Petrologic Study and Fission-track Age of Middle Pleistocene Hirano Volcanic Ash in Northern Nomura Town, Southwestern Shikoku, Japan

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1 /. ( **3) 0.+ /+ FT **3 -. **3 +* +- Petrologic Study and Fission-track Age of Middle Pleistocene Hirano Volcanic Ash in Northern Nomura Town Southwestern Shikoku Japan Masayuki S AKAKIBARA Chisato N AKAMURA Hitomi I WASAKI Michiharu I KEDA Sakae S ANO and Tohru D ANHARA The Hirano volcanic ash in Nomura Town southwestern Shikoku Japan has been characterized by petrography refractive index and geochemical composition of volcanic glass (major and trace elements) and fission-track age of zircon. The Hirano volcanic ash has abundant vesicular glass shards and phenocrysts of biotite hornblende and orthopyroxene. The volcanic glass has a rhyolitic composition. LREE (Light Rare Earth Element) profiles of the volcanic glass are steep with La/Sm N/*. /-. and those of HREE (Heavy Rare Earth Element) are flat with Gd/Yb N They have a weakly developed Eu anomaly. The Hirano volcanic ash has similar values of some incompatible trace element ratios such as La/Y Ba/La and Nb/Zr with the Yufugawa pyroclastic flow deposit in the Ooita Prefecture. The fission-track age of *..0 *.*3 Ma ( + s) was obtained for zircons from the Hirano volcanic ash. The Hirano volcanic ash can be correlated to the Yufugawa pyroclastic flow deposit based on petrographical geochemical and geochronological data. Key words : Hirano volcanic ash Middle Pleistocene Yufugawa pyroclastic flow deposit Nomura Town southwestern Shikoku EPMA + ** ; **+ ; +33+ ( +33+ ) ( +32 ). AT. +32 Ng-+ Department of Geology Faculty of Education Ehime University Matsuyama 13* 2/11 Japan. Department of Earth Sciences Graduate School of Science and Engineering Ehime University Matsuyama Kyoto Fission-Track Co. Ltd.... Omiyaminami- 13* - 2/11 Japan. tajiri-cho Kita-ku Kyoto 0*-22- Japan. Shikoku Research Institute Inc. Takamatsu 10+ *+3 Corresponding author: Masayuki Sakakibara Japan. sakakiba@sci.ehime-u.ac.jp

2 242 Fig. +. Map showing the study area. (a) locality of the study area ; (b) topographical map around the outcrop. Original map is a part of + : /*** topographic map of Nomura published by Geographical Survey Institute of Japan. FT *. 0* *. +- Ma +330 ** No. + : + m **2 No. : / m + +-* +. K-Ah B- + +*. 0 +*. 1 m AT +33* ICP-MS /* 0* **2 ; ** -/ m +/ m Fig. + Fig. + ; **+ ** Fig. + FT

FT 243 +* cm mm - (H- + ) (H- H-- H-+* (H-. H- 0) (H-1 H- 3 ) H-++ H-+ + (Fig. ) -+ +. ++0 / +2 / +2 / +. / +. / + / + / + mm Fig.

3 FT 243 +* cm mm - (H- + ) (H- H-- H-+* (H-. H- 0) (H-1 H- 3 ) H-++ H-+ + (Fig. ) / +2 / +2 / +. / +. / + / + / + mm Fig.. Columnar section with modal compositions of phenocrysts and volcanic glass in the Hirano outcrop. + /. + / mm ++0 / +2 / mm - + /. + / mm *** +. m. m +. +./ m ( +310) H: C: *.. */. m T: m +. It : m *. *. 0 ( +33/ ) m +3.. m *.. m + m Fig / 2 + / +0mm (RIMS***) / cm H-/.* -. EDS

4 244 EDS Table +. Analytical results of obsidian (Astimex Scien- tific Ltd.) and NIST 0* as standard glass samples by EDS. SD: standard deviation RSD: relative () + standard deviation ; Fisher and Schmincke +32. ; +333 ; **. K O Na O **. EDS +333 ; +33+ ; mm X / Si Ti Al Fe Mn Mg Ca Na K 3 Astimex Scientific Limited (NIST 0* ) (**.) (**0) (Table + ) +** wt. */. mm ( +330) Sakakibara and Isono **. *./ mm -/ ICP-MS ICP-MS **0 ELAN0*** + / + mm *./ mm +** mg HNO HF () EDS - HNO (JEOL JSM /.** ) Oxford X -* La Ce Pr Nd Sm EDS Eu Gd Tb Dy Ho Er Tm Yb Lu Co Ni Zn +* kv..** +* A Ga Rb Sr Y Zr Nb Cs Ba Hf Ta Pb Th +* +* +** +* ** +/ kv..** U A Rh In Re Bi.. mm - -

