Bulletin of the Geological Survey of Japan，vo1，43（7），p．421－456，1992 Pe齢g聡曲ic翻幽geoehe亘謡ea』旦磯9一蹴cv蹴醜藍繍胸歪 vo皿c縄醜蚤c r⑪cks o聾琶he vo且ca醜亘c fr⑪魍t of the臨賦一〇9鋸窺w蹴窺（B磯量麟A罵 Makoto YuAsA＊・＊＊and Masato NoHARA＊ YUASA Makoto and NOHARA Masato（1992）Petrographic and geochemical along－arc variations of volcanic rocks on the volcanic front of the Izu－Ogasawara（Bonin） Arc．B％ll．0601．Sκ卿．ノ4ρα銘，vol．43（7），P．421－456，11fig．，3tab． Abstract：The Shichito－lwojima Ridge is（iiv量ded into two parts based on the rock chemistry of volcanic islands and submarine volcanoes．This division corresponds to the topography．The volcanic rocks of the southem part（Nishinoshima to Minami－lwojima volcanoes）are higher in alkali contents than those of the northem part（Oshima to Sofugan volcanoes）． The rocks from seamounts in the Nishinoshima Trough have different chemical characteristics than the surrounding volcanoes．Sr isotope ratios of the rocks are low relative to the surroundings．They are chemically divided into four groups． Variation trends of incompatible elements for each group are different．This fact suggests that there is a difference in primary magma composition among these groups． In the southem part of the ridge，K20，P205，Ba and Sr contents tend to increase gradually from north to south，and reach maximum at Minami－lwojima and Iwojima volcanoes．The SB diagram shows the lowest degree of partial melting of source mantle in those two volcanoes．87Sr／86Sr ratios also vary in the along－arc direction．The highest ratios are at Minami－Iwojima volcano and the lowest in the Nishinoshima Trough．Four segments are defined by the increasing or decreasing trend of the ratios along the arc．In this saw－toothed variation，the lower parts correspond to structural gaps．There is a systematic difference in the ratios between the yomger and older seamounts in the Nishinoshima Trough，in spite of their closeness in geographical location．This suggests temporal changes in the composition of magma source，the fluid phase derived from subducting slab，or the tectonic stress field． 1960）． 1． 亙脆tro幽亙恩et五〇n Island arc volcanism is characterized by central eruptions and formation of volcano 1．l Aeross一蹉岨謡o踊9一蹴e v蹴訟崩⑪壌しof clustersin a chain．This isdifferentfrommid 我re vO且e我囎董S㎜ －oceanic ridge volcanism at divergent plate Island arc volcanoes，produced at conver－ boundaries where fissure eruptions predomi－ gent plate boundaries，distribute as a chain or nate． Hot－spot volcanism，a charac－ a zone parallel to the trench． The most teristic intraplate volcanic activity，shows a intensive and productive part of the arc volcanism is restricted to a narrow belt Keywords：Izu－Ogasawara Arc，Bonin Arc，along－arc called the“volcanic front”（Sugimura，1958， variation， petrography， Shichito－lwojima Ridge， submarine volcano，seamount，SB systematics，Sr isotope ratio，petrochemistry，Sofugan Tectonic Line，Nishino・ ＊Marine Geology Department． shima Trough，Shichiyo Seamounts，caldera，alkaline ＊＊Present address：Japan Nationa10il Corporation，2－2， volcanic rock，volcanic front，Ohmachi Seamomt， Uchisaiwaicho－2，Chiyodaku，Tokyo100Japan． alkalinity of volcanic rocks 一421一 Bz6116〃n（ゾ孟h6G60Jo9∫o召IS％7∂6ッ（ゾノ4ゆ6z％，Vlol．43，〈b．7 similarity to island arc volcanism in that it of backarc basins affecting segnlentation of has a central emption type forming a chain， the arc． but the volcanoes along the chain are not simultaneously active as in the island arc 1．2 Previo粥 st腿〔量ies on a£ross－ ana volcanoes．The arc volcanic chain seems to a且0聰9－arevari＆tionsofIZU一（）gasa・ be accompanied with a plate subduction． w置a（Bo灘1萱聡）Arc Studies of island arc volcanic rocks demon－ The Izu－Ogasawara Arc is one of the arc strated transverse systematic changes in the －trench systems in the westem Pacific and chemical compositions of volcanic rocks in emerges as several volcanic islands and reefs relation to the distances from the trench axis arranged north to south along its central （e．9．Kuno，1966）and to the dip angle of the axis． There are across－ and along－arc Wadati－Benioff Zone（e．g．Gill，1981）．The chemical variations of volcanic rocks in the number of volcanic edifices tends to decrease Izu－Ogasawara arc（Kuno，19591Yuasa and from the volcanic front to the backarc side， Tamaki，1982；Yuasa，1983，19851Uto，1983； suggesting that the magma generation rate Notsu6渉α1．，19831Notsu，1985）．Uto（1983） decreases toward the backarc side． summarized the previous work for volcanic In addition to the above across－arc variati－ rocks from the northem part of the arc and ons，along－al℃ variations in the configura－ showed the across－arc chemical variation of tion of volcanoes and chemistry of volcanic the rocks． His conclusions are as follows． rocks are also important aspects in the study Incompatible elements including K，Rb，Ba， of arc volcanism．