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Geology

Strontium isotopic relations of bimodal volcanic rocks at Kikai volcano in the Ryukyu arc, Japan

Kenji Notsu, Koji Ono and Tatsunori Soya

Geology 1987;15;345-348 doi: 10.1130/0091-7613(1987)15<345:SIROBV>2.0.CO;2

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Strontium isotopic relations of bimodal volcanic rocks at Kikai volcano in the RyukyQ arc, Japan

Kenji Notsu Institute of Chemistry, University of Tsukuba, Sakura-mura Niihari-gun, Ibaraki 305, Japan Koji Ono, Tatsunori Soya Geological Survey of Japan, Yatabe-machi, Tsukuba-gun Ibaraki 305, Japan

ABSTRACT Figure 1 shows the outline of Kikai caldera together with the geographical Bintodal associations of mafic and silicic rocks are observed at distribution of Quaternary volcanoes in Japanese island-arc systems. Kikai volcano in the Ryukyu arc, Japan. We determined ^Sr/^Sr The most important geochemical or petrological feature of volcanic ratios of 16 representative volcanic rocks of this volcano. The ratios rocks from Kikai volcano is their bimodal distribution of Si02 content. scatter in the narrow range between 0.70477 and 0.70508, despite the Ono et al. (1982) pointed out that SiOj content of the volcanic rocks from varying S1O2 content of the rocks, except for one sample with a ratio Kikai volcano ranges from 50% to 57% and from 68% to 72%. Rocks of of 0.70539. This suggests that both basaltic and rhyolitic rocks at intermediate composition are lacking throughout the entire history of the Kikai volcano are generated from the same source material, in con- volcano. Recently, more detailed chemical analyses (Ujike et al., 1986) trast to the separate origin required for the bimodal volcanism typi- confirmed the bimodal nature of this volcano, and a volcanic rock with cally observed in the western United States. The origin of the bimodal Si02 content of 59.48% has been reported. In contrast to Kikai volcano, volcanism in the island-arc setting is distinctly different from that in rocks from all other volcanoes in the Ryukyu arc have mainly andesitic the continental setting, in view of source materials of basaltic and compositions and do not show the bimodal association of basaltic and rhyolitic magmas. rhyolitic rocks. For the rest of the Japanese arc, bimodal volcanism is observed for only two Quaternary volcanoes, as shown in Figure 1. They INTRODUCTION are Usu volcano in the northeast Japan arc (Oba, 1966) and the Higashi- Several Quaternary volcanoes located along the southwest Japan and Izu monogenetic volcano group in the Izu-Ogasawara arc (Hamuro, Ryukyu arcs are related to the subduction of the Philippine Sea plate 1985). Konda (1974) reported that bimodal volcanism took place in beneath the Eurasian plate. Kikai volcano, belonging to the Ryukyu arc, northeastern Honshu, Japan, in Tertiary time. has a large caldera structure and is one of the active volcanoes in Japan. Bimodal volcanism is typically observed for late Cenozoic volcanoes

130°15'E 130°30'E

Figure 1. Outline and lo- cation of Kikai volcano. Solid triangles = land por- tions of central cones in caldera; x = inferred cen- ters of submarine erup- tions (Ono et al., 1982); - 38fN solid circles = Quaternary volcanoes that yield volcanic rocks with bimodal distributions of Si02 content; open circles = other Quaternary volcanoes.

