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,**.  +  +3  ,**/  -  / Petrological Study of Torishima Volcano, Izu Islands, Japan
Takeshi SUGIMOTO῍,῍῍, Hidemi ISHIBASHI῍῍῍ and Takeshi MATSUSHIMA῍
The eruptive activity of Torishima volcano is divided into three stages ; the stratovolcano stage, thecaldera-forming stage and the central cone stage, respectively. Volcanic rocks of the stratovolcano stage arecharacterized by basalt containing abundant plagioclase phenocrysts. The chemical variation of this stage inoxide-oxide diagrams can be explained basically by addition of plagioclase and moderate removal of maﬁc
minerals. Volcanic rocks of the caldera-forming stage are characterized by olivine-bearing dacite. Their
whole-rock compositions lie on a straight line between the rhyolitic glass and the stratovolcano basalt with MgO
2 wt.ῌ. They also show di#erent Ba/Zr and Nb/Zr ratios from stratovolcano-stage basalts, indicating
contamination of crustal materials. So they were most likely to be formed by magma mixing of basaltic magma
and rhyolitic melt formed by partial melting of maﬁc lower crust. Volcanic rocks of the central cone stage areclassiﬁed into two groups ; Komochi-yama basalts and Io-yama basaltic andesites, respectively. Io-yama basaltic
andesites make a tholeiitic trend with stratovolcano-stage basalt in SiO, vs FeO*/MgO diagram and have higher
TiO, and FeO* contents than other volcanic stages. On the other hand, they lie on a mixing line between
caldera-forming-stage dacite and stratovolcano-stage basalt in Zr vs Ba/Zr and Nb/Zr diagrams. Therefore,
their magmatic evolution can be explained by two processes ; (+) TiO, and FeO* enrichment by additional
fractional crystallization of stratovolcano-stage basalt, (,) magma mixing between caldera-forming-stage dacite
and di#erentiated magma with higher TiO, and FeO* content.
Key words : Torishima volcano, Izu islands, magmatic di#erentiation, magma mixing, fractional crystallization

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§B ,ῌ/0.-ῌ,3¨©YYª«¬ª­f®{«¬¯"°±
Institute of Seismology and Volcanology, Faculty of
Sciences, Kyushu University, Shin’yama, Shimabara
2//ῌ*2.-, Japan.
 z²­ : ¦21.ῌ*3*-  §!9 -*22ῌ+10
Y«¬³­f´"«¬µ¶Present address : Institute for Geothermal Sciences,

Kyoto University, Noguchibaru, Beppu 21.ῌ*3*-, Japan.
 ¦2+,ῌ2/2+ #·§¸$¹ 0ῌ+*ῌ+¨©YYª f´%º³»¼
Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 0ῌ+*ῌ+Hakozaki, Fukuoka 2+,ῌ2/2+ Japan.
Corresponding author : Takeshi Sugimotoe-mail : sugimoto@bep.vgs.kyoto-u.ac.jp

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rq 9­2´µ6p¶¤ u_ <·Fig. +. (a) Map showing studied area. (b) Map showing sampling localities. Numerals correspond to sample
number of whole-rock bulk chemical analysis. Topographic map of Torishima, + : ,/,*** in scale (Geographical Survey Institute) is used.

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fghP

Volcanic history of Torishima volcano.

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Table +.

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Iw!dFGDB c_¥¦J¼9Fig. ,. Sketch of the stratovolcano volcanic rocks at cli# of Hatsunezaki. Numerals correspond to sample numberof whole-rock bulk chemical analysis.

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"#$ 2῍,- vol.ῌ bD(JnJ'(,>012nuvt?@^A%`C qr
JP*h?tD FHJ l %m 1 mm

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FH FH I 1 J l %m +* mm

2^ _%`C
+3-3  .ῌ-ῌ,

 No. ..῍.0 ´tP012<9%´
4Z\¤J+ ¦µ , (No. ..῍%u^ _%`C Jgg !

