
ppb, Ba: 0.7ppb). The accuracy and the precision of the m ethod obtained by JB-1 standard
rock sam ple are: Ca = 66,900 i 1,010 ppm (certified value = 66,300ppm ), Sr = 439 :!: 7.6ppm
(C.V. = 435 ppm ) and Ba = 493 :!: 9.0 ppm (C.V.
= 490 ppm ), respectively. T he m ethod is suitable to obtain a set of coherent data from a
given sam ple, rapidly and precisely.
313
from the Higashi-Izu m onogenetic volcano group
m ake a line with gentle slope in order of basalts
(II), basalts (1), andesites (1, II, 111), dacites (II)
and dacite (1). T he line or SB system atics defin ed by the volcano group is sim ilar to that
defined by various volcanic rocks from a stratovolcano (O NUMA et al., 1981; HIRANO et al.,
1982).
O n the SB system atics, basalts (1) and (II)
are discrim inated, w hile andesites (1, II, 111) except D ainoyam a are not discrim inated. The exceptional andesite falls far above the SB system atics. D acites (1 and II) seem to be discrim inated, w hile a granitic xenolith is situated
atthe end ofthe SB system atics.
RESULTS
DISCUSSIONS
T he results obtained in this w ork are show n
in T able I and Fig. 2.
B efore w e discuss
Principle of SB diagram
A s show n in Fig. 2, various volcanic rocks the inform ation contained in the SB system atics
Higashi-lzu M onogenetic V olcano G roup
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163
lc3
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Fig. 2. Sr/Ca-Ba/Ca systematics defined by various volcanic rocks (basalts-andesites-dacites) and a granitic xenolith
from the Higashi-Izu m onogenetic volcano group.
(1: basa lt II, o: basa ltI, o: andesite I, o: and site II, o: andesite 111, e: dacite I & II, o: granitic xenolith)
M. HIRANO et al.
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Sr/Ca-Ba/Ca system atics
315
with addition of the Sr- and Ba-poor m ajor
phases. Ca content of the m elt increases with
addition of Ca from garnet and clinopyroxene,
so Sr/C a and B a/C a ratios of the m elt decrease
w ith constant Sr/B a ratio. The m elt generated
by com plete m elting of the m antle peridotite
shows the sam e ratios as those of the m antle
peridotite.
Therefore, a series of m elts derived directly
from the m antle peridotite w ith different degree
of partial m elting m ake an SB system atics w hich
is a line with slope of 45' through. the m antle
peridotite on the log-log SB diagram . This is the
partial m elting line show n in Fig. 3. T he residual solids left over during partial m elting have
low Sr/C a and BalC a ratios and fall on the line
below the m antle peridotite.
O nce generated m elt, or "prim ary m agm a"
is separated from the residual solids and ascends
to the surface. D uring the ascent, olivine crystals precipitate from the prim ary m agm a. Sr/Ca
and BalCa ratios ofthe m agm a, how ever, do not
change, since olivine does not accept such larger
cations. Thus, both ratios of the prim ary m ag-
in Fig. 2, w e w ould like to explain the principle
of Sr/Ca-Ba/Ca diagram (SB diagram ) w hich
visualizes the origin and evolution of m agm a
generated under a volcano. Figure 3 is a schem atic SB diagram for interpretation of SB system atics in connection with m agm atism (O NUMA
etal., 1981).
W e assum e a garnet peridotite as m antle
m ateri als. The large cation Ca is contained
m ainly in clinopyroxene and garnet w hile the
larger cations Sr and Ba do not enter into the
m ajor phases (olivine, orthopyroxene, clinopyroxene and garnet) by crystal structure control. T he larger cations are contained m ainly in
accessory m inerals situated in interstitial grain
boundaries.
W hen the m antle peridotite is gradually
heated, partial m elting occurs. Sm all degree of
partial m elting produces strong enrichm ent of
Sr and B a in the m elt, since the accessory
mineral is am ong the first com ponent to enter
the m elt. Sr/Ca and B a/C a ratios of the m elt are
the largest ones. Increasing degree of partial
m elting dilutes Sr and Ba contents of the m elt
tse
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lo g (B a/ C a)
Fig. 3. A schem atic Sr/Ca-Ba/Ca diagram to elucidate the origin and evolution of magm a(ONUMA et al., 1981)
316
M. HIRANO etal.
m a are frozen.
