115 12 672 687 2009 12 Jour. Geol. Soc. Japan, Vol. 115, No. 12, p. 672 687, December 2009 Petrography and whole-rock geochemistry of pre-aso lavas from the caldera wall of Aso volcano, central Kyushu Masaya Miyoshi, Kuniyuki Furukawa, Taro Shinmura, Madoka Shimono and Toshiaki Hasenaka 2008 5 17 2009 8 3 Graduate school of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan ; Present address; Beppu Geothermal Research Laboratory, Institute for Geothermal Sciences, Kyoto University, Noguchibaru, Beppu, Oita 874-0903, Japan Faculty of Economics, Aichi University, Kurozasa 370, Miyoshi-cho, Nishikamo-gun, Aichi 470-0296, Japan Faculty of Economics, Kumamoto Gakuen University, Oe 2-5-1, Kumamoto 862-8680, Japan Abstract Pre-caldera volcanism in the Aso area.. Ma, central Kyushu, is characterized by effusive eruptions of multiple lava flows. To clarify the chemical evolution of the magma chamber beneath the Aso area, we investigated the petrological characteristics of these lavas where exposed in the caldera wall. The pre-caldera lavas are divided into eight types with distinct petrographic and compositional characteristics: A. cpx-ol basalt, B. ol- px andesite, C. ol-hb- px andesite, D. px andesite, E. hb- px andesite, F. hb andesite, G. px-hb dacite, and H. bt-hb rhyolite. Incompatible trace element modeling demonstrated that these eight types did not originate via simple fractional crystallization. The phase assemblages and abundances of phenocrysts of the precaldera andesite-rhyolite differ from those of the caldera-forming and post-caldera andesite rhyolite. In addition, the pre-caldera andesiterhyolite contain relatively low concentrations of incompatible trace elements compared to the caldera-forming and post-caldera andesiterhyolite. These observations may indicate that the physical conditions and/or chemical compositions of the source materials that gave rise to the andesite-rhyolite magmas differed between the pre-caldera and caldera-forming stages. Keywords: Aso, pre-caldera volcanism, petrography, bulk-rock geochemistry Corresponding author; M. Miyoshi, miyoshi@bep.vgs.kyoto-u.ac.jp Hunter 1998 270 90 ka;, 2001;, 1991 Aso-1 Aso- 4 90 ka ;,, 2005 2001 270 ka, 1985, 1985 1965 1983 CThe Geological Society of Japan 2009 672
115 12 673 Fig.. Simplified distribution map of the pre-aso volcanic rocks adapted from Geological Survey of Japan Map Series 1: 50,000, Ono and Watanabe, 1985. The pre-aso volcanic rocks are divided into five types by Ono and Watanabe 1985 : biotite rhyolite; hornblende dacite; hornblende andesite; pyroxene andesite; basalt. Data sources of K Ar ages: Kaneoka and Kojima 1970 ; Ono et al. 1982 ; Kamata 1985 ; Watanabe et al. 1989 ; Matsumoto et al. 1991. SiO2 8 25 km 18 km
674 2009 12 Fig.. Sample locality map of pre-aso volcanic rocks. Shaded texts show geographic names.
