No.37 (2002) pp.119-134 1 2 3 1 4 Incompatible Element Chemistry for Basaltic Rocks in the Higashi-Izu Monogenetic Volcano Group. Masaki TAKAHASHI 1, Koji KIKUCHI 2, Tadayuki URUSHIHATA 3, Shigeo ARAMAKI 1 and Kazuchika HAMURO 4 (Received September 30, 2001) Incompatible element chemistry for basaltic rocks in the Higashi-Izu monogenetic volcano group is investigated in order to examine the source rock heterogeneity. Compared with the polygenetic volcano in which the central vent is stable for a long time, the independent monogenetic volcano group is favorable for the research of the regional variation of source rock chemistry, because its vent is unstable and sporadically distributed in the wide area of the volcanic field. The spatial variation of incompatible element ratios is present in the Higashi-Izu monogenetic volcano group; Zr/Y and Rb/Ba ratios are high in central region, and Rb/Zr, Rb/Y, Ba/Zr, Ba/Y ratios are low in southwest portion, however the spatial variation shows no systematic change with time. It is concluded that the spatial variation of incompatible element chemistry is not caused by magma mixing of mafic magma with crustal felsic melt or assimilation of basaltic magma with granitic crustal materials, but it directly reflects the compositional heterogeneity of source rocks in the mantle wedge beneath the Higashi-Izu monogenetic volcano group. Keywords: Izu, island arc, monogenetic volcano, magma, basalt, incompatible element 1. 1 156 8550 3 25 40 2 156 8550 3 25 40 3 310 8512 2 1 1 4 100 0013 2 2 1 Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University: 3-25- 40, Sakurajousui, Setagaya-ku Tokyo 156-8550 Japan 2 Master course, Graduate School of Integrated Basic Sciences, Nihon University: 3-25-40 Sakurajousui, Setagaya-ku Tokyo 156-8550 Japan 3 Department of Earth Sciences, Ibaraki University: 2-1- 1 Bunkyo, Mito 310-8512 Japan 4 Ministry of Foreign Affairs: 2-2 Kasumigaseki, Chiyoda-ku Tokyo 100-0013 Japan 119 95

SiO 2 = 57wt%> 2. Fig.1 Locality map of the Higashi-Izu monogenetic volcano group (revised Aramaki and Hamuro, 1977). solid circle: eruptive center. 20km 25km Fig.1 2 1980 35 km 25 km 15 1992 1995 142.5ka 13 132ka 129ka Fig.3 10 107.5ka 103.5ka 9 8 89ka 94ka 80 ka 6 56ka 31ka 55ka 38ka 3 2 Fig.2 Map showing the distribution of volcanic edifices in the Higashi-Izu monogenetic volcano group (revised Aramaki and Hamuro, 1977). dashed line: eruptive fissure estimated from the alignment of synchronous volcanic vents (Hayakawa and Koyama, 1992; Koyama et al., 1995); numeric number: name of volcanic edifice. 96 120

Fig.3 Eruptive history of the Higashi-Izu monogenetic volcano group (revised Koyama, et al., 1995). Tephra derived from other volcanoes (including wide-spread tephra) is as follows; Kozushima 838: tuff of Kozushima erupted in A.D. 838; Niijima 886: tuff of Niijima erupted in A.D. 886; AT: Aira-Tanzawa tuff; HK-CC4: Hakone-Mishima pumice; HK-TPfl: Hakone-Tokyo pumice; D-5: Hakone-Anjin pumice; K-Tz: Kikai-Tozurahara tuff; On-Pm1: Ontake pumice 1; Da-4: Hakone-KmP9; Ohito-Brown Pumice: Ashitaka-Ohito brown pumice; Ohito-Pink Pumice: Hakone-KlP11; Ohito-Yellow Pumice2: Hakone-KlP10; Da-1: Hakone-KlP9; Ohito-Yellow Pumice1: Hakone KlP6-8; Ohito-Blue Pumice: KlP4 121 97

Table 1 Major and trace element compositions of basalt and basaltic andesite in the Higaishi-Izu monogenetic volcano group. 98 122

