PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - /...
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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+ #·§�¸$¹ 0�+*�+¨©Y�Y�ª�� f´%º³�»¼Department of Earth and Planetary Sciences, Gra-
duate School of Sciences, Kyushu University, 0�+*�+Hakozaki, Fukuoka 2+,�2/2+ Japan.
Corresponding author : Takeshi Sugimoto
e-mail : [email protected]
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Fig. +. (a) Map showing studied area. (b) Map showing sampling localities. Numerals correspond to sample
number of whole-rock bulk chemical analysis. Topographic map of Torishima, + : ,/,*** in scale (Geo-graphical Survey Institute) is used.
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Table +. Volcanic history of Torishima volcano.
Fig. ,. Sketch of the stratovolcano volcanic rocks at cli# of Hatsunezaki. Numerals correspond to sample number
of whole-rock bulk chemical analysis.
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Table
,.W
hole
-rock
chem
ical
com
posi
tion
of
the
Tori
shim
avolc
anic
rock
s.�� ������� �92
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Fig. -. Harker’s variation diagrams for Torishima volcanic rocks. Open symbols indicate the data from Tokyo
Metropolitan Government (+33,).
���������� 93
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Fig. .. Variation diagrams for major and trace elements versus MgO. Symbols are the same as in Fig. -. Broken
lines are the mixing lines proposed for the caldera-forming stage (see text for details).
�� ������� �94
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0�, ������������� (No. -+)��� ������� �
An�0/�3,�� ������ (No. +)����������� (Fig. 0)� ��� !�� An�12�3,����"�#�$%��� An�01�11�� &' (�) (Fig. 1d)���*+�,)� -./0�.1�2� �3'4��� 53�6��7����� ��8���9����8�,)'� ���Mg#' 2.�:������ 02�03��� ,;�'�<=�)� 53�/0��Mg#�03�1+�!�� >?@�6��7���� (Fig. 1e)� ��8����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�)��8O��8��/0�� .%�2� QUILF
(Frost and Lindsley, +33-)���LPQRS�� 3,/T.*E��CD���ML� U�V������ �Fo�21�,) (Fig. 1g)����"7��$%��� A��� An�1,�
1.B ��WVX !"�� #Y. ZJ�L[WVX,-$��\� ���.%&�]� ��� (Table -)�
0�- �� ����������� (No. -0)��� ������� �
An�2,�3/�^?� /0�� � An�0+�20����4����� (Figs. 0, 1h)� ��.'M2(���� An�00�:.��� '_��)4��7���M� ����.`M2� An�3*�:� An�0/�:
.)��abc�$%defbg�!� *���(Fig. 0)���8�� h�+,�i<)(���j2akl�8�,��h����Mg#� 0*�1+��mn4������ KL� �� (An�3.)��o._��M2p�)��8���8�� ���Mg#'2,�,) (Fig. 1i)�
1� � �-.���/q"�� Table +.�ML/�Xrbk
�s.� t0� A� ����u vBOwe�eI AU�wVu vBOt0�xt0�]y�� A�z�{|}1vB �� �~'2M?31�)� M(M4���� �GHI��� 5 �!t0��we�eI.�678!`�'�<=�)*��� t0�xt0�]y��.�9��I���'�<=�) (Figs. -, .)� 5 �t0��we�eI��� 1��*��t0�xt0�]y��.�� �� +8`����N�)��'��J�)� ����� �/�Xrbk�/q"�� 3�����:+.%�2���)�
1�+ ��� ����������Xrbk���.����;'�<8. -*
vol.����=;.�K� (Fig. 3a)� MgO����o.U�V�� ��8fb�'���) (Figs. 3b, 3c)�KL�MgO���.'M2� SiO,'mn*>�,)�.'M (Fig. .a)� Al,O-, Sr'?��) (Fig. .c, .i)��(=� �@�.��@+.�� AA�OB� +322 ; �COB� +322B�D�.���� AE��O�C� ,**,B� ��.�� (Takada et al., +33,)F�#G.� ����t0���� �H_'�¡L��¢=�)� *��MgO����o. CrONi'���) (Figs. .m, .o)���� U�V�O��8fb��3��I£M2�)��¢=�)� h��� ¤¥¦JK (Fig. ,)�§������ (No. +, MgO�1.3wt.�)(=� §¨�©L.�)�XdV�XPQ�� §������ (No. +.,
MgO�/.3wt.�)�ª«�¬L�®"� �� ����No. +������� ���L� RS�� No. +.�� ,..���¢� U�V� -..�� ��8 /.,�� MJ¯)�� No. +..�?:°�¡) (Table /)� �!±²� �Xrbk���4���� 1��fb�� �3��� ���NK������H_�� U�V�O��8� M�³O´P�,)� ��PQRS�KL� No. +���=?�U�V�O��8�µ¶J¯� *�� No. +.'�����H_J¯L� �·¸�)���´P�,)�
1�, ������������Xrbk���� ¹bUbº��� ����u
Fig. /. Variation in MgO content in the strato-
volcano lavas exposed at cli# of Hatsunezaki.
�@-.�����8»¼ 95
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�������� �� ��������� (Fig.
-)� ��������� �� !"�#��$�%&(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���$�%&No. -+�bc] O�PQ�:;�GH�
1Hd=e/�fg�h)*�ijk (Table .)�l@ 2**�3**m�nopq�D�rs�GH (Fig. 2)b[tuvwGH���xGH������ U3�`�yz {;h|��WXW� `7z�wk�fg�h}&�yz���~�67���� !"���� SiO,�
Table -. Representative compositions of phenocrysts and microphenocrysts in the Torishima volcanic rocks.
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������� ������ ���������������������� SiO,�0/�1- wt.����� �� !"#$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�¶+¦� !"#$No. -09�,�-���.,/-�²%� ·7<`�¸
Table .. Compositions of melt inclusions in plagio-
clase and groundmass glass for sample No. -+.
Fig. 0. Histograms of phenocryst and micropheno-
cryst plagioclase composition.
¹º»¼56�8-¯tu 97
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���������� �� (Figs. 0, 1h, 1i)� ��� Zr vs Ba/Zr���Nb�Zr����� ������ ! "#$%&' (�� ) 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
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���������� � 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
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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
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� ������������ ���� ����������������������� ..!��"#$%&��'�(��)*��+,-. /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