Study on the Process of Island Arc Formation Inferred from the Bonin Islands

研究的目的是了解硅质大陆地壳是如何由基性海洋地壳形成的。比较了未成熟的小笠原群岛和成熟的日本弧的火山岩。1998财政年度的重要成果如下：总结了博宁群岛七岛Mikadukiyama火山岩：Mikadukiyama火山岩中除Rb和K外的不相容元素比博宁系岩石更丰富，表明前者在较少的含水条件下形成，来源较少。在琉球群岛久米岛北岸上新世上宿竹组最底层发现了镁铁含量超过16%的苦橄岩玄武岩。其中橄榄石Fo最高，为92.2，铬铁矿Cr2O3最高，为56.5%，岩浆具有难熔性。1认为苦橄玄武岩是由高温地幔柱引起冲绳海槽弧后扩张而形成的。古米岛以东的大尾岛高镁安山岩具有高Sr/Y和Sr/Rb的海相亲缘关系。这是首次在日本群岛发现巴海特。对佐贺县中部的蛇之花山周围的散努基岩进行了研究。其中铌含量高（29 ~ 36ppm），含镁84.7、铬203 0.58%的铜铜矿。它们代表了九州西北部高铌铜安山岩的最东端产状。西野木半岛北部的哈里岛和佐贺县西部的有田的山之岩中铌含量较高（分别为16-21和17-18 ppm），而西野木半岛南部的青铜安山岩中铌含量较低（<10 ppm）。5 .九州西北地区的sanukites中Nb含量由北松浦半岛北部的sanukites （39 ~ 64 ppm）向南递减。对密克罗尼西亚帕劳群岛的火山岩进行了初步调查；自第二次世界大战以来，样本首先从巴伯鲁道布岛的内部收集。高镁安山岩是在巴伯恩道布火山岩中发现的，代表了帕劳群岛的最低层序。来自帕劳群岛的火山岩在化学成分上与来自博宁群岛哈哈哈岛的拉斑岩和钙碱岩相似。少

The purpose of the study is to know how the silicic continental crust forms from the mafic oceanic crust. comparing volcanic rocks from the immature Bonin Islands with those from the mature Japanese arcs. The important results in fiscal year 1998 are as follows.1.Mikadukiyama volcanic rocks in Chichi-jima, Bonin Islands, were summarized : the Mikadukiyama volcanics are richer in incompatible elements except for Rb and K than the boninite series rocks, suggesting that the former have formed under less hydrous conditions from less depleted sources than the latter.2.Picrite basalts with more than 16 % MgO were discovered from the lowest of the Pliocene Uegusukudake formation on the north coast of Kume-jima, Ryukyu Islands. Their highest Fo of olivine is 92.2 and Cr2O3 of chromite is 56.5 %, indicating refractory nature of their magmas. 1 suggested that the picrite basalts were generated by high temperature mantle plume causing Okinawa Trough back-arc spreading.3.The high-Mg andesites from … More Ou-jima, immediately east of Kume-jima, have bahaitic affinities with high Sr/Y and Sr/Rb. This is the first finding of bahaite from the Japanese Islands.4.Sanukitic rocks around Mt. Oninohana-yama, central Saga Prefecture, were investigated. The sanukites have high Nb contents (29-36 ppm) and contain bronzite with Mg^# 84.7 and Cr203 0.58 %. They represent the easternmost occurrence of high-Nb bronzite andesites in northwest Kyushu.5.Sanukites from Hario-jima, northern Nishisonogi Peninsula and Arita, western Saga Prefecture, have moderately high Nb (16-21 and 17-18 ppm, respectively), and bronzite andesites from the southern Nishisonogi Peninsula are low in Nb (<10 ppm). The Nb contents of sanukitoids in northwest Kyushu decrease from sanukites (39-64 ppm) in the northern Kitamatsuura Peninsula to the south.6.Preliminary investigation was carried out on the volcanic rocks of the Palau Islands, Micronesia ; samples were collected first from the inner part of Baberudaob Island since World War II.High-Mg andesites were discovered from the Baberndaob volcanic rocks representing the lowest sequence of the Palau Islands. The volcanic rocks from the Palau Islands are similar in chemical composition to tholeiites and calc-alkali rocks from Haha-jima in the Bonin Islands. Less