長野県北東部烏帽子岳とその周辺の地質と火山形成史

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(4) . Geology and Volcanic History of Mt. Eboshidake and its Adjacent Area, Northeastern Nagano Prefecture, Central Japan Kou TAKAHASHI* Mt. Eboshidake is located in the western part of the Eboshi volcano group (western Eboshi volcano group), central Japan. According to the result of geological survey and the petrographic features, the western Eboshi volcano group is composed of 3 volcanic units such as ; Iwayakan’non, Kokuzo, Kanahara, Omuro, Ofuji, Gotomiki, Lower Eboshidake, Narahara, and Upper Eboshidake lavas. Based on the newly measured paleomagnetic polarities and reported K-Ar radiometric ages, the volcanic activity of the western Eboshi volcano group was reconstructed as follows. The volcanic activity started about + Ma and during the early Pleistocene age, the volcanism was characterized by eruptions of thin basaltic andesite - andesite lavas from scattered vents, which formed small volcanic edifices. Then thick andesite - dacite lavas erupted in the initial period of middle Pleistocene, and formed clusters of lava domes. After that, the volcanic activity of the Eboshi stratovolcano started about *.2 Ma, and after the dormant period (around *.. Ma) when Iwashimizu gravel bed of about -* m thick with two marker tephra beds was piled up, the basaltic andesite - andesite lavas erupted again until about *..*.- Ma. These lavas erupted from central vent and formed main edifice of Eboshi stratovolcano. The whole rock chemistry of 03 samples from the western Eboshi volcano group and eight samples from the eastern Eboshi volcano group was analyzed using XRF. The result shows that the rocks of the western Eboshi volcano group are classified into the basaltic andesite - andesite group (SiO, : /+῍/2ῌ) and the andesite - dacite group (SiO, : /3῍0.ῌ). The basaltic andesite - andesite group is mainly tholeiitic and more depleted in MgO content than the calc-alkaline andesite - dacite group. Most of these rocks are plotted in the medium-K field on the K, O vs. SiO, diagram. This feature is similar with the other volcanoes in the eastern Eboshi volcano group and the Asama volcano. Key words : Eboshi volcano, Asama volcano, Omachi APm tephra, geomagnetic polarity time scale, Middle Pleistocene. +ῌ ῏ ῍ ῐ ῎. >TEUV?WX:<> Y? Z[\]^5. !"#$%. _àbEcW d.gh kmnEUV?#2,. + ῍,-,./0 +/ km 123. Y?oP#2p qrst :<>EuvWX. $456 789 (+3/2) &6:<>?@A. E D>9 (+33+) _`\OPw xy (+333). &'()*. ; = #. BRef * ij. Ml ?. (Fig. +). D>9 (+33+) :<>E. :<>,Yz{5NL _|}&6,P. FGHIJKLMNOP# GH*QRS:<. ~WXd.gh kmn h5:<. BC2. -3*ῌ20*+ q -ῌ+ῌ+ ¡¢z{£ _¤¥¦§¨©£ ª« Division of Enviromental System Science, Graduate School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto, Nagano -3*ῌ20*+, Japan. e-mail : th32.*/@gipac.shinshu-u.ac.jp. B . 2Mij. >,YWX ? _|}&6 P~WX m :<>T5:<> YWX ?OP#2p qz{_ :<> YFE X:<>T?_tXp :<>T (,,*00 m) ERt,. 2 km, -+ 2 km, 0. +,.** m vFMN5Xp qrstv.

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(30) m . Fig. +. Index map of the Eboshi volcano group. a : Location map of the Eboshi volcano group. Open triangles indicate Quatenaly volcanoes (after Ono et al.,+32+). b : Geomorphological map of the Eboshi volcano group and Asama volcano. The areas of the Eboshi volcano group and Asama volcano are quoted from Iijima (+30,) and Aramaki (+302), respectively. The dotted area indicates the strata consisting the Eboshi volcano. The study area is surrounded by thick line. The contour interval is +** m..

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(344) &' Table - ,:% SiO, ¶@ (wtῌ) 4Z tuvZ[ZK /-·/2 wt.ῌ WXYZ[K6. Fig. 0. Zijderveld projections (left) and demagnetization curves (right). Open circles indicate vertical plane. Solid circles indicate horizontal plane. Straight lines of Ziderveld projections show regression lines obtained by the stable components through demagnetization..

(345) 6¸7,-.V¹8º» 3¼ Table ,.. 95. Paleomagnetic results of lavas from the western Eboshi volcano group.. n : number of samples, mINC : average of inclination, mDEC : average of declination, a 3/ : radius of 3/ῌ confidence circle of site mean. SiO, $@!$U V) WXB MgO N TiO, $;Y SiO,-FeO *ῌMgO EQ/ RST/ F. /6Z Miyashiro (+31.) "[\]["# ^$ _àF/6b]c"[\][ "#^def_`@ V) 6b]^g$%&'() *+ SiO,-K, O $hì jk , - lm nop q j r s Btuvw@ V>%&' ` (5$ xy z{|}~ n. . {|}~s 6>g (Aramaki, +30-) B `. Fig. 1. Schmidt’s projection with 3/ῌ confidence circles of lavas. Solid and open squares indicate normal and reversed polarity, respectively.. 1 . . 1ῌ+

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(349) Chemical compositions of the lavas.. ῎ ῌ. The localities of the samples from the western Eboshi volcano group are shown in Fig. ,. Analyses for the rocks from the eastern Eboshi volcano group, the sampling localities of which are shown in the Fig. +b, are also listed. IW : Iwayakan’non lavas, KZ : Kokuzo lava, KH : Kanahara lavas, OM : Omuro lavas, OF : Ofuji lavas, GO : Gotomiki lavas, NH : Narahara lavas, EB : Eboshidake lavas, YU : Yunomaru, SJ : Sajiki, SA : Sanpogamine, KG : Kagonoto and TM : Takamine. Fe, O-* : total Fe as Fe, O-.. Table -.. 96. ῍.