132°E

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in the western United States (Hamilton, 1965; Christiansen and Lipman, ical, geochemical, and geophysical, were carried out; these were reviewed 1972). Contemporaneous association of basalt and rhyolite has also been by Ono et al. 11982). reported everywhere in the world (Yoder, 1973; Sigurdsson and Sparks, A total of 16 samples of representative eruptive products from the 1981). Various kinds of models for the origin of the bimodal volcanism three stages of Kikai volcano were selected and analyzed (Table 1). Petro- have been proposed (see review in Doe et al., 1982), but it seems to be graphical studies and major-element data for these samples were pre- poorly understood. Isotopic studies on bimodal volcanism in the western viously reported (Ono et al., 1982; Ujike et al., 1986). United States show that basalt and rhyolite have distinctly different isotopic compositions (Lipman et al., 1978; Doe et al., 1982; Bacon et al., EXPERIMENTAL PROCEDURES AND RESULTS 87 86 1984). For example, Sr/ Sr ratios of basalts and rhyolites from the Experimental procedures for determining strontium isotopic compo- Yellowstone plateau volcanic field were measured to be 0.7035 to 0.7089 sitions and concentrations of Sr and Rb were reported by Notsu (1983). and 0.7084 to 0.7268, respectively. These results are consistent with the The strontium isotopic compositions were determined on a VG- hypothesis of derivation of the basaltic and rhyolitic magma by partial Micromass MM-30 double-collector-type mass spectrometer at the Chem- melting of distinct source regions in the upper mantle and lower crust, ical Analysis Center of the University of Tsukuba. The 87Sr/86Sr ratio of respectively (Doe et al., 1982). In contrast to Basin and Range tectonism- NBS 987 standard sample was 0.71030 ± 0.00003 from the replicated related volcanism, strontium isotopic compositions were reported from analyses. Concentration of Rb and Sr were measured by X-ray fluores- bimodal volcanism from an island-arc setting. These compositions are cence spectometry. similar for both basaltic and rhyolitic rocks from Usu volcano (Oba et al., Analytical results are shown in Table 1. The 87Sr/86Sr ratios in six 1983). volcanic rocks of the precaldera stage scatter in the range between 0.70485 In this paper, we present strontium isotopic data for volcanic rocks and 0.70539, and the ratios of five of these six samples concentrate in the from Kikai volcano, discuss the origin of bimodal volcanism in the island- range 0.70485 to 0.70508. The ratios of four samples of the caldera- arc setting, and compare it with bimodal volcanism in a continental set- forming stage and six samples of the postcaldera stage also concentrate in ting. We also compare bimodal volcanism with nonbimodal volcanism in the limited ranges 0.70477 to 0.70485 aind 0.70488 to 0.70499, respec- the Japanese arcs. tively. The range of the ratios of the postcaldera stage overlaps that of the precaldera stage. Although the differences of the 87Sr/86Sr ratios in rocks KIKAI VOLCANO AND SAMPLES from the three stages are greater than analytical uncertainties, throughout According to Ono et al. (1982), Kikai volcano formed in late Qua- the whole histoiy of Kikai volcano (with the exception of 751-015) the ternary time. The history of the volcano is divided into three successive ratios vary by only 0.0003. stages: precaldera, caldera-forming, and postcaldera. In each stage, the bimodal distribution of Si02 content in volcanic rocks is observed. The DISCUSSION most recent eruption of Kikai volcano took place between A.D. 1934 and Strontium isotopic relations of bimodal volcanic rocks at Kikai vol- 1935 and formed a new islet, Shin-Iojima, in the caldera. At present, cano are distinctly different from those of Cenozoic volcanoes in the powerful fumaroles are active on and near the summit of Iodake in Iojima western United States (Lipman et al., 1978; Doe et al., 1982; Bacon et al., Island, another postcaldera cone in the caldera. After the 1934-1935 1984). Whereas volcanoes of the western United States require separate eruption, various kinds of investigations, including volcanological, geolog- sources for the b imodal lavas, the data from Kikai volcano indicate that it

TABLE 1. DESCRIPTIONS AND ANALYTICAL RESULTS FOR SAMPLES FROM KIKAI VOLCANO

Sample Description Rock SiO Rb Sr 87Rb/86Sr 87Sr/86Sr$ no. type (%T (ppm) (ppm)

Precaldera stage 75TK38 Lava of Magomeyama volcano Basalt 50.12 7,. 9 268 0.086 0.70505 75TK42-1 Lava of Takahirayama volcano Basalt 50.88 9,. 7 265 0.107 0.70485 7810150 Lava of Yahazuyama volcano Basalt 53.36" 15.. 0 262 0.167 0.70508 751015 Lava of Yahazuyama volcano Andesite 55.88,, 26 265 0.29 0.70539 7810155-2 Takeshimanounose lava of a pre-caldera volcano Andesite 59.48" 37 244 0.44 0.70485 75IÓ8 Nagahama lava of a pre-caldera volcano Rhyolite 71.96 80 167 1.39 0.70489