W hen the prim ary m agm a reaches "m agm a
cham ber", crystal fractionation occurs. Olivine,
orthopyroxene, m agnetite and il m enite cry stallizations in the m agm a cham ber do not change
either Sr/Ca or Ba/C a ratios, since those m inerals
do not accept the three cations. Plagioclase,
clinopyroxene and hornblende crystallization,
how ever, change Sr/C a and BalCa ratios of the
prim ary m agm a. In plagioclase crystallization,
Sr/C a ratio in the evolved m elt hardly changes,
w hile Ba/Ca ratio in the evolved m elt increases
greatly, since plagioclase accepts both Ca and Sr
to sim ilar extents but excludes B a. In clinopyroxene (hornblende) crystallization, both
ratios in the evolved m elt increase. Increase of
Ba/Ca ratio is m uch greater than that of Sr/Ca
ratio, since Ca enters, Sr is hard to enterand Ba
does not enterinto clinopyroxene (h ornblende).
T herefore, a series of the evolved m elt
derived from the prim ary m agm a via plagioclase
and clinopyroxene (hornblende) crystallization
in "m agm a cham ber", m ake another SB system atics w hich is a curv e with a gentle slope through
the prim ary m agm a on the log-10g SB diagram.
This is the cry stal fractionation line show n in
Fig. 3. The slope is controlled by plagioclase/
clinopyroxene ratio precipitated in "m agm a
cham ber". T he cum ulates separated from the
prim ary m agm a via crystal fractionation in
"m agm a cham ber" show sim ilar Sr/Ca ratio and
10w Ba/Ca ratio com pared with the prim ary
m agm a so that they fall on certain positions
extrapolated from the crystal fractionation line
through the prim ary m agm a.
The "crystal fractionation line" in Fig. 3
m ight change its nam e to a "m ixing line" by
som e possibility. For exam ple, a sim ilar SB
system atics could be established by a series of
volcanic rocks w hich are derived from the m ixing process of a prim ary m agm a and the crust
with granitic com position. Therefore, w e m ust
be careful to interpret SB system atics from volcanoes situated on the continental crust.
S B S ystem atics in V olcanoes
parallel tO Izu-O gasaw ara Trench
10 2
Onoharaijma - IP
c'
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(Dacites)
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x (chondrites)
10 3
10 2
B a/ C a
Fl. 4. Sr/Ca-Ba/Ca system atics in Quaterna'y volcanoes in lzu Islands, parallel to the lzu-Ogasawara Trench.
(o.' Oshim a, A.' M ly akejim a,[].' M ikurajim a, A: Onoharafim a, l.' Inambajim a)(HIRAN O et al., 1982).
317
Sr/Ca-Ba/Ca system atics
SB index and the nature of m antle m ateIn Fig. 3, w e call the intersection of
riaIs
the partial m elting line and the cry stal fractionation line (or m ixing line) SB index. The SB
index is considered to correspond to the degree
of partial m elting of m antle m aterials. If w e
could determ ine various SB indices defined by
m any volcanoes situated in a certain region, w e
m ight cast a new light on the nature of m antle
m aterials under the region.
Figure 4 show s series of SB system atics in
volcanoes situated parallel to the lzu-O gasaw ara
Trench (HIRANO et al.. 1 982). T he parallelism
seen in the series of SB system atics indicates
that "m agm a cham bers" under the volcanoes
have sim ilar chem ical environm ents, w hile the
different Sr/C a ratios suggest that degrees of
partial m elting of the m antle m aterials are different.
A Iine nam ed "partial m eltin g line" in Fig.
4 is a hypothetical line w ith a slope of 45'
through chondritic m eteorites w hich are con-
sidered to be a representative solid m aterials in
the solar system . If the m antle under the lzu
Islands region still preserv es the chondritic Sr/C a
and BalCa ratios, prim itive basalt m agm a derived
directly from the m antle without plagioclase and
clinopyroxene precipitation m ust have lined up
on the hypotheticalline.
