溶岩流とテフラの層序関係からみた阿蘇火山中央火口丘西部地域の発達史

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(3) ,**- +* , ,**. 2 ,3 . Geologic History of the Western Part of Post-caldera Central Cones of Aso Volcano, Southwestern Japan, Based on Stratigraphic Relationships between Lava Flows and Airfall Tephra Layers Yasuo MIYABUCHI῍, Naoaki MASUDA῍῍ and Kazunori WATANABE῍῍῍ The western part of post-caldera central cones of Aso Volcano, southwestern Japan, is characterized by wide chemical variation of lava flows (SiO,.3῍1- wt.ῌ) and complicated topography. Geologic history of the area was reconstructed by stratigraphic relationships between lava flows and airfall tephra layers. Stratigraphy and chronology of lavas and tephra distributed on the western slope of the central cones are as follows : Ayugaerinotaki lava, Matsuhata pumice (MhP), Tochinoki lava (1-+* ka ; K-Ar age), Aso central cone pumice . (ACP.), Tateno lava, Aso central cone pumice - (ACP-), Takanoobane lava (/+/ ka), Eboshidake lava, Karisako lava, Akase lava (-* cal ka ; calibrated +. C age), Sawatsuno lava (,10 ka), Kusasenrigahama pumice (Kpfa ; -+ cal ka), Otogase lava and Aso central cone pumice + (ACP+ ; . cal ka), in ascending order. Between about 1*,*** to /*,*** years ago Tochinoki lava, Tateno lava and Takanoobane lava accompanied precursor pumice-fall deposits. A catastrophic eruption occurred in and around the Kusasenrigahama crater at -+ cal ka. The eruption was initiated by the dacite Sawatsuno lava flow and produced the largest plinian pumice-fall deposit (Kpfa ; +., km- in DRE volume). In the western part of post-caldera central cones of Aso Volcano, total discharge volumes of felsic and mafic magmas from about 1*,*** to -*,*** years ago were estimated to be about ,.. km- and *..1 km-, respectively. Key words : Post-caldera central cones, Aso Volcano, lava flows, tephra, geologic history +ῌ ῐ ῎ ῑ ῏. 0vD`P wa x +33+S dyI7zD {|}~. !"# $%&'()*+ ,-./012.. *IEP Aso-.. 345363789:;<=.1> ?.@ABCDE. ?1U ~\]@. 5v<<OF ~ \U +.C t>\/. F )GHI;)JHI;KL7MN:;<)*4OP. 0 \Z7P\Uopt@5vD`P . QR +32/S TU)*U)VIWXYP=.Z. x ,**- aS. >. 0+ {|}~'()*IEP=.1Z. (+31,) @deIfgC [Uh QR (+32/) E. ~ZU\]^_I^YP:znS [U. [U\]^_I<`D+. ab c. (+3-2). ij (,**+) @kCl3ZmnS !"# $. IE`D Tv34fg5vD`P(U=.UAB*. %&opqUrs / <U=.I<`D+ K-Ar t@u. : Y@OPTZ;CnS TUTZ;m. 20*῍*20, a ¡ .῍++῍+0 ¢£¤¥¦§¨©ª«¨ Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami .῍++῍+0, Kumamoto 20*῍*20,, Japan. 203῍,,,/ a¬®¯° ± +,00 "²³ Aso Junior High School, Kurokawa +,00, Asomachi,. Kumamoto 203῍,,,/, Japan. 20*῍2/// a ¡ ,῍.*῍+ a¯²´²( Faculty of Education, Kumamoto University, Kurokami ,῍.*῍+, Kumamoto 20*῍2///, Japan. Corresponding author : Yasuo Miyabuchi e-mail : yasuo@a#rc.go.jp.

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(21) Fig. +. Geologic map of the western part of post-caldera central cones of Aso Volcano, with the surveyed localities. Distribution of most lava flows was revised after Ono and Watanabe (+32/). Nakahono lava, Nagano lava, Otogase lava and Otogase debris-avalanche deposit are from Masuda et al. (,**.)..

(22) 269.

(23) Fig. ,. Stratigraphic relations of lava flows and tephra layers in the western part of Aso caldera. locations of the sites. +.C age of ACP. is from Miyabuchi et al. (,**. b).. See Fig. + for. 270.

