Volcanic and Tectonic Framework of the Hydrothermal Activity of the Izu-Bonin Arc

Abstract

In the Izu -Bonin Arc, hydrothermal activities have been reported from volcanoes along present-day volcanic front, a rear arc volcano and a back-arc rift basin as well as a remnant arc structure now isolated from the Quaternary arc. It is widely known that characteristics of hydrothermal activity (mineralogy, chemistry of fl uid etc.) vary depending upon its tectonic setting. The Izu -Bonin Arc has experienced repeated back-arc or intra-arc rifting and spreading and resumption of arc volcanism. These characteristics make this arc system a suitable place to study the tectonic control on hydrothermal activity. The purpose of the present paper is, therefore, to summarize volcanotectonic setting and history of the Izu -Bonin Arc in relation to the hydrothermal activity. The volcanotectonic history of the Izu -Bonin Arc can be divided into fi ve stages: (i) fi rst arc volcanism (boninite, high-Mg andesite), 48 -46 Ma; (ii) second arc volcanism (tholeiitic, calc-alkaline), 44 -29 Ma; (iii) fi rst spreading of back-arc basin (Shikoku Basin), 25 -15 Ma; (iv) third arc volcanism (tholeiitic, calc-alkaline), 13 -3 Ma; and (v) rifting in the back-arc and tholeiitic volcanism along the volcanic front, 3 -0 Ma. Magmas erupted in each stage of arc evolution show different chemical characteristics from each other, mainly due to the change in composition of slab-derived component and possibly mantle depletion caused by melt extraction during back-arc spreading and prolonged arc volcanism. In the volcanotectonic context summarized here, hydrothermal activity recognized in the Izu -Bonin Arc can be classifi ed into four groups: (i) present-day hydrothermal activity at the volcanic front; (ii) active hydrothermal activity in the back arc; (iii) fossil hydrothermal activity in the back-arc volcanoes; and (iv) fossil hydrothermal activity in the remnant arc. Currently hydrothermal activities occur in three different settings: submarine caldera and stratocones along the volcanic front; a back-arc rift basin; and a rear arc caldera. In contrast, hydrothermal activities found in the back-arc seamount chains were associated with rear arc volcanism in Neogene after cessation of back-arc spreading of the Shikoku Basin. Finally, sulfi de mineralization associated with boninitic volcanism in the Eocene presumably took place during forearc spreading in the initial stage of the arc. This type of activity appears to be limited during this stage of arc evolution. Keywords : hydrothermal activity , Izu -Bonin Arc , magmatism , volcanic history . 207Sea Plate ( Fig. 1 ). Hydrothermal activity related to the volcanism has been found in the volcanic front, active back-arc rifts, back-arc seamount chains and remnant arc structures (e.g. Urabe & Kusakabe, 1990;Usui & Nishimura, 1992;Iizasa et al. , 1999 ). Thus, in the IzuBonin Arc, hydrothermal deposits were formed in different tectonic setting and related to different types of volcanic activity. The current arc -back-arc system is relatively young (<15 Ma) and hydrothermal deposits have no...

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