./) ^!g?k $J  +.῍,- vol.ῌ bDn \5bD JgF FH FH I_%`CDH° !FH°

p@g"#$6*h?t  

1  !g?D $J  -1῍.* vol.ῌ ( (No. ,3῍-.) 7Cbk +3-3 ¶#cp4K·D FD¡ !¢8bk £\012<9%Jl%m - mm  -+῍-- vol.ῌ p+uvt"+ ¤ kP:;¥ ¦t?J%D FHJl %m + mm ,wM "#$uhp<eJg §j¨©%"CDn $J

 - vol.ῌ bD FHJl%m + mm

ºf#$PQ4z»¼ + vol.ῌ   *./ mm  *.- mm  

 ! " - vol.ῌ

(No. .0) #&  '() $%! "*+, - .  /MgO OH+ 012, 

-/ vol.ῌ  , mm  *+  -* vol.ῌ !<=GT'

1\?G

MgO/ wt.ῌ JO1K

9d4

G. OH 2D d48#efMCD p&0LDMq

G

,+ MgO2 wt.ῌ JS

6PQ7GO(Fig. .a῍.g) 2D

8  . 34

  "  

S

Fig. . O.  S 6PQ7 d|V=0sPtuv, vol.ῌ    
 

91

d4

Sr, Cr, Ni O)K S 6PQ7 d|V=0s

Ptuv9d4o5G*+`;(Figs. .i, .m, .o) 56S 6PQ7 d4 MgO ?H84oN78, , 95:+;  

}O)K OP\WX?G6N (Fig.

   + mm  . vol.ῌ  <=>?>@5ABCDEF0CGH

,) TOc8:7 Fig. / O.  37O *+>O*K MgO 5B MgO/./ wt.ῌ J

IJK) 

 *./ mm OQ

 

+ vol.ῌ  

(No. +/῍+3) ,  # & L E OP`¡¢;7012

 MgO..1῍/./ wt.ῌ 5m. 

2D d4  OP ¡¢;701

8  ! "*+, - 

2 (No. ,*῍-+) O)K  £RS^¤

. TM¥4<q O},B MgO,.2῍-./ wt.ῌ 5m ¦/ 

p£8¡¢;7TM¥ (No. -,῍-.)MgO,.0῍,.1 wt.ῌ 5m . !"MN Table , O.  Fig. - O#$PQ4%
 S

 (U)

o (No. -/῍.-)g%, . [=(+33,) OV*%WX?G++*, -, 012Y< (No. ..῍.0) 8

 34

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R&.

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0sPtuv9d4§¨ +3-3 U4P8%4^_),.

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f ,- 12g

(Table .) .h&O1\?G7

012(U) (+33,) OV!"734012 SiO, .2..῍0../ wt.ῌ Oi

7 4Pfjkl8O 5m5n,  S 6P
Q7 d4o p8%%q%%4>+VO©W(*Y+  8­ ®Z¯[\J MgO ,5_) +²QT4!", (MgO0.- wt.ῌ, Fig. -d) S 6PQ7 d !]O,(Fig. .)

34 2D

dr; DC0CG 0sPtuv9dr; 3
4%wQ4 xy4z:;*+%%4<87+ 012 (No. .0 ; Fig. -a) {34

 ,|

0 



Table - O^_," . Table . Oef`a |V=#ef2g .  Fig. 0 O

0sPtuv9d482D d4O |V=G

SiO, O}

O|V=G"µ@ . ~?15=

(SiO,//῍0+ wt.

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SiO, {,+:;,34Hz

>?H

F218 SiO, @OA) Na, O, K, O, Rb, Ba,@ CaO

0ῌ+ 

YZ!"7012 (No. +)   "l8b8 0 !]O5:+;,

5( )7: ¶K.7 µB ;+T'

23῍3. 8·)E @

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Y, Zr, Nb

³fGF´ Fig. 1(Figs. 0, 1a)F21F n, ?H.  S 6PQ7  µ Mg#

d4  SiO, GB,?HOHK Al, O-, MgO,

1b) µ

Sr, Cr, Ni ?H (Fig. 1c)

 0sPtuv9d4  TiO,, FeO* O)KI.  AnAn00῍3- 8¸) 5 10῍2- 8·)5m.

(Fig.

Fo1-῍2/ µ Mg# 8¹75m. Table ,.

Whole-rock chemical composition of the Torishima volcanic rocks.

92ῑ῔ ῎ῌῒ῍῏῕ῌῐΐ ῎

93

Fig. -. Harker’s variation diagrams for Torishima volcanic rocks. Open symbols indicate the data from Tokyo
Metropolitan Government (+33,).

ῑ῔ ῎ῌῒ῍῏῕ῌῐΐ ῎

94Fig. .. Variation diagrams for major and trace elements versus MgO. Symbols are the same as in Fig. -. Brokenlines are the mixing lines proposed for the caldera-forming stage (see text for details).

