ERUPTION OF RHYOLITE AT THE HONEYCOMB HILLS, UTAH - CYCLICAL TAPPING OF A ZONED SILICIC MAGMA RESERVOIR

The Honeycomb Hills volcanic complex, erupted at 4.7 Ma in west central Utah, consists of a 12.5-m-thick tephra deposit that is predominantly fallout tuff, overlain by a 0.15 km3 dome of rhyolitic lava. The Honeycomb Hills rhyolite contains unusually high fluorine (up to 3.4 wt %) and lithophile trace elements (Rb to 1900 ppm). The dominantly fallout tephra deposit has been examined in detail, quantifying textural and compositional parameters. Three distinct eruption phases produced the tephra deposit. Phases I and II display similar changes in clast size, clast speciation, crystal content, pumice vesicularity, bedding style, and glass and mineral composition. The final phase of eruption, phase III, exhibits similar compositional changes while texturally recording the transition from explosive to effusive eruption style. Two-feldspar temperatures yield an average temperature of 570-degrees-C with a weakly developed gradient from 550 to 600-degrees-C upward through the tephra. f(H2O)/f(HF) increases upsection from 560 near the base of the tuff to a maximum of 2400 in the vitrophyre at the top of the pyroclastic section. A corresponding increase exists in the f(HCl)/f(HF) ratio from 0.17 to 5.85. The mineral assemblage topaz-anorthite-annite-fluorite-quartz yields fugacities Of f(H2O) from 250 to 1200 bars and f(HF) from 0.2 to 0.8 bars. Fugacity ratios, as well as major and minor elements in glass and biotite, exhibit cyclic variation within each eruptive phase whose boundaries are defined by physical parameters. The cyclicity is attributed to dynamic relaxation of the magma reservoir during brief pauses between eruptive phases. The patterns of varying eruption energy a-re most strongly correlated with vent widening, crystal content, and gas fugacity ratios. Initially, increasing vent radius and tapping of deeper, higher f(H2O) regions strongly increased eruption vigor. evident in phases I and II. During phase III, continuing crystallization, loss of volatiles, increasing viscosity, and growing conduit size from vent erosion led to the transition from explosive to effusive eruption, culminating in the extrusion of the lava dome.