RAPID THERMAL AND VOLATILE EVOLUTION OF MAGMAS ASSOCIATED WITH A LARGE IGNIMBRITE ERUPTION: POCO CANYON CALDERA SYSTEM, NEVADA

Caldera-forming eruptions may leave behind eruptible melt in shallow intrusive reservoirs. However, it is unclear whether the remaining melt can feed subsequent eruptions without a new influx of magma. To address these two possibilities, we present results of U-Pb zircon geochronology, geothermometry, and hygrometry from the Oligocene Poco Canyon Caldera system and the associated Freeman Creek pluton in the Stillwater Range, central Nevada. This system is ideal for understanding magmatic evolution in response to an eruption as it is exposed in a tilted upper crustal section (maximum 10 km paleodepth; John 1995) such that coeval, and likely cogenetic, intrusive and extrusive magmatic products can be readily compared.

Our field observations suggest the following series of magmatic events: 1) an initial shallow granitic intrusion, 2) the caldera-forming eruption of the rhyolitic Tuff of Poco Canyon (~360 km³), 3) silicic injections into a caldera margin dike (~0.5 by 6 km), and 4) intrusion of the shallowest portions of the granitic Freeman Creek pluton, which is thought to represent a section of the magma storage system that produced the caldera forming eruption (John 1995; Colgan et al., 2018). New CA-ID-TIMS U-Pb zircon geochronology indicate that all of these units intruded or erupted at ~25.3 Ma, with minimal intra- and inter-sample age dispersion indicating that the documented sequence of events occurred over <10⁵ kyr. Two-feldspar and Fe-Ti-oxide mineral thermometry display an ~150°C range in crystallization temperatures over the lifetime of the magmatic system, with the highest temperatures recorded in magmas that ultimately erupted. Hygrometry calculations based on plagioclase and estimated melt compositions suggest degassing associated with the Tuff of Poco Canyon eruption and potential addition of undegassed (i.e., higher H₂O content) magma in intrusions that follow the eruptive phase. We use the thermal and volatile record of magmas in the Poco Canyon Caldera system to assess the effect of large eruptions on magma eruptibility over a relatively short time period (10s to 100s of kyr).