秋田駒ヶ岳火山，後カルデラ活動期における噴火史：火山体構成噴出物と降下テフラ層の対比

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(4) . . . ,**- +*. 1 !"# ,**. +*. 1 !"$%. Eruption History in the Post-caldera Stage of Akita-Komagatake Volcano, Northeastern Japan Arc : Correlation between Eruptives Constituting Volcanic Edifices and Air-fall Tephra Layers Akihiko FUJINAWAῌ, Masayuki IWASAKIῌ, Kyoko HONDAῌ, Akemi NAGAOῌ, Takeshi WACHIῌῌ and Shintaro HAYASHIῌῌῌ Akita-Komagatake volcano is situated at about -* km west of volcanic front in the northeastern Japan arc. Development history of the volcano is divided into - stages ; main strato cone, syn-caldera formation, and post-caldera eruption stages, in descending order. In order to reveal the comprehensive eruption history in the post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed in the concurring stage. Petrological a$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. .,***ῌ-,+** 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 subordinate 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. &-+*ῌ2/+, '()*+ ,-./$/01234567/7 Department of Environmental Sciences, Ibaraki University, Bunkyo, Mito -+*ῌ2/+,, Japan.. &,.-ῌ**+0 89:;<=)>? @A@BCD% Asia Air Survey Co. Ltd., Tamura-Cho, Atsugi ,.-ῌ **+0, Japan.. &*+*ῌ2/*, E)FG/H? E./IJ*K/01/IL Department of Earth Sciences, Faculty of Education and Human Studies, Akita University, Tegata-GakuenCho, Akita *+*ῌ2/*,, Japan. Corresponding author : Akihiko Fujinawa e-mail : fujinawa@mx.ibaraki.ac.jp.

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(172) 344. MNOP=QRST=KUVW=XYOZ=AB [=\]^_. 1

(173) / Fig. 1. Modal compositions of the Kodake first lava flow and the Kodake-Ohyakisuna pyroclastics. Abbreviations refer to those in Fig. /. 5 - #$% Table -. Modal compositions of the tephra.. K+P AK-+- & 6789:;< H,P AK2 &789:=>;/01 s-sc : 6789 :< sc : 789:< pu : >< s-ash : 6?@ 2AB=C< +331 /DE( FGHIJKL 34 K+P and H,P indicate the sandy scoria layer of the AK-+- and mixed pumice/scoria layers of the AK-2, respectively. s-sc : sandy scoria, sc : scoria, pu : pumice, s-ash : scoriaceous ash (referred from Wachi et al., +331). Detailed analytical method is shown in the text.. 2 !"#$%&' a()d( &*+,!-! KK, HO ., HI+, KDP &'.*/01 2 0 34( 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)..

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(259) ´ Fig. +-. Modal compositions for the pairs of eruptives and bulk chemically resembled tephra layers. Abbreviations refer to those in Fig. /.. +. [\ [345 AK-2 XYZ P P >³0 /

(260) ´ Fig. +.. Modal compositions of the Katakuradake pyroclastic cone, Katakuradake-kita lava flow and the tephra AK-2. Abbreviations refer to those in Fig. /..

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(346) LST +. C S ¡ ¢g& -,& ,,+ῌ,,-. 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, .,ῌ0+. £ h (+310) MoLCD

(347) L¤G /& (g¥ i }& no ++, +-*ῌ+-+. Itaya, T., Nagao, K., Nishido, H. and Ogata, K. (+32.) K-Ar age determination of late Pleistocene volcanic rocks. J. Geol. Soc. Japan, 3*, 233ῌ3*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, +ῌ.0. Miyashiro, A. (+31.) Volcanic rock series in island arcs and continental margins. Am. J. Sci., ,1., -,+ῌ-//. J¦4 L)jk §:k` ¨L l ©ª « (+30-) KJK$¬®/0a ( 9¯& 03& +0-ῌ+1+. Kmc2q34 (+312) Kn opq° qrsG± =#" Mog²gbcZ& -+& 0-ῌ13. Kmctuvd2q34 (+32*) Kn o pq°qrsG± =#" 1 , Mog ²gbcZ& --& /1ῌ01..

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