Caldera-forming stage 75I07G-1 Lens of welded tuff in Koabiyama pyroclastic flow deposit Rhyolite 71.74 78 168 1.35 0.70485 75TK22-2 A pumice in Nagase pyroclastic flow deposit Rhyolite 73.26 67 164 1.19 0.70477 75I09P1 A pumice in Takeshima pyroclastic flow deposit Rhyolite 68.46,, 70 185 1.10 0.70485 75TK23B1 A black pumice in Takeshima pyroclastic flow deposit Andesite 58.00 35 268 0.38 0.70479

Postcaldera stage 78IÓ152-1 South lava of Inamuradake volcano Basalt 6.9 274 0. 073 0. 70496 7810167-1 East lava of Inamuradake volcano Basalt 51.46 6.7 267 0. 073 0. 70499 751020 Isomatsuzaki lava of Inamuradake volcano Andesite 54.56,, 16.8 301 0. 162 0. 70488 78IÓ165-1 Asase lava of a post-caldera volcano Rhyolite 71.19" 73 187 1. 14 0. 70488 IW74S-13 A lava of Iodake_volcano Rhyolite 70.50 72 186 1. 13 0. 70497 IW74S-24 A lava of Shin-Iojima volcano Rhyolite 70.60 75 180 1. 21 0. 70495

* Determined by conventional chemical analys is (Ono et al., 1982) , except values with //. + Uncertainties are less than 5%. $ Uncertainty is less than 0.00005 from the replicated analyses of the .standard sample. // Determined by X-ray fluorescence analysis (Ujike et al., 1986).

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0.706 1 1 1 0.707

87Sr/ 88 Sr •7Sr/MSr

\ Kikai volcano

0.705 - /•''.' ° 0.706 1 \ \ o • o s A s V A'

/ Kirishlma volcano

fA \ 0.704 - 0.705 _ I V. - I Usu volcano I •

• — ° o O Oo I Towada volcano 0.703 1 .... 1 ^ 0.704 50 60 70 SiO, <%) Hachijôiima volcano 87 86 Figure 2. Relations between Sr/ Sr ratios and SI02 content for bimodal volcanic rocks from Japanese island-arc volcanoes. Open circles = precaldera stage of Klkai volcano in Ryukyu arc; open triangles = caldera-forming stage of Kikai volcano; solid circles = post- 0.703 caldera stage of_Kikai volcano; solid triangles = Usu volcano in north- 50 60 70 east Japan arc (Óba et al., 1983). Si02 (%)

87 86 Figure 3. Relations between Sr/ Sr ratios and Si02 content for nonbimodal volcanic rocks from Japanese island-arc volcanoes. Solid circles = Kirishlma volcano in Ryükyü arc (Kurasawa et al., 1986); open circles = Towada volcano in northeast Japan arc (Kurasawa et al., 1986); solid triangles = Hachljöjima volcano in Izu-Ogasawara arc (Notsu et al., 1983). does not. If the crustal thickness of the southern Kyushu district, including Kikai volcano, is about 40 km (Ono et al., 1978), this thickness may be sufficient to generate rhyolitic magma by partial melting of the crustal materials under Kikai volcano. The 87Sr/86Sr ratio data of the local crustal materials under the volcano are not available, but we assume the ratios to be above 0.7060, judging from the ratios of Tertiary plutonic rocks in the both basaltic and rhyolitic rocks. Various kinds of 87Sr/86Sr-SiC>2 rela- southern Kyushu district (Yanagi, 1975; Shibata and Ishihara, 1979). If tions are observed for nonbimodal volcanoes. Volcanic rocks from Ha- rhyolitic magma is generated by the partial melting of crustal materials, it chijojima volcano in the Izu-Ogasawara arc have nearly constant 87 86 87 86 is reasonable to expect that Sr/ Sr ratios of rhyolites must be higher Sr/ Sr ratios despite varying Si02 content (Notsu et al., 1983), and than 0.7060. However, our data show that both rhyolitic and basaltic those from Towada volcano in the northeast Japan arc have somewhat 87 86 rocks in Kikai volcano have similar Sr/ Sr ratios distinctly lower than variable ratios (Kurasawa et al., 1986). In contrast to this, the ratios of 0.7060, suggesting that both rocks are derived from the same source volcanic rocks from Kirishima volcano in the Ryukyu arc vary signifi- material, which is not a crustal source. Beneath Kikai volcano, basaltic and cantly and are explained in terms of the combined processes of crystal rhyolitic magmas may be generated by the fractional fusion of the same fractionation and wall-rock assimilation (Kurasawa et al., 1986). In Japa- source material, as proposed by Yoder (1973). Otherwise, rhyolitic mag- nese arcs, many volcanoes yield volcanic rocks with similar or slightly mas may be generated by the fractional crystallization of basaltic magmas variable 87Sr/86Sr ratios, like Hachijojima or Towada volcano. The varia- 87 86 without contamination by other materials with different Sr/ Sr ratios. tions of the ratios in the same volcano are found to be within 0.0002 and The single-source model for the bimodal volcanism at Kikai volcano, 0.0003 in the Izu-Ogasawara arc (Notsu et al., 1983) and the northern and suggested by the strontium isotopic data, is also supported by Ujike et al. central regions of the northeast Japan arc (Notsu, 1983), respectively. A (1986), who concluded that all the Kikai magmas were ultimately derived similar isotopic feature is observed for volcanic rocks generated by bimod- from a single source of the modified mantle material, on the basis of al volcanism in the island-arc setting, except that volcanic rocks that have