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(31). Fig. -.. Geologic cross section of the study area from Tateno to Takanoobane (X-X’ line ; Fig. +).. Fig. .. Stratigraphy and chronology of lava flows and airfall tephra layers distributed in the study area. Solid tielines show stratigraphic relationships confirmed at outcrops and drilling cores. Broken tieline denotes estimated relationship. * Calibrated +.C ages from Miyabuchi and Watanabe (+331) and Miyabuchi et al. (,**- a). ** K-Ar ages (Matsumoto et al., +33+)..

(32) º»¼I*)JK*L. 272. Table +. Petrography of rocks distributed on the western part of post-caldera central cones of Aso Volcano.. -ῌ- . 3 Y~QO,+90S.

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(67) (Figs. +, ,). )7. (PQ+g(("!" . ]@!<:;<?<^ 2 67 4. 4 S h8 :;<? $A*-+-. (+31,) _` AaV ?. (>% S (Fig. +). +i 7(". ῍. bc < +d]e $T.

(68) < Lf ==`. 3 +32/ ; ,**+% ?῍. YZ[Z. N jk ¡lV $\]3^ ,**.%. F889fgV ":# 6 ,*ῌ BH 1:. ": h. . . - . mm $%-G=i jCDEk89. O/)7)* m-=#N¢£u!. F<\=` 5l ":m. + hk¤Hn5 ¥¦Yu@!. GHno?pjH (Table ,) > ./. (W> ¥¦Yu. k'&C¤V. 0_Hq < ./0Ir. ok §=¢£u

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(71) ·¸!¹= !º»O¼. 274. Table ,. Whole rock chemical compositions of lavas distributed on the western part of post-caldera central cones of Aso Volcano.. ,**.

(72) . | K}{| P ~ &. !!"#$%. (k.23 + <1./R-ῌ- ; AR. &

(73) '( )*+ , -. J 2 ; <Hn. .&/0123. o , cm _ 2K&P23 @ABCQRF. .ῌ+ ῌΐ῕῍῎ῑ῏῔ . ῐῒ (ACP.). <! ,**- a ; A32-+ \] + SiO,. 456789:; . <"= +323 ; > ACP.. _ ? 01 wt.ῌ P2 (Table -). ACP. [ld. ? 45@ABCDEF*G2H&.IJK. ? *..- km- ¡K¢£.2 <! ,**- a3. L2MNCO-P23 @ABCQRF .?S. .ῌ, ῌΐ῕῍῎ῑ῏῔ - ῐῒ (ACP-). TNUVAWXYZ[L2 A*-*0 \]^_G. 456789:; - <> ACP- ?~¤¥¦§. \F`?abLNUVAWXYZ?*G. ^_ëLMNCP2 <"= +3233 A*-*0 \]. c Dde fghij^k.23. ? ACP.

(74) +.- m ©ª '(k ,

(75) . A*-*0 \] l2 ACP.

(76) m? +*1 cm P. MNC«¬ *®¯ [KL.23 A*-*0. Hno <MP ; p` l2Hn - qro. \] l2

(77) m?

(78) +, cm P2 n°± ?. stu ? ..- cm P+3 6vwxyz?{R. ²hk³hk.23 ´ µ'k.2 ¶.

(79) "Q_!34 !¹,Dv2²DEº»KO)

(80) à¼ Table -.. 275. Whole rock chemical compositions of pumice in and around Aso caldera.. MP ,., cm

(81) . @83O9~.L {:];. SiO, 03 wt.ῌ . T< ,.-3 km-. (Table -). ACP- . . *.*1 km-

(82) ! "#$ %&'( ,**- a) .ῌ- (Kpfa). !#/F Kpfa Aso-. 01 D! %&'( ,**- a) Kpfa =f.>?X@MA]T!M&. * %( +32, ; + Kpfa) ,. ' %(B +33* ; vB(C +330) M. -

(83) ./$#0"12!#$. (^)

(84) qsW/:;mDWn @6DvsM. 34 567!#89/:;89. T!MF %A*-,* )*) 7 MX. <= >?>@6 %SiO, A00 wt.ῌ) . @$E". (Table -). BC*DEF GDE'. .ῌ. Tn

(85) (AT). %HIJ8 ) /:;K8 L!. A*-*0 )*./$# Kpfa wg + m F@.. M"N#ODPQ.#R#$ %.

