Host: Japan Association of Mineralogical Sciences
The Izu-Bonin-Mariana (IBM) arc is currently active, but this 50-million-year-old subduction-zone system contains Oligocene middle and lower crust below the upper crust, that is in turn covered by Quaternary volcanic rocks. However, remnants of this old crust segments have never been found at the northern end of the Izu-Bonin arc, where it is colliding with the Honshu arc. Two large plutons in the collision zone (the Tanzawa tonalites (and their syn-plutonic dikes) and the Kofu Granitic Complex) were emplaced in the Miocene (4-17 Ma). We show here that major elements, trace element ratios of Zr/Y, and REE patterns indicate that the Kofu Granitic Complex and Tanzawa tonalites (and their syn-plutonic dikes) are more akin to the Oligocene IBM volcanic rocks, and that they are dissimilar to the Neogene and Quaternary Izu-Bonin magmas. One possibility is that Miocene plutonic rocks in the Izu collision zone were derived from the Oligocene middle crust.