115 12 675, 1985, 1965, 1985 Fig. 1;, 1985 1985 5 Fig. 1 5 K Ar : 2 Ma, 1982 0.7 0.6 Ma NEDO, 1991 : 0.8 Ma, 1970 0.8 Ma, 1985 0.8 0.5 Ma, 1989 0.8 0.36 Ma NEDO, 1991 : 0.5 0.8 Ma, 1989 0.6 0.4 Ma NEDO, 1991: 0.6 Ma NEDO, 1991 : 0.4 Ma, 1970, NEDO, 1991 2 Ma 0.8 0.4 Ma 270 90 ka 1985 5 Ma, 1985 1985 90 ka Fig. 1 Fig. 2 43 X Philips PANalytical MagiX PRO Fig.. Reported eruption ages of pre-aso volcanic rocks: A ol cpx basalt; B ol 2px andesite; C ol hb 2px andesite; D 2px andesite; E hb 2px andesite; F hb andesite; G 2px hb dacite; H hb bt rhyolites. Data sources of K Ar ages: Kaneoka and Kojima 1970 ; Ono et al. 1982 ; Kamata 1985 ; Watanabe et al. 1989 ; NEDO 1990 ; Matsumoto et al. 1991. Mori and Mashima 2005 Table 1 2 SiO2 A H Table 3 A : TMPA49, TMPA49B, PAS-BA04, PAS-BA05 0.7 Ma: NEDO, 1991 : PAS- BA07, PAS-BA07B 2 Ma:, 1982; 0.6 Ma: NEDO, 1991 B : TTPA45, TTPA46 : PRAS28 0.5 Ma;, 1989 B C : BCPA40, BCPA41, BCPA43 : PRAS11 D : Daikanbo 0.8 Ma:, 1985 0.5 Ma: NEDO, 1991 : PRAS15 : TTPA44U, TTPA47, TTPA48 : HYPA35, HYPA51 0.6 Ma: NEDO, 1991 : PRAS18, PRAS22, PRAS26 0.8 Ma:, 1970 0.8 Ma:, 1989 D E : PRAS25 :
676 2009 12 Table. Modal analysis of representative rock samples from each type of pre-aso volcanic rocks. : Sample localities are shown in Fig. 2. Abbreviations: ol=olivine; opx=orthopyroxene; cpx=clinopyroxene; hb=hornblende; bt=biotite; gm=groundmass; ph=phenocryst. BCPA54 0.4 Ma: NEDO, 1991 : KDPA38, BCPA42 0.4 Ma: NEDO, 1991 : PRAS12, PRAS13 : HYPA29 HYPA30 0.6 Ma:, 1989 : HB-AN F : PRAS24, PRAS27 1989 0.5 Ma F G : PRAS30, PRAS30B H : BT-RH: PRAS16, PRAS17 0.4 Ma:, 1970, NEDO, 1991 2008, B C D E B C, D, E
115 12 677 Table. Major and trace element data for samples of pre-aso volcanic rocks. : Sample localities are shown in Fig. 2. Fig. 3 C G A 0.8 0.4 Ma Fig. 3 TTPA33 TTPA33B Fig. 2 2 K2O Table 2 K2O TTPA33 TTPA33B 2 2008, 8 0.3 mm A; Fig..A 2.5 mm, 27 46 vol. 1.6 mm, 3 14 vol. 0.8 mm, 5 vol. 0.5 mm, 1 vol.
678 2009 12 Table. continued B; Fig..B 4 mm, 28 39 vol. 1.5 mm, 1 4 vol. 2 mm, 5 8 vol. 2 mm, 1 3 vol. 0.5 mm, 1 vol.
115 12 679 Table. continued Table. Criteria for classifying the pre-aso volcanic rocks into eight types. K Ar age data source: a: NEDO 1991 ; b: Ono et al. 1982 ; c: Watanabe et al. 1989 ; d: Kamata 1985 ; e: Kaneoka and Kojima 1970. Abbreviations are the same as Table 1. C; Fig..C 4 mm, 25 31 vol. 1.5 mm, 1 vol.4 mm, 5 7 vol. 2 mm, 1 3 vol. 3 mm, 1 vol. 0.5 mm, 1 2 vol.
680 2009 12 Fig.. Microscopic photographs of the pre-aso volcanic rocks: A ol cpx basalt; B ol 2px andesite; C ol hb 2px andesite; D 2px andesite; E hb 2px andesite; F hb andesite; G 2px hb dacite; H hb bt rhyolites. Abbreviations: ol = olivine; cpx = clinopyroxene; opx = orthopyroxenea; hb = hornblende; bt = biotite; pl = plagioclase
115 12 681 Fig.. Major elements TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O and P2O5 vs. SiO2 for the pre-aso volcanic rocks. D; Fig..D 4 mm, 22 31 vol. 2 mm, 2 11 vol. 2 mm, 6 vol. 0.5 mm, 1 2 vol. E; Fig..E 2 mm, 21 31 vol. 2 mm, 2 5 vol. 2 mm, 2 vol. 3 mm, 3 vol. 0.5 mm, 2 vol.
682 2009 12 Fig.. Trace elements Ba, Rb, Sr, Ni, V, Zr, Nb, Y, Cr vs. SiO2 for the pre-aso volcanic rocks. F; Fig..F 2 mm, 19 26 vol. 2 mm, 1 vol. 0.5 mm, 2 vol. G; Fig..G 2 mm, 18 20 vol. 1 mm, 1 vol.1 mm, 1 vol. 1 mm, 7 11 vol. 0.5 mm, 2 3 vol.
