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.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.