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˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ !...

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῍῎ .3 (,**.) 0 ----/. 腘腞腒腌腏腎腖腓腆腋腈腊腐腟腑腃腀腁腅腢腎腗: 腎腖腜腔腚腢腙腡腂腕腍腇腉腊腛腄腝腠 腶腸膅腽 膌膋腩腛 膃腵腔腟 腴腻膅腼 膊腱 腄腄 膈 腦腮膉 腄腄腄 ,**- +* 1 腹腣腾,**. +* 1 腹腣膇Eruption History in the Post-caldera Stage of Akita-Komagatake Volcano, Northeastern Japan Arc : Correlation between Eruptives Constituting Volcanic Edifices and Air-fall Tephra Layers Akihiko FUJINAWA , Masayuki IWASAKI , Kyoko HONDA , Akemi NAGAO , Takeshi WACHI ῌῌ and Shintaro HAYASHI ῌῌῌ Akita-Komagatake volcano is situated at about -* km west of volcanic front in the northeastern Japan arc. Development history of the volcano is divided into - stages ; main strato cone, syn-caldera formation, and post-caldera eruption stages, in descending order. In order to reveal the comprehensive eruption history in the post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed in the concurring stage. Petrological a$nities such as whole rock major-element chemistry and mineral assemblages including modal abundances of phenocrystic phases are utilized in identifying the contemporaneous eruptives of various modes of emplacement. To avoid the e#ect of phenocryst-matrix separation, coarse-grained tephra samples were selected to the whole rock chemical analysis. The revealed outline of eruption history is as follows : Tholeiitic magmatism has been dominant through the stage, with episodic calc-alkalic magmatism at the final activitiy in the northern region (ca. .,***-,+** y.B.P.). Relatively large eruptions occurred at the beginning of the stage (ca. +-,*** to +*,*** y.B.P.), resulting in caldera and probably large horse-shoe shaped crater as well, rather than forming volcanic edifices. Formation of the northern volcanic edifice began ca. +*,*** y.B.P. Five pyroclastic cones were built up successively, and subordi- nate lava flows or pyroclastic flows accompanied with them. Composition of magma was abruptly changed from andesitic to basaltic around 2,*** y.B.P., and the basaltic volcanism lasted to 1,*** y.B.P. After ca. -,*** years of dormancy, phreatic explosions occurred successively, probably giving rise to horse-shoe shaped crater at the center of the northern area. Calc-alkali andesitic magma e#used to build up a small pyroclastic cone immediately after the major explosion event. After ceasing the magmatism in the northern area, volcanism in the southern area was initiated by phreato-magmatic explosion in the floor of the south caldera about ,,/** y.B.P., followed by vulcanian eruptions, lasting until ca. +,0** y.B.P. About +,/** y.B.P., eruption center was localized to two major vents at northern part of the caldera. Repeated eruptions from the vents have built up Medake and Kodake cones and their associated lava flows. The latest eruption occurred at the summit of Medake in +31* AD. Key words : Akita-Komagatake vocano, eruption history, tephra, basaltic volcanism, island-arc tholeiite 腄腄 -+*2/+, 腨腙腠膁腒 腉腥腯腍膇腍膀腲腑腪膄腐腓腌腍腌 Department of Environmental Sciences, Ibaraki University, Bunkyo, Mito -+*2/+,, Japan. ,.-**+0 腧腷腫腘腚膆腠腵腭腳 腆腇腆腜腬腂腎腃 Asia Air Survey Co. Ltd., Tamura-Cho, Atsugi ,.-**+0, Japan. 腄腄腄 *+*2/*, 腤腵腠腢腖腍腊腳 腤腵腯腍腕腈膁腋腍膀腲腍腕腡 Department of Earth Sciences, Faculty of Education and Human Studies, Akita University, Tegata-Gakuen- Cho, Akita *+*2/*,, Japan. Corresponding author : Akihiko Fujinawa e-mail : [email protected]
Transcript
Page 1: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

� �� ��� � .3� (,**.)� 0� ---�-/.�

�������� ��� ���������:�������������� !"#

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�,**-� +* 1!"#� ,**.� +* 1!"$%

Eruption History in the Post-caldera Stage of Akita-Komagatake Volcano,

Northeastern Japan Arc: Correlation between

Eruptives Constituting Volcanic Edifices and Air-fall Tephra Layers

Akihiko FUJINAWA�, Masayuki IWASAKI�, Kyoko HONDA�, Akemi NAGAO�,

Takeshi WACHI��and Shintaro HAYASHI���

Akita-Komagatake volcano is situated at about -* km west of volcanic front in the northeastern Japan arc.Development history of the volcano is divided into - stages ; main strato cone, syn-caldera formation, andpost-caldera eruption stages, in descending order. In order to reveal the comprehensive eruption history in the

post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed

air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed in the concurring

stage. Petrological a$nities such as whole rock major-element chemistry and mineral assemblages includingmodal abundances of phenocrystic phases are utilized in identifying the contemporaneous eruptives of various

modes of emplacement. To avoid the e#ect of phenocryst-matrix separation, coarse-grained tephra samples wereselected to the whole rock chemical analysis.

The revealed outline of eruption history is as follows : Tholeiitic magmatism has been dominant through the

stage, with episodic calc-alkalic magmatism at the final activitiy in the northern region (ca. .,***�-,+** y.B.P.).Relatively large eruptions occurred at the beginning of the stage (ca. +-,*** to +*,*** y.B.P.), resulting in calderaand probably large horse-shoe shaped crater as well, rather than forming volcanic edifices. Formation of the

northern volcanic edifice began ca. +*,*** y.B.P. Five pyroclastic cones were built up successively, and subordi-

nate lava flows or pyroclastic flows accompanied with them. Composition of magma was abruptly changed from

andesitic to basaltic around 2,*** y.B.P., and the basaltic volcanism lasted to 1,*** y.B.P. After ca. -,*** yearsof dormancy, phreatic explosions occurred successively, probably giving rise to horse-shoe shaped crater at the

center of the northern area. Calc-alkali andesitic magma e#used to build up a small pyroclastic cone immediatelyafter the major explosion event. After ceasing the magmatism in the northern area, volcanism in the southern

area was initiated by phreato-magmatic explosion in the floor of the south caldera about ,,/** y.B.P., followed byvulcanian eruptions, lasting until ca. +,0** y.B.P. About +,/** y.B.P., eruption center was localized to two majorvents at northern part of the caldera. Repeated eruptions from the vents have built up Medake and Kodake cones

and their associated lava flows. The latest eruption occurred at the summit of Medake in +31* AD.Key words : Akita-Komagatake vocano, eruption history, tephra, basaltic volcanism, island-arc tholeiite

&-+*�2/+, '()*+,-./$/01234567/7Department of Environmental Sciences, Ibaraki

University, Bunkyo, Mito -+*�2/+,, Japan.&,.-�**+0 89:;<=)�>?@A@BC�D%Asia Air Survey Co. Ltd., Tamura-Cho, Atsugi ,.-�**+0, Japan.

