Abstract
The Honeycomb Hills rhyolite represents differentiation in a highly evolved magma. A pyroclastic sequence 12.5 m thick and a dome of ~0.2 km3 occur in western Utah in a region populated with several Tertiary topaz rhyolites. Phenocrysts consist of quartz, sanidine, and albite (10–50% total) in a glassy or fine-grained groundmass. Primary phenocrysts and megacrysts of topaz and fluorsiderophyllite (- lolo total) and accessory phases usually associated with rare-element pegmatites occur: fergusonite, ishikawaite, columbite, fluocerite, thorite, monazite, and zircon. Whole-rock composition (SiO2 = 73.3%, TiO2 = 0.01, Al2O3 = 14.0, Fe2O3 = 0.28, FeO = 0.55, MnO = 0.07, MgO < 0.01, CaO = 0.42, Na2O = 4.59, K2O = 4.44, P2O5 < 0.01, F = 0.61, Cl = 0.10, and maximum values Rb = 1960 ppm, Cs = 78, Li = 344, Sn = 33, Be = 80, and Y = 156) is peraluminous, highly evolved, and comparable to rare element pegmatites. Elevated F contents of up to 2.3% in glass account for low silica and high alumina contents because the granite minimum shifts toward the Ab apex of the Q-Ab-Or ternary with increasing F. Mineralogy and distribution of trace elements with order of eruption indicate evacuation of a cool (570–610 °C), chemically stratified magma chamber. Chemical variation within the erupted volume can be modeled by Rayleigh fractionation of 75% of the phenocryst phases. Spatial variation of some elements, notably Li, Be, B, F, and Cs, may be due to volatile transfer. Enrichment of H2O and F in interstitial melt during crystallization reduced viscosity enough to allow eruption of the highly crystalline lava of the dome.