21ka 29ka 22ka 25ka 2 1 17ka 17.5ka 14.5ka 1 5ka 4ka 3.2ka 2.7ka 1989 2700 14.5ka 3.2 ka 3. Fig.4 1995 1.45 m 3km 6km Okada and Yamamoto 1991 1995 15 20km S Fig.4 Map showing the tectonic situation of the Higashi-Izu monogenetic volcano group (revised Koyama, 1995). solid circle: terrestrial eruptive center; open circle: submarine eruptive center; dashed line: eruptive fissure; THTL: Tanna-Hirayama tectonic line; MMP: Manazuru micro-plate; Philippine Sea Plate. 1992 1995 Hmax 1 123 99

15 0.5 1.5cm/ Koyama and Umino 1991 4. Hirano et al. 1982 Hamuro 1985 1985 1990 Umino et al. 1991 Kawamoto 1992 2000 Hamuro 1985 basalt I basalt II basalt III 3 basalt I basalt II basalt III basalt I basalt II basalt III basalt II basalt I Na 2 O basalt I basalt II Hirano et al. 1982 Hamuro 1985 basalt I basalt II Sr, Ba, Ca basalt I Ba/Ca Sr/Ca Ba/Ca basalt I basalt II 1985 zoned magma chamber 1990 1.0 vol. X-rich 1.0 vol. X-poor X-rich X-poor Umino et al. 1991 Type A Type B Type AB Type C 4 Type A Fo Ni Type B Fo Ni Type AB Type A Type B Type C Type B Fo Ni Type B Fe/Mg Fe/Mg Ni/Mg Cr Type A Type B Kawamoto 1992 Mg An Fe Fe Ab 2000 Hamuro 1985 1990 100 124

5. SiO 2 =57wt% Hamuro 1985 Table 1 SiO 2 SiO 2 TiO 2 FeO MnO MgO CaO Na 2 O K 2 O Fig.5 Al 2 O 3 P 2 O 5 K 2 O vs. SiO 2 SiO 2 < 54wt% low-k SiO 2 > 54wt% medium-k Na 2 O+K 2 O vs. SiO 2 SiO 2 < 54wt% SiO 2 > 54wt% FeO /MgO vs. SiO 2 Fig.5 Silica variation diagrams for major element chemistry of basalt and basaltic andesite in the Higashi-Izu monogenetic volcano group. 125 101

6. XRF 3080 E 3 Table1 Rb= 3.46ppm Sr= 11.8ppm Ba= 23.4ppm Y= 1.77ppm Zr= 9.68ppm V= 8.92ppm Cr= 10.4ppm Ni= 1.69ppm Nb= 1.78ppm Cu= 2ppm Zn= 4ppm Ga= 1.02ppm Pb= 2.12ppm Sc= 1.8ppm Th= 3.1ppm 1987 Rb Ba Nb Zr SiO 2 Fig.6 Rb Ba Zr SiO 2 Nb Y 7. Rb Ba Nb Zr Y Zr Y Rb/Zr Ba/Zr Rb/Y Ba/Y Zr/Y Fig.7 Fig.8 Zr/Y Rb/Ba Rb/Zr Ba/Zr Rb/Y Ba/Y Fig.6 Silica variation diagram for incompatible elements of basalt to basaltic andesite in the Higashi-Izu monogenetic volcano group. Symbols are the same as in Fig.5. Rb/Zr Fig.9 150 120 ka 40 20 ka 20 10 ka 4ka Rb/Zr 102 126

Fig.7 Ratios for incompatible elements of basalt and basaltic andesite in the Higashi-Izu monogenetic volcano group. Symbols are the same as in Fig.5. Numeric numbers show the ratio. A large solid circle in Zr/Y diagram shows Kawagodaira dacite. 127 103

Fig.8 Maps showing the spatial variation for ratios of incompatible element. Dashed lines are estimated eruptive fissures. 104 128

129 105

Fig.9 Maps showing the temporal variation of Rb/Zr ratio. Size of circles and other symbols are the sama as in Fig.8. Dashed lines are eruptive fissures and open triangles are eruptive centers shown in Fig.8. 106 130