major- and trace-element composition. intermediate Si02 content are lacking. Figures 2 and 3 summarize relations between 87Sr/86Sr ratios and Recent extensive studies on strontium isotopic composition of vol-

SiOz content of Japanese arc volcanic rocks with bimodal and nonbimo- canic rocks from Japanese arc volcanoes revealed that each arc is charac- 87 86 dal distributions of Si02 content, respectively. For the Usu volcano, terized by Sr/ Sr ratios that reflect the interaction of the mantle wedge 87 86 87 86 Sr/ Sr ratios of basalt to mafic andesite (Si02 = 49.36%-53.36%) and and subducting oceanic slab. For example, Sr/ Sr ratios of volcanic

dacite to rhyolite (Si02 = 67.07%-72.35%) are 0.70396 to 0.70410 and rocks from the Izu-Ogasawara arc are 0.7033-0.7039 (Notsu et al., 1983), 0.70401 to 0.70407, respectively (Oba et al., 1983). Therefore, two cases and those from the northeast Japan arc are 0.7031-0.7045, except in the of bimodal volcanism are characterized by the similar 87Sr/86Sr ratios for southernmost region where the ratios are anomalously high (Notsu, 1983).

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At present, data are available for only two volcanoes in the Ryukyu arc. Kurasawa, H., Fujinawa, A., and Leeman, W.P., 1986, Calc-alkaline and tholeiitic Sakurajima volcano in Aira caldera, located about 100 km north of Kikai rock series magmas coexisting within volcanoes in Japanese island arcs— volcano, yields volcanic rocks that have 87Sr/86Sr ratios of 0.7051 to Strontium ¿»topic study: Geological Society of Japan Journal, v. 92, p. 255-268. 0.7058 (Kurasawa et al., 1984), and Kirishima volcano, about 140 km Lipman, P.W., Doe, B.R., Hedge, C.E., and Steven, T.A., 1978, Petrologic evolu- north of Kikai volcano, has ratios ranging from 0.7045 to 0.7069 (also see tion of the Sun Juan volcanic field, southwestern Colorado: Pb and Sr isotope Fig. 3) (Kurasawa et al., 1986). These data, combined with Kikai data, evidence: Geological Society of America Bulletin, v. 89, p. 59-82. show that 87Sr/86Sr ratios of volcanic rocks from the Ryukyu arc are Notsu, K., 1983, Strontium isotope composition in volcanic rocks from the North- regionally heterogeneous and are higher than those from either the northeast Japan lire: Journal of Volcanologv and Geothermal Research, v. 18, p. 531-548. east Japan arc or the Izu-Ogasawara arc. This means that the mantle Notsu, K., Isshik , N., and Hirano, M., 1983, Comprehensive strontium isotope 87 86 wedge under the Ryukyu arc has higher Sr/ Sr ratios than under the study of Qut ternary volcanic rocks from the Izu-Ogasawara arc: Geochemical other two arcs. It is interesting that only Kikai volcano lacks volcanic Journal, v. 17, p. 289-302. rocks with intermediate SiC>2 content among all other volcanoes in the O, Y., 1966, Geology and petrology of Usu volcano, Hokkaido, Japan: Hokkaido Ryukyu arc that have dominant amounts of andesitic rocks, despite similar University, Faculty of Science Journal, ser. IV, v. 13, p. 185-236. Oba, Y., Katsui, Y., Kurasawa, H., Ikeda, Y., and Uda, T., 1983, Petrology of plate-tectonic settings throughout the whole region of the arc. The same is historic rhyolite and dacite from Usu volcano, north Japan: Hokkaido Univer- true for Usu volcano in the northeast Japan arc. Further investigations will sity, Faculty of Science Journal, ser. IV, v. 20, p. 275-290. be needed to make clear the detailed mechanisms of bimodal volcanism in Ono, K., Ito, K., liasegawa, I., Ichikawa, K., Iizuka, S., Kakuta, T., and Suzuki, H., island-arc settings. 