Two kinds of variations and light REE increase from the volcanic exist；one is the segmentation of the arc front toward the backarc side，whereas itself and the other is the gradual change in 87Sr／86Sr，206Pb／204Pb，207Pb／204Pb and208Pb／ morphological and petrochemical properties 204Pb ratios decrease toward the backarc． of volcanoes along the arc or within a Yuasa and Tamaki（1982）divided the segment． volcanic rocks along the Shichito－Iwojima The origin of these variations for different Ridge into two groups in terms of their regions has been discussed as shown below． alkalinity；10w－alkaline in the northem part Origin of segmentation （Oshima to Sofugan islands） and high （1） Fingering of subducting plate －alkaline in the southern part（Nishinoshima Central America（Carr6厩1．，1982） to Minami－lwojima islands）．Yuasa（1983 （2） Change of crustal thickness and1985）concluded that a major fault，the South America（Katsui，1972） Sofugan Tectonic，Line，is separating the （3） Rift breaking into arc northern and southern parts on the basis of Philippines（Defant6渉α乙，1988） contrasting geological and geophysical char－ Origin of gradual change in segnlents acteristics in addition to the rock chemis・ （1）Different degree of partial melting tries． He also suggested that the along－al’c Indonesia（Nishimura and Suparka，1986） variation may be ascribed to the different （2）Mantle heterogeneity modes of subduction of the Pacific Plate or Lesser Antilles（Brownασ1．．1977） differential opening of the backarc basins These examples show the effects of （Shikoku and Parece Vela・basins）． These different behaviors of the subducting Plates studies are based mainly on the rock or different properties of the underlying chemistry of emerged volcanic islands， mantle along the arcs。Arc－trench systems in Because the arc is mostly submerged，data the westem Pacific are often accompanied from the submarine part are essential for with backarc basins as one of important discussion of the along－arc variation． elements on their continental side．Yuasa The subducting Pacific Plate is possibly （1985）suggested a possibility that the opening split along the Sofugan Tectonic Line similar 一422一 P6伽09ηψh♂6ごz多z481600hε〃z乞αzl∂召n。αあo％sαプ孟h612z6一（菊αsσ卿ごz名とz／1名6‘y酪σごz％4〈石ohごz名ヒ∂ to the Cocos Plate subducting beneath new island in1952（Morimoto and Ossaka， Central America． The different alkali 1955），and the other is described at the lower contents of volcanic rocks between the slope of Higashiyama Volcano on Hachijoji－ northern and southern parts of the Izu ma Island（lsshiki，1959）．Therefore，1ess acid －Ogasawara Arc may be ascribed to the volcanic rocks had been regarded to be the difference in subduction angle of the descend－ characteristic rock types along the volcanic ing Plate． Notsuαごz乙（1983）stated that the front ofthe Izu－Ogasawara Arc．Subsequent－ contamination of volcanic rocks from cmstal 1y， acid volcanic rocks were dredged and material is negligible and the mderlying grabbed from several seamounts（Omuroda－ mantle is rather homogeneous beneath the shi；Hamuro α 召1．．1983，Kurose Hole； Izu－Ogasawara Arc based on the restricted Yuasa 6渉σ乙，1981），and obtained from range in Sr isotope ratios of volcanic rocks． Kaitoku Seamomt，a newly erupted If the mantle is homogeneous through the arc seamount（Tsuchideα召1．，1985）．Recently， the differences in alkali contents，especially submarine geologic surveys were carried out that of K，would indicate different degree of in this region and five calderas were found partial melting of the mantle．The possibili・ （Higashi－Aogashima，Kita－Bayonnaise， ty，however，has to be examined in comecti－ Sumisu，Minami－Sumisu and Torishima on with submarine geologic data． calderas：Murakami and Ishihara，19851 In this paper the along－arc variation in Yuasa and Murakami，19851Taylor6渉α1．． petrochemical characteristics of seamounts 1990）．Four of them yielded acid volcanics on the volcanic front of the Izu－Ogasawara （dacitic and rhyolitic pumice）．As described Arc is described． in another paper（Yuasaαα1．，1991），these calderas are mostly submerged and only 2． Petrographic and geochemical aescrip・ small parts of the sommas emerge above sea tions of volcanic rocks from seamounts leve1（e．g．Sumisujima Island and Bayomaise Rocks）．These acid rocks had been neglected The samples describ6d here were dredged in the estimation of Aramaki and Ui（1978）． or grabbed from seamounts during the scien－ tific cmises of R／V Hakurei－mam（GH84－2 2．1．1 K％70s6鼠）」6 to GH87－3）for the purpose of research on Large amomts of dacitic pumice and algal hydrothermal activity in the island arc limestone were recovered by dredge D836 （Nakaoασ1．，1990）．Locations of the dredge from the imer wall of Kurose Hole．Dacitic and grab sampling sites are given in Fig．1． pumice is composed of phenocrysts of plagio－ The descriptions of submarine topographic clase and