petrologicalstudyoftorishimavolcano,izuislands,japan, 21 +*+ , ,**. + +3 ,**/ - /...

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ῒΐ /* (,**/) , 21+*+ 腁腌腉腋腂腇腀腄腈腃腊腆腅 , 腈腈 臚膡臈 腈腈腈 臍臷 ,**. + +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, 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 2 wt.. 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 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 dacite and di#erentiated magma with higher TiO, and FeO* content. Key words : Torishima volcano, Izu islands, magmatic di#erentiation, magma mixing, fractional crystallization +ῐ῎ῑ῏ 膇臺臲臷膑膾腡臸膟腠腀腖腛 /1* km 膇臺臌膓膰 膲膑膾腻膄膅腺臎腟膈臭腕腬臉臃膑膾腜腊腬 (Fig. +a, b),**, 腟臲臷膑膾腡 0- 腣腫腟腽腴腽膑腯腍腑 臮膋膑膳膜腜腊腬腍腌 腨腥 膾臱腟臐膑膳腏臘腔 膇臹,**-腅腁 腑腠膑腟臝臲臷膑膾腜腡+3*, 腅腈膑腍腩腢 +3-3 腠腵腺膄膅腼膁臃膑膔臻腏膛腒腭腛腋腬 臯膗+3*, ; 自臑+3*, ; +3*- ; 臙臦臯膗+3*- ; +3*- ; 臵臮膙+3.*腅腁 (+3/+) 腡臲臷膑膾膚腠膚臚膛膺腯膵腌腝 膠腟臬臄臔腯膼臖腓臖臢膑膾臩膦臖腎腪腳膂腹膀臘臖 腯膨腛臮膋膑膳膜膦臖腤腝膉膵腕腬膑膾膔臻膿腯膸腓 (+3/.) 腡膱臬臤腍腩腢臬臄臰膹膩膐腯 腧腝腟 +3-3 腄腐膑腟腍腐腬+.3+* 1 m - 腝膮臛腧腙腗腓腎腓腞腏腪膆臎腠膬腟腍腋腛腡臷膑膾膚腟膘腕腬膚臚膒致臏腟腚腋腛腡膢膎膖膽 腝臟膚臅臖膯臠臡臖腏腮腖腎腟膛膺腒腭腛腋腬腠腦腜 腊腬臸膟臶 (+33,) 腡臲臷膑膾膚.3 腠臟膚臅臖 膯臠臡臖腯膸腓腗腏臧腠腗腌膇臺膲腠膑膾腻膄膅 腺臎腠膑膾臼膑膾膚腏至腳膁腲腾腷膃腰腱腺膧 腈腈 2//*2.- 臷膰臀臐膾 ,/0.-,3 膤臇自膒自膒膊膒膭膞膊臥臬臓膑膾膖臤膭膞腶 膅腸腃 Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin’yama, Shimabara 2//*2.-, Japan. 膱臋臥 : 21.*3*- 腓腊膳膰 -*22+10 膟臶自膒膒膭膞膏臥臬膝膒膭膞臁臜 Present address : Institute for Geothermal Sciences, 腈腈腈 Kyoto University, Noguchibaru, Beppu 21.*3*-, Japan. 2+,2/2+ 膍臀臸膥0+*+ 膤臇自膒自膒膊臬膝臗膏膒臞膴 Department 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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Page 1: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

� �� ��� � /*� (,**/)� ,� 21�+*+�

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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, 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

+� � � ����������� /1* km�� ��������� !"#$%&'()�*+��,-) (Fig. +a,

b). ,**,�%����� 0-�/0%121��3456� 78�9:,-)

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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]

Page 2: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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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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Page 3: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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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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Page 4: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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������ ��� + vol.���� � ����������� *./ mm������ ��� , vol.������ ��������� ���������������������� � !�"����*.- mm������ ��� - vol.���� � �#$�%�� �& ����� �'����()����!�"*+, -� ���. � / +0�12(No. .0)� � ���. ���������3�4���� ��"����� ������ ���*+,�� ���� -/ vol.��� � ���������� , mm�� ��� -* vol.��� � ��56��7��8��,��� ,95�:+; � ������������ + mm������ ��� . vol.��� � ��<=>�?>@�5ABCDE�F0CGH�IJK) � ��������� *./ mm������ ��� + vol.��� � ���������������������, � �#� �& L ����8� �����!�"*+,�� -� ���. �

