小笠原海嶺海溝側斜面および母島海山から採取された MORB 様玄武岩の多様性</br> ─伊豆・小笠原弧前弧域のテクトニクスの検討─

Abstract

Geochemical compositions of the mid-ocean ridge basalt (MORB)-like basalts dredged from the trench slope of the Ogasawara Ridge and the Hahajima Seamount, southern Izu-Bonin fore arc are determined to obtain constraints on the tectonic evolution in the Izu-Bonin arc. The trench slope sample of the Ogasawara Ridge has a distinct enriched (E)-MORB chemistry unlike any other rocks previously sampled in the Izu-Bonin fore arc. The basalt is moderate TiO2, low Zr/Nb, low Ba and is enriched in light rare-earth elements. This sample could represent a trapped remnant of the Philippine Sea plate on which the Izu-Bonin arc was built. On the other hand, two geochemical groups can be identified within the Hahajima Seamount basalts: Low-Ti type basalts and High-Ti type basalts. The Low-Ti type basalts are characterized by depleted in high-field strength (HFS) elements, lower 143Nd/144Nd ratio and high Zr/Nb compared with the High-Ti basalts. Trace element and isotopic compositions of the Low-Ti type basalts are similar to normal (N)-MORB from the West Philippine Basin. In contrast, the High-Ti type basalts show an overall Jurassic Pacific N-MORB signature, marked by high 143Nd/144Nd ratio and enriched in HFS elements. The available MORB-like basalts data show that the Low-Ti type basalts occur in the western side and the High-Ti type ones in the eastern trench side of the seamount. Geochemical signatures and occurrences of the two types of MORB-like basalts suggest that the Hahajima Seamount might compose an uplifted ophiolitic massif of the West Philippine Basin and an accreted piece of subducting Pacific Plate, which have been caused by collision and subduction of the Ogasawara Plateau on the Pacific Plate.