5 FT 245 Eggins et al Table. Analytical results of USGS rock standard BHVO- ard deviation. samples BIR-+ and AGV- +. RSD : relative stand- BIR-+ AGV-+ Table Eggins et al K-Ah AT H-+ H-+ Fig. H- / Fig. H-+ H- - / * H-../ H-/ 1* H-0 H-3 H-+* Fig. H-. / 0 H-1 3 H-. - +* H-/ n /*+ Fig Fig. - H-. H- 0.- ( + ) H-+* EDS H-. / 0.* H-1 3 Table - Fig.. SiO wt. TiO *+. */. wt Al O wt. FeO * wt. MgO C H T *+. *-. wt. CaO * wt. Na O /. -2. wt. K O -0..*. wt. (Table -) FeO-SiO SiO -

$SD: standard deviation. Fig. -. Histgrams of refractive index of volcanic glass from the Hirano volcanic ash.$ .. Harker diagrams of the volcanic glass shards from the Hirano volcanic ash. (Fig.

.. Harker diagrams of the volcanic glass shards from the Hirano volcanic ash. (Fig.

6 246 Table -. Major element composition of volcanic glasses from the Hirano volcanic ash and Yufugawa pyroclastic flow deposit. SD: standard deviation. Fig. -. Histgrams of refractive index of volcanic glass from the Hirano volcanic ash. Yfg data are from Machida and Arai (**-). Yfg : Yufugawa pyroclastic flow deposit. Fig... Harker diagrams of the volcanic glass shards from the Hirano volcanic ash. (Fig..) (Pearce and Parkinson () +33-) (LaCe Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu) K-Ah AT (Co Ni Zn Ga Rb Sr Y CsTh Zr Zr Nb Cs Ba Hf Ta Pb Th U) Lu (Fig. /) Table. Fig. 0 K-Ah AT N-MORB (Hart et al. +333) Fig. / (La/Sm /*. /-. ) K-Ah AT 3 +0 (Gd/Yb ) Eu LIL HFS N N

FT 247 Table.. Trace element composition of volcanic glasses from the Hirano volcanic ash and others. K-Ah: Kikai-Akahoya tephra AT: Aira-Tanzawa tephra Yfg p.f.d: Yufugawa pyroclastic flow deposits.

Deer et al. ( +331) Mg# *02. *13. Ca Leake ( +312) ferroan pargasitepargasite edenite Mg# *01. *1.. An Ab /+.1 Or An Ab Or - 1. - total FeO 21. -22. wt. TiO 0.

7 FT 247 Table.. Trace element composition of volcanic glasses from the Hirano volcanic ash and others. K-Ah: Kikai-Akahoya tephra AT: Aira-Tanzawa tephra Yfg p.f.d: Yufugawa pyroclastic flow deposits. Fig. /. Spiderdiagram showing the compositional di# erence of volcanic glass among the Hirano volcanic ash Yufugawa pyroclastic flow deposit K-Ah and AT tephras. K-Ah AT Gd Lu Eu.. H-H0 Mg# *1+. *13. Deer et al. ( +331) Mg# *02. *13. Ca Leake ( +312) ferroan pargasitepargasite edenite Mg# *01. *1.. An Ab /+.1 Or An Ab Or total FeO wt. TiO wt. TiO FeO Fig. 0. Chondrite normalized REE patterns of volcanic glass from the Hirano volcanic ash Yufugawa pyro- / FT clastic flow deposit and K-Ah and AT tephras. CIchondrite value data are from Sun and Mcdonough + (H-/) (Fig. ) ( +323). ( +333 ) *./ kg

8 248 Table /. Results of fission-track dating. r. and N : density and total number of counted tracks respectively ; Analyses were made by the external detector method applying to external surfaces of zircon (ED method: Gleadow +32+ ); Age was calculated using a dosimeter glass NIST-SRM0+ and age calibration factor zed -/. ( + s) (Danhara et al. **-); P( c ) probability of obtaining the c value for n degrees of freedom ( nnumber of cristals- + ) (Galbraith +32+ ); r : correlation coe$ cient between rs and ri; U: uranium content (Iwano et al. ***); Zircon grains were irradiated with thermal neutrons in the pneumatic tube of the JRR-. unit of the Japan Atomic Energy Research Institute (JAERI) reactor with NIST-SRM0+ dosimeter. /*** FT (*..0 Ma *.*3) 33 FT ED ; Gleadow IUGS (Hurford +33* a b) **- (Danhara et al. **-) ( +33/ ) +33 FT *0*. *+*. Ma KOH-NaOH +33- / /* *./1 *./2 Ma K-Ar **- NISTSRM0+ DAP *./1 *.*3 Ma FT *./ *./- Ma *** JRR. -* n /**. Galbraith ( +32+ ) **- c P( c ) 3+ -* + H-/ (Fig. 1) *..0 *.*3 Ma (Table /) Al O FeO KO 0 0+ SiO Na O K O SiO Fig. 1 Al O K O SiO Na O K O (Fig. 1) Figs. / 0 La/Y-Ba/La La/Y-Ba Nb-Zr - -

FT 249 Fig. 1. Comparison of major elements of volcanic glass compositions among the Hirano volcanic ash and other Middle Pleistocene tephras. Fig. 3.

