Silicic Magmas in the Izu-Bonin Oceanic Arc and Implications for Crustal Evolution

Abundant rhyolite has erupted from the Izu-Bonin-Mariana volcanic arc (IBM arc) from its earliest stage (Eocene) to the present. Geochemically, three types of Quaternary rhyolites exist in the Izu-Bonin arc front, and they are closely related to volcano type and crustal structure. The dominantly basaltic islands of the volcanic front produce small volumes of rhyolites that we call R1. The submarine calderas of the volcanic front erupt mostly rhyolite that we call R2. Seamounts, knolls, and pillow ridges in the backarc extensional zone are mostly basaltic but also contain rhyolites that we call R3. The thickest total crust, and the thickest intermediate composition middle crust, occurs below the dominantly basaltic volcanoes, whereas the intermediate composition middle crust tends to be thinner beneath the submarine calderas. R1 rhyolites may be derived from Quaternary andesitic sources whereas R2 and R3 rhyolites may be derived from Oligocene ones. The higher CaO/Al2O3 in R1 compared with R2 and R3 rhyolites can be attributed to the same difference between Quaternary and Oligocene andesite sources, respectively. Light rare earth element (LREE)-depleted REE patterns of Quaternary andesites versus flat to slightly LREE-enriched patterns of Oligocene andesites are almost parallel to those of R1 and R2, respectively. Partial melts of Quaternary andesite will have low Zr/Y values such as in R1, whereas melts of Oligocene andesites will have the higher Zr/Y values of R2. Lavas from basalt-dominant volcanoes have higher Sr, Pb, and sometimes Nd isotope ratios than those from rhyolite-dominated volcanoes, which are closer to the isotope ratios of the Oligocene arc. Why and how do crustal sources differ systematically and alternately along and behind the Izu-Bonin arc ? If locally developed regions within the mantle wedge (hot fingers) produce large basaltic volcanoes and remain stationary for millions of years, then basalt-dominant volcanoes eventually will overlie thicker crust. Remelting of middle crust to form rhyolite magmas takes place beneath both basaltic and rhyolitic volcanoes (R1 and R2 rhyolite, respectively). However, basalt volcanoes consume new middle crust to produce rhyolite magma whereas rhyolite volcanoes consume old Oligocene middle crust. Moreover, rhyolite volcanoes have no mantle roots beneath the crust. Instead, dikes from basalt volcanoes provide the heat source to partially melt the crust.