1978, Explosion seismic studies in south Kyushu especially around the Sakura- jima volcano: Journal of Physics of the Earth, v. 26, suppl., p. 309-319. Ono, K., Soya, T., and Hosono, T., 1982, Geology of the Satuma-iô-jima district: CONCLUSIONS Geological Survey of Japan Quadrangle Series, 80 p., scale 1:50,000. 87 86 Strontium isotopic data demonstrate that basaltic and rhyolitic rocks Shibata, K, and Ishihara, S., 1979, Initial Sr/ Sr ratios of plutonic rocks from in Kikai volcano are derived from the same source material, in contrast to Japan: Contributions to Mineralogy and Petrology, v. 70, p. 381-390. Sigurdsson, H., and Sparks, R.S.J., 1981, Petrology of rhyolitic and mixed magma the separate sources required for the bimodal volcanism typically observed ejecta from :he 1875 eruption of Askja, Iceland: Journal of Petrology, v. 22, in the western United States. Thus, the nature of bimodal volcanism in p. 41-84. island-arc settings is distinctly different from that in continental settings. Ujike, O., Soya, T., and Ono, K, 1986, Major-element, Rb, Sr, Y and Zr composi- We must postulate different models for the origins of these two types of tion and origin of volcanic rocks from Kikai caldera, south of Kyushu: Japa- bimodal volcanism. nese Association of Mineralogy, Petrography and Economic Geology Journal, v. 81, p. 105-115. Yanagi, T., 1975, Rubidium-strontium model of formation of the continental crust REFERENCES CITED and the granite at the island arc: Kyushu University, Faculty of Science Mem- Bacon, C.R., Kurasawa, H., Delevaux, M.H., Kistler, R.W., and Doe, B.R., 1984, oirs, ser. D, v. 22, no. 2, p. 37-98. Lead and strontium isotopic evidence for crustal interaction and compositional Yoder, H.S., 1973, Contemporaneous basaltic and rhyolitic magmas: American zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of Mineralogist, v. 58, p. 153-171. the Coso volcanic field, California: Contributions to Mineralogy and Petrol- ogy, v. 85, p. 366-375. ACKNOWLEDGMENTS Christiansen, R.L., and Lipman, P.W., 1972, Cenozoic volcanism and plate-tectonic Partially supported by grant 60540363 from the Ministry of Education, evolution of the western United States. II. Late Cenozoic: Royal Society of Science and Cullure, Japan. 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Manuscript accepted December 30,1986 Hamuro, K., 1985, Petrology of the Higashi-Izu monogenetic volcano group: Uni- versity of Tokyo Earthquake Research Institute Bulletin, v. 60, p. 335-400. Konda, T., 1974, Bimodal volcanism in the Northeast Japan arc: Geological Society of Japan Journal, v. 80, p. 81-89. Kurasawa, H., Arai, F., and Machida, H., 1984, Strontium isotopic identification of the widespread Aira-Tn ash-fall deposits (AT) in Japan: Kazan (Volcanologi- cal Society of Japan Bulletin), ser. II, v. 29, p. 115-118.

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