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(Fig. 0) 8  h +,i<)( j2

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8

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2, ,) (Fig. 1i)1 

/q" Table + .ML/Xrbk-.s. t0 A u vBOweeI AU
wVu

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GHI 5 !t0weeI.67
8!`'<=)* t0xt0]y.9 I '<=) (Figs. -, .) 5 t
Fig. /. Variation in MgO content in the stratovolcano lavas exposed at cli# of Hatsunezaki.

0weeI  1* t0xt0]
y.  +8` N)'J)  /Xrbk /q"  3

0ῌ, 

:+.%2)

 (No. -+)  An0/῍3,   (No. +) (Fig. 0) "#$%

1ῌ+  



Xrbk



 !  An12῍3, An01῍11 

&'



.

;'<8. -*vol.ῌ =;.K (Fig. 3a) MgO

o.UV  8 fb ') (Figs. 3b, 3c)() (Fig. 1d) *+,) -./0.1

KL MgO2 3'4 5367 8

'M (Fig. .a) Al, O-, Sr '?) (Fig. .c, .i) ( 98 ,)'

Mg# ' 2. 

.'M2 SiO, 'mn*>,) .

= @. @+. AAOB +322 ; CO:  02῍03  , ;'<=) 53

B +322B D. AEOC ,**,B 

/0 Mg#03῍1+ ! >?@67. (Takada et al., +33,) F#G.   (Fig. 1e) 8t0



Mg#  0,῍0/ /0.1267 (Fig. 1f) 8 
A 

Mg#02῍03B  8 

CD(Lindsley, +32-)  2**῍3**E  (Fig. 2) /0 'FD.GHIJ) KL  M2N)
8

O8



/0

.%2 QUILF(Frost and Lindsley, +33-) LPQRS 3,/T.*E

CDML UV  

Fo21 ,) (Fig. 1g)

MgO



A



An1,῍o. CrONi ') (Figs. .m, .o)  UV O8 fb

ª«¬­L ®"  No. +   L RS No. + .,..ῌ ¢ UVAn2,῍3/ ^? /0  An0+῍20



M 



H_ UM³O´P,)  PQRSKL No. + =? UV O8 µ¶J¯ * No. +. ' H_J¯L 4 (Figs. 0, 1h) .'M2(  An00 :.

/.,ῌ 

4 1fb

3 NK V O8-..ῌ 8

MJ¯) No. +. .?:°¡) (Table /) ,- $ \ .%&]  (Table -) (No. -0)  (No. +.,

MgO/.3 wt.ῌ)!±² Xrbk 

§

(No. +, MgO1.3 wt.ῌ) (= §¨©L.)XdVXPQ § 

1.B  WVX !" #Y. ZJL [WVX
0ῌ-

3I£M2)¢=) h ¤¥¦JK (Fig. ,)"7$%

H_'¡L¢=) *'_)47

.`M2 An3* : An0/ :·¸)´P,)
1ῌ, 



Xrbk   ¹bUbº u   96

 (Fig.

-)   !"#$%&(Fig. 1d) U3V  !">WXWY

Z[\]^_9`Da(No. -+) '()*+,-./0+

$%& No. -+ bc] OPQ:;GH

3  !"+%456

1Hd=e/fgh)*ijk (Table .) l7 (Figs. -aῌ-g) 89 $%& No. -+ ':;

@ 2**ῌ3**m nopqDrsGH (Fig. 2) b12

 <=+> Ca ?@ ABCDEF

GH[tuvwGHxGH U3`yNBCz {;h|WXW `7zwkfgh}DOPQGH (AnI01ῌ11)  , R>ST&yz~67  !" SiO, (AnI12ῌ3,) 

Table -.

<=+> Ca JJKLM

Representative compositions of phenocrysts and microphenocrysts in the Torishima volcanic rocks.

¹º»¼568-¯­tu

97

Table .. Compositions of melt inclusions in plagioclase and groundmass glass for sample No. -+.

  



 SiO,0/῍1- wt.ῌ  !"#$ No. -+ %&' Ca %()*+,
- (An12῍3,) .,/- (Mg#2.) 0""-(Fo21) 12

 3 4567 8 (No. +)9:1;2(Fig. 1a, 1b, 1c) <=+>% Fig. .  -?