(86) 34 G ,3 cal ka %$ ,**,) H Tn D. ( +32, ; S$( +32/) T"NDPQ. vs %IB( +310 +33, ; + AT) M]. UVW!XWYZ[%<\]T!M&' %. T AT ! Kpfa , -. ,**+).

(87) JKL" (^)

(88) ./$#<]. (^)

(89) ./$# Kpfa / _4à bcd@.. AT +*h,* cm

(90) M Mo +*. e ]T!MO fg:Fbcd@ W+hW/ !i. cm N.d Z.]T!".$ W(.. j A*-*0 )*./$# 0fgbcd@ W+ kk. j$m,W (+*YR0/.) <- 014¡ *./ cm

(91) . +l#m,W (+*YR1/2) <-] . 4<=M 1O ¢£ ¤à ¥Dv¦. -, cm (MP,.- cm) bcd@ W, mVW (+*. § / A*,*. )*F AT <. YR//0) n @ % +* cm) F .oM. P$# Dv¦ §!89¨QRSM©ª. p/#$ bcd@ W- 0)

(92) 0"$bcd. IB( (+33,) M\]T«U.zV]T!M. @F % -0 cm) MP " /., cm !1O$ bcd. ¬W#$ %XB( ,**.). @ W. qrsW %/BG) <-] 3 cm tuh2. .ῌ/

(93) (K-Ah). u6Dvs 0wgbcd@ W/ ,0. (^)

(94) ./$# )Y ,h. m

(95) . cm m,W (+*YR1/2) M bcd@ W-. XWZ[®.¯$@X@!. .3x!kkt4 (MP-.- cm) 56My$ z. #/F G°±2²F.

(96) 34 .

(97) 0 _bcd@.e #$. ³´µ¶·Dvs %\](¸I +311 ; IB(. %&'( ,**- a) M {|(^)

(98) !}7.bcd. +312 +33, ; + K-Ah) M^#$ K-Ah .

(99) MN¼q%r%+7s. 276. +*,* cm (1./YR0/2) . 0* cm EF@ &C #@ F9.

(100) . Z[ S. AT K-Ah

(101) !". ;G< (1./R-/-) ,qJr ,. #$ %& '(

(102) )*. cm- -C 6i 3. +" ,% ,**.- ./01. AVL+ 1 X YWEkFBEk , l. K-Ah 234 56"#$ 7. . m- 5 l . m :^_^à ,SiO,b02 wt.ῌ ;. (,./Y1/,) @ (MP-., cm) . 809: *./+ m . Table -- EF@ 4+"i F@. ;< (+*YR-/.) $ #$ ! . HCJ , l 0.2 m- &C . .ῌ0 + (ACP+). #@ 1) $t AVL. . ."=>?# + @ ,A +323 ; $% ACP+-. Jò4XhZ[ @aTU. K-Ah B&C D'()7EF@. < , l +0 m- *4!"i (Fig. ,).. #@"G N1 H "ICJ. .ῌ3 . K*LJ ,+ +33, ; MN%+ +331- ACP+ . EkFLJTU<5. OP,Q- A*-*0 -&C . g#$S l +./..1 m EF@ 4+" (Fig.. 80./01R234+" $S.. ,), #"YC6\*4!" i']. 567T A3./2 - (Fig. +) U J#$. #@^_ka ,SiO,b1, wt.ῌ- . #- VWR X$8

(103) !" YC. BEk !`2e=af . $#Z[9:\W] #@. uS s bcXEk$¡¢ (Tables ,, -).. :^_^à ,SiO,b0- wt.ῌ- (Table -), c;a. 6i AVL+ EkqF +* m m£¤¥. d<$ 2e=fg#$ >? ,A. O¦o4§d 1 BEke¨ ). +323-. © "ª EFTU<«WE. 6hi. .ῌ1

