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

点击翻译按钮获取中文摘要

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