EPHEMERAL MELT EXTRACTION AND SILICIC CUMULATE FORMATION IN AN UPPER CRUSTAL JAPANESE PLUTON

Explosive silicic eruptions are responsible for some of the most catastrophic events in Earth history, yet evidence of their growth and development is mainly derived from the resulting pyroclastic deposits. Discovery of plutonic systems which form by similar processes are important to investigate the timescales of upper crustal rhyolitic melt residence from the subvolcanic perspective. The ideal place to study the intrusive equivalents of silicic volcanic eruptions is within the shallow (≤10 km) plutons exposed at the roofs of young, active subduction zones. One such location is the Tottabetsu plutonic complex in northern Japan which displays petrologic and geochemical evidence of both silicic melt extraction and complementary silicic cumulate formation. Enrichments in Zr concentrations and positive Eu anomalies from diorites (>1250 ppm Zr) are matched by Zr depletions and negative Eu anomalies in the adjacent granites (≤ 250 ppm Zr), recording a non-protracted melt removal process that can be dated. We present the first U-Pb zircon geochronology from this Miocene pluton from three distinct geochemical domains of granite (71-75 wt. % SiO₂), leucodiorite (60-70 wt. % SiO₂), and diorite (53-57 wt. % SiO₂). Zircon dates from all three span ~2 Ma from 19.5 to 17.5 Ma, coeval durations within the resolution of the LA-ICPMS technique. We discuss the implications of this ~2 Ma age dispersion and future plans for investigation of the pluton using petrochronology and thermochronology. Silicic cumulates may be more common in the rock record than currently appreciated, but detailed system-specific petrochronologic studies are needed for their identification, rather than using big data compilations.