(104) ; . @ABCD ,A*-,- -- CjDEkFE. kwOPX .ῌ+*. !" !" . F@ &CJ (Figs. ,, /A). #@F9 l. @AB¬f ,A*,*. -- 1 ¬fEk. +,0 cm ShPmno` 4. Fg@$ ® &C 1 (Fig.. +" "=)ppF l -, cm @fg. ,), ¬f@ ,$% OtP- $}¯" ,%. 2G< (/YR//0) HC qF. ,**.- ® " l 1* cm HCJ. mno` , l -* cm- MP ,.- cm $qXIJK. #$ ")B@ ¬f# + @ ,$. L@ qmno` , l 0. cm- . % OtP+- FB@¬f# , @ ,$% OtP,- $. l! +, cm 2 ShHCJ$. °!" h±$Xc;ad<$ & '. $X JrMsX4+" ,MP,,./cm- @. $N&'fg^_ka ,SiO,b1- wt.ῌ- @. "f6"c;ad<N&'$& 'CjD. ">?B¬fEk$¡¢. Ek$t #@ 801uvO. $S OtP+ LJTU<5g#$. P$wOP i+ xy-z{Q$. S¬fEki]" Ek$7S:. |R@ ,$% MhP- $}~#$J. ) :\iX ,% ,**.-. 6hi. |R@F l +-* cm @ ICJ 2G< (/YR//0) #"@1). /# $%&'()*+,-./012. CjDEk6LJ. j ."=>?²0&CJ. . 4+" i . % @. (MhP)% Ek 7S:TU<$. Ek$OP$ ³´µ (Fig. .) h BX )kS¶. " Vi) CjDEkFDW. 801uqBEk^¬fEk. &CJ#$ X!" #$ ,+ ,**+- . h AT $234 ·l ,\ m , m n-. EkF MhP $!FB DW. < "HCJ#$ ] ,%. 7$!" TU<Z. ,**.- EkF¬f@ ,OtP+ 1). .ῌ2 . OtP,- 9:/0¸@ op¹. @ABF ,A*-+, -- YWEkF l. º»8*

(105) J#$ ¹i.

(106) ῍#;±&'Wg².³´µ¶·¸¹daº#»¼ ῌ

(107) Kpfa . 277. +, ACP- ACP. "efg!. !. KWg wxyz ACP- T$7, #. (Fig. /B, /C) " ῎#$%&'(! )*+. uvg 3 4. ^_` /A*-+/ ab2 +. ,-. /01 ,**+23 45 Kpfa

(108) #&'. .S.T%7#XY"HI+J. +, "

(109) 6$7# Kpfa 89:;<". A*-+. abt"ij. => /+*?13cm2 @AB #C῏DE. g A32,2 abVT A*-,, ab+ #$. F+! GA"HI+J (Fig. /B). K#K

(110). 7VT~OP;B+ Kpfa "efg. ! 3 S. LMNOP;"QRS TUB. .3 Kg#KWg wxyz. VW5 Kpfa "XY.KBZ[ ! 3 \] . ;ij Kpfa ACP- VTt. (! ^_`ῌ /A***+ ab2 #ῑc#. #ῒj K"W-.3 .TU. \d+ Kpfa #;< ,?- cm #OP;. wxyz R"ῐ¡. "efg ῎h#5W"ij. V Sg;!KWg wxy. klm"noS.3 Kg#pqWg

(111) . z\, !!r#LM+, klm". #LMrsf !"

(112) #]". , 3 W"g ῎#$%&'(! . $7, klm"t!uv 3. #¢£+*gW K"+JW-.3. W | .. wxyz ^_`{N} /A32,/ ab2 V!. ~¤¥¦§B¨© ACP- " ~¤¥῍ª+, t. Kpfa #%7~VTOP; /8;< .2. «¬S klm"t!K +t. cm2 B+ij ! (Figs. ,, /D). K#OP;. } (+323) VT®¯ (,**- a) "° ! 3 . Fig. /. Photographs showing stratigraphic relationships between lava flows and airfall pumice layers. (A) Tochinoki lava overlying Matsuhata pumice (MhP) at locality A*-,-. Scale is + m. (B) Sawatsuno lava overlain by Kusasenrigahama pumice (Kpfa) at locality A30+3. Scale is , m. (C) Akase lava overlain by Kpfa at locality A32,3. Scale is , m. (D) Eboshidake lava occurring below Kpfa at locality A32,/. Scale is , m..

(113) ¼rM>?sMR$ct. 278.

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