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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, the

caldera-forming stage and the central cone stage, respectively. Volcanic rocks of the stratovolcano stage are

characterized by basalt containing abundant plagioclase phenocrysts. The chemical variation of this stage in

oxide-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

�2wt.�. They also show di#erent Ba/Zr and Nb/Zr ratios from stratovolcano-stage basalts, indicatingcontamination 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 are

classified 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 daciteand 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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¦2//�*2.- ��§B� ,�/0.-�,3¨©Y�Y�ª��«¬ª�­f®���{«¬¯"°±Institute of Seismology and Volcanology, Faculty of

Sciences, Kyushu University, Shin’yama, Shimabara

2//�*2.-, Japan.z²­ : ¦21.�*3*- � §!9� -*22�+10��Y���«¬³�­f´"�«¬µ¶Present address : Institute for Geothermal Sciences,

���Kyoto University, Noguchibaru, Beppu 21.�*3*-, Japan.¦2+,�2/2+ #·§�¸$¹ 0�+*�+¨©Y�Y�ª�� f´%º³�»¼Department of Earth and Planetary Sciences, Gra-

duate School of Sciences, Kyushu University, 0�+*�+Hakozaki, Fukuoka 2+,�2/2+ Japan.

Corresponding author : Takeshi Sugimoto

e-mail : sugimoto@bep.vgs.kyoto-u.ac.jp

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Table +. Volcanic history of Torishima volcano.

Fig. ,. Sketch of the stratovolcano volcanic rocks at cli# of Hatsunezaki. Numerals correspond to sample number

of whole-rock bulk chemical analysis.

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Table

,.W

hole

-rock

chem

ical

com

posi

tion

of

the

Tori

shim

avolc

anic

rock

s.�� ������� �92

Fig. -. Harker’s variation diagrams for Torishima volcanic rocks. Open symbols indicate the data from Tokyo

Metropolitan Government (+33,).

���������� 93

Fig. .. Variation diagrams for major and trace elements versus MgO. Symbols are the same as in Fig. -. Broken

lines are the mixing lines proposed for the caldera-forming stage (see text for details).

�� ������� �94

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�s.� t0� A� ����u vBOwe�eI AU�wVu vBOt0�xt0�]y�� A�z�{|}1vB �� �~'2M?31�)� M(M4���� �GHI��� 5 �!t0��we�eI.�678!`�'�<=�)*��� t0�xt0�]y��.�9��I���'�<=�) (Figs. -, .)� 5 �t0��we�eI��� 1��*��t0�xt0�]y��.�� �� +8`����N�)��'��J�)� ����� �/�Xrbk�/q"�� 3�����:+.%�2���)�

1�+ ��� ����������Xrbk���.����;'�<8. -*

vol.����=;.�K� (Fig. 3a)� MgO����o.U�V�� ��8fb�'���) (Figs. 3b, 3c)�KL�MgO���.'M2� SiO,'mn*>�,)�.'M (Fig. .a)� Al,O-, Sr'?��) (Fig. .c, .i)��(=� �@�.��@+.�� AA�OB� +322 ; �COB� +322B�D�.���� AE��O�C� ,**,B� ��.�� (Takada et al., +33,)F�#G.� ����t0���� �H_'�¡L��¢=�)� *��MgO����o. CrONi'���) (Figs. .m, .o)���� U�V�O��8fb��3��I£M2�)��¢=�)� h��� ¤¥¦JK (Fig. ,)�§������ (No. +, MgO�1.3wt.�)(=� §¨�©L.�)�XdV�XPQ�� §������ (No. +.,

MgO�/.3wt.�)�ª«�¬­L�®"� �� ����No. +������� ���L� RS�� No. +.�� ,..���¢� U�V� -..�� ��8 /.,�� MJ¯)�� No. +..�?:°�¡) (Table /)� �!±²� �Xrbk���4���� 1��fb�� �3��� ���NK������H_�� U�V�O��8� M�³O´P�,)� ��PQRS�KL� No. +���=?�U�V�O��8�µ¶J¯� *�� No. +.'�����H_J¯L� �·¸�)���´P�,)�

1�, ������������Xrbk���� ¹bUbº��� ����u

Fig. /. Variation in MgO content in the strato-

volcano lavas exposed at cli# of Hatsunezaki.

�@-.�����8»¼ 95

�������� �� ��������� (Fig.

