Zircon geochronological and geochemical insights into pluton building and volcanic-hypabyssal-plutonic connections: Oki-Dōzen, Sea of Japan - a complex intraplate alkaline volcano

The relationship between plutonic and volcanic components of magmatic plumbing systems continues to be a question of intense debate. The Oki-Dōzen Islands, Sea of Japan, preserve outcrops of temporally-associated plutonic, hypabyssal and volcanic rocks. Juxtaposition of these, by post-intrusion uplift, placed Miocene syenites in inferred faulted contact with volcanic trachytes that are cut by rhyolite hypabyssal dikes. This provides a window deep into the timing and origins of magma storage architecture and dynamics. Our aim is to determine what the age and composition of zircon, which is ubiquitous in all samples, can reveal about the plutonic-volcanic connection. Here we show magma source characteristics are recorded in zircon Hf isotopes whereas, in addition to source composition, differentiation processes - assimilation of heterogeneous hydrothermally altered crust and extensive fractional crystallization - are preserved in zircon O isotopes and trace elements, respectively. Combined with new U-Th-Pb SHRIMP zircon ages, 6.4–5.7 Ma, the compositional data show pluton formation was by protracted amalgamation of discrete magma pulses. The rhyolite dike preserves an evolved fraction segregated from these. Synchronous with plutonism was volcanic eruption of trachyte magma derived from the same source, but apparently stalled at a relatively shallow depth. Stalling occurred at least above the zone of amphibole stability since amphibole-compatible Sc and Ti were not depleted in the trachyte melt - resulting in elevated values of these in the volcanic, compared to the plutonic, zircon. Identifying smaller episodic magma pulses in a larger magmatic complex places constraints on potential magma fluxes and eruptible volumes. Distinct from high-flux, large volume, plume-related ocean islands with extensive vertically distributed multi-stage magmatic reservoirs or subduction-related transcrustal magma reservoirs, Oki-Dōzen was a low-flux system with incremental pluton growth and small- to moderate-scale eruptions.