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Geology.Gsapubs.Org on April 15, 2012 Downloaded from geology.gsapubs.org on April 15, 2012 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 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Geological Society of America Downloaded from geology.gsapubs.org on April 15, 2012 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 vol- canic rocks with bimodal distributions of Si02 con- tent; open circles = other Quaternary volcanoes. 132°E GEOLOGY, v. 15, p. 345-348, April 1987 345 Downloaded from geology.gsapubs.org on April 15, 2012 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 iso- topic 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.
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