9 FT 249 Fig. 1. Comparison of major elements of volcanic glass compositions among the Hirano volcanic ash and other Middle Pleistocene tephras. Fig. 3. Nb-Zr diagram of volcanic glasses from the Hirano volcanic ash Yufugawa pyroclastic flow deposit and K-Ah and AT tephras. Fig. 2. La/Y-Ba/La diagrams of volcanic glass from the Hirano volcanic ash and other Middle Pleistocene tephras. (**) +330 Figs. 2 3 Nb Zr (**2) /1./* /1. +/ m -/ cm 0

10 ( +300) Na O ( +33/ ) (FT) ICP-MS REE ( +333).. // Danhara T. Iwano H. Yoshioka T. and Tsuruta T. (**-) Zeta calibration values for fission track dating with a diallyl phthalate detector. J. Geol. Soc. Japan +*3 00/ 002. Deer W. A. Howie R. A. and Zussman J. ( +331) Rock- Forming Minerals. Volume B Double-Chain Silicates. Geol. Soc. London nd edition 10. p. Eggins S. M. Woodhead J. D. Kinsley L. P. J. Mortimer G. E. Sylvester P. McCulloch M. T. Hergt J. M. and Handler M. R. ( +331) A simple method for the precise determination of.* trace elements in 1 geological samples by ICPMS using enriched isotope internal standardisation. Chem. Geo Fisher R. V. and Schmincke H. U. ( +32. ) Pyroclastic Rocks. Springer -Verlag Berlin.1 p. Galbraith R. F.( +32+ )On statistical models for fission track counts. Math. Geol FT Gleadow A. J. W. ( +32+ ) Fission-track dating methods: what are the real alternatives? Nucl. Tracks / -+.. Hart S. R. Blusztajn J. Dick H. J. B. Meyer P. S. and + ) Muehlenbachs K. ( +333) The fingerprint of seawater 0 m circulation in a /**-meter section of ocean crust gabbros. Geochim. Cosmochim. Acta 0-.*/3.*2*. ( +322 ) ) / + FT * pp **3. Ma Hurford A. J. ( +33* a) Standardization of fission track 0* dating calibration : recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission of Geochronology. Chem. Geol. 2* ) Hurford A. J. ( +33* b) International Union of Geological Sciences Subcommission on Geochronology recommendation for the standardization of fission track dating calibration and data reporting. Nucl. Tracks Rad. Meas ( ***) :. AT +- ++* (**+) EDS +*2 -. ICP-MS (**) Ng-+ +*3.0

11 FT 251 (**.) EDS ( +33* ) 0* * 3 +/ ( +33+ ) ( +330 ) EPMA ( +33-) EPMA (WDS) ( +330 ) * Ma Pearce J. A. and Parkinson I. J. ( +33-) Trace element ++ +*3 +* models for mantle melting: application to volcanic arc ( +32 ) petrogenesis : In Magmatic Processes and Plate Tectonics (Prichard H. M. et al. eds.) Geol. Soc. Spec. Pub. 10 D *-. +. Sakakibara M. and Isono Y. ( +330) Middle Miocene ther- (**2) mal metamorphism due to the infiltration of hightemperature fluid in the Sanbagawa metamorphic belt ++..+/./ southwest Japan. Contrib Mineral Petrol +/ -.+ -/2. ( +333 ) Tn ( **2) +*/ 0.+*.*.1 (**) +/ 1 (***) 1/ 3 //- /1* ( +333 ) pp Sun S. S. and McDonough W. F. ( +323) In Magmatism in Leake B. E ( +312) Nomenclature of amphiboles Amer. the Ocean Basins. Spec. Pub. Geol. Soc. Lond Mineral. 0- +*- +*/ -./ ( +33 ) ( +33-) 10 p (**-) ( +330) --0 p /* ( **+) -*--+. ( +332) -+ /2 /3 +*. /12 /23 ( +33+ ) Ng-perhydration ( +33/ ) hydration su- -* /.-/.-2 ( +33+ ) * + ( +310) 20 p 2.13 /+/ (**0) EDS

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