&*+*�2/*, E�)FG/H?E�./IJ*K/01/ILDepartment of Earth Sciences, Faculty of Education

and Human Studies, Akita University, Tegata-Gakuen-

Cho, Akita *+*�2/*,, Japan.Corresponding author : Akihiko Fujinawa

e-mail : [email protected]

Page 2: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

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Page 3: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

� +� ��������� �������� ��������������� ���� �! "#$%&'�

Fig. +. Index map and topographic outline of Akita-Komgatake volcano. Triangles show localities of Quaternary

volcanoes in the Tohoku region of Japan. Point marked with star (�) indicates the site of drilling (see text).

������( )*+,-./01 234�5 : �67849:;<=>?-@AB 335

Page 4: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

����������� ��� ��� ����������� � �!�"#�$%&'(��)*+�����(,- +31+./%0�123��� ���

-� ��������� �������4� �+31+) � 5����,-6(/"78 /�9:;<=�� >?@AB (+321) � +*(��:;<=� +.(�9:;<=�C�D���EF� � %���(GH� !)#�$I�!A(!9�JKL� MN��(I�78OP� QRCD��ST(RC� >?@AB (+321)�UV�WX�Y �Z����[D\ *( ,Y�]�� +� %^_(`LXab�c(def �ghfZi� 2** m(jk +,./* m

lm8n��o���� � �p%����6( �qrskt� �!�� Xu� vw (sintering)D�xyz{|}8~��o�5����6(%&' �def%&'� ����� ,� def%^���o��!�(�� � %^�8����� de��8i���yD�� �(�!� � !9#�$�����C�L� �defZ@ghf�!��� 0�� ����� � ���������(

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§ ,� ®q¯°f%�(%���C�§�Fig. ,. Geomorphological map of Akita-Komagatake volcano.

?±²@��¤³@�q´µ@¶·¸@ ¹ º@�»¼�336

Page 5: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

���������� ����-�+ �������� (KA) ��� : ����������� !"��# �$�%&'()*+���,�-.��� /0�123142�56� +,./* m78 +,*/* m

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Fig. -. Geologic map showing the eruptives at the post-caldera stage of the Akita-Komagatake volcano. + :Tertiary sediments and volcanics, , : Quaternary volcanics erupted prior to the formation of the

Akita-Komagatake volcano, - : Eruptives in the main strato cone stage, . : epiclastic sediments. Other

abbreviations refer to those in the text and also appeared in Fig. .. Solid curves show the caldera or crater

walls, whereas the broken curve indicates the inferred location of the step on the caldera floor.

�����e�# k����¸¹��¥9��eº : e�,-.�>p"ST»¼�M�mn 337

Page 6: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

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G���!H%�� �'1�IJ�K�.� LM;<��� �� +,-/*m�K�N�� � O� +/m�PQ��RSG���T��U�� �0V �'1�I/�$WXY�� �'1�(9!Z[��%����� +��� (HO+) : ����� (+321) �\ + �

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$£$%�� &'�$��<¤����0��u&'��(!� �'�� �'1�(9+&'��(!¥�¦���%�� �>� .§#¨©{��/�0ª�

« .� ¬�­®1��� ¯°±²³��0�´:�«� LF: ��� PC: �(� PFa : ���(� DK: �1�

Fig. .. Block diagram showing stratigraphic rela-

tions among the eruptives at the post-caldera

stage of the Akita-Komagatake volcano. LF:

lava flow, PC: pyroclastic flow, PFa : pyroclastic

fall, DK: dyke.

�234�56µ¶���·¸�¡738�9¹ º�:»¼;338

Page 7: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

��� +,-/* m���� ��� ,��������� ����� ��� ,������ � ,������������� ���������� (OD) : ��� ,������������������������� �������� � ��� +�������� ������ ��������� ���� !� �!""##$�$%��� ������� � ������ %&!'&#������ '&�'( )*�'+����� .� (HO.) : ,(�)- (+321) �� .����./*+�� �,-01�� 2.&� ��/0 +/* m, ��3%0 .** m�4!56��#71��23���� ��� 489% � 56����,�������2.��7%&!�89�:����� +� �� (NC+) : ;<�=�9%>�?6�����:@� ��;�A�7�B� ,(�)- (+321) � � CC�=��<D +., km���=48 E��FGHIJ ��K&!#� >�� ?��L48 1M&N*� O!� +P4Q� � NC-+

R$� 7S���87TU�VN� -W�L48�1M#@%9�N5!�X7A���� ��1�Y���BZ��[� ��\��C��#87��!�DE���� FG!H�#BZ�]S�N5!^IJ_`������ (KC) : ,(�)- (+321) �����FGHI������`!�� 23������� KLNa�M�bc48��1�=�B� 48����23�d7He��4(f!NO9%� ��� +������N*& \�=��7%&!� ���������89���� ��� +��������� � ghY9WP���*�#56&!!$� ���#�&�@@� �����48#Q@R�N5!�i4��� C��Q����0 +,.** m��� jk��#����� l�C���� +������������He�# ������ ,� �� (NC,) : ����=�mS&� ��\��<D0 .** m����`�� ��4Q� -����T!�2.�n6*�� C���� � C���� NV!�� UoV(#W&@� pXq�V(��p���Y�����#rZ��$%��� \���Ps���tu:#`[� [$��#� �����[v&6*�^IJ_`�� wx�yzIA��\P{]9%� C�� [$|�^}"FG��F_���^IJ#`��

-�, ��������� +��� (HI+) ~�T���� : ,(�)-

(+321)���`a�bc'���� ��A�*+���d''��&!`a�b�*�Te�� ,(�)- (+321) � Cc'X7A��UHe��&!#��� ���6*N*� �M�`a�b C��f�_./ghi�jk��&� �f��� 7A��UHe��TeQ�l&� C�%����d''��_���&6*�� WO[� 7A��87U��`a�b�jk�� � +Uc'#f�&m!4Q����*N95!��N�� �56� C��He�#`a�bfQ�f�^I�N5!� FGHI87w� �%@FGHI���=�2.w���#n+����� �56�Cc'���`a�b� ,c'f� wFGHI�UHe��89�� Cc' � `a�bafQ���0** m������oe��A� ,1- m� +z������# ¡¢F�p56���:� �d<6�������� ,��� (HI,) ~�T���� : ,(�)-

(+321)���`a�bc'�a��*+�� `a�b£k��Xq ~��0 3** m� 9%� ¤¥�q ~��0/** m� ���9¦6� `a�b�*�Te�� §¨A� / m�z������#�����#� rs�¤¥�q� ��#0 +/ m�N�� �� 0** m��¦�tuv�ok�� `a�b� +c'f ���#���:������� (MI) : FGHI��s���� ��0 0**

m, �� -** m, P�0 /* m�%b8©c�;48d7�� ,(�)- (+321) ���He���w������� ����� (ME) : ,(�)-

(+321) �������������c'�\P����!�� +��� (KD+) ~�T���� : ,(�)-

(+321)�����c'���� FGHI�Q���T�*+�� FGHI��=� � A�0 , m�G"&!ªcc'�&6oe��!�� ,��� (KD,) ~�T���� : ,(�)-

(+321)�����c'���� FGHI�Q�xT���� FGHI��=�[`a�b«¬0 + kmf����­d7�� Cc'#� ��� +c'f���­�����®��Te8y�C�9%� C�%�am�89�:�� `a�b«¬��� 1+* m4z�¯�'(�{c°��f��� �� 3** m���Xq� � `a�b� ,c'f� ±�²� ���!�"

��

#³:

$N

�%&'� (KO) : �����FGHI�~|;´����Te�{c°�µ¶¡·(��Y� ��}�����'ª9%N[� ,(�)- (+321) �����H)���g��`��

¸~¹º���� wFGHI�U��¦�H�» : ��,d7He��¼�yzIA�\P 339

Page 8: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

����� (KDP) : ������� ������ ����� ����� (+321) ������������� � ������������ ��� +31*�� (MN) : �� !��"��� ����� (+321) � +31*�1+ ������#$� �"�%&'(�� � �"����������)�*+� �,��� (+31+)� -��. (+31+)� ���/� (+31+)� 01 (+31+) ���2%3 ��� ��� (SC) : ����� (+321) 4 ����������3 � ����� �5678� �������9:�+;� �< - km, =< +./ km�>?@�3 � �����A��9BCDEF� %������G4H��������I��J��K������A� LEF�M���������46�NOP� &�O�Q% +**m�RSTU' � VW6� ��� ���I�!X�"6� ���YZ� 4##[+;NQ%\�]' ^_ -� $`a� &�%KA� bc'T����&�'[(�)d� � 4CD� &&e*����&%f8+� NQ���������A� ,

�+�e*�,';ghij�3 k-l%3 � �������@mA� .n"6� o����p�q���

rs� %� ����e*A� o����p�qTts+Db;BT+'Jb�

.� ���������uv�A�/w�. (+331)�xy�z{|b� }xy�zBU~0���+;� ��?Abc'T��������� -km�� � � +12A� �

+cP

�b+

3�A+

��

������ ������4O 30*m?(�� �� AK-+-BU AK--W��xy�z%�5]' �12���A/w�. (+331)� 6� (+330)�+b�W;� � ,12 ^�3� ���BU� +12� �7�b�4O 2.*m?(a �� � -12 ^��7�b�� ����56�� 4O 1.* m?(a �A� �'�'AK--�AK-+z�~0���+;����/w�. (+331)4 }xy���� �����8 ^VEI�a������G4H9:��I P\�����; +M+� ~0��?(G¡ "(¢�<=���� �JG�/w�. (+331)|b� £¤¥�>� ¦§¨?©� ª«�¬ ��­� %� o­�4#� ®�¨J¯m�°@+Jb� W;� ±²³´�«�µ8zAG

; +� ¶�xy����<l�Table +. Eruptive characteristics of the air-fall tephra.

�BCD�EF·¸�G¹©��HCI�/w º��»¼J340

Page 9: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

��� ����������� ��� ������� ������� ������������ ������������ ��� !"#$�%���&�'()*�+� �����,� AK---AK-+�./0����12�345�6�� 7���� 89�3�:;�<�=���>?@A�$B���C D�EF$�%�G�� �2�24�H��%���I�&���AK-+- : J<K�� L�M�KP� NO��P �+QR��� S��%T LU!� +32,P ��+�� "#"!V*:9�W$�'�%X��&�� OY=�� Z2� ��-Z2%[\� (K-P)� ��� ����� LK,P,

�]% +** cmP� Z2� ��� LK+P, �]% /* cmP4&��� ^_�K,P=�� ��% --,* mm�� ����� L�� 3 : +P 0�� K+P=���% +-/

mm�� ��6`a./�'�AK-+, : '(�� (YP) �+QR��� OY=�%[)�6bcd�%[\ (Y-P)� e*f'��g�h� ��5�� LY,P, �] 2* cmP� Z2� ��-d����� ���i� (Y+P)49��� ^_ Y,P=��+-+* mm��� ����� L�� 2 : ,P 6./�'�AK-++ : �]% ,/ cm�Z2� ��5��6�� ��% +-. mm�Z2� ��6./�'�AK-+* : AK-++�+j��]% +/ cm�Z2� ��5��69� �� +-- mm,��Z2� ��6./�'�AK-3 : ,(�� (AP)�+kl� - +./����0$1�� m�6bc�]�% 0* cm, OY�n� � �������� L�]% /* cm, �� 0 : .P� d�%[\��m��234�o�&��� ����g��0�6./�'�AK-2 : 56�� (HP)�+QR��� �]% +1* cm��-��d7�p�� qOY�rst�%[\� (H-P)�u�j�� ������ LH,P, �] /* cm, �� 1 :