T he seri es of SB system atics from volcanoes
parallel to the lzu-O gasaw ara Trench start from
theline w ith different SB indices,indicating that
the m antle under the volcano chain has chondritic com position in term s of Sr/Ca and B a/ca
ratios. T he situation is the sam e in the case of
SB system atics from volcanoes oblique to the
Trench, as show n in Fig. 5 (ISSHIKI et al., 1982).
Therefore, w e can assum e reasonably that the
m antle under the lzu Islands region has chondritic Sr/C a and Ba/C a ratios.
Origin of basalts (1 and II) in the H igashi-Izu
As al ready
m onogenetic volcano group
m entioned in the previous section, H AM URO
SB S ystem atics in V olcanoes
oblique to lzu-O gasaw ara Trench
Jinaijima
Niijim a & Kozushima
IH l !!lgl_I -
102
UdOneijma f
Toshima - ' l
Niiijma - '
6)
6
6shima '
(Basalts)
'1'
u
'L
U)
10-3
/
1d4
" " .- '
""'
.
,
/
[]
o
()o/ (Dacites)
/
(Andesites)
(Rhyolites)
(Chondrites)
1 c3
B a/ C a
I 02
-1
1O
Fig. 5. Sr/Ca-Ba/Ca system atics in Quaternary volcanoes in lzu Islands, oblique to the lzu-Ogasawara Trench.
(o: Oshim a,[]: Niijim a, A: Toshima, A: Udonejim a, l: Kozushim a) (ISSHIKI etal., 1982).
318
M. HIRANO et aL
(1978, 1982) has elucidated the existence of
tw o different basalts (1 and II) in the H igashilzu m onogenetic volcano group.
If w e assum e that Sr/Ca and Ba/Ca ratios of
the m antle under the lzu Peninsula are the sam e
as those of the m antle under the lzu Islands
region, w e can draw the par tial m elting line, as
indicated in Fig. 2. T he basalts (1) are situated
around the intersection of the partial m elting
line and the SB system atics of the Higashi-Izu
volcano group. T herefore, the basalts (1) are
considered to be quenched m elts of the prim ary
m agm a derived from the m antle by partial m elting through olivine crystallization.
Figure 2 also indicates that the basalts (1)
and the basalts (II) are genetically in close relationship. A ccording to the principle m entioned
in the previous section, the basalts (II) are considered to be rem elts of a cum ulus phase (or
gabbroic rocks) Ieft over by a prim ary basalt (1)
type m agm a, or sim ply to be a m ixture of the
cum ulus phase and the basalt (1) type m agm a.
B asalt (1)-andesite series in the H igashi-Izu
m onogenetic volcano group
A s show n in
Fig. 2, the m onogenetic volcano group gave an
SB system atics defined by the basalt-andesitedacite series. There are tw o possible interpretations for the SB system atics: one is fractional
cry stallization process of the basalt (1) type
m agm a within "m agm a cham bers" and the other
is m ixing process of the basalt (1) type m agm a
and the crust with a granitic com position. A s
already pointed out in the previous section, w e
cannot specify w hich process is correct or not,
on the basis of the SB system atics alone.
H AM URO (1978, 1 982) has already presented
m any petrological evidences that the andesites
are m ixtures of the basalts and a rock with
*'ranitic com position sim ilar to the granitic
xenolith collected at lw anokubo. T he granitic
xenolith is situated at the end ofthe SB system atics, as show n in Fig. 2. The petrological
evidences and the position of the granitic
xenolith on the SB diagram suggest that the SB
system atics of the Higashi-Izu m onogenetic volcano group is considered to be, not a "fractional
crystallization line", but a "m ixing line", endm em bers of w hich are the basaltic type I m agm a
and the granitic rock.
The andesite from Dainoyam a w hich is
10cated far above the SB system atics (Fig. 2)
m ight be a m ixture of a m elt derived from a different prim ary m agm a with rather sm all degree
of partial m elting of the m antle peridotite and
the granitic crust.
