Abstract
Systematic measurements were made on initial (87Sr86Sr) ratios and Rb, Sr and K contents of the rocks from Izu, Ogasawara (Bonin), Iwojima, Mariana, Yap and Palau Islands. The initial (87Sr86Sr) ratios for volcanic rocks from Izu, Iwojima and Mariana Islands fall in a range from 0.7030 to 0.7050, which is similar to previously reported values for island-arc volcanics. However, a high initial (87Sr86Sr) ratio (~ 0.7052) was found for rocks from Ogasawara Islands, suggesting that they are genetically different from other islands. This result may be related to the particular topographical feature of the trench discontinuity in the eastern side of the Ogasawara Islands. On the contrary, rocks from the Palau Islands show low initial (87Sr86Sr) ratios (~ 0.7028), which are similar to those of oceanic ridge basalts, and Rb and Sr contents similar to those of normal island-arc volcanics. A low initial (87Sr86Sr) ratio was found for the rock from the Yap Islands, supporting the oceanic origin of them as suggested by Shiraki. It is likely that Palau, Yap and Mariana Island arcs represent the thrëe different stages of development of island arcs. These results suggest that except for the Ogasawara Islands, the island arcs in the western Pacific have not been formed by the splitting or drifting of continental margin, but have grown from oceanic structure.
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