-P� qjY�% +** cm]�d�� 8�� ���i�LH+P, 0�./���]� +- cmP 4&��� H,P=��% ,-,* mm, H+P=���% +-/ mm�0�6./�'�AK-1 : OY�n� ]&% 2 cm�vtZ2� ����]& 2 cm,�9std����� &��9��'w\t%[\�4&��� :�]% .* cm6�� OY ,�=�u�x�� vt� �� L�% +-- mmP� g�hd��� L�% +-. mmP 6./�'�AK-0 : ��% +-2 mm�� �����65���L�� 0 : .P� �] 0* cmy� j#�m�g�h23s

t44������AK-/� AK-. : ;��+��]% / cm, \t-r\t�d�%[\�n9���� �2�2�������� �� +-- mm�%[\6./�'�AK-- : Z2� ��-Z2%[\65���� OY�nwvtZ2%[\ L�]% / cmP� st8�Z2%[\L�]% +*-+/ cmP� rstd�%[\� vtZ2%[\�g�hz\t234=�9��- ,�- -./�g��� OY�Z2� �� L+-- mm�P g�hst8�Z2%[\ L+-0 mm�P 6./�'�AK-, : �]% +* cm�Z2� ��-Z2%[\65���� ,./�g��� �� +-2 mmZ2%[\-� ��6./�'�AK-+ : O#�<st-9st8��Z2%[\� j#��v-v\tZ2� ��49�8=6���]� +/-,* cm�� �� +-+/ mm�Z2� ��6./�'�AK--� AK-.{�� ��>|# : - +./�� AK-.�j?% , cm LAK--�O?% - cmP �� q@] - cm�� ��>|#4}��p'� ��� ,-1 mm�B�� �~���7�� ��+� A�()*���+�./�7��� B��C6�f'� NO� AK--./�����

/� ��������� ��[�89������ D�0�=�EF6�9�� ���'� ��4q+8��G��<H��� �����q@��4 *.*/ mm, I< �� *.*- mm6J��7�4��� �f�� ����� q@K4 *.*/ mmNj��36�L���M����7���'� �F�N9�� EF0�O�O{����� ,,***P��Q��=���� FR�%T�� G��<H� S#- .%T��"��������� +0�� u�N������� ,

--0���F��Q6�f'� ��� FR�SI< �S#- .%T��TU�V- +I< � J�- ,I< �J�%T������ ()*��BWX�'���  Y2M�¡#�N96� P7���nQZ�'� �Q�¢£@¤[\� SEM-EDS��(¥ (JEOL-JSM /0**

LV-Oxford) 6]�� +/ KV, +¦+*§2 A, �Q¨{ +**^�©ª��f'��%[89��«���M��@p&�¬­�_f���� ®K�M��`¯XC°±� *.--+ mm,�� q@��mm�� �#0��� + cm6J��7�+B�� Y2M���� `¯C°±4 *.--*./ mm,�� q@�+ , mm6J������²��B�� ²'� ³Y�Labc��P �� q@�+ *.+ mm,��B��

/�+ �������

´dµ¶�%[� ·"!V*¸�¹�gº��%» : %[�89����¼O()*��B 341

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��� �������� �������������������� +� � ,���� - !"��#$���� %&'���(� )*�� ������ )���� +)����,� ��� %&'��-./������012 ��345�� �#' !"�6#$ !�78�(������� ��� - !"��#'����/� ������9:3���������345� ;< ,=� �>( /*?1/ ;@8= �-ABCD�� ����(� �#'����-.EFGHIJKLKMNOP/QD4R�� S-T-���(2:RU�>VW/X� Y�Z?[YZOP\,�� !�-�>��]�VW(^_�`X� EFGHIJKLKMN?EFGHabLKMNOP\,�;< ,=���� , !"��#$����(� c��d�Oefg�h/i2� j,� ;k /lgm=��#'����m-+)���(� no�pqrFq5� (Wo)/ -1?.+ (mol)��Mg� (+***Mg�(MgsFe) atomic) �t-uv/QD4R� w�pxxFq5� (En) ( -3?./ (mol)�-AByz ;k 0 a=� {|,�)���-Wo( -.3�� En( 0,./?1*�-O5}, ;k 0 a=� ^�� ��� . !"m-+)���(

En/ .*?.0�-AB�,�/� Wo( -3?.+���Mg~�-uv(QD4R12 ;k 0 b=� {|,�)���(Wo�-.+?-./�� En�/1?/3./����#'���m-�u��SR�R����2 ;k 0 b=�

/�, �����������-���(� �Oef-����� ;<

,=� &' !"����'� ,���/��Oefl��U���1����� ��345�� 0�� ������ +)��� )������ ������/��,� ;����� +���� �'���� �'� +

���� �'�����78�� �' !"=� ����� ,������&' +31*����(��������5�R� ���(�X0R������/�0R���>OP(���� !��UVW(����Y�Z?EFGHabLKMNOP0������ ;< ,=��U���1�Oef�,����(� m���� ,� ;< ,� k .=� 0�� �'� +�����'������(� c��14����h/� i2� j,� ;k 1=������ +���m-+)��(� Wo, En/¡�

.-?./ mol�},¢Mg-O5�� Wo�.*?.+��En�.*�£W-O5�� ,��R� ;k 0 c=� {|,�

< ,� �@�5���-¤¥��¦§¨©���ª«¬O5�Table ,. Petrographic features and modal compositions of the eruptives constituting volcanic edifices.

opx : )���� cpx : +)��� ol : ������ bg� : ��� H-P: EFGHabLKMN� H-O: EFGHIJKLKMN� I-G: Y�Z� I-S : [YZ� SE: ­bFMq� P-T: aHx®­LKMNopx : orthopyroxene, cpx : clinopyroxene, ol : olivine, bg. : bearing, H-P : hyalopilitic, H-O: hyaloophitic, I-G:

intergranular, I-S : intersertal, SE : seriate, P-T : pilotaxitic

¯°�±g��²³g�´µ¶g*·�¸g�¹ ºg�»¼�342

Page 11: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

��������Wo�--..� En�/21*���� ��� �� 0 c�� ������������ �!����Wo�-..-� En�.,/+��""#$���� ��� �� 0 d�� ��% ,��� &�'���� �()*)�+�Mg�, 13-*././ 01�

0� ��������� ��������2345*6��78 9���:;<�� AK-/

AK-. =>?6�.@$ABCDEF GH1� I�78 AK-+- (K+P) +, 3 2 0 -./ - ,� 26�+�?JK L�. ,MNO01�PQ R -.S'�TUV��WX�YZ+[�� 6� \]0A^_ 3Z=>� I�` abcde �*.-*.0 mmfg� .�h Iij�1�. 45* klm+n>11o pq'r7.�s T�01� T�` Jt. ,**�uv0 wxNyz{|X+}_~ �F01� �F��1����cd}_��r�����/�+[�� ����/�� ��0A�*����;<�Z ��cd+[���}_�0�0������ @�Z ��+���}_cd !L�s.���T�+o� ���/�.��A$�� �h���.��HA$�$, �<���s� �� ������ � ��}_ =oab�� �cd����"cd}_������  R'�¡�,¢£���� N¤ EFPQ+� ����/��r��¥���� ���� ��,I�¦� //1/�fg ,*�.*�fg �R -� � �y&�_����r��¥�cd�BCD���§�'��cde�()*)��,¨©��145*ª� AK-

+-� AK-3;< AK-+�+�« 3ª+[�� ����

� /� �¬­������% ,���&����r� AK-0�BCD��� opx : ���� cpx :

!��� ol : ()*)� opq : ��}_�Fig. /. Modal compositions of the Katakurazawa

lava flow, Hokubu second pyroclastic cone and

AK-0 tephra. opx : orthopyroxene, cpx :

clinopyroxene, ol : olivine, opq : opaque minerals.

� 0� ®¯��1�y&�°v_���cd���Fig. 0. Chemical compositions of the phenocrystic

pyroxenes in the selected samples of the

eruptives composing volcanic edifices.

±²³´��� `(µ¶*·¸¢.�¹�°�º : ��y&�°v_�»X45*ª�¼� 343

Page 12: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

� 1� ���������� �������������� ���� /���

Fig. 1. Modal compositions of the Kodake first lava

flow and the Kodake-Ohyakisuna pyroclastics.

Abbreviations refer to those in Fig. /.

� 2� �� !"#$%&�'����a()d( &�*+�,!-! KK, HO., HI+,

KDP&�'���.*/01 2� 034(�Fig. 2. Chemical compositions of the phenocrystic

pyroxenes in the selected samples of the tephras.

Enveloped areas in the a) to d) indicate the

concurring phenocrystic compositions in the KK,

HO., HI+, and KDP, respectively (see Fig. 0).

5 -� #$%�������Table -. Modal compositions of the tephra.

K+P� AK-+-&� 6789:;< H,P� AK-

2&�789:=>�;/01� s-sc : 6789:< sc : 789:< pu : >�< s-ash : 6�?@2AB=C< +331/DE(� FGH�IJ�KL34�K+P and H,P indicate the sandy scoria layer of

the AK-+- and mixed pumice/scoria layers of the

AK-2, respectively. s-sc : sandy scoria, sc : scoria,

pu : pumice, s-ash : scoriaceous ash (referred from

Wachi et al., +331). Detailed analytical method

is shown in the text.

MNOP=QRST=KUVW=XYOZ=AB [=\]^_344

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��������������� ��� AK-+*

� AK-1� AK-0�� AK-+*������� � ,�������� � � ����� AK-,���������� ������ ������� !���� AK-2 (H,

P)� AK--./� AK--� AK-,� AK-+�"� � !���� #$����%��&�� AK-2� AK--./� AK--

�'�(����������� AK-,��)*+*��AK-+������%� � ��,&�� AK-2��������Wo--1.-3�� En-.+..-�� ������Wo--.2�� En-0-.0.��� � /�&� 01 2

a2� AK--./���������Wo--3..+�� En-.*..0�� ��� �����Wo--.+.-..�� En-/2./.0*��34�01 2 b2� AK--�������Wo--0..*�� En-.,..-���%5�� 678��� Wo

-.-�� En-.0��� Mg��9�:;��� � 012 c2� <��������Wo� -.0..��/�&� En

� 0+.1-���=%5�� 01 2 c2� AK-,��)*+*��Mg�� 12...13.-�3�� AK-+�������Wo--1..../�� En-.,./..2� ��=%5��01 2 d2�

1� ������������>�?��� �@�A�BC�����3�� �D %E!�FG&�HIJ%K"L�M&� #N$OP�� Q%RS�� T�Q%HIJ%� UV*WU&'�>���� X*YUV*)Z[\]^

_Z`a`��>&��b�� Vc+�'�(�� ()&�*+d,e%FG������ fg�hi��>�?�%j4�� Vc+C�%kZ)Z�- (./&� l0m%n(�op%� m0��q����rs1�� t�� K"L%2��#N$OP%uvrs1�� Q%�� wxg��Vc+�:��8��D�:dy �:%E!�FG��� Ti&(� zv{|&�C�8�� }��~3��:%4�T����� �>5�� B�HIJ����3��B��6C��� U�����`� 0�����H��2 � + : +*�������&� �+UkZ]�&���� �����7� RIGAKU-,1*��� X��,87�hs��&�� 9�C��� ��:�;����<��h��bC��rs1&=��%> .� �� rs1&�¡Rq� �5���:��¢� £� ��?¤¥�� ¦§<�@¨���T�A��

1�+ �� ��BC©�Dª�� «W¬\�­E 0F®£

1 3� ¯G°±²A�� �)`³+´Dª�!BFeO*�MgO-SiO,1� Hµ�1 .��¶� ��Hµ�I���Dª�

Fig. 3. Whole rock FeO*/MgO vs. SiO, for the

eruptives at the post-caldera stage of the

Akita-Komagatake volcano. Abbreviations

refer to those in Fig. .. Italic characters imply

the eruptives of the northern volcanic edifice.

> .� XRF�,§<�Table .. Accuracy and precision of the XRF analysis.

³ZX�� ·¸ 0+331� MS2� JK 0,**,� MS2�Data are from Togari (+331, MS) and Saito

(,**,, MS).

¯G°±²A�� �)`³+¹L´��º�DA» : A���Dª�¼gVc+M�;� 345

Page 14: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

Miyashiro, +31.� ��Gill (+32+) ��� low-K����� �� �� 3� +*�� ��� ��� .������� Medium-K������������ !�� "#$���%���� &'���(�)&'������� *+� ��� +,� ,(�)� -���� -.'��� &/��� (�)-.' 0� 1�2345� �� +* a�� ��������� !���� .���0��6� SiO,�4���&'���7&'�����8� FeO*� TiO,(�)Na,O

�0�9� MgO��:; �� +*� b-d�� <=>�?@0� ����A�BC�(D�EF�GH� ���������I�JK���%��� ��LMKawano et

al., +30+ ; Fujinawa, +322��*+� N�O�� �O����6P+� &/��%��� ,���%0�Q23R�STUQ�VW��XY�Z[\]^_H�>3�-.'��0� 23�`77a;� &'�bc �de /Z[\]^_H 0� Rf�g�; FeO*�MgO��Rhig��j>=+ �� /��

1�, ����������k��l 0�ST�8m��Miyashiro (+31.)