A com parative study betw een the H igashi-Izu
m onogenetic volcano group and volcanoes in the
lzu Islands
Figure 6 show s a com parison of
various SB system atics obtained from the volcanoes of the lzu Islands located near the lzu
Peninsula (O NUMA et al., 1981; ISSHIKI et al.*
1982) and from the Higashi-Izu m onogenetic
volcano group. The basaltic type I m agm a of
the Higashi-Izu m onogenetic volcano group
coincides with the basaltic m agm a of Toshim a
and U donejim a Islands, w hile the basaltic m agm as generated under Niijim a and Oshim a Islands
are different from that of the Higashi-Izu m onogenetic volcano group. The difference corresponds to different degrees of partial m elting
of the m antle peridotite. The degree partial
m elting of the prim ary basaltic m agm a in the
Higashi-Izu m onogenetic volcano (T oshim a and
U donejim a volcanoes) is sm aller than that of the
prim ary basaltic m agm a in Niijim a volcanoes.
The prim ary basaltic m agm a generated under
O shim a indicates the largest degree in this
region.
A s show n in Fig. 5, the various SB system atics from the lzu Islands converge into a point
represented by the Jinaijim a type rhyolite. The
convergence indicates that the prim ary m agm as
with different degrees of partial m elting evolve
into a silicic m agm a by crystal fractionation
processes in "m agm a cham ber". Therefore,
ISSHIKI et al. (1982) considered that the Jinaijim a type rhyolite is a representative silicic m agm a as an end-product of crystal fractionation
processes and m ade a thin crust under these
islands. M ajority of rhyolites from Niijim a and
K 5zushim a m ight have been form ed by rem elting of the silicic crust with com position of
Sr/Ca-Ba/Ca system atics
Jinaijim a type rhyolite and subsequent rem oval
of plagioclase from the silicic m elt.
SB system atics defined by the Higashi-Izu
m onogenetic volcano group also converge into a
point represented by the Jinaijim a type rhyolite,
as show n in Fig. 6. In this case, a dacite (1) from
Caw agodaira of the lzu Peninsula corresponds to
the Jinaijim a rhyolite of the lzu Islands. T he
granitic xenolith from lw anokubo situates near
the Kaw agodair a dacite. The convergence of the
SB system atics suggests that the crusts under the
Higashi-Izu m onogenetic volcano group and also
Niijim a and K 5zushim a have the sam e chemical
com positions represented by the K aw agodaira
dacite and the Jinaijim a rhyolite. T he cru st
m ight be evolved by addition of the end-product of cry stal fractionation processes.
T he SB system atics defined by the Higashilzu m onogenetic volcano group is considered to
represent a m ixing line, end-m em bers of w hich
are the high alum ina basalt m agm a (type I) and
the crust with com position of the dacite (type
319
I), respectively. The basalt (II) m ight be a m ixture of the high alum ina basalt m agm a (1) and a
cum ulus phase (gabbro ?) and the andesites (1,
II and 111) are m ixtures of the high alum ina
basalt m agm a (1) and the cru st to different
extents. The dacite (1) m ight have been derived
from the crust by total m elting w ithout subsequent rem oval of plagioclase. The
, dacite (II)
is a m ixture of the dacite (1) and basaltic rock or
basaltic m agm a.
Ackn owledgements- We th ank Prof. S. ARA MAKI of
Earthquake Research Institute, Tokyo University for
givin g us the constru ctive com m ents on the m anuscript.
The work wassupported by a grant from the M in istry of
Education, Science and Culture,Japan.
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HAMURO, K. (1978) Geology and pctrology of the
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Niijim a & Kozushim a
. ｩ
102
UdOneiim a
,....
l
!
cl'
U
.¥L
V)
Niiijma '-':
,,
Toshima
(Rhyolites)
(Dacites)
(Andesites)
Oshima
(Basalts)
103
l
104
(chondrites)
10 3
10 2
B a lC a
Inu
Fig. 6. A com parison of Sr/Ca-Ba/Ca system atics defined by the Higashi-Izu m onogenetic volcano group and by
Quaterna'y volcanoesof the lzu Islands.
320
M. HIRANO et al.
Higash i-Izu m onogenetic volcano group. PhD Th esis,
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