-)� ��������� �� !"�#��$�%&(No. -+)'��()*�+����,-./0�+��12�3� �� !"����+��%45�67� (Figs. -a�-g)� 89 $�%&No. -+'�:;��� <=+�> Ca�?@ ABC�D�EF GH(AnI12�3,)� <=+�> Ca�JJKLM NBC�D�O�PQ�GH (AnI01�11)� ,R>ST��

(Fig. 1d)� U3��V� �� !"���>WXWYZ�[\]^_9`��Da���$�%&No. -+�bc] O�PQ�:;�GH�

1Hd=e/�fg�h)*�ijk (Table .)�l@ 2**�3**m�nopq�D�rs�GH (Fig. 2)b[tuvwGH���xGH������ U3�`�yz {;h|��WXW� `7z�wk�fg�h}&�yz���~�67���� !"���� SiO,�

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

�� ��������� �96

������� ������ ���������������������� SiO,�0/�1- wt.����� �� !"#$No. -+%&'��� Ca%�()*+,�- (An�12�3,)� .,/- (Mg#�2.)� 0""-(Fo�21)�1�2���� 3�45678 (No. +)

9�:�1;��2�� (Fig. 1a, 1b, 1c)<=+��>%� Fig. .��� -?���<�@AB�8-CDEFGCHI�J%K�L�<� MgO�2 wt.��M�3�4567N�O�8-PN�L�QR"S<TUL�� V� �� WX �Y+3�4567N�O�8- (MgO�2 wt.�, SiO,�.2�.3 wt.�)P� HI�������%Z[L�<\]���� +�� �@AB�8-�QR"SCMgO�2 wt.�'�K�L�<� Ni<Cr�3�4567N�O�8-�^X_`ab�c<+�� d��3�4567N�O�8-<� �@AB�8-�HI������� 0""-efg$��1�Ph+�i�jP\]����Vk%l���mn�� ���������PHI�������<���%opqd<CrsL�� Ba,

Nb, Zr�� �tuvw�x�9��1;�%v`y"z"{@|}$~�<`���'�q�� ��D1������P�Y`q��� d���~����� �<+������� ` `� 0$�N�O�8-<� 3�4567N�O�8-<��� d��y"z"{@|}$~���h+� (Figs. +*a, +*b)� 'q� �@AB�8-� SiO,-FeO*�MgO��� (Miyashiro, +31.)���3�4567N�O���9�5����O�8- �+��R�yQ��QR"S<���%h+^� 0$��$0�����%#��Q��� (Fig. ++)���D1��V��������������<`

�� �����%��������P������ n � !"#$No. -+9��������-?� (Table .)�� k��HI�8Z[���¡¢�£n¤ (Beard and Lofgren, +33+)�opq��8�¥$Q������%#��Q��� (Fig. ++)� ��% Fig.

++%�¦��@AB�8-�� G�r��q!������ §���J� SiO,�1- wt.��������¨ �©�QR"S�%#��Q���� d�ª«��Fig. -��� Fig. .%�¬�� �@AB�8-<-?�� MgO�2 wt.��3�4567N�O�8- �+�FG­QR"S<"®`+¦�

1�- ���������� �@AB�8-%�� ���©�<��D1��CrL8-¯­°±P²%#��������©�CrsL�°±%�Vk�X�P��� '�� �@AB�8-9��1;��� ³�P$�1<`�´µ`� *.- mmV���1�¶+¦� !"#$No. -09�,�-���.,/-�²%� ·7<`�¸

Table .. Compositions of melt inclusions in plagio-

clase and groundmass glass for sample No. -+.

Fig. 0. Histograms of phenocryst and micropheno-

cryst plagioclase composition.

¹º»¼56�8-¯­tu 97

���������� �� (Figs. 0, 1h, 1i)� ��� Zr vs Ba/Zr���Nb�Zr����� ������ ! "#$%&' (�� ) Zr *+,-&'./0� !12 Zr *+3�45/0� ! 6781�9:;< =�>�?< @AB �CDEFGHI9 (Figs. +*a, +*b)� HJCK�3�� (Fig.

-)��� ,-&'./0���3�45/0� !1LC� TiO,, FeO*, Sr, Cr, Ni MNO<P�GQRS /�9� T��U� VWV�:;DEX�Y� ������ !�ZC[\]^_`91abJI9�cd� ������ !� SiO, vs FeO*/MgO���,-&'./0� !1LCce�fQg�GhiGQRS ���jCkb9 (Fig. ++)� ��� K�3���� TiO,, FeO*, Sr, Cr, NiC\^_� ,-&'./0� !���3�45/0� ! lm:;<�CnJ�` (Figs. -a, -c)� HJCMgO vs Oxides����,-&'./0� !1LC� 3�45/0� !1o8p�,-&'./0� !YlmO<qJrI�GQRS /�9 (Figs. .a�.g)� sIJ�tu�� ������ !�VWVvpC� 8wlxyzY

Fig. 1. Core and rim compositions of phenocrysts and microphenocrysts. Solid squares : phenocryst, crosses :

microphenocryst, open squares : plagioclase phenocryst with shieved texture.