(350)

(351) . 97. Fig. 2. Silica variation diagrams. The boundary lines in SiO, vs. K, O diagram are after Gill (+312). FeO* : total Fe as FeO. The rocks from Yunomaruyama and the eastern Eboshi volcano group are also shown on the SiO, vs. K, O diagram. Also, compositional areas for the Asama volcano (the Kurofu, Hotokeiwa and Maekake stages) are shown in the SiO, vs. K, O diagram after Aramaki (+30-). IW : Iwayakan’non, KZ : Kokuzo, KH : Kanahara, OM : Omuro, OF : Ofuji, GO : Gotomiki, LEB : Lower Eboshidake, NH : Narahara, UEB : Upper Eboshidake, YN : Yunomaru and EEB : Eastern Eboshi volcano group..

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(359) x. §¨©ª`LvI1234zvI5]J º\$. | Ax. 4F]56 LM5». Fig. 3. Columnar sections of the Iwashimizu gravel bed (left) and heavy mineral assemblage of the intercalated tephra (right). The localities of IS+ and IS, are shown in Fig. ,. Bi : biotite, Hb : hornblende, Opx : orthopyroxene, Cpx : clinopyroxene and Opq : opaque minerals..

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(361) . STU9 :;<V*WXY. !"#$%&'()*+". )Z[\S)] :;<"^(. Fig. . ,-./01"23$4 $5 6. _^^` +" aOP$Abcdefgh. 789:;<=>)?*(@. ijkel=$4 (] mnohijke"bcd. )*$A%& BCDE)*5F GH9. efghijkepq_rs oth',-u . I@"J*"*KLMINO9P(*4 Q$ R. "gth',-vwxyz{ |}-+". Fig. +*. Summary of the volcanic activity of the western Eboshi volcano group. K-Ar ages are after * Kaneko et al. (+323) and ** Takahashi and Miyake (in prep.). The geomagnetic polarity time scale is after Conde and Kent (+33/). Fission track age is after *** Suzuki et al. (+332). IW : Iwayakan’non, KZ : Kokuzo, KH : Kanahara, OM : Omuro, OF : Ofuji, GO : Gotomiki, LEB : Lower Eboshidake, NH : Narahara and UEB : Upper Eboshidake..

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(407) YZ!" 3 [W G4. Aramaki, S. (+30-) Geology of Asama Volcano. Jour. Fac. Sci. Univ. Tokyo, Sec. II., +., ,,3ῌ..-. » (+302) =Y Y Y¤H% no. +., ./ p. Conde, S. C. and Kent, D. V. (+33/) Revised calibration of the geomagnetic polarity time scale. Jour. Geophys. Res., +**, 0*3-ῌ0*3/..

(408) 102. \ ³. Fisher, R. (+3/-) Dispersion on a sphere. Proc. Roy. Soc., A ,+1, ,3/ῌ-*/. Gill, J. B. (+312) Role of trace element partition coe$cients in models of genesis. Geochim. Cosmochim. Acta, .,, 1*3ῌ 1,.. (+333)

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(416) efg h)"# 00, ,33ῌ-,,. Kaneoka, I., Matsubayashi, O., Zashu, S. and Aramaki, S. (+313) K-Ar ages of Late Tertiary volcanic rocks in the Asama area. Jour. Geol. Soc. Japan., 2/, /.1ῌ/.3. Kirchvink, J. L. (+32*) The least squares line and plane. s analysis of palaeomagnetic data. Geophys. Jour. Roy. Astr. Soc., 0,, 033ῌ1+2. Hi/jk$%/>El & (+32+) ,** ' + &mn , ( opq >rst/uQv/Qw)/xyz* (+330) {| C }+ ~f X 9 , f

(417) -9 %&. d/ -+, +*/ῌ++1. Miyashiro, A. (+31.) Volcanic rock series in island arc and active continental margins. Amer. Jour. Sci., ,1, -,+ῌ-//. 0t (+32/MS) 1.

(418) R %&. P2 3* (,***) 45C6 C7 + 8?Z¡¢£¤¥9 e:& . ; Ser. A, 1-, +ῌ,/. 3*/¦<d (+33*) ¡¢£¤7=§¨Z& APm + 3©

(419) V bcd)* ,3, +*/ῌ +,*. 3*/>] ª/«] (+332) ¬/ ? 7 !aBbcd+ +-®¯°/±-² V D +*1, -.2ῌ-01. \³ s (+331) @ +331 - no. ,, ,+. \³ s/>rst A ^/ $ 1 B @ K-Ar V K?LM (+30.) ´? 9C7Dµ ¶· 7¸?¹9 D 1*, -+/ῌ--2. Zijderveld, J. D. A. (+301) AC demagnetization of rocks : Analysis and results. In Method in Paleomagnetism, ,/.ῌ ,20 p, Elsevier, Amsterdam. &ºE »>¼:.

(420)