115 12 683 Fig.. Rb Nb, Rb Zr and Rb Sr diagrams for the eight types from pre-aso volcanic rocks. Shaded areas and thick dotted lines show the compositional ranges formed by simple fractional crystallization. Symbols are the same as Figs. 5 and 6. H; Fig..H 2 mm, 5 vol. 2 mm, 1 vol. 1 mm, 1 vol. 0.4 mm, 1 vol.
684 2009 12 Fig.. Incompatible trace elements vs. SiO2 for the pre-aso volcanic rocks, caldera forming stage pyroclastic flow deposits Aso-1 to Aso-4 and post-caldera volcanic products. Data sources: Hunter 1998 ; Miyoshi et al. 2005. Thick lines indicate Low K, Medium K, High K and Shoshonite rock series boundary LeMaitre et al., 1989; Rickwood, 1989. Symbols are the same as Figs. 5 and 6. SiO2 Fig. 5 6 TiO2 Al2O3 Na2O MnO P2O5 Sr Ni Nb Y Cr SiO2 54 62 wt. B C D E F A SiO2 53 wt. G SiO2 62 64 wt. G H SiO2 66 72 wt. H SiO2 Rb Nb Rb Zr Rb Sr Fig. 7 A G, Green, 1994; Rollinson, 1993 A: TMPA49, B: PRAS28, C: BCPA40, D: PRAS15, E: HB-AN, F: PRAS27, G: PRAS30Table 1 Rb Nb Zr Sr D A B C D E F Rb Nb Zr D 0.1 Sr D 1 1.4 1.2 1 1.3 1.8 G Rb Nb Zr Sr D 0.1 1.1 0.2 2.4 D Fig. 7.a-7.j Rb Zr Rb Sr A A, B H A Fig. 7.d 7.e 7.f B E C G H E F B Rb B
115 12 685 Fig. 7.g 7.h 7.i C D E C B F G H C B F C Rb C Fig. 7.j 7.k D E Fig. 7.m 7.n 7.o F G H G H B, C, F, G, H A G Fig. 7 B PRAS28 E TTPA44 C BCPA40 D TTPA48 E BCPA54 C G A 0.8 0.4 Ma 40,, SiO2 Figs. 5, 6A H Sr Nd 0.7042 0.7045, 0.5127 0.5128 A 5 40 vol. 5 vol. 10 vol. Aso-4 0.5 4 vol. 1 vol. 1 vol., 1985;, 2005; Kaneko et al., 2007 B, C, D, E, F 5 10 vol. 1 5 vol. B 1 5 vol. C 1 vol. E 3 vol.f 1 vol. D 2 10 vol., 6 vol., 20 30 vol., 1 2 vol. G 3 mm 5 10 vol. H :, 2001 1 vol. 1 vol. A 1, 2005 A A 10 vol. SiO2 K2O,, 1985; Kamata, 1989;, 2001 K2O P2O5, Ba, Rb, Zr, Nb, Y Fig. 8 A SiO2 49 53 wt. SiO2 54 73 wt. Fig. 8 SiO2 K2O Medium-K Medium-K High-K
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115 12 687 ry, 12p. NEDO, 1991, 2 3. NEDO Press 280p Ono, K., 1965, Jour. Geol. Soc. Japan,, 541-553 Ono, K., Shimokawa, K., Soya, T. and Watanabe, K., 1982, 1979 Science and Technology Agency press, 167-189 Ono, K. and Watanabe, K., 1983, Chikyu Monthly,, 73-82 Ono, K. and Watanabe, K., 1985, 5 1 Geol. Map Volcano., Aso 4, Geol. Surv. Japan Rickwood, P.C., 1989, Boundary lines within petrologic diagrams which use oxides of major and minor elements. Lithos,, 247-264. Rollinson, H.R., 1993, Using Geochemical Data; evaluation, presentation, interpretation. Longman Singapore Publ. ltd., 352p. Watanabe, K., 2001, Development of Aso Volcano 7, Ichinomiya, Kumamoto, 241p Watanabe, K., Itaya, T., Ono, K. and Takada, H., 1989, K- Ar Bull. Volcanol. Soc. Japan,, 189-195 English translation from the original written in Japanese Miyoshi, M., Furukawa, K., Shinmura, T., Shimono, M. and Hasenaka, T., 2009, Petrography and wholerock geochemistry of pre-aso lavas from the caldera wall of Aso volcano, central Kyushu. Jour. Geol. Soc. Japan,, 672 687. 8 SiO2