Fig. 2. Chemical compositions of pyroxenes in sample

No. -+. The isothermal lines are from Lindsley

(+32-). Crosses : phenocryst core, open circles :

phenocryst rim.

{� |}!~��}�% |98

���������� � MgO��� �������������� Al, O-������� (Fig. .

c)� !�"#$�%�&' �(��)* (Fig. 3a)�+,-./01,23�4546 7���8� !�"#�� 454�9:;< =>�� ?�@ ABCD

�EF�GHID�JK=*� ���������,L���� 454MN� ABE#O��8P ,Q�RST��UN�GHID� 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.�)tI� u TiO,vFeO*i,�wQnx@y,A454 bn 454 X�e �z{���3| b}6�~������e���454X�� �\[�1,2�[�c�k454���3454MN���D�� ���������i,A��oY@ ��WX3� (Fig. +,)� ��� �RST��o�@�]^��P�� =>�� =*�0@�$[\���3|�GH�

2� � � �����./��.��� -Q��.���� A������������)=f� ��v� '¡A¢��>�EF� J<�����It��>��

+e +,-./01,2����� "#�  £¤pq�3�*� V�,L�pq�454�¥*3� !�"#�¦§)=f� �d�d�v_ `�"#�AB¥��M¨3��WX3��

,e �\[�1,2����� �d�d�"#�©¤[�c�k3�*� V�,L�MgOs2 wt.��+,-./01,2�pq�r454�� ª«¬�­�®!r454�MN3��WX3��

-e 6¯.°±²³2��� b´/ +3-3µ�¶ e�� ·"# £¤pq�ra/�3�*� V���¡@�¸�454MN�ABE#O���l�������454A' ���������� MgOs/ wt.��+,-./01,2�pq�r454�� !�)=f_ `�+0�ABE#O���P��3¹ºnx@�u TiO,vFeO*y,A454�� �\[�1,2�[�c�k454�MN3� ���������i,A����WX3��

Fig. 3. Variation diagrams for modal compositions

versus MgO. Symbols are the same as those in

Fig. -.

����./���¡@»¼ 99

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 plagio-

clase in sample No. -+. Other symbols are the

same as in Fig. -. The discrimination line is

from Miyashiro(+31.). CA : calc-alkali, TH :

tholeiite. Shaded field represents selected rhyo-

litic melt compositions from dehydration experi-

mental data (Beard and Lofgren, +33+). Dottedline is hypothetical mixing curve.

�� ������� �100

� ������������ ���� ����������������������� ..!��"#$%&��'�(��)*��+,-. /0 X1�2�3-4�� 56788��9 :;<8��=>?@<8��� �(��)*��+,-. ,� �����AB�C4DEF,-. ��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/�.*+.Frost, B.R. and Lindsley, D.H. (+33-) Equilibria among

Fe-Ti oxides, pyroxenes, olivine, and quartz : Part II.

Application. Amer. Mineralogist., 11, +**.�+*,*.�\�]^_�`�^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� +/�,-.|}.~/ (+3*,)��01���. 8�� +.� 0-*�

0.*.k���^7��2^c �~^7� �^�� 8^� �!^����^�3��^c���� (,**-)��l� ,**,%&l���. l�� .2� ,-/�,-3.�4b� (+3*-)��~��%5���l�&m�� ���-�. z�67���-�� .-� .-�.2.a� ¡ (+3*-)���01. 8�� +/� ,++�,,*� ,30�

-*..Lindsley, D.H. (+32-) Pyroxene thermometry. Amer. Min-

eralogist., 02, .11�.3-.Miyashiro, A. (+31.) Volcanic rock series in island arcs

and active continental margins. Amer. J. Sci., ,1., -,+�--/.¢ £^8¤9b^�¥j¦ (+322)+320%k$7�l�&m'�§8¨�)O©ª'¨�. l�� --� S

,//-S,0..«¬:;^|}.~/ (+3*-)��01��­-. z�67���-�� .-� -/�.+.7®<= (+3*,)��01­-. 8�� +.� 00/�01.�

1,2�1-1� 2+3�2,0.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, ,--�,/*.¯> r (+3/+)�v,��l���-�. ��-� ,�

/.1�//,.�¢°�~ (+3.*)L? +.%@���&l­-. x�� .1� -12�.*-.�v, (+33,)k$±��)²�l�&l�(x³©l�7��´������µAI � 8��. -1. p.

¶h\·�^; ¸¹ (,**,)5º�«�l��»¼»��§. l�� .1� /1�1,.

��IJ� �KL��

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.

k$��l��=>8n�� 101

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