INFLUENCE OF PRE-EOCENE FEATURES OF THE PHILIPPINE SEA PLATE ON THE EARLY IZU-BONIN-MARIANA ARC

In widely accepted models of the initiation of subduction to form the early Izu-Bonin-Mariana (IBM) arc, arc magmatism begins when old, dense oceanic lithosphere of the Pacific plate subsides and ultimately subducts beneath the younger oceanic lithosphere of the Philippine Sea plate. We have studied the geochemical characteristics of igneous rocks from some of the oldest features of the Philippine Sea plate, including the early Cretaceous Amami Plateau and the Paleocene Daito Ridge, which are remnant island arcs, and the early Cretaceous Huatung Basin, which is oceanic. Geochemical evidence suggests that pre-existing island arc lithosphere of the Philippine Sea plate influenced the development of the early IBM arc. In some areas, the presence of arc-related, low density, felsic material within the lithosphere may have caused enhanced differentiation of primitive early IBM arc magmas, and assimilation of this material also may have occurred, leaving a geochemical imprint.

Isotopic characteristics of early Cretaceous rocks from the Amami Plateau and the Huatung Basin present conflicting evidence about the origin of the Indian Ocean isotopic signature that is characteristic of Philippine Sea plate arc and basin igneous rocks, including those of the IBM arc-basin system. Amami Plateau rocks have Pb, Nd and Hf-isotopic signatures intermediate between those of Indian and Pacific Ocean MORB, whereas Huatung Basin basalts are more typically Indian, especially for Nd and Hf-isotopes. Igneous rocks from the oldest sections of the Philippine Sea plate may have formed in areas that were not strongly influenced by Indian Ocean-type mantle sources.