Musashi High School, Japan

The approximately 50 Myr old Izu-Bonin-Mariana (IBM) arc consists mostly of Oligocene middle and lower crust that underlies the upper crust; these units are in turn covered by Quaternary volcanic rocks. Seismic imaging, forearc geology, Ocean Drilling Program drilling and magnetic anomalies suggest that most IBM arc crust was created in Eocene-Oligocene times. However, remnants of this old crust have never been found at the northern end of the arc, where it is colliding with the Honshu arc (Izu collision zone). Two batholiths in this collision zone (the Tanzawa tonalites and the Kofu Granitic Complex) were emplaced during the Miocene (4-17 Ma). Major elements, Zr/Y, rare earth element ratios and normalized abundance patterns, and Sr-Nd isotopic data indicate that these plutonic bodies are compositionally similar to the Oligocene IBM volcanic rocks, and that they are dissimilar to the Miocene, Pliocene and Quaternary IBM lavas and volcaniclastic rocks. We suggest that the Miocene plutonic rocks in the Izu collision zone were derived from partially melted Oligocene middle crust. A model is proposed in which IBM arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the Honshu and IBM arcs. This resulted in the complete loss of chronological information related to their original source.