CONDITIONS AND TIMESCALES OF RHYOLITE STORAGE IN THE MIOCENE SEARCHLIGHT MAGMATIC SYSTEM, NV

The Searchlight Magmatic System (SMS) is an ideal locality for investigating conditions and timescales of silicic magma storage prior to volcanic eruptions. Previous studies on field relations, geochemistry, and geochronology of the ~90-degree tilted Searchlight pluton and overlying Highland Range volcanics suggest they are cogenetic. In this study, we apply Rhyolite-MELTS geobarometry and Ti-in-quartz diffusion chronometry to two high-silica rhyolite lava units from the Highland Range. We estimate residence pressures of the magmas that produced these lavas using major element compositions of matrix glasses. Results indicate these magmas resided at average 160 MPa (~6 km; ±59 MPa 2σ), which is comparable to the estimated emplacement depths of the middle unit of the pluton. Quartz grains commonly display fine, oscillatory zoning in panchromatic CL images, which we attribute to variations in Ti given the strong correlation with zones in ~450 nm wavelength CL images. The eutectoid nature of the system and euhedral pattern of the fine zoning suggests oscillations are related to processes other than fluctuations in intensive parameters (e.g., P, T). We calculate quartz crystallization times from profiles obtained from CL images, and preliminary results indicate that most quartz crystallization occurred within decades of eruption, with rim timescales of <10 years when applying the Ti-in-quartz diffusion coefficient of Cherniak et al. (2007) at 750 °C. Alkali feldspars and plagioclase in the rhyolites are generally unzoned or have simple major element zoning based on back-scattered electron (BSE) images. In contrast, feldspars from trachydacite lavas erupted earlier in the volcanic sequence contain complex BSE zoning, implying that a range of pre-eruptive magmatic processes must have occurred in the SMS to produce the variety of crystal textures and compositions. We are undertaking additional work into these lower volcanic units to further assess the evolution of the SMS.