LINKING ARC INITIATION TO SILICIC MELT EVOLUTION IN AN OCEANIC ARC SETTING USING DETRITAL ZIRCONS

Understanding the petrologic evolution of oceanic arc magmas through time is important because these arcs reveal the processes of formation and the early evolution of juvenile continental crust. We examined Pb/U and trace element compositions in zircons recovered at IODP Site 351-U1438 located in the proximal back-arc of the northern Kyushu-Palau Ridge at 27.3°N. A 1.2 km thick section of Paleogene volcaniclastic rocks, increasingly lithified and hydrothermally altered with depth, constitutes a proximal rear arc sedimentary record of arc initiation and early evolution of the Izu-Bonin arc. The ages and compositions of U1438 zircons are compatible with provenance in one or more growing submarine edifices of this arc and capture a record of juvenile silicic arc melt evolution. Detrital zircon Pb/U maximum depositional ages range from 28 to 37 Ma, are consistent with depositional age models based on combined magnetostratigraphic and biostratigraphic constraints, and indicate zircon-bearing melts were generated <10 Ma after arc initiation. The abundances of selected trace elements with high native concentrations in zircon provide insight into the petrogenesis of host melts, and indicate short-term variations in melt compositions. The 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. Age similarities and geochemical affinity of some detrital zircons to igneous zircons in Eocene leucotonalite and granodiorite from a partially exhumed intrusive suite at Komahashi-Daini Seamount in the northern Kyushu-Palau Ridge suggests that these zircons also can yield insight into the link between silicic volcanism and mid-crustal intrusion in the Paleogene Izu-Bonin arc.