Fig.10 Diagrams showing the SiO 2 content vs. Rb/Zr and Ba/Zr ratios. Symbols are the same as in Fig.5. Rb/Zr 8. 8. 1 Zr/Y Rb/Zr Rb/Y Ba/Zr Rb/Y Zr vs. Y Zr Y Zr Y Fig.7 SiO 2 SiO 2 Rb/Zr Ba/Zr Fig.10 Umino et al.(1991) TypeB 1990 X-rich Fig.11 Ba/Zr TypeB X-rich 131 107

Ba < Zr Nb<Zr Ba/Zr Nb/Zr Fig.12 Ba/Zr Nb/Zr Ba/Zr Nb/Zr Ba/Zr Nb/Zr r 2 0.12 Fig.13 Fig.11 Map showing the relationship between the spatial variation of Ba/Zr ratio and occurrence of basalt and basaltic andesite with xenocrysts. solid circle: basaltic rocks with xenocrystic quartz; volcano with under line: volcano with X-rich type Miyajima, 1990. Size of circles and other symbols are the same as in Fig.8. Rb/Zr 8. 2 1 2 Fig.12 Partial butch melting model of lherzolite for Ba/Zr and Nb/Zr ratios. The content of Ba, Zr and Nb in lherzolite and bulk-rock partition coefficients of spinel lherzolite and garnet lherzolite for Ba, Zr and Nb are cited from James and Henry (1991). Values in percent show degree of partial melting. solid square and G: garnet lherzolite; open diamond and S: spinel lherzolite. 108 132

Fig.13 Diagram showing the relationship between Ba/Zr and Nb/Zr ratios. Symbols are the same as in Fig.5. Rb/Zr Rb/Y Ba/Zr Ba/Y Zr/Y Rb/Ba Nb/Zr Ba/Zr Ba/Zr Fig.12 Umino et al. 1991 Cr 9. 1 Zr/Y Rb/Ba Rb/Zr Rb/Y Ba/Zr Ba/Y 2 15 3 4 Rb/Zr Rb/Y Ba/Zr Ba/Y 133 109

Zr/Y Rb/Ba 5 Cr 6 1987 1986 8, 32, 57 72 1977 1975 1977, 52, 235 278. Hamuro, K. (1985): Petrology of the Higashi-Izu monogenetic volcano group. Bull. Earthq. Res. Inst., 60, 335 400. 1980 1, 55, 259 297. 1992 1: 0 32ka, 37, 167 181. Hirano, M., Hamuro, K. and Onuma, N (1982): Sr/Ca-Ba/Ca systematics in Higashi-Izu monogenetic volcano group, Izu Peninsula, Japan. Geochem. J., 16, 311 320. James, E.W. and Henry, C.D. (1991): Compositional changes in Trans-Pecos Texas magmatism coincident with Cenozoic stress realignment. J. Geophys. Res., 96 B, 13561 13575. 1992 67, 239 264. Kawamoto, T. (1992): Dusty and honeycomb plagioclase: indicators of processes in the Uchino stratified magma chamber, Izu Peninsula, Japan. J. Volcanol. Geotherm. Res. 49 191 208. 1995, 104, 45 68. Koyama, M. and Umino, S. (1991): Why does the Higashi- Izu monogenetic volcano group exist in the Izu Peninsula?: relationships between late Quaternary volcanism and tectonics in the northern tip of the Izu-Bonin arc. J. Phys. Earth, 39, 391 420. 1995 2: 32 ka 40 191 209. 1990, 85, 315 336. 1985 18 158 174. Okada,Y. and Yamamoto, E. (1991): Dyke intrusion model for the 1989 seismovolcanic activity off Ito, central Japan. J. Geophys. Res., 96B, 10361 10376. 2000 45 149-171. Umino, S., Kato, M. and Koyama, M. (1991) : Diversity of parental magmas of Higashi-Izu monogenetic volcano group. J. Phys. Earth, 39, 371 389. 110 134