<@AB8-CDEFGCHIJ%KL< MgO2 wt.ῌM3 4567N O8-PN LQR"S<TUL V  WX Y+3 4567N O
8- (MgO2 wt.ῌ, SiO,.2῍.3 wt.ῌ) P HI%Z[L<\] +@AB8-QR"SC MgO2 wt.ῌ 'KL< Ni <
Cr 3

4567N

O8-^X_`abc<+ d3 4567N O8-<

Fig. 0. Histograms of phenocryst and microphenocryst plagioclase composition.

 % P n
 !"#$ No. -+ 9

@AB(Table .)  k-?

HI8Z[¡¢£

8-HI  0" "-e fg$n¤ (Beard and Lofgren, +33+) opq8¥$Q

1Ph+ijP\]



Vk%lmn  PHI

<%opqd<CrsL Ba,Nb, Zr 

tuvwx91;%v`y"z%#Q (Fig. ++) % Fig.

++ %¦@AB8-Grq!

 §J SiO,1- wt.ῌ ¨ ©QR"S%#Q dª«"{@|}$~<`'q D1

Fig. -  Fig. . %¬PY`q  d~ <+ ` ` 0$ N O8-< 3

+FG­QR"S<"®`+¦

4567N O8-< dy"z"{@|}$~h+ (Figs. +*a, +*b) 'q

3 4567N O95O8-  MgO2 wt.ῌ 3@AB8-<-?
4567N

O8-

1ῌ- 

@AB

8- SiO,-FeO*ῌMgO  (Miyashiro, +31.) @AB8-% ©<D1C
rL8-¯­°±P²%#
©CrsL°±%VkXP '+RyQQR"S<%h+^ 0$



$0%#Q (Fig. ++)<`´µ` *.- mm V1¶+¦ !"#$

D1V



<`

@AB8-91; ³ P$1

No. -0 9,-.,/-²% ·7<`¸

{ |}!~}% |98

Fig. 1. Core and rim compositions of phenocrysts and microphenocrysts. Solid squares : phenocryst, crosses :microphenocryst, open squares : plagioclase phenocryst with shieved texture.



 (Figs. 0, 1h, 1i)  Zr vs Ba/Zr  NbῌZr  !
'./"#$%&'0( ) Zr

!12 Zr*+,-&*+345/0

! 67 819:;< =>?< @AB C
DEFGHI9 (Figs. +*a, +*b) HJCK3 (Fig.
-)  , -&'./ 0345/ 0

!1LC TiO,, FeO*, Sr, Cr, Ni

MNO<PGQR

S / 9 TU VWV:;DEXY  ! ZC[\]^_`91abJI9
cd  ! SiO, vs FeO*/MgO , -&'./ 0 !1LCcefQgGhi
Fig. 2. Chemical compositions of pyroxenes in sample
No. -+. The isothermal lines are from Lindsley(+32-). Crosses : phenocryst core, open circles :phenocryst rim.

GQRS jCkb9 (Fig. ++)  K3
 TiO,, FeO*, Sr, Cr, Ni C\^_ , -&'./
0 !345/ 0 ! lm:;<CnJ` (Figs. -a, -c) HJC MgO vs Oxides , -&'./ 0 !1LC 345/ 0!1o8p, -&'./ 0 !YlmO<qJ
rIGQRS / 9 (Figs. .aῌ.g) sIJtu  !VWVvpC 8wlxyzY./¡@»¼

99

EFGHID
JK=* ,L 454M
N ABE#O῎8P , QRSTUNGHI
D

V3 + QWXY Z=P454A'$[\]^ Fig. +, _ `a/ bcdR\ No. -0e 6 !)=f_ `6ghi,RSjkCD)* ῔l+῏ABmno3 +,-./01,2A'pqr454 (MgOs/ wt.ῌ) tI u TiO, vFeO* i,wQnx@y,A454 bn 454 X z{3|eb}6~e 454

X  \[ 1,2[ck454345

4MND i,AoY@ WX3

(Fig. +,)  ῔RSTo῕@]^P => =*0@$[\3| GH2ῌ ῎ ῍ ῏./. - Q. A
)=f  v '¡A¢>EF J<It>
+e +,-./01,2 "#£¤pq3* V,Lpq454ῑ¥*3 !
"#¦§)=f d dv_ `"#AB¥M¨3WX3
,e \[ 1,2 d d"#©¤[ck3* V,L MgOs2 wt.ῌ +,./01,2pqr454 ª«ῌ¬­®!
Fig. 3. Variation diagrams for modal compositions
versus MgO. Symbols are the same as those in
Fig. -.

r454MN3WX3
-e 6¯.°±²³2 bΐ´/ +3-3 µ¶ e

 ·"# £¤pqra/3* V¡@¸454MNABE#O῎῔l
454A'

῍
c)

MgO 

῍

MgOs/ wt.ῌ+,-./01,2pqr454!)=Al, O-  (Fig. .

f_ `+0ABE#O῎P3¹º nx!"#$%&' ()* (Fig. 3a)@u TiO,vFeO* y,A454 \[ 1,2+,-./01,234546 7ῒ8 !