/� ������!"MN� Table ,O. � Fig. -O#$PQ4%�

R���&�. � S!T'O�)K� (U)(+33,)OV�*%WX?G�++*, -,0�12�YZ�!",8[O1\?G�75� K, O5]-�O^^_),�. `ab./���.�K) � c7� �2D��d�0�12 (No. -+)0��#e�f ,-1�2g� (Table .)�.h&O1\?G�7�!"�7340�12� SiO,� .2..�0../ wt.�Oi7�� 4Pfj�kl8O�5m5n, � S6PQ7�d�4o� p8%�%q 3�4� �2D��d�r;DC0CG� 0sPtuv9d�r;3�4%wQ4�� xy4�z�:;*+%%4<8�7+0�12 (No. .0 ; Fig. -a)�{3�4�� � ,|�0sPtuv9d�4�8�2D��d�4�O�SiO,O} �~�?15=� (SiO,�//�0+ wt.

�)� >x�xy4�z�:;O#�B -F2�1�SiO,��{,+���:;�,34Hz�>�?H��5���( ��F2�18� SiO,@�OA)� Na,O, K,O, Rb, Ba,

Y, Zr, Nb@��� CaO�B � �;+�T'CD�,G����. � �E� ��T'O�)KF2�1F� n, ?H���. � S6PQ7�d�4��� SiO,�GB,?HOH�K Al,O-, MgO,

Sr, Cr, Ni�?H���� � 0sPtuv9d�4�� TiO,, FeO*O�)KI����. �

MgOOH S!<=G�T'��1\?G�Fig. .O. � S6PQ7�d|V=0sPtuv9d�4�MgO�/ wt.�J�O��1����K�G����. �OH�� �2D��d�4�8�#e�f�M�CD p&0�LD��Mq ��G���O�,+�� MgO�2 wt.�J��S6PQ7�G���8�� (Fig. .a�.g)� �2D��d�4� Sr, Cr, NiO�)K� S6PQ7�d|V=0sPtuv9d�4o�5� G���*+`; (Figs. .i, .m, .o)�S6PQ7�d�4��MgO�?H84o�N�}�O�)K� OP����\�WX?G6N (Fig.

,)�TOc8:7��� Fig. /O. �37�O��*+>�O�*�KMgO5�B�� MgO�/./ wt.�J�O�Q � �E� OP�`� ¡¢;70�12(No. +/�+3)� MgO�..1�/./ wt.��5m�. ��2D��d�4�� OP�� ¡¢;70�12 (No. ,*�-+)O�)K� £��RS^¤ p£�8TM��¥4<q O}�,BMgO�,.2�-./ wt.��5m� ¦�� ¡¢;7TM��¥ (No. -,�-.)MgO

�,.0�,.1 wt.��5m���. �0sPtuv9d�4��§¨� +3-3U4P�8%4o� (No. -/�.-)g%,��.�� [= %%4< (No. ..�.0)8� 34�>�+VO©W�( (Fig. -a)� %%4<O�)K� ª«�¬�4Pv9OX ��*Y+���� 8­�®Z�¯[�\J 7:O°¡�±i,)��8 � MgO,5_) +

0�12(U) (+33,)OV ²�QT4�!",��� (MgO�0.- wt.�, Fig. -d)� S6PQ7�d��!]O�, (Fig. .)�

0� ��� ������Table -O^_�,"���.�� Table .Oe�f

`a��|V=�#e�f�2g��. � Fig. 0O�����|V=G����³fGF�´�� Fig. 1

O���|V=G��"��µ��@����. �0�+ ������������YZ!"�70�12 (No. +)�� �"�l8�b��8� ���0�����!]O�5�:+;,)7:� �¶�K.�7� ������µ��� An

�23�3.8·)�E� @�� An�00�3-8¸)�5m��cde�. (Figs. 0, 1a)�������������� µ��Mg# 10�2-8·)5m�. (Fig.

1b)� ��������µ�� Fo�1-�2/���(Fig. 1c)� ���µ��Mg#8¹75m�. �

ºf#$PQ�4�z�»¼ 91

Page 6: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

Table

,.W

hole

-rock

chem

ical

com

posi

tion

of

the

Tori

shim

avolc

anic

rock

s.�� ������� �92

Page 7: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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

Metropolitan Government (+33,).

���������� 93

Page 8: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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

Page 9: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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

Page 10: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

�������� �� ��������� (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

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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

Page 12: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

���������� �� (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

Page 13: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

���������� � 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

Page 14: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

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

Page 15: PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan, 21 +*+ , ,**. + +3 ,**/ - / PetrologicalStudyofTorishimaVolcano,IzuIslands,Japan TakeshiSUGIMOTO,,HidemiISHIBASHI andTakeshiMATSUSHIMA

� ������������ ���� ����������������������� ..!��"#$%&��'�(��)*��+,-. /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