�EF�GH(�)Gill (+32+) ��� low-K�!k"�#5n=� �� 3�� SiO,� /,�$%�1��>�0*����* � SiO,��o&W23pq_r0W;� .'���(�)s'� ,��0� "#$%���� t�� SiO,�� (�) FeO*�MgO�45��*+� Q23R�u'� ��LM Basaltic Volcanism

Study Project, +32+� `� "#$�'�%(vPW;���k�l �w>=�)xig �� +* a-e� 0�����A�BC��Z[yH*z�{�������EF�GH����+�ig%��� ��LM� Kawano

et al., +30+�� +|� SiO,� /3�0*�,-��� 0�./"}� ,���� .' +31*0���~`K,O

�1�� TiO,, FeO*, Na,O�2P; �� +* a-d�� *+��O� 2��,����6P+� s'���%s'�+��%0� Q23`ST�3���

1�- �4�5����O��'6���� Q23��T

���4�7��+j� �8��� ��1� AK-3�AK-0��Z��94:6P� 8-;�Q234<����P+� ��,��n�+�94 .mm�R� ,mm�R .mm$%� +mm�R ,mm$%� +mm$%� .Z����y�8-�'P+� �Z����y%`������=�P� <�'���P+� *+� AK-3� AK-0%` ¡�� �¢£8� %¤ �¥£8� �¦§���

¨£8©I�23���+j�¦�?�ª� �;«=`¬.mm�R�� ¢£8,¥£8���Z����y`­3� <�'�4®¯+�°�4� 0�5�� >e%`�¦�? �8¬ .mm

�R�Z����y%� ,±.mm�?²�Z����y

� +*� ���³�´�l �Q SiO,)x��"µ0� 3%@¶�

Fig. +*. Whole rock silica variation diagrams for

the eruptives at the post-caldera stage. Symbols

are same as those in Fig. 3.

ABCD,EF·¸,�G¹©,HICJ,K º,L»¼M346

Page 15: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

����� ������� � �� +mm���������������� ������ +�,mm����������� !� ""#�������$%������ &��'( �)*+,-�� ./0�����123-4553�6�7�8.9 (AK-3) :$;<�9 (AK-0) ����� => ++� +,?� �� @ABC �DE FG���HI� *+,-$%JKL� SiO,� �M�123-4553�6$% CaO, Na,O, K,O���N� OP� #��QRS�� QT��U��� ��V��QRWX�� ""*+,-��$%������ => ++�+,? ��Y��QRS��$%Y���Z[�*+,-\ABQRW(]^EP���_` =� 0?�aX(]� bcUd�P���e� f���g9���� �h +mm����V�:Ni� �j�Bk��l N�AB\*+,-N^E�J�� �m�V�Q��no�KL�&��'( � &�no� +�, mm������Jpq�r]E � �� ,mmaX��V��NEsXt�no��uv � $wi� �xy�� h , mmaX��V��Ni�z{��U|�P� AK--�AK-+�`Ni�} ,\} -� ,��� ~E

a��} +��(]�{��z���P� �:� ���.��/� AK-/:$; AK-.�`Ni�� �z��'�N�AK-+,� AK-++:$; AK-2�� N^EJ*+,-�

1�aX��r *+,-\ABQRS���� AK-3�0�Xt� %QRS�� *+,-\AB~E�E���� AK-1����(]���P*+,-� AB~E�E���� AK-+-� AK-+*� AK--./� AK--�+�*+,-�1�S����)�!��P�./0��"#� $�f��J23-45�%9����� ���-��,9� AK--./��l � f���g9���:� ��%���� a��$��O�r]E �

+) ��$%� AK-+-(] AK-+*O� SiO,�� /,�/-�&'�<�9� SiO,� /0�&'�<�9�8.9����$�� => ++� +,?� <�9�(�� SiO,���� ./0��$%�N FeO*�MgO���� =>++?�

,) AK-3� 2�KL��N FeO*�MgO����� �)������ �i¡��P*¢���� AK-3�SiO,� /,�/-�&'�<�9� � AK-2�� SiO,J

� /� +£¤�¥�7�:� ¦§¤,9-��¨©ª«¬5�g9����� ­� .®���X� X���¯�°��

Table /. Relationship between flow sequence and whole-rock compositional variation among the Onamedake

lava flow at the rim of the Katakuradake crater. Arrows show the ascending order of stratigraphy.

� 0� AK-3� AK-0f���/�HI� *+,-\AB:$;QRS�g9��±��� QRS��'�$% .��� �²��z�P�

Table 0. Whole sample major element chemical compositions of the scoria, pumice and mixtures in the AK-3and AK-0. As for the mixtures, four fractions sifted in size were separately analyzed.

³0´µ¤�.� ¶��·�¸1¹�:� $�º : �./0�$���»�f����¼� 347

Page 16: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

/3����� ������� ������������������� � ++!"

-) AK-1� 0#$�%&� ' FeO*�MgO����()�*+ � ++!" AK-1#$� ,-�./012�3 SiO,

� /.�45�6��789�� :-�;/<�=>$SiO,�� /*��6��()���" AK-0$� ./012��?�()#$ SiO, � /,�� @AB#$ SiO, �/*�45�6��()#� AK-1�;/=>��C�()���" ./012�$���� � ��' FeO*�MgO��� D�()$9EF) ��' FeO*�MgO��GH�� � ��I ,JKL�MN%O�+"

.) AK--./$��%�PQmedium-K�RST�SR289�()��� � +, a!"

