The Izu Peninsula, Japan: Zircon geochronology reveals a record of intra-oceanic rear-arc magmatism in an accreted block of Izu–Bonin upper crust

The Izu Peninsula, central Japan, is situated in a zone where the active intra-oceanic Izu–Bonin arc has been colliding end-on with the mainland Honshu arc for the past 15 million years. As a result of this arc–arc collision, parts of the submarine Izu–Bonin upper crustal sequences have been accreted and uplifted to form the Izu Peninsula, exposing seafloor volcaniclastic deposits, associated lava flows, and coeval intrusive bodies. Parts of this sequence have been subjected to extensive hydrothermal alteration, and these altered rocks have previously been interpreted as representative of hypothetical widespread Middle Miocene basement that presumably underlay northern Izu–Bonin arc volcanoes. New zircon U–Pb ages presented here, however, show that both fresh and altered volcanic sequences exposed in Izu Peninsula are broadly contemporaneous and were products of the same Late Miocene to Pleistocene magmatism. Geochemical characteristics of these sequences show them to have formed in the Izu–Bonin rear-arc environment, providing an unusual opportunity to investigate in detail the growth and architecture of a rear-arc region in an active intra-oceanic arc. Moreover, zircon ages from altered basal units of Kozushima and Niijima, Quaternary volcanic islands in the northern Izu–Bonin rear-arc, show that these islands rest on units only slightly older (< 1 Ma) than the main body of these subaerial edifices, not, as previously believed, part of a regional older Miocene basement. The near-continuum growth of these arc volcanoes and their underlying successions, plus the absence of a distinctly older basement underlying the Izu Peninsula and northern Izu–Bonin arc, provide new insight into upper crust development in an intra-oceanic, convergent margin environment.