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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 : fujinawa@mx.ibaraki.ac.jp

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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).

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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

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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

��� �������� �������������������� +� � ,���� - !"��#$���� %&'���(� )*�� ������ )���� +)����,� ��� %&'��-./������012 ��345�� �#' !"�6#$ !�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

��������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

� 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

��������������� ��� 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

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

����� ������� � �� +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

/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

������������ �� ���� ������������������������������ ������ !"�#$�%"� � &' (��)*�+,-�� ./�0���� !"�123+����� 454�� 6789��7�:;<�=>?@ABC�DE���FGH-I� 454�J���KLM�N�� �&O-���P�6Q�RSTM=&#9��7��L� :;�� �UP��MF�L-I��454V�L�N���WX*�� Y0��&O��� �Z��[��\��]^+ _������6Q� FeO*, TiO,��G� `abTM!�GH�cd�� �� Al,O-��G� SiO,�ef�g"���G�cd�+��� CaO, Na,O-I�#$�-�� �$���� %?h!���FGH�2�����F��i�jX*�!"k�&l-mn'�.���o-�� Y0� 9p�L�q�!"�M�123�N�()�.��*r��-m� +st�+u�-vwa�����-�� 7x���N��,��454!"�y)���� �� ��F����i�!"k�q+��454�J���KLM�*z� ����F�����F�������m� {|O��-�}o-+���-��~�*���� �^���B�!"�� �KLM�.���454�/K!"����*�� �/��� ���!"��L\��7��*z����L� ������!"�454!"���.��-�� ..�� ����0�(20��p��2���)*+�����1���6_� �^��!"�����23�!"���� ����23��^���!"k�� �^�4� ����5��+R� AK-3� AK-0��W� #$�%"����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

+�������� 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

�����������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<�

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

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¡ +/! =��>?B¢�F £�! ¤¥ ¦§^¨©�8ª�[«¬! �§^¡ .3J­�� AK-+�AK-+-^���� �+331� $�E1�VW? ®¯[«¬!

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-�,+-.

2q34 (+330) Mo��)��`CLDEFP���RG� ���HI}�/&���R& +/*.

2q34�JK-���L�M (+32-) MoLD& NO�L��PQR$� +. C�S� �Mo�L�ST+1��UP�VWT� +. C�S (1� +��� Mo�XYT�bcZ�& no +, ,3�-..

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.)

K-Ar age determination of late Pleistocene volcanic

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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