/) AK-- UV AK-+W#$� XYZ�9EF)3[)�+� \]^ low-K��������()��+ � ++!" AK--$ SiO, 3 /1�45�6��789�� AK-,$ SiO,_/+�45�6��� W AK-+$��`� a� SiO,_/.�45�6��789�()#b+"

2� � �2�+ ������� ������� c�def +(Yg� hi�jkl� him�n*Vlfo�()�pC�� JKq�� �(rs3tuVZ+" ��I ,JKL��GH�� � jkl� vwxyz�>�vwI +�� #b+ � /�1!" {ZV�(rs$� |Xf�}~�-�� �-����+" ��I ,JKL� �GH�� ��f� �}~3��I ,��J~�f���Z� �GwJKLjkl��I +JKL����+" %�� vwxyz�>�vwI +�� ����}�$��fvw#b+" W�vwxyz�>��73vw9E���f�VZ*X�$� {Z3���}���� vw,�f�A�+u����Z+" gW�� �`��� vw����}���� �-3%O�+���3�X" ���� :� ,

(��}$DZ�Z� �%�J#���� �*+�������f���+�}���# +"

2�, ��� ��������9EF)����()¡n�¢£f¤�¥� 012��=>@A�f¦�� ��*o�()���;/0

++" §RS¨���}���o� FeO*�MgO-SiO, " ©?$ª�*9EF)�()«©���" ¬­$�%��� �AK-3, AK-0! ��012��=>�@rB�@r­���"

Fig. ++. Whole sample FeO*/MgO vs. SiO, for the tephra erupted at the post-caldera stage. Enveloped areas

indicate the compositional variation ranges for the representative eruptives composing volcanic edifices.

Dashed lines indicate the mixing lines between the coexisting scoria, pumice and mixtures within a single

tephra layers AK-3 and AK-0, respectively.

®¯�°���±²��³´µ�¶·�¸� ¹ º�!»¼"348

Page 17: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

������������ �� ���� ������������������������������ ������ !"�#$�%"� � &' (��)*�+,-�� ./�0���� !"�123+����� 454�� 6789��7�:;<�=>?@ABC�DE���FGH-I� 454�J���KLM�N�� �&O-���P�6Q�RSTM=&#9��7��L� :;�� �UP��MF�L-I��454V�L�N���WX*�� Y0��&O��� �Z��[��\��]^+ _������6Q� FeO*, TiO,��G� `abTM!�GH�cd�� �� Al,O-��G� SiO,�ef�g"���G�cd�+��� CaO, Na,O-I�#$�-�� �$���� %?h!���FGH�2�����F��i�jX*�!"k�&l-mn'�.���o-�� Y0� 9p�L�q�!"�M�123�N�()�.��*r��-m� +st�+u�-vwa�����-�� 7x���N��,��454!"�y)���� �� ��F����i�!"k�q+��454�J���KLM�*z� ����F�����F�������m� {|O��-�}o-+���-��~�*���� �^���B�!"�� �KLM�.���454�/K!"����*�� �/��� ���!"��L\��7��*z����L� ������!"�454!"���.��-�� ..�� ����0�(20��p��2���)*+�����1���6_� �^��!"�����23�!"���� ����23��^���!"k�� �^�4� ����5��+R� AK-3� AK-0��W� #$�%"����3!6�jX*�!"7�89�:�� �� �x�� ��� ����23� �^�� )*�!"����+� !"�0��/]^��;�1��.��-�� ��W��P�Bvp�� #$%"!�� �3!��� ���� !"�(<���� �.�� � !"�=�V�q�o� �0�>�!^�� _¡¢.���0� ��£�� ¤%??678%??0�=�V¥X� 7_¦^V�R�#$%"!���p�!^���0������p��/§� |¨7_� AK-+-@AK-3� /���� !"£� �©�#$%"!�@ªX*-� «¬++� +,­�� !"¥X� �0�>�-���� (+) AK-2�� A®¯&'°+�m�A®¯B C� (,) AK-0�� �_�Q±²������ B�D ,&'°678A®E C� (-) AK--./�� B�D .&'°� (.) AK--�� FG6³D + C� (/) AK-,�´¯D , C� (0) AK-

¬ +,� P�Bvp��3��p�� SiO,�M¬� µ2� H¶� I·�¬ ++����

Fig. +,. Whole sample silica variation diagrams for

the tephra erupted at the post-caldera stage.

Envelops, dashed lines and abbreviations are

same as those in Fig. ++.

¸J¹º¯&#� P�Bvp»K��6Q��&¼ : &#$%"�3!�{|��p���0 349

Page 18: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

+�������� 0�� �� ++� +,��+� +��� .����� ��������������� !"�# � $%��&�� '��()�*+# ,-��./0*12& � 345���67'89:��0;���� � <�<� �=:�>� ?��@ <A��!B#0C�� � ,-0�.�D?���E�F<A����:�����6�!�< �� +-�� �G���7HI89 �

+� ,���JK�&L� (,)0� ,��MNOP!�QR�S�TU<A� 19� -G0��6��7?���E�F< �� /�� �Q�S�<����V � W � (+)�0� XYZP��0� AK-2��[���\��7���V �� +.���NOP����]�: /^�&L� AK-+-� AK-

+,��� AK-3!DA�<A�0� �C9D_`aYb���� �2cd AK-+-0���efg5hP��&D��TU89 � ijk� +331�� 19�0� MN����0lmh!hP<AAn� ��# MNOP7:

���Di9:�� AK-+,0� ijk (+331) ��^op� � ijk +331� � . b� ��0� ��efg5�q��Sr7s�]��� �efg5�q�� tu�� -� � +3� hP�v�wd�9 ]� 7� � (x���S<� ���y��345� zj!"��� �C9D{fe|b��#$�89A AK-++� AK-+*

0� }\P~� AK-+-� AK-+,�;9�9�F# ����� y�0���8�D��� AK-+-� +,��EFA���P�� � %&'�����A]� 7� � ��QR���}\P��� efg5hPR� D<E0;�(���)����V� ���A:�MN7hP89�D*�D<E0+�89A]� 0��� �A� _`aYb���89A AK-3���# lmh�<�� ^��345����Sr�����0� ���+,-���7�.�:2& �./� AK-/��� AK-.00�'�� 1=<A�2��M�!�N�# � ��� ijk (+331)�� !"���/�!�3# �� 19����# lmh�0� ��� ,,-���7s�89 ��A� AK-.~/�4n�9A��`�5�� (AK-

-./)���� .��� (HO.)�P������� ,62�TU7�V � .�0� HO.0#��� <A7�AK-.¡¢� £V¤�A�y�¥¦§%&��2*�89A]� � D&.�0� AK-.�&¨<AHO.7� hP(���y�:%&'���� *�!7�A]� �� � A�©� HO.7 AK-.&¨Q�� <� *�!89A�<�D� 9§1=ª�«����7:1=�� R¬0­�0C�� � ���� AK-/� .&¨Q� HO

.0;®� <:��A]� 7��� 1�<�� ��� ,,-���0� v�# ¯���� =°±�¥¦§%&�� hP7�©�9� ����mh'0��²

� +-� }\P�F6�AMNOP�345��XYZP� >³0� /��´�

Fig. +-. Modal compositions for the pairs of

eruptives and bulk chemically resembled tephra

layers. Abbreviations refer to those in Fig. /.

