Generation of Silicic Melts in the Early Izu-Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea

A 1.2 km thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation, and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U, and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved. Plain Language Summary How does subduction of tectonic plates begin, and how does this process lead to the formation of new explosive volcanoes on the sea floor? Understanding the early stages of these important geologic processes is challenging, because their rock record is usually hidden, buried deep beneath younger rock and sediment. International Ocean Discovery Program Expedition 351 set out to answer these questions by recovering a continuous, 1.6 km long drill core from beneath the Philippine Sea floor that contains a rock record of these geologic events. Sedimentary rocks recovered from deep below the sea floor during the expedition preserve a remarkable record of submarine volcanoes above a new subduction zone. Radioactive isotopes and trace elements in minerals from these sediments reveal a 15 million year-long account of the growth and evolution of explosive submarine volcanoes.