[ck454MN3 i,A"# 4549:;< => ?ῐ@ ABCD

WX3

100

ῑ῔ ῎ῌῒ῍῏῕ῌῐΐ ῎Table /. A result of least square calculation for fractional di#erentiation. Plag :plagioclase, Oliv : olivine, Cpx : clinopyroxene. RSS : residual sum of squares.

Fig. +*. Variation diagrams for Ba/Zr and Nb/Zr
versus Zr. Symbols are the same as in Fig. -.
Dotted line is hypothetical mixing curve.

Fig. ++. Variation diagrams for FeO*/MgO versus
SiO,. Small dots are melt inclusions of plagioclase in sample No. -+. Other symbols are thesame as in Fig. -. The discrimination line isfrom Miyashiro(+31.). CA : calc-alkali, TH :tholeiite. Shaded ﬁeld represents selected rhyolitic melt compositions from dehydration experimental data (Beard and Lofgren, +33+). Dottedline is hypothetical mixing curve.

k$l=>8n

101ῌ ῑ ῐ ῍Fig. +,. Variation diagrams for TiO, and FeO*
versus MgO showing relationship among volcanic
rocks of stratovolcano stage, caldera-forming
stage and central cone stage. Pl : plagioclase,
Cpx : clinopyroxene.

῏῎

 

%&'()*+.. !"#$,-. /0 X 1

23-4 5678 89 : ;<8>?@<8

()*+

,-

=
. ,

ABC4D EF,- . G
HG IJKLMNOP4 7FQRSTU,- . VWXYQZ[D -,.

Beard, J. S. and Lofgren, G. E. (+33+) Dehydration melting
and water-saturated melting of basaltic and andesitic
greenstones and amphibolites at +, -, and 0.3 kb. J.
Petrol., -,, -0/ῌ.*+.
Frost, B. R. and Lindsley, D. H. (+33-) Equilibria among
Fe-Ti oxides, pyroxenes, olivine, and quartz : Part II.
Application. Amer. Mineralogist., 11, +**.ῌ+*,*.
\]^_`^ab !^cdHe^f"gh^#ijL (+322) k$7l +320 %&l&m'=>8n(o. l -- S,-.-S,/..
p )^q* r^sH+^pt u (+3/.) v,l w x. yz{- +3 +/ῌ,-.
|}.~/ (+3*,) 01.
8 +. 0-*ῌ0.*.
k^7 2^c ~^7 ^ 8^ ! ^  ^ 3 ^ c (,**-)
l ,**, %&l. l .2 ,-/ῌ,-3.
4b (+3*-) ~%5l&m
-. z67- .- .-ῌ.2.
a ¡ (+3*-) 01. 8 +/ ,++ῌ,,* ,30ῌ-*..
Lindsley, D. H. (+32-) Pyroxene thermometry. Amer. Mineralogist., 02, .11ῌ.3-.
Miyashiro, A. (+31.) Volcanic rock series in island arcs
and active continental margins. Amer. J. Sci., ,1., -,+ῌ--/.
¢£^8¤9b^¥j¦ (+322)+320 %k$7
l&m'§8¨)O©ª'¨. l -- S,//-S,0..
«¬:;^|}.~/ (+3*-) 01­-. z67- .- -/ῌ.+.
7®<= (+3*,) 01­-. 8 +. 00/ῌ01.
1,2ῌ1-1 2+3ῌ2,0.
Takada, A., Oshima, O., Aramaki, S., Ono, K., Yoshida, T.
and Kajima, K. (+33,) Geology of Aogashima Volcano,
Izu Islands, Japan. Bull. Volcanol. Soc. Japan, -1, ,--ῌ,/*.
¯> r (+3/+) v,l-. - ,/.1ῌ//,.
¢°~ (+3.*) L? +. %@&l­-.
x
 .1 -12ῌ.*-.
v, (+33,) k$±)²l&l(x³©l7´µAI  8. -1. p.
¶h\·^; ¸¹ (,**,) 5º«l»¼»
§. l .1 /1ῌ1,.
IJ KL