� +.� �[���\�� �[�����345AK-2�XYZPP� >³0� /��´�

Fig. +.. Modal compositions of the Katakuradake

pyroclastic cone, Katakuradake-kita lava flow

and the tephra AK-2. Abbreviations refer to

those in Fig. /.

?@AjBCµ¶j�D·¸j¹EFj i ºjG»¼H350

Page 19: ˘ ˇˆ˙˝˛˚˜+,-/*m , , , (OD): , ˘ ˇˆ˙˘ ˇ˝ ˆ + ˛ ˙ ˛˙˝ ˛˚ ˚ ˚ ˛˚˜˜ ! ˘!""##$ˆ$% ˇˆ ˇ ˚ %&!’&#˝ˆ ’&’()* ’+ . (HO.): ,( )-(+321) . ./ *+ ˙,-01

�����������2�- ������AK-2������ (KK)� AK--./�� .� � (HO.)� AK--��� +�� (HI+)��������������� AK-,�� ,�� (KD,)

�������� AK-+��� � (KDP) ��������������� !"�#���$���� � !"%�&�����'��()*+�� ������,-./��� �*01 AK-

2�KK2�����!�� ��������3,4�� �����Wo� En5����6,7�8�,79:�� ;� 2 a<� AK--./�HO.��������Wo

, En� 6=�>?@!�7�8�,2���� A"�BCDEFG) En�H� AK--./"��������IJ� Mg�#!�$����� HO."���������,@%�� ;� 2 b<� AK--�HI+��� ����� En��3,K�8�� �������,BCDEFG�7�8��2�!�&L:�� ;� 2 c<� AK-,�KD,"����������4����3�@%���AK-+�KDP"������������3,K��'��>?()*+I ;� 2 d<�

2�. ��'M(�N)�OP� QR� .M�S*,T+�7�;� +/� +0<�

a) ��� +-,/**U+,,***,-� VEI .U/V.�WXYEZ(�,[8*� �\]��^:_"� 8�(�N)���/0 � ��(`�1a�� 23b��Gc��d�/_"� ++,3**U++,0**,-� �Gc��4e�f5(�,[8a":!�� 67]��(`�1�� �Gc���8g,/h"T+�,7��A"�9 +*,***U3,***,-� �3b�[i"� VEI-V.�jG�kZ(��� � +:;l�md��6<!"n�!�)��

b) ������ ����� +*,***U3,***,-� � +:;l�m �[8a

"jG�kZ(��� opq�rXZ(�s�=t!���� ��d�!"� 8�u� � ��>u?/�@A/��� u55v�$nw� ���m � VEI.

V.�WXYEZ(�,[8*� x7]��^R/_"� 8�u����m�yB/�"T+�,7�� (�z� yB/�"���m ��{:��m|,�i"�

2,3**U1,2**,-}� ���m�(�,[8a"�'�mC��|�DE�,EF��8�n:� ~�G��H?�a"���� �m �I�!"J��DE��

|G������:��� ��� VEI.V.�WXYEZ(�,Ki�LM]��^R/_"�8��-z!�����m n:��������,��!"�

2,.**U1,/**,A��5�� ���m �opq�rXZ(�,[i� � +� �,d�/�"�+-,***,-n:8�uNA����������w�����Cp�O�����7��

2,.**U1,/**,-}n:� (`�������Cp��P���)a"� ���m�2Q� � +� ��2�� VEI-V.�jG�kZ(� (AK-1) $��opq�rXZ(�,$i� � -� ��� R�� �,d�/�� ����`!"� ��@����m���� ���S��R!"�

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Fig. +/. Subdivision of formative history in the post-caldera stage. Numbers at the top show the assumed order

of formation. Abbreviations are same as those in Fig. ., and AK-+ to AK-+- are the names of tephra layers

defined by Wachi et al. (+331).

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Basaltic Volcanism Study Project (+32+) Island arc basalts.

In Basaltic volcanism on the terrestrial planets, +3-�,+-.

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Fujinawa, A. (+322) Tholeiitic and calc-alkaline magma

series at Adatara volcano, northeast Japan : +. Geochem-

ical constraints on their origin. Lithos, ,,, +-/�+/2.

:[�\�]^_`�a)�� (,**+) Kb_c�L�K-Ar�S : Kb_c�L(� ���d� �L& .0&3/�+*0.

:e�l�U]f4 (+331)Ng`��� � �L& .,&+3/�,+,.

Gill, J. B. (+32+) Orogenic andesites and plate tectonics.

Springer, Berlin, -3* pp.

qK�  (+312)�)��`�L�ST� +. C�S� ¡¢g& -,& ,,+�,,-.

Inoue, K. (+32*) Stratigraphy, distribution, mineralogy

and geochemistry of late Quaternary tephras erupted

from tne Akita-Komagatake volcano, notheastern Japan.

Soil Sci. Plant. Nutr., ,0, .,�0+.

£ h (+310) MoLCD��L¤G� /& (g¥i�}& no ++, +-*�+-+.

Itaya, T., Nagao, K., Nishido, H. and Ogata, K. (+32.)

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rocks. J. Geol. Soc. Japan, 3*, 233�3*3.

Kawano, Y., Yagi, K. and Aoki, K-I. (+30+) Petrography

and petrochemistry of the volcanic rocks of Quaternary

volcanoes of northeastern Japan. Sci. Rep. Tohoku Univ.,

Ser. III, 1, +�.0.

Miyashiro, A. (+31.) Volcanic rock series in island arcs

and continental margins. Am. J. Sci., ,1., -,+�-//.

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Fig. +0. Eruption history in the post-caldera stage.

Abbreviations are same as those in Fig. .. AK-+to AK-+- excluding AK--./ were quoted from

Wachi et al. (+331). The AK--./ was tentatively

named in this study (see text).

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